Electrochemical CO2 conversion can be coupled with a photovoltaic cell and provide a pathway to utilize solar energy for the chemical synthesis. Ideally, such artificial photosynthesis system want to use CO2 and H2O as feed-stock molecules to produce value-added chemicals such as fuels or raw chemicals. My research team reported a monolithic and stand-alone device composed of a photovoltaic cell module, an Au CO2 reduction, a cobalt oxide anode accomplishing over 4 % conversion efficiency for CO2 conversion to CO production. To improve the solar to chemical conversion efficiency and to increase the feasibility further, we have developed efficient electrocatalysts and replaced the photovoltaic cell with Si modules, achieving ~ 8% of solar-to-CO conversion efficiency.

In addition, in this talk, metal-based electrocatalysts interacting with p-block elements or surface mediated molecules will be discussed for selective CO or C2+ (i.e. ethylene) production from CO2 reduction. The experimental results and theoretical simulation with various different types of metal catalysts (Ag, Zn, and Cu) give insights how to suppress the hydrogen evolution reaction (HER) is crucial to achieve efficient CO2 reduction catalysts. Monodispersed Ag nanoparticles are suggested to have the special interaction between the surface Ag and the surface mediated molecules which can modify the local electronic structure favoring for the selective CO production (up to 95 % of Faradaic efficiency). In addition, in the case of selective ethylene production, special Cu nanostructure formed by in-situ electrochemical fragmentation is demonstrated to be effective for increasing C-C bond coupling (up to 73 % of Faradaic efficiency) and selective ethylene production (up to ~ 60 % of Faradaic efficiency). In-situ X-ray absorption spectroscopy (XAS) studies are performed to understand the catalyst activity. Our series of studies suggests the modification of the metal nanoparticle surface by oxygen atom or surface mediated molecules can be effective strategies to increase CO2 reduction reaction activity and stability.

Chemically synthesized quantum dots (QDs) can potentially enable new classes of highly flexible, spectrally tunable lasers processible from solutions [1,2]. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage and an important challenge - realization of lasing with electrical injection - is still unresolved. A major complication, which hinders the progress in this field, is fast nonradiative Auger recombination of gain-active multicarrier species such as trions (charged excitons) and biexcitons [3,4]. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination by “softening” the electron and hole confinement potentials [5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [6], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a “zero-threshold-optical-gain” concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [7]. Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting recent developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices operating across a wide range of wavelengths and fabricated on virtually any substrate using a variety of optical-cavity designs.

[1] Klimov, V. I.et al., Optical gain and stimulated emission in nanocrystal quantum dots. Science290, 314 (2000).

[2] Klimov, V. I.et al., Single-exciton optical gain in semiconductor nanocrystals. Nature447, 441 (2007).

[3] Klimov, V. I. et al., Quantization of multiparticle Auger rates in semiconductor quantum dots. Science287, 1011 (2000).

[4] Robel, I., et al., Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett.102, 177404 (2009).

[5] Y.-S. Park, et al., Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).

[6] Lim, J., et al., Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater.17, 42 (2018).

[7] Wu, K., et al., Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol.12, 1140 (2017).

The need for better energy storage and conversion devices has never been so urgent in order to enable the rapid deployment of renewable energies and reduce our use of fossil fuels. While batteries spurred the spread of portable electronics, their limited energy density hampers their use for large scale applications. Hydrogen has long been envisioned as a viable energy carrier owing to its very large energy density, nevertheless its electrochemical production by electrolysis, the most efficient carbon-free production route at large scale, greatly suffers from the slow kinetics associated with the oxygen evolution reaction (OER). The key challenges that need to be addressed to improve the OER kinetics are well-spotted and researchers eagerly pushed to better understand the reaction leading to numerous progresses since our early vision of this reaction. Hence, while some works were devoted to finding physical descriptors capable of describing the OER activity following a Sabatier principle, recent developments in the field point towards the complexity of such reaction. Indeed, a substantial body of evidence now points towards the involvement of the bulk chemistry of the most active transition metal oxides in the OER mechanism. Hence, we recently found that bulk oxygen atoms are evolved under OER conditions for cobalt-based perovskites materials,¹ eventually triggering a new mechanism into which chemical steps and proton exchange are rate limiting. We could then demonstrate that the origin for the often observed activity-stability relationship for OER electrocatalysts is nested into the existence of a common intermediate which is reactive surface oxygen in the form of oxyl-group.² Overall, the line becomes blurrier between heterogeneous and homogeneous catalysis when using transition metal oxides as OER catalysts for which complex surface dynamics are at play.³ Hence, efforts must be paid to understanding and stabilizing these intermediates in order to break this relationship and further enhance the stability of OER electrocatalysts. We will therefore discuss in this talk our recent efforts at designing chemical approach to counterbalance the chemical reactivity of the most active transition metal oxides through a combined crystallographic and electrolyte engineering approach.

The increasing replacement of fossil fuels by renewable energies from wind and sun requires large energy storage capabilities, because of the only intermittent availability of these sources. Such big capacities can only be provided by the storage in chemical bonds, the simplest being the hydrogen molecule formed by direct electrochemical water splitting. For this purpose, expensive and rare electrocatalysts made from Platinum series metals will have to be replaced by cheaper, more abundant and hazard-free materials. Nickel metal and its oxides are known since many years for their good activity for water splitting.

In our studies Nickeloxide nanoparticles and thin films are prepared by electrochemical deposition as well as by magnetron sputtering. The composition is optimised towards the achievement of an optimum activity for the hydrogen evolution reaction (HER) as well as the oxygen evolution reaction (OER). In order to gain a deeper understanding of the critical parameters for the catalytic activity during the electrochemical reaction, we investigate the chemical composition by XPS and SEM before and after electrochemical testing as well as by in-operando Raman-spectroscopy. The achieved activities are comparable to Platinum for the HER and to RuO2 for the OER. They depend strongly on the chemical composition of the catalyst, which in turn is heavily influenced by the chosen preparation method. Furthermore, we investigate the stability of our catalysts in relation to their activity and composition. The highly active Nickel (oxide/hydroxide) mixture for the HER degrades over time by the complete transformation to the less active pure Nickel-dihydroxide compound. For the OER the pre-treatment of the Nickel compound is of extreme importance in order to form a large amount of the catalytically active NiO(OH) species on the surface during the electrochemical reaction. These nanoparticles show a nearly constant activity for a testing period of 26 hours at a current density of 10 mA/cm2.

The study of electrocatalysts for water splitting is pivotal to improving the efficiency of fuel production. This is an attractive field since electrocatalysts can be used: (1) as part of a photoelectrode, for direct sunlight to fuel transformation, or (2) can be used in an electrolyser to transform the electricity generated through a solar panel to produce a fuel. In a complete system it is thought that water oxidation is the limiting process. Therefore, a lot of effort is devoted to finding a better catalyst for this reaction in order to enhance the final performance of the system.

Recently, iron and nickel oxyhydroxides have been appointed as good candidates for the water oxidation reaction in basic conditions. In this context, interest for understanding the origin of their behaviour has grown in recent years. In 2014, Boettcher and co-workers published for the first time that the high activity towards water oxidation of NiOOH was due to Fe incorporation from the electrolyte.[1] Since then, there has been a lot of effort to determine which species are involved in the catalytic reaction, with no consensus reached. Some experiments suggest the presence of Fe(IV) during the catalysis,[2] while on the contrary, other works cannot detect such iron species and suggest that nickel centres are the active sites.[3] Despite this debate, some structural differences have been found due to iron incorporation which leads to an improved performance. However, the mechanistic and kinetic analysis of metal oxide based electrocatalysts, in general, is hampered by the non-ideal nature of these materials. Most of these electrocatalysts present different redox states and a non-planar and dense structure. Thus the interpretation of the traditional electrochemical techniques, such as tafel plots, is more complicated and sometimes not possible.[4]

In this work, we used spectroelectrochemical methods to analyse three different samples: Pure FeOOH, a mixed FeOOHNiOOH sample composed of different layers, and finally Ni(Fe)OOH with spontaneous Fe incorporation. From these measurements, we are able to determine the active species and study the kinetics under catalytic conditions to yield a basic mechanistic picture.

References

[1]          L. Trotochaud, et al. JACS, 2014, 136, 6744-6753.

[2]          J. Y. C. Chen, et al. JACS, 2015, 137, 15090-15093.

[3]          M. Görlin, et at, JACS, 2017, 139, 2070-2082.

[4]          E. Pastor, et al, Nat. Commun., 2017, 8, 14280.

Photoelectrochemical water splitting is one of the most interesting alternatives to produce hydrogen in a clean way by solar energy conversion.(1) Despite the huge potential and great advances, new materials need to be developed in order to take this technology to a commercial level. At present, different materials as oxides, oxisulfides and metal chalcogenides are being investigated as photoelectrodes in photoelectrochemical cells. However, achieving high energy conversion efficiencies by using a single material is a very tricky objective. Therefore, hybrid materials are getting a lot of attention lately. (2)

In this work, we present a hybrid material formed by the heterojunction of a novel synthesized organic conductive polymer and TiO2 nanocrystals. The nanostructured conjugated porous polymer is based on dithiothiophene moiety (Nano-CMPDTT) and was synthesized by Sonogashira cross coupling reaction from precursors in mini-emulsion conditions. In order to elucidate the electronic structure and the ability of this material to be used as a photocatalyst, HOMO and LUMO positions were determined by cyclic voltammetry. The energy diagram shows an ideal position of the energy bands in order to use the synthesized polymer as an electron injector to TiO2 in photocatalytic reactions. TiO2 NCs and organic polymer suspensions have been deposited by spin coating in ITO glasses. The formed films have been characterized by X-ray diffraction, SEM, EDX and AFM. Photoelectrochemical measurements were performed in a three electrode cell configuration, using the hybrid material as the working electrode. The hybrid material presents an enhancement in photovoltages and photocurrents values. Electrochemical Impedance Spectroscopy (EIS) was performed to confirm the improved charge transfer observed when illuminating the hybrid material in comparison to the TiO2 nanocrystals alone. In fact, a decrease in the resistance associated with this phenomenon was found. This confirms that the presence of the polymer in the hybrid material increases the absorption of light, charge transfer and reduces electron-hole recombination, making this hybrid a good candidate to be used as a photoelectrode for the hydrogen evolution reaction. In fact, recent results show an improvement in the energy conversion efficiency by using this new hybrid material as electrode compared with regular TiO2.

[1] Z. Chen, H. N. Dinh, E. Miller, Photoelectrochemical Water Splitting (Springer New York, New York, NY, 2013; http://link.springer.com/10.1007/978-1-4614-8298-7), SpringerBriefs in Energy.

[2] M. P. Arciniegas et al., Self-Assembled Dense Colloidal Cu2Te Nanodisk Networks in P3HT Thin Films with Enhanced Photocurrent. Adv. Funct. Mater. 26, 4535–4542 (2016).

Transition metal oxides are amongst the most widely studied materials for solar water oxidation owing to their earth abundance, good aqueous stability and facile syntheses.¹ Understanding the dynamics of the photogenerated charge carriers in these materials is key to highlighting properties that need to be targeted to improve water splitting performance. With a narrower band gap than TiO₂, tungsten trioxide (WO₃) can absorb a larger proportion of the solar spectrum and has a deeper valence band energy to provide a large thermodynamic driving force for water oxidation.^2-3 WO₃ is also often reported to have high electrical conductivity, leading to its frequent implementation as an electron transporting layer in various heterojunction photoanodes.^2-4 This high conductivity is considered to be a result of a large density of charge carriers, caused by intrinsic oxygen vacancies which can act as n-type dopants.⁵ However, deviations from stoichiometry have also been suggested to introduce chemical defects that can result in increased trapping of charges which, rather than boost performance, can often introduce additional recombination pathways.⁶

Using transient diffuse reflectance spectroscopy and transient photocurrent measurements, I will discuss the dynamics of the photogenerated charges in WO₃ photoanodes, synthesised by chemical vapour deposition (CVD). The synthesis method generates heavily doped monoclinic WO_3-x needles, which are annealed in air at elevated temperature to remove most of the oxygen vacancies. These photoanodes exhibit an early photocurrent onset and a high faradaic efficiency (>85%) for water oxidation. Compared to other transition metal oxide photoanodes, we observed rapid water oxidation (>1 ms) but found the rate of electron extraction is significantly slower (>10 ms). We investigated the effect of oxygen vacancy states on electron transport and found that electron trapping in the needles is significant, proposing a trap-mediated mechanism of electron transport to the back contact. We then used ultrafast transient absorption spectroscopy to examine the bias dependence on bulk recombination processes. Finally, we altered the oxygen vacancy content to determine the overall effect of these defects on charge transport and performance.

[1] G. Wang, et al. Energy Environl Sci, 2012, 5, 6180–6187.

[2] Z. Huang, et al. Advanced Materials, 2015, 27 (36), 5309-5327.

[3] X. Liu, et al. PCCP, 2012, 14 (22), 7894-7911.

[4] C.X. Kronawitter, et al. Energy Environl Sci 2011, 4 (10), 3889-3899.

[5] T. Zhu, et al. ChemSusChem, 2014, 7, 2974-2997

[6] M.B. Johansson, et al. J Phys Condens Matter, 2016, 28, 475802.

Organometallic complexes with reactive metal centers are promising candidates for photocatalysis. To improve their performance, insight into the nature of excited states and control thereof is crucial. Ultrafast x-ray spectroscopy is a powerful method to achieve mechanistic insight into processes at catalytic metal moieties and is particularly useful to detect light-induced changes in oxidation state and coordination geometry. It can also be applied to probe the essential charge transfer step to the catalyst in real-time, including the potential involvement of atomic rearrangements.
We recently developed a series of new Ru-metal photocatalysts, of which the RuPt derivative showed a H2 turnover number of 80 after 6 h of irradiation at 470 nm. The photodynamics of RuPt studied by femtosecond transient absorption are highly complex and differ significantly from the RuPd analogue [1], indicating an important role of the catalytic moiety and motivating ultrafast x-ray absorption studies performed at the Advanced Photon Source at Argonne. This work particularly focuses at light-induced redox processes at the Pt moiety and the timescales thereof. Another question to be answered involves the local structure of the catalytic moiety and possible temporal or permanent changes.
The earliest differential absorption spectrum at 30 ps indicates that the Pt moiety is already reduced at this timescale. The signal intensity partially decays on a timescale of ca. 930 ps. The observation of a differential absorption signal at timescales far beyond indicates branching. The spectra at late timescales can be modelled by a hexa-coordinated PtIV (or possibly PtIII) species. This intermediate species may be formed by oxidative addition of iodine, is long-lived (>10 microseconds) and ultimately recovers into the original ground state structure. Hence, activation of the catalyst by a single photon may lead to withdrawal of two electrons. This mechanism presents a new paradigm for H2 generation by supramolecular photocatalysts.

[1] Q. Pan, F. Mecozzi, J.P. Korterik, J.G. Vos, W.R. Browne, A. Huijser, "The Critical Role Played by the Catalytic Moiety in the Early-Time Photodynamics of Hydrogen-Generating Bimetallic Photocatalysts", ChemPhysChem 17, 2654-2659 (2016).   

Developing energy storage mechanisms is a crucial step to secure a sustainable energy economy. This is particularly important for solar power because there is a mismatch between the periods of peak energy harvesting and peak consumption. Photo-electrocatalytic energy storage has traditionally been dominated by inorganic systems. In particular, transition metal oxides are attractive due to their stability in liquid electrolytes and the remarkable catalytic flexibility of metallic centres. However, inorganic systems often underperform and only poor quantum yields are achieved, even when synthetic care is taken to eliminate defects and impurities.  In the dark, oxides like Fe₂O₃ and BiVO₄ are poor conductors due to the localization of charges on specific atomic sites forming small polarons. Under illumination, transport improves but the mechanisms behind it are unknown. Recent x-ray and terahertz studies have hinted at the existence of light-induced small polarons and their link with structural properties. ^1-3 However, these measurements lack device sensitivity and it is difficult to know if such states really influence performance. In this talk, I will present ultrafast optical studies with photocurrent detection of photoelectrochemical devices. These measurements are capable of probing light-initiated changes that control actual device activity.⁴ I will present data demonstrating that, in oxides such as efficient Fe₂O₃ thin-films,⁵ charges intrinsically self-trap within femtoseconds of photo-generation hindering transport. Importantly, I will show how in efficient systems, crossover between molecular-type, localised transport and band-like transport is critical and necessary for activity. This behavior is remarkably similar to that observed in molecular crystals and some of the strategies used to improve molecular semiconductors might apply to metal oxides. I will discuss the material implication of these findings and how it might be possible to achieve synthetic control of charge hopping and delocalisation rates in oxides.

References:

¹Carneiro L. et al. Nat Mater. 2017, 16, 8, 819 

²Biswas S. et al. Nano Lett. 2018,18,1228

³Butler  K.T. et al. J.Mat.Chem.A, 2016,4,18516

⁴Bakulin A. et al. J.Phys.Chem. Lett. 2016, 7, 2, 250

⁵Steier L. et al. ACS Nano, 2015,9,11775

Mimicking photosynthesis and producing solar fuels is an appealing way to store the huge amount of renewable energy from the sun in a durable and sustainable way. Hydrogen production through water splitting has been set as a primary target for artificial photosynthesis,¹ which requires the development of efficient and stable catalytic systems, only based on earth abundant elements, for the reduction of protons from water to molecular hydrogen. We will report on our contribution to the development of various series of catalysts for H₂ evolution,^2-4 including the reinvestigation of amorphous molybdenum sulfide⁵ and to the establishment of methodologies towards the rational benchmarking of their catalytic activity. Besides, we will also describe our effort towards the combination of such catalysts with various photoactive motifs for the preparation of photoelectrode materials^6-10 that can be implemented into photoelectrochemical (PEC) cells for water splitting.

References

1.         N. Queyriaux, N. Kaeffer, A. Morozan, M. Chavarot-Kerlidou and V. Artero, J. Photochem. Photobiol. C, 2015, 25, 90-105.

2.         D. Brazzolotto, M. Gennari, N. Queyriaux, T. R. Simmons, J. Pécaut, S. Demeshko, F. Meyer, M. Orio, V. Artero and C. Duboc, Nat. Chem., 2016, 8, 1054-1060.

3.         N. Kaeffer, M. Chavarot-Kerlidou and V. Artero, Acc. Chem. Res., 2015, 48, 1286–1295.

4.         T. N. Huan, R. T. Jane, A. Benayad, L. Guetaz, P. D. Tran and V. Artero, Energy Environ. Sci., 2016, 9, 940-947.

5.         P. D. Tran, T. V. Tran, M. Orio, S. Torelli, Q. D. Truong, K. Nayuki, Y. Sasaki, S. Y. Chiam, R. Yi, I. Honma, J. Barber and V. Artero, Nat. Mater., 2016, 15, 640-646.

6.         J. Massin, M. Bräutigam, N. Kaeffer, N. Queyriaux, M. J. Field, F. H. Schacher, J. Popp, M. Chavarot-Kerlidou, B. Dietzek and V. Artero, Interface Focus, 2015, 5, 20140083.

7.         N. Kaeffer, J. Massin, C. Lebrun, O. Renault, M. Chavarot-Kerlidou and V. Artero, J. Am. Chem. Soc., 2016, 138, 12308−12311.

8.         T. Bourgeteau, D. Tondelier, B. Geffroy, R. Brisse, C. Laberty-Robert, S. Campidelli, R. de Bettignies, V. Artero, S. Palacin and B. Jousselme, Energy Environ. Sci., 2013, 6, 2706-2713.

9.         T. Bourgeteau, D. Tondelier, B. Geffroy, R. Brisse, R. Cornut, V. Artero and B. Jousselme, ACS Appl. Mater. Interfaces, 2015, 7, 16395–16403.

10.       A. Morozan, T. Bourgeteau, D. Tondelier, B. Geffroy, B. Jousselme and V. Artero, Nanotechnology, 2016, 27, 355401.

One of the most promising future sources of alternative energy involves water-splitting photoelectrochemical cells (PECs) – a technology that could potentially convert sunlight and water directly to a clean, environmentally-friendly, and cheap hydrogen fuel. Practical PEC-mediated hydrogen production requires robust and highly efficient semiconductors, which should possess good light-harvesting properties, a suitable energy band position, stability in harsh condition, and a low price. Despite great progress in this field, new semiconductors that entail such stringent requirements are still sought after. Over the past few years, graphitic carbon nitride (g-CN) has attracted widespread attention due to its outstanding electronic properties, which have been exploited in various applications – including in photo- and electro-catalysis, heterogeneous catalysis, CO2 reduction, water splitting, light-emitting diodes, and PV cells. gCN comprises only carbon and nitrogen, and it can be synthesized by several routes. Its unique and tunable optical, chemical, and catalytic properties, alongside its low price and remarkably high stability to oxidation (up to 500 °C), make it a very attractive material for photoelectrochemical applications. However, to date, only a few reports regarded the utilization of g-CN in PECs, due to the difficulty in acquiring a homogenous g-CN layer on a conductive substrate and to our lack of basic understanding of the intrinsic layer properties of g-CN. In this talk I will introduce new approaches to grow g-CN layers with altered properties on conductive substrates for photoelectrochemical application. The growth mechanism as well as their chemical, photophysical, electronic and charge transfer properties will be discussed.

While the field of sunlight-driven fuel generation has traditionally been dominated by inorganic materials, organic photocatalysts are currently gaining substantial momentum - particularly due to their much higher synthetic flexibility. For instance, their optical band gap can be tuned continuously throughout large parts of the solar spectrum by copolymerising suitable monomers in defined ratios.¹ This tunability has sparked intense research interest in organic photocatalysts,^2,3 however, the fundamental understanding of photoinduced processes in these systems has stayed behind the rapid development of new materials. Some parallels can be drawn to organic photovoltaics where comparable materials are used, but especially the aqueous environment makes polymer photocatalysts distinct from other applications. To understand what dictates their performance and how structurally similar polymers can exhibit very different degrees of hydrogen evolution activity,⁴ photophysical processes in these materials require further investigation.

The combined study presented here is the first in-depth investigation of hydrogen evolution activity of linear conjugated polymers and combines materials development with spectroscopic characterisation and computational modelling. We investigate a series of polymers with strikingly different hydrogen evolution activity, including some of the highest performing photocatalysts reported to date in this class of materials. A comparison to structurally related polymers with significantly lower activity allows us to identify the key determinants of hydrogen evolution activity in this series. To this end, we use transient absorption spectroscopy to monitor photogenerated reaction intermediates on time sales of femtoseconds to seconds after light absorption and correlate the type and yield of observed intermediates with the hydrogen evolution activity of the respective polymer. Computational simulations and calculations build on this transient data and extend the observations to the role of the solvent environment in the photoinduced reaction sequence. The presented results can provide design strategies for new materials and thus have implications for the development of more efficient organic photocatalysts.

References

1. Sprick, R. S. et al. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution. J. Am. Chem. Soc. 137, 3265–3270 (2015).

2. Zhang, G., Lan, Z.-A. & Wang, X. Conjugated Polymers: Catalysts for Photocatalytic Hydrogen Evolution. Angew. Chemie Int. Ed. 55, 15712–15727 (2016).

3. Vyas, V. S., Lau, V. W. & Lotsch, B. V. Soft Photocatalysis: Organic Polymers for Solar Fuel Production. Chem. Mater. 28, 5191–5204 (2016).

4. Sprick, R. S. et al. Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts. Angew. Chemie Int. Ed. 55, 1792–1796 (2016).

The concept of dye-sensitized  photoelectrosynthesis cells (DS-PECs) has recently been proposed as an alternative water splitting cell. The DS-PEC architecture involves an oxide semiconductor sensitized by an organic/inorganic chromophore and a water oxidation catalyst (WOC) either connected to the chromophore (a dyad assembly) or directly to the semiconductor. In this assembly, chromophore produces an electron-hole pair upon excitation with visible light followed by the injection of electrons to the semiconductor and then to the cathode compartment while holes are transferred to the catalytic site to activate the catalyst for water oxidation.

Despite the recent promising studies, achieving high stability for both the catalyst and the chromophore under DS-PEC operating conditions, extending the light absorption to visible and near IR region, and developing entirely earth abundant assemblies still remain as key challenges. 

Our recent research efforts have recently been devoted to developing dye-sensitized photoanodes involving cyanide-based assemblies. For this purpose, pentacyanoferrate building block was used not only as donor system for the construction of a new donor-acceptor chromophore but also as a cyanide precursor to produce a heterogeneous cobalt-iron Prussian blue electrocatalyst film. The photoelectrochemical performance and superior stability of the Prussian blue film coupled with TiO₂ will be presented in detail.

Integrated wireless monolithic solar water splitting devices, i.e. monoliths submerged in the electrolyte, is a promising approach for low-cost photoelectrochemical solar water splitting [1]. However, such a device design poses a significant limitation: the ion transport distances around the monolith are long and consequently, the ionic Ohmic losses become high. This turns out to be a bottleneck for reaching high efficiencies and maintain optimum performance when it comes to up-scaling.

In this work, we present a novel approach to tackle the aforementioned limitation. Our device design is based on the concept of porosity as an ionic shortcut, implemented in silicon-based monoliths for low-cost and scalable solar water splitting [2, 3]. Simulation and experimental results towards enabling the proof of concept device consisting of porous multi-junction thin-film silicon solar cells on perforated substrates are presented. Based on simulations, we highlight how porous monoliths can benefit from lower ionic Ohmic losses compared to dense monoliths for various pore geometries and monolith thicknesses. As a result, the overpotentials to drive the water splitting reaction can be reduced by more than 400 mV. Experimentally, the impact of porosity (square array of holes with a period of 100 μm and a diameter of 20 μm) on single-junction and multi-junction amorphous and microcrystalline thin-film silicon solar cells is explained. A minimal decrease in V_OC is seen, with porous triple-junction thin-film silicon solar cells reaching a value of 1.98 V. Additionally, we discuss the implementation of surface coatings by atomic layer deposition to i) alleviate the material degradation that occurs during silicon etching (electrical passivation) and ii) enable a chemically stable operation (protection against material corrosion). Finally, to demonstrate the beneficial effect of porosity on the hydrogen production we focus on two systems: i) a well-known simplified system based on platinum nanoparticle decorated porous silicon photocathodes (V_on ≈ 475 mV) immersed in a sulfite scavenger (cell potential ΔΕ⁰ = 170 mV) in absence of electrical bias and ii) the envisage device of porous multi-junction silicon solar cell monoliths in unbiased solar water splitting conditions. Overall, keeping a device oriented point of view, we discuss the results and challenges in our approach as well as some design guidelines.

References:

[1] S. Y. Reece et al., Science 334, 645-648 (2011).

[2] T. Bosserez et al., J. Phys. Chem. C 120 (38), 21242 – 21247 (2016)

[3] C. Trompoukis et al., Sol. Energy Mater. Sol. Cells 182, 196–203 (2018).

Photocatalytic conversion of CO2 and H2O is an interesting route to produce fuels and chemicals [1]; this process is also known as Artificial Photosynthesis (AP). In last years, extensive efforts have been made to develop efficient catalytic systems capable of harvesting light absorption and reducing CO2 especially when using water as the electron donor. Several modification pathways of inorganic semiconductors have been performed to improve the reaction efficiencies, including tailoring of the band structure, doping with metals and non-metallic elements and deposition of metal nanoparticles, etc [1-2].

Herein, we report different strategies and modifications of photocatalysts to increase process performance. Among them, an interesting approach to improve charge separation in photocatalytic systems is the use of heterojunctions. In this line, the combination of different semiconductors with noble metal nanoparticles or organic semiconducting polymers leads to a separation of the photogenerated charge carriers and thus to increasing their life time, facilitating charge transfer to adsorbed molecules.

The main products, using bare TiO2, were CO and H2, with low concentrations of CH4. The deposition of surface plasmon nanoparticles (SP-NPs) leads to changes in the selectivity to higher electron-demanding products, such as CH4. TAS measurements confirm that this behaviour is due to the electron scavenging ability of SP-NPs.

The H2 evolution rates of organo-inorganic hybrid materials, is increased with the polymer content reaching the optimum with IEP-1@T-10 which improves the activity of TiO2 by a factor of 40. These hybrid materials also show a dramatic reactivity improvement in CO2 photoreduction. Hybrid materials also show a great change in the selectivity, enhancing the relative production of methane vs. carbon monoxide, and largely promoting selectivity towards hydrogen. Meanwhile, bare TiO2 gives rise to the formation of CO, a small amount of CH4 and H2 and traces of CH3OH and higher hydrocarbons (C2 mainly). To explain this behaviour a combination of in-situ NAP-XPS, FTIR, TAS spectroscopies and theoretical tools has been used, showing a more efficient light absorption and charge transfer in the hybrid photocatalyst compared with bare materials.

References

[1] V. A. de la Peña O’Shea, D. P. Serrano, J. M. Coronado, “Current challenges of CO2 photocatalytic reduction over semiconductors using sunlight”, in Molecules to Materials—Pathway to Artificial Photosynthesis, Ed. E. Rozhkova, K. Ariga (Eds.), Springer, London, 2015.

[2] L. Collado, A. Reynal, J.M. Coronado, D.P. Serrano, J.R. Durrant, V.A. de la Peña O'Shea, Appl. Catal. B: Environ. 178, 177, 2015

There is a need to rationalise the catalytic mechanisms occurring during solar fuels production if the design rules for improved materials are to be identified. With both direct solar to fuels materials (e.g. photoelectrodes) and indirect systems (e.g. PV driven electrolysis) there is a common challenge – how to characterise short-lived intermediates that are present only at the electrode surface in the presence of a high concentration of bulk solvent/reagent.

IR-Vis Sum-Frequency Generation (SFG) spectroscopy specifically probes molecules at interfaces offering a sensitive probe of only the catalytically relevant species at the electrode surface. SFG has been widely used to study model electrochemical systems but rarely to explore the mechanisms of carbon dioxide reduction and water oxidation. Here I will describe both early experiments to examine photoelectrode surfaces and our recent reports on the mechanisms of carbon dioxide reduction by group 6 and 7 transition metal electrocatalysts which allow for the identification of new catalytic intermediates and the first in-situ observation of the “protonation-first” pathway by [Mn(bpy)(CO)₃Br]. We will also discuss how the potential and light dependence of the non-resonant response recorded during the SFG experiment can provide important insights into the changing nature of the double layer structure during the study of solar fuels materials.

Storing the energy of sunlight into feedstock chemicals or energy-rich compounds, such hydrogen, appears as an enticing option to broaden the utilization of solar energy far beyond classical photovoltaics. Among different emerging technologies, photoelectrochemical (PEC) water splitting stands out with the promise of competitive solar-to-hydrogen conversion efficiencies (predicted to be above 20 %) and a conveniently simple design.¹ These devices, comprising two photoactive electrodes wired-stacked together, to leverage their complementary light absorption, and directly immersed in an aqueous electrolyte, generate under illumination a voltage greater than 1.23 V, driving the photoelectrosynthetic reactions of H₂ and O₂ separately at the different electrode-electrolyte interfaces. Unfortunately, current conversion efficiencies are far below the expectations. In recent years, there has been an encouraging progress on the refinement of the bulk properties of the semiconducting electrodes (viz. nanostructuring, doping, band gap engineering, etc.) as well as on the design of more active electrocatalyst, both contributing to an improved performance. However, another key component of these devices, the semiconductor-liquid junction (SCLJ), where the complex reactions occur and therefore, whose electrochemical and catalytic properties (namely, surface potential, overpotential, kinetics of charge transfer and recombination) control the conversion efficiency, remains hazy posing a major bottleneck for enhancing the conversion efficiency. A better understanding on the electrocatalytic properties of the SCLJ could not only nail down the processes limiting the performance of these devices but also provide specific routes to patch them.

Over the last few years, a wide variety of in-operando electrochemical based techniques have burst in the field of electrochemical-based solar fuel production offering new insights on the nature of intermediate species, the functioning of electrocatalysts, the carrier dynamics within the electrodes, etc.² Here, we introduce a new technique where a network-type electrical-contact at the electrode-electrolyte interface afford direct probing of the surface carrier dynamics when combined with a transient photocurrent/photovoltage technique. This technique applied to model semiconducting materials incorporating state-of-the-art electrocatalyst demonstrates to provide unprecedented access to steady-state (interfacial energetics) and transient (interfacial kinetics) electrochemical information of the SCLJ in-operando. We believe that these new tools will help to forge a better understanding on the SCLJ and to establish the designing principles for a next generation of more efficient electrodes.

[1] M. S. Prévot, K. Sivula. J. Phys. Chem. C, 2013, 117, 17879-17893

[2] W. A. Smith, I. D. Sharp, N. C. Strandwitz, J. Bisquert. Energy Environ. Sci. 2015, 8, 2851-2862

Due to their potential long term stability, ease of synthesis, and low production cost, semiconducting metal oxide materials have received much attention for use as photoanode materials in photoelectrochemical water splitting devices. To date, most research has focused on binary semiconducting oxide materials. Since no binary oxide material has currently met all the criteria listed above, researchers have expanded the materials search database to include more complex multinary oxides. Among the multinary oxides investigated, bismuth vanadate, BiVO₄, is the highest performing material. However, charge carrier recombination at the BiVO₄/electrolyte interface remains a limitation. Reactions at the BiVO₄/electrolyte interface may give rise to surface states that can act as relay sites for charge injection into the electrolyte, but also as electron and hole traps that can enhance recombination rates. A detailed understanding of the chemical composition at the BiVO₄/electrolyte interface and its dependence on specific conditions (applied potential and illumination) would provide valuable input for strategies to suppress surface recombination and to further optimize BiVO₄-based photoanode materials.

We have used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to gain a molecular-level understanding of the BiVO₄/aqueous electrolyte interface. With soft X-rays water adsorption from the gas phase at pressures up to a few Torr can be studied providing information about the early stages of solid/electrolyte interface formation. The tender X-ray form (AP-HAXPES) can be used to directly interrogate a solid surface under a bulk-like electrolyte film that is tens of nanometers thick. Our AP-XPS measurements on the BiVO₄(010) single crystal surface indicate that the surface is significantly hydroxylated by ~0.5 Torr. Surface hydroxylation is accompanied by reduced vanadium at the surface which leads to occupied states above the valence band maximum. These states may act as recombination centers on BiVO₄-based photoanodes. Using AP-HAXPES we have studied the open-circuit behaviour of the thin-film BiVO₄/potassium phosphate electrolyte interface when illuminated with a solar simulator. Upon illumination we observe spectral changes consistent with the formation of a thin bismuth phosphate layer and significant restructuring of the electrolyte near the interface. Bismuth phosphate formation under illumination may quench surface states that have been observed in capacitance versus voltage scans. Surface state suppression by bismuth phosphate formation is a also potential explanation for the increase in performance of BiVO₄ photoanodes that have undergone light soaking. In general, these results provide fundamental information about the complex chemical behaviour of semiconductor/electrolyte interfaces in water splitting devices.

The development of efficient photoelectrochemical (PEC) cells for solar hydrogen production is necessary for a prospective renewable energy supply of mankind. Semiconducting films in PEC cells are used as absorber layers in a triple system of electrolyte-catalyst-semiconductor for generating hydrogen which can be stored and afterwards reburned to water in gas turbines, fuel cells or for the production of chemicals [1]. Due to the low efficiency and poor stability of the absorber layers in the electrolyte their performance must be considerably improved.

We were successful in the preparation of highly (001)-textured polycrystalline WSe_2+x thin films with a Pd-promoter on TiN:O back contact substrates [2]. In this process, in a first step X-ray amorphous Se-rich WSe_x films have been deposited at room temperature by a reactive magnetron sputtering onto a thin metal-promoter Pd film which were afterwards annealed in an H₂S(e)/Ar atmosphere. During the crystallization process a liquid promoter PdSe_x forms and migrates to the surface of the growing WSe₂ film. It acts as a solvent agent and accumulates at the non van-der-Waals-planes finally initiating the lateral growth of WSe₂ platelets and their coalescence. A maximum hole mobility of 70 cm² V^-1s^-1 was reached for our polycrystalline films, which is close to the value for a single crystalline WSe₂ (100 cm² V^-1s^-1).

Recently, we improved the photoelectrochemical performance using Pt [3], Rh and ammonium thiomolybdate (ATM: (NH₄)₂Mo₃S₁₃) layers on top of the WSe₂ photocathode for hydrogen evolution reaction in an acidic solution. The surface states of WS₂ and WSe₂ crystallites can be passivated by photodeposition of the metal catalysts such as Pt, Rh and Ag. We show that the ATM can be used instead of expensive conventional metal catalysts on the WSe₂ photocathode. In addition, the ATM layer forms a heterojunction with the WSe₂ film, which improves the charge carrier separation, and allows lowering the prize for preparation of catalyst layers on top of different types of semiconductors.

1. Fichtner, M. J. Alloys Comp. 509S, S529-S534 (2012).

2. Bozheyev, F., Friedrich, D., Nie, M., Rengachari, M. & Ellmer, K. Phys. Stat. Sol. A 211, 2013-2019 (2014).

3. Bozheyev, F., Harbauer, Zahn, C., Friedrich D., & Ellmer, K. Sci. Rep. 7, 16003 (2017).

The synthesis of solar fuels and chemicals through artificial photosynthesis does not only require the coupling of solar light absorption and charge separation, but also the direct pairing with chemical redox processes. This approach is a one-step and versatile alternative to the more indirect coupling of a photovoltaic cell with electrolysis and enables potentially the synthesis of a wide range of fuels and feedstock chemicals. A common drawback in most artificial photosynthetic systems and organic photocatalysis is their reliance on expensive materials and device architectures, which challenges the development of ultimately scalable devices. Another limitation in many approaches is their inefficiency and reliance on sacrificial redox reagents, which may be system damaging and often prevent truly energy-storing chemistry to proceed. This presentation will give an overview about our recent progress in developing semiconductor hybrid materials to perform efficient full redox cycle solar fuel catalysis with inexpensive components, and our first steps in extending this approach for sustainable biomass photoreforming and fine chemical synthesis.

Representative recent references

(1) “Solar Hydrogen Generation from Lignocellulose”

Kuehnel, Reisner, Angew. Chem. Int. Ed., 2018, 57, 3290.

(1) “Photocatalytic CO₂ Reduction in Water through Anchoring of a Molecular Ni Catalyst on CdS Nanocrystals”

Kuehnel, Orchard, Dalle, Reisner, J. Am. Chem. Soc., 2017, 139, 7217.

(2) “Solar-driven reforming of lignocellulose to H₂ with a CdS/CdO_x photocatalyst”

Wakerley, Kuehnel, Orchard, Ly, Rosser, Reisner, Nature Energy, 2017, 2, 17021.

(3) “Enhancing Light Absorption and Charge Transfer in Carbon Dots through Graphitization and Core N-doping”

Martindale, Hutton, Caputo, Prantl, Godin, Durrant, Reisner, Angew. Chem. Int. Ed., 2017, 56, 6459.

(4) “Carbon Dots as Versatile Photosensitizers for Solar-Driven Catalysis with Redox Enzymes”

Hutton, Reuillard, Martindale, Caputo, Lockwood, Butt, Reisner, J. Am. Chem. Soc., 2016, 138, 16722.

(5) “Solar-driven Reduction of Protons Coupled to Alcohol Oxidation with a Carbon Nitride-Catalyst System”

Kasap, Caputo, Martindale, Godin, Lau, Lotsch, Durrant, Reisner, J. Am. Chem. Soc., 2016, 138, 9183.

(6) “Clean Donor Oxidation Enhances H₂ Evolution Activity of a Carbon Dot-Catalyst Photosystem”

Martindale, Joliat, Bachmann, Alberto, Reisner, Angew. Chem. Int. Ed., 2016, 55, 9402.

(7) “Electrocatalytic and Solar-driven CO₂ Reduction with a Mn Catalyst Immobilized on Mesoporous TiO₂”

Rosser, Windle, Reisner, Angew. Chem. Int. Ed., 2016, 55, 7388.

Semiconductor nanocrystals (quantum dots) emerged in the last years as an appealing alternative to molecular photosensitizers, owing to their superior stability, intense light absorption and bright luminescence. In particular, non-cadmium quantum dots seem to be a particularly interesting choice as they display good optoelectronic properties while containing no toxic elements with respect to CdSe and CdTe.[1] In the field of artificial photosynthesis, very efficient ”hybrid” photocatalytic systems for hydrogen production in water were obtained by associating Cd-based quantum dots as photosensitizers with molecular H₂-evolving catalysts in presence of a sacrificial reductant. [2,3]

In this communication, we describe new hybrid systems associating environmentally friendly (Cd-free) quantum dots with molecular catalysts based on earth-abundant metals, in order to perform photocatalytic H₂ production in purely aqueous environment.

Core-shell CuInS₂/ZnS nanocrystals capped with glutathione were synthesized in the aqueous phase, and their structural and optical properties were fully characterized. The nanocrystals exhibit a broad absorption throughout the visible range with orange luminescence in aqueous solution. For the photocatalysis process the colloidal solution was mixed with a cobalt macrocyclic catalyst and a sacrificial reductant, and the H₂ production under irradiation was quantified by gas chromatography. This hybrid system exhibited remarkable activity for hydrogen production under visible light irradiation at pH 5.0 with up to 7700 and 1010 turnover numbers versus catalyst and QDs, respectively. The CIS/ZnS nanoparticles were also compared to widely studied CdSe nanocrystals, using the same catalyst, and the former give remarkably better performances in terms of TON. [4]

1. M. Sandroni et al. ACS Energy Lett., 2017, 2, 1076–1088.

2. C. Gimbert-Suriñach et al., J. Am. Chem. Soc. 2014, 136, 7655-7661.

3. Z.J. Han et al., Science 2012, 338, 1321-1324

4. M. Sandroni et al. Energy Environ. Sci., 2018, DOI: 10.1039/c8ee00120k.

Artificial photosynthesis is a very promising method to turn solar energy into fuels.¹ In a photoelectrochemical (PEC) cell, the generation of H₂ by water splitting takes place on two different electrodes, which facilitates the separation of the products by the addition of a proton exchange membrane.  PEC cells have led to some of the highest solar-to-hydrogen efficiencies achieved to date.² The setup of a successful PEC cell requires the preparation of efficient and stable photoelectrodes.

Subestequiometric copper telluride, Cu_2-xTe, is a very interesting material that, as far as we know, has not been used in a PEC cell yet. It has already been used in photovoltaic cells due to its small band gap of around 1.5 eV and its high conductivity.³ In this work, we used Cu_2-xTe nanocrystals synthesized by a colloidal method⁴ to prepare thin films and use them as photoelectrodes. After their organic capping was successfully removed by a thermal treatment in Ar, two different crystallographic phases were obtained depending on the heating temperature. Then, the photoelectrochemical and optical properties of both phases were measured. Electrochemical impedance spectroscopy was used to build the Mott-Schottky diagrams and determine the flat band potential, charrier density and conduction type, which is p-type for both phases. This conductivity type was ratified by photopotential measurements. Both electrochemical impedance spectroscopy and UV-Vis-nIR spectroscopy allowed us to build the bands energy diagram, which confirms the ability of this material to reduce water to hydrogen. This means that Cu_2-xTe thin films are good candidates to be used as photocathodes, which was also confirmed by the measurement of their photocurrents. As they show some stability problems, two strategies were proved to solve this: the addition of an amorphous TiO₂ layer and the change of the electrolyte pH. Moreover, H₂ generation by water splitting in a two-electrode PEC cell was observed and quantified.

References

1.           Sivula, K. & van de Krol, R. Semiconducting materials for photoelectrochemical energy conversion. Nat. Rev. Mater. 1, 1–16 (2016).

2.           Young, J. L. et al. Direct solar-to-hydrogen conversion via inverted metamorphic multijunction semiconductor architectures. Nat. Energy 17028, 1–8 (2017).

3.           Rungtaweechai, N. & Tubtimtae, A. Cu_2-xTe/MnTe co-sensitized near-infrared absorbing liquid-junction solar cells. Mater. Lett. 158, 70–74 (2015).

4.           Arciniegas, M. P. et al. Self-Assembled Dense Colloidal Cu₂Te Nanodisk Networks in P3HT Thin Films with Enhanced Photocurrent. Adv. Funct. Mater. 26, 4535–4542 (2016).

All-Inorganic Halide Perovskite Quantum Dots (QDs) have emerged as a new class of fascinating nanomaterials with outstanding optoelectronic properties, with promise to revolutionize different disciplines like photovoltaics, lasing and emission.[1] In the present talk, we will discuss about the application of these materials for solar-driven environmental remediation. To that aim, it is essential to elucidate the energy levels of CsPbxSn1-xX3 QDs by different techniques and to identify candidate target molecules, which can be oxidized with photogenerated holes in these QDs. As a proof of concept, we demonstrate the efficient photocatalytic and photoelectrochemical oxidation of 2-Mercaptobenzothiazol with CsPbBr3 QDs.

The efficient production of solar fuels requires catalysts capable of accelerating complex multi-electron reactions at electrified interfaces. These reactions can be carried out at the metallic surface sites of heterogeneous electrocatalysts or via redox mediation at molecular electrocatalysts. Molecular catalysts yield readily to synthetic alteration of their redox properties and secondary coordination sphere, permitting systematic tuning of their activity and selectivity. Similar control is difficult to achieve with heterogeneous electrocatalysts because they typically exhibit a distribution of active site geometries and local electronic structures, which are recalcitrant to molecular-level synthetic modification. However, metallic heterogeneous electrocatalysts benefit from a continuum of electronic states which distribute the redox burden of a multi-electron transformation, enabling more efficient catalysis. We have developed a simple synthetic strategy for conjugating well-defined molecular catalyst active sites with the extended states of graphitic solids. Electrochemical and spectroscopic data indicate that these graphite-conjugated catalysts do not behave like their molecular analogues, but rather as metallic active sites with molecular definition, providing a unique bridge between the traditionally disparate fields of molecular and heterogeneous electrocatalysis. Our efforts to deploy these new hybrid materials for solar fuels production will be discussed.

Dimensionally stable electrodes that are able to sustain the harsh chemical environment involved during electrochemical reactions is a requirement for materials to reach industrial applications. Dimensionally stable anodes (DSA) have been obtained previously for the electrochemical generation of chlorine gas and for water splitting applications.¹ So far the most successful approach has been the generation of a thin layer of a stable electrocatalytic and conductive coating deposited on a base metal, usually Ti. Unfortunately, this approach renders materials with a small contact surface area between catalyst and electrolyte which could limit the overall activity of the reaction. The current presentation will focus on how DSA can be prepared from powder metallurgy processes. The composition can be controlled to modify the porosity of the electrode. This type of electrodes can be used in combination with photovoltaic materials by using different photoelectrochemical configurations for generation of solar fuels.^2-4

References

1.         Mousty, C.; Fóti, G.; Comninellis, C.; Reid, V., Electrochemical behaviour of DSA type electrodes prepared by induction heating. Electrochim. Acta 1999, 45, 451-456.

2.         Guerrero, A.; Bisquert, J., Perovskite semiconductors for photoelectrochemical water splitting applications. Current Opinion in Electrochemistry 2017, 2, 144-147.

3.         Guerrero, A.; Haro, M.; Bellani, S.; Antognazza, M. R.; Meda, L.; Gimenez, S.; Bisquert, J., Organic photoelectrochemical cells with quantitative photocarrier conversion. Energy Environ. Sci. 2014, 7, 3666-3673.

4.         Haro, M.; Solis, C.; Blas‐Ferrando Vicente, M.; Margeat, O.; Dhkil Sadok, B.; Videlot‐Ackermann, C.; Ackermann, J.; Di Fonzo, F.; Guerrero, A.; Gimenez, S., Direct Hydrogen Evolution from Saline Water Reduction at Neutral pH using Organic Photocathodes. ChemSusChem 2016, 9, 3062-3066.

The morphology of copper electrodes significantly affects the performance of electrochemical CO₂ reduction devices. Porous morphologies are important to increase the active reaction sites and to tune the selectivity towards desired products, but it can also limit the mass transport through the pore space. A better understanding of morphology-induced performance limitations or enhancements in porous electrodes is needed.

We used a coupled experimental-numerical approach to quantitatively characterize morphologically-complex porous electrodes (micrometer thick macroporous films with mesoporous structural details). We utilized a 3D-microscopy method, FIB-SEM tomography [1] with a high resolution of 4x4x4 nm³, to obtain a grey value array representing the photoelectrode morphology. The digital structure was segmented based on trainable machine-learning algorithms to subsequently quantify performance-related morphological parameters.

We applied this method to a hierarchically structured indium−tin oxide (ITO) electrode, covered by electrodeposited copper. The ITO scaffold has a macroporous inverse opal (IO) architecture and a mesoporous skeleton to increase the effective surface area [2]. Various samples with IO film thickness of 10 – 30 μm and pore diameters of 0.5 – 2 μm were scanned.

The digitalized electrode morphologies were then used in direct pore-level simulations to understand the mass transport within the macro- and mesopores. The diffusive transport of reactants and products in the electrolyte were investigated with a finite volume solver on a meshed representation of the exact geometries. Local current densities at the solid-liquid interface and pH distributions in the pore space were determined for the different film thickness and pore diameters.

The FIB-SEM tomography, with its nanometer-scale resolution, and the advanced pore-level simulations allowed for direct linking of the multi-physical transport to the morphological parameters of the porous ITO/Cu films. This study lays the ground for the optimization of the CO₂ reduction efficiency by tuning the morphological parameters on digitally modified electrode representations (e.g. modified pore diameters, pore distribution and film thickness).

References

[1]         M. Cantoni and L. Holzer, “Advances in 3D focused ion beam tomography,” MRS Bull., vol. 39, no. 4, pp. 354–360, 2014.

[2]         D. Mersch et al., “Wiring of Photosystem II to Hydrogenase for Photoelectrochemical Water Splitting,” 2015.

The electrocatalytic reduction of CO2 has attracted high attention recently, not only to reduce the carbon footprint, but for the possibility to produce value-added chemicals, such as carbon monoxide, formic acid, methanol or more complex organic molecules like ethanol or ethylene, under ambient temperature and pressure conditions. However, the real application of these electrochemical techniques are still a challenge, and efforts have to be made in decrease the overpotential, improve the product selectivity and stability of the electrocatalyst. Among all the CO2 reduction products that can be obtained, we have focused our research in formic acid, as it has been receiving great industrial attention for its several applications in fuel cells, textile or chemical industries.

We have studied two different metals and the combination of both as electrocatalysts for CO2 reduction to formic acid: tin (Sn)1 and palladium (Pd)2. Electrodeposited Sn has shown a potential around -0,8 VRHE at 10 mA/cm2 and a faradaic efficiency to formate around 70% using KHCO3 as electrolyte. On the other hand, under the same experimental conditions, the electrode containing Pd nanoparticles supported on carbon black shows a potential around -0.25 VRHE at 10 mV/cm2 and more than 80% of faradaic efficiency. The cathode potential obtained with Pd electrodes in comparison with Sn is 500 mV lower however, the catalyst working at such high current density is rapidly poisoned with CO (subproduct of the CO2 reduction) and the faradaic efficiency to formic is decreased dramatically. In this work, we present how the combination of both catalysts can be used to modulate the overpotential of the CO2 electroreduction reaction, the selectivity and the electrode stability under continuous operation. Also, a novel technique of co-electrodeposition of both metals is achieved, showing an alternative method to produce easily tunable electrodes, which facilitates and enhance the possible coupling to a photo-absorbing element in a complete photoelectrochemical cell.

References:

1. J. Mater. Chem. A,2016, 4, 13582–13588

2. ACS Energy Lett. 2016, 1, 764−770

The transformation of solar energy into chemical energy stored in the form of hydrogen, through photoelectrochemical water splitting is a promising method that has the important advantages of being environment friendly and free from carbon dioxide emission. Metal oxides are promising candidates for photoanode but the strong electron-hole recombination may explain their low efficiency. It has been recently proposed to use the spontaneous electric field of a ferroelectric compound to separate photogenerated charges in photoanode. In our laboratory we have been developing model oxide thin films for solar water splitting prepared by oxygen assisted molecular beam epitaxy for several years, in order to understand the relevant parameters to improve photoanodes performance. In this context, in order to unravel the role of ferroelectricity we have been investigating epitaxial TiO₂ films and TiO₂/BaTiO₃ heterojonctions deposited on Pt(001) where BaTiO₃ is the prototypical ferroelectric material. We have studied the growth, the electronic structure, the ferroelectric properties and the photoelectrochemical performance (photocurrent and impedance spectroscopy) as a function of the position of the ferroelectric layer in the film (above, in the middle and below) and of the orientation of ferroelectric polarization. Thereby, we show that the TiO₂ films adopt a TiO₂-B (001) structure for thickness lower than 3 nm and an anatase (001) structure for higher thicknesses, while the BaTiO₃ films adopt a tetragonal (001) structure with a natural electrical polarization perpendicular to the surface. We show that the performance of the TiO₂ photoanode can be improved by a polarized layer of BaTiO₃, the best improvement is when the ferroelectric film is below the TiO₂ film and when the electrical polarization is downward polarized. We explain this finding by the presence of an internal electric field which favors the separation of photo-generated charges, and explain how the electronic structure is modified by the electrical polarization in each case.

In this contribution, we report on strategies for high yield syngas production from solar CO₂ recycling. To make CO₂ re-use available at industry level, versatile and cost-effective processes at large-scale are in dire need. Therefore, we focused our study on efficient processes, which are scalable to large areas and compatible with state-of-the-art photovoltaics and electrocatalysts. Efficient processes include low overpotentials for the CO₂ reduction reaction (CO₂RR) and the oxygen evolution reaction (OER), respectively, along with excellent selectivity for the desired products (in particular for the case of CO₂RR). We demonstrate that the coating of three-dimensional metallic foam electrodes with adapted nanosized catalysts resulted in high yield CO₂ conversion to syngas. Furthermore, we provide evidence that photovoltaic structures based on cheap and mature silicon technology can be adapted to provide the required photovoltage to break the CO₂ molecule into the targeted product.

In particular, we have investigated the application of silicon heterojunction photovoltaic cells as photoanodes, which requires meeting challenges, such as increasing the photovoltage without impairing the photovoltaic efficiency; protection of the solar cell by robust coatings to increase the stability in aqueous electrolytes; and the decoration with catalysts ensuring an efficient OER. We show that photovoltages up to 2.5 V with photocurrent densities up to 7.5 mA/cm² can be reached by connecting four HIT cells in series. Furthermore, for the rear contact of the HIT cells we explored the applicability of metallic foams (e.g. Ni and Cu) loaded with metallic particles as OER catalyst. We demonstrate OER overpotentials below 300 mV (for 10 mA/cm²).

The CO₂RR was performed by using large-scale metallic foam electrodes as highly conductive catalyst scaffolds. In this context, we developed a deposition process, which enables tunable coating of Cu and Ni foams, respectively, with highly active nanosized metal catalysts, such as Zn or Ag, for selective syngas production. The performance of the as-produced electrodes has been evaluated in terms of product selectivity, Faradaic efficiency, overpotentials, and stability. The developed electrodes exhibit overpotentials below 400 mV for CO₂RR. Furthermore, stable and tunable H₂:CO ratios between 5 and 1 along with high CO Faradaic efficiencies of up to 96% and CO current densities of 40 mA/cm² were measured. Finally, we demonstrate a bias-free operation of the complete  device (2-electrode configuration) providing a photocurrent density of 5.0 mA/cm² measured under 100 mW/cm² illumination. This corresponds to a solar-to-syngas conversion efficiency of 4.3%.

In this talk, I will discuss our group’s recent experimental and computational work on understanding electronic and phononic structure nanocrystal thin films and charge transport in these thin films. Using electrochemical-based approaches, we show that we can quantify the electronic density of states and also examine charge-transfer processes across interfaces. Using inelastic x-ray scattering, we quantify the phononic denisty of states. We combine density functional theory calculations and kinetic Monte Carlo simulations to develop a first-principles model for charge transport in nanocrystals solids. We show that these simulations explain temperature-dependent time-of-flight measurements of electron and hole mobility performed on lead sulfide nanocrystal thin films. The combination of experimental and computational work highlights the importance of electron-phonon interactions in nanoscale transport and enables us to determine the relative impact of energetic and positional disorder on transport, providing us with design guidelines on parameters to consider when optimizing nanocrystal synthesis, nanocrystal surface treatments, and nanocrystal thin film preparation for different device applications.

For the fabrication of an integrated solar-to-chemical system, different components should be interfaced together in an orchestrated manner. Photoelectrodes need to absorb in the visible range, with a valence and a conduction band suited for the target reaction. Moreover, the presence of catalysts is required to manage the intrinsic energetic hurdle. Herein, we address the study of the major challenges, namely performance, stability, and interfaces to enable fabrication of integrated solar-to chemical systems. Novel scientific directions for the synthesis of functional interfaces and the development of new tools for their characterization will be addressed. Specifically, we will present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. These findings are confirmed by in situ electrochemical atomic force microscopy (EC-AFM), which reveals that degradation under operating conditions occurs via dissolution of the film, starting at exposed facets of grains in polycrystalline thin films. In addition, we will present the correlation between morphological and functional heterogeneity in this material by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate.

Insights into corrosion mechanisms and nanoscale heterogeneity aid development of protection strategies and provide information on how local functionality affects the macroscopic performance. 

During this presentation we will introduce the A-LEAF project: "An artificial leaf: a photo-electro-catalytic cell from earth-abundant materials for sustainable solar production of CO2-based chemicals and fuels" (FET-PROACT-732840). This project, funded by the European Union, is a collaborative effort by thirteen partners from eight European countries, coordinated by the Institute of Chemical Research of Catalonia (ICIQ), in Spain.

Our A-LEAF initiative aims to design and develop a fully functional scheme to transform sunligh, water and carbon dioxide into useful, sustainable and environmentally neutral fuels and/or fine chemicals. We have gathered a truly multidisciplinary consortium to cover all scientific expertise needed to overcome the major challenges: physicists to achieve sunlight capture; surface scientists to use this energy to oxidize water, extracting protons and electrons; electrochemists to use these equivalents to reduce carbon dioxide into useful stock; and system engineers to implement and fine-tune all components into a viable and cost-effective process. Because, in addition to the general scientific challenge, we have committed to use, exclusively, scalable processes, earth abundant non-critical raw materials and inexpensive platforms. Only with these added values, we may dream of bringing artificial photosynthesis, and solar fuels, to the future Energy pull, where societal impact is the ultimate target.

Our  thirteen partner institutions: ICIQ, ETH Zürich, Universitet Leiden, IMDEA Nanociencia, Technische Universität Wien, Universitat Jaume I, Imperial College, Technische Universität Darmstadt, Forschungzentrum Jülich, Université de Montpellier, INSTM and COVESTRO; started the collaborative work in Januay 2017, and the major results and current state of the project (successful achievements and unexpected problems) will be highlighted and discussed.

Efforts in the solar water splitting field for the past decade have established BiVO₄ as the highest performing metal oxide photoanode. Within this period, the AM1.5 photocurrents of BiVO₄ have been increased from hundreds of mA/cm² to ~7 mA/cm².^[1] However, high photocurrents (> 5 mA/cm²) are only achieved with nanostructuring, which poses additional complexities (e.g., light scattering) for the design and scalability of a tandem device for water splitting. Non-nanostructured BiVO₄ is limited by its poor carrier transport properties. In addition, with reported photocurrents already within 90% of the theoretical maximum, the solar-to-hydrogen (STH) efficiency of BiVO₄ is limited to < 9% due to its relatively wide bandgap of 2.4-2.5 eV.

Here, we demonstrate that these limiting factors can be alleviated by controlled anion and cation substitution in BiVO₄. To overcome the bandgap limitation, we incorporated sulfur into BiVO₄ by post-annealing in sulfur-rich atmosphere. As a result, the bandgap is reduced by up to ~0.3 eV, which increases the theoretical maximum STH efficiency to ~12%. We confirmed that sulfur substitutes oxygen in the lattice of BiVO₄ by a series of structural and chemical characterization (e.g., XRD, Raman, XPS). Moreover, hard X-ray photoelectron spectroscopy (HAXPES) reveals that the bandgap decrease is a result of an upward shift of the valence band maximum. Simultaneously, time-resolved microwave conductivity (TRMC) measurements reveal an improvement of charge carrier transport by the incorporation of sulfur; the mobility increases by a factor of ~5. Wavelength-dependent TRMC measurements confirm the photoactivity of the sulfur-incorporated BiVO₄ up to 560 nm, well beyond the bandgap of typical BiVO₄ films. In addition, we successfully introduced calcium into BiVO₄ thin films;^[2] calcium substitutes bismuth and acts as an acceptor-type dopant. Although it seems counterintuitive to introduce an acceptor dopant into n-type BiVO₄, this would allow the fabrication of p-type, and eventually p-n homojunctions based on BiVO₄. HAXPES measurements reveal that calcium out-diffuses towards the surface of the film, thereby creating a spontaneous p-n homojunction within the film. As a result of the internal electric field, the carrier separation efficiency was enhanced by a factor of ~2. Overall, this work underlines the importance of controlled ionic substitution in complex metal oxides, which may not only improve the performance but also enable new device architectures.

[1] Abdi and Berglund, J. Phys. D. Appl. Phys. (2017)

[2] Abdi et al. ChemPlusChem (2018)

Recently, Ham et al. reported that aluminum incorporation into SrTiO₃ microparticles followed by modification with Rh_2-yCryO₃ produces a superior overall water splitting catalyst with 30% apparent quantum efficiency at 350 nm. Based on transient IR spectroscopy, the improved activity was attributed to the removal of Ti³⁺ recombination sites. Here we use results from photoelectron spectroscopy (XPS) and density functional theory to show that Al³⁺ incorporation into the perovskite lattice not only reduces the number of Ti³⁺ deep recombination sites and but also promotes the formation of oxygen vacancies in the material. The oxygen vacancies form a mini-band 0.2 eV above the valence band whose energy position strongly depends on the spatial separation between Al³⁺ sites and oxygen vacancies. The vacancy band is believed to aid photochemical charge separation in this metal oxide. Particle suspensions of the material inside of a baggie reactor promote overall water splitting under direct sunlight illumination with 0.1% solar to hydrogen efficiency.

References

Ham, Y., T. Hisatomi, Y. Goto, Y. Moriya, Y. Sakata, A. Yamakata, J. Kubota, and K. Domen, Flux-Mediated Doping of SrTiO₃ Photocatalysts for Efficient Overall Water Splitting. Journal of Materials Chemistry A, 2016. 4(8): p. 3027-3033. http://dx.doi.org/10.1039/C5TA04843E

Cu(I)-based delafossites (CuMO2) have garnered significant attention for their small optical bandgaps, strong light absorption characteristics, and native p-type conductivities, which make them suitable photocathode candidates for solar fuel production. While resolving the inherently complex electronic structures of these ternary oxides is often challenging, doing so is imperative for understanding optical excitations and carrier transport mechanisms, as well as for furthering material design and discovery. Combining density functional theory (DFT) calculations and X-ray spectroscopy techniques, this research gives a detailed portrait of the band structure of rhombohedral 3R-CuFeO2, prepared by reactive co-sputtering. In particular, element-specific contributions from 3d orbitals of Fe and Cu, as well as 2p orbitals of O, in the valence and conduction bands are revealed by resonant inelastic X-ray scattering (RIXS) measurement. By combining this knowledge of electronic structure, with measurements of optical properties, carrier relaxation dynamics, and photoelectrochemical performance characteristics, this research provides insights into current limitations of this novel photocathode material and informs strategies to overcome them.

The nanostructured W-doped BiVO₄ photoanodes were prepared by electrospinning. The role of surface states (SS) during water oxidation for the as-prepared photoanodes was investigated by using electrochemical, photoelectrochemical, and impedance spectroscopy measurements. An optimum 2% doping is observed in voltammetric measurements with the highest photocurrent density at 1.23 V_RHE under back side illumination. It has been found that a high PEC performance requires an optimum ratio of density of surface states (N_SS) with respect to the charge donor density (N_d), to give both good conductivity and enough surface reactive sites. The optimum doping (2%) shows the highest N_d and SS concentration, which leads to the high film conductivity and reactive sites. The reason for SS acting as reaction sites (i-SS) is suggested to be the reversible redox process of V⁵⁺/V⁴⁺ in semiconductor bulk to form water oxidation intermediates by electron trapping process. Otherwise, the irreversible surface reductive reaction of VO₂⁺ to VO²⁺ by electron trapping process raises the surface recombination. W doping does have an effect on the surface properties of BiVO₄ electrode. It can tune the electron trapping process to obtain high concentration of i-SS and less surface recombination. This work gives a further understanding for the enhancement of PEC performance caused by W doping in the field of charge transfer at the semiconductor/electrolyte interface.

One of the critical challenges for efficient solar water splitting is the identification of a stable photoelectrode material with a bandgap of 1.6 - 1.9 eV that can be used as a top absorber in an efficient D4 tandem device. Recently, n-type α-SnWO₄ was identified as a potential photoanode candidate due to the combination of its ideal bandgap (~1.9 eV) and a very negative photocurrent onset potential (~0 V vs. RHE) [1-3]. However, up to now, α-SnWO₄ photoelectrodes have shown very low photoconversion efficiencies. The reason for this is not fully understood, and many of the essential material parameters are still elusive.

In this study, we identify the major limiting parameters of pulsed laser-deposited α-SnWO₄ photoanodes: (I) the oxidation of Sn²⁺ to Sn⁴⁺ at the surface creating a hole blocking layer and pushing the photoconversion efficiency to almost zero, and (II) the relatively poor charge carrier dynamics resulting in a mismatch between the charge carrier diffusion length and the light penetration depth. To address these challenges, we explored several strategies such as the deposition of a hole-conducting NiO_x protection layer resulting in a new benchmark sulfite oxidation photocurrent of 0.75 mA cm^-2 at 1.23 V vs. RHE. We furthermore show that a high-temperature treatment enhances the charge carrier transport by improving the film crystallinity, resulting in a charge carrier diffusion length comparable to state-of-the-art BiVO₄ photoanodes. Finally, the remaining challenges for oxygen evolution using our α-SnWO₄ films will be discussed.

Our findings provide important insights into the limitations and key properties of α-SnWO₄ photoelectrodes and will help to further improve the performance of this promising photoanode material.

[1] Ziani et al., APL Materials, 3 (2015) 096101

[2] Zhu et al., ACS Appl. Mater. Interfaces, 9 (2017) 1459-1470

[3] Pyper et al., Chin. Chem. Lett., 26 (2015) 474-478

The development of technologies for H2 production or CO2 reduction strongly relies on an abundant supply of protons and electrons liberated by water oxidation. [1-2] Therefore, photoelectrochemical (PEC) water oxidation is an important anodic half-cell process in the development of a sustainable artificial solar fuel system. In the PEC devices design, coupling water oxidation catalysts with active photoanode materials has become the most promising methodology, since the attachment/integration of the catalyst on the semiconductor light absorbers could kinetically facilitate interfacial charge transfer reactions.

In this contribution, we have fabricated ITO/Fe2O3/Fe2TiO5/FeNiOOH multi-layers nanowire heterostructures via combination of sputtering, hydrothermal, ALD, photo-electrodepositon methods for photoelectrochemical (PEC) oxygen evolution application. Structural, spectroscopic and electrochemical investigations disclose that the origin of the superior catalytic performance is owing to the interfacial coupling effect of ITO underlayer (Sn doping and conductivity promoter), ultrathin Fe2TiO5 coating (Ti doping, energetics and surface state density modulation) and FeNiOOH eletrocatalyst (varying surface state energy level). [2]

Meanwhile, an alternative earth-abundant CoFe prussian blue analogues (CoFe PBA) is incorporated in Fe2O3/Fe2TiO5 core-shell type II heterojunction nanowires as photoanodes for PEC water oxidation. The observed photocurrent is improved from 0.12 mA cm-2 to 1.25 mA cm-2 at 1.23 V vs. RHE under illumination by involvement of ultrathin Fe2TiO5 layer and CoFe PBA WOCs coating. Further investigation of the PEC mechanisms via photoelectrochemical impedance spectroscopy unveils that the enhanced PEC performance is attributed to the enhanced charge transfer efficiency owing to the tuned energy level and density of surface state. [3-4]

References

[1] Félix Urbain, Pengyi Tang, Nina M. Carretero, Teresa Andreu, Luís G. Gerling, Cristóbal Voz, Jordi Arbiol, Joan R. Morante, Energy & Environmental Science, 10, 2256-2266 (2017).

[2] Pengyi Tang, HaiBing Xie, Carles Ros, LiJuan Han, Martí Biset-Peiró, Yongmin He, Wesley Kramer, Alejandro Perez-Rodriguez, Edgardo Saucedo, Jose Galan-Mascaros, Teresa Andreu, Joan R. Morante, Jordi Arbiol, Energy & Environmental Science, 10, 2124-2136 (2017).

[3] Lijuan Han, Pengyi Tang, Alvaro Reyes-Carmona, Barbara Rodriguez-Garcia, Mabel Torrens, Joan Ramon Morante, Jordi Arbiol, Jose Ramon Galan-Mascaros, Journal of the American Chemical Society, 138, 16037-16045 (2016).

[4] PengYi Tang, LiJuan Han, Paul Paciok, Marti Biset Peiro, Hong-Chu Du, Xian-Kui Wei, Lei Jin, Hai-Bing Xie, Qin Shi, Teresa Andreu, Joan Ramon Morante, Mónica Lira-Cantú, Marc Heggen, Rafal E. Dunin-Borkowski, José Ramón Galán-Mascarós, Jordi Arbiol, to be submitted.

The n-type semiconductor bismuth vanadate (BiVO₄) is a promising material as photoanode for light induced water oxidation. Its absorption in the visible range (band gap energy of 2.4 eV), its suitable band edge positions compared to the OER half reaction, its stability against photo-corrosion as well as its low cost make BiVO₄ one of the most interesting ternary oxide materials for light-induced oxygen evolution from water.¹ One major drawback for BiVO₄ is its poor electronic conductivity, which can however be overcome by applying three strategies: i) adjustment of thin film properties (especially thickness), ii) n-type doping by cation and more recently anion substitution and iii) heterojunction design (type II). Within the present talk, I will give an overview of the improvements we achieved for novel sol-gel based BiVO₄ thin film photoanodes following each of these strategies allowing us to go all the way from low to high performance BiVO₄ photoanodes.

First we developed a new, sol-gel-based synthesis involving simple dip-coating and calcination allowing the easy and reproducible fabrication of porous BiVO₄ and Mo‑doped BiVO₄ thin film photoanodes.² The obtained thin films crystallize in the monoclinic scheelite structure in micrometre large, two-dimensional, single-crystalline porous domains with wall features in the range of the hole diffusion length of BiVO₄. Optimization of the electron transport properties resulting in higher PEC performance was realized by cation and anion doping using Molybdenum and Fluorine, respectively.³ Finally, our new synthesis approach could be easily applied for the fabrication of BiVO₄/WO₃ type II heterojunctions following a simple layer-by-layer deposition. It is shown that precise control of the layer morphology and the overlapping interface between the layers is an indispensable prerequisite for high performance WO₃/BiVO₄ heterojunction photoanodes.

This work was funded by the DFG SPP1613 program.

¹ Z.-F. Huang, L. Pan, J.-J. Zou, X. Zhang, L. Wang, Nanoscale 2014, 6, 14044.

² M. Rohloff, B. Anke, S. Zhang, U. Gernert, C. Scheu, M. Lerch, A. Fischer, Sustainable Energy Fuels 2017, 1, 1830.

³ B. Anke, M. Rohloff, M. G. Willinger, W. Hetaba, A. Fischer, M. Lerch, Solid State Sci. 2017, 63, 1.

Recently, due to promising results in the fields of photocatalysis and photoelectrocatalysis, the attention has been focused on ferrites as new visible light-active materials [1,2]. Properties such as narrow band gap energies (≈ 2 eV), high stability, abundance, and low cost make ferrites promising for solar energy production and solar remediation. Zinc ferrite (ZnFe₂O₄) is one of the most widely studied compounds belonging to the spinel ferrite family. However, various and even contradictory results regarding the photocatalytic activity of ZnFe₂O₄ have been reported in the scientific literature [3]. ZnFe₂O₄ crystallize in a phase-centered cubic spinel structure with Fe³⁺ and Zn²⁺ ions occupying tetrahedral or octahedral sites[1]. When the Fe³⁺ and Zn²⁺ ions are arranged in octahedral and tetrahedral sites, respectively, the ferrite exhibits a so-called normal spinel structure (^T[Zn]^O[Fe₂]O₄). However, when all the Zn²⁺ ions at the tetrahedral sites are exchanged by Fe³⁺ ions from octahedral sites, the compound adopts a so-called inverse spinel structure (^T[Fe]^O[ZnFe]O₄). The degree of inversion, x, defined as the fraction of Zn²⁺ ions occupying octahedral sites, can consequently adopt values from 0 (normal structure) to 1 (inverse structure) according to ^T[Zn_1-xFe_x]^O[Zn_xFe_2-x]O₄ with 0 ≤ x ≤ 1. The degree of inversion closely depends on the synthetic route.

For the first time, the effect of the degree of inversion on the physicochemical properties affecting the photocatalytic behavior of ZnFe₂O₄ has been investigated. Interestingly, as the degree of inversion increases, the conductivity of the material increases exponentially and an influence on the photocatalytic activity is observed. Thus, the degree of inversion plays a fundamental role and is a parameter of utmost importance to be investigated in order to get a meaningful approach regarding the photocatalytic performance of spinel ferrites.

[1] R. Dillert, D. H. Taffa, M. Wark, T. Bredow and D. W. Bahnemann, APL Materials, 2015, 3, 104001.

[2] D. H. Taffa, R. Dillert, A. C. Ulpe, K. C. Bauerfeind, T. Bredow, D. W. Bahnemann and M. Wark, Journal of Photonics for Energy, 2016, 7, 012009.

[3] A. Arimi, L. Megatif, L. I. Granone, R. Dillert and D. W. Bahnemann, Journal of Photochemistry and Photobiology A: Chemistry, 2018, DOI: 10.1016/j.jphotochem.2018.03.014.

Using a combination of experimental and theoretical methods we explore the beneficial effect of Sn(IV) doping in ultrathin hematite photoanodes for water oxidation. A series of hematite photoanodes with tailored Sn-doping profiles were prepared by alternating atomic layer deposition. Using data from spectrophotometry and intensity-modulated photocurrent spectroscopy we deconvoluted the overall efficiency and obtained the individual process efficiencies for light harvesting, charge separation and charge transfer. Photoanodes with Sn-doping both on the surface and in the subsurface region show the best performance with enhanced charge separation and charge transfer efficiency. Density functional theory calculations with a Hubbard U parameter were performed to investigate the causes of the efficiency improvement considering both Fe₂O₃ (0001), as well as Fe₂O₃ (11-26) surface orientation, as identified from micrographs at atomic resolution. The energetics of surface intermediates during the oxygen evolution reaction reveal that while Sn-doping decreases the overpotential on the (0001) surface, the Fe₂O₃  (11-26) orientation shows a significantly lower overpotential, one of the lowest reported for hematite so far. Electronic structure calculations demonstrate that Sn-doping on the surface also enhances the charge transfer efficiency by elimination of surface hole trap states (passivation). Moreover, the subsurface Sn-doping introduces a band bending that helps to improve the charge separation efficiency.

We acknowledge funding by SPP1613 and computational time at MagntUDE.

[1] A. G. Hufnagel, H. Hajiyani, S. Zhang, T. Li, O. Kasian, B. Gault, B. Breitbach, T. Bein, D. Fattakhova-Rohlfing, C. Scheu and R. Pentcheva, Adv. Funct. Mater. (accepted).

We report on the carrier transport properties of CVD-grown CuWO₄ films for solar water splitting with varying copper-to-tungsten stoichiometry within the borders of the binary metal oxides CuO and WO₃. Time-resolved terahertz (TRTC) and microwave conductivity (TRMC) measurements addresses the bulk dynamics in different time windows from sub-ps to ms. In addition, photo-induced absorption (PIA) in a photo-electrochemical cell is applied to study the temporal behavior of excited carriers in the vicinity of the semiconductor/electrolyte interface under varying bias. A charge carrier mobility of ∼6× 10⁻³ cm² V⁻¹ s⁻¹ and a diffusion length of and 30 nm is determined for a CuWO₄ absorber film deposited with a copper-to-tungsten ratio (Cu/W) of 1.1 which also provides the best photocurrents in assembled photo-electrochemical cells. This value is comparable to undoped BiVO₄ where poor carrier transport is governed by small polaron formation leading to slow mobility values. We compare the experimental results with measurements performed on dip-coated CuWO₄ samples and other metal oxides such as BiVO₄ and Cu₂O. Our findings establish new insights into the advantages and limits of CuWO₄-based photoanodes that can be possibly used in a tandem configuration on top of a highly absorbing semiconductor with optimal electronic properties.

[1] D. Peeters,  O. Mendoza Reyes,  L. Mai,  A. Sadlo,  S. Cwik,  D. Rogal, lH.-W. Becker, H. M.  Schütz, J. Hirst, S. Müller, D. Friedrich, D. Mitoraj, M. Nagli, M. Caspary Toroker, R.Eichberger, R. Beranek, A. Devi, J. Mater. Chem. A, adv. article (2018)

[1] M. Ziwritsch, S. Müller, H. Hempel, T. Unold, F. F. Abdi, R. v. d. Krol, D. Friedrich, R. Eichberger, ACS Energy Lett., 1 , 888 (2016)

[2] F. F. Abdi, T. J. Savenije, M. May, B. Dam, R. van de Krol, J. Phys. Chem. Lett. 4, 2752 (2013)

[4] J. Ravensbergen, F. F. Abdi, J. H. van Santen, R. N. Frese, B. Dam, R. van de Krol, J. T. M. Kennis, J. Phys. Chem. C, 118, 27793 (2014)

The application of multijunction solar cells in photoelectrochemical (PV-EC) devices for hydrogen production is addressed. Integrated PV-EC devices are composed of a ‘traditional’ photovoltaic (PV) cell combined with an electrochemical (EC) cell, presenting a promising approach to produce hydrogen and other fuels. The requirements for PV-EC devices and strategies for the solar cell development will be discussed. The results on the photovoltaic development of multijunction silicon based cells will be presented, focusing on a wide range of photovoltages and photocurrents in various systems, including the adoption of the cells to function on either cathode or anode sides of the system. A prototype integrated PV-EC system based on silicon multijunction solar cells can yield solar-to-hydrogen efficiencies (STH) of 9.5%.

The paths of device upscaling beyond laboratory size and the influence of varied illumination conditions close to obtained outdoor will also be discussed. This includes the effects of spectral quality, intensity and incident angle on the performance of both photovoltaic cells and PV-EC devices.

The efficient catalysis of the four-electron oxidation of water to molecular oxygen (oxygen evolution reaction, OER) is a central challenge for the development of devices for the conversion of electrical energy, ideally from renewable sources, into storable chemical energy (e.g. by electrolysis or “artificial leaves”). Up to now, a plethora of earth-abundant, non-toxic catalysts (e.g. Ni/Fe, Co, Mn oxides)¹ for OER is known. Most of these possess high activities and stabilities under strongly alkaline pH conditions. In contrast, the only convincing materials for OER in acidic media are based on the scarce elements Ru and Ir, while especially Ni and Co based electrodes suffer from corrosion and/or show little activity.²

Inspired by the oxygen evolving complex (OEC) of Photosystem II, the biological catalyst for this reaction, several manganese oxide (MnO_x) polymorphs have been tested as heterogeneous water oxidation (electro-)catalysts. Here we found that amorphous layered or tunnelled manganese oxides show good activities and stabilities also for strong acidic reaction media.³ Recently, we developed a route to directly prepare coatings of amorphous MnO_x on free-standing carbon based electrode supports (e.g. carbon-fiber-paper, CFP) by a simple, scalable redox deposition approach.⁴

This presentation will deal with a study where the electrocatalytic performance of such MnO_x/CFP-anodes was tested under different electrolyte conditions. A special focus was laid on the influence of the pH on activity and stability. Additionally, the MnO_x/CFP-electrodes were characterized before and after electrolysis by means of XRD, SEM/EDX, vibrational- and X-ray absorption spectroscopy. Details of the preparation, characterization, electrocatalytic performance and corrosion stability will be discussed together with possible reasons for the different behavior of the electrocatalyst at different pHs. We will report on an efficient catalyst for OER at acidic and near-neutral solution, a pH range highly desirable for (photo-)electrochemical devices, such as polymer electrolyte membrane (PEM) electrolysers or artificial leaves.

References

¹ N.-T. Suen, S.-F. Hung, Q. Quan, N. Zhang, Y.-J. Xu and H. M. Chen, Chem. Soc. Rev., 2017, 46, 337–365.

² L. C. Seitz, C. F. Dickens, K. Nishio, Y. Hikita, J. Montoya, A. Doyle, C. Kirk, A. Vojvodic, H. Y. Hwang, J. K. Nørskov and T. F. Jaramillo, Science, 2016, 353, 1011–1014.

³ C. E. Frey and P. Kurz, Chem. A Eur. J., 2015, 21, 14958–14968.

⁴ J. Melder, W. L. Kwong, D. Shevela, J. Messinger and P. Kurz, ChemSusChem, 2017, 4491–4502.

Semiconducting metal oxide thin films are promising candidates for photoelectrochemical (PEC) solar water-splitting applications due to their abundance, light absorption properties and stability in aqueous media. To identify materials with optimized properties, thin-film materials libraries, exhibiting combined thickness and compositional gradients, were synthesized by combinatorial reactive co-sputtering from elemental targets on platinized 100 mm diameter wafers in several complex multinary oxide systems: Fe-W-Ti-O, Fe-Cr-Al-O, Cu-Si-Ti-O, V-X-O, and Bi-V-X-O. High-throughput measurements of compositional, structural and functional data on the materials libraries were performed by automated thickness measurements, energy-dispersive x-ray analysis (EDX), X-ray diffraction (XRD) and PEC analyses using an optical scanning droplet cell in 342 measurement areas on each of the materials libraries. Furthermore, the microstructure of selected thin films was characterized by electron and atomic force microscopy. The analysis of the obtained data enables to establish correlations between composition, crystallinity, morphology, thickness, and photocurrent density. Several promising compositions were identified using the combinatorial approach. Furthermore, we demonstrate the combinatorial glancing angle sputter deposition (GLAD) approach for the fabrication of thin film materials libraries consisting of columnar nanostructures.

Colloidal lead halide perovskite nanocrystals (APbX_3, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) emerge as promising materials for optoelectronic applications such as in television displays, light-emitting devices, and solar cells. The sponaneous and stimulated emission spectra of these NCs are readily tunable over the entire visible spectral region of 410-700 nm [1-2]. The photoluminescence of these NCs is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, and high quantum yields of up to 100%. Cs_1-xFA_xPbI₃ and FAPbI₃ reach the near-infrared wavelengths of 800 nm [3].  A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. A promising approach lies in the formation of multinary compositions such as Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs. We show that droplet-based microfluidics can successfully guide the synthesis of such complex compositions [4]. We could fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission linewidths (to below 40nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. Most importantly, we demonstrate the excellent transference of reaction parameters from microfluidics to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices. As an example, Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting high external quantum efficiency of 5.9% and very narrow electroluminescence spectral bandwidth of 27 nm.

The processing and optoelectronic applications of perovskite NCs are, however, hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, we have developed a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules, resulting in much improved chemical durability [5].

             Perovskite NCs also readily form long-range ordered asssemblies known as superlattices. These assemblies exhibit accelerated coherent emission (superfluorescence), not observed before in semiconductor nanocrystal superlattices [6].

L. Protesescu et al. Nano Letters 2015, 15, 3692–3696

M. V. Kovalenko et al. Science 2017, 358, 745-750

L. Protesescu et al. ACS Nano 2017, 11, 3119–3134

I. Lignos et al. ACS Nano 2018, DOI: 10.1021/acsnano.8b01122

F. Krieg et al. ACS Energy Letters 2018, 3, 641–646.

Raino, M. Becker, M. Bodnarchuk et al. 2018, submitted

Novel semiconducting nanostructured oxides have gained interest in photocatalytic water splitting where they can act as electrode material. One candidate is the n-type semiconductor Nb₃O₇(OH), which can be fabricated as 3D nanoarray using a hydrothermal synthesis approach [1]. The 3D nanoarray consists of nanowires arranged perpendicular to each other. The growth defects within the Nb₃O₇(OH) nanostructure such as stacking faults are the key parameters which determine the functionality as will be discussed in the talk. The nanostructures have been studied in-depth using advanced transmission electron microscopy including electron energy loss spectroscopy to determine the oxidation state of the individual atoms as well as to analyze the band gap on the nanometer scale. In addition, electron tomography and focused ion beam slicing have been used to obtain the 3D morphology of the Nb₃O₇(OH) array after various growth stages. The functional properties of the Nb₃O₇(OH) arrays can be improved by the incorporation of Ti within the orthorhombic crystal structure which leads to a higher hydrogen production rate in light driven water splitting experiments [2]. For these measurements, a Pt co-catalyst is deposited on the nanowire array. In order to understand the stability and degradation behavior of the co-catalyst we plan to perform identical location transmission electron microscopy measurements similar as we have done for a Pt/Ru electrocatalyst [3]. Such measurements enable tracking of individual nanoparticles and allow to determine the dominating degradation mechanisms such as catalyst dissolution or Oswald ripening down to the atomic scale. 

[1] S. B. Betzler, A. Wisnet, B. Breitbach, C. Mitterbauer, J. Weickert, L. Schmidt-Mende, and C. Scheu, J. Mater. Chem. A, 2014, 2, 12005.

[2] S. B. Betzler, F. Podjaski, K. Bader, M. Beetz, K. Hengge, A. Wisnet, M. Handloser, A. Hartschuh, B. V. Lotsch, C. Scheu, Chemistry of Materials, 2016, 28, 7666.

[3] K. Hengge, T. Gänsler, E. Pizzutilo, C. Heinzl, M. Beetz, K. J. J. Mayrhofer, C. Scheu, International Journal of Hydrogen Energy 2017, 42 (40), 25359.

[4] The author would like to thank the colleagues and co-workers who contributed to this work and the German Research Foundation (DFG) for financial support.

Finding electro-catalysts for driving the oxygen evolution reaction (OER) at the minimum overpotential remains the bottle neck in water splitting. Birnessite is a promising earth-abundant electrode material, since its layered calcium manganese oxide structure with intercalated crystal water possibly allows bulk OER activity [1,2].

We study a nanocrystalline type of highly active electrode, prepared by dropping ink solutions on top of a substrate. The structure of the highly porous electrodes with a grain size down to a few nanometres are analysed with High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Electron Diffraction (SAED). To gain more information about the mechanism of water oxidation we investigated the interaction of the electrode with the electrolyte. Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-Ray Spectroscopy (EDX) indicate an intercalation of the electrolyte cation phosphorous while calcium was depleted from the electrode. The exchange mainly happens at the surface and at grain boundaries.

To further explore the active centres of water oxidation with Birnessite we took steps towards in-situ TEM studies in water vapour. The Environmental TEM (ETEM) offers to investigate Birnessite active states in electric potentials close to operando conditions. Based on our studies we present conclusions on the involved OER mechanism and design strategies for highly active and stable electro-catalysts.

[1] S. Y. Lee, D. González-Flores, J. Ohms, T. Trost, H. Dau, I. Zaharieva and P. Kurz, ChemSusChem, 2014, 7, 3442-3451

[2] C. E. Frey and P. Kurz, Dalton Trans. , 2014, 43, 4370-4379

Titania-based photocatalysts represent a promising class of materials to  split water in its elementary components due to their high abundancy and stability [1]. Albeit their low solar visible light exploitation, their crystal  structure and electronic properties are well-known and facilitate the  investigation of the several fundamental aspects involved in photocatalytic water splitting. A detailed understanding of these key steps will subsequently allow for the design of an appropriate photocatalyst.
Most recently, the anatase modification of titanium dioxide (a-TiO₂) emerged as a widely applied material since it is the majority phase of TiO₂-nanoparticles and shows enhanced photocatalytic activity [1,2]. The intermediates during the water oxidation pathway on the a-TiO₂(101) surface were already identified on GGA-PBE level of theory [3]. The key step was determined to consist of the first proton removal induced through a photogenerated hole (H₂O + h⁺ → OH^• + H⁺ ). The influence of the photogenerated hole on the dissociation process was addressed controversially by theoretical studies: whereas combined PBE/HSE06 studies indicate that hole-trapping only occurs after dissociation through the OH-anionic species [4], recent work employing the B3LYP functional gives evidence for a concerted proton/hole transfer [5]. In addition, as observed by experimental TPD and TOF methods, OH^• radicals are found to be ejected from a well-defined a-TiO₂(101) surface after irradiation [6].
In order to get a more sophisticated insight into this yet indetermined reaction, we use two different theoretical approaches in this contribution: We firstly identify the active sites of the H₂O/a-TiO₂ system through periodic slab calculations using hybrid DFT functionals (PBE0/HSE06). Afterwards, these data will provide the basis for an embedded cluster approach allowing for accurate post-HF methods. As a result, we will present potential energy surfaces of a single water molecule on a-TiO₂(101).

[1] F. De Angelis, C. Di Valentin, S. Fantacci et al., Chem. Rev. 114 (2014) 9708.
[2] A. Barnard, P. Zapol, L. Curtiss, Surf. Sci. 582 (2005), 173.
[3] Y.-F. Li, Z.-P. Liu, L. Lui, W. Gao, J. Am. Chem. Soc. 132 (2010) 13008.
[4] W.-N. Zhao, Z.-P. Liu, Chem. Sci. 5 (2014) 2256.
[5] C. Di Valentin, J. Phys.: Condens. Matter. 28 (2016) 074002.
[6] Z. Geng, X. Chen, W. Yang et al., J. Phys. Chem. C 120 (2016) 26807.

New semiconductor metal oxides capable of driving water-splitting reactions by solar irradiation alone are required for sustainable hydrogen production. Whereas most metal oxides only marginally deliver the photochemical energy to split water molecules, uranium oxides are efficient photoelectrocatalysts due to their absorption properties (Eg ~ 2.0 - 2.6 eV) and easy valence switching among uranium centers that additionally augment the photocatalytic efficiency. Although considered a scarce resource, the abundance of uranium compounds in the environment is manifested in the huge quantities of stored UF6 gas, produced as waste streams in the nuclear fuel enrichment process. Here we demonstrate that thin films of depleted uranium oxide (U3O8) and their bilayers with hematite (a-Fe2O3) are high activity water oxidation catalysts due to electronically coupled phase boundaries. The electronic structure of uranium oxides showed an optimal band edge alignment in U3O8//Fe2O3 bilayers (DFT calculations) resulting in improved charge-transfer at the heterojunction as supported by TAS and XAS measurements. The enhanced photocurrent density of the heterostructures with respect to well-known hematite offers unexplored potential of uranium oxide in artificial photosynthesis.

The development of semiconductors that split water photocatalytically under visible-light irradiation is a path to the efficient conversion of solar energy. Various oxides with d⁰ and d¹⁰ electron configuration possess such photocatalytic activity, but suffer from poor oxygen and hydrogen evolution or work only in the ultraviolet regime ^[1]. Domen et. al. developed gallium-oxynitride (Ga_1-xZn_x)(N_1-xO_x) as such a material by nitriding mixture of Ga₂O₃ and ZnO in a solid solution. It is capable of absorbing visible light efficiently with a bandgap of 2,6 eV ^[2,3].

In the first step we are producing nanoparticles by Chemical Vapor Synthesis. By adjusting process parameters such as temperature, pressure and precursor evaporation rate, we vary the characteristics of the materials, such as: surface area, crystallinity and particle size ^[1]. Exploiting the advantages of the synthesis from the gas phase we are able to obtain pure phase or mixture of ZnO and β-Ga₂O₃, as well as the complex spinel phase ZnGa₂O₄. In second step these powders are nitrided thermally or with assistance of a microwave plasma to obtain the desired oxynitrides. It is known that photocatalytic activity of such a material strongly depends on crystallinity and its composition. Varying the nitridation time from 0.17 h to 10 h zinc and oxygen concentration in zinc-gallium oxyntrides decreases, which is associated with a change in band-gap energy.

Incorporation of nitrogen and elimination of oxygen and zinc during nitridation process should take place at the surface of β-Ga₂O₃ and ZnO nanoparticles, parallel with diffusion of the constituent ions to form the stoichiometric solid solution. X-Ray diffraction analyzed by Rietveld refinement reveals crystal phases, cell parameters, as well as atomic composition. These results are supported by High-Resolution Scanning-Electron Microscopy in combination with Energy Dispersive Spectroscopy. The specific surface area of the samples is analyzed by using low temperature nitrogen adsorption. The bandgap is determined by using Ultraviolet-visible Spectroscopy.

[1] S. Lukic et al., ChemSusChem, 10, (2017), 4190-4197.

[2] K. Maeda et al., J. Am. Chem. Soc. 127, (2005), 8286-8287.

[3] K. Maeda et al., J. Phys. Chem. B 109, (2005), 20504-20510.

We have developed a straightforward microwave synthesis protocol using acetylacetonate and acetate precursors to produce nanocrystals of the earth-abundant cubic spinel ferrites MgFe₂O₄ and ZnFe₂O₄[1,2], which are promising materials for both photoelectrochemical and photocatalytic water splitting under visible light irradiation due to their narrow band gaps (~ 2.0 eV) and matching band positions. The crystallite size can be tailored by post-synthetic heat treatment or seed-mediated growth method. Samples were characterized employing transmission electron microscopy (TEM), X-ray diffraction (XRD), dynamic light scattering (DLS), Raman spectroscopy and N₂ physisorption, indicating highly-crystalline, single phase nanoparticles with specific surface areas of around 200 m²/g and good colloidal stability in non-polar solvents. Phase transfer into aqueous medium has been performed using different organic capping ligands, resulting in stable dispersions with a narrow size distribution. First results of photocatalytic experiments will be presented.

In addition, well-ordered mesoporous ZnFe₂O₄ thin films were fabricated by sol-gel synthesis, using a polymer-templating approach previously reported by Haetge et al.[3]. By means of the amphiphilic diblockcopolymer poly(isobutylene)-block-poly(ethylene oxide) (PIB₅₀-b-PEO₄₅), ordered mesopores are obtained after dip-coating by evaporation-induced self-assembly [4] followed by heat treatment. Scanning electron microscopy (SEM) confirms the porous morphology with average pore diameters of 12-15 nm. Raman spectroscopy and XRD Rietveld analysis revealed phase pure ZnFe₂O₄ with a crystallite size of 15 nm of. Furthermore, photocurrent and Mott‑Schottky measurements were performed at different pH values to determine the flat band potential and photocurrent density of the thin film electrodes calcined at various temperatures.

References

[1] C. Suchomski, B. Breitung, R. Witte, M. Knapp, S. Bauer, T. Baumbach, C. Reitz, T. Brezesinski, Beilstein J. Nanotechnol. 2016, 7, 1350

[2] K. Kirchberg, A. Becker, A. Bloesser, T. Weller, J. Timm, C. Suchomski, R. Marschall, J. Phys. Chem. C 121 (2017) 27126−27138

[2] J. Haetge, C. Suchomski, T. Brezesinski, Inorg. Chem. 2010, 49, 11619.

[3] C. J. Brinker et al., Adv. Mater. 1999, 11, 579.

Over the past years, α-Fe2O3 (hematite) has been considered as a promising photoanode material in photoelectrochemical (PEC) water splitting. In spite of significant success in obtaining relatively high PEC performance, the main drawbacks hindering practical application at hematite are related to an intrinsically poor charge transport and an inferior kinetics of the oxygen evolution reaction on the photoelectrode surface. In this presentation, we will discuss a strategy for reducing onset potential of hematite photoanode by coupling of gradient Sn4+ ion doping and Zn-Co LDH serving as a highly active OER catalyst. The gradient Sn doping in hematite influences the space charge layer (SCL), which is critical for charge separation and thus for an enhanced photoelectrochemical water splitting performance (PEC). The OER catalyst, i.e. Zn-Co LDH nano-sheets have been synthesized by a simple microwave treatment. The synergistic effect of Zn-Co LDH decoration and gradient Sn4+ doping results in decreasing the onset potential of more than 300 mV, from 0.86 VRHE to 0.54 VRHE, and in increasing the photocurrent density from 0.60 mA/cm2 to 2.00 mA/cm2 at 1.50 VRHE. Our approach demonstrates strategies to overcome onset potential limitations as well as poor OER properties of hematite and leads to a remarkably improved PEC water splitting performance.

The electro-oxidation of water to oxygen (oxygen evolution reaction, OER) in (photo-)electrolytic devices yields the electrons and protons required to form molecular fuels^1,2: This is why it is expected to play a major role in the development of future (photo-)electrochemical energy conversion and storage technologies. However, the slow rate of water oxidation remains a key challenge that requires fundamental understanding and the design of more active and stable OER electrocatalysts^3,4.

To further this development, we probe the local geometric ligand environment and the electronic metal states of O-coordinated Ir centers in Ni-leached IrNi@IrOx metal-oxide core-shell nanoparticles⁵ — one of the most active OER electrocatalysts known to date — under catalytic oxygen evolution condition using operando spectroscopic techniques, resonant high-energy XRD and differential atomic pair correlation analysis, with support of density functional theory (DFT) calculations. Ni-leaching generates lattice vacancies, which in turn produce uniquely shortened Ir-O metal-ligand bonds and an unusually large number of d-band holes in the Ir oxide shell under OER. DFT calculations show this increase in formal Ir oxidation state drives the formation of O 2p holes on the oxygen ligands in direct proximity to lattice vacancies, resulting in the highly covalent and uniquely short Ir-O bonds seen experimentally. We argue the electrophilic character of these ligands renders them susceptible to nucleophilic acid-base-type O-O bond formation at reduced kinetic barriers, resulting in strongly enhanced OER reactivities. Together, our findings advance both our fundamental understanding of the exceptional reactivity of bimetallic core-shell Ir catalysts and provide a roadmap to tailoring of other “hole-doped” core-shell catalysts for water oxidation at a molecular level.

References

Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. Chemcatchem 2010, 2, 724.

Olah, G. A.; Goeppert, A.; Prakash, G. K. S. J. Org. Chem. 2009, 74, 487.

Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. Science 2011, 334, 1383.

Reier, T.; Nong, H. N.; Teschner, D.; Schlögl, R.; Strasser, P. 2017, 7, 1601275.

Nong, H. N.; Gan, L.; Willinger, E.; Teschner, D.; Strasser, P. Chem. Sci. 2014, 5, 2955.

For semiconductor nanowire structures the ratio of the minority charge carrier diffusion length over the light absorption depth is significantly reduced compared to bulk materials. In addition, nanostructured semiconductors exhibit larger surface-to-volume ratio. The dimensions of the nanostructures and their geometrical arrangement influence relevant processes such as light absorption, as well as charge separation and transport. Ion-track nanotechnology combined with electrodeposition and atomic layer deposition, enables the controlled synthesis of 3D architectures of semiconductor nanostructures with tailored composition, size and density.

Among the various materials studied as photocathodes for solar hydrogen production, Cu₂O is a promising candidate with a predicted solar-to-hydrogen conversion efficiency of ~18%. Moreover, Cu₂O is cheap, earth-abundant, non-toxic, and is also scalable and compatible with low-cost fabrication processes. Currently, the main challenge for Cu₂O-based photocathodes is their chemical instability in aqueous solution, which can be improved by the use of suitable passivation coatings.

Here, we present the synthesis and characterization of two types of semiconductor nanowire-based photocathodes: (i) highly textured single-crystalline p-type Cu₂O nanowire arrays prepared by electrodeposition in etched ion-track polymer membranes, (ii) Au/Cu₂O core-shell nanowire arrays prepared by electrodeposition of Cu₂O on arrays of free-standing single-crystalline Au nanowires. Polymer membranes with controlled nanochannel density (typically 10⁸ - 10¹⁰cm^-2) and channel diameter (~20 - 250 nm) are fabricated by swift heavy ion irradiation and selective chemical etching. Subsequently, either Cu₂O or Au nanowires with lengths up to 10 µm are synthesized by electrodeposition in the etched nanochannels. Optimized deposition conditions yield single-crystalline nanowires of both materials. After electrodeposition, the polymer membranes are dissolved in an organic solvent. The from the template released Cu₂O nanowire arrays are directly coated with a thin and conformal TiO₂ passivation film by atomic layer deposition (ALD). The Au nanowire arrays, in turn, are first coated with a Cu₂O layer by electrodeposition and then with a TiO₂ film by ALD. For optimization, the thickness of both layers is systematically varied during the synthesis.

The photoelectrochemical performance of both types of nanowire-based photoelectrodes is studied as a function of parameters such as nanowire length, diameter, and density. In particular, the influence of the core-shell geometry, and the resulting minimization of the diffusion length for both minority and majority charge carriers is discussed.

LaTaON₂^1,2 is a well-known perovskite-type material with defined composition, oxidation state, and physical properties, which make it favorable for solar water splitting.³ This Ta⁵⁺-containing oxynitride can be synthesized, e.g. from LaTaO₄. The crucial synthetic step for perovskite-type oxynitrides is the ammonolysis, typically a thermal treatment in flowing NH₃. Several parameters such as temperature and heating ramp, reaction time, and gas flow rate have to be carefully adjusted.^4,5 Besides, the anionic composition can be altered via the adjustment of the precursor reactivity. A thorough tuning of all above mentioned parameters allowed us a controlled variation of the anionic composition from LaTaON₂ to LaTaO₂N. This consequently leads to a reduction of Ta⁵⁺ to Ta⁴⁺. A great influence of this oxidation state change on the physical properties, particularly the light absorption, the charge separation, and the surface redox activity, is expected.⁶

References:

(1)         Marchand, R.; Pors, F.; Laurent, Y. Ann. Chim. Fr. 1991, 16, 553–560.

(2)         Marchand, R.; Antoine, P.; Laurent, Y. J. Solid State Chem. 1993, 107, 34–38.

(3)         Liu, M.; You, W.; Lei, Z.; Takata, T.; Domen, K.; Li, C. Chinese J. Catal. 2006, 27 (7), 556–558.

(4)         Ebbinghaus, S. G.; Abicht, H. P.; Dronskowski, R.; Müller, T.; Reller, A.; Weidenkaff, A. Prog. Solid State Chem. 2009, 37 (2-3), 173–205.

(5)         Widenmeyer, M.; Peng, C.; Baki, A.; Xie, W.; Niewa, R.; Weidenkaff, A. Solid Sate Sci. 2016, 54, 7–16.

(6)         Bubeck, C.; Widenmeyer, M.; Richter, G.; Coduri, M.; Salas Colera, E.; Yoon, S.; Osterloh, F.; Weidenkaff, A. in preparation.

Fossil fuels need to be replaced quickly and on a large scale to render our energy system compatible with current greenhouse gas emission targets. The scale of the transition is enormous and makes efficiency a key factor for its feasibility. The required pace of the transition, on the other hand, renders the modification of materials, that are already established in photovoltaics, for use in solar water splitting devices attractive.

In the case of light-driven water splitting, multi-junction absorbers are essential to provide enough photovoltage and -current for efficiencies beyond 10%. Challenges arising from the distribution of the solar spectrum over multiple sub-cells include appropriate internal band alignment, spectral shaping by (metallic) co-catalysts and the electrolyte as well as the reduction of reflection losses. In this talk, I will discuss current developments in the field of high-efficiency, immersed water splitting systems of currently up to 19% solar-to-hydrogen and outline the route towards efficiencies beyond 20%. Furthermore, I will outline the impact of efficiency on the potential of photoelectrochemical systems as a negative emission technology.

The hydrogen production will play a major role for the energy transition. Recently, for water splitting context [1], [2], a demonstration of the efficiency enhancement of BiVO₄ photoanodes has been shown in PEC devices[3] by simply a texturation of surfaces. Moreover, in the study of the semiconductor photocatalyst materials[4], GaP semiconductor appears to be a good candidate for photoelectrode in PEC devices[5]. One strong argument is to have at least 1.73 eV photopotential requirements for water splitting and its bandgap is larger and about 2.26 eV.

In this aim of water splitting applications, we propose a surface energy engineering for a large scale textured GaP template monolithically integrated on Si [6]. Based on experimental analysis and theory, the stability of the {114} facets is scrutinized by scanning tunneling microscopy images and also supported by density functional theory calculations. We then show that change of the surface energy for experimentally promoting the GaP(114) surface texturation can be achieved through (i) destabilizing the GaP(001) surface by using a vicinal Si substrate or through (ii) favoring the {114} facets formation by changing the group-V atmosphere above the surface on a miscut-free GaP substrate.

This work is supported by the French National Research Agency project ANTIPODE (Grant no. 14-CE26-0014-01) and Région Bretagne. The ab initio simulations have been performed on HPC resources of CINES under the allocation 2017-[x2017096724] made by GENCI (Grand Equipement National de Calcul Intensif).

[1] M. G. Walter et al., ‘Solar Water Splitting Cells’, Chem. Rev., vol. 110, no. 11, pp. 6446–6473, Nov. 2010.

[2] A. Fujishima and K. Honda, ‘Electrochemical Photolysis of Water at a Semiconductor Electrode’, Nature, vol. 238, no. 5358, p. 37, Jul. 1972.

[3] J. Zhao et al., ‘High-Performance Ultrathin BiVO₄ Photoanode on Textured Polydimethylsiloxane Substrates for Solar Water Splitting’, ACS Energy Lett., vol. 1, no. 1, pp. 68–75, Jul. 2016.

[4] A. Kudo and Y. Miseki, ‘Heterogeneous photocatalyst materials for water splitting’, Chem. Soc. Rev., vol. 38, no. 1, pp. 253–278, 2009.

[5] E. E. Barton, D. M. Rampulla, and A. B. Bocarsly, ‘Selective Solar-Driven Reduction of CO₂ to Methanol Using a Catalyzed p-GaP Based Photoelectrochemical Cell’, J. Am. Chem. Soc., vol. 130, no. 20, pp. 6342–6344, May 2008.

[6] I. Lucci at al., ‘A Stress‐Free and Textured GaP Template on Silicon for Solar Water Splitting’, Adv. Funct. Mat. Hot Topic: Water Splitting, 1801585, 2018

Unassisted water splitting by this approach requires efficient, stable and low-cost photoelectrode materials. Of the material candidates for photocathodes, Cu₂O stands out as best performing oxide for hydrogen production currently available. Although numerous experimental investigations greatly enhanced performance and stability of Cu₂O photocathode, there is lack of detailed theoretical understanding of charge 
transport mechanism in Cu₂O buried junction photocathodes. On the macroscopic scale within the drift-diffusion approximation of charge transport in semiconductors, device simulation studies so far studied Cu₂O photovoltaics. The charge transport in photoelectrochemical Cu₂O buried junctions has not been addressed so far by device simulation study.

In our work, we discuss existing belief that separation of the valence band of p-type Cu₂O to conduction band of n-type buffer layer limits the available photovoltage, providing example calculation with Al:ZnO buffer layer, which does not follow this rule. To explain the measured low photovoltage obtained with Al:ZnO buffer layer, we identify recombination at defect-rich Al:ZnO/Cu₂O interface as a responsible mechanism. Furthermore, our device simulations of onset voltage of TiO₂/Ga₂O₃/Cu₂O photocathode correspond well with the measured value. We describe how the two energy barriers for electron transport at Cu₂O/Ga₂O₃ and at TiO₂/Ga₂O₃ interfaces cause anodic shift of the onset voltage. Numerical quantification of the drift and diffusion currents throughout the TiO₂/Ga₂O₃/Cu₂O photocathode brings us to conclusion that electron diffusion in Cu₂O bulk close to Cu₂O/Ga₂O₃ dominates the electron current at the onset voltage. Variation of electron affinity, mobility and doping of Ga₂O₃ buffer layer in our model result in important improvements on onset voltage up to 1.65 V vs RHE and ratiometric power-saved of the Cu₂O photocathode, providing future ideas for experimental developments.

Our constantly increasing society’s need for energy has triggered a pressing need for the development of new materials for renewable sources. Concerning materials for energy harvesting, the most promising approaches for high-performance and low-cost photovoltaics rely in inhomogeneous compounds, such as perovskites and polycrystalline thin films (e.g. CIGS and CdTe). Thus, resolving their electrical and optical behavior at the nanoscale is imperative to advance their understanding. In this talk, I will share our scientific findings to image and quantify the local voltage response of nano- and mesoscale inhomogeneities in perovskites [1,2], CIGS [3], and CdTe through a variant of KPFM and NSOM [4-6]. By submitting the samples to illumination and humidity treatments under controlled conditions, we map the dynamic physical behavior of MAPI and triple-cation perovskites.

References:

[1] J. M. Howard et al. J. Phys Chem Letters, in press (2018)

[2] J. L. Garrett et al. Nano Letters 17, 2554 (2017).

[3] E. M. Tennyson et al. ACS Energy Letters 1, 899 (2016).

[4] E. M. Tennyson et al. ACS Energy Letters, 2, 2761 (2017). Invited Review

[5] E. M. Tennyson et al. ACS Energy Letters 2, 1825 (2017). Invited Perspective

[6] E. M. Tennyson et al. Advanced Energy Materials 5, 1501142 (2015).

Understanding water splitting at nanostructured electrodes presents some formidable challenges. Here, regular nanostructured photoelectrodes are considered using hematite nanorod arrays as an example. Since Mott Schottky plots are often reported for nanostructured electrodes, we revisit the effects of the cylindrical nanorod geometry on the capacitance-voltage behaviour. The limiting case of complete depletion is discussed in terms of the residual geometric capacity at the base of the nanorods. Since nanorod arrays generally leave areas of the substrate exposed, it is also necessary to consider the parallel capacitance associated with the fraction of uncovered surface. We then turn to the enhancement of external quantum efficiency (EQE) achieved by nanostructuring, again using hematite nanorod arrays as experimental examples. We show that, although very substantial EQE enhancement should be achieved by simple geometric effects, the performance of nanostructured hematite electrodes in the visible region of the spectrum is considerably lower than predicted if all charge carriers generated in the space charge region (SCR) were collected. Further analysis reveals that the internal quantum efficiency increases with photon energy, suggesting that the probability of generating free, rather than bound, electron-hole pairs in hematite depends on the excess energy hν - E_gap.

The performance of semiconducting materials as photoanodes is continually optimised overtime. With the aid of nanostructuring, noteworthy performance enhancements for the water oxidation reaction are obtained when coupled with electrocatalysts such as FeOOH/NiOOH and Co-Fe Prussian Blue.^1,2 In the longstanding compromise between the light penetration depth and charge diffusion lengths, nanostructuring provides an effective tool. However, it is not often efficacious unless coupled with co-catalyst or in presence of hole scavengers. This highlights the presence of significant surface recombination that is especially critical for kinetically demanding reactions such as the four hole oxidation of water, requiring the accumulation of holes on the surface.

In this talk, we will discuss the structure performance relationship between densely packed and mesoporous structures of bismuth vanadate alongside other n-type materials such as titania and hematite. Using techniques such as transient photocurrent measurements (TPC) and photo induced absorption spectroscopy (PIAS), we can directly probe the influence of porosity and increased surface area on electron transport and photo-catalysis.

References:

1         T. W. Kim and K.-S. Choi, Science (80-. )., 2014, 343, 990–994.

2         F. S. Hegner, I. Herraiz-Cardona, D. Cardenas-Morcoso, N. López, J. R. Galán-Mascarós and S. Gimenez, ACS Appl. Mater. Interfaces, 2017, 9, 37671–37681.

Solar water splitting is a sustainable and in principle environmentally sound approach to solve the high energy demand of mankind using renewable energies[1], because of the high energy density of hydrogen and no CO₂ emission when used as a fuel. To provide hydrogen in large quantities, nontoxic, earth abundant and cheap catalysts are in demand to replace costly platinum for reducing the overpotential in the hydrogen evolving reaction (HER) process[2].

In our study, molybdenum sulfide layers of different crystallinity and morphology have been prepared by reactive magnetron sputtering and tested as HER catalyst. Best performance was obtained starting from an amorphous MoS_x layer deposited at room temperature on FTO (η₁₀=180mV at pH 0 sulfuric acid electrolyte), which in Raman spectroscopy shows similar vibrations as the highly efficient HER catalyst, (NH₄)₂Mo₃S₁₃[3]. However, during electrochemical measurement, production of hydrogen was proved along with the release of H₂S especially during the first 20min of CV (activation step). Afterwards, only hydrogen was evolved from the electrode. A structural change of the material from amorphous MoS_x to MoS₂ layered structure happens during CV was proved by Raman measurement with the disappearance of terminal and bridging [S₂]^2- units and the emergence of the characteristic vibration mode A_1g (out of plane S bonding) from layer structured MoS₂. Mo atoms at the edges of formed MoS₂ nano-islands are oxidized to Mo(VI) after removal from the electrolyte. From the ratio of Mo (IV) to Mo(VI) in the XPS spectra, the size of MoS₂ nano-islands could evaluated. The high number of this Mo atoms at the edges explains the good performance of the HER electrode. For stability test, room temperature sputtered MoS_x electrode was measured under CV conditions for 10h exhibiting an increase in overpotential of 46 mV.

1.            Lewis, N.S. and D.G. Nocera, Powering the planet: chemical challenges in solar energy utilization. Proc Natl Acad Sci U S A, 2006. 103(43): p. 15729-35.

2.            McCrory, C.C., et al., Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J Am Chem Soc, 2015. 137(13): p. 4347-57.

3.            Kibsgaard, J., T.F. Jaramillo, and F. Besenbacher, Building an appropriate active-site motif into a hydrogen-evolution catalyst with thiomolybdate Mo3S13 (2-) clusters. Nature Chemistry, 2014. 6(3): p. 248-253.

Even after ca. four decades of R&D effort, we still do not have a "magic bullet" inorganic semiconductor to photoelectrochemically generate fuels or chemicals from sunlight in a sustainable, efficient and environment-friendly manner. While it is unlikely that a single semiconductor candidate will emerge that simultaneously satisfies all the optical, electrical, surface chemical, and electrochemical prerequisites for efficient solar conversion, complex oxides or chalcogenides (or derivatives thereof, e.g., oxynitrides) do provide a versatile framework for rational design of the "perfect beast" in a chemical architectural sense. Ultimately two or more such semiconductor compositions can be combined in a composite design much like complementary functionalities are combined in photosynthetic assemblies in Nature. In such designs, the semiconductor(s) and the photoactive junction can even be separated from the electrolyte and the electrocatalyst component in a "buried junction" design. In this vein, the author's laboratory has been engaged in the development of time- and energy-efficient methods for synthesizing new families of photoelectrode or photocatalyst materials. In this particular talk, the author will provide first a context for the key role that solid-state chemistry paradigms and principles can play in photoelectrode designs for driving multi-electron processes typical of solar fuel generation. A representative ternary semiconductor system, namely, M-Ln-X (M = divalent metal, e.g., Ba, Ln = lanthanide element, e.g., Ce, and X = chalcogen, e.g., S) will be discussed in this talk.

The recombination of photogenerated electron-hole pairs is one of the major limiting factor in photo­electrocatalysts absorbing in the visible region of the solar spectrum. The performance as expressed by the incident photon-to-current efficiency (IPCE) strongly depends on the loading and thickness of the active material. Especially for BiVO₄ the slow electron transport to the back contact facilitates charge carrier recombination and thus does not allow to exploit the entire potential of the photo­absorber. Hence, thin layers are favoured to avoid excessive charge carrier recombination, however, on the expense of a low absorption of the incident light with the result of low photocurrent output of such electrodes.

We present the development of different strategies to address the limitations evoked by charge carrier recombination of BiVO₄-based photoabsorbers. The electrochemically-induced deposition of Pt-nanoparticles enhanced the contact between electrochemically deposited BiVO₄ films and fluorine doped tin oxide (FTO) electrodes which showed significantly lower recombination rates during frontside illumination¹. Modifications of BiVO₄ films by doping with different metals showed enhancements of the IPCE by more than 30% at 1.2 V vs. RHE for Mo/Zn and Mo/B doped systems during light driven oxygen evolution reaction (OER)¹. High-throughput screening showed highest improvement of photocurrents by a factor of 10 for Bi(V-Mo-X)O₄ material libraries as compared to a BiVO₄ reference material². To further improve the performance of BiVO₄-based photoabsorbers a systematic characterisation of different co-deposited OER catalysts onto pre-modified Mo-doped BiVO₄ films was performed revealing the necessity for an optimal deposition technique and loading of the co-catalyst in question such as electrodeposition, photoassisted electrodeposition and photo­deposition. Moreover, an alternative technique for simple electrode preparation and modification based on an air brush-type spray-coater system is proposed which allows to easily tune the layer thickness of photoabsorber films and is able to deposit gradients of additives or OER co-catalysts. Using the spray-coating technique in combination with an optical scanning droplet cell allowed for a quick and easy but precise tuning of the investigated systems for further enhancing the performance of BiVO₄-based photoabsorbers.

References

R. Gutkowski, D. Peeters, W. Schuhmann, J. Mater. Chem. A, 2016, 4, 7875–7882.

R. Gutkowski, C. Khare, F. Conzuelo, Y. Kayran, A. Ludwig, W. Schuhmann, Energy Environ. Sci., 2017, 10, 1213–1221.

Acknowledgements

The authors are grateful to the financial support of the DFG within the framework of the SPP1613 (SCHU929/12-1 and 12-2).

The development of photoelectrochemical strategies for the production of added-value chemicals and fuels using solar light is particularly attractive to overcome the dependence of fossil fuels at a global scale.[1] Specifically, the photoelectrochemical oxidation of H2O to produce solar H2 as a clean energy vector or valuable chemical precursor stands out as one of the most promising approaches in this direction. Different approaches have been followed to achieve competitive devices, targeting Solar To Hydrogen (STH) efficiency of 10%, durability of 10 years and cost of 2-4 $/kg of dispensed Hydrogen.[2] For this purpose, the use of low-cost, earth-abundant, stable materials synthesized by easily up-scalable methods is essential.[3] In the present talk, we will discuss about the suitability of earth-abundant metal oxides to achieve these targets. Different examples of the synergistic combination of metal oxides (Fe2O3[4], and BiVO4[5],[6]) with catalytic layers (Fe-Co Prussian Blue, Ag3PO4, etc…) will be described, emphasizing the mechanistic insights leading to enhanced performance. Our studies focus on the correlation of the photoelectrochemical response of the materials with a detailed structural and mechanistic characterization carried out by different microscopic and spectroscopic tools.

The interest in the direct storage of solar energy as a chemical fuel in a semiconductor based photoelectrochemical system started with the demonstration of solar photoelectrolysis of water with large band gap oxide semiconductor electrodes in the late 1970s. In the last decade or so there has been both and increased interest and increased funding towards achieving a goal of efficiently producing cost effective fuels from solar energy with either water or carbon dioxide as a feedstock. This talk will review the progress towards this goal considering recent developments. One of these developments is that the cost of photovoltaic systems has been decreasing rapidly to where currently the cost of the solar panels is now exceeded by balance of systems cost. The cost of electrolyzers will be also decrease as they are improved and scaled. Connecting these two existing technologies has the advantage of producing hydrogen where and when it is needed and at pressure. These facts mean that the window for direct solar photoelectrolysis is rapidly closing. One possible breakthrough is that a new stable, efficient, inexpensive, defect-tolerant and scalable new materials are identified that can quickly improve the efficiency of photoelectrolysis much like the hybrid perovskites are have done for photovoltaic devices. This talk will review the progress in combinatorial approaches to discover new materials for photoelectrolysis with some examples including one from the Solar Hydrogen activity Research Kit (SHArK) Project, a distributed science project that provides undergraduates and high school students with the resources to produce and screen metal oxide semiconductors for photoelectrolysis activity. In addition the reasons for producing hydrogen from water rather than direct carbon dioxide reduction to produce fuels will be reviewed. A new system for storing solar energy directly as redox equivalents, a solar chargeable redox flow battery, will also be introduced and its advantages and disadvantages compared to solar hydrogen generation will be discussed.

Electrochemical CO2 conversion can be coupled with a photovoltaic cell and provide a pathway to utilize solar energy for the chemical synthesis. Ideally, such artificial photosynthesis system want to use CO2 and H2O as feed-stock molecules to produce value-added chemicals such as fuels or raw chemicals. My research team reported a monolithic and stand-alone device composed of a photovoltaic cell module, an Au CO2 reduction, a cobalt oxide anode accomplishing over 4 % conversion efficiency for CO2 conversion to CO production. To improve the solar to chemical conversion efficiency and to increase the feasibility further, we have developed efficient electrocatalysts and replaced the photovoltaic cell with Si modules, achieving ~ 8% of solar-to-CO conversion efficiency.

In addition, in this talk, metal-based electrocatalysts interacting with p-block elements or surface mediated molecules will be discussed for selective CO or C2+ (i.e. ethylene) production from CO2 reduction. The experimental results and theoretical simulation with various different types of metal catalysts (Ag, Zn, and Cu) give insights how to suppress the hydrogen evolution reaction (HER) is crucial to achieve efficient CO2 reduction catalysts. Monodispersed Ag nanoparticles are suggested to have the special interaction between the surface Ag and the surface mediated molecules which can modify the local electronic structure favoring for the selective CO production (up to 95 % of Faradaic efficiency). In addition, in the case of selective ethylene production, special Cu nanostructure formed by in-situ electrochemical fragmentation is demonstrated to be effective for increasing C-C bond coupling (up to 73 % of Faradaic efficiency) and selective ethylene production (up to ~ 60 % of Faradaic efficiency). In-situ X-ray absorption spectroscopy (XAS) studies are performed to understand the catalyst activity. Our series of studies suggests the modification of the metal nanoparticle surface by oxygen atom or surface mediated molecules can be effective strategies to increase CO2 reduction reaction activity and stability.

Chemically synthesized quantum dots (QDs) can potentially enable new classes of highly flexible, spectrally tunable lasers processible from solutions [1,2]. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage and an important challenge - realization of lasing with electrical injection - is still unresolved. A major complication, which hinders the progress in this field, is fast nonradiative Auger recombination of gain-active multicarrier species such as trions (charged excitons) and biexcitons [3,4]. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination by “softening” the electron and hole confinement potentials [5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [6], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a “zero-threshold-optical-gain” concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [7]. Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting recent developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices operating across a wide range of wavelengths and fabricated on virtually any substrate using a variety of optical-cavity designs.

[1] Klimov, V. I.et al., Optical gain and stimulated emission in nanocrystal quantum dots. Science290, 314 (2000).

[2] Klimov, V. I.et al., Single-exciton optical gain in semiconductor nanocrystals. Nature447, 441 (2007).

[3] Klimov, V. I. et al., Quantization of multiparticle Auger rates in semiconductor quantum dots. Science287, 1011 (2000).

[4] Robel, I., et al., Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett.102, 177404 (2009).

[5] Y.-S. Park, et al., Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).

[6] Lim, J., et al., Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater.17, 42 (2018).

[7] Wu, K., et al., Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol.12, 1140 (2017).

Cadmium chalcogenide (CdX, X=Se, S, Te) colloidal quantum wells or nanoplatelet (NPLs) have atomically precise thickness of a few CdX layers (1-2 nm) and uniform exciton confinement energy across lateral dimensions of 10s of nanometer and larger. This unique property has led to speculation of coherent delocalization of the exciton center-of-mass over the entire NPL, which would have profound effect on fundamental properties of excitons (including its oscillator, transport mechanism and Auger annihilation) and their applications (such as lasing threshold). In this talk, I will summarize a series of recent studies on fundamental exciton properties in 2D NPLs. We show that at room temperature, exciton transport can be described by 2D diffusion with diffusion constants near the bulk crystal values and hot exciton diffusion completes with exciton relaxation. In CdSe NPLs, the biexciton Auger recombination lifetime does not depend linear on its volume, deviating from the “Universal Volume” scaling law that has been reported for 0D quantum dots. Instead, the Auger lifetime scales linearly with the lateral size because of the size dependent collision frequency, and the Auger lifetime depends sensitively (nonlinearly) on the NPL thickness due to change in the degree of quantum confinement. We suggest Auger lifetime in for other 2D NPLs and 1D nanorods can also be expected to deviate from the volume scaling law because of the different dependences on the quantum confined and non-confined dimensions. We have developed an optical gain model that accounts for the nature of 1D confinement in NPLs and revealed the origin of low gain thresholds in these materials. Finally, we will also discuss conditions under which coherent delocalization of excitons and the Giant Oscillator Strength Transition (GOST) effect can be observed.

Hybrid nanoparticles (HNPs) combine disparate materials onto a single nanosystem thus providing a powerful approach for bottom-up design of novel architectures. Beyond the fundamental development in synthesis, the interest in HNPs arises from their combined and often synergetic properties exceeding the functionality of the individual components. These ideas are well demonstrated in hybrid semiconductor-metal nanoparticles, which are the focus of this talk. The synergistic optical and chemical properties of hybrid nanoparticles resulting in light-induced charge separation and charge transfer, allow photocatalytic activity which can promote surface chemistry redox reactions, and open a pathway for converting solar energy to chemical energy stored in a fuel. An additional area of interest is in use of the HNPs for light-induced generation of radicals opening options for light-induced on-demand radicals formation.

We will report on the effects of the surface coating and the co-catalyst metal size on the photocatalytic function of metal tipped semiconductor nanorods as a model hybrid nanoparticle system. Both tested parameters were found to influence the photocatalytic efficiency and charge transfer dynamics. The work combines advances in synthesis of well-controlled hybrid nanoparticles, hydrogen evolution efficiency measurements, steady state and time resolved emission measurements, as well as ultrafast transient absorption measurements to gain a complete view on the effects of these parameters on photocatalysis with metal tipped semiconductor nanorods. A model was devised to capture the essential effects of the size of the metal tip on the photoctalytic efficiency. An additional effect concerns the HNPs functionality under high excitation fluence in the regime of multiexcitons. The understanding of the effects of the hybrid nanosystems properties on the photocatalytic processes contribute to the rational design of hybrid nanostructures in photocatalytic applications. Moreover, use of the HNPs in generation of reactive hydrogen species and its application for controlling enzymatic activity and additional processes will also be highlighted.

Delayed exciton emission has been extensively studied in several types of low dimensional semiconductors but is still not fully understood. The photon energy of delayed emission is identical to that of the spontaneous exciton emission, but it occurs on an extended time scale, from the life time for spontaneous emission (typical 20 ns) to the microsecond and even millisecond regime. In a heuristic way, we might say that the exciton is stored as non-radiative state in the nanocrystal for a variable time without (much) energy loss. The stored energy finally returns to the radiative state and emission can occur.

I will review our work on the characterization of delayed emission of low dimensional CdSe systems (core-shell quantum dots, rods, and platelets) which was performed on the ensemble level as well as on the single-dot level. I will also relate the phenomenon of delayed emission to another extensively studied phenomenon in these systems, namely blinking. In another contribution from our group the interplay between spontaneous emission, delayed emission and energy transfer as observed in QD supraparticles will be presented.

Reduced Auger Recombination in Single CdSe/CdS Nanorods by One-Dimensional Electron Delocalization." Nano Letters 13(10): 4884-4892.

Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots." Nano Letters 15(11): 7718-7725.

Dynamics of Intraband and Interband Auger Processes in Colloidal Core-Shell Quantum Dots." Acs Nano 9(10): 10366-10376.

Temporary Charge Carrier Separation Dominates the Photoluminescence Decay Dynamics of Colloidal CdSe Nanoplatelets." Nano Letters 16(3): 2047-2053.

Composite Supraparticles with Tunable Light Emission." Acs Nano 11(9): 9136-9142.

Generating multiple excitons by a single high-energy photon is a promising third generation solar energy conversion strategy.  I will discuss recent progress within the Center for Advanced Solar Photophysics (CASP) on both improving the multple exciton generation (MEG) efficiency in heterostructured nanostructures as well as in the using MEG in the production of solar fuels.  We are exploring MEG in PbE|CdE (E = S, Se) Janus-like hetero-nanostructures and find that MEG is enhanced over that of single-component and core/shell nanocrystal architectures. The enhanced MEG arrises due to the asymmetric nature of the hetero-nanostructure that results in an increase in the effective Coulomb interaction that drives MEG and a reduction of the competing hot exciton cooling rate. We find that slowed cooling occurs through effective trapping of hot-holes by a manifold of valence band interfacial states having character of both PbS and CdS. The Janus-like NCs retain their symmetric structure and thus can be easily incorporated as the main absorber layer in functional solid-state solar cell architectures. Finally, based upon our analysis, we provide design rules for the next generation of engineered nanocrystals to further improve the MEG characteristics. In addition, we have developed a PbS QD photoelectrochemical cell that is able to drive a hydrogen evolution reaction with a peak external quantum efficiency (EQE) of over 100%, with the highest EQE at 114±1.3%.  Our results show that the extra carriers produced via MEG can be used to drive a chemical reaction with above unity quantum efficiency thus demonstrating a new direction in exploring high efficiency approaches for solar fuels. I will discuss the potential for using MEG to drive a photochemical reaction as opposed to use in a solar cell. 

Ternary CuInX₂ (X= S, Se) nanocrystals (NCs) have attracted increasing attention as promising alternatives for CdX and PbX NCs due to their low toxicity, large absorption cross-sections across a broad spectral range, and unparalleled photoluminescence tunability, spanning a spectral window that extends from the green to the NIR (~550 to ~1100 nm for X= S). To achieve properties that are inaccessible to single component NCs (such as high PL quantum yields, spatial charge carrier separation, etc.), researchers have been synthesizing colloidal CuInX₂-based hetero-NCs (HNCs) (e.g., CuInS₂/ZnS concentric core/shell HNCs, CuInSe₂/CuInS₂ dot-in-rod HNCs). Anisotropic CuInX₂-based HNCs are particularly interesting, since they are expected to exhibit novel properties, such as polarized NIR photoluminescence (PL) and spatial charge separation, which are attractive for many applications (e.g., polarized LEDs, photocatalysis and artificial photosynthesis, luminescent solar concentrators, solar cells). Nevertheless, reports on the synthesis of anisotropic CuInX₂-based HNCs are scarce. 

In this work, we report a novel two-step pathway that yields CuInS₂/ZnS dot core/ rod shell heteronanorods. The wurtzite CuInS₂ NCs used as seeds are obtained by cation exchange in template Cu_2-xS NCs. The CuInS₂ NC seeds are injected together with the S precursor into a hot solution of the Zn precursor and suitable coordinating ligands, which leads to heteroepitaxial growth of ZnS primarily on the cation-rich polar facet of the seeds, as demonstrated by high-angle annular dark-field scanning transmission electron microscopy and electron tomography. The colloidal wurtzite CuInS₂/ZnS dot-in-rod heteronanorods have large molar extinction coefficients, and photoluminescence in the NIR (~800 nm) with PLQYs ~20%. Moreover, they exhibit multi-exponential PL decay that is initially rather fast (a few ns), and then slows down to several hundreds of ns, similar to the behavior previously reported for both chalcopyrite and wurtzite isotropic CuInS₂/ZnS core/shell HNC, which has been attributed to radiative recombination of a conduction band electron with a hole localized at a Cu ion. The slow radiative recombination dynamics are potentially beneficial for photovoltaic and photocatalytic applications, since long carrier lifetimes are of great importance for effectively extracting charge carriers. 

III-V semiconductor quantum wells have obtained a central place in advanced logics and opto-electronics. In more recent research directions heading towards materials with entirely new functions, the effects of a nano scale geometry forming a periodic scattering potential in the lateral directions of the quantum well has been discussed and calculated. In case of a nano scale honeycomb geometry, and entirely new band structure emerges in which the highest valence and lowest conduction bands become Dirac cones at the K-points, while the semiconductor quantum well band gap remains nearly unaltered.

In this presentation we report on the fabrication of a 10 nm thick InGaAs quantum well (QW) on a n-type InP substrate with a honeycomb symmetry structure by creating a triangular anti-lattice inside the QW using high-resolution electron beam lithography. The morphology of the samples are studied using atomic force microscopy (AFM), elementary diffraction spectroscopy (EDS) and cross-section transmission electron microscopy (TEM). The quality of the samples is characterized using scanning electron microscopy (SEM), which is used for an extensive statistical analysis to determine the disorder inside the lattices. The results are supported by theoretical simulations on the bandstructure and density of states (DOS).

Nanocrystals with type-II heterostructures are attractive candidates for applications in which separation of excited charge carriers are exploited, such as photovoltaics [1], upconversion [2], or quantum dot lasing [3]. These properties are in competition with processes involving interfacial trap states, usually associated with the particle surface or strained interfaces. To use semiconductor nanocrystals in technical applications it is of utmost importance to characterise and control these effects.

Here we present a model system made of CdTe-tipped CdSe/CdS seeded nanorods, in which the CdTe particle is separated in space from the CdSe seed while being in electronic contact. The CdTe/CdS interface has type-II character, but charge carrier delocalisation is likely affected by the large lattice mismatch of 10% between CdTe and CdS [4]. We performed ultrafast (femto to picosecond) transient absorption spectroscopy to map carrier dynamics after exciting one or both recombination centres, with seed size and rod length as variable parameters. Conduction band electrons, after exciting the CdTe/CdS interface, are expected to localise in the CdSe seed, which has the lowest energy band edge and acts as a reporter particle. However, while states in the CdS rod get bleached by the electron no signal corresponding to CdSe is detected. We employ electron and hole scavengers to determine the roles of the individual charge carriers, and by this the contribution of Coulomb interactions and interfacial trap states.

This experiment gives fundamental insight into the behaviour of strained nano-heterointerfaces, as well as the effects from size quantisation and mean free path of the carriers. The experimental data is compared to effective mass approximation-based simulations for free carriers. While the basic electronic structure is predicted with sufficient accuracy the comparison highlights the importance to simulate carrier interactions and interfaces on the atomic scale.

[1] Itzhakov, S.; Shen, H.; Buhbut, S.; Lin, H.; Oron, D.; J. Phys. Chem. C 2013, 117 (43), 22203–22210.

[2] Deutsch, Z.; Neeman, L.; Oron, D.; Nat. Nanotech. 2013, 8, 649–653.

[3] Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I. V.; Mcguire, J. A.; Piryatinski, A.; Nature 2007, 447, 441–446.

[4] Jing, L.; Kershaw, S. V.; Kipp, T.; Kalytchuk, S.; Ding, K.; Zeng, J.; Jiao, M.; Sun, X.; Mews, A.; et al.; J. Am. Chem. Soc. 2015, 137, 2073–2084.

It was recently shown that single colloidal quantum dots (QDs) may exhibit large sensitivities to external electric fields in the form of a spectral shift of the emission wavelength. This opens the possibility of designing QDs’ based local electric field sensors, for example for neuronal membrane action potential (AP) sensing. One of the hurdles on the way to such a goal is fast detection. Typically, slow spectrometers are used to detect the spectral shifts however, AP sensing requires significantly faster detection rates of the order of a millisecond.

The underlying mechanism for the QDs’ response to external electric fields is the quantum confined Stark effect. From symmetry considerations spherical QDs exhibit only a red-shifted emission spectrum in the presence of an electric field. APs consist of a change from negative to positive potential thus, symmetric QDs are of limited value. Asymmetric type II ZnSe/CdS nanorods, on the other hand, exhibit a large linear response, namely both red- and blue-shifts, depending on the orientation of the QD in the electric field. Moreover, the spectral shifts are expected to be correlated with changes in the decay rates.

Here, we demonstrate an experimental setup designed at achieving shot-noise limited sensitivity to emission spectral shifts on time scale suitable for AP sensing. We present experimental results of these phenomena as well as characterize the performance of single QDs as sensors for short millisecond voltage pulses comparable to APs in both duration and amplitude.

Two-dimensional colloidal nanomaterials represent very exciting optoelectronic properties. The formation of ordered and densely packed surface layers of amphiphilic ligand molecules on certain crystal facets can drive the normally isotropic into a two-dimensional crystal growth, resulting in semiconducting nanosheets. Such structures combine good lateral conductivity with solution-processability and electronic confinement in height, which allows tuning the effective bandgap of the materials. I will present the syntheses of the materials and the analyses of the optoelectronic transport through these materials in field-effect transistors, solar cells, and spintronic devices. It turns out that the electronic confinement allows for an optimization of their performances.

Two-dimensional colloidal nanomaterials represent very exciting optoelectronic properties. The formation of ordered and densely packed surface layers of amphiphilic ligand molecules on certain crystal facets can drive the normally isotropic into a two-dimensional crystal growth, resulting in semiconducting nanosheets. Such structures combine good lateral conductivity with solution-processability and electronic confinement in height, which allows tuning the effective bandgap of the materials. I will present the syntheses of the materials and the analyses of the optoelectronic transport through these materials in field-effect transistors, solar cells, and spintronic devices. It turns out that the electronic confinement allows for an optimization of their performances.

The solar-driven photocatalytic splitting of water into hydrogen and oxygen is a potential source of clean and renewable fuels. However, four decades of global research have proven this multi-step reaction to be highly challenging. The design of effective artificial photo-catalytic systems will depend on our ability to correlate the photocatalyst structure, composition, and morphology with its activity.

I will present our strategies, and most recent results, in taking photocatalyst production to new and unexplored frontiers. I will focus on unique design of innovative nano scale particles, which harness nano phenomena for improved activity, and methodologies for the construction of sophisticated heterostructures. I will share our design rules and accumulated insights, which enabled us to obtain a perfect 100% photon-to-hydrogen production efficiency, under visible light illumination, for the photocatalytic water splitting reduction half reaction. Finally, I will describe our future designs of systems capable of overall water splitting and genuine solar-to-fuel energy conversion.

One of the potential pathways for exceeding the Shockley-Queisser efficiency limit of photovoltaic cells is via incoherent upconversion of low-energy photons. An ‘ideal’ upconversion system would have high efficiency, a broad response and a low saturation intensity, all encapsulated in an easily deposited system. Past colloidal upconversion nanocrystals were based on rare-earth doped oxides or on organic triple-triplet annihilation polymers. Several years ago we introduced double quantum dots as an alternative upconversion mechanism. In this system a low bandgap dot is coupled to a high band gap dot through a tunneling barrier. Interband absorption in the low bandgap dot is followed by intraband absorption which enables to transfer the excitation to the higher bandgap dot, eventually leading to emission of a higher energy photon. The formation of the latter system and recent advances in the design of double quantum dots will be discussed. We also introduce a new hybrid organic-inorganic particle where upconversion is mediated by ligand induced mid-gap states. Importantly, this latter system relies on direct excitation of charge transfer state and can therefore be potentially extended deeper into the infrared. The state of the art of all these designs will be discussed, as well as the potential for low cost upconversion add-ons to present day solar cells.

The maximum efficiency reachable by a photovoltaic device based on a single absorber is thermodynamically limited to ~33%; the Shockley−Queisser (SQ) limit. To a large extent, this efficiency limit is determined by waste heat in the absorber; waste heat which is generated by the thermalization of “hot” charge carriers generated in the material following the absorption of above-bandgap high-energy photons. Several approaches have been proposed in order to bypass thermal losses in solar cell devices, among them, hot carrier solar cells (HCSCs) are distinctly the most promising concept when considering photo-conversion efficiency limits (reaching ~74%). However, practical implementation of operational HCSCs prototypes remains a big challenge; this is in part due to the difficulties on finding/engineering systems where hot carriers are efficiently collected at room temperature.

In this communication, by employing time resolved THz spectroscopy (TRTS), we demonstrate highly-efficient room-temperature hot electron transfer (HET) at QD/mesoporous oxide interfaces. The emergence of HET is directly apparent from photon-energy dependent TRTS measurements. When the samples are irradiated with photon energies matching the QD bandgap, the ET dynamics are monophasic and defined by a time constant of ~10ps. When the samples are irradiated with above QD bandgap photon energies, the ET dynamics become bi-phasic with characteristic time constants of ~10ps and <1ps respectively (representing cold and hot ET components respectively). For even higher photon energies (~3eV photons onto ~1eV bandgap PbS QDs) the ET dynamics become again monophasic with sub-ps time constants (≤0.1ps, limited by the TRST setup resolution). In this case, the HET collection efficiency for photo-generated carries reaches unity quantum yield. Finally, from temperature dependent analysis of interfacial QD-oxide dynamics, we resolve that HET rates (and hence efficiency) are substantially enhaced as the temperature of the system is reduced. This observation is fully consistent with the “a pirori” expectation that HET efficiency is determined by kinetic competition between QD-to-oxide HET rate and hot electron cooling rate within the QD.

Our results reveal the effect and interplay of key parameters governing hot electron transfer at QD-oxide interfaces. The foreseen potential and constraints for the analyzed systems for the realization of hot carrier solar cells prototypes will be briefly discussed.

Nanocrystals of heavily-doped semiconductors have recently emerged as very promising materials for plasmonics. In contrast to nanocrystals of noble metals, their Localized Surface Plasmon Resonance (LSPR) can be easily tuned in energy by controlling the carrier concentration through doping. In addition, due to the low concentration of carriers compared to metals, the LSPR can be extended to infrared and near-infrared ranges. Recent experimental studies have demonstrated the existence of LSPR in doped nanocrystals of Si and different types of oxides (ZnO, SnO₂, In₂O₃). However, the physics of the LSPR in these NCs is not totally understood. In this talk, I will review recent theoretical studies performed to clarify a certain number of issues. The evolution with doping concentration of the optical processes from single-electron transitions to collective excitations will be described. The conditions required for the emergence of plasmonic modes will be discussed. The results of atomistic calculations will be compared with those of more classical approaches. The intrinsic mechanisms at the origin of plasmon damping in doped ZnO nanocrystals will be analyzed. In this case, the theoretical simulations show that the intrinsic line width of the LSPR can be below 80 meV, in agreement with recent experiments [1]. These results confirm that doped ZnO nanocrystals are very promising for the development of IR plasmonics.

[1] Delerue C. “Minimum Line Width of Surface Plasmon Resonance in Doped ZnO Nanocrystals”. Nano Letters 17 (12), 7599-7605 (2017).

Colloidally prepared, quantum-confined, semiconductor nanocrystals offer tunable energy gaps, strong photoluminescence, and, in some cases, optical gain and lasing [1]. We report ultrafast optical pump, X-ray diffraction probe experiments performed at Argonne National Lab’s Advanced Photon Source with CdSe nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump intensity. Shifts of diffraction peaks relate lattice heating and peak amplitude reduction conveyed transient lattice disordering (or melting). For smaller NCs, melting was observed upon absorption of as few as ∼15 electron–hole pair excitations per NC on average (0.89 excitations/nm³ for a 1.5 nm radius) with a similar electron-hole pair density inducing disordering for all examined NCs. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. Zincblende and wurtzite NCs revert to initial structures following intense photoexcitation suggesting melting occurs primarily at the surface, as supported by simulations. These findings suggest a need to take into account nanomaterial physical stability and transient electronic structure for high intensity excitation applications such as lasing and solid-state lighting.
[1] Klimov et al. Science, 290, 314 (2000).; Kazes et al. Adv. Mater. 14, 317 (2002).
[2] Kirschner et al. Nano Lett. 17, 5314 (2017).

Decades of investigation show that inter-band photoexcitation of quantum dots is followed by rapid relaxation of hot carriers to the quantized band edge states within one or two picoseconds. Due to the large oscillator strength and low degeneracy of the band edge exciton transition, evolution in its intensity and spectrum have played a pivotal part in probing quantum dot exciton cooling. These changes start as a bi-exciton spectral shift while carriers are hot, changing to a bleach due to state filling once the exciton relaxes. Accordingly, kinetics of the BE bleach buildup has served to characterize the final stages of carrier cooling, and its amplitude per cold exciton the degeneracy of underlying electronic states.

Hot multi-excitons (MX) add a new relaxation process to this scenario. Auger recombination (AR) reduces an N exciton state to N-1 plus heat, initially. Again, investigation of AR dynamics is based on the amplitude and decay kinetics of the BE bleach. Interpretation of such data was based on the following assumptions: 1) that ultrafast cooling of hot excitons leads directly to occupation of the lowest electron and hole states (in accordance with the lattice temperature and the state degeneracy), and 2) that aside from mild spectral shifts induced in the remaining band edge transitions, after carrier cooling is over the BE bleach increases linearly with N until state filling is complete.

To test these assumptions, three pulse pump-probe experiments were conducted in our lab, measuring fs transient transmission (TT) of PbSe nanocrystals in the presence and absence of single cold spectator excitons. Results show that the bleach introduced by a second hot exciton falls significantly from that introduced by the spectator. Later we will describe a recent extension to CdSe which shows that adding an additional hot exciton to the cold spectator reduces the band edge bleach merely by a half! We conclude that the source of incomplete bleaching by the second exciton are hitherto unrecognized random spin orientation conflicts between the two conduction electrons in the crystallites. The presence of this effect both in lead salts and in CdSe NCs demonstrates its generality. This new discovery imposes new restrictions on the utility of the BE exciton transition as a universal “exciton counter” in experiments with all kinds of semiconductor NCs
 

Using a combination of steady-state and ultrafast deep-ultraviolet (UV) we have identified the nature of the transitions at the optical gap of the much studied anatase TiO₂ nanoparticles. The first excitation is a strongly bound 2-dimensional exciton in the 3D lattice of the material.[1] We also find that coherent acoustic phonons confined in the nanoparticles selectively modulate the oscillator strength of the 2D exciton, and theory shows that the deformation potential is at the origin of the coherent phonon wavepackets.[2] Further studies of the charge electron and hole trapping in anatase TiO2[3] and in ZnO[4] using ultrafast X-ray spectroscopy will be presented.

Finally, time resolved X-ray studies of CsPbBr₃ perovskite NPs reveal the nature of the electron-hole recombination across the band gap, with the electrons being delocalized in the conduction band, while holes form small polarons in the valence band.[5]

[1] Strongly bound excitons in anatase TiO₂ single crystals and nanoparticles
E. Baldini, L. Chiodo, S. Moser, J. Levallois, E. Pomarico, G. Auböck, A. Magrez, L. Forro, M. Grioni, A. Rubio and M. Chergui. Nature Communications 8 (2017) 13

[2] Phonon-Driven Selective Modulation of Exciton Oscillator Strengths in Anatase TiO₂ Nanoparticles

E. Baldini, T. Palmieri, A. Dominguez, P. Ruello, A. Rubio and M. Chergui. Nanoletters (under review)

[3] Femtosecond X-ray absorption study of electron localization in photoexcited anatase TiO₂

F. G. Santomauro, A. Lübcke, J. Rittmann, E. Baldini, A. Ferrer, M. Silatani, P. Zimmermann, S. Grübel, J. A. Johnson, S. O. Mariager, P. Beaud, D. Grolimund, C. Borca, G. Ingold, S.L. Johnson, M. Chergui

Scientific Reports 5 (2015) 14834-1-6

[4] Revealing hole trapping in ZnO nanoparticles by time-resolved X-ray spectroscopy

Thomas J. Penfold, Jakub Szlachetko, Fabio G. Santomauro, Alexander Britz, Wojciech Gawelda, Gilles Doumy, Anne Marie March, Stephen H. Southworth, Jochen Rittmann, Rafael Abela, Majed Chergui and Christopher J. Milne. Nature Communications 9 (2018) 478

[5] Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy

Fabio G. Santomauro, Jakob Grilj, Lars Mewes, Georgian Nedelcu, Sergii Yakunin, Thomas Rossi, Gloria Capano, André Al Haddad, James Budarz, Dominik Kinschel, Dario S. Ferreira, Giacomo Rossi, Mario Gutierrez Tovar, Daniel Grolimund, Valerie Samson, Maarten Nachtegaal, Grigory Smolentsev, Maksym V. Kovalenko, and Majed Chergui. Structural Dynamics 4 (2017) 044002

This presentation will focus on the excited state behavior of semiconductor nanocrystals as light absorbers that, coupled with redox catalysts, drive light-driven multi-electron transfer reactions. Reactions of interest include hydrogen generation, carbon dioxide reduction, nitrogen fixation, and water oxidation. We have demonstrated that nanocrystal excited state behavior, charge transfer dynamics, and surface chemistry play a governing role in the overall photochemistry of nanocrystal-catalyst complexes. This presentation will describe our most recent findings about how the reactions of interest can be driven and controlled through manipulation of nanocrystal excited state dynamics. In particular, the presentation will focus on: (i) measurement (by transient absorption spectroscopy), kinetic modeling, and control of electron transfer kinetics for injection of photoexcited electrons into redox enzymes; and (ii) elucidation of the motion of photoexcited holes on nanocrystal surfaces, using a combination of transient absorption measurements, modeling, and theory, and the implications of this motion on oxidation photochemistry.

In this talk, I will discuss our group’s recent experimental and computational work on understanding electronic and phononic structure nanocrystal thin films and charge transport in these thin films. Using electrochemical-based approaches, we show that we can quantify the electronic density of states and also examine charge-transfer processes across interfaces. Using inelastic x-ray scattering, we quantify the phononic denisty of states. We combine density functional theory calculations and kinetic Monte Carlo simulations to develop a first-principles model for charge transport in nanocrystals solids. We show that these simulations explain temperature-dependent time-of-flight measurements of electron and hole mobility performed on lead sulfide nanocrystal thin films. The combination of experimental and computational work highlights the importance of electron-phonon interactions in nanoscale transport and enables us to determine the relative impact of energetic and positional disorder on transport, providing us with design guidelines on parameters to consider when optimizing nanocrystal synthesis, nanocrystal surface treatments, and nanocrystal thin film preparation for different device applications.

For the fabrication of an integrated solar-to-chemical system, different components should be interfaced together in an orchestrated manner. Photoelectrodes need to absorb in the visible range, with a valence and a conduction band suited for the target reaction. Moreover, the presence of catalysts is required to manage the intrinsic energetic hurdle. Herein, we address the study of the major challenges, namely performance, stability, and interfaces to enable fabrication of integrated solar-to chemical systems. Novel scientific directions for the synthesis of functional interfaces and the development of new tools for their characterization will be addressed. Specifically, we will present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. These findings are confirmed by in situ electrochemical atomic force microscopy (EC-AFM), which reveals that degradation under operating conditions occurs via dissolution of the film, starting at exposed facets of grains in polycrystalline thin films. In addition, we will present the correlation between morphological and functional heterogeneity in this material by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate.

Insights into corrosion mechanisms and nanoscale heterogeneity aid development of protection strategies and provide information on how local functionality affects the macroscopic performance. 

We use tunable nanoscale defect cavities to create zero-dimensional exciton-polaritons [1]. Data on the strong coupling as well as mode engineering is shown as a function of cavity detuning. At ambient conditions, we observe non-equilibrium exciton-polariton condensation [2] with strong lateral confinement on the wavelength scale [3]. Threshold and line narrowing are analyzed as a function of excitation density. Both, in real and momentum space we observe the distinct signature of strong transversal confinement in the condensation. First order coherence properties are investigated by means of a Michelson interferometer.

As a building block towards extended lattices, we realize two coupled cavities by focused ion beam milling and thermal scanning probe lithography. These photonic molecules are investigated by means of atomic force microscopy and optical measurements, to compare both fabrication methods. Furthermore, we investigate different ways to improve the optical properties of the active material e.g., photo-degradation and inhomogeneity.

These are the initial steps towards studying quantum fluids in extended, arbitrary potential landscapes at ambient conditions.

References

[1] D. Urbonas et al., ACS Photonics 3 (9), 1542–1545 (2016).

[2] J. D. Plumhof et al., Nat. Mater. 13, 247 (2014).

[3] F. Scafirimuto et al., ACS Photonics 5 (1), 85–89 (2018).

Spectroscopy of single colloidal quantum dots (QDs), especially at cryogenic temperature, helps to understand the inherent properties of nanocrystals that are often hidden in ensemble level studies. This applies in particular to InP-based QDs, which attract increasing interest as Cd-free alternative nanocrystals yet were hardly investigated at the single QD level. Here, we discuss the photoluminescence properties of single InP/ZnSe QDs, both at room temperature and at cryogenic temperature. While ensemble level measurements feature a luminescent linewidth of around 50 nm, we find that emission spectrum of single InP/ZnSe QDs can have a linewidth as narrow as 14 nm (50 meV). Hence, the relatively broad emission line that characterizes ensembles of InP-based QDs is by no means an intrinsic material property. In addition, we found that InP/ZnSe QDs combine a nearly blinking free emission with a high purity single photon emission (g²(0)<0.03), also well beyond the saturation intensity.[1] Cryogenic single QD spectra, on the other hand, consist of zero-phonon lines that can be as narrow as 40 µeV. Polarization resolved spectra point to a linearly polarized spectral doublet from the bright exciton. At lower excitation intensities, jitter was negligible and spectra could be integrated for tens of seconds without erasing the doublet splitting of 1.2 meV. At higher excitation intensities, jitter becomes more severe and switching between emission from the exciton-doublet and the trion-singlet can be observed. This indicates that spectral jitter finds its origin in changes of the local electric field caused by the temporal trapping of one charge carrier. In summary, we find conclude that single InP QDs have emission characteristics similar to the extensively studied CdSe-based QDs. Moreover, the narrow emission lines, limited jitter and fluorescence intermittency of single InP/ZnSe QDs holds great promise to further explore these materials as a solution-processable single-photon emitter and improve the ensemble level characteristics of these materials.

Reference:
[1] Chandrasekaran, V.; Tessier, M. D.; Dupont, D.; Geiregat, P.; Hens, Z.; Brainis, E. Nano Letters 2017, 17 (10), 6104–6109.

Colloidal semiconductor quantum dots (CQDs) have been at the forefront of scientific research for more than two decades, based on their size tunable properties. Although implementation of CQDs in opto-electronic devices already occurs, various fundamental issues with a direct impact on technology are left as open questions. Recent years showed an interest in the investigation of magneto-optical properties of various CQDs with substantial importance for opto-electronic and spin-based devices.

Here we include the study of two different CQD platforms: (1) Synthesis and magneto-optical characterization of spectrally stable pure and diluted magnetic semiconductor CQDs from the II-VI semiconductor family (e.g., CdTe/CdSe and Mn@CdTe/CdSe); (2) Magneto-optical characterization of perovskite CQDs of the type APbBr3 (A - methylamonium or Cs+). Both systems show intriguing spin properties of special scientific and technological interests. The uniqueness of the spin properties and their novelty will be the focus during the talk.

CdTe/CdSe colloidal quantum dots with special composition, including soft boundary (alloying) at the core/shell interface or a giant core or a shell, possess quasi type-II configuration and show blinking-free behavior. The Mn+2 doping induces internal spin interactions between photo-generated species and the dopant spins, leading to giant magnetization or to an internal energy transfer into the dopant orbitals, and consequence emission from host-dopant hybrid- or from dopant atomistic-states. The current study developed a method to position the Mn ions selectively either at the core or at the shell. The magneto-optical measurements, including the use of optically detected magnetic resonance, exhibited resonance transitions related to the coupling of the Mn electron and the nuclear spins with the individual photo-generated carriers. The work was done in collaboration with the laboratory of Prof. Volkan Hilmi Demir from Bilkent and University and NTU.

The perovskites are minerals that have been studied extensively in the past. They are the focus of new interest in recent years, due to their exceptional performance in photovoltaic cells. Perovskites semiconductors possess high absorption coefficients as well as long-range transport properties. Currently, they are also prepared in the form of CQDs with very interesting properties including ferroelectricity, magnetism and exciton effects. The magneto-optical measurements of excitons in CsPbBr3 as individuals were investigated by monitoring the micro-photoluminescence spectra in the presence of an external magnetic field, while monitoring either the circular or linear polarization components.

Fully inorganic cesium lead halide (CsPbX₃, where X = Cl, Br, I) perovskite-type nanocrystals are colloidal quantum dots with bright, narrowband emission that is tunable by composition and size over a wide spectral range with room-temperature photoluminescence quantum yield of up to 90%^[1]. A pecularity of this material is the almost blinking-free emission at low temperature^[2] that originates from a triplet state with exceptionally high oscillator strength^[3].

We use densely packed arrays of up to several millions of these nanocrystals, known as superlattices, produced by means of solvent-drying-induced spontaneous assembly^[4]. Such ensemble of emitters behaves dramatically different from its individual constituents due to coherent interaction enabled by the strong light-matter interaction and the excellent monodispersity of the quantum dots. The collective coupling gives rise to an intriguing many-body quantum phenomenon, resulting in short, intense bursts of light: so-called superfluorescence. We observe a comprehensive set of key signatures like dynamically red-shifted emission with more than twenty-fold accelerated radiative decay, extension of the first-order coherence time by more than a factor of four, photon bunching, and delayed emission pulses with Burnham–Chiao ringing behaviour at high excitation density.

This is the first demonstration of optical collective behaviour and extended coherent states with nanocrystals, opening up new opportunities for high-brightness and multi-photon quantum light sources.

References:
[1] Protesescu et al., Nano Lett. 15, 3692–3696 (2015)
[2] Rainò et al., ACS Nano 10, 2485–2490 (2016)
[3] Becker et al., Nature, 553, 187-193 (2018)
[4] Rainò et al., arXiv 1804.01873 (2018)

Coherent quantum dynamics of excitons in semiconductor quantum dots (QDs) are of key interest, besides fundamental physics, for many applications ranging from quantum computing to advanced photonic devices. With the advances in colloidal synthesis, high-quality semiconductor nanocrystals can be fabricated at lower cost, and more flexibility in size, shapes and compositions. Despite its importance, measuring the exciton dephasing time in colloidal nanocrystals is technically challenging. Using three-beam transient resonant four-wave mixing technique in heterodyne detection not affected by spectral diffusion, we have measured the temperature-dependent ground-state exciton dephasing dynamics in CdSe/ZnS wurtzite QDs, CdSe/CdS spherical zincblende and rod-shape wurtzite QDs with variable core diameter and shell thickness / rod length, CdSe nanoplatelets, PbS QDs, InP/ZnSe QDs, and perovskite (CsPbBr₂Cl) QDs.

In these structures, the importance of phonon-assisted transitions, and the zero-phonon line (ZPL) dephasing by phonon-mediated spin-relaxation and radiative decay at low temperature are vastly varying. In PbS QDs, the peculiar band structure allows coupling with phonons at the zone edge (X-point), resulting in dominating phonon assisted transitions [1] even at low temperatures with sub-picosecond dephasing and a ZPL weight of less than 7%. In CdSe QDs [2,3] and dots in rods of similar size, the ZPL weight instead is above 50% since only zone-center phonons can couple, and the ZPL dephasing is limited by spin-relaxation into dark states on a 10-1000ps time scale, faster than the radiative lifetime of about 10ns. In InP QDs, a non-toxic alternative to CdSe, a similar behaviour is observed. In CdSe nano-platelets, the quasi two-dimensional confinement leads to quantum-well type behaviour, with a large exciton coherence area, such that the exciton dephasing is dominated by a fast radiative decay in the 1ps range [4]. In perovskite CsPbBr₂Cl QDs, the exciton ground state is bright, and a radiatively limited dephasing in the 10-100ps range is observed at low temperatures.  

[1] F. Masia et. al., Phys. Rev. B 83, 201309(R) (2011) DOI:10.1103/PhysRevB.83.201309
[2] F. Masia et al. Phys. Rev. Lett. 108, 087401 (2012) DOI:10.1103/PhysRevLett.108.087401
[3] N. Accanto et al., ACS Nano 6, 5227-5233 (2012) DOI:10.1021/nn300992a
[4] A. Naeem et al., Phys. Rev. B 91, 121302(R) (2015)  DOI 10.1103/PhysRevB.91.121302

Recent progress in colloidal chemistry have led to the synthesis of new types of two-dimensional (2D) lattices of PbX (X=Se, S, Te) nanocrystals [1,2]. In these lattices, each nanocrystal is epitaxially connected to its neighbors, the 2D materials are single crystalline in absence of disorder. Remarkably, square and honeycomb lattices can be synthesized using the same initial nanocrystals, allowing to investigate the role of the lattice geometry on the electronic properties. In addition, cation exchange processes can be used to transform PbX lattices into CdX ones. Theoretical studies accompanying these experiments have already demonstrated that these 2D lattices are characterized by very interesting band structures, including in some cases Dirac and non-trivial flat bands. In the present talk, theoretical works on the optical properties of 2D lattices will be presented. The effects of the epitaxial bonds between neighbor nanocrystals on the optical properties will be discussed. The theoretical calculations will be compared to recent experimental studies [3]. The differences between IV-VI (PbX) and II-VI (CdX) materials, between square and honeycomb lattices, will be reviewed.

[1] W. H. Evers, B. Goris, S. Bals, M. Casavola, J. de Graaf, R. van Roij, M. Dijkstra, and D. Vanmaekelbergh, Nano Lett. 13, 2317 (2013).

[2] M. P. Boneschanscher, W. H. Evers, J. J. Geuchies, T. Altantzis, B. Goris, F. T. Rabouw, S. A. P. van Rossum, H. S. J. van der Zant, L. D. A. Siebbeles, G. Van Tendeloo, I. Swart, J. Hilhorst, A. V. Petukhov, S. Bals, and D. Vanmaekelbergh, Science 344, 1377-1380 (2014).

[3] M. Alimoradi Jazi et al., Nano Lett. 17, 9 5238-5243 (2017).

Ever since the exfoliation of graphite into an atomically thin monolayer, known today as graphene [1], two dimensional (2D) materials have been of central interest for a variety of electronic applications. 2D materials belong to a large family of anisotropically active compounds which have strong, covalent bonds within a layer while in between layers there are only weak van der Waals interactions that can be overcome, obtaining molecularly thin sheets. Such a reduction of dimensionality has a profound impact on properties, known to vary strongly with respect to the number of atomic layers.

As graphene applications in electronics has thus far been hindered by its non-existent band gap, layered semiconductors are studied as potential candidates for future devices. Many Transition Metal Dichalcogenides (TMDs) including MoS₂, MoSe_2, WS₂, WSe₂ have been thoroughly investigated, however in order to meet rising demands new families of 2D semiconductors are studied. One family of such is the transition metal thiophosphates, denoted MPS_x, for x=3 or 4; for example bulk crystals of CrPS₄ - chromium thiophosphate – which has been examined in the past for applications in lithium batteries. Nowadays, this compound has once again gained scientific interest due to its optical anisotropic properties and the possibility to obtain and study its few- and monolayer systems [2].

In this work, bulk crystals of CrPS₄ were obtained by vapor transport synthesis (furnace method), followed by structure and composition confirmation via different techniques, for example Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM/EDX), Powder X-Ray Diffraction (PXRD) and Raman spectroscopy. Optical properties, such as band gap and optical transitions were investigated by Solid State UV-VIS Spectroscopy, PhotoAcoustic Spectroscopy (PAS) [3] and Modulated Spectroscopy (MS) [3]. Later, bulk crystals of chromium thiophosphate (CrPS₄) were exfoliated in liquid to obtain few layers systems and photoconductivity measurements were used to ascertain photoactive properties, both of re-stacked films and bulk crystals.

Acknowledgments:
This work was supported by the European Comission via the Marie-Sklodowska Curie action Phonsi (H2020-MSCA-ITN-642656)
This work was performed within the grant of the National Science Centre Poland (OPUS 11 no. 2016/21/B/ST3/00482).
S.J.Z. also acknowledges the support within the ETIUDA 5 grant from National Science Center Poland (no. 2017/24/T/ST3/00257).

References:
[1] Novoselov et.al., Science, vol 306, no. 5696 (2004)
[2] Lee et.al., ACS Nano vol. 11, no. 11 (2017)
[3] Zelewski & Kudrawiec, Scientific Reports, vol. 7 no. 15365 (2017)

The emission of fully inorganic cesium lead halide (CsPbX₃, where X = I,Br,Cl) perovskite-type nanocrystals is tunable over a wide energy range with ultrahigh photoluminescence quantum yields of up to 90%^[1] and exhibits narrow emission lines. Due to their facile solution processability and their potential for high-efficiency photovoltaics and light sources they have gained enormous interest.

Experiments on single perovskite quantum dots reveal a unique energetic level structure with a lowest bright triplet state^[2], thus enabling photon emission rates ~20 and ~1000 times higher compared to any other conventional semiconductor nanocrystals at room and cryogenic temperatures, respectively. We investigate the nature of this exceptionally fast photon emission by temperature dependent quantum yield measurements. Furthermore we discriminate it from composition dependent “A-type” blinking behaviour in intensity-decay time correlation measurements and demonstrate stable, narrowband emission, with suppressed blinking and small spectral diffusion^[3] for single CsPbBr₂Cl nanocrystals. By means of polarization dependent high resolution spectroscopy, the complex nature of the exciton fine structure splitting and charged exciton emission has been characterized.

Based on these measurements, supported by effective-mass models and group theory calculations, we conclude that the triplet exciton state is responsible for the extraordinary photon emission properties of lead halide perovskites. Our results can assist to identify other semiconductors that exhibit bright triplet excitons, with potential implications for improved optoelectronic devices.

References:

[1] Protesescu et al., Nano Lett. 15, 3692–3696 (2015)

[2] Becker et al., Nature, 553, 187-193 (2018)

[3] Rainò et al., ACS Nano 10, 2485–2490 (2016)

Two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications due to their pronounced excitonic features leading to unique properties in terms of light emission. However, only a few studies focus on the use of these materials for light amplification or net optical gain development and the ensuing high carrier density photo-physics. The beneficial nature of the strong excitonic effects on optical gain remain hence unquantified and , despite the large binding energies, it remains unclear what the involvement of is at the concomitant high carrier densities. Here, we use colloidal 2D CdSe nanoplatelets as a model system and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several distinct and carrier density-dependent optical gain regimes exist for these materials. At low density, optical gain is found to originate from excitonic molecules delivering large material gains up to 20.000 cm^-1, yet with an Auger limited lifetime of few hundred picoseconds. At increasing pair density, we observe a surprising transition to a combined regime of blue-shifted and disruptively large optical gain, combined with the typical exciton mediated gain. We show that this peculiar situation originates from a carrier cooling bottleneck at high density. Surprisingly, the insulating (multi-)exciton gas is found to co-exist with the conductive phase in a density regime nearly one order of magnitude beyond the expected Mott transition.  The ensuing exciton ground state absorption even counter-acts the development of net optical gain in certain spectral regions. Our results shed a new light on the disruptive photo-physics of high binding energy excitons in strongly excited 2D materials and pave the way for the development of more efficient broadband optical gain media and/or high density excitonic devices such as polariton lasers.

Colloidal lead halide perovskite nanocrystals (APbX_3, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) emerge as promising materials for optoelectronic applications such as in television displays, light-emitting devices, and solar cells. The sponaneous and stimulated emission spectra of these NCs are readily tunable over the entire visible spectral region of 410-700 nm [1-2]. The photoluminescence of these NCs is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, and high quantum yields of up to 100%. Cs_1-xFA_xPbI₃ and FAPbI₃ reach the near-infrared wavelengths of 800 nm [3].  A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. A promising approach lies in the formation of multinary compositions such as Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs. We show that droplet-based microfluidics can successfully guide the synthesis of such complex compositions [4]. We could fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission linewidths (to below 40nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. Most importantly, we demonstrate the excellent transference of reaction parameters from microfluidics to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices. As an example, Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting high external quantum efficiency of 5.9% and very narrow electroluminescence spectral bandwidth of 27 nm.

The processing and optoelectronic applications of perovskite NCs are, however, hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, we have developed a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules, resulting in much improved chemical durability [5].

             Perovskite NCs also readily form long-range ordered asssemblies known as superlattices. These assemblies exhibit accelerated coherent emission (superfluorescence), not observed before in semiconductor nanocrystal superlattices [6].

L. Protesescu et al. Nano Letters 2015, 15, 3692–3696

M. V. Kovalenko et al. Science 2017, 358, 745-750

L. Protesescu et al. ACS Nano 2017, 11, 3119–3134

I. Lignos et al. ACS Nano 2018, DOI: 10.1021/acsnano.8b01122

F. Krieg et al. ACS Energy Letters 2018, 3, 641–646.

Raino, M. Becker, M. Bodnarchuk et al. 2018, submitted

Controlling the flow of light at length scales below the diffraction limit is the quest of nanophotonics. The experimental mapping of electromagnetic modes could provide fundamental insights into nanophotonic systems and facilitate their rational design.

We propose NaYF₄ nanocrystals doped with Eu³⁺ ions as nanoscopic probes for electromagnetic modes at optical frequencies. Eu³⁺ ions feature several electronic transitions throughout the visible spectrum with electric- or magnetic-dipole character. This enables the nanoprobes to sense both the electric and magnetic components of optical modes at the probes’ location. We verify our concept by mapping the photonic modes close to a metallic mirror. Further, we adapt the method to study surface plasmon polaritons (SPPs), electromagnetic modes confined to metal–dielectric interfaces. By placing the nanoprobes locally at varying distances from a plasmonic reflector, we study plasmonic modes with a resolution well beyond the diffraction limit of light. Our results highlight how a well-designed plasmonic environment can be utilized to control the emission directionality of SPP sources and to selectively enhance electric-dipole-forbidden optical transitions of quantum emitters.

The formation of bound excitons via the spatial localization of charge carriers has long been a goal for luminescent semiconductors. Often, this has been accomplished through the formation of nanocrystals either through top-down or bottom-up methods. Alternatively, zero dimensional (0D) materials structurally impose carrier localization and result in the formation of highly localized Frenkel excitons. Recent works on perovskite-derived, hybrid organic-inorganic, 0D Sn(II) materials have demonstrated that high quantum yield emission from self-trapped excitons is possible when octahedra are isolated. As a new entry to the family of luminescent 0D materials, the fully-inorganic, perovskite-derived Cs₄SnBr₆ exhibits broad-band photoluminescence centred at 540 nm with a quantum yield of 15±5% at room temperature.[1] A compositional series, following the general formula Cs_4-xA_xSn(Br_1-yI_y)₆ (A = Rb, K; x ≤ 1,y ≤ 1), can be synthesized by solid-state methods. Furthermore, the emission of these materials ranges from 500 nm – 620 nm with the possibility to compositionally tune the Stokes shift and the self-trapped exciton emission bands. Finally, utilizing density functional theory calculations, the self-trapped exciton was ascribed to pseudo-Jahn-Teller distorted octahedra.

[1] Benin, B.M.,*; Dirin, D.N.*; Morad, V.; Woerle, M.; Yakunin, S.; Raino, G.; Nazarenko, O.; Fischer, M.; Infante, I.; Kovalenko, M.V. submitted.

The excellent optoelectronic properties of perovskite nanocrystals (NCs) such as enhanced photoluminescence quantum yield (PLQY) and tunable emission wavelength has stimulated a widespread investigation of this class of semiconductors.^[1] Very recently, it has been demonstrated that CsPbBr₃ NCs can reach near-unity PLQY in solution. Yet, retaining the PLQY in film is not trivial; since the NCs are not as well passivated as in solution and close packing can lead to energy-transfer to trap-states and increased self-absorption.

Here, a room temperature synthesis of perovskite NCs displaying near-unity PLQY in solid state films is presented.^[2]  Spin-coated films of the obtained CsPbBr₃ NCs show PLQY values approaching unity (>95%), thanks to the combination of a novel synthesis at room temperature, and a post synthetic treatment. The as-obtained NCs show PLQY = 80% in spin-coated films. Further enhancement of the PL efficiency is obtained via addition of PbBr₂. Following the synthesis, the obtained NCs were employed in optoelectronic devices. Efficient solar cells based on mixed-halide (CsPbBrI₂) NCs obtained via anion exchange reactions under ambient conditions were fabricated.^[3]. Solar cell devices operating in the wavelength range 350−660 nm were fabricated in air with two different deposition methods: single step (SP) and layer-by-layer (LbL). The solar cells display a photoconversion efficiency of 5.3%, independently of the active-layer fabrication method, and open circuit voltage (V_oc) up to 1.31 V, among the highest reported for perovskite-based solar cells with bandgap below 2 eV, clearly demonstrating the potential of this material.

The high potential of the material is further tested in light-emitting diodes (LEDs) employing an inverted structure comprising of ZnO nanoparticles as an electron-transport layer and a conjugated polymer hole-transport layer. The LEDs demonstrate an external-quantum-efficiency of 6.04%, with luminance of 12998 Cd/m² and low efficiency droop (around 10%). Importantly, such high efficiency was achieved by substituting Cesium with Formamidinium in line with our synthetic procedure. These results show the versatility of our synthetic protocol while the material quality is pointed out by the high performance of the optoelectronic devices. 

References

[1]       L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H. Hendon, R. X. Yang, A. Walsh, M. V Kovalenko, Nano Lett. 2015, 15, 3692.

[2]       F. Di Stasio, S. Christodoulou, N. Huo, G. Konstantatos, Chem. Mater. 2017, 29, 7663.

[3]       S. Christodoulou, F. Di Stasio, S. Pradhan, A. Stavrinadis, G. Konstantatos, J. Phys. Chem. C 2018, 122, 7621-7626.

Semiconductor colloidal quantum dots (QDs) offer a realm of opportunities especially, in terms of tuning the band gap, manipulating the trap states in a very precise manner, controlling their electronic doping character and their ease of manufacturing and processing in devices. Moreover, when combined with 2D materials, the resultant structures can be used for a diverse range of applications offering photodetectors with exceptionally high responsivity and sensitivity.¹ During my presentation, I will talk about quantum dot-graphene hybrid photodetector devices for food inspection, spectrometers, imaging purposes, monitoring vital health parameters and night vision leveraging the unique opportunity of detecting photons from the UV up to the short-wave infrared in a single material platform.  In this photodetector, we make use of the large absorption cross section of QDs and high carrier mobility of graphene to demonstrate our complementary metal-oxide–semiconductors (CMOS) compatible infrared image sensors.² Unlike commercial detectors, the hybrid detector can be used simultaneously in UV, visible and infrared light conditions at room temperature with measured detectivity upto 10¹² Jones. Such detectors exhibit response times of 0.1–1 ms that make them suitable for video frame rate as Stijn Goossenswell as spectrometry applications.

[1] G. Konstantatos, et al., Nature Nanotechnol., 7 (June 2012)

[2] Goossens et al., Nat. Phot. 11 (June 2017)

Optical switches are key components for data processing on the basis of “silicon photonics”, in which they perform the crucial conversion of a photonic information from an optical fiber into an electric information for a silicon-based processing unit. The status of the switch is controlled by an external light source, emitting at a wavelength suitable to be absorbed by the conductive channel to photo-induce additional charge carriers and modulate the current output of the switch in close analogy to a classic transistor. This presentation details how hybrid superlattices of semiconducting nanocrystals and organic pi-systems with long-range order are applied as active layers in functional optical switches. The particular novelty for optical switching is an activated absorption mechanism, in which stimulation with one optical signal sensitizes the material towards an amplified recognition of a second optical stimulus. Several examples with different material combinations are presented and the importance of exciton formation as well as charge transfer across the inorganic-organic interface is discussed.

The rapid growth of colloidal quantum dot (CQD) opto-electronics establishes it as one of the most promising new generation technology related to photo-detection, photovoltaic (PV) and light emitting diodes (LEDs) [1]. Although CQD based LEDs enjoy tremendous success in the visible range [2], not so can be claimed about their infrared counterpart, which has a number of applications, including night vision, remote sensing, spectroscopy and biological imaging. The main reason for the poor performance in the infrared is the low photoluminescence quantum yield (PLQY) of ligand exchanged CQD solids. Several techniques like synthesis of core-shell structures [3], chemical passivation with perovskite matrix [4] etc. improve the efficiency but still they are far from their potential. We report here a passivation technique based on suprananocrystalline matrix engineering which leads to record external quantum efficiency (EQE).

Mixed ligand treatment on short-wave infrared (SWIR) based PbS quantum dot solar cell leads to record high PV performance [5]. Yet, the ligand treated SWIR QD solids showed a mere 2% PLQY due to a large amount of non-radiative recombination. The matrix presented here, passivates the non-radiative recombination channels and improve the PLQY over 60%. Careful optimization of the matrix and device architecture leads to record peak EQE of ~7.9% and peak power conversion efficiency of ~9.3% with emission at 1400 nm. Furthermore, PV devices based on this passivation technique showed record high open circuit voltage (0.69 V corresponding to 0.92 eV QD bandgap) confirming the effectiveness of the passivation.

References:

[1] Kagan, C. R., Lifshitz, E., Sargent, E. H., & Talapin, D. V. Buliding devices from colloidal quantum dots. Science 353, aac5523 (2016).

[2] Shirasaki, Y., Supran, G. J., Bawendi, M. G. & Bulović, V. Emergence of colloidal quantum-dot light-emitting technologies. Nat. Photonics 7, 13–23 (2013).

[3] Supran, G. J. et al. High-performance shortwave-infrared light-emitting devices using core–shell (PbS–CdS) colloidal quantum dots. Adv. Mater. 27, 1437–1442 (2015).

[4] Gong, X. et al. Highly efficient quantum dot near-infrared light-emitting diodes. Nat. Photonics 10, 253–257 (2016).

[5] Bi, Y. et al. Infrared solution‐processed quantum dot solar cells reaching external quantum efficiency of 80% at 1.35 µm and J_SC in excess of 34 mA cm^-2. Adv. Mater. 30, 1704928 (2018).

Pushing nanocrystals optical features toward the infrared range can be a material challenge. Two strategies can be explored. Either the use of narrow band gap materials for the design of narrow interband transitions, either the use of doped semiconductor presenting intraband transition in the mid infrared [1]. Here, I will focus on this second strategy while using mercury chalcogenides compounds as optically active material.

At first, I will present our strategy to explore the synthesis of HgSe [2] and HgTe [3] self-doped nanocrystals which has been used to tune absorption up to the THz range, typically from 3 µm to 60 µm for peak absorption and up to 200µm for cut-off wavelength.

Because the doping plays a key role in this material, I will discuss the origin of self-doping and how surface chemistry can be used to tune accurately its magnitude [4].

In the last part of the talk, I will present some results relative to the use of this intraband transition for mid IR photodetection. I will show how an heterostructure made of HgSe and HgTe can be used to uncouple the problem of absorption and charge transport [5]. This paves the way for the design of even more complex colloidal heterostructure on the model of quantum cascade system develloped for III-V material.

[1] Emergence of intraband transitions in colloidal nanocrystals, A. Jagtap, C. Livache, B. Martinez, J. Qu, Audrey Chu, C. Gréboval, N. Goubet, E. Lhuillier, Opt. Mater. Express 8(5), 1174-1183 (2018)

[2] Infrared photo-detection based on colloidal quantum-dot films with high mobility and optical absorption up to the THz, E. Lhuillier, M. Scarafagio, P. Hease, B. Nadal, H. Aubin, X. Z. Xu, N. Lequeux, G. Patriache, S. Ithurria, B. Dubertret, Nano Lett 16, 1282 (2016)

 [3] Terahertz HgTe nanocrystals: beyond confinement, N. Goubet, A. Jagtap, C. Livache, B. Martinez, H. Portales, X. Zhen Xu, R.P.S.M. Lobo, B. Dubertret, E. Lhuillier, J. Am. Chem. Soc. 140, 5053 (2018).

[4] Surface Control of Doping in self-doped Nanocrystals, A. Robin, C. Livache, S. Ithurria, E. Lacaze, B. Dubertret, E. Lhuillier, ACS Appl. Mat. Interface 8, 27122−27128 (2016).

 [5] Wavefunction engineering in HgSe/HgTe colloidal heterostructures to enhance mid infrared photoconductive properties, N. Goubet, C. Livache, B. Martinez, X. Z. Xu, S. Ithurria, S. Royer, H. Cruguel, G. Patriarche, A. Ouerghi, M. Silly, B. Dubertret, E. Lhuillier, Nano Lett 18 (2018)

State-of-the-art imagers use silicon circuitry for pixel readout, in combination with either a silicon absorber (visible region) or flip-chip bonded III-V materials (infrared region). Thin-film layers show a promise to replace these absorber layers, as the optical cross-talk can be reduced, and a heterogeneous integration on silicon chips enables further downscaling of the pixel size. In this talk we will discuss our progress and the challenges to incorporate colloidal quantum dot materials into a fab compatible process flow. Challenges lay in translating the chemical vocabulary and incorporating the silicon production fab restrictions into the device optimization. We will show initial results of incorporating infrared PbS based materials (950 nm and 1450 nm quantum peak absorption) into a silicon fab compatible stack. From a device aspect, the focus lays on high EQE values in combination with low noise (= dark current limited). From an integration aspect, the available contact materials are limited, and all layers need patterning using photo-lithography to enable processing on 200 or 300 mm wafers. These two aspects (device and integration) should be looked at jointly. A screening and optimization method will be presented, including a full opto-electronic characterization to determine the optimal stack constitution. This method enables a quick uptake of next generation quantum dot (or other thin-film) materials into wafer scale imager production.

Our constantly increasing society’s need for energy has triggered a pressing need for the development of new materials for renewable sources. Concerning materials for energy harvesting, the most promising approaches for high-performance and low-cost photovoltaics rely in inhomogeneous compounds, such as perovskites and polycrystalline thin films (e.g. CIGS and CdTe). Thus, resolving their electrical and optical behavior at the nanoscale is imperative to advance their understanding. In this talk, I will share our scientific findings to image and quantify the local voltage response of nano- and mesoscale inhomogeneities in perovskites [1,2], CIGS [3], and CdTe through a variant of KPFM and NSOM [4-6]. By submitting the samples to illumination and humidity treatments under controlled conditions, we map the dynamic physical behavior of MAPI and triple-cation perovskites.

References:

[1] J. M. Howard et al. J. Phys Chem Letters, in press (2018)

[2] J. L. Garrett et al. Nano Letters 17, 2554 (2017).

[3] E. M. Tennyson et al. ACS Energy Letters 1, 899 (2016).

[4] E. M. Tennyson et al. ACS Energy Letters, 2, 2761 (2017). Invited Review

[5] E. M. Tennyson et al. ACS Energy Letters 2, 1825 (2017). Invited Perspective

[6] E. M. Tennyson et al. Advanced Energy Materials 5, 1501142 (2015).

Electrochemical CO2 conversion can be coupled with a photovoltaic cell and provide a pathway to utilize solar energy for the chemical synthesis. Ideally, such artificial photosynthesis system want to use CO2 and H2O as feed-stock molecules to produce value-added chemicals such as fuels or raw chemicals. My research team reported a monolithic and stand-alone device composed of a photovoltaic cell module, an Au CO2 reduction, a cobalt oxide anode accomplishing over 4 % conversion efficiency for CO2 conversion to CO production. To improve the solar to chemical conversion efficiency and to increase the feasibility further, we have developed efficient electrocatalysts and replaced the photovoltaic cell with Si modules, achieving ~ 8% of solar-to-CO conversion efficiency.

In addition, in this talk, metal-based electrocatalysts interacting with p-block elements or surface mediated molecules will be discussed for selective CO or C2+ (i.e. ethylene) production from CO2 reduction. The experimental results and theoretical simulation with various different types of metal catalysts (Ag, Zn, and Cu) give insights how to suppress the hydrogen evolution reaction (HER) is crucial to achieve efficient CO2 reduction catalysts. Monodispersed Ag nanoparticles are suggested to have the special interaction between the surface Ag and the surface mediated molecules which can modify the local electronic structure favoring for the selective CO production (up to 95 % of Faradaic efficiency). In addition, in the case of selective ethylene production, special Cu nanostructure formed by in-situ electrochemical fragmentation is demonstrated to be effective for increasing C-C bond coupling (up to 73 % of Faradaic efficiency) and selective ethylene production (up to ~ 60 % of Faradaic efficiency). In-situ X-ray absorption spectroscopy (XAS) studies are performed to understand the catalyst activity. Our series of studies suggests the modification of the metal nanoparticle surface by oxygen atom or surface mediated molecules can be effective strategies to increase CO2 reduction reaction activity and stability.

Chemically synthesized quantum dots (QDs) can potentially enable new classes of highly flexible, spectrally tunable lasers processible from solutions [1,2]. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage and an important challenge - realization of lasing with electrical injection - is still unresolved. A major complication, which hinders the progress in this field, is fast nonradiative Auger recombination of gain-active multicarrier species such as trions (charged excitons) and biexcitons [3,4]. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination by “softening” the electron and hole confinement potentials [5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [6], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a “zero-threshold-optical-gain” concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [7]. Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting recent developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices operating across a wide range of wavelengths and fabricated on virtually any substrate using a variety of optical-cavity designs.

[1] Klimov, V. I.et al., Optical gain and stimulated emission in nanocrystal quantum dots. Science290, 314 (2000).

[2] Klimov, V. I.et al., Single-exciton optical gain in semiconductor nanocrystals. Nature447, 441 (2007).

[3] Klimov, V. I. et al., Quantization of multiparticle Auger rates in semiconductor quantum dots. Science287, 1011 (2000).

[4] Robel, I., et al., Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett.102, 177404 (2009).

[5] Y.-S. Park, et al., Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).

[6] Lim, J., et al., Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater.17, 42 (2018).

[7] Wu, K., et al., Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol.12, 1140 (2017).

Colloidal semiconductor nanocrystals have gained interest since their optical and electronic properties can be tuned by varying their shape, size and composition. Recently, 2D square and honeycomb superlattice of lead- and cadmium-chalcogenide quantum dots (QDs) have been prepared. These superstructures are formed by assembling PbSe nanocrystals in a monolayer at the toluene suspension air/interface after which the nanocrystals attach via their four vertical {100} facets [1],[2]. Afterward, cation exchange transforms PbSe into zinc blend CdSe. Theoretical studies show that these 2-D systems have distinct band structures compared to continuous nanosheets, with the appearance of Dirac cones in the case of the honeycomb [3]. Strong electronic coupling via the atomic connections of the QDs in the superstructure may result in a higher mobility compared to the self-assembled lead chalcogenide QDs that are less strongly coupled due to the (in) organic ligands [4].  

In our research, we use electrolyte-gated transistors to study the optoelectronic properties and transport characteristics of 2-D PbSe and CdSe superstructures [5]. The potential of the gate electrode determines the Fermi level with respect to the conduction band (CB) or valence band (VB) of the superstructure. First, to monitor the stability of the superlattice under electron injection we measure the differential capacitance as a function of gate voltage. Second, the conductivity of the network is measured as a function of the Fermi level position. To quantify band occupation into the superlattice, the optical absorption quenching employed. Finally, the mobility of the system is calculated from conductivity and charge density.

We reported the first study of electron transport in a 2-D PbSe system with a square geometry in which band occupation is assured by the electron density of 8 electrons per nanocrystal . The electron mobility between 5 and 18 cm²/Vs is observed for these supersructures [6].­­

In our recent work, we study the electron transport of CdSe superlattices with square and honeycomb geometry. The band occupation is assured by the number of 2 electrons per nanocrystal. The electron mobility of 1 and 10 cm²/Vs is achieved for square and honeycomb geometry respectively.   

1) W.H. Evers et al., Nano Lett., 13, (2013).

2) M.P. Boneschanscher et al., Science, (2014).

3) E. Kalesaki et al., Phys. Rev. B 88, (2013).

4) W.H. Evers et al., Nature Communications 6, (2015).

5) D. Vanmaekelbergh et al., Electrochemica Acta, 53, (2007).

6) M. Alimoradi Jazi et al., Nano Lett., 17, (2017)

Nanocrystal building blocks can be assembled to make an artificial, nanocrystal solid. The choice of building block and the way they are assembled set up pathways to make new and unique materials with tailored properties. A case in point are superlattices of semiconductor nanocrystals or quantum dots (QDs), which find applications in, e.g., photodetectors, solar cells and field-effect transistors. Quantum dots offer the appealing combination of a tunable band gap, a high absorption coefficients, and a suitability for solution-based processing. QD films are typically produced through, e.g., spincoating, dropcasting or spraycoating. This results in disordered nanocrystal stacks, where poor electronic transport can be caused by excessive surface defects or restricted dot-to-dot hopping. To disentangle such effects, we analyzed the delocalization and transport of charge carriers in 2D superlattices of epitaxially connected QDs. In the case of PbS and PbSe QDs, such superlattices can be formed over several square micrometer. Using elemental analysis and structural analysis by in-situ XRF and GISAXS, respectively, we show that such lattices keep their structural integrity in a wide temperature window, ranging up to 310 ºC and more; an ideal starting point to assess the effect of gentle thermal annealing on the superlattice properties. We find that annealing such superlattices at temperatures ranging from 75-150 ºC induces a marked redshift of the QD band-edge transition. In fact, the band-edge found after annealing agrees, opposite from state-of-the-art literature, with theoretical predictions on charge carrier delocalization in such epitaxially connected superlattices. In addition, we observe a 1000-fold increases of the charge carrier mobility after mild annealing. While the superstructure remains intact at these temperatures, an XRD rocking curve analysis indicates that annealing markedly decreases the density of grain boundaries. This indicates that the presumably epitaxial connections between QDs in as-synthesized superlattices still form a major source of grain boundaries and defects, to an extend that carrier delocalization over multiple QDs is prevented and dot-to-dot transport remains strongly restricted.

The dynamics of photoluminescence (PL) from nanocrystal quantum dots (QDs) is significantly affected by reversible trapping of photo-excited charge carriers. This process occurs after up to 50% of the absorption events, depending on the type of QD considered, and can extend the time between photo-excitation and relaxation of the QD by orders of magnitude. Although many opto-electronic applications require QDs assembled into a QD solid, until now reversible trapping has been studied only in (ensembles of) spatially separated QDs. Here, we study the influence of reversible trapping on the excited-state dynamics of CdSe/CdS core/shell QDs when they are assembled into close-packed “supraparticles”. Time- and spectrally resolved PL measurements reveal competition between spontaneous emission, reversible charge carrier trapping, and Förster resonance energy transfer between the QDs. While Förster transfer causes the PL to redshift over the first 20–50 ns after excitation, reversible trapping stops and even reverses this trend at later times. We can model this behavior with a simple kinetic Monte Carlo simulation by considering that charge carrier trapping leaves the QDs in a state with zero oscillator strength from which no energy transfer can occur. Our results highlight that reversible trapping significantly affects the energy and charge carrier dynamics for applications where QDs are assembled into a QD solid.

Hot carriers refer to electrons (holes) that are formed from the thermalization of non-equilibrium photoexcited carrier populations following the absorption of above-bandgap photons. These hot carriers (HCs) later equilibrate within few picoseconds with the semiconductor lattice through carrier cooling processes such as carrier phonon scattering, Auger process, etc. The mechanisms and dynamics of HC cooling in semiconductors are of fundamental importance for enhancing device functionalities. Colloidal lead halide perovskite nanocrystals recently emerged as promising candidate materials for many optoelectronic applications. Building efficient and long-lasting devices from perovskite nanocrystals however remains a challenge. In this work, we study the size and temperature dependent cooling process of HC in CsPbBr₃ nanocrystals using time-domain density functional theory. We provide detailed insights into the mechanism of cooling by analyzing the effect of the passivating ligands (alkyl ammonium) in this process. We demonstrate that the inorganic lattice plays a much larger role in mediating the thermalization to the band edge.

Colloidal semiconductor nanocrystals, specifically quantum dots (QDs), are of interest to numerous scientific disciplines due to their highly tunable optical and electronic properties. For many years, various chemical treatments have been developed to fabricate conductive QD arrays for use in solar cells.  While such investigations have lead to increased solar cell performance there is much about how the chemical treatments modify the optical and electrical properties that are not well understood.  We have developed a simple, robust, and scalable solution-phase X-type ligand exchange method for PbS QDs that replaces native surface ligands with functionalized cinnamate ligands, yielding highly tunable, well-defined organic/inorganic hybrid chemical systems. We explore a library of functionalized cinnamic acid molecules to systematically tune PbS QD surface chemistry, and find that thin films of fully ligand exchanged QDs exhibit remarkable band edge shifts: the band edge position of QDs can be tuned over 2.0 eV.  We have also developed simple methods to impurity dope PbS and PbSe QDs with both n and p-type dopants.  We show how the dopants are incorporated and result in doped QDs with well behaved properties.   

Nanocrystals can be used to achieve transition in the mid and far infrared. This is in particular promising for the design of low cost infrared photodetectors. However more need to be understood on the dynamic of carriers in narrow band gap nanocrystals. This question is nevertheless quite challenging since many conventional experimental methods such as time resolved photoluminescence cannot be used because of the low PL efficiency in the infrared nanocrystals and because of difficulties to build infrared optical setup. We have investigated two complementary ways to probe carrier dynamics in narrow band gap colloidal materials which are time resolved photoemission and transient photocurrent. To illustrate these experiments, two systems have been investigated. First I will show that pump probe experiment can be conducted while exciting 2D HgTe nanoplatelets in the near IR and while looking at relaxation with a photoemission probe. This method is very efficient to determine without contact majority and minority carrier lifetime.In the second part of the talk, I will discuss how it is possible to determine band structure parameter such as the Urbach energy from transient photocurrent measurements conducted on HgTe nanocrystals. To do so, we have built a broad bandwidth setup (from ns to ms) and bring evidence for the multi-trapping transport regime.These two methods are extremely complementary to optical time resolved spectroscopy often limited to short dynamics (ns and less)

Nonresonant excitation of colloidal quantum dots (QDs) creates hot carriers that subsequently cool down to the band edges or are trapped in localized states. Carrier cooling and trapping typically happens on timescales from femtoseconds to picoseconds, orders of magnitude faster than the nanosecond to microsecond timescales of radiative recombination. Understanding cooling and trapping is relevant for (hot) carrier extraction in photovoltaics and to increase the luminesence output of QDs used as phosphor.

We investigate carrier cooling and trapping in InP QDs with a ZnSe_1-xS_x shell. Undoped QDs are compared to Cu⁺-doped QDs, where the Cu⁺ ion serves as a designed hole trap. Using pump probe transient absorption spectroscopy with femtosecond time resolution, we are able to monitor the population of electrons in the conduction band.

Our comparative study shows that hot electron cooling is almost an order of magnitude slower in the Cu⁺-doped QDs than in the undoped QDs. We ascribe this to rapid hole trapping on the Cu⁺ ion. This confirms the model in which hot electron cooling goes via an Auger-like process, where the hot electron transfers its excess energy to the hole which subsequently relaxes by phonon coupling. In our Cu⁺-doped QDs the hole is trapped on a Cu⁺ ion on sub-picosecond timescales, so the Auger cooling pathway is unavailable to the hot electron. Instead it must cool down via another, slower, pathway, most likely by coupling to high-energy vibrations at the surface of the QDs. This must also mean that hole trapping on the Cu⁺ ion is faster than the Auger cooling timescale of the undoped QDs, which is of the order of 300 fs. Our results provide insight in the behaviour of hot electrons and holes in the short time period after excitation of both Cu⁺-doped and undoped InP QDs.

The operational lifetime and scale-up of quantum dot sensitized solar cells (QDSCs) are largely limited by the hole transport layer, which is usually an aqueous polysulfide solution. The liquid, and alkaline nature of this electrolyte makes it difficult to develop laminated sealed devices. Further, the polysulfide electrolyte composition alters with use (there are observable color and viscosity variations!) thus impacting the cell performance. Besides the photoanode, the counter electrode (CE) also plays a key role in controlling cell response. While a variety of CEs have been attempted in the past: metallic coatings, carbon nanomaterials and conducting polymers, poly(3,4-ethylenedioxypyrrole) or PEDOP has rarely been used in QDSCs.

In view of the above described issues, in this report, QDSCs with a photoanode comprising of N-doped graphene quantum dots (N-GQDs) and cadmium sulfide (CdS)/titania (TiO₂) based solar cells with electropolymerized PEDOP@carbon cloth as CE and a polysulfide/SiO₂ gel with an electrolyte additive, namely, sodium poly(styrene sulfonate) or NaPSS were developed. The proportion of NaPSS is optimized on the basis of cell performance. A significant improvement in QDSC performance is obtained by incorporating NaPSS in the gel. By using impedance spectroscopy, the role of NaPSS in improving the cell performance is determined. NaPSS increases the recombination resistance for back electron transfer at the photoanode/electrolyte interface, thus increasing the power conversion efficiency to nearly 7% from 6.1% (when no NaPSS is present). NaPSS also imparts an enhanced operational life to the QDSC. Apart from NaPSS, the effectivity of PEDOP as a CE for QDSCs is studied by comparing impedance parameters, electrocatalytic activities and electrical conductivities of PEDOP films with different dopants. Ionically conducting and electrically conducting dopants were attempted. These studies provide a deeper understanding of factors that limit QDSC performances, and help in overcoming them.

Control over the energy alignment of Quantum Dots (QD) heterostructures can be used to unlock new functionalities for QD based optoelectronic devices: from improved carrier separation in type-II heterostructure, to control over hot-carrier transfer in type I heterostructures and lowered Carrier Multiplication threshold in quasi-type-II heterostructures.

We investigated the carrier dynamics in QD heterojunction films composed of PbSe and CdSe QDs. We demonstrate that such films tend to form a type I band alignment in which fast and efficient hot electron transfer from PbSe QDs to CdSe QDs is observed by transient absorption (TA) measurements. ­ The efficiency of the hot electron transfer process increases with excitation energy as a result of the more favorable competition between hot-electron transfer and electron cooling. The experimental picture is supported by time-domain density functional theory calculations, showing that electron density is transferred from lead selenide to cadmium selenide quantum dots on the sub-picosecond timescale. Hot-electron solar cells have been proposed as a route towards higher efficiency solar cells, and our observation reveals the possibility to achieve and control hot-electron transfer via energy-structure engineering in QD heterojunctions.

We next attempted to switch the energy alignment between the PbSe QDs and CdSe QDs, using the size-dependence of their energy structure as well as using tailored ligands to shift the energy levels through their surface dipoles. Spectroelectrochemical measurements reveal that we can shift the type I alignment to a type II alignment and TA measurements demonstrate a much-improved efficiency of “cold” electron transfer.

We thus proved that a combination of size-variation and control over surface-passivation allows to span the range between type-I and type-II alignment. One particularly interesting configuration is that of quasi-type-II alignment, where the conduction electron levels are resonant, as this could potentially be used for optimal Carrier Multiplication.

Metal-organic frameworks (MOFs) are coordination polymers consisting of metal ions connected by organic ligands. Besides the traditional applications in gas storage and separation as well as catalysis, the long-range crystalline order in MOFs and the tunable coupling between their organic and inorganic constituents have recently led to the design and synthesis of (semi-)conducting MOFs, opening the path for their application in opto-electronics. Yet, despite being a critical aspect for the development of MOF based electronics, the true nature of charge transport in MOFs, i.e. whether hopping or band-like transport occurs, has remained unresolved to date.

Here we report a time-resolved high-frequency (terahertz) conductivity study of a newly developed Fe₃(THT)₂ (THT=2,3,6,7,10,11-hexathioltriphenylene) two-dimensional MOF.  The novel π-d conjugated samples, synthesized through interfacial method at room temperature, are obtained as a large-area, free-standing film with tunable geometry (size and thickness). The Fe₃(THT)₂ films are conductive (~1 S/cm), porous (specific surface area of 526±5 m²/g) and semiconducting (with a ~250 meV direct bandgap). We demonstrate for the first time band-like charge transport in MOFs. This finding is directly apparent from the Drude-type high-frequency (terahertz) photo-conductivity response obtained in the samples; revealing free-moving, delocalized charge carriers displaying ~200 cm²/Vs mobilities at room temperature; a record mobility for MOFs. The temperature dependence of the mobility reveals that the main scattering mechanism limiting the mobility and hence band-like charge transport in this material is related to impurity scattering, so that material improvements may further increase the mobility.

The demonstration of band-like charge transport in MOFs reveal the potential of (porous) conductive MOFs to be employed as active materials in opto-electronics devices.

Understanding the charge transport rates across molecules and molecule-electrode interfaces  is important in many areas of research including chemistry, biology, and nanoscience.^1-2 A crucial parameter is the tunnelling decay coefficient (β, in Å^-1 or n_C^-1) which determines how quickly the current across the junction decreases as a function of the length of the molecule. Usually, the value of β can be changed by changing the chemical structure of the molecular backbone,^3-4 but β also depends on the type of the binding with the electrodes for conjugated systems.³ We have reported that SAMs of S(CH₂)_nX where X = H, F, Cl, Br, or I, have increasingly high currents with increasing polarizability of X.⁵ Here we report a new approach to tune β by simply changing X. In this system, eutectic gallium-indium alloy (EGaIn) was used as the top electrode, a monolayer of S(CH₂)_nX was self-assembled on a Ag surface which also served as the bottom electrode. We found that as the polarizability of X increases from X = F to I, β decreased from 0.97 ± 0.04 n_C^-1 to 0.34 ± 0.01 n_C^-1 and the dielectric constant ε_r increased from 2.5 ± 0.6 to 8.9 ± 1.6, respectively. DFT calculations show that the electrostatic potential profile of the SAM depends on X. More specifically, we found that the HOMO-1 is the dominant conduction orbital that is highly effected by X resulting in the tunnelling barrier height and thus the decay coefficient. In other words, the value of β can be controlled by using one polarizable atom without the need to change the molecular backbone.

References:

(1)           Stubbe, J. et al. Chem. Rev. 2003, 103, 2167.

(2)           Heitzer, H. M. et al. Acs Nano 2014, 8, 12587.

(3)           Kim, B. et al. Am. Chem. Soc. 2011, 133, 19864.

(4)           Xie, Z. et al. Acs Nano 2015, 9, 8022.

(5)           Wang, D. et al. Adv. Mater. 2015, 27, 6689.

Among many other applications, room temperature ionic liquids (ILs) are used as electrolytes for storage and energy conversion devices. In this context, rechargeable batteries are extended and useful devices to store energy. Since their discovery in 1970s, Li-ion batteries have become popular energy storage solutions and nowadays represent a promising alternative to conventional devices. The increasing requirements and power of these batteries, and disadvantages such as degradation or high flammability, make essential the search of alternative materials. Thanks to the high abundancy of Na as a raw material, Na-ion batteries have attracted intense attention as potential candidates for the replacement of Li-ion batteries. Other alternatives to Li-ion batteries based on multivalent metal cations are emerging in recent years. The ability of these metal species to transfer more than one electron can be useful to obtain faster charge rates. In this work, we investigate at microscopic level the structural and dynamical properties of 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide [C₄PYR]⁺[Tf₂N]^- IL-based electrolytes for metal-ion batteries. We carried out molecular dynamics simulations of electrolytes mainly composed of [C₄PYR]⁺[Tf₂N]^- IL with the addition of Mⁿ⁺-[Tf₂N]^- metal salt (M = Li⁺, Na⁺, Ni²⁺, Zn²⁺, Co²⁺, Cd²⁺, and Al³⁺, n = 1, 2, and 3) dissolved in the IL. The addition of low salt concentration lowers the charge transport and conductivity of the electrolytes. This effect is due to the strong interaction of the metal cations with the [Tf₂N]^- anions, which allows for molecular aggregation between them. We analyze how the conformation of the [Tf₂N]^- anions surrounding the metal cations determine the charge transport properties of the electrolyte. We found two main conformations based on the size and charge of the metal cation: monodentate and bidentate (number of oxygen atoms of the anion pointing to the metal atoms). The microscopic local structure of the Mⁿ⁺-[Tf₂N]^- aggregates influences the microscopic charge transport as well as the macroscopic conductivity of the total electrolyte.

Acknowledgements. The research leading to these results has received funding from the Andalucía Region (FQM-1851).

While charge transport in highly ordered system is generally well understood, correct modelling of conductivity in highly disordered system quite often presents an important theoretical but also experimental challenge. Slow conductance relaxation has been studied in many disordered insulators using field effect measurements. After a quench at low temperatures, a change in the gate voltage is accompanied by a sudden increase in the conductivity, slowly decreases with a roughly logarithmic dependence on time. Memory effects and aging are often seen in the same type of experiments. These glassy behaviour has been interpreted in different ways, but there is a growing tendency to explain them in terms of electron glasses, i.e., systems with states localized by the disorder and long-range Coulomb interactions between carriers.

The use of local probe techniques, such as scanning Kelvin probe microscopy (SKPM), presents two advantages as compared with the conductance measurements performed so far: i) it allows a study of the phenomena at the nanometer scale, and ii) samples with larger resistances can be measured. In the present work Dynamic AFM is used to characterize the electronic properties of two quite different nanoscale highly disordered and low conductivity systems. On the one hand highly resistive granular metal grains[1], and on the other Graphene Oxide islands[2].

We apply Electrostatic and Kelvin Force Microscopy to these samples. In addition, their time evolution is studied using “movies” where topography and surface potential are acquired simultaneously. Our AFM studies suggest evidence of the formation of an electron glass on the materials studied. This evidence includes the presence of domains on the surface potential, uncorrelated with topograph. The fluctuations of the surface potential are compatible with variations of the Coulomb energy of a single charge over the distance between domains. At the same time, the fact that the fluctuations are larger than kT and that time correlations are dominated by a broad distribution of characteristic times can be naturally explained within the electron glass model. When the conducting polymers are excited with light the surface potential relaxes logarithmically with time, as usually observed in electron glasses.

[1] M. Ortuño, E. Escasaín, E. López-Elvira, A. Somoza, J. Colchero and E. Palacios-Lidón. Scientific Reports, Scientific Reports 6, article #21647 (2016).

[2]M.F. Orihuela, A.M. Somoza, J. Colchero, M. Ortuño, E. Palacions-Lidón, Physical Review B 95(20) 205427 (2017).

Constructing a materials-specific theory of charge dynamics in organic single materials is a complex prob- lem, where the computation of accurate structural and vibrational properties needs to be coupled to ways of determining the charge mobility characteristics. In particular one needs an accurate method for describing excitations, which is also scalable to reasonably large systems. Here I will discussed how different flavours of constrained density functional theory (CDFT) can achieve such goal.

Firstly I will consider the most conventional form of CDFT, which allows one to calculate the energy of systems with displaced electron densities (e.g. in a charge transfer process). Such scheme can be used to extract a number of quantities important for charge dynamics. Here I will make examples of the calculation of 1) the charge transfer energies of molecules on surfaces, so to derive accurate level alignments [1,2]; 2) the quasi-particle gap renormalisation in molecular crystals [3]; 3) the reorganisation energy of molecules in the gas phase and on surfaces [4].

Then I will move to show a recently implemented scheme, which uses CDFT to compute elementary excitations in molecules [5]. This method, which we have named excitonic DFT (XDFT), calculates the M-particle excited state of an N-electron system, by optimizing a constraining potential to confine N−M electrons within the ground-state Kohn-Sham valence subspace. The efficacy of XDFT will be demonstrated by calculating the lowest single-particle singlet and triplet excitation energies of the well-known Thiel molecular test set, with results which are in excellent agreement with time-dependent density functional theory (TDDFT).

[1]A.M.Souza, I.Rungger, C.D.Pemmaraju, U.Schwingenschloegl and S.Sanvito, Constrained-DFTmethod for accurate energy-level alignment of metal/molecule interfaces, Phys. Rev. B 88, 165112 (2013).

[2]  Subhayan Roychoudhury, Carlo Motta and Stefano Sanvito, Charge transfer energies of benzene physisorbed on a graphene sheet from constrained density functional theory, Phys. Rev. B 93, 045130 (2016).

[3] A. Droghetti, I. Rungger, C.D. Pemmaraju and S. Sanvito, Fundamental gap of molecular crystals via constrained Density Functional Theory, Phys. Rev. B 93, 195208 (2016).

[4] Subhayan Roychoudhury, David D. O’Regan and Stefano Sanvito, Wannier-function-based constrained DFT with nonorthogonality-correcting Pulay forces in application to the reorganization effects in graphene- adsorbed pentacene, Phys. Rev. B 97, 205120 (2018).

[5] Subhayan Roychoudhury, Stefano Sanvito and David D. ORegan, XDFT: an efficient first-principles method for neutral excitations in molecules, arXiv:1803.01421 (2018).

The development of robust, chemically-sensitive techniques is crucial for the advancement of single-molecule electronics. Studies in single-molecule junctions largely rely on indirect electrical characterization to statistically evaluate the chemistry and quality of the established circuits. One fundamental challenge is the direct, quantitative determination of charge-vibrational coupling for well-defined single-molecule junctions. The ability to record molecular charge-vibrational coupling for individual species grants access to the determination of maximal charge transport efficiencies for specific molecular configurations and currents. Here we explore the charge-vibrational coupling for current-carrying tethered molecules by combined vibrational and metal-molecule-metal junction current-voltage spectroscopy. By inspecting the steady-state vibrational distribution during charge transport in a bis-phenyl-ethynyl-anthracene derivative by Raman scattering, we deduce a coupling constant of ≈0.35 vibrational excitations per charge carrier. Furthermore we follow the conformational response of a two-state molecular switch. Specifically, we remove the ground state polarizability and symmetry of a known p-terphenyl-4,4´´-dithiol (TPD) molecule by employing the 2,2´,5´,2´´-tetramethylated (TM-TPD) derivate. Whereas the highly sterically hindered, non-planar TM-TPD, lacking π-conjugation, in its pristine conformation does not exhibit a Raman signature, a marked on/off modulation of the single-molecule Raman signal exceeding a factor of 100 is achieved via redox state control by means of the applied voltage.

Support by the Deutsche Forschungsgemeinschaft (DFG) via SPP 1234 (Grant RE2592) & Munich Centre for Advanced Photonics (MAP), the European Research Council via Advanced Grant MolArt (n° 247299) and Chinese Scholarhip Council (H.B., Y.G.) is gratefully acknowledged.

Key refs.: JACS 140 (2018) 4835 | Nature Comm. 7 (2016) 10700 | Nature Nanotechn. 7 (2012), 673

Electron transport (ETp), i.e., electronic conduction, across protein monolayers in a solid state–like configuration is surprisingly efficient, comparable, length-normalized, to completely conjugated molecules. This is amazing as apparently nature does not use this capability, except for electron transfer, ET, within and some times between redox proteins, a process that is coupled to ionic transport.

Nature regulates ET via redox chemistry, while for ETp a redox process is not a necessary condition. This allows studying ETp via non-redox proteins, such as rhodopsins and albumins; remarkably, ETp is quite efficient also across these proteins.

If contact to the external world does not limit ETp, i.e., intra-protein transport dominates, there seems to be no barrier for transport. As ETp is temperature-independent, it may well be coherent…. This may be important also for electron transfer, ET, which involves injection and extraction of electrons, the analogue of which for ETp is the coupling to the electrodes. Then, efficient ETp via non-redox proteins suggests why there are redox centres in ET proteins, ito protect it from high reducing power.

I will discuss experimental data that illustrate our main results as well as some more recent ones on multi-heme proteins and on protein multilayers, which raise even more questions.^1,2

* work with Mordechai Sheves & Israel Pecht, Weizmann Inst.

References

C. Bostick et al.  Rep. Prog.Phys., 81 (2018) 026601, “Protein bioelectronics:a review of what we do and do not know” doi.org/10.1088/1361-6633/aa85f2 ,

N. Amdursky et al., Adv. Mater. 42, (2014) 7142 “Electronic Transport via Proteins” 10.1002/adma.201402304

We report recent experimental and theoretical results for molecular junctions based on \pi conjugated wires. These wires represent a class of linear molecules whose transport properties can be understood in the framework of the topological Su-Schrieffer-Heeger model for polyacetylene. We present an in-depth theoretical analysis based on tight-binding and ab-initio simulations of their coherent transport properties and show that, under certain conditions and depending on the chain parity (even /odd) and length, the conductance at the Fermi level can depend very weakly or even increase with the wire length. For short odd chains, we also provide experimental evidence of the role of the external environment in their charge transport properties: we study conductance trends in single-molecular junctions of polymethine dyes and prove that the trends can also be altered by the choice of the embedding solvent. Overall, our results suggest a way of enabling efficient electron transport at the nanoscale with one-dimensional wires.Iryna Davydenko

The extraordinary absorption cross-section and high photoluminescence (PL) quantum yield of perovskite nanocrystals make this type of material attractive to a variety of applications in optoelectronics. For the same reasons, nanocrystals are also ideally suited to function as nanoantennae to excite nearby single dye molecules by fluorescence resonant energy transfer (FRET). Here, we demonstrate that FAPbBr3 perovskite nanocrystals, of cuboidal shape and approximately 10 nm in size, are capable of selectively exciting single Cyanin 3 molecules at a concentration a hundredfold higher than standard single-molecule concentrations. This FRET antenna mechanism increases the effective brightness of the single dye molecules one hundredfold. Photon statistics and emission polarization measurements provide evidence for the FRET process by revealing photon-antibunching with unprecedented fidelity and highly polarized emission stemming from single dye molecules. Remarkably, the quality of single-photon emission improves two-fold compared to emission collected directly from the nanocrystals since the higher excited states of the dye molecule act as effective filters to multiexcitons. The same process gives rise to efficient deshelving of the molecular triplet state by reversed intersystem crossing, translating into a reduction of the PL saturation of the dye, thereby increasing the maximum achievable PL intensity of the dye by a factor of five.

Perovskite based solar cells have shown impressive progress in recent years. Now that high efficiencies are in reach, more and more effort is put into understanding the underlying device physics and to improve the devices intrinsic and extrinsic stability.

Touching on both of these subjects, I want to first discuss recent findings regarding the implementation of metal oxide layers into perovskite solar cell device stacks. These can be used to form impenetrable barrier layers which prevent the ingress of humidity as well the egress of perovskite decomposition products. Using this strategy, the overall decomposition of perovskite can be significantly suppressed, leading to outstanding solar cell device stability.

On the other hand, we find for a variety of systems that directly interfacing the perovskite to metal oxide layers can trigger a complex variety of reactions, which significantly alter the composition of the perovskite at the interface and lead to the presence of degradation products.  These thin interlayers play an important role for film formation and charge extraction and will therefore influence the overall device performance.

Photovoltaic devices based on hybrid metal halide perovskites are rapidly improving in power conversion efficiency. While these materials are generally viewed as three-dimensional, the effects of nano-scale interfaces within the bulk films are still poorly understood. Such interfaces may appear between the perovskite layers and the charge extraction layers, or within the perovskite layers, at grain boundaries, of when passivating or hydrophobic interlayers are included.

We show here that photon reabsorption in lead iodide perovskite layers is strongly influenced by layer thickness and interfaces formed with charge-extraction layers^[1]. Such photon re-absorption is found to reduce the apparent bi-molecular charge-carrier recombination rate constant with increasing film thickness, while the intrinsic value can be fully explained as the inverse process of absorption^[2].

In addition, we discuss the role of nanoscale interfaces^[3,4], energetic disorder^[5], and passivating interlayers^[6] on the dynamics of charge-carriers in various metal halide perovskites. Lowering of the perovskite dimensionality is shown to have effects similar to those known for classic inorganic semiconductors, such as enhancing bimolecular and Auger recombination and reducing trap-mediated recombination through surface passivation. Such extrinsic effects will also reduce the charge-carrier mobility below the maximum attainable value near 100cm²/(Vs) for MAPbI₃^[7].

[1] T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P. Müller-Buschbaum, M. B. Johnston, and L. M. Herz,Nano Lett. 17, 5782 (2017).

[2] C. L. Davies, M. R. Filip, J. B. Patel, T. W. Crothers, C. Verdi, A. D. Wright, R. L. Milot, F. Giustino, M. B. Johnston, and L. M. Herz, Nature Communications 9, 293 (2018).

[3] D. P. McMeekin, Z. Wang, W. Rehman, F. Pulvirenti, J. B. Patel, N. K. Noel, S. R. Marder, M. B. Johnston, L. M. Herz, and H. J. Snaith, Adv. Mater., 29 (2017), p. 1607039.

[4] R. L. Milot, R. J. Sutton, G. E. Eperon, A. A. Haghighirad, J. M. Hardigree, L. Miranda, H. J. Snaith, M. B. Johnston, and L. M. Herz, Nano Letters, 16 (2016), pp. 7001-7007.

[5] A. D. Wright, R. L. Milot, G. E. Eperon, H. J. Snaith, M. B. Johnston, and L. M. Herz, Adv. Func. Mater., 27 (2017), p. 1700860.

[6] Z. Wang, Q. Lin, F. P. Chmiel, N. Sakai, L. M. Herz, and H. J. Snaith, Nature Energy, 2 (2017), p. 17135.

[7] L. M. Herz, ACS Energy Lett., 2 (2017), pp. 1539-1548.

The optical and transport properties of lead-halide perovskites (LHPs) have been used as a basis for new solar cell technologies showing record improvements in efficiencies. In the search for the microscopic origins of this success, many recent studies suggest that structurally dynamic effects are active already at room temperature and standard operating conditions and may affect device performance and/or stability. Here, we explore this issue using first-principles calculations based on density functional theory. In particular, we focus on ion migration and dynamic distortions.

The optical and transport properties of lead-halide perovskites (LHPs) have been used as a basis for new solar cell technologies showing record improvements in efficiencies. In the search for the microscopic origins of this success, many recent studies suggest that structurally dynamic effects are active already at room temperature and standard operating conditions and may affect device performance and/or stability. Here, we explore this issue using first-principles calculations based on density functional theory. In particular, we focus on ion migration and dynamic distortions.

In this talk, I will discuss our group’s recent experimental and computational work on understanding electronic and phononic structure nanocrystal thin films and charge transport in these thin films. Using electrochemical-based approaches, we show that we can quantify the electronic density of states and also examine charge-transfer processes across interfaces. Using inelastic x-ray scattering, we quantify the phononic denisty of states. We combine density functional theory calculations and kinetic Monte Carlo simulations to develop a first-principles model for charge transport in nanocrystals solids. We show that these simulations explain temperature-dependent time-of-flight measurements of electron and hole mobility performed on lead sulfide nanocrystal thin films. The combination of experimental and computational work highlights the importance of electron-phonon interactions in nanoscale transport and enables us to determine the relative impact of energetic and positional disorder on transport, providing us with design guidelines on parameters to consider when optimizing nanocrystal synthesis, nanocrystal surface treatments, and nanocrystal thin film preparation for different device applications.

For the fabrication of an integrated solar-to-chemical system, different components should be interfaced together in an orchestrated manner. Photoelectrodes need to absorb in the visible range, with a valence and a conduction band suited for the target reaction. Moreover, the presence of catalysts is required to manage the intrinsic energetic hurdle. Herein, we address the study of the major challenges, namely performance, stability, and interfaces to enable fabrication of integrated solar-to chemical systems. Novel scientific directions for the synthesis of functional interfaces and the development of new tools for their characterization will be addressed. Specifically, we will present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. These findings are confirmed by in situ electrochemical atomic force microscopy (EC-AFM), which reveals that degradation under operating conditions occurs via dissolution of the film, starting at exposed facets of grains in polycrystalline thin films. In addition, we will present the correlation between morphological and functional heterogeneity in this material by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate.

Insights into corrosion mechanisms and nanoscale heterogeneity aid development of protection strategies and provide information on how local functionality affects the macroscopic performance. 

Metal-halide perovskites are promising materials for opto-electronic applications. Their mechanical and electronic properties are directly connected to the nature of their lattice vibrations. Whereas the mid infrared (IR) range contains mainly information on the internal vibrations of the methylammonium cation, the lead-halide lattice vibrations are located in the far IR.

We will report far-IR spectroscopy measurements of lead halide perovskite thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra.¹ We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites. We determine the frequencies of both the transversal and longitudinal optical phonons, and use them to calculate the static dielectric constants, polaron masses, and upper limits for the phonon-scattering limited charge carrier mobilities. Furthermore, we compare our experimental results to molecular dymanics simulations of lead halide perovskites. Our findings are important for the basic understanding of charge transport processes and mechanical properties in metal halide perovskites.

(1) Sendner, M.; Nayak, P. K.; Egger, D. A.; Beck, S.; Müller, C.; Epding, B.; Kowalsky, W.; Kronik, L.; Snaith, H. J.; Pucci, A.; Lovrincic, R. Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport. Mater Horiz 2016, 3 (6), 613–620.

Solar cells based on metal-halide perovskite absorber layers have resulted in outstanding photovoltaic devices with long non-radiative lifetimes as a crucial feature enabling high efficiencies. Long non-radiative lifetimes occur if the transfer of the energy of the electron-hole pair into vibrational energy is slow, due to, e.g., a low density of defects, weak electron phonon coupling or the release of a large number of phonons needed for a single transition. Here, we discuss the implications of the known material properties of metal-halide perovskites (such as permittivities, phonon energies and effective masses) and combine those with basic models for electron-phonon coupling and multiphonon-transition rates in polar semiconductors. We find that the low phonon energies of MAPbI₃ lead to a strong dependence of recombination rates on trap position, which can be readily deduced from the underlying physical effects determining non-radiative transitions. Here, we show that this is important for the non-radiative recombination dynamics of metal-halide perovskites, as it implies that these systems are rather insensitive to defects that are not at midgap energy. This can lead to long lifetimes, which indicates that the low phonon energies are likely an important factor for the high performance of optoelectronic devices with metal halide perovskites. 

In conventional solar cells (SCs), above-bandgap “hot” carriers (HCs) rapidly lose their excess energy to vibrations in the semiconductor lattice via electron-phonon coupling. This thermalisation to the band edge forms the main part of the Shockley-Queisser limiting efficiency. Semiconductors with reduced carrier cooling rates are desirable to exceed this limit via a hot-carrier architecture, where the hot carriers are extracted before they are fully cooled. Lead-halide perovskites are solution processible solar cell materials, which exhibit exceptional power conversion efficiencies and a tunable band gap. Recent measurements indicate slow cooling at high carrier densities in these material systems. Here we use ultrafast infrared intraband spectroscopy to directly compare the dynamics of carrier cooling in a range of five commonly studied lead-halide perovskites: FAPbI₃, FAPbBr₃, MAPbI₃, MAPbBr₃ and CsPbBr₃. We measure this cooling as occuring within ~100-900 fs, depending on both the carrier density, n^hot (slower cooling at higher n^hot) and choice of cation (with the slowest cooling in the all-inorganic Cs-based system). These observations support the existence of a “hot-phonon bottleneck” and assert the role of lattice vibrations towards HC cooling.

Metal-halide perovskites show exceptional optoelectronic properties for next generation photovoltaics and light-emitting diodes. Recently, monovalent cation substitution has been reported to generate luminescence very efficiently, yet the underlying photo-physics remain to be understood.

Here, we study the origin of this increased brightness by combining transient absorption and photoluminescence (PL) to track charge carrier dynamics in thin films. Unexpectedly, we find that the recombination regime changes from the previously-reported second to first order regime dynamically within tens of nanoseconds after excitation, in line with fluence-dependent PLQE measurements. In temperature-dependent PL we find a redshift of the luminescence with decreasing temperature, directly mapping localized shallow traps. Supported by DFT calculations and transistor measurements we propose that energetic disorder in the distribution of electronic states leads to spatial accumulation of charges, creating n- and p-type regions. Our results indicate that strong luminescence can be achieved in mixed-cation perovskites even at low carrier densities and thereby provides a roadmap for highly efficient LEDs.

Colloidal nanoplatelets are atomically flat, quasi-two-dimensional sheets of semiconductor that can exhibit efficient, spectrally pure fluorescence. These materials were discovered when liquid-phase chemical syntheses of spherical nanocrystals were modified. However, despite intense interest in their properties, the mechanism behind their highly anisotropic shape and precise atomic-scale thickness remained unclear, and even counter-intuitive for commonly studied nanoplatelets that arise from isotropic crystal structures (such as zincblende CdSe and lead halide perovskites). Here we describe an intrinsic instability in growth kinetics that can lead to such highly anisotropic shapes. By combining experimental results on the synthesis of CdSe nanoplatelets with theory predicting enhanced growth on narrow surface facets, we develop a model that explains nanoplatelet formation as well as observed dependencies on time and temperature. Based on standard concepts of volume, surface, and edge energies, the resulting growth instability criterion can be directly applied to other crystalline materials. Thus, knowledge of this previously unknown mechanism for controlling shape at the nanoscale can lead to broader libraries of quasi-two-dimensional materials.

Two-dimensional (2D) semiconductors are finding a renewed interest in recent years, due to their combination of physical properties, e.g., mobility, fluorescence and spin selectivity, with a potential implementation in new and emerging opto-electronic and spin-electronic devices. The current work discusses various magneto-optical phenomena, found in two different 2D systems: The transition metal phosphorous trichalcogenides, indium chalcogenides and magnetically doped colloidal nanoplatelets. The magneto-optical properties are investigated by following optical polarization in the presence of an external magnetic field, as various temperatures, as well as implementing the use of optically detected magnetic resonance spectroscopy.

The transition metal phorphorous trichalcogenides resemble the most common layered dichalcogenides, but one-third of the metals are replaced with a P-P pair; hence, the chemical formula is written as M2/3(P-P)1/3X2 or M2P2X6. The dilution of metal site by non-magnetic atoms, endows a column like arrangement of the remaining metals, leading to a special magnetic properties, from a full antiferromagnetic Neel, through antiferromagnetic zigzag to ferromagnetic character. The various arrangements can be tuned by variation of the metal cations (among the first row of transitions metal atoms). The work focuses on the influence of the created magnetism on the magneto-optical properties of the M2P2X6 semiconductors. In addition, the talk will report about the synthesis and characterization of In2S3 layered compounds, exploring the various possible structural phases and their magnetically doped derivatives.

Transition metal dopant embedded in colloidal semiconductor nanoplatelets (NPLs) exhibit special magnetic properties, resemble the bulk diluted magnetic semiconductors. However, the NPLs confined thickness induces an extremely intense spin-exchange interaction between the resident photo-excited carriers and the guest magnetic spins. Such an interaction leads to a giant magnetization and g-factor, and consequently endows the materials with special magneto-optical properties. The work emphases the investigation of the spin-exchange interaction while varying the magnetic dopants, by following variation in the magneto-optical properties, when detecting either an ensemble of NPLs or a focus on a single platelet. **

** Magnetically doped NPLs project was carried out in collaboration with Prof. Volkan Hilmi Demir and his groups from Nanyang Technological University – NTU Singapore 639798, Singapore and from Bilkent University, Ankara 06800, Turkey.

We introduce the electronic and exciton dynamic properties of these nanoplatelets and their hetero structures and demonstrate for example that ligand induced strain in these colloidal quantum wells with finite size results in a strong alteration of the transition energies. We show that intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy efficient display technology and propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic and directed photoluminescence.  Our analysis of the two-dimensional k-space distribution of the nanoplatelet absorption and emission reveals the underlying internal transition dipole distribution. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters for photonic applications. We also demonstrate by 2D k-space spectroscopy that two-photon absorption (TPA) is highly anisotropic in CdSe nanoplatelets, thus promoting them as a new class of directional two-photon absorbers with extremely large cross sections.

References:                                                                                                                     1. Scott and Achtstein et al., Nature Nanotechnology 12, 1155 (2017)                        2. Heckmann and Achtstein et al., Nano Letters 17, 6321, (2017)                                3. Antanovich and Achtstein et al., Nanoscale, (2017); DOI: 10.1039/c7nr05065h

Solution-processed semiconductor nanocrystals have attracted great interest in photonics including high-purity color conversion and enrichment in quality lighting and display backlighting. These nanocrystals span different types and heterostructures of semiconductors in the forms of colloidal quantum dots and rods to a more recently emerging class of colloidal quantum wells. Here we will talk about colloidal photonics using the family of quasi-2D, tightly-confined, atomically flat nanocrystals. Here we will show that custom-design 2D heteronanoplatelets uniquely offer record high optical gain coefficients and ultra-low threshold stimulated emission. In addition, we will show that controlled stacking of these nanoplatelets provides us with the ability to further tune and master their excitonic properties. Also, we will discuss doping of these nanoplatelets with Cu for high-flux solar concentration properties and with Mn for precise wavefunction-engineered magnetic properties. Given their most recent accelerating progress, these solution-processed quantum materials hold great promise to challenge their epitaxial counterparts in semiconductor optoelectronics in the near future.

Colloidal PbS nanosheets represent an important infrared 2D nanomaterial. They have tunable energy gaps and much higher charge carrier mobility than quantum-dot films, which makes them ideal for optoelectronic and electronic applications including photovoltaic devices and field-effect transistors.

The growth of PbS nanosheets is confirmed by electron microscopy and photoluminescence spectroscopy. Their thickness can be tuned by changing the reaction temperature during the synthesis. Recent research also demonstrates that the lateral size of the nanosheets can be systematically tuned between 20 nm to a few hundred nanometers. Core/shell PbS/CdS nanosheets can be synthesized using a cation exchange method. This method can protect the PbS core as well as further tuning the core thickness. Surface passivation of the nanosheets using organic molecules is also proven to be very effective to suppress the surface defects and improve the optical properties.

The energy gap of the PbS nanosheets can be tuned by changing their thickness. The thickness dependent energy gap is a unique feature of the exciton under one-dimensional confinement. In contrast to quantum dots, the confinement energy can be achieved in a typical nanosheet is smaller than a quantum dot. However, the maximum energy gap of the nanosheet can still reach 1 eV which is about more than twice of the energy gap of the bulk PbS.

The optical absorption of typical PbS nanosheets shows a step-like spectrum. The step edge is nearly coincident with the photoluminescence peak, indicating a negligible Stokes shift. The absorption spectrum of the nanosheets of 20 nm in lateral size shows an excitonic peak similar to quantum dots, which is possibly due to the additional lateral confinement.

The absolute photoluminescence quantum yield of the PbS nanosheets can be measured accurately by using an integrating-sphere technique. Well-passivated PbS nanosheets typically show 20% to 30% photoluminescence quantum yield. The maximum photoluminescence quantum yield has reached 60%, which is about twice of the quantum yield from PbS quantum dots of the same energy gap.

The time-resolved photoluminescence of the PbS nanosheets shows a distinct fast decay followed by a slow decay. The fast decay accounts for more than 90% of the total luminescence intensity. The exciton radiative lifetime derived from the time-resolved photoluminescence and the quantum yield is much shorter than the PbS quantum dots. It is an indication of giant oscillation strength transition which is a consequence of large exciton coherence volume in a nanosheet.

Colloidal lead halide perovskite nanocrystals (APbX_3, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) emerge as promising materials for optoelectronic applications such as in television displays, light-emitting devices, and solar cells. The sponaneous and stimulated emission spectra of these NCs are readily tunable over the entire visible spectral region of 410-700 nm [1-2]. The photoluminescence of these NCs is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, and high quantum yields of up to 100%. Cs_1-xFA_xPbI₃ and FAPbI₃ reach the near-infrared wavelengths of 800 nm [3].  A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. A promising approach lies in the formation of multinary compositions such as Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs. We show that droplet-based microfluidics can successfully guide the synthesis of such complex compositions [4]. We could fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission linewidths (to below 40nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. Most importantly, we demonstrate the excellent transference of reaction parameters from microfluidics to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices. As an example, Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting high external quantum efficiency of 5.9% and very narrow electroluminescence spectral bandwidth of 27 nm.

The processing and optoelectronic applications of perovskite NCs are, however, hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, we have developed a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules, resulting in much improved chemical durability [5].

             Perovskite NCs also readily form long-range ordered asssemblies known as superlattices. These assemblies exhibit accelerated coherent emission (superfluorescence), not observed before in semiconductor nanocrystal superlattices [6].

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Raino, M. Becker, M. Bodnarchuk et al. 2018, submitted

Tin(II)sulfide nanosheets with lateral sizes in the range of several micrometers and thicknesses of only tens of nanometers are synthesized by injection of S-oleylamine into a hot solution of oleylamine, oleic acid, HMDS and tin chloride. We will present structural data to follow the nanosheets growth and show how additional octadecene can influence the lateral growth direction to form either rectangular or hexagonal nanosheets.

In the second part we focus on the electrical properties of individual hexagonal SnS-nanosheets. Here we present data from combined Kelvin Probe Force Microscopy (KPFM) with simultaneous Scanning Photocurrent Measurements (SPCM) to attribute the electro-optical properties to mutual interaction between locally generated charge carriers and electric potentials. For example the occurrence of zero-bias photocurrent can directly be attributed to changes of the band bending due to the optically generated charge carriers.

The surface ligation of semiconductor nanocrystals determines their optical and transport properties, and purposeful control of surface chemistry is under active investigation.  Colloidal 2D CdSe nanoribbons or quantum belts provide an ideal system for the study of rapid, reversible, and complete exchange of surface ligation at room temperature.  L-type primary-amine ligation is readily replaced by Z-type Lewis acid ligation of type MX₂ (M = Zn, Cd; X = carboxylate or halide).  Re-exposure to primary amines removes the Z-type ligands and restores L-type ligation.  Primary-amine ligation is readily converted to bound-ion-pair X-type ligation upon exposure of the quantum belts to acids HX (X = halide, carboxylate, or nitrate).  Salts of type R₄NX or NaX (X = carboxylate or halide) are unable to displace L-type ligation, but readily substitute for Z-type ligation, forming bound-ion-pair X-type ligation.  This bound-ion-pair X-type ligation is readily exchanged to L-type ligation upon exposure to primary amines.  The surface exchanges are characterized by a variety of means, and the absorption and emission spectra of the quantum belts are particularly sensitive to surface ligation.  Such surface exchanges promise to optimize the properties of the 2D nanocrystals.

One important feature of two-dimensional semiconductors (e.g., transition metal
dichalcogenide monolayers or phosphorene) is the strong Coulomb and Exchange interactions between photo-generated charge carriers resulting from the unusual carrier screening and its environmental sensitivity.   Particular interesting is the formation of bound states involving multiple carriers, such as positively and negatively charged trions (3 particles),  biexcitons and more. The low screening situation leads to quasiparticles that can remain stable even near-room temperature. I will address the corresponding situation describe some of the atomistic theoretical approaches available. En emphasis will be placed on the effects of the environment on the quasiparticle binding energies (exciton binding energies and the shift of exciton transitions relative to trions and biexcitons). Empirical theoretical approaches will be contrasted to more ab-initio descriptions, which still suffer from the high computational demand. Finally, the results will be compared to —the rather well established— situation in semiconductor quantum dots where screening effects tend to be rather well described by bulk screening.

The nanocrystal-ligand interface of colloidal semiconductors has attracted significant research interest owing to their dominant role in tuning and tailoring the physio-chemical properties of such functional nanomaterials. For metal chalcogenide quantum dots (QDs), a comprehensive picture has emerged over the past decade on surface passivation and photoluminescence relationship. The surface of QDs is metal rich passivated by X-type ligands. Displacing them from the surface results in a marked drop of PLQY, which could be reversed by readsorption of ligands. DFT studies indicate that this is linked to the formation of under-coordinated surface Se which behave as traps for electrons or holes. QDs are nanometre-size crystallites with crystal facets of few square nanometres. This differs markedly from 2-D nanocrystals which are few monolayers thick with atomically flat top and bottom surfaces measuring hundreds of square nanometre e.g. CdSe nanoplatelets. A pronounced interplay between the ligand capping and optical properties of nanoplatelets has been reported. But, research on CdSe nanoplatelets is often hampered by their limited colloidal stability and unwanted stacking in bundles, two aspects that are linked to nanoplatelet-ligand interface. This combination of promising properties and cumbersome processing calls for an in-depth study of their surface chemistry. Here, the question as to whether concepts developed for multifaceted QDs can be transferred to nanoplatelets, which are terminated solely by atomically flat interfaces, stands out.

In this work, detailed study of the surface-chemistry of CdSe nanoplatelets is reported. We first introduce an improved synthesis strategy for nanoplatelets that yields colloidally stable and aggregation free nanoplatelets suspensions. Despite large surface area, these core-only nanoplatelets have a PLQY as high as 55%. Elemental analysis show nanoplatelets are Cd rich.¹H NMR analysis show that Cd excess comes with a surface termination by X-type carboxylate ligands, a binding motif similar to QDs. Addition of an L-type ligand displaces cadmium-carboxylate complexes from surface. The displacement isotherm unravelled that the surface features adsorption sites with different binding energies for CdX₂. We further emphasize the validity of a multiple-site model by DFT calculations, which yield a variation in binding sites from edges to the centre of the facet. Moreover, we analysed different types of mid-gap trap states and site dependent surface reconstruction the nanoplatelet undergoes with displacement of the ligands. Finally, this surface-model for CdSe nanoplatelets is most likely not restricted to CdSe only and should be considered when analysing the surface reactions of another 2-D nanoplatelet system.

We present our recent results on the optical and magneto-optical studies of charged and neutral excitons in ensemble of CdSe-based colloidal nanoplatets (NPLs). The influence of the temperature and external magnetic field on the time-resolved dynamics of photoluminescence (PL) allows us to distinguish between the recombination originating from neutral and changed excitons. We compare the dependence of the fine structure energy splitting of the neutral exciton on the NPLs thickness determined by different experimental methods with the theoretical calculations. We show that at low temperatures the PL in CdSe bare NPLs is determined by the radiative recombination of the lowest dark (spin-forbidden) exciton. We propose an effective mechanism of the dark exciton activation via the exchange interaction with surface dangling bonds. We discuss the mechanisms of the dangling bond spins alignment in external magnetic field and its effect on the exciton spin polarization [3].

The sign of the circular polarization of the PL induced in magnetic field allows us to distinguish between the recombination originating from negative or positive trions. The theoretical analysis of the magnetic field dependences of the degree of circular polarization (DCP) and of the Spin Flip Raman Scattering (SFRS) energy shift allows us to determine the effective g-factor controlling the Zeeman splitting of the spin sublevels of resident electrons, excitons and holes in the negative trion in external magnetic field. The maximum value of the DCP and the SFRS intensity allow us to extract an information about the preferable orientation of the NPLs in the ensemble.

This work was supported in part by the Russian Foundation for Basic Research (Grant No. 17-02-01063 and Grant No. 15-52-12015).

[1] E. V. Shornikova, L. Biadala, D. R. Yakovlev, V. F. Sapega, Y. G. Kusrayev, A. A. Mitioglu, M. V. Ballottin, P. C. M. Christianen, V. V. Belykh, M. V. Kochiev, N. N. Sibeldin, A. A. Golovatenko, A. V. Rodina, N. A. Gippius, A. Kuntzmann, Ye Jiang, M. Nasilowski, B. Dubertret, M.Bayer, Nanoscale 10, 646 (2018).

[2] E. V. Shornikova, L. Biadala, D. R. Yakovlev, D. H. Feng, V. F. Sapega, N. Flipo, A. A. Golovatenko, M. A. Semina, A. V. Rodina, A. A. Mitioglu, M. V. Ballottin, P. C. M. Christianen, Y. G. Kusrayev, M. Nasilowski, B. Dubertret, M. Bayer, Nano Letters 18, 373 (2018).

[3] A.V Rodina, A.A. Golovatenko, E.V. Shornikova,  D.R. Yakovlev, Al.L. Efros, Journal of Electronic Materials 4, 2018

It has been reported that drop casting of a suspension of PbX (X=S, Se, Te) nanocrystals on an ethylene glycol liquid substrate results in the formation of atomically coherent 2-D sheets of a nanocrystal monolayer in thickness. The Klinke group reported the formation of PbS NC sheets. We reported PbSe (S, Te) sheets with a superimposed honeycomb and square geometry.

I will present here our recent progress in this field which is based on extremely slow solvent evaporation under a constant (nearly saturated) solvent vapor phase. We present silicene type honeycomb sheets with lateral dimensions in the 100 micrometer range. We also prepared silicene structures that extend in the vertical dimension over several unit cells. Finally, we studied the "atomic-like" and orientation defects in these systems.

The mechanism of this remarkable self-assembly process was studied by in-situ GISAX and GIWAX and by molecular dynamic simulations.

Low-Dimensional Semiconductor Superlattices Formed by Geometric Control over Nanocrystal Attachment." Nano Letters 13(6): 2317-2323.

Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlattices." Science 344(6190): 1377-1380.

In situ study of the formation mechanism of two-dimensional superlattices from PbSe nanocrystals." Nature Materials 15(12): 1248-1254.

Mono- and multilayer silicene-type honeycomb lattices by oriented attachment of PbSe nanocrystals: synthesis, structural characterization, and analysis of the disorder.

Submitted

The rapid development of colloidally synthesized, two-dimensional nanoplatelets with precise thickness-tunable, narrow band-edge absorption and photoluminescence unlock several classes of investigations. We have examined charge and energy transfer involving these structures for purposes of energy capture and conversion and lighting. Particularly fast transfer of excitations between self-assembled, co-facial arrangements of thinner donor and thicker acceptor structures is found and modeled. Electron transfer rates for four isoenergetic donor–acceptor pairs comprising a well-known molecular electron acceptor and controlled lateral extents of nanoparticles, examined via ultrafast photoluminescence, relate a dependence of charge transfer rate on the spatial extent of the electron–hole pair wave function explicitly. A nonlinear dependence of rate with surface area is attributed to exciton delocalization within each structure, which we show via temperature-dependent absorption measurements remains constant.

The rapid development of colloidally synthesized, two-dimensional nanoplatelets with precise thickness-tunable, narrow band-edge absorption and photoluminescence unlock several classes of investigations. We have examined charge and energy transfer involving these structures for purposes of energy capture and conversion and lighting. Particularly fast transfer of excitations between self-assembled, co-facial arrangements of thinner donor and thicker acceptor structures is found and modeled. Electron transfer rates for four isoenergetic donor–acceptor pairs comprising a well-known molecular electron acceptor and controlled lateral extents of nanoparticles, examined via ultrafast photoluminescence, relate a dependence of charge transfer rate on the spatial extent of the electron–hole pair wave function explicitly. A nonlinear dependence of rate with surface area is attributed to exciton delocalization within each structure, which we show via temperature-dependent absorption measurements remains constant.

A solid-state assembly of nanoparticles for the use as a thermoelectric generator¹ is presented. The colloidal synthesis of the two-dimensional lead sulphide nanosheets was performed by oriented attachment of PbS nanoparticles². A self-supporting network of these anisotropic nanoplatelets was formed by slow destabilization of the colloidal solution by altering the pH value. Supercritical drying of such gel-type networks resulted in aerogels. These aerogel materials exhibit extremely low density, high surface to mass ratio and mesoporosity. The physical properties were characterized by transmission electron microscopy, scanning electron microscopy and N₂-physisorption measurements. The thermoelectric properties around room temperature were measured with a hot probe setup utilising multifunctional probes. Thus, the application of a temperature difference and the simultaneous measurement of the temperature difference and the induced thermovoltage was possible³. The material features the nanostructural benefits of aerogels while retaining the Seebeck coefficient of the bulk phase, enabling promising approaches for the use in thermoelectrical devices.

Semiconductor nanoparticle (NP) based photoelectrochemical sensors exhibit several advantages over other sensor types, such as a wide analyte range, fast responses, and high sensitivies.¹ For the development of systems for multi-analyte detection, not only a spatial structuring of the photoelectrodes, but also the improvement of their sensitivity and their detection range is necessary. Due to their high surface-to-volume ratio, semiconductor nanoplatelets (NPLs) are promising candidates for the application in (multi-analyte) photoelectrochemical sensing devices. The high surface area of the NPLs potentially increases the absolute number of surface trap states per particle, which may lead to an enhanced photoresponse of the NP covered electrode.³ In addition, CdSe NPLs were already shown to assemble into highly porous non-ordered network structures with large surface areas.² Our work reports on the preparation of different CdSe NPL based 3D assemblies on conductive glass electrodes and the photoelectrochemical characterization of the charge transfer processes across these structures. Electron microscopy revealed that porous NPL gels with different morphologies were obtained via the applied gelation processes. Photocurrents more than one magnitude larger than for simple particle monolayers were detected and the transport of charge carriers was proven by means of intensity modulated photocurrent spectroscopy (IMPS).⁴

References:

(1) Yue Z. et al., ACS Appl. Mater. Interfaces 2013, 5, 2800−2814.

(2) Naskar, S. et al., Chem. Mater. 2016, 28 (7), 2089–2099.

(3) Spittel, D. et al., ACS Nano 2017, 11 (12), 12174–12184.

(4) Jan F. Miethe, Anja Schlosser et al., submitted.

Colloidal nanoplatelets (NPLs) are quasi-two-dimensional nanocrystals with atomically precise thickness in one dimension. Due to their highly anisotropic shape, they offer favorable optical properties such as narrow emission linewidths and large absorption cross-sections.^1,2 However, as-synthesized, NPLs exhibit poor photo- and chemical stability. Thus, strategies have been sought to improve their properties by adding a shell on the NPLs. For spherical quantum dots, recently developed recipes for growing high-quality shells are performed at high temperatures. However, to date, this strategy has not been extended to NPLs because of their low thermal stability compared to quantum dots.³ Here, we present a method for obtaining CdSe/CdS core/shell NPLs in which the shell is added at high temperatures (~300 °C).⁴ This enables the growth of uniform and thick CdS shells, which is not possible with existing continuous-growth protocols. We further extend this protocol to produce alloyed Cd_xZn_1-xS shells with varying composition and thickness. We obtain high-quality monodisperse and stable core/shell NPLs with narrow emission linewidths, high QYs exceeding 70-80%, and suppressed blinking. Such samples exhibit tunable emission peaks that can result in improvements for a wide range of applications in optics and optoelectronics relying on efficient and narrow emitters.

1)  S. Ithurria et al., Nat. Mater., 10, 936 (2011)

2)  A. Yeltik et al., J. Phys. Chem. C, 119, 26768 (2015)

3)  A. Riedinger et al., Nat. Mater., 16, 743 (2017)

4)  A. A. Rossinelli et al., Chem. Commun., 53, 9938 (2017)

One of the most attractive areas of research in Nanoscience is the combination of 2D materials through van der Waals forces to form heterostructures (Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. Science 2016, 353, DOI: 10.1126/science.aac9439). Here, we present strategies beyond dispersion forces to interface 0D (molecules) and 2D materials.

We will show examples of 0D-2D heterostructures connected through covalent and noncovalent chemistry. In the first example, we will show a method to functionalize graphene covalently with exquisite atomic selectivity and yield (NanoLett. 2016, Chem. Commun. 2017). In the second example, we will show that the photoresponse of MoS2 photodetectors can be enhanced by simple and reversible supramolecular functionalization with organic dyes (Chem. Commun. 2016). We will also show the example of a naturally occurring van der Waals heterostructure, franckeite, an air stable, p-doped semiconductor with photoresponse in the far IR (Nat. Comms. 2017). Finally, we will present strategies for the self assembly of liquid-phase exfoliated 2D materials from suspension into functioning electronic devices (Chem. Commun. 2017 and Nanoscale 2018).

Our constantly increasing society’s need for energy has triggered a pressing need for the development of new materials for renewable sources. Concerning materials for energy harvesting, the most promising approaches for high-performance and low-cost photovoltaics rely in inhomogeneous compounds, such as perovskites and polycrystalline thin films (e.g. CIGS and CdTe). Thus, resolving their electrical and optical behavior at the nanoscale is imperative to advance their understanding. In this talk, I will share our scientific findings to image and quantify the local voltage response of nano- and mesoscale inhomogeneities in perovskites [1,2], CIGS [3], and CdTe through a variant of KPFM and NSOM [4-6]. By submitting the samples to illumination and humidity treatments under controlled conditions, we map the dynamic physical behavior of MAPI and triple-cation perovskites.

References:

[1] J. M. Howard et al. J. Phys Chem Letters, in press (2018)

[2] J. L. Garrett et al. Nano Letters 17, 2554 (2017).

[3] E. M. Tennyson et al. ACS Energy Letters 1, 899 (2016).

[4] E. M. Tennyson et al. ACS Energy Letters, 2, 2761 (2017). Invited Review

[5] E. M. Tennyson et al. ACS Energy Letters 2, 1825 (2017). Invited Perspective

[6] E. M. Tennyson et al. Advanced Energy Materials 5, 1501142 (2015).

Scattering-type scanning near-field optical microscopy (s-SNOM) has emerged as a key technology to gain chemical information, study structural properties and observe charge carrier distributions on the 10 nm length-scale. s-SNOM employs a metallic AFM tip which is illuminated to create a 10 nm small optical hot-spot at the tip apex independent from the wavelength of incident light. The concentrated light in the optical hotspot interacts with the sample surface below the tip and is modified by the local dielectric properties (absorption, reflection) of the sample. Detection of the elastically scattered light simultaneous to AFM imaging yields near-field images and broadband near-field spectra (nano-FTIR) with <20 nm spatial resolution. [1]

Using infrared s-SNOM imaging, the free carrier distribution in Bi2Se3 plates could be determined and it could be revealed that polyol-synthesized plates feature a non-uniform distribution of Se vacancies which cannot be cured by post-annealing. CVD-grown Bi2Se3 plates on the contrary don’t show such inhomogeneities. [2]

Furthermore s-SNOM images, recorded using mid-IR light, reveals the highly symmetric optical pattern of solvothermally grown Sb2Te3 hexagonal nanoplatelets. The superordinate optical patterns of the spiral growth patterns are shown to represent domains of distinct electronic properties, resulting from so-called growth twins with different densities of antisite defects. [3]

References:

[1] F. Huth et al., Nano Lett. 2012, 12, 3973

[2] X. Lu et al., Adv. Electron. Mater., 2018, 4, 1700377

[3] B. Hauer et al., Nano Lett., 2015, 15 (5), 2787

Colloidal two-dimensional (2D) tin (II) sulfide (SnS) nanocrystals are now emerging as potential and promising nanomaterials for electronic and optoelectronic applications. This is due to lower toxicity compared to other metal chalcogenides, such as PbS, PbSe, CdS, CdSe. We present a new and simple method for the preparation of colloidal 2D SnS nanosheets with large size, tunable thickness and single-crystallinity. The synthesis is performed by using tin (II) acetate as precursor to replace the common used tin halides (e.g. tin chloride) and harmful precursor (bis[bis(trimethylsilyl)amino] tin(II). The lateral size of synthesized square nanosheets can be tuned from 150 nm to 500 nm, and the thickness in a range of 25 to 30 nm. In addition, hexagonal shaped SnS nanosheets can also be synthesized (lateral size: 200-1700 nm, thickness: 15-50 nm). We control the shape and size by varying the amounts of ligands and precursors, which is also supported by DFT simulations. The crystal phase can also be optimized from the mixture of pseudotetragonal structure (PT, from nanoparticle byproducts) and orthorhombic structure (OR, from main product nanosheets) into single-crystalline OR structure. The optoelectronic measurements show their impressive conductivity and highly sensitivity to light. These materials are thus promising regarding electronic and optoelectronic applications.

Two-dimensional metal chalcogenide (CdSe, PbSe, PbS) materials were synthesized by processing from solution. Typical thicknesses range from one to more than ten nanometers. We studied the photogeneration, mobility and decay dynamics of charge carriers and excitons in: 1) CdSe nanosheets, 2) superlattices of connected PbSe QDs with square or honeycomb geometry, and 3) PbS nanosheets. The studies were performed using ultrafast pump-probe laser spectroscopy with optical or terahertz conductivity detection.

The composition and nanogeometry of the material were found to have pronounced effects on the relative yield of free mobile charges and neutral excitons. The relative yield of excitons was found to increase with excitation density. This effect could be described on the basis of the Saha equation, which accounts for more charge recombination at higher photoexcitation density.

The mobility of charge carriers depends strongly on the nanogeometry and material composition. In PbSe honeycomb superlattices mobilities are of the order of 1 cm2/Vs, while in square superlattices and PbS nanosheets values as high as a few hundred cm2/Vs were found. The frequency dependence of the mobility could be described theoretically by the Drude-Smith model, which includes effects of charge scattering on phonons as well as static defects.

Colloidal CdSe nanoplatelets (NPLs), also known as colloidal quantum wells, exhibit interesting properties such as narrow emission lines and atomic controlled thicknesses. To selectively tune the electrical and optical properties of these highly interesting nanomaterials, several methods have been developed to yield either core/shell or core/crown NPLs. Moreover, it has recently been shown that those NPLs can also be converted into nanorings with a toroidal topology.¹ It is hypothesized that this new geometry exhibits interesting properties such as the presence of magnetoexcitonic states and terahertz absorption. Until know, not much knowledge has been obtained about this new geometric shape of CdSe.

In our research, we optimize the synthesis procedure of NPLs and subsequently etch the NPLs to obtain the characteristic ring-like geometry of nanorings. To reveal information about the obtained particles, we use a wide range of techniques such as time-resolved PL spectroscopy, single dot spectroscopy, AFM and TEM. From these experiments, the electrical and optical properties can be determined. As such, we take the first steps to extend the knowledge about this unexplored geometry of CdSe.

1.             Fedin, I.; Talapin, D. V., Colloidal CdSe Quantum Rings. J Am Chem Soc 2016, 138 (31), 9771-4.

Nanocrystal (NC) solids are commonly prepared from non-polar organic suspensions, with preservation of the original capping. On the other hand, NC superstructures with a direct crystalline connection between the NCs have been reported too. Here, we present large-scale uniform superstructures of attached PbSe NCs with a silicene-type honeycomb geometry, resulting from solvent evaporation under nearly reversible conditions. Those superstructures were prepared by self-assembly of the nanocrystals on a liquid substrate. The nanocrystals are subsequently oriented attached to form a superstructure with preferred geometry. Besides  the hoenycomb lattice, we also prepared multilayered silicene honeycomb structures by using larger amounts of PbSe NCs. Using HAADF-STEM tomography we show that the bilayer and multilayered silicene structures of attached PbSe nanocrystals form slices of the simple cubic superstructure. We describe the disorder in the silicene lattices in terms of the nanocrystals position and their atomic alignment. The silicene honeycomb sheets are large enough to be used in transistors and opto-electronic devices.

Semiconducting monolayers hold many promises for the development of optoelectronic and spintronic devices, but are comparatively less explored for light harvesting. This situation is partly due to the lack of adequate materials: current production strategies (exfoliation and epitaxial growths) are limited to explore the wide range of potential two-dimensional (2D) materials for light conversion applications. Using colloidal chemistry to synthesize monolayers opens up the possibility to achieve an unprecedented control over the size, edges nature, composition and structure of the nanosheets. This control would allow to precisely engineer their electronic and optical properties to produce highly efficient monolayer-based nanomaterials for light conversion applications.

Using tungsten disulfide as a prototype material, we explored the transition metal dichalcogenide (TMDC) colloidal synthesis, through a strategy relying on precursors decomposition in high boiling point organic solvent. After having identified conditions to reproducibly synthetize colloidal WS₂ monolayers in the metallic 1T crystal structure,¹ we systematically modify the reaction parameters. The obtained protocols allow us to tune the crystal structure as well as the shape and the aggregation of WS₂ nanosheets. These first steps demonstrate the potential of colloidal synthesis to prepare TMDCs nanosheets of controlled crystal structure, shape and thickness. Other materials and alloys can also be prepared through this approach, as recently demonstrated by other groups for WSe₂² and Mo_xW_1-xS₂.³

(1)           Mahler, B.; Hoepfner, V.; Liao, K.; Ozin, G. A. J. Am. Chem. Soc. 2014, 136, 14121.

(2)           Jung, W.; Lee, S.; Yoo, D.; Jeong, S.; Miró, P.; Kuc, A.; Heine, T.; Cheon, J. J. Am. Chem. Soc. 2015, 137, 7266.

(3)           Sun, Y.; Fujisawa, K.; Lin, Z.; Lei, Y.; Mondschein, J. S.; Terrones, M.; Schaak, R. E. J. Am. Chem. Soc. 2017, 139, 11096.

Following the discovery of graphene, a vibrant research area on two-dimensional (2D) transition metal chalcogenides (TMDs) layered materials has emerged in recent years due to their exciting and diverse properties.^1-4 The existing approaches to make TMDs are mainly exfoliation, substrate growth and colloidal synthesis. Among them, colloidal wet-chemistry methods are particularly promising as the as-synthesized colloidal dispersions are directly fit for solution-based processing, opening a gateway for a straightforward and cost-efficient introduction into various technology platforms. However, compared to the other two preparation schemes, colloidal synthesis is relatively underdeveloped with only a few examples being known.^2-4

In this contribution, we describe the preparation of WSe₂ nanocrystals using a hot injection colloidal synthesis in the presence of a capping ligand as the solvent. We find that the choice of capping ligands influences the shape of the nanocrystals to great extent. Nano-flowers were obtained in the presence of oleic acid as capping ligand, whereas, nanosheets were obtained in the presence of oleylamine. Moreover, modification of the reaction temperature allowed to control the size of the nanocrystals. A thorough investigation of the reaction yield by means of quantitative XRF analysis enabled us to rationalize the reaction process. This synthesis strategy might provide a versatile approach to synthesize a wide range of TMDs such as WS₂ and MoSe₂.

[1] Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., ... & Firsov, A. A. (2004). Electric field effect in atomically thin carbon films. Science, 306(5696), 666-669.

[2] Mahler, B., Hoepfner, V., Liao, K., & Ozin, G. A. (2014). Colloidal synthesis of 1T-WS₂ and 2H-WS₂ nanosheets: applications for photocatalytic hydrogen evolution. Journal of the American Chemical Society, 136(40), 14121-14127.

[3] Jung, W., Lee, S., Yoo, D., Jeong, S., Miró, P., Kuc, A., ... & Cheon, J. (2015). Colloidal synthesis of single-layer MSe₂ (M= Mo, W) nanosheets via anisotropic solution-phase growth approach. Journal of the American Chemical Society, 137(23), 7266-7269.

[4] Jin, H., Ahn, M., Jeong, S., Han, J. H., Yoo, D., Son, D. H., & Cheon, J. (2016). Colloidal single-layer quantum dots with lateral confinement effects on 2D exciton. Journal of the American Chemical Society, 138(40), 13253-13259.

We have developed new families of highly ordered two dimensional covalent organic framework (2D-COF) materials. The materials have a hexagonal aromatic backbone with nanopores with controlled sizes that can be functionalized with any number of functional groups. We make them in high yields from condensation reactions from easy to obtain starting materials that in some cases produce very sharp x-ray diffraction patterns as well as showing triangular shaped crystallites with sharp diffraction spots in the TEM indicating a high degree of order. Single metal binding sites can also be designed within the framework structures. We have initially concentrated on putting charged functional groups in the pores to use as membrane materials and have demonstrated very specific ion charge and size selectivity with membranes made from these 2D-COFs. We have also produced 2D-COF materials that have less order but selectively absorb and release hydrogen at near room temperature. Other applications such as size exclusion filtration will also be shown.

In recent years, monolayers of transition metal dichalcogenides have attracted a great deal of research interest. These two-dimensional (2D) materials typically have direct bandgaps in the visible or near-infrared, making them attractive for both classical and quantum optoelectronic applications [1]. Many different techniques have been developed to isolate monolayers from bulk; among them liquid assisted exfoliation is preferred by many, as it is simple, scalable, convenient and cost-effective [2]. However, the flakes produced using this class of technique are often not suitable for many applications, having poor uniformity and optical properties due to unintentional doping and other impurities.

In this work we employ ultrasonic bath sonication of bulk MoS₂, starting with powders of grain sizes ranging from 6 to 40 µm, in an isopropanol/water mixture (70/30 vol %), following the procedure according to [3]. This produces suspensions of few-layer MoS₂ flakes. Photoluminescence (PL) measurements from drop-cast samples on silicon substrates are found to have full widths at half maxima of 200 meV, indicating an average thickness of 2-3 layers. This agrees with independent atomic force microscopy measurements [4]. However, PL intensities are found to be weak – a typical result for flakes produced with this technique.

Super acid treatment of dry mechanically-exfoliated monolayers of MoS₂ has been shown to vastly improve internal quantum efficiency, and with it, PL intensity [5] But it impractical to implement in a manufacturing process. In this work we will detail a new all-wet superacid treatment process for MoS₂ flakes. We show that it enhances PL emission intensity by over 200 times. It thus represents a promising technique for the practical application of 2D inks for optoelectronics.