Electronic Relaxation Processes Research Articles

Real-Time Ab Initio Investigation on Hot Electron Relaxation Dynamics in Silicon.

The relaxation of hot electrons in semiconductors is pivotal for both energy harvesting processes and optoelectronics. Utilizing a self-developed non-adiabatic molecular dynamics simulation approach in the momentum space (NAMD_k), we have examined the dynamics of hot electrons in silicon. Whether excited from the Γ or L point, the relaxation dynamics exhibit two distinct stages. Initially, within 100 fs, electrons scatter with phonons throughout the Brillouin zone. Subsequently, over a few picoseconds, they further relax toward the conduction band minimum as a whole. This picture of hot electron relaxation is highly consistent with the quasi-equilibrium hot electron ensemble (HEE) concept. Throughout the hot electron relaxation process, energy transfer to phonons is efficient, leading to time-dependent phonon excitation, which is thoroughly analyzed. This investigation provides a novel perspective on hot electron relaxation in silicon, which holds substantive implications for the realm of photovoltaic and optoelectronic device applications.

Electron-phonon coupling dictates electron mean free paths and negative thermal diffusion in metals

The spatiotemporal dynamics of the fundamental energy carriers in metals underpin the efficiency of a wide array of technologies. Although much progress in our understanding of the temporal response of electronic relaxation processes following an external field perturbation has been achieved, the spatial relaxation dynamics of electrons after excitation remains unclear. Here, we employ a scanning ultrafast microscopy with high spatial resolution to directly measure the mean free paths of electrons in different metals. We show that the strength of electron-phonon coupling not only dictates the mean free paths, but also controls a peculiar spatial shrinking of the electronic temperature profile immediately after the electrons have thermalized with the ‘colder’ lattice. More specifically, from our experimental tracking of the spatial relaxation profiles, a clear observation of an effective negative thermal diffusion is apparent for metals with weaker electron-phonon coupling, while for metals with stronger electron-phonon coupling, the negative thermal diffusion effect is not observable. Our spatiotemporal modeling based on the two-temperature approach provides evidence that although negative diffusion occurs universally for materials with two different species that have varying diffusivities and are thermodynamically coupled, effective negative diffusion is masked for cases with stronger coupling between the species.

Crystal structure, optical characterization, conduction and relaxation mechanisms of a new hybrid compound (C6H9N2)2[Sb2Cl8

Hybrid materials play a crucial role in the construction of flexible electronic devices due to the advantages of both organic and inorganic components. To this end, a new hybrid compound (C6H9N2)2[Sb2Cl8] was successfully fabricated using the slow evaporation solution growth approach at room temperature. In-depth research has been done on the structural, optical, and dielectric characteristics. This compound adopts the triclinic symmetry and crystallizes in the centrosymmetric space group P1̄. The inorganic and organic components respectively form anionic and cationic layers parallel to the ac-plane and alternate along the crystallographic b-axis. The [Sb2Cl8]2- dimeric units are bound to the 2-amino-5-picolinium cations [(C6H9N2)]+ through N-H⋯Cl hydrogen bonds. Optical absorption measurements showed a semiconductor behavior with a band gap of approximately 3.57 eV. In addition, DFT calculations were performed to investigate the absorption spectrum, wavelength, and HOMO-LUMO gap. The analysis of complex impedance spectra shows that the electrical conductivity of the sample is strongly frequency and temperature dependent, indicating a relaxation phenomenon and semiconductor-type behavior. Dielectric data obtained from complex impedance spectroscopy and ac conductivity with the use of the Maxwell-Wagner equivalent circuit model, and the universal power law have been investigated to explore the basic components of the electronic transport and relaxation process in our material.

(Keynote) Experimental and Computational Insights into Electronic Structure and Redox Properties of Bare and Doped Co3O4 Nanocrystals of Euhedral Morphology and Their Heterojunctions with Alien Oxides and Carbon Materials

One of the most attractive oxide materials with remarkable record of widespread applications are cobalt spinels and their derivatives obtained by doping with alien redox (3dn) and nonredox (Li) cations or via hybridization with other oxides (CeO2, TiO2, a-Al2O3) or carbon support. They have recently received a great deal of theoretical and practical attention because of an outstanding activity in many redox processes important for photo/electro/catalysis. Among many potential applications, their relevance as promising substitutes for platinum in oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) can be mentioned, in particular. The properties of nanostructured cobalt spinel materials depend on their size, shape, doping and interfacing features. In this context, euhedral spinel nanocrystals that expose well defined crystallographic planes, allow for sensible investigations into establishment of the surface/bulk structure-redox reactivity relationships in real conditions.In this presentation application of S/TEM and inverse Wulff hull matching techniques supported by image simulation was discussed for elucidation the mechanism of hydrothermal synthesis of euhedral cobalt spinel nanocrystals the their shape retrieving. Periodic spin unrestricted DFT-PW91+U and hybrid DFT/HSE06 calculations were applied for modeling the morphology, structure and electronic properties (DOS structure) relevant for redox behavior of bare and doped cobalt spinel nanocrystals. The results allowed for thorough characterization of the structure and reconstruction of the exposed low index (100), (110), and (111) planes, and explain the influence of dopants. The effect of the valence state of lithium (Li+ or Li0), and its locus on the valence pining of the cobalt cations was determined. DFT calculations together with the ab initio thermodynamic modeling were used to study the electronic structure of the created defects, and stability of different terminations of cobalt spinel and Li-doped Co3O4 under various redox conditions. The associated electronic and spin relaxation processes were discussed as well. For the most stable stoichiometric (100) and (111) facets, the surface redox state diagrams in a wide stoichiometry range were constructed, and analyzed in detail. Intrinsic charge transfer processes involving electron (n-conductivity) and hole (p-conductivity) transfer between the octahedral and tetrahedral cobalt cations were modeled using a small polaron approximation (Marcus-Dupuis model). Formation of n-p junctions (Co3O4|CeO2, Li-Co3O4|CeO2) and MxCo3-xO4/carbon hybrids were analyzed by S/TEM, XPS spectroscopy (band alignment), contactless electronic conductivity and work function measurements, corroborated by DFT modeling. The redox properties of the interface and spinel faceting, and the role of electric conductivity and the nature of the carbon support functional groups were discussed in the context of the ORR, OER performance, and catalytic redox reactivity (activation of O2).AcknowledgmentsS.W. thanks for the financial support of the project “High-performance electrocatalyst for oxygen reduction reaction with low platinum content” carried on within the Program “Pearls of Science” of the Ministry of Education and Science of Poland.

Study of excited-state dynamics of D-π-A thiophene derivatives: Enhanced excited-state absorption and refraction

Two D-π-A-type thiophene derivatives Br-TH and 3-TH with different donor units are designed and synthesized to investigate the effect of intra-molecular charge transfer (ICT) on their excited state absorption (ESA) and excited state refraction (ESR). The photodynamic evolution of these compounds in different polar solvents is analyzed by transient absorption spectra (TAS), and the electronic relaxation process from local excited-state (LES) to charge transfer state (CTS) is observed. Z-scan experiments indicate that ESA and ESR of these molecules can be significantly enhanced by extending the π-conjugated length. Phase object pump–probe (POPP) experiments further describe the photophysical mechanisms of CTS-based ESA and ESR of 3-TH. ESA cross section and ESR volume are obtained by establishing the equivalent five-energy level with corresponding rate equations. Results indicate that ICT performance can be effectively modulated by extending the π-conjugated length of these thiophene derivatives, and further improve their ESA and ESR properties. Furthermore, nanosecond optical limiting (OL) experiments indicate that compound 3-TH has excellent OL performance, which has potential application prospects in protecting sensitive devices or human eyes.

Near-Infrared II Hemicyanine Dye with Large Stokes Shift Designed by TICT Regulation for Boosting Imaging-Guided Photothermal Therapy.

The serious threat that cancer poses to human health highlights the significance of early detection and effective treatment. The integration of fluorescence diagnosis and photothermal therapy in NIR-II has gained attention due to its high sensitivity, fast response, and noninvasiveness. Fluorescence, produced by the radiative relaxation process of electrons in a molecule, and photothermal, generated by the nonradiative relaxation process of electrons in a molecule, are competing photophysical processes. Hence, it is a challenge for the molecule to balance between the properties of fluorescence and photothermal. In this study, a NIR-II hemicyanine with TICT character is designed to obtain molecules with both better fluorescence and photothermal properties, utilizing positively charged pyridine salt and triphenylamine as electron acceptor and donor, respectively, and oxole as the conjugated π-bridge. HCY-995, one of the synthesized compounds, has a quantum yield of 0.09%, photothermal conversion efficiency of 54.90%, and a significant Stoke shift of 232nm, which makes it appropriate for the integration of photothermal therapy and high-resolution imaging. This study provides new insights into the development of NIR-II molecules with fluorescent and photothermal integrated properties.

Great Influence of Pressure and Isotope Effects on Nonradiative Charge Loss in Hybrid Organic-Inorganic Perovskites.

The intrinsic softness of hybrid organic-inorganic perovskites (HOIPs) allows their lattice and optoelectronic performance to be tunable to external pressure. Using nonadiabatic (NA) molecular dynamics, we demonstrate that a mild pressure accelerates hot electron relaxation and suppresses nonradiative electron-hole recombination in CH3NH3PbI3. Both processes are governed by NA coupling, which is enhanced between the electronic states of the quasi-continuous bands while is decreased between the band-edge states by reducing the electron-hole wave function overlap. Hydrogen/deuterium isotope exchange alleviates the pressure-induced NA coupling by increasing lattice rigidity and decreasing wave function overlap, slowing down both the hot electron relaxation and electron-hole recombination processes. The simulated time scales of sub-3 ps for hot electron relaxation and half nanoseconds for recombination agree well with the experiments. The study suggests that the isotope exchange can mitigate the pressure-caused fast losses of hot electrons and further prolong the charge carrier lifetime in HOIPs.

Elucidating the Role of Hydrogen Bonding in the Optical Spectroscopy of the Solvated Green Fluorescent Protein Chromophore: Using Machine Learning to Establish the Importance of High-Level Electronic Structure.

Hydrogen bonding interactions with chromophores in chemical and biological environments play a key role in determining their electronic absorption and relaxation processes, which are manifested in their linear and multidimensional optical spectra. For chromophores in the condensed phase, the large number of atoms needed to simulate the environment has traditionally prohibited the use of high-level excited-state electronic structure methods. By leveraging transfer learning, we show how to construct machine-learned models to accurately predict the high-level excitation energies of a chromophore in solution from only 400 high-level calculations. We show that when the electronic excitations of the green fluorescent protein chromophore in water are treated using EOM-CCSD embedded in a DFT description of the solvent the optical spectrum is correctly captured and that this improvement arises from correctly treating the coupling of the electronic transition to electric fields, which leads to a larger response upon hydrogen bonding between the chromophore and water.

Photon energy dependence of the photoelectron spectra of the anthracene anion: On the influence of autodetaching states.

Vibrationally resolved photoelectron spectra of anthracene anions have been measured for photon energies between 1.13 and 4.96eV. In this energy range, photoemission mostly occurs via autodetaching electronically excited states of the anion, which strongly modifies the vibrational excitation of the neutral molecule after electron emission. Based on the observed vibrational patterns, eight different excited states could be identified, seven of which are resonances known from absorption spectroscopy. Distinctly different photon energy dependencies of vibrational excitations have been obtained for different excited states, hinting at strongly different photoemission lifetimes. Unexpectedly, some resonances seem to exhibit bimodal distributions of emission lifetimes, possibly due to electronic relaxation processes induced by the excitation of specific vibrational modes.

Microwave-free J-driven dynamic nuclear polarization: A proposal for enhancing the sensitivity of solution-state NMR.

J-driven dynamic nuclear polarization (JDNP) was recently proposed for enhancing the sensitivity of solution-state nuclear magnetic resonance (NMR), while bypassing the limitations faced by conventional (Overhauser) DNP at magnetic fields of interest in analytical applications. Like Overhauser DNP, JDNP also requires saturating the electronic polarization using high-frequency microwaves known to have poor penetration and associated heating effects in most liquids. The present microwave-free JDNP (MF-JDNP) proposal seeks to enhance solution NMR's sensitivity by shuttling the sample between higher and lower magnetic fields, with one of these fields providing an electron Larmor frequency that matches the interelectron exchange coupling J_{ex}. If spins cross this so-called JDNP condition sufficiently fast, we predict that a sizable nuclear polarization will be created without microwave irradiation. This MF-JDNP proposal requires radicals whose singlet-triplet self-relaxation rates are dominated by dipolar hyperfine relaxation, and shuttling times that can compete with these electron relaxation processes. This paper discusses the theory behind the MF-JDNP, as well as proposals for radicals and conditions that could enable this new approach to NMR sensitivity enhancement.

Origin of the giant persistent photoconductivity in LaAlO3/SrTiO3 heterostructures probed by noise spectroscopy

• The origin of the giant persistent photoconductivity in LaAlO 3 /SrTiO 3 heterostructures is explained by the photo-induced valence change in oxygen vacancies. • The change in the ionization state distribution is successfully probed by the noise spectroscopy technique. • The noise spectra correlate the ionization state distribution and the charge trapping behavior of oxygen vacancies. • The neutralization of the ionized oxygen vacancies near the LaAlO 3 /SrTiO 3 interface hinders the fast and complete charge relaxation, resulting in the strong persistent photoconductivity. LaAlO 3 /SrTiO 3 (LAO/STO) heterostructures have shown a strong persistent photoconductivity (PPC) at room temperature. The abnormally strong PPC has attracted immense research interest due to its possible applications in optically-tunable electronic devices. Despite its promise, the fundamental understanding of the PPC in the LAO/STO heterostructures is still elusive. Herein, we report that the giant PPC originates from the photo-induced valence change in oxygen vacancies near the LAO/STO interface. Our spectral analysis of the photocurrent and the model-fitting study consistently show that the ionized oxygen vacancies near the interface are neutralized during the electron relaxation process. They hinder the complete relaxation of the photoexcited electrons by the deeply-located oxygen vacancies and result in the strong PPC. The change in the ionization state distribution of the oxygen vacancies is probed by the persistent noise behavior at the frequency between 1 kHz and 20 kHz regime. These results provide insight into the role of oxygen vacancies in influencing the internal charge distribution and triggering the PPC phenomena in complex oxide heterostructures.

Basic aspects of gold nanoparticle photo-functionalization using oxides and 2D materials: Control of light confinement, heat-generation, and charge separation in nanospace.

Although the optical properties of localized surface plasmon resonance and the relaxation processes of excited hot electrons in gold nanoparticles (AuNPs) have been well understood, the phenomena that occur when AuNPs relax on solid surfaces of semiconductors or insulators remain largely unknown. Thermal energy diffusion and electron transfer are relatively simple physical processes, but the phenomena they induce are interesting because of a variety of new application developments. In this Perspective, we introduce the fundamental aspects as well as advanced applications of several new physical phenomena induced by AuNPs-based hybrid materials with oxides or 2D materials. Localized heat can induce a great force on the surrounding medium to control mass transport, and plasmon-induced charge transfer reactions are expected to have applications in photocatalysis and solar cells. We also review increasing reports on the development of nano-optical sensors, transistors, and nano-light sources based on precisely controlled device structures utilizing AuNPs.

Modeling of ultrafast X-ray induced magnetization dynamics in magnetic multilayer systems

In this work, we report on modeling results obtained with our recently developed simulation tool enabling nanoscopic description of electronic processes in X-ray irradiated ferromagnetic materials. With this tool, we have studied the response of Co/Pt multilayer system irradiated by an ultrafast extreme ultraviolet pulse at the M-edge of Co (photon energy ~60 eV). It was previously investigated experimentally at the FERMI free-electron-laser facility, using the magnetic small-angle X-ray scattering technique. Our simulations show that the magnetic scattering signal from cobalt decreases on femtosecond timescales due to electronic excitation, relaxation, and transport processes both in the cobalt and in the platinum layers, following the trend observed in the experimental data. The confirmation of the predominant role of electronic processes for X-ray induced demagnetization in the regime below the structural damage threshold is a step toward quantitative control and manipulation of X-ray induced magnetic processes on femtosecond timescales.

Unconventional Photoconversion from In‐Plane 2D Heterostructures of 2D Transition Metal Carbides/Semiconductors

2D transition metal carbides/nitrides (MXenes) are thought of promising photothermal materials due to their broadband localized surface plasma resonance (LSPR) and attractive electromagnetic interference (EMI) properties. However, the weak oxidation resistance and unclear photothermal mechanism of MXenes impede their photothermal modulations. Herein, the in‐plane 2D Ti3C2/TiO2 heterostructures are fabricated by partial oxidation and an unconventional photothermal effect under near infrared irradiation, even if the oxidation compromises the EMI and LSPR capacities, is demonstrated. Such performance is ascribed to the faster transfer of photoexcited electrons across the seamless in‐plane Ti3C2/TiO2 heterointerface than the reported out‐of‐plane one, and the 1–2 orders of magnitude faster relaxation processes of electrons than the ground bleaching in the Original MXene. These experimental results are well‐supported by theoretical calculations. The conceptual advances broaden the fundamental understanding of the photoconversion of MXenes, which would be extended into a variety of applications, such as biomedical therapy, photosynthesis, and photovoltaics.

Delayed Onset and Directionality of X-Ray-Induced Atomic Displacements Observed on Subatomic Length Scales.

Transient structural changes of Al_{2}O_{3} on subatomic length scales following irradiation with an intense x-ray laser pulse (photon energy: 8.70keV; pulse duration: 6fs; fluence: 8×10^{2} J/cm^{2}) have been investigated by using an x-ray pump x-ray probe technique. The measurement reveals that aluminum and oxygen atoms remain in their original positions by ∼20 fs after the intensity maximum of the pump pulse, followed by directional atomic displacements at the fixed unit cell parameters. By comparing the experimental results and theoretical simulations, we interpret that electron excitation and relaxation triggered by the pump pulse modify the potential energy surface and drives the directional atomic displacements. Our results indicate that high-resolution x-ray structural analysis with the accuracy of 0.01Å is feasible even with intense x-ray pulses by making the pulse duration shorter than the timescale needed to complete electron excitation and relaxation processes, which usually take up to a few tens of femtoseconds.

The Kinetics of Electron Transfer from CdS Nanorods to the MoFe Protein of Nitrogenase

Combining the remarkable catalytic properties of redox enzymes with highly tunable light absorbing properties of semiconductor nanocrystals enables the light-driven catalysis of complex, multielectron redox reactions. This Article focuses on systems that combine CdS nanorods (NRs) with the MoFe protein of nitrogenase to drive photochemical N2 reduction. We used transient absorption spectroscopy (TAS) to examine the kinetics of electron transfer (ET) from CdS NRs to the MoFe protein. For CdS NRs with dimensions similar to those previously used for photochemical N2 reduction, the rate constant for ET from CdS NRs competes with other electron relaxation processes, such that when a MoFe protein is bound to a NR, about one-half of the photoexcited electrons are delivered to the enzyme. The NR–MoFe protein binding is incomplete with more than one-half of the NRs in solution not having a MoFe protein bound to accept electrons. The quantum efficiency of ET (QEET) in these ensemble samples is similar to previously reported efficiencies of product (NH3 and H2) formation, suggesting that the enzyme utilizes the delivered electrons without major loss pathways. Our analysis suggests that QEET, and therefore the photochemical product formation, is limited at the ensemble level by the NR–MoFe protein binding and at the single-complex level by the competitiveness of ET. We characterized ET kinetics for several CdS NRs samples with varying dimensions and found that for CdS NRs with an average diameter of 4.2 nm the ET efficiency dropped to undetectable levels, defining a maximum NR diameter that should be used to photochemically drive the MoFe protein. The work described here provides insights into the design of systems with increased control of photochemical N2 reduction catalyzed by the MoFe protein of nitrogenase.

Study of ultrafast nonlinear optical response and transient dynamics of pyrene derivatives with intramolecular charge transfer characteristics

Two pyrene derivatives 1906 and 1936 with typical charge transfer characteristics were designed and synthesized to study the influence of intramolecular charge transfer (ICT) on the ultrafast nonlinear optical (NLO) absorption. Frontier molecular orbital distribution and hole-electron analysis show that compound 1906 has stronger ICT characteristics than 1936 by introducing the halogen effect. Intense ESA signals in the green region of these pyrene derivatives were observed in the transient absorption spectrum (TAS). The excited state dynamics clearly show singlet-based electronic transitions and relaxation processes. Femtosecond Z-scan experiment at 480 nm indicates the ultrafast NLO absorption performance of these pyrene derivatives can be significantly improved by enhancing the ICT characteristics. The photophysical mechanism is one-photon absorption (OPA) induced excited-state absorption (ESA). As the input intensity is 46.8 GW cm−2, the effective NLO absorption coefficient of 1906 is 7.6 × 10−13 W m−1 which is 6.3 times that of 1936. Our results indicate that these pyrene derivatives are promising potential candidates for ultrafast laser protection applications.

Formative period in the x-ray-induced photodissociation of organic molecules

The authors combine x-ray free electron laser pulses and infrared laser pulses to characterize the femtosecond-scale electronic relaxation, ring opening and fragmentation processes on a thiophene molecule.

Tailoring submicrometer periodic surface structures via ultrashort pulsed direct laser interference patterning

Direct laser Interference Patterning (DLIP) with ultrashort laser pulses (ULP) represents a precise and fast technique to produce tailored periodic sub-micrometer structures on various materials. In this work, an experimental and theoretical approach is presented to investigate the previously unexplored fundamental mechanisms for the formation of unprecedented laser-induced topographies on stainless steel following proper combinations of DLIP with ULP. DLIP is aimed to determine the initial conditions of the laser-matter interaction by defining an ablated region while double ULP are used to control the reorganisation of the self-assembled laser induced sub-micrometer sized structures by exploiting the interplay of different absorption and excitation levels coupled with the melt hydrodynamics induced by the first of the double pulses. A multiscale physical model is presented to correlate the interference period, polarization orientation and number of incident pulses with the induced morphologies. Special emphasis is given to electron excitation, relaxation processes and hydrodynamical effects that are crucial to the production of complex morphologies. Results are expected to derive new knowledge of laser-matter interaction in combined DLIP and ULP conditions and enable enhanced fabrication capabilities of complex hierarchical sub-micrometer sized structures for a variety of applications.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.