Electron Excess Energy Research Articles

Interfacial Charge Transfer Circumventing Momentum Mismatch at Two-Dimensional van der Waals Heterojunctions.

Interfacial charge separation and recombination at heterojunctions of monolayer transition metal dichalcogenides (TMDCs) are of interest to two-dimensional optoelectronic technologies. These processes can involve large changes in parallel momentum vector due to the confinement of electrons and holes to the K valleys in each layer. Because these high-momentum valleys are usually not aligned across the interface of two TMDC monolayers, how parallel momentum is conserved in the charge separation or recombination process becomes a key question. Here we probe this question using the model system of a type-II heterojunction formed by MoS2 and WSe2 monolayers and the experimental technique of femtosecond pump-probe spectroscopy. Upon photoexcitation specifically of WSe2 at the heterojunction, we observe ultrafast (<40 fs) electron transfer from WSe2 to MoS2, independent of the angular alignment and thus momentum mismatch between the two TMDCs. The resulting interlayer charge transfer exciton decays via nonradiative recombination with rates varying by up to three-orders of magnitude from sample to sample but with no correlation with interlayer angular alignment. We suggest that the initial interfacial charge separation and the subsequent interfacial charge recombination processes circumvent momentum mismatch via excess electronic energy and via defect-mediated recombination, respectively.

Persistent Energetic Electrons in Methylammonium Lead Iodide Perovskite Thin Films.

In conventional semiconductor solar cells, carriers are extracted at the band edges and the excess electronic energy (E*) is lost as heat. If E* is harvested, power conversion efficiency can be as high as twice the Shockley-Queisser limit. To date, materials suitable for hot carrier solar cells have not been found due to efficient electron/optical-phonon scattering in most semiconductors, but our recent experiments revealed long-lived hot carriers in single-crystal hybrid lead bromide perovskites. Here we turn to polycrystalline methylammonium lead iodide perovskite, which has emerged as the material for highly efficient solar cells. We observe energetic electrons with excess energy ⟨E*⟩ ≈ 0.25 eV above the conduction band minimum and with lifetime as long as ∼100 ps, which is 2-3 orders of magnitude longer than those in conventional semiconductors. The energetic carriers also give rise to hot fluorescence emission with pseudo-electronic temperatures as high as 1900 K. These findings point to a suppression of hot carrier scattering with optical phonons in methylammonium lead iodide perovskite. We address mechanistic origins of this suppression and, in particular, the correlation of this suppression with dynamic disorder. We discuss potential harvesting of energetic carriers for solar energy conversion.

Electronic structure and ultrafast dynamics of FeAs‐based superconductors by angle‐ and time‐resolved photoemission spectroscopy

AbstractIn this article, we review our angle‐ and time‐resolved photoemission studies (ARPES and trARPES) on various ferropnictides. In the ARPES studies, we focus first on the band structure as a function of control parameters. We find near optimally “doped” compounds a Lifshitz transition of hole/electron pocket vanishing type. Second, we investigated the inelastic scattering rates as a function of the control parameter. In contrast to the heavily discussed quantum critical scenario, we find no enhancement of the scattering rate near optimally “doping.” Correlation effects which show up by the non‐Fermi‐liquid behavior of the scattering rates, together with the Lifshitz transition offer a new explanation for the strange normal state properties and suggests an interpolating superconducting state between BCS and BE condensation. Adding femtosecond time resolution to ARPES provides complementary information on electron and lattice dynamics. We report on the response of the chemical potential by a collective periodic variation coupled to coherent optical phonons in combination with incoherent electron and phonon dynamics described by a three temperature heat bath model. We quantify electron phonon coupling in terms of and show that the analysis of the electron excess energy relaxation is a robust approach. The spin density wave ordering leads to a pronounced momentum dependent relaxation dynamics. In the vicinity of , hot electrons dissipate their energy by electron–phonon coupling with a characteristic time constant of 200 fs. Electrons at the center of the hole pocket exhibit a four time slower relaxation which is explained by spin‐dependent dynamics with its smaller relaxation phase space. This finding has implications beyond the material class of Fe‐pnictides because it establishes experimental access to spin‐dependent dynamics in materials with spin density waves.

Effects of model approximations for electron, hole, and photon transport in swift heavy ion tracks

The event-by-event Monte Carlo code, TREKIS, was recently developed to describe excitation of the electron subsystems of solids in the nanometric vicinity of a trajectory of a nonrelativistic swift heavy ion (SHI) decelerated in the electronic stopping regime. The complex dielectric function (CDF) formalism was applied in the used cross sections to account for collective response of a matter to excitation. Using this model we investigate effects of the basic assumptions on the modeled kinetics of the electronic subsystem which ultimately determine parameters of an excited material in an SHI track.In particular, (a) effects of different momentum dependencies of the CDF on scattering of projectiles on the electron subsystem are investigated. The ‘effective one-band’ approximation for target electrons produces good coincidence of the calculated electron mean free paths with those obtained in experiments in metals. (b) Effects of collective response of a lattice appeared to dominate in randomization of electron motion. We study how sensitive these effects are to the target temperature. We also compare results of applications of different model forms of (quasi-) elastic cross sections in simulations of the ion track kinetics, e.g. those calculated taking into account optical phonons in the CDF form vs. Mott’s atomic cross sections. (c) It is demonstrated that the kinetics of valence holes significantly affects redistribution of the excess electronic energy in the vicinity of an SHI trajectory as well as its conversion into lattice excitation in dielectrics and semiconductors. (d) It is also shown that induced transport of photons originated from radiative decay of core holes brings the excess energy faster and farther away from the track core, however, the amount of this energy is relatively small.

Excess electron states in fluid methane: Density-functional versus Lanczos approaches

We compare density-functional theory (DFT) electronic structure calculations at the hybrid B3LYP level for fluid methane with experiments and a pseudo-potential Lanczos method. We generate fluid configurations from classical/ab initio molecular dynamics and use DFT to determine one-particle orbital and total energies. Our results show that DFT predicts excess electron energies that qualitatively agree with experiments over a density range, provided these values are determined from total energy differences between charged and neutral systems. By contrast, orbital energies of the lowest unoccupied state of the N-electron system provide qualitatively incorrect excess electron energies as a function of the fluid density.

Unique characteristics of nonequilibrium carrier transport dynamics in an undoped GaAs/n-type GaAs epitaxial structure

We have investigated nonequilibrium carrier dynamics in an undoped GaAs/n-type GaAs epitaxial structure at room temperature using reflection-type pump–probe spectroscopy at different pump photon energies and Monte Carlo simulation. It was found that the transport process of photogenerated electrons in the undoped layer is characterized by the valance between the quasiballistic motion passing through the undoped layer and the intervalley scattering from the Γ valley to the L one as a function of the excess energy of photogenerated electrons. The Γ–L scattering component exhibits threshold-like appearance and then gradually increases with increasing excess energy.

On the applicability of one- and many-electron quantum chemistry models for hydrated electron clusters.

We evaluate the applicability of a hierarchy of quantum models in characterizing the binding energy of excess electrons to water clusters. In particular, we calculate the vertical detachment energy of an excess electron from water cluster anions with methods that include one-electron pseudopotential calculations, density functional theory(DFT) based calculations, and ab initio quantum chemistry using MP2 and eom-EA-CCSD levels of theory. The examined clusters range from the smallest cluster size (n = 2) up to nearly nanosize clusters with n = 1000 molecules. The examined cluster configurations are extracted from mixed quantum-classical molecular dynamics trajectories of cluster anions with n = 1000 water molecules using two different one-electron pseudopotenial models. We find that while MP2 calculations with large diffuse basis set provide a reasonable description for the hydrated electron system, DFT methods should be used with precaution and only after careful benchmarking. Strictly tested one-electron psudopotentials can still be considered as reasonable alternatives to DFT methods, especially in large systems. The results of quantum chemistry calculations performed on configurations, that represent possible excess electron binding motifs in the clusters, appear to be consistent with the results using a cavitystructure preferring one-electron pseudopotential for the hydrated electron, while they are in sharp disagreement with the structural predictions of a non-cavity model.

Charge Separation and Recombination at Polymer-Fullerene Heterojunctions: Delocalization and Hybridization Effects.

We address charge separation and recombination in polymer/fullerene solar cells with a multiscale modeling built from accurate atomistic inputs and accounting for disorder, interface electrostatics and genuine quantum effects on equal footings. Our results show that bound localized charge transfer states at the interface coexist with a large majority of thermally accessible delocalized space-separated states that can be also reached by direct photoexcitation, thanks to their strong hybridization with singlet polymer excitons. These findings reconcile the recent experimental reports of ultrafast exciton separation ("hot" process) with the evidence that high quantum yields do not require excess electronic or vibrational energy ("cold" process), and show that delocalization, by shifting the density of charge transfer states toward larger effective electron-hole radii, may reduce energy losses through charge recombination.

Thickness-dependent electron–lattice equilibration in laser-excited thin bismuth films

Electron–phonon coupling processes determine electronic transport properties of materials and are responsible for the transfer of electronic excess energy to the lattice. With decreasing device dimensions an understanding of these processes in nanoscale materials is becoming increasingly important. Here we use time-resolved electron diffraction to directly study energy relaxation in thin bismuth films after optical excitation. Precise measurements of the transient Debye–Waller-effect for various film thicknesses and over an extended range of excitation fluences allow to separate different contributions to the incoherent lattice response. While phonon softening in the electronically excited state is responsible for an immediate increase of the r.m.s. atomic displacement within a few hundred fs, ‘ordinary’ electron–phonon coupling leads to subsequent heating of the material on a few ps time-scale. The data reveal distinct changes in the energy transfer dynamics which becomes faster for stronger excitation and smaller film thickness, respectively. The latter effect is attributed to a cross-interfacial coupling of excited electrons to phonons in the substrate.

Electronic and atomic kinetics in solids irradiated with free-electron lasers or swift-heavy ions

In this brief review we discuss the transient processes in solids under irradiation with femtosecond X-ray free-electron-laser (FEL) pulses and swift-heavy ions (SHI). Both kinds of irradiation produce highly excited electrons in a target on extremely short timescales. Transfer of the excess electronic energy into the lattice may lead to observable target modifications such as phase transitions and damage formation. Transient kinetics of material excitation and relaxation under FEL or SHI irradiation are comparatively discussed. The same origin for the electronic and atomic relaxation in both cases is demonstrated. Differences in these kinetics introduced by the geometrical effects (μm-size of a laser spot vs nm-size of an ion track) and initial irradiation (photoabsorption vs an ion impact) are analyzed. The basic mechanisms of electron transport and electron-lattice coupling are addressed. Appropriate models and their limitations are presented. Possibilities of thermal and nonthermal melting of materials under FEL and SHI irradiation are discussed.

Time-resolved electron kinetics in swift heavy ion irradiated solids

The event-by-event Monte Carlo model, TREKIS, was developed to describe the excitation of the electron subsystems of various solids by a penetrating swift heavy ion (SHI), the spatial spreading of generated fast electrons, and secondary electron and hole cascades. Complex dielectric function formalism is used to obtain relevant cross sections. This allows the recognition of fundamental effects resulting from the collective response of the electron subsystem of a target for excitation that is not possible within the binary collision approximation of these cross sections, e.g. the differences in the electronic stopping of an ion and in the electron mean free paths for different structures (phases) of a material. A systematic study performed with this model for different materials (insulators, semiconductors and metals) revealed effects which may be important for an ion track: e.g. the appearance of a second front of excess electronic energy propagation outwards from the track core following the primary front of spreading of generated electrons. We also analyze how the initial ballistic spatial spreading of fast electrons generated in a track turns to the diffusion ~10 fs after ion passage. Detailed time-resolved simulations of electronic subsystem kinetics helped in understanding the reasons behind enhanced silicon resistance to SHI irradiation in contrast to easily produced damage in this material by femtosecond laser pulses. We demonstrate that the fast spreading of excited electrons from the track core on a sub-100 fs timescale prevents the Si lattice from nonthermal melting in a relaxing SHI track.

Efficient charge generation by relaxed charge-transfer states at organic interfaces

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

Controlled and Selective Area Growth of Monolayer Graphene on 4H-SiC Substrate by Electron-Beam-Assisted Rapid Heating

A facile method for direct and selective area conversion of 4H-SiC substrates into homogeneous epitaxial graphene is demonstrated by the irradiation of a low energy (5 keV) electron beam (e-beam). The localized interactions like scattering, excitation, and ionization between the primary electrons and the SiC surface result in Si–C bond breaking, and the excess kinetic energy of electrons is dissipated as heat that results in the selective sublimation of the Si ion favoring the formation of epitaxial graphene. The thickness of the graphene layer is precisely controlled by varying the e-beam energy and the irradiation time. The number of graphene layers increases with irradiation due to thermally induced Si sublimation over the depth of a few atomic layers depending on the incident energy of the e-beam. The Hall mobility of large area single layer graphene formed on the Si-face is ∼6450 cm2V–1s–1 with a surface carrier density (n-type) of 1.5 × 1013 cm–2. Our results demonstrate that the e-beam irradiation ...

State-specific heavy-atom effect on intersystem crossing processes in 2-thiothymine: A potential photodynamic therapy photosensitizer

Thiothymidine has a potential application as a photosensitizer in cancer photodynamic therapy (PDT). As the chromophore of thiothymidine, 2-thiothymine exhibits ultrahigh quantum yield of intersystem crossing to the lowest triplet state T(1) (ca. 100%), which contrasts with the excited-state behavior of the natural thymine that dissipates excess electronic energy via ultrafast internal conversion to the ground state. In this work, we employed high-level complete-active space self-consistent field and its second-order perturbation methods to explore the photophysical mechanism of a 2-thiothymine model. We have optimized the minimum energy structures in the low-lying seven electronic states, as well as ten intersection points. On the basis of the computed potential energy profiles and spin-orbit couplings, we proposed three competitive, efficient nonadiabatic pathways to the lowest triplet state T(1) from the initially populated singlet state S(2). The suggested mechanistic scenario explains well the recent experimental phenomena. The origin responsible for the distinct photophysical behaviors between thymine and 2-thiothymine is ascribed to the heavy-atom effect, which is significantly enhanced in the latter. Additionally, this heavy-atom effect is found to be state-specific, which could in principle be used to tune the photophysics of 2-thiothymine. The present high-level electronic structure calculations also contribute to understand the working mechanism of thiothymidine in PDT.

THz excitation spectra of AIIIBV semiconductors

The dependence of terahertz (THz) radiation on the excitation wavelength of femtosecond pulses (from 640 to 2600 nm) was investigated. Four different materials, InAs, InSb, InN and GaAs, were analyzed. The generated THz amplitude increases linearly with the photon energy due to the reduced absorption length and the increased quasi-ballistic transit distance. When the electron excess energy reaches the position of the subsidiary conduction band valleys, the intervalley scattering sets in and the THz amplitude drops. Thus, this method allows us to determine intervalley separation of various semiconductors. THz generation due to subsurface band bending in GaAs was observed as well.

Effect of interaction of atomic electrons on ionization of an insulator in swift heavy ion tracks

Effects of applications of two different models of atomic ionizations in MC simulations describing excitation of the electronic subsystem of an insulator in tracks of swift heavy ions decelerated in the electronic stopping regime are investigated. The first mechanism is based on binary collisions of a fast projectile with atomic electrons considered as independent. The second one assumes redistribution of the energy transferred to an atom from an ion between the atomic electrons before autoionization of the excited atom. It is demonstrated that an appreciable difference occurs only in the kinetics of the most energetic electrons appeared during the ionization. This difference affects only a small fraction of the excess electronic energy in the far periphery of a track.

Energy Partitioning of Tunneling Currents into Luttinger Liquids

Tunneling of electrons of definite chirality into a quantum wire creates counterpropagating excitations, carrying both charge and energy. We find that the partitioning of energy is qualitatively different from that of charge. The partition ratio of energy depends on the excess energy of the tunneling electrons (controlled by the applied bias) and on the interaction strength within the wire (characterized by the Luttinger-liquid parameter κ), while the partitioning of charge is fully determined by κ. Moreover, unlike for charge currents, the partitioning of energy current should manifest itself in dc experiments on wires contacted by conventional (Fermi-liquid) leads.

Electron spin coherence and effect of spin polarization on electron relaxation dynamics in GaAs

Time-resolved circularly and linearly polarized pump-probe spectroscopy is used to study the evolution of the electron spin coherence and electron relaxation dynamics in bulk GaAs at 9.6 K. In particular, their dependence on photon energy (or electron excess energy) is carefully investigated. The absorption quantum beats which are observed in circularly polarized pump-probe spectroscopy are obtained, reflecting the dephasing of the electron spin coherence. A circularly dichromatic pump-probe model is developed with both the spin-polarization-dependent band-filling and band-gap renormalization effects being taken into account. The model is used to simulate the differential transmission spectra for the collinearly polarized, co-helicity circularly polarized and cross-helicity circularly polarized pump-probe configurations, respectively. It is found that the model simulates well the features of the absorption quantum beats for a spin-dependent thermalized distribution of the photocreated carriers by a circularly polarized pump pulse, such as the variation of the oscillatory amplitude and phase reversal of the absorption quantum beats with photon energy increase. The simulation is in good agreement with our experimental results and reveals the effect of spin polarization on electron relaxation dynamics.

Charge transport and localization in nanocrystalline CdS films: A time-resolved terahertz spectroscopy study

Assessment of characteristic length and time scales of the charge localization in nanostructured semiconductors is a key point for understanding the initial stage of carrier transport after photoexcitation. A concerted use of time-resolved terahertz spectroscopy and Monte Carlo simulations of the motion of confined electrons allow us to obtain this information and develop a quantitative microscopic model of the electron transport in a nanocrystalline CdS film. A weak localization is observed inside individual nanocrystals while much stronger localization stems from the existence of nanocrystal clusters partially surrounded by voids. The efficiency of the short-range transport is controlled by the excess energy of electrons: Its increase enhances the conductive coupling between adjacent nanocrystals and clusters. Relaxation of electrons with high excess energy then leads to a decrease of their mobility on a subpicosecond time scale. Filling of conduction-band states by increasing the optical pump fluence allows us to maintain a high level of conductive coupling even at later times.

White radiation upon breakdown of an nSi— pCdiamond heterojunction and upon limiting efficiency of a silicon solar element

White radiation was registered upon breakdown of nSi— pCdiamond heterojunction resulting from collision ionization and recombination of electron-hole pairs without thermalization of an excess energy of electrons. The probabilities of collision ionization and radiation transitions of photoelectrons in the region of a conduction band generated by photons with energy above 2Eg and of enhancement of the limiting efficiency of a silicon solar element are discussed.