Cesium-Coated Halide Perovskites as a Photocathode Material: Modeling Insights.

Photocathodes emit electrons when illuminated, a process utilized across many technologies. Cutting-edge applications require a set of operating conditions that are not met with current photocathode materials. Meanwhile, halide perovskites have been studied extensively and have shown a lot of promise for a wide variety of optoelectronic applications. Well-documented halide perovskite properties such as inexpensive growth techniques, improved carrier mobility, low trap density, and tunable direct band gaps make them promising candidates for next-generation photocathode materials. Here, we use density functional theory to explore the possible application of pure inorganic perovskites (CsPbBr3 and CsPbI3) as photocathodes. It is determined that the addition of a Cs coating improved the performance by lowering the work function anywhere between 1.5 and 3 eV depending on the material, crystal surface, and surface coverage. A phenomenological model, modified from that developed by Gyftopoulos and Levine, is used to predict the reduction in work function with Cs coverage. The results of this work aim to guide the further experimental development of Cs-coated halide perovskites for photocathode materials.

Bright triplet excitons in caesium lead halide perovskites.

Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund's rules state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules predict an analogue of this triplet state known as the 'dark exciton'. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX3, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin-orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX3 nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.

Energy Transfer Sensitization of Luminescent Gold Nanoclusters: More than Just the Classical Förster Mechanism.

Luminescent gold nanocrystals (AuNCs) are a recently-developed material with potential optic, electronic and biological applications. They also demonstrate energy transfer (ET) acceptor/sensitization properties which have been ascribed to Förster resonance energy transfer (FRET) and, to a lesser extent, nanosurface energy transfer (NSET). Here, we investigate AuNC acceptor interactions with three structurally/functionally-distinct donor classes including organic dyes, metal chelates and semiconductor quantum dots (QDs). Donor quenching was observed for every donor-acceptor pair although AuNC sensitization was only observed from metal-chelates and QDs. FRET theory dramatically underestimated the observed energy transfer while NSET-based damping models provided better fits but could not reproduce the experimental data. We consider additional factors including AuNC magnetic dipoles, density of excited-states, dephasing time, and enhanced intersystem crossing that can also influence ET. Cumulatively, data suggests that AuNC sensitization is not by classical FRET or NSET and we provide a simplified distance-independent ET model to fit such experimental data.

Temperature and magnetic-field dependence of radiative decay in colloidal germanium quantum dots.

We conduct spectroscopic and theoretical studies of photoluminescence (PL) from Ge quantum dots (QDs) fabricated via colloidal synthesis. The dynamics of late-time PL exhibit a pronounced dependence on temperature and applied magnetic field, which can be explained by radiative decay involving two closely spaced, slowly emitting exciton states. In 3.5 nm QDs, these states are separated by ∼1 meV and are characterized by ∼82 µs and ∼18 µs lifetimes. By using a four-band formalism, we calculate the fine structure of the indirect band-edge exciton arising from the electron-hole exchange interaction and the Coulomb interaction of the Γ-point hole with the anisotropic charge density of the L-point electron. The calculations suggest that the observed PL dynamics can be explained by phonon-assisted recombination of excitons thermally distributed between the lower-energy "dark" state with the momentum projection J = ± 2 and a higher energy "bright" state with J = ± 1. A fairly small difference between lifetimes of these states is due to their mixing induced by the exchange term unique to crystals with a highly symmetric cubic lattice such as Ge.

Nonradiative Auger recombination in semiconductor nanocrystals.

We calculate the rate of nonradiative Auger recombination in negatively charged CdSe nanocrystals (NCs). The rate is nonmonotonic, strongly oscillating with NC size, and sensitive to the NC surface. The oscillations result in nonexponential decay of carriers in NC ensembles. Using a standard single-exponential approximation of the decay dynamics, we determine the apparent size dependence of the Auger rate in an ensemble and derive CdSe surface parameters consistent with the experimental dependence on size.

Synthesis and spectroscopy of PbSe fused quantum-dot dimers.

We report the synthesis and characterization of Pb-chalcogenide fused quantum-dot (QD) dimer structures. The resulting QD dimers range in length from 6 to 16 nm and are produced by oriented attachment of single QD monomers with diameters of 3.1-7.8 nm. QD monomers with diameters exceeding about 5 nm appear to have the greatest affinity for QD dimer formation and, therefore, gave the greatest yields of fused structures. We find a new absorption feature in the first exciton QD dimer spectra and assign this to a splitting of the 8-fold degenerate 1S-level. The dimer splitting increases from 50 to 140 meV with decrease of the QD-monomer size, and we present a mechanism that accounts for this splitting. We also demonstrate the possibility of fusing two QDs with different sizes into a heterostructure.

Dark and photo-conductivity in ordered array of nanocrystals.

A theory of photo- and dark-band conductivities in semiconductor supercrystals consisting of nanocrystals is developed by assuming scattering by structural defects in the supercrystals. A new proposed mechanism of photoexcitation, which is triggered by an efficient Auger ionization of charged nanocrystals, provides explanation for the measured photocurrent being 2-3 orders of magnitude larger than the dark current. For dark conductivity, the metal-insulator transitions and temperature dependence of mobility in the metal phase are considered.

Efficiency of multiexciton generation in colloidal nanostructures.

Solar energy production, one of the world's most important unsolved problems, has the potential to be a source of clean, renewable energy if scientists can find a way of generating cheap and efficient solar cells. Generation of multiple excitons from single photons is one way to increase the efficiency of solar energy collection, but the process suffers from low efficiency in bulk materials. An increase of multiexciton generation efficiency in nanocrystals was proposed by Nozik in 2002 and demonstrated by Schaller and Klimov in 2004 in PbSe nanocrystals. Since then, scientists have observed efficient multiexciton generation in nanostructures made of many semiconductors using various measurement techniques. Although the experimental evidence of efficient carrier multiplication is overwhelming, there is no complete theory of this phenomenon. Researchers cannot develop such a theory without a self-consistent description of the Coulomb interaction and a knowledge of mechanisms of electron and hole thermalization in nanostructures. The full theoretical description requires the strength of the Coulomb interaction between exciton and multiexciton states and the thermalization rates, which both vary with the dimensionality of the confining potential. As a result, the efficiency of multiexciton generation depends strongly on the material and the shape of the nanostructure. In this Account, we discuss the theoretical aspects of efficient carrier multiplication in nanostructures. The Coulomb interaction couples single excitons with multiexciton states. Phenomenological many-electron calculations of the evolution of single-photon excitations have shown that efficient multiexciton generation can exist only if the rate of the Coulomb mixing between photo-created exciton and biexciton states is significantly faster than the rate of exciton relaxation. Therefore, to increase multiexciton generation efficiency, we need to either increase the exciton-biexciton mixing rate or suppress the exciton relaxation rate. Following this simple recipe, we show that multiexciton generation efficiency should be higher in semiconductor nanorods and nanoplatelets, which have stronger exciton-biexciton coupling due to the enhancement of the Coulomb interaction through the surrounding medium.

Ground-state features in the THz spectra of molecular clusters of ß-HMX.

We present calculations of absorption spectra arising from molecular vibrations at THz frequencies for molecular clusters of the explosive HMX using density functional theory (DFT). The features of these spectra can be shown to follow from the coupling of vibrational modes. In particular, the coupling among ground-state vibrational modes provides a reasonable molecular-level interpretation of spectral features associated with the vibrational modes of molecular clusters. THz excitation from the ground state is associated with frequencies that characteristically perturb molecular electronic states, in contrast to frequencies, which are usually substantially above the mid-infrared (mid-IR) range, that can induce appreciable electronic-state transition. Owing to this characteristic of THz excitation, one is able to make a direct association between local oscillations about ground-state minima of molecules, either isolated or comprising a cluster, and THz absorption spectra. The DFT software program GAUSSIAN was used for the calculations of the absorption spectra presented here.

A general framework for numerical simulation of improvised explosive device (IED)-detection scenarios using density functional theory (DFT) and terahertz (THz) spectra.

We have developed a general framework for numerical simulation of various types of scenarios that can occur for the detection of improvised explosive devices (IEDs) through the use of excitation using incident electromagnetic waves. A central component model of this framework is an S-matrix representation of a multilayered composite material system. Each layer of the system is characterized by an average thickness and an effective electric permittivity function. The outputs of this component are the reflectivity and the transmissivity as functions of frequency and angle of the incident electromagnetic wave. The input of the component is a parameterized analytic-function representation of the electric permittivity as a function of frequency, which is provided by another component model of the framework. The permittivity function is constructed by fitting response spectra calculated using density functional theory (DFT) and parameter adjustment according to any additional information that may be available, e.g., experimentally measured spectra or theory-based assumptions concerning spectral features. A prototype simulation is described that considers response characteristics for THz excitation of the high explosive ß-HMX. This prototype simulation includes a description of a procedure for calculating response spectra using DFT as input to the Smatrix model. For this purpose, the DFT software NRLMOL was adopted.

Quantum simulation of multiple-exciton generation in a nanocrystal by a single photon.

We have shown theoretically that efficient multiple-exciton generation (MEG) by a single photon can be observed in small nanocrystals. Our quantum simulations that include hundreds of thousands of exciton and multiexciton states demonstrate that the complex time-dependent dynamics of these states in a closed electronic system yields a saturated MEG effect on a picosecond time scale. Including phonon relaxation confirms that efficient MEG requires the exciton-biexciton coupling time to be faster than exciton relaxation time.

Highly efficient multiple exciton generation in colloidal PbSe and PbS quantum dots.

We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.