Quantum dots (QD) are regarded as ideal light emitters for current and next-generation displays. Hence, there is an urgent need to produce environmentally friendly QDs that show high efficiency and better color purity. From this perspective, a strategy of tuning the wavelength and spectral width is discussed to optimize the brightness and color space agreement. The critical parameters affecting photophysical properties, such as the uniformity of the InP QD core, the thickness and shape of the ZnSe shell, the electron/hole distribution, surface dangling defects, oxidative phase, and the stacking faults in the crystalline structure, are examined. In addition, quantitative analyses are suggested to understand the nature of the ligands so that practical applications can be diversified. Recently, QD-LEDs using InP-based QDs with controlled shell structure showed potential for future commercialization. For further development, improvement in the stability via the control of inorganic and organic passivating structures is required.

Quantum dots are expected to play a unique as emissive materials with various excitation means, including photo-excitation for photoluminescence, electro-excitation for electroluminescence, and electrochemical-excitation for electrochemiluminescence. Each of these types of applications has its unique requirements for quantum dots. This talk intends to briefly discuss these requirements and tailored design for each type of applications. One thing is in common for any realistic applications as emitters, that the QDs must be core/shell ones. While photoluminescence relies on absorbing a single photon and generating an electron-hole pair within a core/shell QD, electroluminescence and electrochemiluminescence both rely on injection of electron and hole separately from the environment. As a result, electroluminescence and electrochemiluminescence are both sensitive to the interface structure between the inorganic nanocrystal and organic ligands/solvents. Given QDs must be diffuse freely in solution after receiving one of the carriers and before receiving the other, electroluminescence is further more sensitive to the interface structure.

Understanding the structure of cesium lead halide perovskites CsPbX₃ (X = Cl, Br, I) has been a major research thrust. Magnetic resonance spectroscopy has emerged as a powerful tool for probing the structure and structural dynamics of perovskites, in both bulk and nanocrystalline forms. Applying ²⁰⁷Pb NMR on CsPbBr₃ it was confirmed that the PbBr₆-octahedra experience greater structural disorder at the nanoscale, a feature that cannot be readily captured by diffraction-based techniques.[1] The quadrupolar nature of the halides makes them highly sensitive to subtle structural variations, both static and dynamic. The quadrupole interaction can resolve structural changes with accuracies commensurate with synchrotron X-ray diffraction and scattering. Halide NMR (³⁵Cl, ⁷⁹Br) and NQR (⁷⁹Br, ¹²⁷I) spectroscopy show greatly enlarged space-averaged site-disorder in the nanocrystals compared to the bulk, while the dynamics of nuclear spin relaxation indicates enhanced structural dynamics in the nanocrystals.[2] The findings from NMR and NQR were corroborated by ab-initio molecular dynamics, which point to the role of the surface in causing the radial strain distribution and disorder. These findings showcase a great synergy between solid-state NMR or NQR and molecular dynamics simulations in shedding light on the structure of soft lead-halide semiconductors.

The synthesis of monodisperse nanocrystals (NCs) in high yields is a challenge that has only been mastered in few material systems. For lead halide perovskite NCs this problem is particularly challenging, due to the low synthesis temperature, fast reaction kinetics and weak bonding of both the ligands to the NC and within the NCs. The combination of these contributions leads to a finite polydispersity of the crude solution. In fact monodisperse lead halide perovskite NCs have only been reported for colloids after some degree of purification.[1] Zwitterion capped lead halide perovskites are no exception, [2, 3] however, the large synthesis scale, high isolated yields and their robustness against anti-solvent based purification, allows for their size selection into several, stable, monodisperse fractions (up to >100 mg inorganic-core-mass of monodisperse NCs in one fraction), which can be purified of all other solutes and stored for months without deterioration [4]. Such pure, stable, versatile and well defined colloids are needed for the study of intrinsic properties of the NCs such as optical properties and their dependence on NC shape and size. Assemblies of zwitterion capped NCs only form from monodisperse colloidal solutions and exhibit larger domain sizes and higher degree of order compared to superlattices formed from NCs with oleic acid and oleylamine ligands and exhibit super-fluorescence at cryogenic temperatures.

Lead halide perovskites (LHPs) demonstrate great potential for optoelectronic applications such as light emitting devices, solar energy harvesting and spintronics. Understanding the spin dynamics of charge carriers in this material could help improving the efficiencies of those applications. Perovskites inhibit large spin-orbit coupling, leading to a significant fine structure splitting, which makes both, the conduction and valence band two-fold degenerate. This allows for efficient optical orientation of charge carriers, making perovskites an ideal material to investigate the spin dynamics of photoinduced charge carriers.

We elucidate the spin dynamics of photoexcited charge carriers and the underlying spin relaxation mechanisms in CsPbI₃ nanocrystals by employing time-resolved differential transmission spectroscopy (DTS) [1]. We find that the photoinduced spin polarization significantly diminishes during thermalization and cooling of charge carriers toward the respective band edges. Temperature-dependent DTS reveals a decay in spin polarization that is more than 1 order of magnitude faster at room temperature (3 ps) than at cryogenic temperatures (32 ps). As a result of comparing our experimental data to theoretical models, we propose that spin relaxation of free charge carriers in large-SOC materials like LHPs occurs as a result of carrier−phonon scattering, as described by the Elliott−Yafet mechanism.

Anisotropic nanomaterials exhibit unique properties, which allow their implementation into new enhanced optical devices. Dot-in-rods (DRs) exhibit polarized emission with high degrees of polarization (DOPs) and high quantum yields. The energy-efficient generation of polarized light is of major interest for display technologies. Aligned DR nanostructures have been reported as photoluminescent light sources for efficient LCD technologies. Aligned DR films can also find use as color converters with polarized photoluminescence.[1,2]

One of the main challenges is the alignment of DRs for their use in macroscopic devices. This contribution offers insight into the large-scale fabrication of CdSe/CdS DRs by continuous flow processes and their subsequent alignment. The produced films exhibit high DOPs in ensemble, and can be further used as polarized photoluminescent layers in color converters and electroluminescent quantum-based LEDs.

High emission quantum yield, easily tuned emission colors, and high color purity of chemically synthesized lead halide perovskite nanocrystals make this class of materials particular attractive for applications in light-emitting devices [1,2]. I will introduce some of our recent synthetic strategies leading to highly luminescent perovskite nanorods with a strong polarized emission [3-5]. I will also provide some examples of use of perovskite nanocrystals in efficient charge injection LEDs [6], lasing [7], and for temperature imaging in microfluidics [8].

Solution-processed semiconductor nanocrystals have been attracting increasingly greater interestin photonicsincluding spectrally pure color conversion and enrichment in quality lighting and display backlighting [1,2]. These nanocrystals span different types and structures of semiconductors in the forms of colloidal quantum dots and rods to a more recently developing class of colloidal quantum wells. In this talk, we will introduce the emerging field of nanocrystal optoelectronics using solution-processed, efficient, quantum emitters: through the journey of colloidal quantum dots to wells. In particular, we will present a new concept of all-colloidal lasers developed by incorporating nanocrystal emitters as the optical gain media, intimately integrated into fully colloidal cavities [3]. In the talk, we will then focus on our recent work on the latest rising star of tightly-confined atomically-flat nanocrystals, the quasi-2D colloidal quantum wells (CQWs), also popularly nick-named ‘nanoplatelets’. Among various extraordinary features of these CQWs, we will present our most recent discovery that the CQWs uniquely enable record high optical gain coefficients among all colloids [4]. In addition, we will show our results on the controlled stacking and assemblies of these nanoplatelets, which provides us with the ability to tune and master their excitonic properties [5], present the first accounts of doping them for high-flux solar concentration and precise wavefunction-engineered magnetic properties [6], and our record high-efficiency LEDs [7]. Given their current accelerating progress, these solution-processed quantum materials hold great promise to challenge their epitaxial thin-film counterparts in semiconductor optoelectronics in the nearfuture.

Colloidal semiconductor quantum dots (CQDs) are attractive materials for realizing highly flexible, solution-processable optical gain media with readily tunable operational wavelengths [1, 2]. However, CQDs are difficult to use in lasing due to extremely short optical gain lifetimes limited by nonradiative multicarrier Auger recombination [3]. This, in particular, is a serious obstacle for realizing cw optically and electrically pumped lasing devices. Recently, we have explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell CQDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination [4, 5]. Using these specially engineered CQDs, we have been able to realize optical gain with direct-current electrical pumping [4], which has been a long-standing goal in the field of colloidal nanostructures. Further, we have applied these dots to practically demonstrate the viability of a ‘zero-threshold optical gain’ concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [5, 6]. Such charged QDs are optical-gain-ready without excitation, which has allowed us to reduce the lasing threshold to record-low values that are well below the fundamental single-exciton-per-dot limit [6]. Most recently, we have developed CQD devices that operate as both an electroluminescent (EL) structure and a distributed feedback optically pumped laser [7]. By carefully engineering a refractive-index profile across the device stack, we have been able to demonstrate low-threshold lasing even with a very thin EL-active region, which comprises only three monolayers of the QDs.  Yet another recent advance has been the realization of CQD-LEDs that achieve ultrahigh current densities exceeding 1,000 A cm^-2. This has allowed us to inject ~10 excitons per dot and thereby realize population inversion of both the ground-state (1S) and the excited-state (1P) transitions. All of these recent developments suggest that CQD laser diodes (QLDs) are just around the corner. The availability of such devices will benefit numerous fields from integrated photonic circuits and optical interconnects to lab-on-a-chip platforms and wearable devices.

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.^1,2 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.

We report on time-resolved differential transmission measurements on colloidal InP/ZnS and InP/ZnSe core/shell QDs. We can distinguish between electron and hole relaxation processes by optically exciting and probing individual transitions. This, in turn, allows us to determine how the initial excess energy of the charge carriers affects the relaxation processes. An efficient Auger-like electron−hole scattering mechanism circumvents the expected phonon bottleneck for the electron. Accordingly, electrons relax faster than holes. Moreover, the hole relaxation is slowed down significantly for two reasons. First, a small wave function overlap between core and shell states reduces the scattering probability. Second, holes can be trapped at the core/shell interface. This leads to either slow detrapping or nonradiative recombination, thus, reducing the radiative efficiency.

Overall, these results demonstrate that it is crucial to consider charge carrier relaxation when constructing devices. An energetic alignment of the relevant states (in charge transport materials and QDs) should help to maximize radiative efficiency.

Colloidal nanocrystal superlattices are highly ordered aggregates of particles. Crystals are highly ordered aggregates of atoms. However, nanocrystal superlattices are not conventionally considered crystals. But where does the border lie? Previously, we reported that CsPbBr₃ nanocrystal superlattices have a structural perfection comparable with that of epitaxially grown multilayers, which can be considered as full-fledged single-crystals.[1]

In this work, we will discuss a novel approach to the characterization of periodic colloidal nanostructures self-assembled on flat substrates. Our method takes advantage of the modulation of the Bragg peaks profile by the superlattice periodicity, which enriches them in structural information. Nanocrystal superlattices are often studied by grazing-incidence scattering techniques, which require dedicated instruments or synchrotron beamlines and the development of complex and sample-tailored fitting programs. Instead, our approach enables to extract as much detailed information with the help of a common lab-grade diffractometer and a straightforward fitting routine imported from epitaxial multilayers research.[2] The versatility of the analysis is demonstrated on a variety of nanocrystal compositions and shapes (CsPbBr₃ and PbS, nanocrystals and nanoplatelets) with excellent results.

By using our approach, we could accurately determine all the superlattice structural parameters, track their evolution upon treatment and demonstrate that our colloidal superlattices feature nanocrystal displacements in the range 0.5 – 1.3 Å. This opens to promising and easily accessible perspectives for future research in the field of structural and optoelectronic characterization on those and similar systems, such as the 2D layered perovskites. Furthermore, there is a broad interest in nanocrystal superlattices for applications such as quantum light sources, miniband-based electronics and magnetic mesostructures, and a possible bottleneck in their development is the high demands for their structural investigation. The development of a simple and accessible characterization method can significantly speed up the process of turning those fascinating system into real-life applicable devices. 

Lead halide perovskites attract the interest of researchers from a variety of fields due to a unique combination of properties: optoelectronic tolerance to intrinsic defects, particularly long diffusion length of carriers, and highly dynamic crystal lattice. These materials successfully advance towards application in solar cells, light-emitting devices, and high-energy radiation detectors. There was also significant progress in the understanding of their optoelectronic properties on both bulk- and nano-scale levels. This new understanding indicates that finding an alternative lead-free material with similar optoelectronic properties among other, non-group-14 metal halides can be very challenging if possible at all. On the other hand, recent advances in metal halide perovskite photovoltaics indicate that the closest analog to lead halide perovskites, namely formamidinium tin iodide (FASnI₃), could exhibit similarly good performance when synthesized with a low number of trap states. The main challenge with this material originates from the easiness of trap states generation during the processing of tin (II) iodide perovskites. Typically, these trap states are ascribed to the oxidation of Sn(II) to Sn(IV). In this work, we present a colloidal synthesis of FASnI₃ nanocrystals with a high degree of monodispersity and show that their oxidation is not the only reason for low photoluminescence quantum yield. 

Lead halide perovskites have emerged as promising new semiconductor materials for high-efficiency photovoltaics, light-emitting applications and quantum optical technologies. Their luminescence properties are governed by the formation and radiative recombination of bound electron-hole pairs known as excitons, whose bright or dark character of the ground state remains debated [1, 2].

Spectroscopically resolved emission from single lead halide perovskite nanocrystals at cryogenic temperatures provides unique insight into physical processes that occur within these materials. At low temperatures the emission spectra collapse to narrow lines revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level and in bulk materials.

In this talk, I will discuss how magneto-photoluminescence spectroscopy provides a direct spectroscopic signature of the dark exciton emission of single lead halide perovskite nanocrystals [3]. The dark singlet is located several millielectronvolts below the bright triplet, in fair agreement with an estimation of the long-range electron-hole exchange interaction. Nevertheless, these perovskites display an intense luminescence because of an extremely reduced bright-to-dark phonon-assisted relaxation [4]. Resonant photoluminescence excitation spectroscopy allows the determination of the optical coherence lifetimes in these nanocrystals and to assess their suitability as sources of indistinguishable single photons [5]. Memories in the Photoluminescence Intermittency of Single Cesium Lead Bromide Nanocrystals are observed [6].

References:

[1] M. Fu, P. Tamarat, J. Even, A. L. Rogach, and B. Lounis, “Neutral and Charged Exciton Fine Structure in Single Lead Halide Perovskite Nanocrystals Revealed by Magneto-optical Spectroscopy” Nano Lett., 17, (2017) 2895–2901.

[2] G. Nedelcu, A. Shabaev, T. Stöferle, R. F. Mahrt, M. V. Kovalenko, D. J. Norris, G. Rainò, and A. L. Efros, “Bright triplet excitons in cesium lead halide perovskites” Nature  553 (2018) 189–193.

[3] P. Tamarat, M. I. Bodnarchuk, J.-B. Trebbia, R. Erni, M. V. Kovalenko, J. Even, and B. Lounis, “The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state,” Nat. Mater., 18 (2019) 717-724.

[4] M. Fu, P. Tamarat, J.-B. Trebbia, M. I. Bodnarchuk, M. V. Kovalenko, J. Even, and B. Lounis, “Unraveling exciton-phonon coupling in individual FAPbI3 nanocrystals emitting near-infrared single photons" Nat. Commun. 9 (2018) 3318.

[5] P. Tamarat et al., submitted (2020)

[6] L. Hou, C. Zhao, X. Yuan, J. Zhao, F. Krieg, P. Tamarat, M. V. Kovalenko, C. Guo, B. Lounis, "Memories in the Photoluminescence Intermittency of Single Cesium Lead Bromide Nanocrystals" Nanoscale 12 (2020) 6795-6802

The incorporation of diluted concentrations of magnetic impurities in bulk or epitaxially grown semiconductors has been of great interest in the past, for the regulation of magneto-optical properties. The topic received a renewed interest in recent times in colloidal nanostructures, in which a giant enhancement of carrier-to-dopant spin interactions led to new physical phenomena, like gaint magnetization and spin polarized emission. Magnetic doping was implemented extensively in colloidal quantum dots, but with alimited way in anisotropic structures.  The discussed work deals with a control and characterization of  spin degrees of freedom of photo-generated carriers in colloidal seeded nanorods (sNRs) upon implementation of magnetic doping. The material under consideration is CdSe/CdSe:Mn sNRs, including diluted concentration of Mn²⁺ ions across the rod. 

The spin degrees of freedom in the mentioned materials were monitored by an optically detected magnetic resonance (ODMR) spectroscopy, providing a significant information on exact location of host carriers and dopants, as well as examine the interaction between them. The extracted physical parameters from the ODMR experiments included: g-factors and their anisotropy, spin exchange interactions, angular momentum, carrier-dopant coupling constants, radiative and spin-lattice relaxation times.

The temporaly resolved ODMR measurements deconvoluted a few recombination events: band-to-band, trap-to-band and trap-to-trap processes, where carriers' trapping occurred at the seed/rod interface.  Those trapped carriers already possess unpaired spins, endowing selective magneto-optical properties along with a relatively long radiative and spin-relaxation times. The dominant interaction with the magnetic dopant takes place along the seed/rod interface, leading to further enhancement of spin helicity and moreover, bestows an order of magnitude extension of spin-lattice relaxation time in doped sNRs (0.1 msec) with respect to that of the pristine material (0.01 msec), with an extreme importance for practical applications.  The mentioned spin-properties in confined systems, undoubtedly can play an important role in the development of new spin-based technologies.

Light-emitting nanocrystals are highly attractive for novel lighting applications and quantum optics. To exploit their full potential, precise knowledge of their excitation and emission spectra as well as their underlying physical mechanism is crucial. Ensemble inhomogeneity washes out many phenomena such as fine-structure splitting, blinking, and spectral diffusion. Consequently, single-particle studies are pivotal to deepen the understanding. Over the past years, these studies have focused on measuring the emission lifetime and emission spectra of individual particles which restricts extracted information to the lowest excited state. While excitation spectra can provide additional insight into higher excited states, they are inherently much more difficult to measure on single particles. A sequential probing of the different wavelengths is necessary to obtain spectral information and especially blinking thus obscures the single-particle excitation spectrum. We present a novel approach to measure excitation spectra based on a broadband light source and a rapidly tunable narrow filter. We perform repeated spectral excitation scans at rates above 100 Hz to average out effects of blinking. During our scans, we record not only the wavelength that caused the emission of specific photons but also the corresponding arrival times. We identify moments in time where the individual quantum dot was in a “bright” or in a “dim” state. The rapid scanning allows us then to extract the separate excitation spectra of transiently occurring states of the quantum emitter. We leverage this to gain further insight into the origin of blinking in CdSe/CdS/ZnS quantum dots that show multiple discrete dim states. We observe a bright, a grey, and a dark state that occur in the same quantum dot. We attribute the bright and the dark state to excitonic emission while the grey state shows a change in the excitation spectrum that we attribute to a trion state.

Assemblies of lead halide perovskite nanocrystals are promising quantum emitters due to the electronic coupling and collective emission of nanocrystals, which reveal themselves through spectroscopic observables such as low energy narrow photoluminescence peaks and accelerated radiative decays. In order to bring these intriguing phenomena closer towards applications, it is crucial to understand how the stability of notoriously unstable lead halide perovskites affects them. Surprisingly, we found that similar spectroscopic features could arise due to the nanocrystal reactivity with the environment under the common sample storage conditions such as under vacuum or in air. The optical response of individual CsPbBr₃ nanocube assemblies was studied with steady-state and time-resolved micro-photoluminescence techniques. The observed changes at T = 4 K include the appearance of low energy narrow emission peaks (with redshifts as large as ~105 meV and fwhm <20 meV) with lifetimes of ~10-40 ps, a nearly an order of magnitude faster than those of pristine assemblies. These results point to the reactivity of CsPbBr3 nanocrystals with themselves and their environment as a significant roadblock for studies of cooperative emission and highlight the need to develop strategies of lead halide perovskite nanocrystal stabilization which are compatible with high-quality self-assembly.

Band-edge excitons of semiconductor nanocrystals feature a complex set of energy sublevels. Those sublevels affect the light-emission properties of the nanocrystals. At low temperatures, when the thermal energy of the system is comparable to the energy differences between those sublevels, multi-exponential decay dynamics of the fluorescent emission is probed with time-resolved measurements [1]. Often temperature-dependent decay studies are used to gain insight into the transition rates between sublevel states, recombination rates, and energetic ordering of the sublevels. However, such studies can sometimes yield multiple interpretations on the dynamics of the band-edge exciton. Here we show that control of the local density of optical states (LDOS) of the nanocrystal environment is another technique to probe exciton fine-structure states. As initially shown by Drexhage on europium complexes [2], the LDOS modifies the rate of radiative transitions according to Fermi’s golden rule. This is experimentally achieved by placing the nanocrystals at different distances from a reflecting surface, therefore exposing them to a different LDOS. We record low-temperature time-resolved photoluminescence, which reflects changes in radiative decay rates. Using CdSe-based nanocrystals, we show that our method provides a complementary tool to investigate the dynamics between excited-state energy sublevels and to estimate the radiative recombination efficiency of the energy sublevels in the low-temperature regime. Our approach is also easily extended to other fluorescent colloidal nanocrystals.

For state-of-the-art ensembles of colloidal CdSe quantum dots with size dispersion less than 5 % typical width of the photoluminescence (PL) spectrum under nonresonant excitation exceeds 100 meV. It results in spectral overlap between zero phonon (ZPL) and phonon-assisted (1PL) emission lines of dark excitons from quantum dots of different sizes within an ensemble. Recently it was shown theoretically that while ZPL emission is circularly polarized, 1PL emission is predominantly linearly polarized [1]. We propose a theoretical model which demonstrates that polarization properties of these recombination channels of the dark exciton are responsible for the long-standing contradiction between intensities and spectral positions of the σ⁺ and σ^– polarized PL peaks in the external magnetic field [2-5].

The proposed theoretical model is used to describe the experimental data on circularly polarized PL in magnetic fields up to 30 Tesla for CdSe nanocrystals with diameter 3.3-6.1 nm embedded in silica glass matrix. It is shown that the saturation level of degree of circular polarization of PL (DCP) at high magnetic fields is controlled by the ratio between ZPL and 1PL intensities, which is found to be higher in small nanocrystals.  We demonstrate that 1PL emission results in an increase of DCP towards the high energy side of PL spectra due to the gradual number decrease of NCs which can provide this contribution.  Finally, we show that shifts of σ+ and σ– polarized PL peaks in the external magnetic field are also controlled by 1PL emission and not by the Zeeman shift of the dark exciton sublevels.

Strong interaction of charge carriers with a soft and polar lattice challenges our understanding and exploitation of charge-carrier dynamics in lead halide perovskites. For example, surprisingly slow electron cooling in metal-halide perovskites were reported recently [1] and may enable efficient hot-electron charge extraction [2], hot-electron plasmonics and catalysis [3], or carrier multiplication. Colloidal perovskite nanocrystals add yet additional knobs of control to leverage high efficiencies: their facile tuning of composition, size, and shape, and a synthetically very accessible surface all may be used to optimize charge-carrier lifetimes. Here, we combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 nm to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites [1], no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling of electrons in the vicinity (within ~ 0.1 eV) of the conduction band edge. Instead, we suggest that the observed cooling process may be rationalized by fast phonon-mediated intra-band transitions driven by strong and size-dependent electron-phonon coupling. Our ab initio simulations allow a direct comparison to a variety of time-resolved spectroscopies, intuitively explains the persistent ‘warm’ electrons, and yields the spectrum of phonons modulating the excited electron and hole states. The presented experimental and computational methods may easily be extended to a wider range of material systems and thus guide the development of devices utilizing hot charge carriers.

The binding of ligands to nanometer-sized surfaces is a central aspect of II-VI colloidal nanocrystal research, for which CdSe nanocrystals have been used as the main model system to evaluate different surface chemistries. In this work, we revert this approach and analyze the binding of a single ligand to two different materials. In this work, we present a detailed comparison of the binding of a particular Z-type ligand, cadmium oleate (CdOA₂), to colloidal CdSe and CdS NCs. We make use of quasi-spherical CdSe and CdS NCs with similar sizes and zinc blende crystal structures. Using solution ¹H nuclear magnetic resonance (NMR) spectroscopy, we demonstrate that in both cases, as-synthesized, purified NCs are capped by cadmium oleate. On the other hand, we find that CdS has a significantly higher ligand surface concentration (4.6 nm^-2) than CdSe (3.6 nm^-2). In both cases, the addition of BuNH₂ results in cadmium oleate displacement and the corresponding isotherms point towards binding site heterogeneity. Describing cadmium oleate displacement using a weakly and a strongly binding pool, we find that the weak bindings sites exhibit the same displacement energy for both CdSe and CdS. On the other hand, we find that CdS exhibits a considerably larger fraction of strongly bound ligands. These findings contrast with DFT calculations on NC model systems, which show that the adsorption energy of cadmium oleate is, on average, 32 kJ/mol larger for CdS than for CdSe. To account for this discrepancy, we argue that the NC purification that precedes the displacement analysis removes ligand up to a critical displacement energy threshold. As a result, purified CdSe NCs will have a smaller ligand concentration than CdS NCs, yet the adsorption energy of the weakest binding ligands remaining is identical in both cases. This conclusion highlights the interplay between NC processing and the actually observed surface chemistry of purified NCs.

Semiconductor nanocrystals (NCs) can exhibit narrow emission across the visible and NIR spectrum but suffer from photoluminescence intermittency (blinking) that is associated with irreversible photochemical damage.^[1] Even under continuous excitation NC emission randomly switches between ON and OFF states, with both lacking characteristic lifetimes due to their power-law probability distributions.^[2] It is a challenge to spectroscopically distinguish between proposed mechanisms due in part to the very different timescales of a single excitonic life-cycle and the governing intermittent dynamics.  Particularly, simple first-order kinetics are unable to capture the observed ON- and OFF-time distributions.  Blinking presents an intriguing fundamental challenge as mechanistic origins of the observed scale-free distributions remain unclear.

Here, we demonstrate all-optical two-colour modulation of blinking statistics in individual CdSe/ZnS NCs using sub-bandgap light. Motivated by single-NC measurements, we use a continuous wave source tuned to the stimulated emission transition to alter blinking statistics in individual CdSe/ZnS NCs. Accounting for background, we compare the modulated/unmodulated photon streams and discover that sub-bandgap modulation steepens the power-law slope of the ON-time probability distribution by Δα_ON=0.46±0.09. This effect is characteristic (a consistent relative increase is observed across numerous NCs), selective (no changes with longer-wavelength, off-resonance modulation), and robust (data reduction and statistical truncation had negligible effects on the slope change).  This slope change is also pervasive and is observed in 19/20 of the NCs examined at an irradiance of 2.6 kW/cm². Intriguingly, we observe neither a concurrent change in the OFF-time slope, nor a significant effect on the truncation times of the ON- or OFF- probability distributions—as would be expected for a simple stochastic process. Our technique provides an all-optical method with an internal standard to monitor and modulate the emission from individual NCs and will facilitate mechanistic insight into the causes of blinking. 

Lead halide perovskites have gained a lot of popularity in the field of optoelectronics in recent times. Where the toxicity of lead remains a major issue, an emerging non-toxic alternative proposed is lead-free double perovskites with the generic stoichiometric formula A₂M^IM^IIIX₆. In double perovskite, the divalent lead is replaced with one monovalent (M^I) and one trivalent (M^II) metal cation. Cs₂AgBiBr₆ is one of the stable double perovskite with an indirect bandgap where the optical properties and the charge carrier relaxation processes are not fully understood. We applied time-resolved photoluminescence and differential transmission spectroscopy to explore the photo-excited charge carrier dynamics within the indirect band structure of Cs₂AgBiBr₆ nanocrystals. We observed a high energetic emission at the direct bandgap, alongside the emission from the indirect bandgap transition. We assign this emission to the radiative recombination of the direct bound excitons originating due to trapping of holes. Due to the electron intervalley scattering process, the emission maximum from this direct bound excitons redshifts over 1 eV within 10 ps, leading to its transfer from direct to indirect bound exciton. We conclude that this direct bound exciton has giant oscillator strength which causes the higher energetic emission to occur at the direct bandgap despite the prevailing intervalley scattering process. These results expand the current understanding of the optical properties and the charge carrier relaxation process in double perovskites family, thus, facilitating the further development of optoelectronic devices harnessing lead-free perovskites. [1]

Lead-halide perovskite APbX₃ (A=Cs or organic cation; X=Cl, Br, I) nanocrystals (NCs) are subject of intense research due to their exceptional properties as both classical [1] and quantum light sources [2-4]. Many challenges often faced with this material class concern the long-term optical stability, a serious intrinsic issue connected with the labile and polar crystal structure of APbX₃ compounds. When conducting spectroscopy at a single particle level, due to the highly enhanced contaminants (e.g., water molecules, oxygen) over NC ratio, deterioration of NC optical properties occurs within tens of seconds, with typically used excitation power densities (1-100 W/cm²), and in ambient conditions.

Here [5], we demonstrate that choosing a suitable polymer matrix is of paramount importance for obtaining stable spectra from a single NC and for suppressing the dynamic photoluminescence (PL) blueshift. In particular, polystyrene (PS), the most hydrophobic amongst four tested polymers, leads to the best optical stability, one-to-two orders of magnitude higher than that obtained with poly-(methyl methacrylate) (PMMA), a common polymeric encapsulant containing polar ester groups. Molecular mechanics simulations based on a force-field approximation corroborate the hypothesis that PS affords for a denser molecular packing at the NC surface. These findings underscore the often-neglected role of the sample preparation methodologies for the assessment of the optical properties of perovskite NCs at a single-particle level and guide the further design of robust single photon sources operating at room temperature. 

We demonstrate that non-chiral perovskite nanostructures can exhibit circular dichroism (CD)  in the absence of a magnetic field and without their activation by chiral molecules. The exciton dispersion in orthorhombic perovskites in the presence of Rashba spin-orbit terms creates   helical exciton states, which are split from each other.   The splitting can be  described  as a  Zeeman  effect in an effective magnetic field,  whose direction  and magnitude depend on the exciton momentum.   The selective excitation of these states by helical light gives rise to CD.  Using experimentally determined material parameters, we calculate significant circular dichroism  of order 10%  in orthorhombic perovskites under  off-normal, top illumination.  These calculations  suggest the effect is observable  and CD can be measured  in non-chiral perovskite nanostructures such as layered-2D perovskites or nanoplatelets.

Active imaging is an infrared mode of imaging where an eye invisible source is used to illuminate a scene and the scattered light is collected on a sensor. Such applications used for industrial vision or LIDAR detection requires an infrared source and a detector. These latters typically operate in the short wave infrared (1-3 µm range).

Here i will review different steps of progress required to achieve nanocrystal based infrared active imaging.

I will start with recent progresses relative to HgTe nanocrystal growth using liquid mercury and enabling mass scale and greener synthetic route for this material. [1]

Compared to conventional epitaxially grown semiconductor, nanocrystals consideraly ease the coupling to the read out circuit and i will discuss how conductive ink can be deposited to obtain a large absorption from VGA format focal plane array. [2]

In the last part, i will discuss progresses obtained on the source side. While HgTe appears as a versatile platform for detection, its use for electroluminescence remains limited. I will present some new results in this direction and show that the obtained LED is compatible with application such as moisture detection.

The ability to control the charge carrier transport in the colloidal NC assemblies is fundamental for altering their electronic and optical properties for the desired applications. While surface doping and ligand manipulations have been very effective, they generally suffer from the sample-to-sample variability and insufficient long-term retention of the surface stoichiometry due to environmental effects. Wherever possible and applicable, the formation of a more stable core-shell morphology wherein shell acts as a dopant and carrier-regulating region is preferred. For many applications in devices where charge carrier transport functionality is one of the most prominent, the role of shells in the core@shell NCs are still not well understood. The shell might not only provide the NCs with better stability but may also result in vastly different physical properties of the overall heterostructure. Here we demonstrate a strategy to render the solids of narrow-bandgap NC assemblies exclusively electron-transporting by creating a type-II heterojunction via shelling. Electronic transport of molecularly cross-linked PbTe@PbS core@shell NC assemblies is measured using both a conventional solid gate transistor and an electric-double-layer transistor. The transport characteristics are compared with those of core-only PbTe NCs. In contrast to the ambipolar characteristics demonstrated by many narrow-bandgap NCs, the core@shell NCs exhibit exclusive n-type transport, i.e., drastically suppressed contribution of holes to the overall transport. The PbS shell that forms a type-II heterojunction assists the selective carrier transport by heavy doping of electrons into the PbTe-core conduction level and simultaneously strongly localizes the holes within the NC core valence level. Any efforts to use various kinds of hole dopants are not strong enough to counteract the influence of the shell. Furthermore, the use of electric-double-layer transistor to scan broadly the Fermi level provides a clear pictorial related to the absence of hole transport and the enhanced electron transport. This strongly enhanced n-type transport makes these core@shell NCs suitable for applications where ambipolar characteristics should be actively suppressed to enhance their performance, in particular, photodetectors, thermoelectric devices and the selective electron-transporting layer in solar cells and the other optoelectronic devices.

Colloidal Quantum Dots (CQDs) are a broadly tunable building block for low-cost optoelectronic devices and particularly infrared (IR) photodetectors.¹ However, due to hooping transport the carrier diffusion length remains short, typically few tens of nanometers², which is more than one order of magnitude shorter than the absorption depth (few µm). Consequently, to efficiently collect charge, only thin film can be used and only few percent of the light is absorbed. To tackle this issue, we have designed light-matter coupling devices using plasmonic resonator to drastically increase the absorption in photoconductive devices and boost responsivity.

The light-matter coupling is based on sub-wavelength resonator and relies on guided mode resonance (GMR): a metallic grating focuses the incident light in a thin slab of nanocrystals that acts as a wave guide.³ The structure is designed to achieve 100 % light absorption at the targeted wavelength. A key challenge is to make the absorption within the CQD film and not in the metal, to avoid losses.

This structure is versatile and can be applied to different materials (PbS and HgTe) at different wavelengths (1.55 and 2.5 µm, respectively) in short wave IR and extended short wave IR.⁴ There is an excellent agreement between electromagnetic simulation and the measured photocurrent. The responsivity is increase of several orders of magnitude due to the enhancement of absorption (factor 3-6) and of photoconductive gain as well. This method can also be applied to a film of mixed CQDs⁵ (FAPI/PbS) in order to increase the absorption in a low-dark current IR system.

Beyond their use as light sources for displays, nanocrystals also appear as promising candidates to design low cost infrared sensors. In such devices the carrier density is a key parameter driving the signal-to-noise ratio. The carrier density can be controlled thanks to the gate in a field effect transistor configuration. Most common gates are SiO₂ and electrolyte^[1] which are respectively limited by their low capacitance and (only) room temperature operation. Here, we explore a high capacitance solid state gating from ionic glass (LaF₃). The method is versatile and ca be applied PbS and HgTe NCs thin films with ionic glasses.^[2] We show that by tuning the operating gate bias the signal to noise ratio can be improved by a factor of 100.

In a second step this high capacitance gate is coupled to graphene electrodes enabling (i) IR transparency, (ii) tunable work function of the contacts and (iii) propagation of the gate induced doping to the film thanks to the large quantum capacitance of graphene. We demonstrate the formation of a p-n junction improving charge extraction.^[3] The latter enable operating condition which simultaneously maximizes the response and reduces the dark current enhancing the detectivity by two orders of magnitude.

Applications in quantum optics have stringent performance requirements on single-photon sources and detectors, in addition to the need for low-loss passive components. Integrating the building blocks on a chip-based platform can enable scaling up to larger experiments, whilst improving phase stability. A key property of single-photon sources is their radiative lifetime, which can be modified through the high local density of states created by plasmonic antennas.

In recently published work we demonstrate an enhancement of the microsecond lifetime of IR-emitting colloidal PbS/CdS quantum dots (QDs), with a concurrent increase of the count rate. We further show wavelength-resolved measurements of the Purcell enhancement of QDs deterministically positioned in the gap of plasmonic antennas, performed on a single photonic chip. This provides an in-depth analysis of the enhancement and holds promise to wavelength-multiplex multiple single-photon emitters on the same chip. Making use of low-fluorescence silicon nitride with a waveguide loss smaller than 1 dB/cm, we implemented high extinction ratio optical filters and planar concave grating spectrometers. Waveguide-coupled superconducting nanowire single photon detectors (SNSPDs) allowed for highly efficient and low time-jitter single-photon detection. Through a careful analysis of the different contributions to the count rate at the SNSPDs we predict our method to scale down to single QDs. In addition, newly developed emitters can be readily integrated on the chip-based platform. This furthermore enables lifetime-spectroscopy of solution-processed nano-materials at cryogenic temperatures, using a single photonic chip.

I employ a topotactic method called cation exchange to produce semiconductor nanocrystals (NCs) in novel morphologies, compositions, and crystallographic phases. My dissertation research focused on the understanding of the physical properties and phase transitions of these new nanomaterials prepared by cation exchange1–3. In this talk, I shall describe the countless possibilities of the exploration of physicochemical properties and applications of molecularly precise semiconductor nanoclusters, a class of materials that we were able to expand with the help of cation exchange. In particular, I shall discuss how ultrasmall copper selenide (Cu2-xSe) NCs prepared by cation exchange of cadmium selenide NCs exhibit a disordered cationic sub-lattice under ambient conditions1. Known superionic materials, such as AgI, Cu2Se etc. in their bulk form, display this phase transition at high temperatures and/or pressures, making them unsuitable for many applications. As a follow-up to this study, I shall describe my investigations of Li-doping of Cu2-xSe NCs and how this doping influences the crystal structure and consequently the phase transition behavior2. For this study to pave the way to fundamental understanding on ion transport behavior in solids, and applications as solid-state electrolytes, thermoelectrics and ultrafast electronic switches, the possible mechanism of ionic transport in these NCs remains to be investigated. As a separate demonstration on the synthesis and stabilization of a non-natural phase in NCs, I shall explain on the basis of optical spectra measurements and density functional theory (DFT) calculations how HgSe NCs3, prepared using cation exchange is found to have an inverted band structure along with a finite band-gap, making it a potential 3D topological insulator.

Improved mechanistic understanding of the colloidal synthesis of PbS nanocrystals will guide synthetic progress, and advance their use in near-infrared optoelectronic applications. Here we show that the conventional nucleation and growth of PbS nanocrystals per Hines exhibits two-step kinetics involving an intermediate species [1].  The metastable intermediate is small, lead-rich, and has characteristic, reproducible visible-wavelength emission—all consistent with a PbS pre-nucleation cluster. Growth in the presence of the cluster permits a long size-focusing period and has historically yielded large nanocrystals (⌀ > 4nm, hν_peak,abs<1.05eV, λ_peak,abs>1180nm) with the narrowest reported ensemble excitonic absorption peak widths. However, we find that the competing generation of the meta-stable cluster suppresses the nucleation rate of nanocrystals. This ultimately requires that the reaction be quenched when small nanocrystals are desired—short-circuiting size-focussing growth and affecting size-dispersity, yield, and scalability.

We then show that the introduction of Lewis bases to the reaction can selectively disrupt the cluster, offering a new route for synthetic control. By tuning the strength of the interaction through the pK_a of amines or the multidentate interaction of glycol ethers, the cluster can be effectively suppress in favor direct nucleation from a molecular intermediate, and permitting the synthesis of PbS nanocrystals as small as (⌀~1.7 nm, hν_peak,abs=2.2eV, λ_peak,abs=560 nm) with ensemble linewidths that are up to 25% narrower than previous leading reports.

The colloidal synthesis of III-V quantum dots (QDs) has since its very beginning lagged behind development of other classes such as II-V and IV-VI QDs. Precursors employed in their synthesis are often highly reactive, pyrophoric and commercially unavailable, posing a persistent challenge to synthesis and commercialization of these nanomaterials. Considering the case of InAs, many potential applications such as biological imaging, lighting and sensing have been explored due to its high-performing optoelectronic properties in the near-infrared and short-wave infrared.

Addressing this gap between arduous, small-scale syntheses and a rich application landscape, we report on a new synthetic scheme of InAs QDs[1]. Recently popularized tris(amino)arsines are combined with In(I)Cl as the key precursor. In(I) is proposed to sequentially reduce As-N bonds, yielding As(-3) and In(III). These species go on to smoothly form InAs QDs with first excitonic absorption features in the range of 700–1400 nm. In(I)Cl forms a dynamic equilibrium with In(0) and InCl3 under reaction conditions, delivering the active In(I) species in a controlled manner. The synthesis exhibits straightforward execution, gram scale batch sizes with maintained nanoparticle quality and a facile workup procedure under ambient conditions. The as-prepared particles are shown to incorporate well into existing CdSe shell growth schemes, furnishing core-shell QDs emissive in the window 1000–1500 nm with a narrow photoluminescence full-width-at-half-maximum of about 120 meV throughout.

In summary, harnessing benign, commercially available precursors with well matched reactivity opens up a new paradigm towards the synthesis of colloidal III-V QDs. Ease of execution, scalability and flexibility of this approach are likely to spur further research and advance commercial viability of colloidal nanomaterials in a diverse set of applications in the infrared. The chemistry described here is expected to translate to other indium-based materials, serving as an alternative platform to previously reported routes.

Colloidal quantum dots are attractive building blocks for highly miniaturized and integrated components in optoelectronic applications. In combination with rather simple processability at room temperature, this has led to considerable interest in quantum dot photodetectors in recent years [1]. In particular, PbS quantum dot film photodetectors have been realized with record performance in the visible and near infrared spectral range [2]. Here, we investigate highly miniaturized PbS photodetectors involving only a few to a few hundred quantum dots, aiming at efficient light-to-current conversion with a nanoscale footprint.

We investigate PbS-MAPbI3 quantum dots in lithographically tailored gold electrodes as a controlled platform for characterizing the photocurrents from small quantum dot ensembles. The electrode gaps filled with quantum dots are varied between 15 nm and 1.5 µm, the generated currents are in the pA-nA range for nW-µW light power. The MAPbI3 ligands enable carrier tunneling between the individual quantum dots when deposited as closely packed ensembles by spin coating.

We demonstrate that PbS-MAPbI3 quantum dots are reliable nanoscale light/current converters and correlate the measured photocurrents to the quantum dot number, the gap voltage and light irradiance. For the latter, we find a photocurrent power law dependence with an exponent of 2/3. Furthermore, we probe the role of plasmonic effects in the gold electrodes and image by scanning photocurrent microscopy the spatial dependence of photocurrent generation.

PbS nanocrystals are among the most-studied infrared-active QD materials, with potential applications ranging from solar cells to IR LEDs to photon up/down conversion systems and thermoelectrics. Despite their rich history we continue to learn new things about PbS QD chemistry, photophysics, and self-organization into ordered solids. In this talk, I will highlight recent work from my lab at MIT on the structure of PbS QD surfaces. In particular, I will demonstrate neutron scattering as a powerful emerging tool for the characterization of colloidal QDs and the detailed information that small-angle neutron scattering (SANS) can provide about the ligand layers surrounding QDs. Additionally, I will demonstrate the underappreciated role of unbound – or “free” – ligand in mediating the structure of self-assembled QD superlattices.

Imaging and sensing in the short-wave infrared (SWIR) range has been crucial in high-end applications (surveillance, defense, low-light imaging) and is now gaining importance in new consumer applications (automotive, AR/VR, smartphone depth sensing). As opposed to incumbent InGaAs and HgCdTe photodetectors, organic and quantum dot materials enable pixel stacks for integration directly on top of CMOS readout circuits [1,2]. This allows monolithic infrared imagers beyond the cut-off wavelength of silicon (above 1100 nm) and a compact sensor form factor. Patterning by photolithography makes it possible to pixelate the stack and integrate different types of pixels side-by-side. As for manufacturability, fabrication on wafer-scale enables high throughput and the resulting low cost of imagers. Monolithic integration offers high pixel density (even below 1 micrometer) and multi-megapixel resolution. In this presentation, we will summarize recent advances on pixel and integration concepts and present our pixel and sensor results. Imec prototype imagers demonstrate sensitivity at 940 and 1450 nm wavelength, with a roadmap towards multispectral arrays, opening a multitude of new applications. 

Infrared photodetection is an interesting research and applications area for colloidal quantum dot.  Within the present technology, photon detection with infrared epitaxial materials is very expensive while bolometric detection suffers from lower sensitivity and speed.  CQDs, with wide spectral tunability, molecularly passivated surfaces, high throughput fabrication and direct coating of silicon read-out chips, could change the infrared technology.   Pioneering work on PbS and HgTe quantum dots showed early promises, and commercialization of PbS CQD imaging systems started. Raising the performance drives exciting research on many generic issues of colloidal quantum dots, including size and shape control, surface passivation, mobility, doping, non-radiative processes, Auger relaxation, etc. 

Colloidal quantum dots (CQDs) have emerged as prime contenders for the next-generation thin-film optoelectronics and tandem photovoltaic (PV) technologies, thanks to their solution-processability, tunable bandgaps and strong optical absorption. This rapid rise is largely owed to: robust surface ligand exchange recipes and efficient device architectures. The state-of-the-art CQD solar cells have an n-i-p device architecture that involves a solution-phase ligand exchanged CQD absorber sandwiched between a ZnO electron transport layer (ETL) and a solid-state ligand exchanged CQD hole transport layer (HTL).

In the first part of this talk, I will present our latest insights on solid-state ligand exchange and highlight the abrupt phase transition the CQDs undergo as the ligand shell gets exchanged due to ligand-ligand coupling signaling an optimized exchange.[1] These results have direct implications on the optimization of efficient n-i-p solar cells, and are an answer to the question - what embodies an optimized ligand exchange of CQD solids? - that has traditionally been addressed via ‘trial-and-error’.

In the second part, I will discuss scalable fabrication of CQD PV and methods to boost its long-term shelf-life, operational stability and UV-tolerance. >1-year shelf-life is demonstrated for blade-coated solar cells with >10% power conversion efficiency fabricated in a high-humidity ambient environment.[2] This result highlights that solution-phase ligand exchange has made CQD PV directly compatible with low-cost, roll-to-roll fabrication. Finally, the usually-employed ZnO electron transport layer is shown to hold back further improvements in stability. An efficient replacement with ultrathin oxide bilayers is suggested.[3]

Colloidal lead based nanocrystals (NCs) have attracted broad interest for applications in solution-processed voltovoltaic devices.[1] In this presentation, I will discuss several different matierals and approaches for colloidal metal halide, chalcogenide and chalcohalide nanocrystals in energy harvesting and detection devices.

First, I will discuss the room-temperature synthesis of inks based on CsPbBr₃ perovskite nanocrystals using short, low boiling-point ligands and environmentally friendly solvents that are easy to process into high quality thin-films.[2]. The robustness of these films is further demonstrated by the fabrication of the first CsPbBr3 NC-based solar cells, with density of short circuit current higher than 6 mA cm^-2 and open circuit voltages as high as 1.5 V.

We also explored rectangular-shaped PbS nanosheets with a highly monodisperse thickness of thickness of 1.2 nm.[3] In contrast to rocksalt PbS nanocrystals, these nanosheets have an orthorhombic crystal structure and lack both excitonic absorption features and photoluminescence. The PbS nanosheets are though highly photoconductive in films, with a responsivity up to 0.1 A W–1 and a detectivity of 1.3 × 10^9 Jones and perform significantly beter under bending stress compared to that of films of PbS quantum dots, indicating their use in flexible devices.

Finally we studied a series of surfactant-stabilized lead chalcohalide nanocrystals.[4] The Pb₄S₃Br₂ NCs feature a remarkably narrow size distribution a good size tunability (from 7 to∼ 30 nm), an indirect bandgap, photoconductivity (responsivity= 4±1 mA/W) and stability for months under air. We could also prepare NCs of Pb₃S₂Cl₂ and Pb₄S₃I₂. Although not directly usable for energy harvesting, it highlights the important role of colloidal chemistry in the discovery of new materials otherwise not stable, and motivates further exploration into metal chalcohalides NCs.

[1] Q. A. Akkerman, et al. Nat. mater. 17, 394 (2018)

[2] Q. A. Akkerman, et al.Nature  Energy 2, 16194 (2017)

[3] Q. A. Akkerman, et al.Chem. Mater. 2019, 31, 19, 8145–8153

[4] S. Toso, Q. A. Akkerman et al. J. Am. Chem. Soc. 2020, 142, 22, 10198–10211

Semiconductor quantum dots (QDs) are an easily tunable family of materials with excellent optoelectronic properties. With appropriated QD synthesis and surface passivation is possible to decrease the non-radiative recombination pathways, increasing significantly the photoluminescence quantum yield (PLQY) in comparison with their bulk counterparts. These properties give quantum dots an immense potential for the development of optoelectronic devices, including energy applications in photovoltaic devices. Beyond the direct application of colloidal QDs in the light harvesting, PbS QDs can interact synergistically with halide perovskite in order to produce halide perovskite solar cells with enhanced properties. We show that the interaction of PbS quantum dots and nanoplatelets can produce the stabilization of FAPbI3 perovskite black phase and also the increase of the efficiency, stability and reproducibility of the photovoltaic devices prepared with FAPbI3. The use of PbS nanoplatelets also improve the long term stability of CsFAPbI3 perovskite. Incorporation of PbS QDs allows the dramatic decrease of the annealing temperature for the formation of black FAPbI3 phase perovskite thin film, from the 170ºC required without QDs to 85ºC when QDs are present. This result points the interest of Perovskite-Quantum Dot Nanocomposites, for further development of advanced optoelectronic devices.

Opto-electronic devices based on all-inorganic perovskite systems are an energy-efficient source of lighting due to their high photoluminescence quantum yield (QY). However, dominant surface trapping continues to plague the field, despite their high defect tolerance, as evidenced by the several fold improvements in the external quantum efficiency of perovskite nanocrystals (NCs) upon appropriate surface passivation or physical confinement between high bandgap materials. Here, we introduce the concept of drip-feeding of photo-excited electrons from an impurity-induced spin-forbidden state to address this major shortcoming.  An increased and delayed (about several milliseconds) excitonic QY, and density functional theory establish the electron back-transfer signifying efficient recombination.  We term this electron back-transfer from Mn²⁺ to the host conduction band in this prototypical example of Mn-doped CsPbX₃ (X = Cl, Br) NCs through vibrational coupling as Vibrationally Assisted Delayed Fluorescence (VADF).

The urgency for affordable and reliable detectors for ionizing radiation in medical diagnostics, nuclear control and particle physics is generating growing demand for innovative scintillator devices. Plastic scintillators consisting of polymeric matrices embedding scintillating materials offer valuable advantages over conventional inorganic scintillating crystals owing to their design flexibility, low fabrication cost and light weight. Key features to be attained are efficient scintillation, fast emission lifetime, high interaction probability with ionizing radiation - enhanced by the presence of high atomic number (Z) elements – as well as mitigated reabsorption capability, so as to minimize optical losses in large volume/high-density detectors. Lead halide perovskite nanocrystals (NCs) have very recently emerged as valid scintillation materials that can be produced at room temperature via large-scale, cost-effective routes. In this talk, I will show how we can advantage of such appealing features to realize low reabsorption, fast and efficient plastic scintillator exploiting CsPbBr₃ perovskite NCs as high-Z sensitizers for a conjugated organic dye specifically designed so as to feature a large Stokes shift and fast emission lifetime. Binary NC:dye blends incorporated into poly methyl methacrylate (PMMA) nanocomposites show essentially loss-free sensitization resulting in radioluminescence performance comparable to high quality commercial bismuth germanate crystals and emission lifetime of ~3.4 ns that is competitive with established fast scintillators based on rare earths. Scintillation measurements using 5.5 MeV α-particles emitted by ²⁴¹Am isotopes suggest an achievable light yield larger than 1000 ph/MeV_α. Finally, proof-of-concept large size plastic scintillators consisting of optical grade PMMA waveguides coated with our NC:dye nanocomposites show nearly reabsorption-free scintillation for long optical distances under both X-rays and α-particle excitation.

The solar-driven photocatalytic splitting of water into hydrogen and oxygen is a potential source of clean and renewable fuels. Yet it is known, following four decades of global research, to be highly challenging. Perhaps the framework of solar-to-chemical energy conversion may be revolutionized by the examination of alternative oxidative pathways. Here, I will present a demonstration of efficient and robust endothermic full-cycle redox transformations, synthesizing solar chemicals. Utilizing nanoscale particles, we obtained hydrogen production with photon to hydrogen quantum efficiencies of up to ~70%, under visible light and mild conditions, while simultaneously harvesting solar chemical potential for valuable oxidative chemistries, realizing genuine solar-to-fuel energy conversion, with state of the art efficiencies of up to 4.2%.

Colloidal quantum dots have benefited from impressive progress in their synthesis, surface management, colloid engineering, and materials processing into well-passivated, sharp-bandedge, CQD solids. At the device level, the global community has an increased understanding of how these properties are best exploited in the realization of increased-performance solar cells and sensors. I will overview advances and also talk about the roadmap for the field.

Singlet-fission (SF) is a carrier multiplication process in organic materials where a photo-excited singlet state decays into two triplet excitons, each with roughly half the excitation energy. Integrated properly with a photovoltaic (PV) device the singlet-fission material can generate two charge carrier pairs per absorbed photon, leading to a significant increase in device performance. However, major challenges remain in how to integrate the SF material efficiently. A promising solution is to re-emit the exciton energy from the two triplets as two low energy photons that can be re-absorbed by the PV device. This scheme allows for the decoupling and separate optimization of the PV device and SF-photon multiplier material. Unfortunately, triplet excitons are inherently dark. By transferring triplet excitons into emissive PbS quantum dots (QDs) we are able to convert these dark states into photons.

Here I will present our latest work on PbS-TIPS-Tetracene hybrid materials that shows a 60% enhanced QD photoluminescence when exciting the SF material in solution and in film. I will focus on the triplet energy transfer from the SF material TIPS-Tetracene to PbS quantum dots, and how it can be optimized through surface engineering with various ligands and quantum dot size. I will also touch upon the role of surface ligands on the film forming properties of the QD-organic hybrid materials. The fundamental understanding of the triplet energy transfer process to QDs discussed here will benefit the future design of other optoelectronic organic-inorganic hybrid nanomaterials

Currently, intensive research efforts focus on the fabrication of meso-structures of assembled colloidal quantum dots (QDs) with original optical and electronic properties. Such collective features originate from the QDs coupling, depending on the number of connected units and their distance [1]. However, the development of general methodologies to assemble colloidal QD with precise stoichiometry and particle-particle spacing remains a key challenge. Dimers of CdSe QDs, stable in solution, can be obtained by engineering QD surface chemistry, reducing the surface steric hindrance and favoring the link between two QDs. The connection is made by using alkyl dithiols as bifunctional linkers and different chain lengths are used to tune the interparticle distance from few nm down to 0.5nm. The spectroscopic investigation highlights that coupling phenomena between the QDs in dimers are strongly dependent on the interparticle distance and QD size, ultimately affecting the exciton dissociation efficiency [2].

Semiconductor quantum dots (QDs) are prime candidates as harvesters of light and donors of excited charge carriers for solar energy conversion. The unique properties of QDs can be exploited to generate desirable energetic offsets to promote interfacial charge transfer between QDs. Our group’s recent efforts have established the validity of utilizing carbodiimide-mediated coupling chemistry to selectively tether two QDs through the formation of an amide bond between the terminal functional groups of capping ligands [1]. We previously reported on excited-state hole transfer in colloidal CdS/CdSe QD heterostructures, which exhibit quasi-type-I interfacial energetic offsets [1]. Type-I energetics rely on the excitation of one QD component, which results in unidirectional charge transfer and hindered charge separation.

This presentation reports on our efforts to improve and expand upon our previous work in two ways. First, we synthesized and characterized covalently tethered colloidal CdSe/CdTe QD heterostructures via formation of amide bonds. These heterostructures exhibit type-II energetics that promote interfacial charge separation, irrespective of which constituent QD is initially excited, and afford enhanced control over the thermodynamic driving forces for charge transfer. Within these heterostructures, photogenerated electrons are transferred from CdTe to CdSe, and photogenerated holes are transferred from CdSe to CdTe, on timescales of 10^-8 s. Second, we prepared ternary CdSe/CdTe heterostructures by immobilizing a covalently-linked bilayer of these QDs on a metal oxide substrate. When compared to colloidal heterostructures, thin films consisting of QDs adsorbed to a metal oxide substrate, introduce the possibility of an additional stepwise excited-state charge transfer process. We hypothesized that a stepwise process such as this should facilitate extended spatial separation of charge carriers and longer charge-separated state lifetimes, such that energy is harvested more efficiently and desirable processes can more effectively compete with recombination. Dynamic quenching of emission was observed in heterostructure-modified thin films, consistent with excited-state charge transfer. Rate constants for photoinduced electron and hole transfer between QDs are on the order of 10⁸ s^-1 and 10⁷ s^-1_, respectively.

The bidirectional interfacial charge transfer within these type-II QD heterostructures, both in dispersion and within films, further reveals the potential of this system for use in light harvesting and solar energy conversion. This presentation will highlight these recent results as well as our ongoing time-resolved spectroscopic characterization of photoinduced charge transfer in CdSe/CdTe QD heterostructures.

Recently, a growing interest has been devoted to the synthesis and organization of nanostructured materials, Intensive research has focused on the fabrication of QDs based organized hybrid structures in solid-state, which have shown new properties due to the collective interactions of nanoparticles or at the interface with molecular dyes. In particular, QDs with controlled size and shape coupled and/or functionalized with suitable organic chromophores demonstrated efficient energy transfer (ET) and processes [1]. The aim of this work is the fabrication of a high FRET efficiency system, both in solution and in solid-state, formed by colloidal CdSe QDs and fluorescent Bodipy dyes. Careful engineering of the systems is required to minimize the distance between the organic and inorganic fluorophores for promoting effective energy transfers both in solution and in solid-state. In this perspective, dyes bearing amino terminated groups are properly designed in virtue of the high affinity of such functionalities in coordinating the QD surface. Monoaminostyryl substituted BODIPY (MASB), with an amino moiety, and diaminostyryl functionalized BODIPY (DASB), a bidentate dye bearing two amino groups, are selected as acceptors in the FRET systems with respect to the donors CdSe QDs. A pre-functionalization procedure of the QDs surface with a short amine ligand demonstrates to favor the interaction with the organic fluorophores in solution. The coupled hybrid system with MASB shows a very short donor-acceptor distance and an effective energy transfer already in solution, with an efficiency of 76%. The efficiency further enhances up in solid state when the fluorophores are deposited as single coordinated units from solution, demonstrating the effectiveness of the coupling strategy. The overall results demonstrate the effectiveness of the tight coupling of the dye at the QD surface in obtaining a FRET system with high efficiency both in solution and in solid-state, which can be successfully employed in sensor devices. In addition, the very short distance between donor and acceptor can promote the investigation of possible coherent effects in the energy transfer process involving the two fluorophores.

Tetrasubstituted cyclobutyl structures are precursors to, or core components of, many important bioactive molecules, including prospective drugs. Light-driven [2+2] cycloaddition is the most direct strategy for construction of these structures. [2+2] photocycloadditions that proceed through the triplet excited state are advantageous because (i) their scope is not limited by the electrochemical potentials of the substrate, (ii) triplets have long enough lifetimes to mediate intermolecular cycloadditions, and (iii) triplets can be accessed using visible (as opposed to UV) light through excitation of a triplet sensitizer, such as a transition metal complex or organic chromophore, followed by triplet-triplet energy transfer (TT EnT). Synthetic applications of [2 + 2] photocycloadditions however also demand high selectivity, not only for specific coupling products, but also for particular stereo- and regioisomers of those products. Achieving selectivities for (i) a particular regioisomer of the coupled product, (ii) a particular diastereomer of the coupled product, and (iii) homo- vs. hetero-coupling within a mixture of reactive olefins still remains a challenge. Here, we discuss the use of colloidal CdSe quantum dots (QDs) as visible light absorbers, triplet exciton donors, and scaffolds to drive homo- (photodimerization) and hetero- (cross coupling) intermolecular [2+2] photocycloadditions of 4-vinylbenzoic acid derivatives, with (i) perfect and switchable regioselectivity and (ii) 97-98% diastereoselectivity for the previously minor syn-head-to-head (HH) or syn-head-to-tail (HT) configurations of the adducts. The diasteromeric ratios (d.r.) we achieve are a factor of 5 - 10 higher than those reported with all other triplet sensitizers. Furthermore, the size-tunable triplet energy of the QD enables regioselective hetero-intermolecular couplings through selective sensitization of only one of the reagent olefins.

Halide perovskites have experienced a tremendous amount of rapid development now as one of the most unique semiconductor systems because of incredible tolerance to structural and atomic defects. Most notably, perovskites have been slated to revolutionize the photovoltaics industry, and many signs now point to the fact that this may indeed happen. 

Halide perovskites can also be synthesized in solution into colloidal nanocrystals again with exceptional qualities which could apply to PV, but also to other novel technologies. Using perovskite nanocrystals, NREL has pioneered new types of devices in photovoltaic, sensing, and light emitting systems. This talk will show what makes colloidal perovskites distinct from thin films, how surface effects can be leveraged, and open opportunities at the forefront of development. Perovskite nanocrystals can alleviate some stability issues that thin film materials exhibit, most notably, they show stable phase characteristics with fully inorganic compositions. However, they also provide a fantastic platform to discover what fundamental properties hybrid semiconductors possess and how to engineer desired characteristics. I will discuss surface and structural properties, quantum confinement characteristics, film formation, device fabrication and performance.