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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Scientific Reports 5 (2015) 14834-1-6

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

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

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

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

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