Electron Excess Energy Research Articles

Experimental High-Resolution Observation of the Truncated Double-Icosahedron Structure: A Stable Twinned Shell in Alloyed Au-Ag Core@Shell Nanoparticles.

Given the binary nature of nanoalloy systems, their properties are dependent on their size, shape, structure, composition, and chemical ordering. When energy and entropic factors for shapes and structure variations are considered in nanoparticle growth, the spectra of shapes become so vast that even metastable arrangements have been reported under ambient conditions. Experimental and theoretical variations of multiply twinned particles have been observed, from the Ino and Marks decahedra to polyicosahedra and polydecahedra with comparable energetic stability among them. Herein, we report the experimental production of a stable doubly truncated double-icosahedron structure (TdIh) in Au-Ag nanoparticles, in which a twinned Ag-rich alloyed shell is reconstructed on a Au-Ag alloyed Ino-decahedral core. The structure, chemical composition, and growth pathway are proposed on the basis of high-angle annular dark-field scanning transmission electron microscopy analysis and excess energy calculations, while its structural stability is estimated by large-scale atomic molecular dynamics simulations. This novel nanostructure differs from other structures previously reported.

Phonon-Mediated Ultrafast Electron Relaxation Dynamics in Monolayer Black Phosphorus: Instantaneous Coherent Delocalization.

Understanding carrier relaxation processes in semiconductors is crucial for designing high-performance optoelectronic and photocatalytic devices. Recent transient spectroscopic experiments on two-dimensional materials have revealed ultrafast optical responses within several tens of femtoseconds, which are usually ascribed to electron-electron scattering. Here, by conducting quantum dynamics simulations for monolayer black phosphorus, we show that electron-phonon scattering also profoundly influences the early stage of carrier dynamics. The photogenerated electron generally undergoes phonon-mediated instantaneous coherent delocalization in reciprocal space, accompanied by an entropy-driven sharp change in electronic energy. The distribution of the density of states controls the energy exchange between the electron and lattice vibrations. The phonon-induced quantum coherence significantly suspends the energy relaxation time, which is very beneficial for harvesting electron excess energy. These findings offer novel insights into the ultrafast carrier dynamics and energy flow in two-dimensional materials and may prompt new opportunities for regulation of carrier dynamic behaviors.

Role of Vibronic Couplings and Energy Gap in the Internal Conversion Process of a Molecule

Abstract An analytical expression for the rate constant of internal conversion (IC) in a molecule was derived based on the crude adiabatic representation. All vibrational modes were considered to be on an equal footing in the rate constant expression. Based on this expression, we investigated the role of vibronic couplings and electronic energy gap in IC processes, using 9-fluorenone as an illustrative example. Vibrational modes with strong off-diagonal vibronic coupling constants (VCCs) give rise to non-radiative transitions. In contrast, vibrational modes with strong diagonal VCCs constitute the final vibronic states that accept the excess electronic energy between the initial and final electronic states. Therefore, vibrational modes are classified into promoting and accepting modes based on their roles. We identified important promoting modes responsible for one-phonon emission/absorption and accepting modes that contribute greatly to the final state. A Franck-Condon envelope, which describes the density of final vibronic states, explains the dependence of the rate constant on the electronic energy gap. VCC can be visualised as a spatial distribution of its density form, i.e., vibronic coupling density (VCD). The VCD concept is expected to facilitate the design of functional molecules with IC processes understood in terms of electronic states and vibrational modes.

Spin-polarized hot electron transport versus spin pumping mediated by local heating

A ‘toy model’—aimed at capturing the essential physics—is presented that jointly describes spin-polarized hot electron transport and spin pumping driven by local heating. These two processes both contribute to spin-current generation in laser-excited magnetic heterostructures. The model is used to compare the two contributions directly. The spin-polarized hot electron current is modeled as one generation of hot electrons with a spin-dependent excitation and relaxation scheme. Upon decay, the excess energy of the hot electrons is transferred to a thermalized electron bath. The elevated electron temperature leads to an increased rate of electron-magnon scattering processes and yields a local accumulation of spin. This process is dubbed as spin pumping by local heating. The built-up spin accumulation is effectively driven out of the ferromagnetic system by (interfacial) electron transport. Within our model, the injected spin current is dominated by the contribution resulting from spin pumping, while the hot electron spin current remains relatively small. We derive that this observation is related to the ratio between the Fermi temperature and Curie temperature, and we show what other fundamental parameters play a role.

Ultrafast relaxation of photoinjected nonthermal electrons in the Γ valley of GaAs studied by time- and angle-resolved photoemission spectroscopy

We study ultrafast relaxation of nonthermal electrons confined within the \ensuremath{\Gamma} valley of GaAs, based on the momentum-resolved transient electron distribution functions determined using time- and angle-resolved photoemission spectroscopy at 293 K. To elucidate the fundamental processes that lead to the electronic quasithermalization at the nonthermal regime of relaxation, the dynamics in both $p$- and $n$-type samples of GaAs are investigated. In $p$-type GaAs, photoinjected electrons form the electron ensembles quasiequilibrated only in the momentum space within 100 fs of excitation by the Coulomb interaction among photoinjected electrons. However, the ensembles maintain strong nonthermal populations in the energy space. They are quasithermalized only after certain time delays of a few hundred femtoseconds, which depend on the excess energy of photoinjected electrons. These features in $p$-type samples are not sensitively dependent on the excitation density in the range from $1.5\ifmmode\times\else\texttimes\fi{}{10}^{16}$ to $5\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}3}$. On the other hand, in $n$-type samples, which include the cold electrons populated near the conduction-band minimum with the density of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}3}$, photoinjected electrons are quasithermalized within 170 fs of excitation. The main interaction for the fast quasithermalization is attributed to the inelastic scattering among the cold electrons and photoinjected electrons by a dynamics screened Coulomb interaction like the electronic thermalization in metals.

Thioredoxin-like protein CDSP32 alleviates Cd-induced photosynthetic inhibition in tobacco leaves by regulating cyclic electron flow and excess energy dissipation

Thioredoxin-like protein CDSP32 (Trx CDSP32), a thioredoxin-like (Trx-like) protein located in the chloroplast, can regulate photosynthesis and the redox state of plants under stress. In order to examine the role of Trx CDSP32 in the photosynthetic apparatus of plants exposed to cadmium (Cd), the effects of Trx CDSP32 on photosynthetic function and photoprotection in tobacco leaves under Cd exposure were studied using a proteomics approach with wild-type (WT) and Trx CDSP32 overexpression (OE) tobacco plants. Cd exposure reduced stomatal conductance, blocked PSII photosynthetic electron transport, and inhibited carbon assimilation. Increased water use efficiency (WUE), cyclic electron flow (CEF) of the proton gradient regulation 5 pathway (PGR5-CEF), and regulated energy dissipation [Y(NPQ)] are important mechanisms of Cd adaptation. However, CEF of the NAD(P)H dehydrogenase pathway (NDH-CEF) was inhibited by Cd exposure. Relative to control conditions, the expression levels of violaxanthin de-epoxidase (VDE) and photosystem II 22 kDa protein (PsbS) in OE leaves were significantly increased under Cd exposure, but those in WT leaves did not change significantly. Moreover, the expression of zeaxanthin epoxidase (ZE) under Cd exposure was significantly higher than that in WT leaves. Thus, Trx CDSP32 increased Y(NPQ) and alleviated PSII photoinhibition under Cd exposure. Trx CDSP32 not only increased PGR5-like protein 1A and 1B expression, but also alleviated the down-regulation of NAD(P)H-quinone oxidoreductase subunits induced by Cd exposure. Thus, Trx CDSP32 promotes CEF in Cd-exposed tobacco leaves. Thus, Trx CDSP32 alleviates the Cd-induced photoinhibition in tobacco leaves by regulating two photoprotective mechanisms: CEF and xanthophyll cycle-dependent energy dissipation.

Charge-carrier thermalization in bulk and monolayer CdTe from first principles

While cadmium telluride (CdTe) thin films are being used in solar cell prototyping for decades, the recent advent of two-dimensional (2D) materials challenges the fundamental limit for thickness of conventional CdTe layers. Here, we report our theoretical predictions on photocarrier dynamics in an ultimately thin (about 1 nm) CdTe slab. It corresponds to a layer that is just a single unit cell thick, when the bulk parent crystal in the zinc blende phase is cleaved along the [110] facet. Using an \textit{ab-initio} method based on density functional theory (DFT) and the Boltzmann equation in the relaxation time approximation (RTA), we determine the thermalization time for charge carriers excited to a certain energy for instance through laser irradiation. Our calculations include contributions arising from all phonon branches in the first Brillouin zone (BZ), thus capturing all relevant inter- and intraband carrier transitions due to electron-phonon scattering. We find that the photocarrier thermalization time is strongly reduced, by one order of magnitude for holes and by three orders of magnitude for electrons, once the CdTe crystal is thinned down from the bulk to a monolayer. Most surprisingly, the electron thermalization time becomes independent of the electron excess energy up to about 0.5~eV, when counted from the conduction band minimum (CBM). We relate this peculiar behavior to the degenerate and nearly parabolic lowest conduction band that yields a constant density of states (DOS) in the 2D limit. Our findings may be useful for designing novel CdTe-based optoelectronic devices, which employ nonequilibrium photoexcited carriers to improve the performance.

Crystal Symmetry and Static Electron Correlation Greatly Accelerate Nonradiative Dynamics in Lead Halide Perovskites.

Using a recently developed many-body nonadiabatic molecular dynamics (NA-MD) framework for large condensed matter systems, we study the phonon-driven nonradiative relaxation of excess electronic excitation energy in cubic and tetragonal phases of the lead halide perovskite CsPbI3. We find that the many-body treatment of the electronic excited states significantly changes the structure of the excited states' coupling, promotes a stronger nonadiabatic coupling of states, and ultimately accelerates the relaxation dynamics relative to the single-particle description of excited states. The acceleration of the nonadiabatic dynamics correlates with the degree of configurational mixing, which is controlled by the crystal symmetry. The higher-symmetry cubic phase of CsPbI3 exhibits stronger configuration mixing than does the tetragonal phase and subsequently yields faster nonradiative dynamics. Overall, using a many-body treatment of excited states and accounting for decoherence dynamics are important for closing the gap between the computationally derived and experimentally measured nonradiative excitation energy relaxation rates.

Emergence of new materials for exploiting highly efficient carrier multiplication in photovoltaics

In conventional solar cell semiconductor materials, the excess energy of electrons and holes beyond the bandgap is wasted as heat, because they cool down to the band edge due to phonon emission. If the excess energy is more than the bandgap, it can in principle be utilized through a process known as carrier multiplication (CM) in which a single photon generates two (or more) electron-hole pairs. In this way, CM can enhance the photocurrent of a photovoltaic device. We provide an overview of experimental and theoretical methods used to study CM. Next, we consider the effects of composition and nanostructure of materials, on the threshold photon energy and efficiency of CM. Results for percolative networks of coupled PbSe quantum dots, Sn/Pb based halide perovskites, and two-dimensional transition metal dichalcogenides such as MoTe2 are discussed. Based on our current understanding, the CM threshold can get close to the minimal value of twice the bandgap in materials where a photon induces an asymmetric electronic transition from a deeper valence band or to a higher conduction band. We then address the effects of the exciton binding energy and charge carrier mobility on the photogeneration of free charges and their extraction at external electrodes of a photovoltaic device. Finally, we discuss future directions toward the development of new materials to realize a low threshold photon energy and high efficiency of CM.

Resonant low-energy electron attachment to O2 impurities in dense neon gas

We report measurements of resonant low-energy electron attachment to O2 molecular impurities in neon gas in the temperature range . The reduced attachment frequency shows a well defined peak as a function of the gas density N when the electron energy is resonant with the 4th vibrational level of . For a second peak has been detected at a much higher density, which is due to the formation of ions in the 5th vibrational level. The temperature dependence of the first peak position can be explained within an ionic bubble model by computing the electron excess free energy as a function of T and N. The peak shape is rationalized by taking into account the density dependent shift of the electron energy distribution function and the density of states of excess electrons in a disordered medium, and by assuming that electrons sample the gas density over a region of the order of the ionic bubble radius.

Reactions of radiation-induced electrons with carbon dioxide in inert cryogenic films: matrix tuning of the excess electron interactions in solids.

A single-electron reduction of carbon dioxide is supposed to be an important basic step in various processes, ranging from interstellar chemistry to photocatalytic transformations. In this work, we report an FTIR spectroscopic study on the reactions of carbon dioxide (12CO2 and 13CO2) with the radiation-induced excess electrons in deposited cryogenic matrices with different physical characteristics (Ne, N2, Ar, Xe) occurring at 6 K. The reaction was monitored by the observation of carbon dioxide radical anions. It was found that attachment of excess electrons to CO2 occurred in neon and nitrogen matrices, but not in argon and xenon. In the case of nitrogen, the formation of matrix-related cationic species (N4+˙ and NNCO+˙) was also observed. Since the CO2 molecules have a negative intrinsic electron affinity, it was suggested that the electron attachment to CO2 is controlled by the energy of excess electrons in the solid matrix, which is determined by the value of the corresponding conduction band bottom energy (V0). The implications of the obtained results are discussed.

Long-Range Order in the Dislocation Structure of Martensite Crystals

Optimal estimation of the diffraction observations over the object reliably detects periodicity in the dislocation structure of martensitic transformation as an exhibition of its wave nature. The period along normal to the slip planes is comparable with the radius of dislocation loops in crystals. The measured degree of one-dimensional long-range order in the arrangement of the loops is close to the upper limit equal to unity. Subject to the theory of metals, the observed structure could be generated by quantum lattice vibrations, which actuate a jump-like phase transition. A simple explanation exists: after a sharp fall in temperature, the excess energy of conduction electrons causes the crystal to expand instantly with the transformation of translational symmetry. Internal shifts of the crystal lattice caused by electron-phonon interactions concurrently trigger the wave process of formation of thin martensitic plates in the surrounding matrix, which are observed in metallography. Based on an in-depth analysis of the dislocation structure of martensite crystals, a physically founded concept is advanced in which the martensitic transformation is a macroscopic quantum phenomenon connected with the symmetry properties of a crystal system in metals.

Effect of the electronic kinetics on graphitization of diamond irradiated with swift heavy ions and fs-laser pulses

Diamond preliminary damaged with neutrons was irradiated with swift heavy ions (SHI, 1030 MeV 209Bi) decelerated in the electronic stopping regime as well as with fs-laser pulses. The initial excess electronic energy densities appearing in the nanometric vicinity of the SHI trajectories and within the absorbing layers in laser spots were comparable (∼1024 eV cm−3).Graphitization of diamond in the central parts of the lased spots was observed above the threshold fluence of 15–30 J/cm−2. It was also found that the lower threshold fluence is required for initiating graphitization as well as destruction of the pre-damaged crystal by laser pulses in comparison to that for undamaged diamond. This indicates a noticeable effect of an existing defect ensemble on the kinetics of diamond transformations in laser spots.However, X-ray diffraction, atomic-force microscopy, and electron microscopy detected no graphitic domains within the SHI-irradiated pre-damaged crystal.The research demonstrated that the density of the initial excess electronic energy cannot be treated as the sole parameter governing subsequent structure transformations in diamond. Large differences between the spatial as well as temporal scales finally results in different pathways of the relaxation kinetics of this excess energy in laser spots and SHI tracks in diamond.

Accurate Predictions of Electron Binding Energies of Dipole-Bound Anions via Quantum Monte Carlo Methods.

Neutral molecules with sufficiently large dipole moments can bind electrons in diffuse nonvalence orbitals with most of their charge density far from the nuclei, forming so-called dipole-bound anions. Because long-range correlation effects play an important role in the binding of an excess electron and overall binding energies are often only on the order of 10s-100s of wave numbers, predictively modeling dipole-bound anions remains a challenge. Here, we demonstrate that quantum Monte Carlo methods can accurately characterize molecular dipole-bound anions with near-threshold dipole moments. We also show that correlated sampling Auxiliary Field Quantum Monte Carlo is particularly well-suited for resolving the fine energy differences between the neutral and anionic species. These results shed light on the fundamental limitations of quantum Monte Carlo methods and pave the way toward using them for the study of weakly bound species that are too large to model using traditional electron structure methods.

Site-Specific Photodecomposition in Conjugated Energetic Materials.

Nonadiabatic excited-state molecular dynamics (NEXMD) has been used to study photodecomposition in a class of recently synthesized bicyclic conjugated energetic materials (CEMs) composed of fused tetrazole and tetrazine derivatives with increasing oxygen substitutions. Modification by oxygen functionalization has already been demonstrated to increase the two-photon absorption intensity in the target CEMswhile simultaneously improving oxygen balance. Photodecomposition mechanisms in materials that undergo nonlinear absorption could be used to achieve controlled, direct optical initiation. Here, we use NEXMD simulations to model the nonradiative relaxation and photodecomposition in CEMs following photoexcitation by a simulated Nd:YAG laser pulse. Excess electronic energy is quickly converted into vibrational energy on a sub-100 fs time scale resulting in bond dissociation. We find that, for the studiedtetrazine derivatives, the bicyclic framework is an important structural feature that enhances the photochemical quantum yield and the high atomic oxygen content increases the relaxation lifetime and opens additional photodissociation pathways targeting the oxygen-substituted sites. The presented analysis scheme based on bond orders in the swarm of NEXMD trajectories is a useful tool for determining photochemical reactions.

Effect of plasmonic near field on the emittance of plasmon-enhanced photocathode

The surface plasmon polaritons make the emittance of the electron beam emitted from the photocathode complicated. In this article, numerical investigations are carried out to study the influence of the plasmonic near field (PNF) excited by a square hole array on the beam emittance. The characteristics of the emittance induced by PNF are studied. The coupling between the emittance caused by PNF and the original intrinsic emittance due to the excess energy of electrons and the surface roughness of the cathode is discussed. The results provide insights into the suppression of the emittance caused by PNF and new possibilities to break the limits of the nature of the materials, improving the performance of a photocathode.

The excess electron in polymer nanocomposites.

We have used ab initio molecular dynamics and density-functional theory (DFT) calculations at the B3LYP/6-31G** level of theory to evaluate the energy and localisation of excess electrons at a number of representative interfaces of polymer nanocomposites. These modelled interfaces are made by combining liquid water and amorphous slabs of polyethylene and silica. The walls of the amorphous silica slabs are built with two surface chemistries: Q4 or fully-dehydroxylated and Q3/Q4 or partially-hydroxylated with a silanol content between 1.62 and 6.86 groups per nm2. Our results indicate that in silica/polyethylene systems an excess electron would sit at the interface with energies between -1.75 eV with no hydroxylation and -0.99 eV with the highest silanol content. However, in the presence of a free water film, the chemistry of the silica surface has a negligible influence on the behaviour of the excess electron. The electron sits preferentially at the water/vapour interface with an energy of minus a few tenths of an eV. We conclude that the moisture content in a wet polymer nanocomposite has a profound influence on the electron trapping behaviour as it produces much lower trapping energies and a higher excess-electron mobility compared to the dry material.

The excess electron at polyethylene interfaces.

This work investigates the energy and spatial properties of excess electrons in polyethylene in bulk phases and, for the first time, at amorphous vacuum interfaces using a pseudopotential single-electron method (Lanczos diagonalisation) and density functional theory (DFT). DFT calculations are made employing two approaches: with pseudopotentials/plane waves and the local-density approximation; and with all-electron Gaussian basis functions at the B3LYP level of theory, supplemented with a lattice of ghost atoms. All three methods predict similar spatial localisation of the excess electron, but a reliable comparison of its energy can only be made between the Lanczos and DFT using Gaussian bases. While Lanczos predicts that an excess electron would preferentially localise in nanovoids with diameters smaller than 1 nm, DFT suggests that it would localise on surfaces in nanovoids larger than 1 nm. Overall we conclude that in DFT studies of polyethylene/vacuum interfaces at the current level of theory, orbital-based methods provide a useful representation of excess electron properties.

Ultrafast Intersystem Crossing in Acetylacetone via Femtosecond X-ray Transient Absorption at the Carbon K-Edge

Molecular triplet states constitute a crucial gateway in the photochemical reactions of organic molecules by serving as a reservoir for the excess electronic energy. Here, we report the remarkable sensitivity of soft X-ray transient absorption spectroscopy for following the intricate electronic structure changes accompanying the non-adiabatic transition of an excited molecule from the singlet to the triplet manifold. Core-level X-ray spectroscopy at the carbon-1s K-edge (284 eV) is applied to identify the role of the triplet state (T1, 3ππ*) in the ultraviolet-induced photochemistry of pentane-2,4-dione (acetylacetone, AcAc). The excited-state dynamics initiated at 266 nm (1ππ*, S2) is investigated with element- and site-specificity using broadband soft X-ray pulses produced by high harmonic generation, in combination with time-dependent density functional theory calculations of the X-ray spectra for the excited electronic singlet and triplet states. The evolution of the core-to-valence resonances at the carbon K-edge establishes an ultrafast population of the T1 state (3ππ*) in AcAc via intersystem crossing on a 1.5 ± 0.2 ps time scale.

Breaking the Kasha Rule for More Efficient Photochemistry.

This paper provides a systematic review and analysis of different phenomena that violate a basic principle, Kasha's rule, when applied to photochemical reactions. In contrast to the classical route of ultrafast transition to the lowest energy excited state and photochemical reaction starting therein, in some cases, these reactions proceed directly from high-energy excited states. Nowadays, this phenomenon can be observed for a number of major types of excited-state reactions: harvesting product via intersystem crossing; photoisomerizations; bond-breaking; and electron, proton, and energy transfers. We show that specific conditions for their observation are determined by kinetic factors. They should be among the fastest reactions in studied systems, competing with vibrational relaxation and radiative or nonradiative processes occurring in upper excited states. The anti-Kasha effects, which provide an important element that sheds light on the mechanisms of excited-state transformations, open new possibilities of selective control of these reactions for a variety of practical applications. Efficient utilization of excess electronic energy should enhance performance in the systems of artificial photosynthesis and photovoltaic devices. The modulation of the reporting signal by the energy of excitation of light should lead to new technologies in optical sensing and imaging.