Electronic Relaxation Rate Research Articles

In English

The paper considers the problem of choosing the composition, structure, and size of spherical catalyst nanoparticles for carrying out plasmon-induced polymerization reactions. The concept of reducing the activation energy of the reaction in the presence of a catalyst and, accordingly, increasing the rate of a chemical reaction during heating due to the excitation of surface plasmon resonance is presented. Using the Drude model for the dielectric function, relationships were obtained for the frequency dependences of such characteristics as the real and imaginary parts of the polarizability, heating and the rate of chemical reactions when monometallic and bimetallic nanoparticles are used as catalysts, as well as the amplification of fields in their vicinity. The concepts developed in this work take into account the classical size dependence of the effective electron relaxation rate in monometallic and bimetallic nanoparticles under the assumption of diffuse scattering of electrons. Changes in the positions of the maxima of the imaginary part of the polarizability, heating, and reaction rate are analyzed with a change in the radii of monometallic and bimetallic nanoparticles. It is shown that the maxima of the dependences under study correspond to dipole surface plasmon resonances, and their number depends on the particle morphology. Changes in the amplification of electric fields in the vicinity of nanoparticles of different morphology have been studied. It has been found that the enhancement of the fields in all considered cases is maximum on the surface of the nanoparticle and decreases with distance from it. Practical recommendations are formulated regarding the size, composition and structure of nanoparticles for plasmon catalysis, which provide the highest rates of chemical reactions. Thus, all obtained frequency dependences have one maximum for monometallic and two maxima for bimetallic nanoparticles.

Thickness-dependent electronic relaxation dynamics in solution-phase redox-exfoliated MoS2 heterostructures.

Electronic relaxation dynamics of solution-phase redox-exfoliated molybdenum disulfide (MoS2) monolayer and multilayer ensembles are described. MoS2 was exfoliated using polyoxometalate (POM) reductants. This process yields a colloidal heterostructure consisting of MoS2 2D sheet multilayers with surface-bound POM complexes. Using two-dimensional electronic spectroscopy, transient bleaching and photoinduced absorption signals were detected at excitation/detection energies of 1.82/1.87 and 1.82/1.80eV, respectively. Approximate 100-fs bandgap renormalization (BGR) and subsequent defect- and phonon-mediated relaxation on the picosecond timescale were resolved for several MoS2 thicknesses spanning from 1 to 2 L to ∼20 L. BGR rates were independent of sample thickness and slightly slower than observations for chemical vapor deposition-grown MoS2 monolayers. However, defect-mediated relaxation accelerated ∼10-fold with increased sample thicknesses. The relaxation rates increased from 0.33 ± 0.05 to 1.2 ± 0.1 and 3.1 ± 0.4ps-1 for 1-2 L, 3-4 L, and 20 L fractions. The thicknesses-dependent relaxation rates for POM-MoS2 heterostructures were modeled using a saturating exponential function that showed saturation at thirteen MoS2 layers. The results suggest that the increased POM surface coverage leads to larger defect density in the POM-MoS2 heterostructure. These are the first descriptions of the influence of sample thickness on electronic relaxation rates in solution-phase redox-exfoliated POM-MoS2 heterostructures. Outcomes of this work are expected to impact the development of solution-phase exfoliation of 2D metal-chalcogenide heterostructures.

Propagation Characteristics of Terahertz Waves in Magnetized Dusty Plasma

The propagation characteristics, including reflectivity, transmissivity, and absorptivity, of terahertz (THz) waves in magnetized dusty plasmas sheath are investigated theoretically by the analytic method. Both the right-and left-hand circularly polarized (RCP and LCP) waves are considered. The calculation results illustrate that external magnetic field, plasma slab thickness, electron collision frequency, dust particles, and electron relaxation rate can affect the propagation characteristics of THz waves in plasmas. When dust particles are absent, a strong external magnetic field will cause the transmissivity of RCP waves to produce a transmissivity valley at the lower frequency band. Due to the action of external magnetic field and dust particles, a transmissivity peak appears at the lower frequency band for RCP waves. Increasing the density or radius of dust particles can cause an increase in transmissivity peak. For LCP waves, the transmissivity increases monotonously as the frequency of THz wave increases, whether the dust particles exist or not. These simulation results may provide a significant theoretical basis for solving the problem of “blackout.”

Photoluminescence in Cerium-Doped Fluoride Borate Crystals

Antizeolite fluoride borates LiBa12(BO3)7F4 (LBBF) doped by Ce3+ ions demonstrate photoluminescence (PL) and hold promise for use as phosphors in white light-emitting diodes. In this study, we present the results of modeling the electronic and optical properties of Ce-containing LBBF structures where doping atoms are located at different sites of the host lattice. We also consider the presence of F-centers, which exhibit trap states located within the band gap. We further investigate the nonadiabatic excited state electronic dynamics to elucidate electron-relaxation pathways. The nonadiabatic couplings (NACs) calculations provide transition probabilities facilitated by the nuclear movement. The relaxation rates of electrons and holes are calculated using Redfield’s theory of the formalism of the reduced density matrix (RDM). The PL spectra are calculated using molecular dynamics (MD) sampling and time-integrated methods along the excited state trajectory based on NACs. Mechanisms of PL in LBBF:Ce3+ crystals are interpreted using both computational and experimental observations. This work illustrates the dependence of transition energies, intensities, and relaxation rates of Ce-containing LBBF crystals on a selection of doping sites. In addition to analysis of emission band contributed by cerium, this work allows to identify and reproduce spectral lines hypothetically corresponding to the interband and intraband transitions in F-centers of borate crystals, available in the absence of a metal center.

THz Surface Plasmons in Wide and Freestanding Graphene Nanoribbon Arrays

Graphene is a thin-film carbon material that has immense potential as a key ingredient in new nanoelectronic and nanophotonic devices due to its unique characteristics. In particular, plasmons in graphene appear as a practical tool for the manipulation of light with potential applications from cancer treatment to solar cells. A motivating tunability of graphene properties has been observed in graphene nanoribbons (GNRs) due to their geometrically controllable bandgaps that, in turn, influence the plasmonic properties. The formidable effort made over recent years in developing GNR-based technologies is, however, weakened by a lack of predictive approaches that draw upon available semi-analytical electromagnetic models. An example of such a framework is used here, focusing on experimentally realized GNRs from 155 to 480 nm wide and organized as two-dimensional (2D) GNR arrays. The results show that the plasmon frequency behavior is highly affected by the experimental setup or geometrical factors. In particular, the bandgap of the analyzed systems is of the order of a few meV with a density of states opening around zero energy (Fermi level) in contrast to what is observed in graphene. From the plasmonic part, it is observed in all 2D GNR arrays that the frequency–momentum trend follows a q-like plasmon dispersion whose plasmon frequency can be increased substantially by increasing the ribbon width or charge density concentration. Forbidden plasmon regions are observed for high values of plasmon excitation angle or electron relaxation rate. From a sensing point of view, the important finding is the fact that 2D GNR arrays of 155 nm wide with high values of electron relaxation rate have plasmon responses similar to those observed for α−thrombin in water. Our predictions are projected to be of fast support for detecting plasmons in more complex designs of ribbon nanodevices with potential applications in molecular sensing of aqueous molecules.

Four-Photon Absorption Iron Complex for Magnetic Resonance/Photoacoustic Dual-Model Imaging and an Enhanced Ferroptosis Process.

Four-photon absorption (4PA) multimodal therapeutic agent applied to tumor ferroptosis process tracking is rarely reported. In this paper, two functionalized terpyridine iron complexes (TD-FeCl3, TD-Fe-TD) with four-photon absorption properties were designed and synthesized. The four-photon absorption cross sections of TD-FeCl3 reached 6.87 × 10-74cm8·s3·photon-3. Due to its strong near-infrared absorption, TD-FeCl3 has excellent photoacoustic imaging (PAI) capability for accurate PA imaging. TD-FeCl3 has an efficient longitudinal electron relaxation rate (r1 = 2.26 mM-1 s-1) and high spatial resolution, which can be applied as T1-weighted magnetic resonance imaging (MRI) contrast agent for tumor imaging in vivo. In addition, Fe3+ as a natural ferroptosis tracer, TD-FeCl3, is able to deplete glutathione (GSH) effectively, which can further enhance the ferroptosis process. We found that the series of cheap transition metal complexes has four-photon absorption activity and can be used as multimodal (MRI/PAI) diagnostic agents for tumor tracing processes.

Spectral and kinetic properties of silver sulfide quantum dots in an external electric field

The effect of an external electric field on the luminescence characteristics of silver sulfide nanoparticles embedded in a film based on an optically passive dielectric matrix has been studied. The luminescence characteristics were studied using methods of optical and time-resolved spectroscopy involving the time-correlated single-photon counting technique. The morphology of the nanoparticles was studied using transmission electron microscopy. It was shown that in an external electric field, an increase in the intensity of the recombination luminescence band is observed for silver sulfide nanoparticles, together with an increase in the electronic relaxation rate. This effect is explained by the fact that the electric field enhances the transport of free holes to electron traps which are radiative recombination centers. The observed effects indicate that silver sulfide nanoparticles can be effectively used as active layers of organic light-emitting diodes, where an external field of the order of 500 kV/cm will not lead to a deterioration in their operating luminescence characteristics.

Fast electron and slow hole spin relaxation in CsPbI3 nanocrystals

Spin-dependent properties of lead halide perovskites (LHPs) have recently gained significant attention paving their way toward spin-optoelectronic applications. However, separate measurements of the electron and hole spin relaxation rates are so far missing in LHPs. The knowledge of the electron and hole spin relaxation timescales is necessary to understand the spin-dependent properties of LHPs. Here, we report on the spin polarization dynamics in CsPbI3 nanocrystals (NCs). We employ polarization dependent ultrafast differential transmission spectroscopy (DTS) at room temperature to study the spin polarization dynamics in this system. In the case of pure CsPbI3 NCs, it is not possible to measure separately electron and hole spin relaxation rates from the polarization dependent DTS. Here, we introduce the soluble fullerene derivative PC60BM as an electron acceptor along with CsPbI3 to create an imbalance between the photoexcited electrons and holes in the NCs and, thus, affecting their spin-dependent carrier distribution. CsPbI3:PC60BM blend sample shows a distinct difference in the spin dependent kinetics of the DTS spectra as compared to the NCs-only sample. With the help of a kinetic model for the spin-dependent charge carrier distributions, we separately determine the electron and hole spin relaxation times in CsPbI3 NCs. We find that the room temperature hole spin lifetime (τh ∼ 5 ps) is ∼13 times longer than the electron spin lifetime (τe ∼ 0.4 ps). We ascribe the fast electron spin relaxation to the presence of strong spin–orbit coupling in the conduction band, which is ineffective for holes in the s-type valence band.

Spin polarization characteristics of hybrid optically pumped comagnetometers with different density ratios.

We investigate the effects of the density ratio of K-Rb hybrid cells on the alkali metal-noble gas comagnetometers. Bloch equations simplified with the density ratio and average-pumping-rate model are presented for numerical simulation, which simplifies equations of complete hybrid spin ensemble and problem of polarization gradient. The spin polarizations of electron and nucleon, total electronic relaxation rates, and the spin-exchange efficiencies are measured with cells of different density ratios. The results are in good agreement with our equivalent model. Based on our theoretical analysis, the K-Rb-21Ne comagnetometer achieves maximum output signal by optimizing the combination of density ratio and optical power density. The density ratio is critical to the homogeneity of spin polarization and efficiency of hyperpolarization. The method in this work finds a way to optimize the sensitivity of comagnetometers, which is significant for angular-rotation sensors and new physics research.

Nonadiabatic Molecular Dynamics with Extended Density Functional Tight-Binding: Application to Nanocrystals and Periodic Solids.

In this work, we report a new methodology for nonadiabatic molecular dynamics calculations within the extended tight-binding (xTB) framework. We demonstrate the applicability of the developed approach to finite and periodic systems with thousands of atoms by modeling "hot" electron relaxation dynamics in silicon nanocrystals and electron-hole recombination in both a graphitic carbon nitride monolayer and a titanium-based metal-organic framework (MOF). This work reports the nonadiabatic dynamic simulations in the largest Si nanocrystals studied so far by the xTB framework, with diameters up to 3.5 nm. For silicon nanocrystals, we find a non-monotonic dependence of "hot" electron relaxation rates on the nanocrystal size, in agreement with available experimental reports. We rationalize this relationship by a combination of decreasing nonadiabatic couplings related to system size and the increase of available coherent transfer pathways in systems with higher densities of states. We emphasize the importance of proper treatment of coherences for obtaining such non-monotonic dependences. We characterize the electron-hole recombination dynamics in the graphitic carbon nitride monolayer and the Ti-containing MOF. We demonstrate the importance of spin-adaptation and proper sampling of surface hopping trajectories in modeling such processes. We also assess several trajectory surface hopping schemes and highlight their distinct qualitative behavior in modeling the excited-state dynamics in superexchange-like models depending on how they handle coherences between nearly parallel states.

Calibration of Fermi Velocity to Explore the Plasmonic Character of Graphene Nanoribbon Arrays by a Semi-Analytical Model.

We present an analysis of the electronic and plasmonic behavior of periodic planar distributions of sufficiently wide graphene nanoribbons, for which a thorough ab initio investigation is practically unfeasible. Our approach is based on a semi-analytical model whose only free parameter is the charge carrier velocity, which we estimate by density-functional theory calculations on graphene. By this approach, we show that the plasmon resonance energies of the scrutinized systems fall in the lower THz band, relevant for optoelectronic and photonic applications. We further observe that these energies critically depend on the charge carrier concentration, ribbon width, electron relaxation rate, and in-plane transferred momentum angle, thus, suggesting a tunability of the associated light-matter modes.

Hot electron relaxation and energy loss rate in silicon: Temperature dependence and main scattering channels

In this work, we revisit the density functional theory (DFT)-based results for electron–phonon scattering in highly excited silicon. Using the state-of-the-art ab initio methods, we examine the main scattering channels, which contribute to the total electron–phonon scattering rate and the energy loss rate of photoexcited electrons in silicon as well as their temperature dependence. Both temperature dependence and the main scattering channels are shown to strongly differ for the total electron–phonon scattering rate and the energy loss rate of photoexcited electrons. While the total electron–phonon scattering rate increases strongly with temperature, the temperature dependence of the energy loss rate is negligible. Also, while acoustic phonons dominate the total electron–phonon scattering rate at 300 K, the main contribution to the energy loss rate comes from optical modes. In this respect, DFT-based results are found to disagree with conclusions of Fischetti et al. [Appl. Phys. Lett. 114, 222104 (2019)]. We explain the origin of this discrepancy, which is mainly due to differences in the description of the electron–phonon scattering channels associated with transverse phonons.

Controlling optical properties and drag of photon and surface plasmon polaritons in triple quantum dot molecules and dots-metal plasmonic interface via tunneling-assisted quantum coherence

Optical properties of surface plasmon polaritons (SPPs) and drag of photon and SPPs attracted great attention owing to multi-fold potential fundamental and technological implications. In this paper, we investigate the nature of light propagation, optical properties of probe field and SPPs and drag of photon and SPPs in a four-level triple quantum dot molecule (TQDM) well and a dots-metallic plasmonic interface (DMPI). The quantum coherence in TQDM and coupled DMPI interface is achieved via interbands tunneling-assisted quantum coherence namely- classical electromagnetically induced transparency (EIT) and plasmonic induced transparency (PIT). In contrast to earlier studies, herein we employed Drude-model type dielectric constant of metal with inclusion of size-dependent correction term to account for the reduced electron relaxation rate near the thin metal interface. In particular, after solving the master density-matrix equation of the quantum dot system for the eigenvalues, we interpreted the calculated theoretical results on light dragging and optical properties of probe field and SPPs via tunneling-assisted dressed-state picture of EIT/PIT in the proposed TQDM system and DMPI interface. The tunneling parameter considerably tuned the nature of light propagation from subluminal to superluminal and vice versa, optical properties and drag of photon and SPPs. The maximum positive (negative) value of the group index for photon is of order 3.68×106 (2.685×106) while for SPPs it is of order 3×105. The maximum value of lateral drag for photon and SPPs is ±5×10-2m and ±1.173×10-3m, respectively. The calculated propagation length and wavelength for SPPs are 6.25×10-7m and 3.78×10-7 m which correspond to the quality factor of 10.38.

Atomic-Level Structure Determines Electron-Phonon Scattering Rates in 2-D Polar Metal Heterostructures.

The electron dynamics of atomically thin 2-D polar metal heterostructures, which consisted of a few crystalline metal atomic layers intercalated between hexagonal silicon carbide and graphene grown from the silicon carbide, were studied using nearly degenerate transient absorption spectroscopy. Optical pumping created charge carriers in both the 2-D metals and graphene components. Wavelength-dependent probing suggests that graphene-to-metal carrier transfer occurred on a sub-picosecond time scale. Following rapid (<300 fs) carrier-carrier scattering, charge carriers monitored through the metal interband transition relaxed through several consecutive cooling mechanisms that included sub-picosecond carrier-phonon scattering and dissipation to the silicon carbide substrate over tens of picoseconds. By studying 2-D In, 2-D Ga, and a Ga/In alloy, we resolved accelerated electron-phonon scattering rates upon alloy formation as well as structural influences on the excitation of in-plane phonon shear modes. More rapid cooling in alloys is attributed to increased lattice disorder, which was observed through correlative polarization-resolved second harmonic generation and electron microscopy. This connection between the electronic relaxation rates, far-field optical responses, and metal lattice disorder is made possible by the intimate relation between nonlinear optical properties and atomic-level structure in these materials. These studies provided insights into electronic carrier dynamics in 2-D crystalline elemental metals, including resolving contributions from specific components of a 2-D metal-containing heterojunction. The correlative ultrafast spectroscopy and nonlinear microscopy results suggest that the energy dissipation rates can be tuned through atomic-level structures.

Evaluating the anharmonicity contributions to the molecular excited state internal conversion rates with finite temperature TD-DMRG.

In this work, we propose a new method to calculate molecular nonradiative electronic relaxation rates based on the numerically exact time-dependent density matrix renormalization group theory. This method could go beyond the existing frameworks under the harmonic approximation (HA) of the potential energy surface (PES) so that the anharmonic effect could be considered, which is of vital importance when the electronic energy gap is much larger than the vibrational frequency. We calculate the internal conversion (IC) rates in a two-mode model with Morse potential to investigate the validity of HA. We find that HA is unsatisfactory unless only the lowest several vibrational states of the lower electronic state are involved in the transition process when the adiabatic excitation energy is relatively low. As the excitation energy increases, HA first underestimates and then overestimates the IC rates when the excited state PES shifts toward the dissociative side of the ground state PES. On the contrary, HA slightly overestimates the IC rates when the excited state PES shifts toward the repulsive side. In both cases, a higher temperature enlarges the error of HA. As a real example to demonstrate the effectiveness and scalability of the method, we calculate the IC rates of azulene from S1 to S0 on the ab initio anharmonic PES approximated by the one-mode representation. The calculated IC rates of azulene under HA are consistent with the analytically exact results. The rates on the anharmonic PES are 30%-40% higher than the rates under HA.

Role of Electronic Relaxation Rates in the Casimir Force between High-Tc Superconductors

We revisit the problem of the Casimir force between high-Tc superconductors below and above the critical temperature for the superconducting transition. Ceramic superconductors exhibit a different temperature dependence of the reflectivity when switching from the normal to the superconducting state. We leverage this unique characteristic with respect to ordinary metals to claim that these kind of materials can prove useful as an alternative system where the long-standing discussion on the role of electronic relaxation can be addressed. Furthermore, we show that the two main damping mechanisms associated with free and mid-infrared electrons dominate at very distinct scales, meaning that they can be considered separately when the Casimir force is measured as a function of slab distance. This facilitates the experimental identification of the role of the two electronic relaxation contributions to the force.

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.

Ab initio modeling of phonon-assisted relaxation of electrons and excitons in semiconductor nanocrystals for multiexciton generation

Electron-phonon and exciton-phonon interactions in nanoclusters are formulated and computed under the framework of GW-BSE (Bethe-Salpeter equation) approach. The phonon effect is modeled with the two-particle representation for the first time. The nonradiative relaxation rates of electrons and excitons are calculated. It is uncovered that both single-phonon relaxation and multiple-phonon relaxation are significant in nanocrystals, and correspond to two types of physical processes that have totally different spectral lineshapes. Furthermore, the multiple-phonon relaxation always occurs and its rates are comparable to the corresponding single-phonon relaxation rates for both electrons and excitons in the system studied (Si46). The inelastic scattering rates of electrons and excitons are also calculated based on many-body Green function theory. For the electronic states in Si46, the inelastic scattering decay is predicted to be a primary decay mechanism for multiexciton relaxation, and nonradiative relaxation rates are larger than inelastic scattering rates for most excitonic states in Si46.

Excited state dynamics in a sodium and iodine co-doped lead telluride nanowire

ABSTRACT Materials that convert wasted heat into electricity are needed to help solve global warming and other climate challenges. Thermoelectric nanowires are novel metamaterials for such applications. Non-adiabatic coupling computations are critical in understanding thermally activated charge transfer in thermoelectric materials. Here, non-adiabatic computations are used to evaluate electron relaxation rates in lead telluride nanowires. This work reports results on PbTe (lead telluride) atomistic models doped with sodium and iodine that contain 288 atoms in simulation cells with periodic boundary conditions. The calculations are performed on the basis of ground-state DFT under the VASP software. The transitions between states are modelled in terms of Redfield equation of motion parameterised by on-the-fly non-adiabatic couplings along thermalised molecular dynamic trajectory. The initial states are approximated by the promotion of an electron from occupied to unoccupied Kohn–Sham orbital. In each transition, the change of the energy and spatial charge distribution with respect to time were calculated, demonstrating formation of charge transfer. The trends of electron and hole relaxation rates comply with the energy gap law.

Emergence of massless Froḧlich polarons in two-dimensional semi-metals on polar substrates

We consider the polaron dynamics driven by Froḧlich type, long wavelength dominated electron-phonon interaction at zero temperature, for three different semi-metals: single and bilayer graphene, and semi-Dirac, all grown on polar substrates such as, SiC. Single layer graphene (henceforth called SL graphene), bilayer graphene (henceforth called BL graphene), and semi-Dirac have two dimensional band-structures with point Fermi surfaces in their natural undoped conditions. When these materials are grown on polar substrates, their electrons can interact with the optical phonons (LO) at the surface of the substrates. That gives rise to the possibility of polaron formation in the context of these semi-metals, although they themselves are non-polar. Starting from the Froḧlich type electron-phonon interaction Hamiltonian, perturbation theory is employed to calculate the self energy of the electron due to polaron formation for the three aforementioned systems. The electron self energy, or the polaron energy, calculated analytically for BL graphene, is shown to vary linearly with the electron momentum for small electron momenta. Whereas for ordinary polar crystals (both two and three dimensional), for small electron momentum, the polaron energy is quadratic leading to the mass correction of the electron, for BL graphene the polaron energy disperses linearly, rendering the massive BL graphene electrons effectively massless. Energies for Froḧlich polarons in SL graphene and semi-Dirac on polar substrates, are numerically evaluated. Also, the electron relaxation rate, related to the imaginary part of the analytically continued electron self energy expression, is calculated for the three systems.