Until recently, originating from covalent nature in crystal bonding and the surface dangling bond as well as oxophillic nature of the element, synthesis and surface chemistry of III-V (InP, InAs, InSb) nanocrsytals are lagging behind compared to other ionic nanocrystals. In this presentation, I will present two specific challenges in III-V nanocrystal synthesis. First, we present covalent tetrapod InAs and InP nanocrystals as a crystalline “late intermediate” that warrants atomically controlled colloidal nanocrystal growth. We present the use of the late intermediate with well-defined facets at the sub-10 nm scale for directional growth with atomic control and highlight the potential for the new directed approach of nanocrystal synthesis. Second, we present a route of synthesizing highly mono-dispersed InAs colloidal quantum dots in wide size-range by continuously introducing amorphous InAs nanoclusters as a single-source precursor on nanocrystal seeds. Our synthesis using InAs nanoclusters as a source of monomer resulted in induced focusing otherwise challenging with conventional precursors used for III-V quantum dot synthesis. Further, using a modified Fick’s law,, we find that the growth of InAs QD is mainly dominated by the monomer diffusion dynamics and a continued precursor injection reduce the effective monomer concentration in the solution, lead to a decline of the monomer concentration gradient thus slow down the growth. Diffusion-dynamics-controlled chemical process finally provides a route of creating highly monodispersed InAs colloidal quantum dots enabling short-wavelength IR photon over 1700 nm.¹

Colloidal lead halide perovskite nanocrystals (LHP NCs_, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) have become a research spotlight owing to their spectrally narrow (<100 meV) fluorescence, tunable over the entire visible spectral region of 400-800 nm, as well as facile colloidal synthesis. These NCs are attractive single-photon emitters as well as make for an attractive building block for creating controlled, aggregated states exhibiting collective luminescence phenomena. Attaining of such states through the spontaneous self-assembly into long-range ordered superlattices (SLs) is a particularly attractive avenue. In this regard also the atomically-flat, sharp cuboic shape of LHP NCs is of interest, because vast majority of prior work had invoked NCs of rather spherical shape. Long-range ordered SLs with the simple cubic packing of cubic perovskite NCs exhibit sharp red-shifted lines in their emission spectra and superfluorescence (a fast collective emission resulting from coherent multi-NCs excited states). When combined with spherical dielectric NCs, perovskite-type ABO₃ binary NC SLs form, wherein CsPbBr₃ nanocubes occupy B- and/or O-sites, while with spherical dielectric Fe₃O₄ or NaGdF₄ NCs reside on A-sites.¹ When truncated-cuboid PbS NCs are added to these systems, ternary ABO₃-phase form (PbS NCs occuoy B-sites). Such ABO₃ SLs, as well as other newly obtained SL structures (binary NaCl- and AlB₂-types, columnar assemblies with disks etc.), exhibit a high degree of orientational ordering of CsPbBr₃ nanocubes. These mesostructures exhibit superfluorescence as well, characterized, at high excitation density, by emission pulses with ultrafast (22 ps) radiative decay and Burnham-Chiao ringing behaviour with a strongly accelerated build-up time.

The rational design and development of new protocols for the colloidal synthesis of multicomponent nanocrystals (NCs) represent an important research direction to expand the functionalities of single-component counterparts¹. In these systems, the functionality of the composite NCs is ultimately determined by the atomic interactions between constituent domains, which essentially relies on the precise and deliberate control of the NC architecture. In quest of developing advanced functional NCs, the design of the nanostructures has become quite sophisticated. However, a long-standing barrier has been the development of simple and cost-effective aqueous approaches with a fine adjustment of the final composition, location engineering, dimensionality, and surface structure of each individual domain. We present the model case of noble metal/CeO₂ NCs multicomponent NCs and their hollow counterparts^2-4. The combination of noble metal and CeO₂ domains in a well-defined architecture represents the possibility to tune the optical and catalytic properties of the final structure, thereby presenting interesting applicability in biomedicine, catalysis and sensing^5-6.

Nanocrystals of intermetallic compounds are a large family of emerging materials with highly promising catalytic, plasmonic, magnetic, and energy storage and conversion applications. However, generalized synthetic approaches for intermetallic nanocrystals are lacking. Here we report the development of a colloidal synthesis based on amalgamation of monometallic nanocrystal seeds with low-melting point metals. We use this approach to achieve crystalline and compositionally uniform intermetallic nanocrystals of Au-Ga, Ag-Ga, Cu-Ga, Ni-Ga, Pd-Ga, Pd-In and Pd-Zn compounds. We demonstrate a compositional tunability across phases (e.g., AuGa2, AuGa, Au7Ga2, and Ga-doped Au nanocrystals for the bimetallic Au-Ga system), while each phase can be prepared with accurate size control and excellent size uniformity. TOC Graphic shows monodisperse and compositionally uniform Pd-bimetallic nanocrystals, illustrating the applicability of amalgamation seeded growth towards intermetallic nanocrystals with unprecedented quality and flexibility of materials design. Our new synthetic method is simple, predictive, and generalizable, giving access to a large family of size and composition controlled intermetallic nanocrystals, thus opening up a multitude of possibilities for these materials.

This work has been accepted to the publication (Science Advances) on 20. May 2021.

Lead halide perovskite has attracted intensive research interests for its optoelectronic properties in the recent years, with applications in light emission, energy harvesting, solid state lasing, etc. One particular example is the quantum-confined single-halide CsPbBr₃ perovskite nanoplatelets which is favourable for blue LEDs because of sharp emission linewidth whilst keeping phase stability [1,2]. However, their traditional batch synthesis approach faces problems of reproducibility and small production capacity [3].

In this work, we present the reproducible and size controlled continuous synthesis of CsPbBr₃ nanoplatelets with sharp blue emission. Novel 3D reactor geometries are designed to precisely control mass transfer during the synthesis. Supported by computational fluid dynamics studies and on-line photoluminescence and absorption spectroscopy, we showed that enhanced mixing efficiency leads to more homogenous nucleation and hence smaller particle size. Using different reactors, CsPbBr₃ nanoplatelets thickness can be tuned between 2.2 and 3.3 nm showing that mixing is important in both nucleation and growth steps. In this way, the photoemission can be effectively tuned from 500 to 472 nm. The processing rate of a single reactor is as high as 8.02 g per day of nanoplatelets and can be simply multiplied by setting up parallel reactors.

1D heterostructure NCs are typically synthesized through colloidal methods, solution-liquid-solid growth (SLS), solution-solid-solid growth (SSS), and cation exchange processes. However, the transformation of 1D heterostructures to alloyed multi-element NCs requires cation diffusion in NC templates or seeds using complex ligand-cation interactions and high annealing temperatures. A facile approach and mechanistic insights into the transformation of heterostructures especially for metal-semiconductor heterostructure to alloyed multi-element NCs is needed. Here we demonstrate the combination of metallic seeded (SLS) and cationic diffusion mediated growth in a conventional hot-injection system for colloidal Cu-Bi-Zn-S nanorods (NRs) formation from Bi-Cu_2-xS heterostructures. The transformation of the Bi seed and Cu_2-xS stem into ternary metal sulfides and materialization of a transitional segment at the heterointerface lead to the formation of the tri-segmented heterostructure NCs with Bi_xCu_yS_z phases. The NR evolution is driven by the dissolution of the Bi-rich seed and recrystallization of the Cu-rich stem into the transitional segment, followed by the incorporation of Zn²⁺ to form the quaternary Cu-Bi-Zn-S NRs. This study highlights the importance of the integration of seeded growth into colloidal chemistry to access new multi-element alloyed NRs. The present study also revealed the variation of Zn concentration in the NRs modulates the aspect ratio and affects the nature of majority charge carriers. The NRs exhibit promising thermoelectric properties with significantly low thermal conductivity values of 0.45 W/mK and 0.65 W/mK at 775 K and 605 K respectively for Zn poor and Zn rich NRs.

Lead halide perovskite (LHP) nanocrystals are the rising star in the quantum dots field.[1] Their remarkable optical properties made them a promising material for several applications such as lighting and LEDs.[2] Although chemists can produce high quality LHP nanocrystals, deep understanding of the formation mechanism underlying this material is still limited, thus hampering the research on this topic. The main drawbacks to a complete and deep investigation of this process are: the fast reaction kinetics, the extensive range of by-products, and the high temperatures required for the synthesis. These reasons substantially hamper the possibility to use conventional characterization techniques such as, for example, ex situ absorption. In this regard, there are few reports on the formation mechanism of LHP nanocrystals but either only via indirect measurements or in experimental conditions far from lab standards.

Recently synchrotron-based X-ray scattering techniques have proven to be a valid platform for the study of the formation mechanism of nanocrystals.[3] The high time resolution and the high signal-to-noise ratio, combined with the possibility to probe the average nanocrystal population, in situ, in chemical environments identical to those used in the labs, promotes this technique as an ideal candidate for the study of LHP nanocrystals.

In order to shed light on the formation mechanism of LHP nanocrystals, we performed synchrotron-based time-resolved in-situ small- and wide-angle X-ray scattering (SAXS & WAXS) measurements on forming nanocrystals in a custom-made three neck flask. The high time resolution (~ ms), combined with the high brilliance of synchrotron radiation, allowed us to follow the formation of the nanocrystals from nucleation to full growth. In particular, we were able to extract the evolution of particle size distribution and concentration throughout the synthesis. The extracted information are combined to give full description of the formation process, allowing us to propose a formation mechanism and a path toward the production of monodispersed LHP nanocrystals.

Thickness and anisotropy of nanoscale materials has been shown to strongly influence photophysical properties. For example, reducing the thickness or number of crystalline layers improves photocurrent collection efficiency of many semiconductors. While different exfoliation strategies for 2D materials have been proposed, many can cause damage or chemical modifications, and control over size and aggregation can be problematic. In this talk, templating the synthesis of semiconductors using colloidal particles that are anisotropic in both shape and surface charge is presented as a means to achieve 1D and 2D nanomaterial hybrids. Initial studies on shape-templating the growth of gold nanoparticles on layered silicate clays [1]  led to the colloidal synthesis of ultrathin layers of semiconducting materials such as MoS₂[2], In₂S₃, [3] and ZnIn₂S₄ [4] in the organically-modified interlayer space. In the case of Aurivillius-type perovskite Bi₂MoO₆, self-assembly of template particles with dianionic precursors results in micron-scale hybrids with a switched crystalline growth direction, providing optimal surface facets for molecular adsorption [5]. These nanohybrids show promising enhancements in catalytic and photophysical properties, and current directions in ‘facet engineering’ and colloidal synthesis of 1D semiconductor wires in tubular templates continue to keep us on our toes.

Quantum dots have emerged as technologically relevant materials for efficient optoelectronic devices, such as LED televisions and solar cells. However, they are often unstable as thin films or as inks in the environment or during their processing.

In this talk, I will discuss how oxide shells can help to overcome such issues. I will focus on our recently developed colloidal atomic layer deposition (c-ALD) approach to grow tunable alumina shells around quantum dots.[1] I will briefly compare the c-ALD with the previously developed gas-phase ALD in film.[2] This newly developed synthesis has the advantage of preserving the colloidal stability of the nanocrystalline core while controlling the shell thickness from 1 to 6 nm.[1] I will describe the details on how the aluminum precursor interacts with the surface of the QDs during the nucleation step and how the shell eventually grows. Finally, I will also discuss some of our recent founding on the impact of a thin metal oxide shell on the dynamicity of the quantum dots ligands.

Wet chemical synthesis is a very versatile method to obtain Au and Ag particles with an astonishing shape diversity, largely through the use of various additives, each of them fulfilling one or several roles. In this framework, I will focus on bimetallic Au/Ag nanoparticles, whose properties are rendered even more diverse by the combination of the two constituents. For instance, depositing Ag on pentatwinned Au bipyramids (AuBPs) is an efficient way to form Ag nanorods (AgNRs) with controllable aspect ratio which can be increased significantly upon controlled Ag addition, shifting LSPR from the visible to the infrared region. In a first work, we have shown an unexpected double role of ascorbic acid in this synthesis by using a combination of time resolved techniques.(1) AgNRs were prepared in which the gold bipyramid was located at the center of mass of the NPs. In a recent work, we developed a method in mixed solvent to break the inversion symmetry of the AgNRs by displacing the seed position at one tip of the AgNR. We propose a mechanism to understand the Ag deposition on the gold surface which is supported by experimental evidence and molecular dynamic (MD) simulations.(2)

Colloidal semiconductor nanocrystals, also called quantum dots (QDs), have exceptional optical properties, such as high absorption cross section and quantum yield, as well as emission spectra that are tunable by changing their size, shape or composition. They are finding numerous applications eg. in displays, imaging… Recently, our group reported the plasmonic coupling of single QD emitters embedded in silica coated with a gold nano-shell with a Purcell factor of 6. The resulting emitters showed enhanced photostability and reduced blinking rates^[1]. Here we explore the optical properties of similar objects containing not one but hundreds of QDs in their core. We first synthesize CdSe/CdS/ZnS core/multishell QDs and assemble them into aggregates of controlled sizes (typically 230 +/- 60 nm in diameter) by emulsion/evaporation. The aggregates are then coated with a ca. 10 nm silica shell, functionalized and coated with gold seeds. Various thicknesses of continuous gold shell are then grown using reduction of gold salts in solution.

We have investigated the optical properties of QD assemblies with and without gold nanoshells. These objects display high quantum efficiency, stable and Poissonian emission at room temperature. In addition, enhancement of the photoluminescence decay rate through Förster resonance energy transfer (FRET) is observed from bluer to neighboring redder QDs within the assemblies^[2]. We show that polyvinylpyrrolidone can be used to tune the gold reduction rate and the morphology of the gold nano-shell. We are currently examining the coupling effects of the emission from the QD assembly with the gold nano-shell plasmonic resonator.

The chemistry of non-noble metal nanocrystals (NCs) is far less advanced compared to noble metals. Yet, tuning their composition, size and shape is important for various applications, spanning from plasmonics to catalysis.

In this talk, I will present our recent group efforts towards the synthesis via colloidal chemistry of Cu NCs and Cu-based NCs.

First of all, I will focus on our studies on nucleation and growth. A fundamental understanding of the formation mechanisms is indeed crucial to rationally approach the design of new or underexplored classes of materials. Here, in-situ investigations by X-Ray spectroscopies and scattering allowed us to identify the key reaction intermediates and to direct the growth towards shaped-controlled Cu NCs.

I will then illustrate how these NCs can help to identify selectivity rules at the branching nodes which lead to C₁ and C₂₊ reduction products in the challenging electrochemical CO2 reduction reaction. I will conclude by sharing our latest results which illustrate that the above discussed catalysts are not only model systems but can be implemented in a gas-fed electrolyzer and sustain the same selectivity at technologically relevant conditions with currents up to 300 mA/cm².

Photocatalysis is a pathway to direct conversion of CO₂ to CO, one step within light-powered reaction networks that could, if efficient enough, transform the solar energy conversion landscape. To date, the best performing photocatalytic CO₂ reduction systems operate in nonaqueous solvents, but technologically viable solar fuels networks will likely operate in water. In this talk, we demonstrate photoreduction of CO₂ to CO in pure water at pH 6-7 with an unprecedented combination of performance parameters: turnover number >80,000, quantum yield >5%, and selectivity >99%, using CuInS₂ colloidal quantum dots (QDs) as photosensitizers and a Co-porphyrin catalyst. The performance of the QD-driven system greatly exceeds that of the benchmark aqueous system due primarily to: (i) electrostatic attraction of the QD to the catalyst, which promotes fast multielectron delivery and co-localization of protons, CO₂, and catalyst at the source of photoelectrons, and (ii) termination of the QD’s ligand shell with free amines, which capture CO₂ as carbamic acid that serves as a reservoir for CO₂, effectively increasing its solubility in water, and lowers the onset potential for catalytic CO₂ reduction by the Co-porphyrin. The breakthrough efficiency achieved in this work represents a non-incremental step in the realization of reaction networks for direct solar-to-fuel conversion.

Semiconductor nanocrystals have attracted great interest for color conversion and enrichment in quality lighting and displays. Optical properties of these solution‑processed nanostructures are conveniently controlled by tailoring their size, shape and composition in an effort to realize high‑performance light generation and lasing. These colloids span different types and heterostructures of semiconductors in the forms of quantum dots and rods to the latest sub‑family of nanocrystals, the colloidal quantum wells (CQWs). In this talk, we will introduce the emerging field of semiconductor nanocrystal optoelectronics with most recent examples of their photonic structures and optoelectronic devices employing such atomically‑flat, tightly‑confined, quasi‑2‑dimensional CQWs, also popularly nick‑named ‘nanoplatelets’. Among various extraordinary features of theirs, we will show that these CQWs enable record high optical gain coefficients [1] and can achieve gain thresholds at the level of sub‑single exciton population per CQW on the average [2], empowered by carefully engineering their heterostructure [3]. Next, we will present a new, powerful, large-area self‑assembly tool for orientation‑controlling of these nanoplatelets [4], which provides us with the ability to tune and master their excitonic properties in their ensemble as well as the level of achievable energy transfer among them and with other nearby species. Using three‑dimensional constructs of face‑down self‑assembled slabs of CQWs with monolayer precision, we will demonstrate ultrathin optical gain media and lasers of these oriented CQW assemblies [5]. Finally, we will show record high‑efficiency colloidal LEDs using CQWs employed as the electrically‑driven active emitter layer [6] and record low-threshold solution lasers using the same CQWs employed as the optically‑pumped fluidic gain medium [7]. Given their current accelerating progress, these solution‑processed quantum well materials hold great promise to challenge their epitaxial thin‑film counterparts in semiconductor optoelectronics in the near future.

Hybrid perovskites like (C₄H₉NH₃)₂PbI₄ have fascinating layered crystal structure with periodic nanoscale interfaces between the inorganic {PbI₄}^2- and organic C₄H₉NH₃⁺ layers. Because of these interfaces, electron and hole are confined in atomically thin {PbI₄}^2- inorganic well layers. Therefore, these layered perovskites are considered as electronically 2D systems, irrespective of their crystallite sizes.^1,2 Importantly, the crystal structure is flexible, allowing a number of combinations of different organic cations and inorganic anions. So a rational design of the nanoscale interfaces, and hence, tunable optoelectronic properties are feasible. For example, excitonic binding energy can be controlled over an order of magnitude from a few tens of meV to a few hundreds of meV, with simple variation of composition of organic cations. So for solar cell and photocatalytic applications, one can choose the composition with lower excitonic binding energies, whereas for LED, higher excitonic binding energy is preferred. Interestingly, chiral organic cations can impart optical non-linearity and chiral optoelectronic properties. In this talk, I will discuss about how controlling nanoscale interface between organic and inorganic layers can yield interesting optical and optoelectronic properties.³ But note that the nanoscale properties will be discussed using millimeter sized single crystals.

Light-emitting field-effect transistors (LEFETs) are an emerging type of optoelectronic devices which combine electrical switching and light emission. Application of lead chalcogenide colloidal quantum dots (CQDs) in LEFETs allows to tune the emission of these devices continuously in the near-infrared region, reaching spectral regions which are impossible to reach with other solution-processable materials. In a recent work, we have demonstrated the first fully solid PbS CQD based LEFET showing an electroluminescence (EL) quantum efficiency of 1.3×10⁻⁵ at room temperature and about 1% below 100 K. In this work, we present devices exhibiting an order of magnitude higher EL quantum efficiency, obtained by using an active material comprising two sequentially deposited layers, the first of PbS CQDs, the second of polymer-wrapped semiconducting carbon nanotubes. The combination of these two materials results in well-balanced ambipolar transport and high charge carrier mobility of about 0.2 cm²/Vs for both electrons and holes. Maximum EL external quantum efficiency of 1.2×10^-4 is achieved at room temperature.

Broadening of multi-exciton emission from colloidal quantum dots (QDs) at room temperature is important for their use in high-power applications but in-depth characterization has not been possible until now. We present and apply a novel spectroscopy method to quantify the biexciton linewidth and biexciton binding energy of single CdSe/CdS/ZnS colloidal QDs at room temperature. In our method, which we term “cascade spectroscopy”, we select emission events from the biexciton cascade and reconstruct their spectrum. The biexciton has an average emission linewidth of 83 meV on the single-QD scale, similar to the exciton. Variations in the biexciton repulsion (4.5 ± 3.1  meV; mean biexciton binding energy ± standard deviation of 15 QDs) are correlated with but more narrowly distributed than variations in the exciton energy (10.5 meV standard deviation). Using a simple quantum-mechanical model we conclude that inhomogeneous broadening in our sample is primarily due to variations in the CdS shell thickness.

It is well known that the optoelectronic properties of colloidal nanoplatelets are largely set by the strong electron-hole Coulomb attraction, which is enabled by the dielectric confinement and 2D geometry, and gives rise to large exciton binding energies or giant oscillator strength, to name a few effects.[1,2]

Much less is known about the role of electron-electron or hole-hole repulsions. Naturally, the same conditions that lead to strong attractions in nanoplatelets, should lead to strong repulsions as well. In combination with the weak lateral confinement, such repulsions can trigger severe electronic correlations, which are found neither in  nanocrystals (owing to the strong confinement) nor in bulk (owing to the weak Coulomb interaction).

In this work, we explore theoretically the opportunities of exploting Coulomb repulsions to engineer the band structure of colloidal nanoplatelets. We study type-I and type-II CdSe-based nanoplatelets charged with up to 4 electrons or holes. Several remarkable phenomena are then predicted

1) The giant oscillator strength effect, present for neutral excitons, vanishes for trions.

2) Addition energies exceeding 100 meV are required to introduce extra charges in the nanoplatelets, which implies that carriers can be injected electrochemically one-by-one at room temperature

3) High electron spin states are populated even at low temperatures, which provides multi-electron platelets with an enhanced magnetic moment and paramagnetic response.

4) In type-II core/crown nanoplatelets, large and reversible changes in the emission intensity and energy (~100 meV) can be achieved when switching from X2+ to X3- excitonic species.

5) Biexciton binding energies are highly tunable depending on the lateral dimensions of the platelet, from highly repulsive (40 meV) to neutral (~0 meV).

It is concluded that the charging of nanoplatelets with a few interacting electrons or holes  is a promising route to develop novel magnetic and optical functionalities.

Natural high carrier concentration and electron occupancy in the conduction band are remarkable properties of self-doped colloidal quantum dots (CQDs). It has been revealed that the silver chalcogenides and mercury chalcogenides CQDs have intraband transitions that are induced by the nature of self-doped CQDs. The self-doping property promotes the intraband transition but forbids the interband transition of the material. Thus, the self-doped CQDs are an applicable material that could carefully probe the electronic dynamics and energy structure in nanocrystals. Further understanding of the intraband transitions can extend the nanocrystal application.

The Ag₂Se CQDs is a fascinating MWIR active material that has a high potential for future application. The Ag₂Se CQDs intraband transition is reported for the first time by Prof. Jeong’s group in ACS photonics. A spectro-electro-chemistry (SEC) method has disclosed the intraband transition of Ag₂Se CQDs and proves that the interband transition of Ag₂Se CQDs exists at higher energy.  Interestingly, the silver selenide CQDs have revealed a broken degeneracy of the p-state. The Ag₂Se CQDs undergo the crystal structural transformation from cubic to tetragonal nanocrystal structure with increasing size. Accordingly, the corresponding degeneracy broken of the 1P_e states is optically observed along with the structural transformation. Strikingly, the Ag₂Se CQDs plasmonic character coexists with the steady-state intraband electronic transition, which indicates the quantum-plasmon characters in the self-doped Ag₂Se CQDs. Considering that the unsophisticated intraband transitions of self-doped CQDs can be achieved from the HgS CQDs. Thus, the HgS conduction band was thoroughly investigated with MWIR pump-probe spectroscopy. The pico-second time decay shows three decaying components: exciton lifetime, Auger recombination, and, interestingly, electron-vibration energy transfer (EVET) to surface ligands. The HgS decay shows unexpectedly fast Auger component ~ 1 ps and slow EVET process ~ 500 ps. The intraband transition study could elucidate the challenging observations such as blinking of QDs, excessive charging in QDs device, and hot electron dynamics in conduction band coupled to nanocrystals’ surface states.

Fully inorganic lead halide perovskites nanocrystals (NCs) are emerging as extremely interesting active materials for a wide variety of optoelectronic and photonic devices, due to their capability to combine easy synthesis in solution, high photoluminescence quantum yield, ultra-wide color gamut, very high optical gain at room temperature, simple deposition in thin films by using wet techniques and improved stability with respect to organic-inorganic NCs. Among these materials CsPbBr3 NCs are particularly interesting for light emitting devices in the visible, thanks to their bright emission in the green that candidates them as very promising semiconductors able to close the so called "green-gap" of semiconductors. A deep understanding of their photophysics is thus fundamental to properly understand the origin of the material active properties and provide strategies to improve them.

In this frame, a particularly powerful approach is the investigation of the temperature dependence of the Photoluminescence (PL) spectra and of the PL relaxation dynamics that allows to explore several features, like the interplay between radiative and non-radiative relaxation processes, the origin of the emitting states (free carriers or excitons), and the coupling with phonons.

To date the PL temperature dependence has been often used to determine fundamental quantities of CsPbBr3 NCs, like the LO phonon energy and the exciton binding energy, interestingly obtaining widely scattered, and inconsistent each other, values.

In this work we exploit the local morphology variations of a drop cast CsPbBr3 nanocrystals thin film to evidence that NCs aggregation has strong effects on the PL spectra peak wavelength, linewidth and intensity temperature dependence. We demonstrate that an analysis based on frequently used models in literature lead to completely different conclusions about the intrinsic NCs emission properties extracted from spectra measured in points with different contribution of the PL from the aggregates. A more careful analysis instead allows to ascribe the inconsistencies to the different contribution of NCs aggregates to the total PL and to determine in which conditions the PL is mainly due to the NCs and in which ones to their aggregates. Our results demonstrate that extreme care has to be used in order to correctly correlate the PL spectral features to the intrinsic emission properties of lead halide perovskites NCs films, and that the investigation of the local morphology is fundamental to avoid wrong conclusions.

A range of optoelectronic and photochemical applications have driven interest in CdS nanocrystals over the last few decades. Of particular note is the use of semiconductor nanocrystals in both photooxidative and photoreductive systems, where the high molar absorptivity of CdS nanocrystals, and the tunable redox potentials of the conduction and valence band can be utilized. In most photoreduction systems using Cd chalcogenide nanocrystals, an external chemical reductant is necessary to generate photocharged states. Here we present an intrinsic photocharging process in CdS nanocrystals without the need for an external reductant. We observe photocharging in CdS nanocrystals across a range of capping ligands, solvent environments, and nanocrystal morphologies. This photocharging process is mediated via hole transfer to the organic surface capping ligand (oleate or octadecyl phosphonate) which then subsequently dissociates. The remaining photoexcited electron persists for many minutes, and the photocharging process is entirely reversible over multiple illumination and recovery cycles. In contrast, nanocrystals with more tightly bound ligands with thiol binding moieties (eg. 3-mercaptopropionate) do not exhibit intrinsic photocharging. This process has significant implications for the photophysical study of CdS nanocrystals, a common model system, as photoreduced states can accumulate even under room lights. However, the long-lived nature of the photoexcited electron also provides a promising avenue for high efficiency electron transfer processes.

All-inorganic perovskite nanocrystals (NCs) are emerging as a new class of materials with advanced optical properties, depending on both the fabrication techniques and the diverse strategies of surface passivation. In particular, due to their intrinsic sensitivity to the ambient conditions (often inducing critical fast optical irreversible quenching) the development of novel strategies for effective surface stabilization is a current challenge. On the other hand, some results in literature evidence the presence of reversible environmental effects on the emission properties, that could open the way for sensing applications (for instance, atmosphere composition/pressure), if properly engineered.

Here, we present a study on the Amplified Spontaneous Emission (ASE) properties of lecithin-capped CsPbBr₃ NCs[1] and on their dependence on the environmental conditions. The NCs, deposited on quartz substrates and placed under two different air pressure (10^-1 mbar and atmospheric pressure), have ASE threshold in line with the state of art for similar materials. We demonstrate a clear environmental sensitivity of ASE consisting in a reversible and reproducible variation of the intensity under vacuum and ambient conditions (up to 20 %), against a variation of a few percent observed for the spontaneous emission.

We explain our results by an air-induced ASE quenching, followed by a sort of signal restoring assigned to the peculiar role played by the lecithin-based surface passivation. These results suggest the possibility to exploit the ASE environmental dependence for the development of high sensitivity optical gas sensors.

Halide perovskite nanocrystals (NCs) are a promising platform for light-emitting devices, including LEDs and single-photon emitters. Excitonic properties can be precisely tuned via the NC shape for improved device performance. Control of dimensionality is particularly powerful since excitons show dramatically different properties in lower-dimensional structures than 3D NCs. We explore the thickness-dependent fine structure of excitons in square Cs_n−1Pb_nBr_3n+1 nanoplatelets through time- and temperature-resolved photoluminescence measurements. In parallel, we build a two-dimensional effective-mass model of these excitons, which are weakly confined in-plane but strongly confined in the out-of-plane direction. The model introduces the critical effect of shape anisotropy on the band-edge Bloch functions and presents a new numerical approach for calculating long-range exchange interaction. The predicted fine structure is very different from that observed in 3D NCs and is in good agreement with observed spectral shifts. Notably, the bright excitons show a shape anisotropy-induced splitting of several meVs, meaning they cannot be considered a single degenerate state. Taking this into account, we expand the traditional two-level decay model to include both the in-plane-polarized and out-of-plane-polarized bright exciton states. Using the three-level model, we show that the experimentally determined decay rates are consistent with the calculated fine structure. The results of this work may be readily extended to other materials and NC shapes, and will help to unleash the full potential of dimensionality control in halide perovskite optoelectronics.

Organic-inorganic halide perovskite nanocrystals (PNCs) are gaining increasing attention in contemporary research due to their promising performance in light-emitting as well as solar technology. Photoexcitation of these PNCs with an ultrashort laser pulse can produce coherent phonons along the lattice displacement coordinate, leading to lattice vibrations. Here, we employ femtosecond pump-probe spectroscopy to initiate the photophysics by the formation of coherent phonons and to observe the subsequent coherent vibrational dynamics of highly luminescent formamidinium lead halide (FAPbX3, X=Br, I) PNCs.[1] We find that the FAPbX3 PNCs generate halide-dependent coherent vibronic wave packets upon non-resonant excitation, and the dominant contributions are attributed to the Pb–X bending and stretching modes of PbX64- octahedral units of the lattice. More importantly, for the first time, we observe higher harmonic vibrational modes in FAPbI3 PNCs, which points to a more anharmonic potential energy surface in the case of FAPbI3 as compared to FAPbBr3 PNCs. This is likely due to the weaker interaction between the central FA moiety (which sits in a larger octahedral interstitial site) and the inorganic cage for FAPbI3 PNCs, and thus the PbI64- unit can vibrate more freely (Fig. 1a). This weakening reveals the intrinsic anharmonicity in the Pb-I framework, and thus facilitating the energy transfer into overtone and combination bands (Fig. 1b). Furthermore, our control experiment with MAPbBr3 reveals the energy transfer between framework phonons due to the intrinsic anharmonicity of the lead-halide framework is indeed influenced by the interaction between the framework and the organic molecules, and not only by the halide nature. The insights interestingly not only unravel the underlying reason for the halide-dependent stability of these materials but also shed light on their charge-carrier mobility and light emission properties.

Colloidal metallic and magnetic nanocrystals (NCs) are known for their size- and shape-dependent optical and magnetic properties and their solution-based printing and imprinting in device fabrication. We use NCs as building blocks of assemblies and exploit their chemical and physical (electrical, optical, magnetic, mechanical, thermal) tailorability to design and fabricate optical metamaterials. Chemical exchange of the long ligands used in NC synthesis with more compact ligand chemistries brings neighboring NCs into proximity, increasing interparticle coupling. For metal NCs, ligand-controlled coupling allows us to tune through a dielectric-to-metal phase transition, seen by a 10¹⁰ range in DC conductivity and a dielectric permittivity ranging from everywhere positive to everywhere negative across the whole range of optical frequencies [1], and is useful in the design of materials that are strong, ultrathin film optical absorbers [2] or strong optical scatterers [1], [3]. Addition of magnetic NCs, allows actuation of magnetic or mixed magnetic-metallic NC superstructures by external fields [4], [5]. Compact ligand exchange and thermal annealing of NC films also drives a large volume shrinkage in NC thin films, allowing a 10X tailorability in their Young’s modulus [5], [6]. By juxtaposing plasmonic NCs and bulk materials, we exploit their different chemical and mechanical properties to create misfit strain that drives the folding of NC/bulk bilayer heterostructures and the transformation of lithographically-defined two-dimensional structures into three-dimensional structures [5], [6]. We use the three-dimensional structures to demonstrate the scalable fabrication of large-area metamaterials with chiroptical responses of ~40% transmission difference between left-hand and right-hand circularly polarized light and that are suitable broadband circular polarizers [6], [7].

HgTe is a special material, with a zero gap, an inverted band structure, low electron mass, singly degenerate conduction band, strong spin orbit coupling, and good stability in air.   Colloidal quantum dots can be made by simple protocols and size tuning provides full coverage for photodetection between 1 and 12 microns. These quantum dots are explored by a few groups and are still the only ink-based material to achieve background-limited thermal photon imaging.[1] With further refinement, HgTe colloidal quantum dots are poised to disrupt the very high cost/gram of infrared imaging, with exciting and new fabrication processes. The material also offers many topics of basic interest. (i) A joint effort with Dmitri Talapin explored films of 10% size distribution HgTe colloidal quantum dots and achieved bandlike mobility within the 1Se miniband.[2] The same but improved system may then be the most promising to search for delocalized transport in colloidal quantum dot solids. (ii) HgTe has a strong Te spin-orbit coupling and this splits the 1Se-1Pe intraband multiplet. The theory by Delerue and Allan introduces the new concept of the interplay between spin-orbit splitting and quantum dot shape and this can be explored with experiments.[3] (iii) Strong luminescence is the key to most colloidal quantum dot applications including detectors.  Bright visible quantum dots universally use the core/shell strategy long ago demonstrated in my group by Margaret Hines.[4] With mid-infrared quantum dots, theory predicts that multiphonon relaxation limits quantum yields to 1/1000 or 1/10000,[5]  and experiments are needed to test these predictions.

We have investigated theoretically the transport of electrons and holes in crystalline solids consisting of three-dimensional arrays of semiconductor nanocrystals passivated by two types of organic ligands, linear chain carboxylates and functionalized aromatic cinnamates.¹ We focus on a critical quantity in transport: the quantum-mechanical overlap of the strongly confined electron and hole wavefunctions on neighboring nanocrystals. Using results from density-functional-theory (DFT) calculations, we construct a one-dimensional model system whose analytic wavefunctions reproduce the full DFT numerical overlap values. By investigating the analytic behavior of this model, we reveal several important features of electron transport. The most significant is that the wavefunction overlap decays exponentially with ligand length, with a characteristic decay length that depends primarily on properties of the ligand and is almost independent of the size and type of nanocrystal. Functionalization of the ligands can also affect the overlap by changing the height of the tunneling barrier. The physically transparent analytic expressions we obtain for the wavefunction overlap and its decay length should be useful for future efforts to control transport in nanocrystal solids.

1. A. R. Khabibullin, Al. L. Efros  and S. C. Erwin,  Nanoscale,  2020, DOI: 10.1039/d0nr06892f

Although colloidal nanocrystals of many different materials can be synthesized in high quality in respect of size, shape and crystallinity, our understanding of their formation and the involved chemical reactions is still rather poor. We will present detailed studies on nucleation and growth as well as ion exchange processes in nanocrystals. These include mass spectrometric, optical, electron microscopic and x-ray synchrotron experiments.

Almost all applications of nanocrystals require the control of surface properties in respect of solubility, miscibility, biocompatibility, passivation of surface states as well as electronic and magnetic interaction with the environment. We will show various examples for ligand exchange and encapsulation of quantum dots, plasmonic and magnetic nanocrystals and will report on applications as high-performance ceramics, for display and lighting, electrocatalysis as well as for biolabeling and drug delivery.

In solution, nanoparticles may be conceptually compartmentalized into cores and engineered surface coatings. Recent advances allow for simple and accurate characterization of nanoparticle cores and surface shells. After introduction into complex biological environment, adsorption of biological molecules to the nanoparticle surface as well as a loss of original surface components occur. Thus, colloidal nanoparticles in the context of biological environment are hybrid materials with complex structure, which may result in different chemical, physical, and biological outcomes as compared to the original engineered nanoparticles. In this perspective, the way to synthesize selected inorganic nanoparticles, as a model for discussion, will be highlighted. Then, we will discuss the environmental changes upon exposure of these nanoparticles to biological media and their uptake by cells. Finally, the intracellular fate of nanoparticles and their degradation will be discussed.

For the last 20 years, I have been focused on the synthesis and medical & energy applications of uniform-sized nanocrystals and related nanomaterials (Nature Mater. 2004, 3, 891). We reported that uniform 2 nm iron oxide nanoclusters can be successfully used as T1 MRI contrast agent for high-resolution MR angiography of monkeys (Nature Biomed. Eng. 2017, 1, 637). We demonstrated that ceria nanoparticles and ceria–zirconia nanoparticles can work as therapeutic antioxidants to treat various nasty diseases including ischemic stroke, Alzheimer’s disease, sepsis, and Parkinson’s disease (Angew. Chem. Int. Ed. 2012, 51, 11039; ACS Nano, 2016, 10, 2860; Angew. Chem. Int. Ed. 2017, 56, 11399; Angew. Chem. Int. Ed. 2018, 57, 9408; Adv. Mater. 2018, 30, 1807965). CeO₂/Mn₃O₄ nanocrystals possessing surface strains protect tissue-resident stem cells from irradiation-induced ROS damage, significantly increasing the survival rate of the animals (Adv. Mater. 2020, 32, 2001566). We report a highly sensitive and selective K⁺ nanosensor that can quantitatively monitor extracellular K⁺ concentration changes in the brains of freely moving mice experiencing epileptic seizures (Nature Nanotech. 2020, 15, 321).

We present a synthesis of highly durable and active electrocatalysts based on ordered fct-PtFe nanoparticles and FeP nanoparticles coated with N-doped carbon shell (J. Am. Chem. Soc. 2015, 137, 15478; J. Am. Chem. Soc., 2020, 142, 14190; J. Am. Chem. Soc. 2017, 139, 6669). We also report on the design and synthesis of highly active and stable Co-N₄(O) moiety incorporated in nitrogen-doped graphene (Co₁-NG(O)) that exhibits a record-high kinetic current density (2.84 mA cm^-2 at 0.65 V vs. RHE) and mass activity (277.3 A g^-1 at 0.65 V vs. RHE) with unprecedented stability (>110 h) for electrochemical hydrogen peroxide (H₂O₂) production (Nature Mater. 2020, 19, 436). We report on the design and synthesis of highly active TiO₂ photocatalysts incorporated with site-specific single copper atoms (Cu/TiO₂) that exhibit reversible & cooperative photoactivation process, and enhancement of photocatalytic hydrogen generation activity (Nature Mater. 2019, 18, 620). We synthesized multigrain nanocrystals consisting of Co₃O₄ nanocube cores and Mn₃O₄ shells. At the sharp edges of the Co₃O₄ nanocubes, we observed that tilt boundaries of the Mn₃O₄ grains exist in the form of disclinations, and we obtained a correlation between the defects and the resulting electrocatalytic behavior for the oxygen reduction reaction (Nature 2020, 359, 577).

Clinical modality based on light activation and photosensitizers, phototherapy, has been recognized as a novel alternative for cancer therapy owning to its high selectivity, safety and compatibility with other tumor ablation modalities. However, key challenges such as low light penetration, low generation rate of reactive oxygen species (ROS) for photodynamic therapy (PDT) or of heat for photothermal therapy (PTT), and poor distribution specificity of photosensitizers have restricted its widespread clinical use, which require rational design and improvement on photosensitizers. Herein we propose a strategy that combines plasmonic gold nanorods, which can generate hot electrons under laser irradiation, with semiconductor materials such as ZnO or TiO2. The generated hot electrons can be injected into the conduction band of semiconductors and eventually produce ROS (·OH and 1O2), which can lead to cell apoptosis. In addition, by adjusting the size and the aspect ratio of gold nanorods, we extend the irradiation window to NIR-II region where light can go deeper into the tissues. Finally, we use zwitterionic polymers to cover the surface of the synthesized nanoparticles to prevent nonspecific interactions with biomolecules and cells, and prolong their circulation time in vivo. We continue working on designing different zwitterionic block copolymers, modifying the end groups of polymer chains and adding functional groups to enable bioconjugating targeting ligands for better tumor specificity.

Although the reported environmental concentrations of nonsteroidal anti-inflammatory drugs (NSAIDs) have been mainly found to be at the µg/L to g/L range in seawaters and surface waters, and in the lower ng/L level in groundwaters and drinking waters, they tend to increase due to their wide usage and irresponsible disposal [1]. Even at these low concentrations, pollutants can impact aquatic ecosystems causing chronic harm. Among them, Ibuprofen (IBP) and Diclofenac (DCF) are considered Class I pharmaceuticals that must be dealt with in urgency while estimated to be consumed in several kilotons per year globally[2].

Activated carbon, carbon nanotubes, clays, and others have been used for absorption of pharmaceutical pollutants but graphene and graphene oxide have shown better results and behavior since they can remove persistent pollutants and organics due to their high surface area (∼2630 m² g⁻¹), large delocalized pi (π) electrons and tunable chemical properties which make them potential outstanding adsorbents for environmental decontamination applications [3]. The addition of magnetic nanoparticles to graphitic structures has recently been studied to help remove the adsorbent materials from the treated water easily such as metals and dyes with outstanding results [4]. 

In this work, a graphene-based magnetic hybrid material decorated with Fe₃O₄ nanocrystals (NCs) is designed for the effective removal of emerging pollutants when they are present at low concentrations. Several variations of the Fe₃O₄/Graphene hybrid were fabricated such as Fe₃O₄/Graphene oxide and a multimodal Fe₃O₄-TiO₂/Graphene hybrid which explored the photocatalytic degradation of the captured drugs. Green preparation of this hybrid, a study of its application and performance, and a simple, direct method of detection and quantification by UV-Vis are presented.

One of the most attractive points of synthetic nanochemistry is the possibility to precisely engineer the nanocrystal’s shape. For most metallic oxide compounds, however, this is not an easy task.

Cerium oxide (CeO₂) is a rare-earth semiconductor capable of switching valence states between Ce³⁺ and Ce⁴⁺ without affecting the material structure when working at the nanoscale. The increase of surface area provided by the nanometric regime gives rise to the reversible removal of oxygen atoms from the exposed surface, generating a higher density of surface defects in the crystal structure. Electrons left behind by released oxygen localize on empty f states of cerium atoms (formally reduced from Ce⁴⁺ to Ce³⁺). The ability to work as an oxygen buffer is the core of the specific tasks it can be applied to, both in the nanocatalysis field and in biomedicine, where it is manly used as an antioxidant-like substance capable of the modulation of oxidative stress and inflammation-related processes .

Here we present how, through a deep mechanistic description of the synthetic process and the control of its key synthetic parameters, we can develop a strategy to obtain complex CeO₂ nanostructures, like hollow nanocrystals or 2D nanosheets, using 3 nm CeO₂ nanocrystals synthesized in situ as building blocks without surfactants or templates.

Quantification of specific biomarkers is an important diagnostic tool. Standard immunoassays such as ELISA require extensive washing steps and signal amplification, in particular when the biomarker of interest is only present at very low concentrations. On the other hand, non-radiative Fôrster resonance energy transfer (FRET) has been used to design one-step bioassays which do not require any washing steps, where the biomarker enables the formation of a sandwich complex involving donor-labeled and acceptor-labeled antibodies. FRET from the donor to the acceptor then provides an optical signature of the complex formation, hence of the biomarker of interest. However, the large size of this complex limits the efficiency of energy transfer, preventing sensitive detection. Here we propose a novel energy transfer modality using solution-phase optical microcavities to enhance energy transfer. To this aim, we have designed structures in which fluorescent colloidal quantum dots are located within dielectric microspheres to enable strong coupling of their fluorescence emission with the whispering gallery modes (WGMs) of the microspheres. We characterize the energy transfer between these modes and acceptor dye-loaded nanoparticles present in the evanescent field, within a few tens of nanometers above the microsphere surface. Compared to FRET, WGM-enabled energy transfer occurs over a much more extended volume, thanks to the delocalization of the mode over a typically 105 times larger surface and to the extension of the WGM electromagnetic field to larger distances (>100 nm vs 5-8 nm) from the surface of the microcavity. This enables combining the sensitivity of WGM assays with the simplicity and specificity of FRET assays into a novel biosensing modality. We finally demonstrate DNA detection as a proof-of-concept biomolecular assay.

Nanocrystals are important for a wide range of applications because of their unique properties, which are strongly connected to their three-dimensional (3D) structure. Electron tomography has therefore been used in an increasing number of studies. Most of these investigations resulted in 3D reconstructions with a resolution at the nanometer scale, but also atomic resolution was achieved in 3D. However, the increasing complexity of nanomaterials has driven the development of even more advanced 3D characterization techniques, which will be discussed in this contribution.

For example, 3D characterization of structural defects in nanoparticles by transmission electron microscopy is far from straightforward since the presence of diffraction contrast in a tilt series of images violates the projection requirement for tomography. However, being able to visualize defects is of great importance to understand e.g. the initial growth of metallic nanoparticles or the effect of pulsed laser irradiation on the crystal structure. By simultaneous acquisition of tilt series using different annular detectors, we were able to visualize both the morphology and the defect structure of several types of nanostructures [1]. 

In order to preserve the carefully designed morphologies and functionalities, understanding the stability of nanomaterials during application is of equal importance. It is hereby important to note that most electron tomography investigations have been performed at the conventional conditions of an electron microscope. An emerging challenge is therefore to fully understand the connection between the 3D structure and properties under realistic conditions, including high temperatures as well as in the presence of liquids and gases. Therefore, innovative methodologies are required to track the fast 3D changes of nanomaterials that occur under such conditions.

Recently, we proposed an acquisition approach where a tilt series of projection images is acquired within a few minutes. By continuously tilting the holder and simultaneously acquiring projection images while focusing and tracking the particle, we were able to reduce the total acquisition time for a tilt series by a factor of ten. In this manner, we were able to study the 3D morphological evolution of anisotropic Au(Pd) nanocrystals as a function of both heating time and temperature [2,3]. Moreover, we measured the elemental diffusion dynamics of individual anisotropic Au-Ag nanoparticles in 3D [4]. We conclude that for a given composition, the shape of the nanoparticle does not influence the alloying process significantly and that other factors such as surface diffusion need to be taken into account.

Colloidal nanocrystals are hybrid objects in which the properties of core and surface both determine the characteristics of the entire nanocrystal. The surface is often capped by (in)organic ligands which determine colloidal stability and the physical and chemical properties. As a result, nanocrystal surface chemistry, i.e., the understanding of and control over the ligand shell, has become one of the central themes in nanocrystal research.

Here, we focus on three aspects. First, we will discuss NMR spectroscopy as a tool for surface chemistry analysis. We will explain the NMR line broadening of nanocrystal-bound ligands (see figure) and show how it can be manipulated or used as a diagnostic tool for assessing solvent-ligand interactions. Second, we discuss the non-innocence of solvents used in nanocrystals synthesis. In particular, 1-octadecene (ODE) and trioctylphosphine oxide (TOPO) are popular solvents but they tend to polymerize or decompose, respectively. Finally, we discuss our recent efforts in designing, synthesizing and evaluating ligands with a very high affinity for the nanocrystal surface. Studying a series of phosphonates and multidentate phosphoric acids, we uncover rules for optimizing the binding affinity and nanocrystal solubility. 

Colloidal semiconductor nanocrystals (NCs) are characterized by a large surface-to-volume ratio that renders them extremely sensitive to surface processes. Passivating ligands, employed to stabilize NCs in organic solvents, play a pivotal role in influencing the structure and the optoelectronic properties of these materials. Despite major progresses attained in the last years to model the surface of NCs, there are still several key questions to be answered on the nature of the NC-ligand interactions.

Recently, our group has been developing a set of programs interfaced with available computational chemistry software packages that allow to simulate atomistically the thermodynamic controlling factors in the NC surface chemistry by including explicit solvent molecules, ligands, and NC sizes that match the experiments.

To fully take advantage of the power of these computational tools, we decided to embed the thermodynamics behind the dissolution/precipitation of nanocrystal–ligand complexes in organic solvents and the crucial process of binding/detachment of ligands at the NC surface into a unique chemical framework [1]. We show that formalizing this mechanism with a computational bird’s eye view helps in deducing the critical factors that govern the stabilization of colloidal dispersions of NCs in organic solvents as well as the definition of those key parameters that need to be calculated to manipulate surface ligands. This approach has the ultimate goal of engineering surface ligands in silico, anticipating and driving the experiments in the lab.

Flexible semiconductor materials, where structural fluctuations and transformation are tolerable and have low impact on electronic properties, focus interest for future applications. Two-dimensional thin layer lead halide perovskites are hailed for their unconventional optoelectronic features  and high degree of compliance. I will  show structural deformation via thin layer buckling in colloidal CsPbBr₃ nanobelts adsorbed on carbon substrates. The microstructure of buckled nanobelts is determined using transmission electron microscopy and atomic force microscopy. We measured significant decrease in emission from the buckled nanobelt using cathodoluminescence, marking the influence of such mechanical deformations on electronic properties.  By employing plate buckling theory, we approximate adhesion forces between the buckled nanobelt and the substrate, to be  Fadhesion~0.12μN, marking a limit to sustain such deformations. This work highlights detrimental effects of mechanical buckling on electronic properties in halide perovskite nanostructures and points towards the capillary action that should be minimized in fabrication of future devices and heterostructures based on Nano-perovskites.  

Since their introduction in 1991 by Arduengo, N-heterocyclic carbenes (NHCs) have attracted much attention as versatile metal-coordinating groups. This mode of interaction has also been actively investigated for surface passivating nanocrystal surfaces.

In this contribution, we probe the interactions between colloidal gold nanoparticles (AuNPs), or luminescent quantum dots (QDs) and three structurally distinct NHC-based ligands: two monodentate and one multidentate ligands [1,2]. The latter combines multiple NHC groups and several poly (ethylene glycol) (PEG) solubilizing moieties. We find that NHC-based ligands rapidly coordinate onto both sets of nanocrystals (requiring ~ 10 min of reaction time), which reflects the soft Lewis base nature of these groups, with their two electrons sharing capacity. We combine NMR spectroscopy, fluorescence spectroscopy, high-resolution transmission electron microscopy and dynamic light scattering to characterize the nature of the binding interactions. Furthermore, the long term stability of the NHC-stabilized nanocolloids have been tested after phase transfer to water, a highly challenging chemical venue for such groups. Data show that our NHC-polymers exhibit long-term colloidal stability in buffer media with no sign of degradation or aggregation build up for at least one year. Additionally, the benefits of the NHC-polymer ligand design have also been demonstrated for the coating of other transition metal core colloids (e.g., magnetic iron oxide and silver NPs). We will discuss the ligand design, characterization and potential use of these colloids in few specific applications.

Surface ligands affect many properties of colloidal nanocrystals, including solubility, reactivity, and electronic structure. Therefore, unveiling their nature is crucial to design and control nanocrystal properties. Here, we investigated the synthesis of metal oxide nanocrystals by heating up metal nitrates in the presence of oleylamine as the only surfactant. Based on the precursor choice, the expected ligand in these syntheses is oleylamine. However, we found that the ligand shell of the so-produced CeO₂ nanocrystals is only comprised of oleic acid, even though it is not added to the reaction. Thorough characterization with NMR spectroscopy of the particles before and after purification unveiled how oleic acid is formed, and we proposed a reaction path for the oxidation of oleylamine. We further showed that NiO, CoO, and ZnO prepared from nitrates and amines are also capped exclusively by carboxylic acids. Our findings reaffirm the importance of studying the nanocrystals surface chemistry, as one cannot assume that the ligands on the surface are the starting ones.

CsPbBr₃ nanocrystals (NCs) suffer from instabilities caused by the dynamic and labile nature of both the inorganic core and the organic-inorganic interface. Weak and dynamic binding between the NC surface and capping ligands causes rapid ligand desorption upon isolation and purification of colloids, eventually leading to a loss of structural integrity and sintering of NCs into bulk polycrystalline materials. Surface ligand engineering therefore remains an imminent research topic. Much progress in obtaining purifiable and stable colloids was achieved with a recent experimental discovery of new capping ligands, such as dimethyldidodecylammonium halides,[1,2] alkylphosphonic acids,[3] and long-chain zwitterionic ligands.[4] However, comprehensive understanding of the NC–ligand–solvent interface and the atomistic origins of the observed differences lags behind. In this study, we use classical molecular dynamics simulations to gain insights into the inherent binding properties of three different alkylammonium ligands – primary dodecylammonium (DA), secondary didodecylammonium (DDA) and quaternary dimethyldidodecylammonium (DMDDA) – in a mixture of nonpolar (toluene) and polar (acetone) solvents, the medium which is typically encountered during purification of the NCs. Our simulations uncover three main factors that govern effective ligand–substrate interactions: (i) the ability of the head-group to penetrate into the binding pocket, (ii) the strength of head-group's interactions with the polar solvent, and (iii) higher barrier for ligand adsorption/desorption in the case of multiple alkyl chains. The interplay between these factors causes the following order of the binding free energies: DDA < DA ≈ DMDDA, while surface capping with DDA and DMDDA ligands is additionally stabilized by the kinetic barrier. These findings are in agreement with experimental observations, wherein DDA is found to loosely bind to the CsPbBr₃ surface, while DMDDA capping is more stable than capping with primary oleylammonium ligand. The presented mechanistic understanding of the ligand-NC interactions is then used to design new cationic ligands that are expected to make perovskite NC surfaces more robust. We anticipate that the methodology, which is used in this study, can be extended to other types of inorganic materials with a predominantly noncovalent nature of interactions with capping ligands.

Nanocrystal micro-/nanoarrays with multiplexed functionalities are of broad interest in the field of nanophotonics, cellular dynamics, and biosensing due to their tunable electrical and optical properties. This work focuses on the multicolor patterning of two-dimensional nanoplatelets via two sequential self-assembly and direct electron-beam lithography steps. First, CdSe/ZnyCd1-yS and CdSe1-xSx/ZnS core/shell nanoplatelets (NPLs) are synthesized as building blocks. They are utilized later as red- and green-emitting layers, respectively. Next, large-area NPL thin films with controlled orientation are fabricated based on Langmuir-type self-assembly at the liquid/air interface. By varying the concentration of ligands in the subphase, we investigate the effect of interaction potential on the film's final characteristics to prepare thin superlattices suitable for the patterning step. Equipped with the ability to fabricate a uniform superlattice with a controlled thickness, we finally perform nanopatterning on a thin film of NPLs utilizing direct electron-beam lithography technique. The effect of acceleration voltage, aperture size, and e-bam dosage on the nanopattern's resolution and fidelity is investigated for both of the presented 2D NPLs. Our results indicate that clear and high-resolution nanopatterns can be obtained at an acceleration voltage of 10 kV, the electron dosage of 300 μC cm^-2, the aperture size of 30 μm, and under an e-beam current of 174 pA. By using scanning electron microscopy (SEM), atomic force microscopy (AFM), and fluorescence microscopy, we demonstrate the successful fabrication of multicolor fluorescent nanoarrays with the thickness of only 2-3 monolayers (7-10 nm) and the feature line width of ~40 nm, which is three to four NPLs wide. The obtained multicolor micro-/nanoarrays provide us an innovative experimental platform to investigate biological interactions as well as Föster resonance energy transfer.

Crystallization through attachment on nanoscale building blocks represents a novel way of creating ‘designer materials’. The opportunity to tune the structure, and hence the quality and degree of interaction between the building blocks creates a wide parameter space for tailoring the ensemble-level properties, eventually reaching truly purpose-built materials. Traditionally, bottom-up synthesis meant using nanocrystals as a feedstock for conventional materials processing. The approach of controlled assembly through intermolecular forces, however, seems more promising for the ability to restrict particular degrees of freedom and to shift the energy- and timescales into the soft matter regime.[1] Moreover, working at interfaces of liquids (solvent-solvent or solvent-air), confines the assembly into a plane, enabling the formation of 2D-confined structures.[2]

Assembly and oriented attachment of colloidal quantum dots into a superlattice with long range order both at the atomic and multi-nanometer scale carries the promise of creating novel properties, particularly for semiconductor devices and related applications.[3,4] Additionally, the established reaction mechanism, control approaches and general behavior can be translated into many other systems. The general approach to created ordered lattices has been established, but a knowledge gap persists in our ability to control the assembly and attachment process.[5,6] Filling these knowledge gaps is essential to establish the energetic-kinetic framework of a representative system that can be later generalized.

Here we report the results of our efforts to understand and control the formation of superlattices of PbSe colloidal nanocrystals.[7-9] Using predominantly structural characterization techniques, such as electron microscopy and synchrotron-based X-ray scattering, we describe the process from the introduction of a droplet of a colloidal dispersion through an assembly of nanoparticles at a liquid surface into a correlated superlattice to an epitaxially connected hierarchical system. We describe the physics and chemistry of spreading, drying and reactive assembly, and establish the guidelines for applying the method to other systems.

Caesium lead halide perovskite nanocrystals, owing to high oscillator strength of bright triplet excitons, slow dephasing (coherence times of up to 80 picoseconds) and minimal inhomogeneous broadening of emission lines, are promising building blocks for creating superlattice structures that exhibit collective phenomena in their optical spectra. Thus far, only single-component superlattices with the simple cubic packing have been devised from these novel nanocrystals, which have been shown to exhibit superfluorescence – a collective emission resulting in a burst of photons with ultrafast radiative decay (ca. 20 ps) that could be tailored for use in ultrabright (quantum) light sources [1]. However, far broader structural engineerability of superlattices, required for programmable tuning of the collective emission and for building a theoretical framework can be envisioned from the recent advancements in colloidal science [2]. We show that co-assembly of cubic and spherical steric-stabilized nanocrystals is experimentally possible and that the cubic shape of perovskite nanocrystals leads to a vastly different outcome compared to all-spherical systems. We present perovskite-type (ABO₃) binary nanocrystal superlattices, besides expected NaCl-type or common AlB₂-type superlattices. In binary ABO₃ superlattices, larger spherical nanocrystals occupy the A sites and smaller cubic CsPbBr₃ nanocrystals reside on B and O sites. The deformability of ligand shell on cube corners makes the lattice stable over broad nanocrystal size ratio range. Targeted substitution of B-site nanocubes by truncated cuboid PbS nanocrystals leads to the exclusive formation of ternary ABO₃ superlattice. All synthesized superlattices exhibit a high degree of orientational ordering of the CsPbBr₃ nanocubes. We also demonstrate the effect of superlattice structure on superfluorescent behaviour. Our work paves the way for further exploration of complex, ordered and functionally useful perovskite mesostructures.

We study the assembly kinetics of surfactant-stabilized gold nanoparticles in the presence of sulfate ions. The reaction proceeds in two steps: very rapid (a few minutes) formation of amorphous aggregates, followed by slow reordering (over several hours). The latter process is the only one detectable via absorbance spectroscopy and results in the formation of intimate contacts between the objects, with interparticle distances below the thickness of a surfactant bilayer. The rate-limiting step of the reaction could be related to surfactant expulsion from the initial aggregates, which allows the particles to come in close contact and form chains. There are marked differences in reaction yield and rate constant between spheres, rods and bipyramids, highlighting the role of surface curvature in contact formation. Once formed, the assemblies are very sturdy and stable under centrifugation and dialysis. The contact interaction is strong and highly directional, as shown by liquid-cell transmission electron microscopy.

Quantum mechanics teaches that electrons exhibit a dual behavior of particles and waves. This feat has well-known implications for the optical properties of low-dimensional semiconductors: confinement of the electronic excitation (exciton wave) in the limited space of the crystallite increases the energy of the excitation. The effects of quantum confinement on the exciton energetics is well established, and has resulted in numerous optical applications of colloidal nanocrystals. However, the optical absorption strength of excitons with different degrees of confinement has not been considered quantitatively. Here, we report generality in the band-edge light absorptance of semiconductors, independent of their dimensions. First, we provide atomistic tight binding calculations that show that the absorptance of semiconductor quantum wells is equal to m πα (m=1 or 2 with α  the fine structure constant) per allowed transition, in agreement with reported experimental results. Then, we show experimentally that a monolayer superlattice of quantum dots has very similar absorptance, suggesting an absorptance quantum of m πα  per (confined) exciton diameter. Extending this idea to bulk semiconductors, we experimentally demonstrate that an absorptance quantum equal to m πα  per exciton Bohr diameter explains the widely varying absorption coefficient of bulk semiconductors. We thus provided compelling evidence that the absorptance quantum πα  per exciton diameter rules the band-edge absorption of all direct semiconductors, regardless of their dimension.

The synthesis of monodisperse colloidal nanocrystals (NCs) with controlled composition, size, and shape provides ideal building blocks for assembling new thin films and devices. These monodisperse colloidal NCs act as "artificial atoms" with tunable electronic, optical, magnetic properties that allow the development of a new periodic table for design at the Mesoscale. In this talk, I will briefly outline the current state of the art in synthesis, purification, and integration of single-phase NCs and core-shell (heterostructures) NCs emphasizing the design of semiconductor building blocks with tunable shapes (spheres, roads, cubes, discs, octahedra, and heterodimers, etc. I will then share how these tailored NCs can be directed to assemble into single-component, binary, ternary NC superlattices providing a scalable route to the production of multifunctional thin films. The modular assembly of these NCs allows the desirable features of the underlying quantum phenomena to be enhanced even as the interactions between the NCs allow new delocalized properties to emerge. Synergies in the electronic and optical coupling between NCs will be emphasized, along with the potential of complimentary assembly for systematic chemical transformations. I will share specific case studies in catalysis in supported films, and I will also share progress in microfluidic superparticle assembly approaches. Creating mesoscale structures that span 100s of nanometer to 10s of microns as the following scale of building units.

In this talk, I will describe recent experimental and computational work to understand the phononic properties in chalcogenide and perovskite nanocrystals and their impact on optoelectronic properties, including linewidths, carrier cooling, and recombination [1-5]. I will explain how we can measure and calculate phonon density of states and electron-phonon coupling strengths in nanocrystals and investigate trends with nanocrystal size and defects.   Across systems, we find that large emission linewidths and efficient intraband transitions result from strong coupling, in particular, to surface phonons.  We describe strategies to mitigate this through surface engineering, and, in the perovskite system (ABX₃), we also examine the role of the A-cation in the phononic properties of the system [3, 6-7].

[1] D. Bozyigit, et al., Nature, 2016, 531, 618.

[2] N. Yazdani, et al. J. Phys. Chem. Lett., 2018, 9, 1561–1567.

[3] N. Yazdani, et al. Nano Lett. 2018, 18, 2233–2242.

[4] N. Yazdani, et al. ACS Photonics 2020, 7, 1088–1095.

[5] L. Piveteau, et al. ACS Cent. Sci. 2020, 6, 1138–1149.

[6] G. Raino, et al. submitted.

[7] N. Yazdani, et al. in preparation.

Colloidal synthesis routes allow precise control over material parameters at the nanometer scale with moderate capital or operating costs and with high-throughput production and material yields. Stimulated by its simplicity and huge potential, countless groups all around the world have developed an extensive library of synthetic strategies and routes to produce nanocrystals with almost any composition, size and shape. However, for such control over material parameters at the nanoscale to truly impact real applications, colloidal nanocrystals need to be arranged into functional patterns, thin films, porous nanomaterials or highly dense nanocomposites, depending on the application. Additive manufacturing technologies based on layer-by-layer deposition of material ejected from a nozzle in the form of ink are well-suited to produce macroscopic structures from colloids, but are limited in terms of printing speed and resolution. Electrohydrodynamic (EHD) jetting uniquely allows generating submicrometer jets that can reach speeds above 1 m s^-1, but such jets cannot be precisely collected by too slow mechanical stages. In this talk, I will present our progress in the control of the jet trajectory in EHD jetting technologies through a voltage applied to electrodes located around the jet. This method allows to continuously adjust the jet trajectory with lateral accelerations up to 10⁶ m s^-2. Through electrostatically deflecting the jet, 3D objects with submicrometer features can be printed by stacking material layers on top of each other at layer-by-layer frequencies as high as 2000 Hz. The fast jet speed and large layer-by-layer frequencies achieved translate into printing speeds up to 0.5 m s^-1 in-plane and 0.4 mm s^-1 in the vertical direction, three to four orders of magnitude faster than techniques providing equivalent feature sizes.

We have studied bismuth oxyiodide (BiOI) nanoplatelets, a material which combines intriguing optical and structural properties for photocatalysis. In a pump-probe experiment, we show that excitation by a femtosecond laser pulse creates coherent phonons which leads to an oscillating modulation of the differential optical density. We found that the two underlying frequencies originate from lattice vibrations along the [001] crystallographic axis, the stacking direction of oppositely charged layers in BiOI. This is consistent with a sub-picosecond charge separation in the built-in dipolar field, which screens the electric field partially and thus creates coherent phonons. Furthermore, we determine the two major dephasing mechanisms that lead to the loss of vibronic coherence: (i) the anharmonic decay of an optical phonon into two acoustic phonons and (ii) phonon-carrier scattering.
Our results provide the first direct demonstration of the presence of an electric field in BiOI along the [001] axis and show its role in efficient charge separation that is crucial for photocatalytic applications of BiOI.

Fast neutrons offer high penetration capabilities for both light and dense materials due to their comparatively low interaction cross sections, making them ideal for the imaging of large-scale objects such as large fossils or as-built plane turbines, for which X-rays or thermal neutrons do not provide sufficient penetration. However, inefficient fast neutron detection limits widespread application of this technique. Traditional phosphors such as ZnS:Cu embedded in plastics are utilized as scintillators in recoil proton detectors for fast neutron imaging. However, these scintillation plates exhibit significant light scattering due to the plastic–phosphor interface along with long-lived afterglow (on the order of minutes), and therefore alternative solutions are needed to increase the availability of this technique. Here, we utilize colloidal nanocrystals (NCs) in hydrogen-dense solvents for fast neutron imaging through the detection of recoil protons generated by neutron scattering, demonstrating the efficacy of nanomaterials as scintillators in this detection scheme. The light yield, spatial resolution, and neutron-vs-gamma sensitivity of several chalcogenide (CdSe and CuInS2)-based and perovskite halide-based NCs are determined, with only a short-lived afterglow (below the order of seconds) observed for all of these NCs. FAPbBr3 NCs exhibit the brightest total light output at 19.3% of the commercial ZnS:Cu(PP) standard, while CsPbBrCl2:Mn NCs offer the best spatial resolution at ∼2.6 mm. Colloidal NCs showed significantly lower gamma sensitivity than ZnS:Cu; for example, 79% of the FAPbBr3 light yield results from neutron-induced radioluminescence and hence the neutron-specific light yield of FAPbBr3 is 30.4% of that of ZnS:Cu(PP). Concentration and thickness-dependent measurements highlight the importance of increasing concentrations and reducing self-absorption, yielding design principles to optimize and foster an era of NC-based scintillators for fast neutron imaging.

To improve their quantum yield and stability, metal chalcogenide nanoplatelets (NPLs) are often synthesized as core-shell or core-crown heterostructures. Unfortunately, in doing so the emission linewidth becomes broad and asymmetric. The asymmetric linewidth has been recently ascribed to shakeup processes, a partly radiative Auger process [1,2]. Strategies are then needed for the narrowing of the emission linewidth on these colloidal materials.

In this work [3], we supply a theoretical point of view about shakeup processes in order to assess on the origin of the multi-peak emission observed on Refs [2,3}. Besides, we suggest strategies that would allow minimization of such peaks, therefore reducing the emission linewidth. The conclusions we obtain are:

(1) Shakeup processes are expected in colloidal NPLs charged with trions, unlike in strongly confined nanocrystals. Since Coulomb interactions are considerably strong one could expect sizeable radiative Auger processes.

(2) Off-centred impurities [4], reducing the system symmetry, are needed to observe shakeup processes in NPLs. This result allows us to connect the apparently different interpretations of the broadening reported in Refs [2,5], namely surface defects or shakeup processes.

(3) Multi-peaked emission in CdSe/CdS NPLs [2] cannot be explained by shakeup processes only. Therefore, we propose an alternative interpretation involving emission peaks from metastable spin triplet trion states.

The past several years witnessed an explosion of chemical preparation methods dedicated to the synthesis of CsPbBr₃ perovskite nanocrystals. In contrast, the methods of making CsPbBr₃-based nanoheterostructures are scarce. In this contribution, we will discuss non-perovskite zero-dimensional metal halide Cs₄PbBr₆ as a precursor for CsPbBr₃-Cs₄PbBr₆ and CsPbBr₃-SiO₂ nanoheterostructures. Upon addition of an organic anhydride to the colloidal dispersion of oleylammonium/oleate-capped Cs₄PbBr₆ nanocrystals, the amine-anhydride condensation reaction takes place. That reaction destabilizes the surface of Cs₄PbBr₆ nanocrystals, produces maleamic acid derivatives, and results in the Cs₄PbBr₆ to CsPbBr₃ transformation. If a small molecule reactive anhydride is used for the condensation, the transformation happens fast and results in the acidic conditions suitable for hydrolysis of alkoxysilanes and overcoating of the resulting CsPbBr₃ nanocrystals with a shell of amorphous silica. If a macromolecular anhydride is used for the condensation, the transformation takes time at room temperature, allowing to capture CsPbBr₃-Cs₄PbBr₆ nanoheterostructures and characterize them with electron microscopy and optical spectroscopy. More generally, the anhydride-induced transformation of Cs₄PbBr₆ nanocrystals opens up a strategy for the chemical modification of metal halide NCs initially passivated with nucleophilic amines.

The recent surge of interest in ns² (e.g. Sn²⁺, Sb³⁺, Bi³⁺) based metal halides within the nanocrystal community is part of an extensive effort to seek for non-toxic alternatives to the well-known lead halide perovskites. Sb³⁺ cations have attracted particular attention due to the interesting optical properties they confer to both 3D (i.e. Cs₂NaInCl₆, Cs₂KInCl₆) and 0D (Rb₃InCl₆) metal chloride hosts when introduced as dopants [1-4]. A new Sb-based compound, Rb₇Sb₃Cl₁₆, characterized by a peculiar 0D crystal structure composed of isolated [SbCl₆]^3- octahedra and [Sb₂Cl₁₀]^4- dimers, is currently under the spotlight since the origin of its optical features, both in the bulk and at the nanoscale, are still to be completely understood [5-6]. Here, we propose the use of density functional theory (DFT)  as the most accurate way to unravel the absorption and emission properties of Sb-based materials depending on the [SbCl₆]^3- octahedra’s surrounding and connectivity.

Metal halide perovskites with low content of Pb have been synthesized with the purpose to make available the commercialization of less toxic and highly efficient devices. However, these materials do not preserve the intrinsic features of their Pb-based counterparts, giving a dilemma about the convenience of Pb substitution.[1] Here, we analyze the impact of Sr as potential dopant on the photophysical and structural properties of FAPbI₃ quantum dots (QDs). The replacement of Pb by 7 at. % Sr facilitates the preparation of FAPb_1-xSr_xI₃ QDs with 100% photoluminescence quantum yield, long-term stability for 8 months under relative humidity of 40-50%, and T₈₀ = 6.5 months. This value is one of the highest values reported for halide QDs under air ambient. This material also depicts photobrightening under UV irradiation for 12 h, recovering the 100% PLQY 15 days after synthesis. The suppression of Schottky defects (Pb- and I-vacancies) by the presence of Sr restrains the non-radiative channels for electron relaxation.[2] Nevertheless, the increase of the Sr fraction during the QDs growth induces the emergence of 2D nanoplatelets/3D nanocubes mixture, caused by a high Pb deficiency during QDs synthesis. This work provides a novel insight about how the adequate/poor Pb substitution dictates the photophysical properties of QDs potentially applicable in optoelectronics.

The tunability of semiconductors quantum dots (QDs) is generally restricted by Fermi’s Golden Rule — altering the bandgap concomitantly alters photoluminescent (PL) lifetime. Herein, we present a strategy to circumvent this restriction, which can be realized with the soft and flexibile crystal lattice of  perovskite nanocrystals (NCs). Through the partial substituion of formamidinium (FA) for ethylenediammonium {en}, “hollow” {en}FAPbBr₃ NCs can be generated and emission wavelength matched to CsPbBr₃ NCs while maintaining drastically different PL lifetimes. We attribute this to two potentially co-existing effects: increased phonon-exciton interaction and additional energy levels for the excitonic transition. This unique materials system allows us to push the young yet promising field of lifetime-encoded security tags forward. Our proof-of-principle security system is based on high-resolution electrohydrodynamically printed unicolour multi-fluorescent-lifetime codes that can be deciphered with either commercially available time-correlated single-photon counting fluorescence-lifetime imaging (TCSPC-FLI) microscopy or our time-of-flight (ToF)-FLI prototype. We believe that this innovative approach may provide a new tool for securing global trade against counterfeit goods and currency.

Hybrid metal lead halide quasi-2D (two-dimensional) perovskites are considered as innovative materials in photonic and photoelectronic applications thanks to their high photoluminescence yield and improved stability compared to the three-dimensional (3D) counterparts. Nevertheless, despite their outstanding emission properties, which indicate them as competitive active materials for light emitting devices, like LED and lasers, few works have been reported in literature on amplified spontaneous emission (ASE) in these materials.

In our work, we investigate the temperature dependence of photoluminescence (PL) and ASE in multilayered quasi-2D BA₃MA₃Pb₅Br₁₆ films in order provide a thorough understanding of the photophysics which distinguishes these innovative materials.

By analyzing the temperature-dependent PL spectra of the films, characterized by a mixture of wells with different thickness, and by comparing the PL spectra under high energy nanosecond pumping with standard spectra under weak excitation, we identified two temperature regimes, with different emission properties, which are ascribed to the coexistence of two crystalline phases (High Temperature - HT and Low Temperature-LT phases).

In particular, we demonstrated that the PL and ASE properties are strongly affected by the presence, above 190 K, of a minoritary fraction of the HT-phase, which dominates the PL spectra at low excitation density, while the emission of the LT phase dominates the PL spectra at high excitation density. We also observed that ASE is only present at low temperatures in the range between 13 K and 230 K, whereas at higher temperatures the ASE disappears, due to the competition between ASE and reabsorption from charge transfer states of the HT phase.

Our results provide a novel insight into the emission properties of quasi-2D lead bromide perovskites and are expected to be a guide for possible material improvement in order to exploit excellent emission properties of these quasi-2D materials for the realization of low threshold optically pumped lasers.

In photon upconversion the wavelength of the light emitted upon irradiation is shortened, resulting in a gain in photon energy. To comply with energy conservation laws, this process occurs by combining two or more low energy photons. We take advantage of long-lived triplet states to store the energy. However, as triplet states are ‘spin-forbidden’, they are only weakly directly optically accessible. Therefore, so-called sensitizers are required to indirectly populate the triplet state. Triplet sensitizers span a broad range of material classes including metal-organic complexes, nanomaterials and bulk perovskite films. Understanding the energy transfer mechanism is crucial for the further advancement of optoelectronic devices based on upconversion.

The exact triplet sensitization mechanism varies depending on several factors including: (i) the absolute alignments of the sensitizer and acceptor energy levels. (ii) The exciton binding energy in the sensitizer, resulting in excited states in form of excitons or free carriers. (iii) Energetic polydispersity of a sample, which varies the energetic driving force for triplet transfer.

I will present the current status of triplet sensitization using sensitizer materials with varying dimensionalities including 0D quantum dots, 1D nanorods and 2D nanoplatelets.

Materials displaying electron photoemission under visible-light excitation are of great interest for applications in advanced electron beam sources, free electron lasers, electron microscopy, and photochemistry. We demonstrate that using CdSe colloidal quantum dots (CQDs) heavily doped with manganese (Mn) [1], we can realize highly efficient electron photoemission under excitation with visible-light pulses. This effect is enabled by extremely fast (<300 femtoseconds) spin-exchange Auger energy transfer from excited Mn ions to an intrinsic CQD exciton [2]. Since the rate of this process outpaces that of intra-band cooling, the high-energy ‘hot’ electron produced by the first Auger-excitation step can be efficiently promoted further into the external ‘vacuum’ state via one more Mn-to-CQD energy-transfer step. This CQD ionization pathway exploits exceptionally large up-hill energy gain rates associated with the spin-exchange Auger process (>10 eV ps^-1) and leads to photoemission efficiencies of more than 3%, orders of magnitude greater than in the case of undoped CQDs. High photoelectron yields along with extremely short timescales of spin-exchange Auger ionization could enable ultrabright sources of sub-picosecond electron pulses with unprecedented current densities of tens of kiloamperes per cm². In addition to photocathodes, other prospective uses of this phenomenon include advanced hot-carrier photovoltaics, ‘high-energy’ photochemistry, and ultrafast photodetectors enabled by long-range hot-electron transport.