Multiple exciton generation in nano-crystals revisited: consistent calculation of the yield based on pump-probe spectroscopy

Exciton and biexciton transient absorption spectra of CdSe quantum dots with varying diameters

Transient absorption (TA) spectroscopy of semiconductor nanocrystals (NCs) is often used for excited state population analysis, but recent results suggest that TA bleach signals associated with multiexcitons in NCs do not scale linearly with exciton multiplicity. In this manuscript, we probe the factors that determine the intensities and spectral positions of exciton and biexciton components in the TA spectra of CdSe quantum dots (QDs) of five diameters. We find that, in all cases, the peak intensity of the biexciton TA spectrum is less than 1.5 times that of the single exciton TA spectrum, in stark contrast to a commonly made assumption that this ratio is 2. The relative intensities of the biexciton and exciton TA signals at each wavelength are determined by at least two factors: the TA spectral intensity and the spectral offset between the two signals. We do not observe correlations between either of these factors and the particle diameter, but we find that both are strongly impacted by replacing the native organic surface-capping ligands with a hole-trapping ligand. These results suggest that surface trapping plays an important role in determining the absolute intensities of TA features for CdSe QDs and not just their decay kinetics. Our work highlights the role of spectral offsets and the importance of surface trapping in governing absolute TA intensities. It also conclusively demonstrates that the biexciton TA spectra of CdSe QDs at the band gap energy are less than twice as intense as those of the exciton.

FeS/FeS2 nanoscale structures synthesized in one step from Fe(ll) dithiocarbamate complexes as a single source precursor

Nanoscale FeS and FeS2 mixed phases were synthesized by one-pot decomposition of (N-anil-N-piperldtc)Fe1 as FeS#1), (N-piperldtc)Fe2 as FeS#2) and (N-anildtc)Fe3 as FeS#3) complexes as precursors, with the help of tri-n-octylphosphine oxide (TOPO) coordinating solvent. Their morphology, stability, size, optical and structural characteristics were observed using various material characterization instruments. In comparison to the FeS#2 nano-flower shape, FeS#1 and FeS#3 have a uniform nano-rod shape. A one-step decomposition pattern was obtained from the thermal gravimetric analysis (TGA) results with 3% final mass residual. The high-resolution transmission electron microscopy (HRTEM) image reveals an aggregation and size diameter of around 14.47–30.25 nm for the three samples. The optical response between 3.8 and 4.2 eV from the three samples shows that they are inconsiderable materials for solar cells application. The diffraction peaks for the three samples matched well with the FeS/FeS2. These nanoscale materials can be used in a variety of applications, including lithium-ion batteries, biosensors, hydrogen evolution, and multifunctional nanocomposite materials.

Numerical Analysis of the Detailed Balance of Multiple Exciton Generation Solar Cells with Nonradiative Recombination

In this study, we analyzed the nonradiative recombination impact of multiple exciton generation solar cells (MEGSCs) with a revised detailed balance (DB) limit. The nonideal quantum yield (QY) of a material depends on the surface defects or the status of the material. Thus, its QY shape deviates from the ideal QY because of carrier losses. We used the ideal reverse saturation current variation in the DB of MEGSCs to explain the impact of nonradiative recombination. We compared ideal and nonideal QYs with the nonradiative recombination into the DB of MEGSCs under one-sun and full-light concentration. Through this research, we seek to develop a strategy to maintain MEGSC performance.

Coherent Exciton Dynamics in Ensembles of Size-Dispersed CdSe Quantum Dot Dimers Probed via Ultrafast Spectroscopy: A Quantum Computational Study

Interdot coherent excitonic dynamics in nanometric colloidal CdSe quantum dots (QD) dimers lead to interdot charge migration and energy transfer. We show by electronic quantum dynamical simulations that the interdot coherent response to ultrashort fs laser pulses can be characterized by pump-probe transient absorption spectroscopy in spite of the inevitable inherent size dispersion of colloidal QDs. The latter, leading to a broadening of the excitonic bands, induce accidental resonances that actually increase the efficiency of the interdot coupling. The optical electronic response is computed by solving the time-dependent Schrodinger equation including the interaction with the oscillating electric field of the pulses for an ensemble of dimers that differ by their size. The excitonic Hamiltonian of each dimer is parameterized by the QD size and interdot distance, using an effective mass approximation. Local and charge transfer excitons are included in the dimer basis set. By tailoring the QD size, the excitonic bands can be tuned to overlap and thus favor interdot coupling. Computed pump-probe transient absorption maps averaged over the ensemble show that the coherence of excitons in QD dimers that lead to interdot charge migration can survive size disorder and could be observed in fs pump-probe, four-wave mixing, or covariance spectroscopy.

Multiphoton Excitation of CsPbBr3 Perovskite Quantum Dots (PQDs): How Many Electrons Can One PQD Donate to Multiple Molecular Acceptors?

Metastable multiexcitonic states (MESs) of semiconductor quantum dots can be involved in multielectron transfer reactions, which opens new perspectives in nanomaterials-based optoelectronic applications. Herein, we demonstrate the generation of a MES in CsPbBr₃ perovskite quantum dots (PQDs) and its dissociation dynamics through multiple electron transfers to molecular electron acceptors, anthraquinones (AQs), bound to the PQD surface by a carboxylic anchor. As many as 14 excitons are produced at an excitation density of roughly 220 μJ cm^-2 without detectable PQD degradation. Addition of AQ results in the formation of PQD-AQ hybrids with excess of AQs (PQD:AQ ≈ 1:20), which opens the possibility of multielectron transfer acts from MES to AQs. We found that the electron transfer saturates after roughly five transfer acts and that the first electron transfer (ET) time constant is as short as 1 ps. However, each ET increases the Coulomb potential barrier for the next ET, which decreases the rate of ET, resulting in a saturation after five ETs.

Enhanced Size Selection in Two-Photon Excitation for CsPbBr3 Perovskite Nanocrystals

Cesium lead bromide (CsPbBr₃) perovskite nanocrystals (NCs), with large two-photon absorption (TPA) cross-section and bright photoluminescence (PL), have been demonstrated as stable two-photon-pumped lasing medium. With two-photon excitation, red-shifted PL spectrum and increased PL lifetime is observed compared with one-photon excitation. We have investigated the origin of such difference using time-resolved laser spectroscopies. We ascribe the difference to the enhanced size selection of NCs by two-photon excitation. Because of inherent nonlinearity, the size dependence of absorption cross-section under TPA is stronger. Consequently, larger size NCs are preferably excited, leading to longer excited-state lifetime and red-shifted PL emission. In a broad view, the enhanced size selection in two-photon excitation of CsPbBr₃ NCs is likely a general feature of the perovskite NCs and can be tuned via NC size distribution to influence their performance within NC-based nonlinear optical materials and devices.

Drastic difference between hole and electron injection through the gradient shell of CdxSeyZn1-xS1-y quantum dots

Ultrafast fluorescence spectroscopy was used to investigate the hole injection in Cd_xSe_yZn_1-xS_1-y gradient core-shell quantum dot (CSQD) sensitized p-type NiO photocathodes. A series of CSQDs with a wide range of shell thicknesses was studied. Complementary photoelectrochemical cell measurements were carried out to confirm that the hole injection from the active core through the gradient shell to NiO takes place. The hole injection from the valence band of the QDs to NiO depends much less on the shell thickness when compared to the corresponding electron injection to n-type semiconductor (ZnO). We simulate the charge carrier tunneling through the potential barrier due to the gradient shell by numerically solving the Schrödinger equation. The details of the band alignment determining the potential barrier are obtained from X-ray spectroscopy measurements. The observed drastic differences between the hole and electron injection are consistent with a model where the hole effective mass decreases, while the gradient shell thickness increases.

Size- and Wavelength-Dependent Two-Photon Absorption Cross-Section of CsPbBr3 Perovskite Quantum Dots

All-inorganic colloidal perovskite quantum dots (QDs) based on cesium, lead, and halide have recently emerged as promising light emitting materials. CsPbBr₃ QDs have also been demonstrated as stable two-photon-pumped lasing medium. However, the reported two photon absorption (TPA) cross sections for these QDs differ by an order of magnitude. Here we present an in-depth study of the TPA properties of CsPbBr₃ QDs with mean size ranging from 4.6 to 11.4 nm. By using femtosecond transient absorption (TA) spectroscopy we found that TPA cross section is proportional to the linear one photon absorption. The TPA cross section follows a power law dependence on QDs size with exponent 3.3 ± 0.2. The empirically obtained power-law dependence suggests that the TPA process through a virtual state populates exciton band states. The revealed power-law dependence and the understanding of TPA process are important for developing high performance nonlinear optical devices based on CsPbBr₃ nanocrystals.

Time-resolved terahertz spectroscopy reveals the influence of charged sensitizing quantum dots on the electron dynamics in ZnO

Photoinitiated charge carrier dynamics in ZnO nanoparticles sensitized by CdSe quantum dots is studied using transient absorption spectroscopy and time-resolved terahertz spectroscopy.

Theory of highly efficient multiexciton generation in type-II nanorods

Multiexciton generation, by which more than a single electron–hole pair is generated on optical excitation, is a promising paradigm for pushing the efficiency of solar cells beyond the Shockley–Queisser limit of 31%. Utilizing this paradigm, however, requires the onset energy of multiexciton generation to be close to twice the band gap energy and the efficiency to increase rapidly above this onset. This challenge remains unattainable even using confined nanocrystals, nanorods or nanowires. Here, we show how both goals can be achieved in a nanorod heterostructure with type-II band offsets. Using pseudopotential atomistic calculation on a model type-II semiconductor heterostructure we predict the optimal conditions for controlling multiexciton generation efficiencies at twice the band gap energy. For a finite band offset, this requires a sharp interface along with a reduction of the exciton cooling and may enable a route for breaking the Shockley–Queisser limit.

Multiple exciton generation could help limit thermalization losses in solar cells, but the efficiency of the process is still limited. Here, the authors show by atomistic calculations that type-II interfaces in nanostructures along with a change in exciton cooling rate favour multiple exciton generation.

Surface plasmon inhibited photo-luminescence activation in CdSe/ZnS core-shell quantum dots

In a composite film of Cd x  Se y  Zn1-x  S1-y gradient core-shell quantum dots (QDs) and gold nanorods (NRs), the optical properties of the QDs are drastically affected by the plasmonic nanoparticles. We provide a careful study of the two-step formation of the film and its morphology. Subsequently we focus on QD luminescence photoactivation-a process induced by photochemical changes on the QD surface. We observe that even a sparse coverage of AuNRs can completely inhibit the photoactivation of the QDs' emission in the film. We demonstrate that the inhibition can be accounted for by a rapid energy transfer between QDs and AuNRs. Finally, we propose that the behavior of emission photoactivation can be used as a signature to distinguish between energy and electron transfer in the QD-based materials.

Hot exciton cooling and multiple exciton generation in PbSe quantum dots

PbSe QDs show high multiple exciton generation (MEG) quantum yield. Here we have investigated the role of theΣtransition in slowing down the hot exciton cooling, which can help MEG to take over phonon relaxation.

Hot electron and hole dynamics in thiol-capped CdSe quantum dots revealed by 2D electronic spectroscopy

State-specific relaxation and trapping of excitations in thiol-capped CdSe QDs is followed by 2D electronic spectroscopy.

Ultrafast Charge Dynamics in Trap-Free and Surface-Trapping Colloidal Quantum Dots

Ultrafast transient absorption spectroscopy is used to study subnanosecond charge dynamics in CdTe colloidal quantum dots. After treatment with chloride ions, these can become free of surface traps that produce nonradiative recombination. A comparison between these dots and the same dots before treatment enables new insights into the effect of surface trapping on ultrafast charge dynamics. The surface traps typically increase the rate of electron cooling by 70% and introduce a recombination pathway that depopulates the conduction band minimum of single excitons on a subnanosecond timescale, regardless of whether the sample is stirred or flowed. It is also shown that surface trapping significantly reduces the peak bleach obtained for a particular pump fluence, which has important implications for the interpretation of transient absorption data, including the estimation of absorption cross-sections and multiple exciton generation yields.

Sandwiched confinement of quantum dots in graphene matrix for efficient electron transfer and photocurrent production

Quantum dots (QDs) and graphene are both promising materials for the development of new-generation optoelectronic devices. Towards this end, synergic assembly of these two building blocks is a key step but remains a challenge. Here, we show a one-step strategy for organizing QDs in a graphene matrix via interfacial self-assembly, leading to the formation of sandwiched hybrid QD-graphene nanofilms. We have explored structural features, electron transfer kinetics and photocurrent generation capacity of such hybrid nanofilms using a wide variety of advanced techniques. Graphene nanosheets interlink QDs and significantly improve electronic coupling, resulting in fast electron transfer from photoexcited QDs to graphene with a rate constant of 1.3 × 10(9) s(-1). Efficient electron transfer dramatically enhances photocurrent generation in a liquid-junction QD-sensitized solar cell where the hybrid nanofilm acts as a photoanode. We thereby demonstrate a cost-effective method to construct large-area QD-graphene hybrid nanofilms with straightforward scale-up potential for optoelectronic applications.

Electron relaxation in the CdSe quantum dot--ZnO composite: prospects for photovoltaic applications

Quantum dot (QD)-metal oxide composite forms a “heart” of the QD-sensitized solar cells. It maintains light absorption and electron-hole separation in the system and has been therefore extensively studied. The interest is largely driven by a vision of harvesting the hot carrier energy before it is lost via relaxation. Despite of importance of the process, very little is known about the carrier relaxation in the QD-metal oxide composites. In order to fill this gap of knowledge we carry out a systematic study of initial electron dynamics in different CdSe QD systems. Our data reveal that QD attachment to ZnO induces a speeding-up of transient absorption onset. Detailed analysis of the onset proves that the changes are caused by an additional relaxation channel dependent on the identity of the QD-ZnO linker molecule. The faster relaxation represents an important factor for hot carrier energy harvesting, whose efficiency can be influenced by almost 50%.

Multiexciton Generation in Seeded Nanorods

The stochastic formulation of multiexciton generation (MEG) rates is extended to provide access to MEG efficiencies in nanostructures containing thousands of atoms. The formalism is applied to a series of CdSe/CdS seeded nanorod heterostructures with different core and shell dimensions. At energies above 3Eg (where Eg is the band gap), the MEG yield increases with decreasing core size, as expected for spherical nanocrystals. Surprisingly, this behavior is reversed for energies below this value, and is explained by the dependence of the density of states near the valence band edge, which increases with the core diameter. Our predictions indicate that the onset of MEG can be shifted to lower energies by manipulating the density of states in complex nanostructure geometries.

Anisotropic absorption in PbSe nanorods

We present absorption anisotropy measurements in PbSe nanostructures. This is accomplished via a new means of measuring absorption anisotropy in randomly oriented solution ensembles of nanostructures via pump-probe spectroscopy, which exploits the polarization memory effect. We observe isotropic absorption in nanocrystals and anisotropic absorption in nanorods, which increases upon elongation from aspect ratio 1 to 4 and is constant for longer nanorods. The measured volume-normalized absorption cross section is 1.8 ± 0.3 times larger for parallel pump and probe polarizations in randomly oriented nanorods as compared to nanocrystals. We show that this enhancement would be larger than an order of magnitude for aligned nanorods. Despite being in the strong quantum confinement regime, the aspect ratio dependence of the absorption anisotropy in PbSe nanorods is described classically by the effects of dielectric contrast on an anisotropic nanostructure. These results imply that the dielectric constant of the surrounding medium can be used to influence the optoelectronic properties of nanorods, including polarized absorption and emission, phonon modes, multiple exciton generation efficiency, and Auger recombination rate.

Multiexciton Absorption Cross Sections of CdSe Quantum Dots Determined by Ultrafast Spectroscopy

Multiexciton absorption cross sections are important for analysis of a number of experiments, including multiple exciton generation and stimulated emisson. We present a rigorous method to determine these cross sections using transient absorption (TA) measurements. We apply the method to CdSe quantum dots (QDs) and core-shell (CdSe)ZnS QDs. The method involves measuring TA dynamics for various excitation intensities over a broad time range and analyzing the experiments in terms of a kinetic multiexciton model taking into account all contributions to the signal. In this way, we were able to quantify exciton and multiexciton absorption cross sections at different spectral positions. The absorption cross sections decrease with increasing number of excitations, qualitatively in agreement with the state-filling effective mass model but showing a slower decrease. The cross sections for single-exciton to biexciton absorption range between 57 and 99% of the ground to single-exciton cross section.