Electronic Relaxation Research Articles

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.”

Interaction of Stacking Faults with point/extended defects in Fe-He irradiated 6H-SiC

The study explored the microstructure evolution of 6H-SiC that underwent sequential iron and helium ion irradiation with energies of 2.5 MeV and 500 keV, respectively, at room temperature, followed by annealing at 1500°C for two hours. Following irradiation, the entire damaged layer underwent amorphization. However, during subsequent annealing, epitaxial recrystallization took place, resulting in the formation of defected polycrystalline 6H-SiC characterized by the presence of Fe-rich clusters, cavities, and stacking faults. Fe-rich cavities were found to predominantly form at the edges of the stacking faults, as revealed by XTEM. The interaction of microstructural defects is further investigated via first-principles calculations. The periphery of the stacking faults has been identified as the primary location for the emergence of vacancy clusters, serving as favorable sites for the accumulation of point defects, including Fe atoms. This behavior can be attributed to the combined effects of mechanical and electronic energy relaxation mechanisms. Mechanically, the presence of stacking faults allows for the release of elastic energy that had been stored at the boundary. Electronically, the energy relaxation arises from the saturation of C- and Si-dangling bonds. Both of these processes contribute to the observed behavior, highlighting the intricate interplay between mechanical and electronic factors in the system. The low point defect migration energy barriers in the vicinity of the stacking faults promise high recombination, which can limit cavity growth and enhance radiation resistance. The study not only offers valuable insights into the mechanism of cavity/stacking faults interaction, contributing to a better understanding of radiation damage in 6H-SiC but also demonstrates that 6H-SiC material containing stacking faults could serve as a viable alternative to 3C-SiC for nuclear application.

Structural Evolution of Paramagnetic Lanthanide Compounds in Solution Compared to Time- and Ensemble-Average Structures.

Anisotropy in the magnetic susceptibility strongly influences the paramagnetic shifts seen in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) experiments. A previous study on a series of C3-symmetric prototype MRI contrast agents showed that their magnetic anisotropy was highly sensitive to changes in molecular geometry and concluded that changes in the average angle between the lanthanide-oxygen (Ln-O) bonds and the molecular C3 axis due to solvent interactions had a significant impact on the magnetic anisotropy and, consequently, the paramagnetic shift. However, this study, like many others, was predicated on an idealized C3-symmetric structural model, which may not be representative of the dynamic structure in solution at the single-molecule level. Here, we address this by using ab initio molecular dynamics simulations to simulate how the molecular geometry, in particular the angles between the Ln-O bonds and the pseudo-C3 axis, evolves over time in the solution, mimicking typical experimental conditions. We observe large-amplitude oscillations in the O-Ln-C̃3 angles, and complete active space self-consistent field spin-orbit calculations show that this leads to similarly large oscillations in the pseudocontact (dipolar) paramagnetic NMR shifts. The time-averaged shifts show good agreement with experimental measurements, while the large fluctuations suggest that an idealized structure provides an incomplete description of the solution dynamics. Our observations have significant implications for modeling the electronic and nuclear relaxation times in this and other systems where the magnetic susceptibility is exquisitely sensitive to the molecular structure.

Photoreduced graphene oxide recovers graphene hot electron cooling dynamics

Reduced graphene oxide (rGO) is a bulk-processable quasi-amorphous 2D material with broad spectral coverage and fast electronic response. rGO sheets are suspended in a polymer matrix and sequentially photoreduced while measuring the evolving optical spectra and ultrafast electron relaxation dynamics. Photoreduced rGO yields optical absorption spectra that fit with the same Fano lineshape parameters as monolayer graphene. With increasing photoreduction time, rGO transient absorption kinetics accelerate monotonically, reaching an optimal point that matches the hot electron cooling in graphene. All stages of rGO ultrafast kinetics are simulated with a hot-electron cooling model mediated by disorder-assisted supercollisions. While the rGO room temperature 0.31 ps$^{-1}$ electronic cooling rate matches monolayer graphene, subsequent photoreduction can rapidly increase the rate by ~10-12$\times$. Such accelerated supercollision rates imply a reduced mean-free scattering length caused by photoionized point-defects on the rGO sp$^2$ sub-lattice. For visible range excitations of rGO, photoreduction shows three increasing spectral peaks that match graphene quantum dot (GQD) transitions, while a broad peak from oxygenated defect edge states shrinks. These three confined GQD states donate their hot carriers to the graphene sub-lattice with a 0.17 ps rise-time that accelerates with photoreduction. Collectively, many desirable photophysical properties of 2D graphene are replicated through selectively reducing rGO scaffolded within a 3D bulk polymeric network.

Bi0.5Sr0.5FeO3-δ perovskite B-site doped Ln (Nd, Sm) as cathode for high performance Co-free intermediate temperature solid oxide fuel cell

The commercialization of the Co-free solid oxide fuel cell (SOFC) cathode was hindered due to its insufficient oxygen reduction reaction (ORR) activity. In this paper, the ORR activity of the cathode material is effectively increased by doping a small amount of low-valence Ln3+ (Nd3+, Sm3+) ions at the B-site of Fe-based perovskite Bi0.5Sr0.5FeO3-δ (BSF). Nd3+ increases the free volume (Vfree) per unit cell of perovskite due to its larger ionic radius, thus increasing the oxygen vacancy concentration of Bi0.5Sr0.5Fe0.95Nd0.05O3-δ (BSFN). Compared with BSF, BSFN has greatly improved the surface oxygen exchange and volume diffusion rate through the electronic conductivity relaxation (ECR) method. At 700 °C, the area-specific resistance (ASR) of BSFN reached 0.062 Ω cm2, which was 65% lower than that of BSF (0.177 Ω cm2), and the peak power density (PDD) of BSFN-based single cell reached 1.10 W cm−2, which was 1.7 times higher than that of BSF-based single cell (0.64 W cm−2). Through DRT analysis, it is concluded that BSFN can accelerate the adsorption/dissociation process of oxygen due to its higher oxygen vacancy concentration compared with BSF, so that ORR is not limited to the three-phase interface but extends to the entire cathode surface. In addition, in the 80 h long-term stability test at 600 °C, the BSFN-based single cell has better long-term stability because the thermal expansion coefficient of BSFN (14.1 × 10−6 K−1) is lower than that of BSF (14.4 × 10−6 K−1). In general, BSFN is expected to become a Co-free base high-performance intermediate temperature SOFC cathode.

Hot-Electron-Induced Photochemical Properties of CdSe/ZnSe Core/Shell Quantum Dots under an Ambient Environment.

Using CdSe/ZnSe core-shell quantum dots (QDs) as a model, we systematically investigate the photochemical properties of QDs with the ZnSe shells under an ambient environment, which show almost opposite responses to either oxygen or water in comparison with CdSe/CdS core/shell QDs. While the ZnSe shells provide an efficient potential barrier for photoinduced electron transfer from the core to the surface-adsorbed oxygen, they also act as a stepping stone for hot-electron transfer directly from the ZnSe shells to oxygen. The latter process is so effective and competes favorably with ultrafast relaxation of hot electrons from the ZnSe shells to the core QDs, which can completely quench the photoluminescence (PL) with saturated adsorption of oxygen (1 bar) and initiate oxidation of the surface anion sites. Water can slowly eliminate the excess hole to neutralize the positively charged QDs, partially canceling the photochemical effects of oxygen. Alkylphosphines─through two distinctive reaction pathways with oxygen─stop the photochemical effects of oxygen and completely recover PL. With limited thickness (around two monolayers), the ZnS outer shells substantially slow down photochemical effects on CdSe/ZnSe/ZnS core/shell/shell QDs but cannot fully stop PL quenching by oxygen.

The Metal-Oxide Nanoparticle-Aqueous Solution Interface Studied by Liquid-Microjet Photoemission.

ConspectusThe liquid-microjet technique combined with soft X-ray photoelectron spectroscopy (PES) has become an exceptionally powerful experimental tool to investigate the electronic structure of liquid water and nonaqueous solvents and solutes, including nanoparticle (NP) suspensions, since its first implementation at the BESSY II synchrotron radiation facility 20 years ago. This Account focuses on NPs dispersed in water, offering a unique opportunity to access the solid-electrolyte interface for identifying interfacial species by their characteristic photoelectron spectral fingerprints. Generally, the applicability of PES to a solid-water interface is hampered due to the small mean free path of the photoelectrons in solution. Several approaches have been developed for the electrode-water system and will be reviewed briefly. The situation is different for the NP-water system. Our experiments imply that the transition-metal oxide (TMO) NPs used in our studies reside close enough to the solution-vacuum interface that electrons emitted from the NP-solution interface (and from the NP interior) can be detected.We were specifically exploring aqueous-phase TMO NPs that have a high potential for (photo)electrocatalytic applications, e.g., for solar fuel generation. The central question we address here is how H2O molecules interact with the respective TMO NP surface. Liquid-microjet PES experiments, performed from hematite (α-Fe2O3, iron(III) oxide) and anatase (TiO2, titanium(IV) oxide) NPs dispersed in aqueous solutions, exhibit sufficient sensitity to distinguish between free bulk-solution water molecules and those adsorbed at the NP surface. Moreover, hydroxyl species resulting from dissociative water adsorption can be identified in the photoemission spectra. An important aspect is that in the NP(aq) system the TMO surface is in contact with a true extended bulk electrolyte solution rather than with a few monolayers of water, as is the case in experiments using single-crystal samples. This has a decisive effect on the interfacial processes that can occur since NP-water interactions can be uniquely investigated as a function of pH and provides an environment allowing for unhindered proton migration. Our studies confirm that water is dissociatively adsorbed at the hematite surface and molecularly adsorbed at the TiO2 NP surface at low pH. In contrast, at near-basic pH the water interaction is dissociative at the TiO2 NP surface.The liquid-microjet measurements presented here also highlight the multiple aspects of photoemission necessary for a full characterization of TMO nanoparticle surfaces in aqueous environments. For instance, we exploit the ability to increase species-specific electron signals via resonant photoemission, so-called partial electron yield X-ray absorption (PEY-XA) spectra, and from valence photoelectron and resonant Auger-electron spectra. We also address the potential of these resonance processes and the associated ultrafast electronic relaxations for determining charge transfer or electron delocalization times, e.g., from Fe3+ located at the hematite nanoparticle interface into the aqueous-solution environment.

Optimizing Hot Electron Harvesting at Planar Metal-Semiconductor Interfaces with Titanium Oxynitride Thin Films.

Understanding metal-semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where electrons in the metal can be excited by sub-bandgap photons and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO2 and titanium oxynitride (TiON)/TiO2-x interfaces, where in the latter case the spontaneously forming oxide layer (TiO2-x) creates a metal-semiconductor contact. Time-resolved pump-probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO2, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO0.5N0.5) exhibits the highest carrier extraction efficiency (NFC ≈ 2.8 × 1019 m-3), slowest trapping, and an appreciable hot electron population reaching the surface oxide (NHE ≈ 1.6 × 1018 m-3). Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and prolonging electron lifetimes, providing an optimized metal-semiconductor interface using only the native oxide of titanium oxynitride.

Ultrafast Laser Control of Antiferromagnetic-Ferrimagnetic Switching in Two-Dimensional Ferromagnetic Semiconductor Heterostructures.

Realizing ultrafast control of magnetization switching is of crucial importance for information processing and recording technology. Here, we explore the laser-induced spin electron excitation and relaxation dynamics processes of CrCl3/CrBr3 heterostructures with antiparallel (AP) and parallel (P) systems. Although an ultrafast demagnetization of CrCl3 and CrBr3 layers occurs in both AP and P systems, the overall magnetic order of the heterostructure remains unchanged due to the laser-induced equivalent interlayer spin electron excitation. More crucially, the interlayer magnetic order switches from antiferromagnetic (AFM) to ferrimagnetic (FiM) in the AP system once the laser pulse disappears. The microscopic mechanism underpinning this magnetization switching is dominated by the asymmetrical interlayer charge transfer combined with a spin-flip, which breaks the interlayer AFM symmetry and ultimately results in an inequivalent shift in the moment between two FM layers. Our study opens up a new idea for ultrafast laser control of magnetization switching in two-dimensional opto-spintronic devices.

Engineering-Physical Method for Determining the Thermal Conductivity of Objects with Micrometric Thickness and a Complex Structure

Introduction. The application of functional coatings on products, the performance properties of which are localized in the surface layer is a trend in modern mechanical engineering and materials science. The issues considered in this regard are relevant, in particular, for thermal-barrier coatings of turbine blades of steam and gas turbine engines. It is worth mentioning the materials that experience significant thermal loads during operation. In this case, the lack of reliable methods for predicting the thermophysical properties of the coating seems to be a problem. The work objective is to create a computational and analytical methodology for determining the thermal conductivity of coatings. This approach is based on experimental data and takes into account structural parameters of the material.Materials and Methods. The experiments were carried out with the blades of a high-speed gas turbine of a locomotive engine made of heat-resistant chromium-nickel alloy Inconel 713LC. An experimental multiphase coating of the Nb-Ti-Al intermetallic system with a thickness of about 80 microns was applied using vacuum ion-plasma technology. The two-beam scanning electron microscope Zeiss CrossBeam 340 was used in the work. The thermal conductivity of the coatings was determined by an experimental technique based on the measurement of the contact potential difference (CPD). Numerical values of this difference were obtained using a mirror galvanometer with high voltage sensitivity. A special signal amplifier and a USB oscilloscope were used to record the readings.Results. The calculation apparatus of the thermal conductivity determination technique is based on the experimental values of ∆φ CPD:– for the base metal (Inconel 713LC) +846 mkV;– for the coating Nb-Ti-Al – 90 mkV.The solution to the problem of the distribution of particles in a force field with a potential difference ∆φ is described by the Boltzmann distribution. Starting from the obtained result, we get:– CPD at the boundary of the contacting metals;– energy and thermal conductivity of the Fermi level;– electron relaxation time.The multidirectional influence that the dimensional differences of the particles of the second phase have on the effective thermal conductivity is considered. For this case, a dimensionless value of the effective thermal conductivity in the direction of each axis and the effective thermal conductivity of the composite are found. Porosity is taken into account according to the Maxwell – Aiken dependence and introduced into the general calculation system. The thermal conductivity of Nb-Ti-Al is established: λNbTiAl = 4,76 W/m.K. Thus, the thermal barrier coating Nb-Ti-Al fully meets its functional purpose.Discussion and Conclusion. The method of determining thermal conductivity described in the article is applicable only to conductive consolidated materials or composites with a continuous conductive matrix. The presented work completes the initial stage of creating a computational and analytical model for predicting the thermal conductivity of materials and coatings. The results of testing the model for materials with a complex structure showed its satisfactory accuracy. This indicates the expediency of using the two considered elements of the model. The first one is the instrumental measurement of the CPD. The second one is taking into account the features of the structural and phase state of the material. With the development of the model, it is expected to overcome its weaknesses:– the impossibility of using non-conductive objects to determine the thermal conductivity;– a significant decrease in the accuracy of determining thermal conductivity for materials and coatings with a gradient structure.

Atmosphere-Dependent Electron Relaxation of the Ag-Decorated TiO2 and the Relations with Photocatalytic Properties

In the current research, the atmosphere effects on the photoinduced electron relaxations of the undecorated TiO2 and Ag-decorated TiO2 (Ag/TiO2) were carefully studied by means of the in situ photoconductance and diffuse reflection measurements. In pure N2 atmosphere, the results showed that the electron relaxation mainly occurs through the transfer to the residual O2, and the Ag nanoparticles form a fast electron transfer pathway. It was seen that the apparent activation energy of the electron transfer to O2 was greatly reduced by the Ag decoration. In the methanol-containing N2 atmosphere, the electron relaxation can still occur via the transfer to residual O2 in the case of the undecorated TiO2, while the relaxation mechanism changes for the Ag/TiO2 as the relaxations are decreased with the temperatures. It is possible that the methanol molecule adsorbed on the Ag/TiO2 perimeters could act as the bridge for the recombination of the holes and the electrons stored in the Ag nanoparticles. Reducing the Ag nanoparticle size from 15 nm to 3 nm can greatly increase the electron relaxations due to the increase in Ag dispersion and Ag/TiO2 interconnection. Although the electron transfer to O2 was increased, both the photocatalytic oxidations of acetone and isopropanol showed a decrease after the Ag decoration. The results indicated that the photocatalytic oxidation was not limited by the electron transfer to O2. The increased electron transfer to O2 contributed to the recombination around the Ag/TiO2 perimeters, and the photocatalytic activities were decreased.

The role of shear waves in electron-phonon relaxation and electrical resistivity of noble metals

• Shear waves make a main contribution to the resistivity of noble metals • At low temperatures, it is 95, 91, and 95% for crystals Au, Ag, and Cu, respectively • Results is aligns well with experiment in the temperature range from 10 to 1000 K The influence of elastic energy anisotropy on electron-phonon relaxation and electrical resistivity of noble metals has been studied within the Bloch-Grüneisen theory. Accounting for the influence of elastic anisotropy on spectrum, phonon polarization vectors, and electron relaxation by quasi-transverse modes makes it possible to quantitatively put in agreement the outcomes of calculating the temperature dependence of the electrical resistivity of noble metals with experimental data in the temperature range from 10 to 1000 K. A constant characterizing the interaction of electrons with shear waves in noble metals has been determined. The role of shear waves in electrical resistivity has been analyzed. It has been shown that the relaxation of electrons by shear waves, which was not previously taken into account, makes a dominant contribution to the electrical resistivity of noble metals in the entire temperature range from 10 to 1000 K. At temperatures well below the Debye temperature, this contribution is 95%, 91%, and 95% of the electrical resistivity for crystals Au, Ag, and Cu, respectively, and at T = 1000 K it decreases to 73%, 44%, and 68%.

Ab initio real-time quantum dynamics of charge carriers in momentum space.

Application of the non-adiabatic molecular dynamics (NAMD) approach is limited to studying carrier dynamics in the momentum space, as a supercell is required to sample the phonon excitation and electron-phonon (e-ph) interaction at different momenta in a molecular dynamics simulation. Here we develop an ab initio approach for the real-time charge carrier quantum dynamics in the momentum space (NAMD_k) by directly introducing e-ph coupling into the Hamiltonian based on the harmonic approximation. The NAMD_k approach maintains the zero-point energy and includes memory effects of carrier dynamics. The application of NAMD_k to the hot carrier dynamics in graphene reveals the phonon-specific relaxation mechanism. An energy threshold of 0.2 eV-defined by two optical phonon modes-separates the hot electron relaxation into fast and slow regions with lifetimes of pico- and nanoseconds, respectively. The NAMD_k approach provides an effective tool to understand real-time carrier dynamics in the momentum space for different materials.

Charge transfer and electronic relaxation effects in the photoemission of EMIM-DCA ionic liquid vapor

Ultraviolet photoelectron spectroscopy (UPS) measurements of EMIM-DCA vapors have been conducted. Density functional theory (DFT), Møller–Plesset perturbation theory (MP2) and many-body perturbation theory calculations (GW) have been used to interpret the experimentally measured spectra. The vapor of ionic liquids normally consists of isolated ion-pairs and the corresponding photoemission spectra should reflect the electronic binding energies of the molecular orbitals. The UPS spectra have revealed that many photolines are shifted when compared to ab initio calculation results. The hybrid DFT functionals, which performed very good in the description of the UPS spectra of many other ionic liquid vapors have shortcomings in the EMIM-DCA case. The MP2 calculation and the quasiparticle GW approach are shown to offer a better description of the experimental UPS spectra. The charge distribution analysis showed that there seems to be a favorable charge transfer from the cation to the anion upon ionization. The new data provides strong support to the argument that in some cases DFT still has difficulties in the modeling of IL ion-pairs.

Role of La 5p in Bulk and Quantum-Confined Solids Probed by the La 5p54f1 3D1 Excitonic Final State of Resonant Inelastic X-ray Scattering

The varied electronic localization of rare earth elements is essential to functional materials and a key to tailoring their properties. We establish with unprecedented spectral resolution the excitonic nature of the lanthanum 5p54f1 3D1 and 3D2 final states of resonant inelastic X-ray scattering (RIXS) at the La N4,5 edges. We extract the intrinsic lifetime, energy distance, and relative intensity ratio from single crystal LaAlO3 and construct an empirical model. With help of the model, we precisely determine the RIXS 3D1 final state position and identify La 5p as a descriptor of covalency with the host material. For metallic lanthanum, La3+ ions in mixed-covalent-ionic simple oxides and phosphates, and ionic salts alike, we find a sizable chemical shift, indicating band-like and free-ion-like La. The different electronic relaxation of the La 5p5 hole and the La 4f1 electron is discussed with local and nonlocal screening contributions. In addition, the energetics of the excitonic La 5p54f1 Coulomb attraction is quantified in its variation from lanthanum metal to mixed-covalent-ionic La2O3 and the ionic LaF3 salt. The power of the approach and analysis is applied to map the influence of geometric quantum confinement to La 5p sharing within quantum dots and quantum wires in comparison to bulk-like microrods of monoclinic LaPO4.

An ab initio molecular dynamics study on Ti2AlN(0001) surfaces

The structures, surface energies, electronic properties of Ti2AlN(0001) surfaces at 0 K-1273 K were systematically investigated by ab initio molecular dynamics and first-principles calculations. It is found that the structural relaxations of Ti(N)- and Ti(Al)-terminated surfaces at all temperatures are mainly confined between outermost two layers, and those of N(Ti)- and Al(Ti)-terminated surfaces with Ti atoms at second layer extends into third layer. N(Ti)-terminated surface experiences the largest relaxation, and Al(Ti)-terminated surface possesses the least relaxation and is the most stable surface at all temperatures. The structural shrinkage relaxation of outermost two layers and surface energy for N(Ti)- and Ti(N)- terminated surfaces keep relatively stable from 0 K to 933 K, whereas dramatically increase from 933 K to 1273 K. The relaxation of outermost two layers for Al(Ti)- and Ti(Al)-terminated surfaces changes owing to the thermal expansion of the d-spacing of Al-Ti layers. The larger thermal expansion coefficient of the d-spacing of Al-Ti layers and faster surface energy decline of Ti(Al)-terminated surface than those of Al(Ti)-terminated surface indicate stronger metallic properties. The influence of redistribution of Ti-3d, N-2p and Al-3p orbitals on the electronic properties and structural relaxations of different termination surfaces at all temperatures range is elucidated.

Hot electron relaxation in Type-II quantum wells.

A photovoltaic device fabricated with conventional zincblende materials can use the Type-II quantum well structure, which spatially separates electrons and holes, to reduce their recombination rate. In order to obtain higher power conversion efficiency, it is desirable to preserve more energetic carriers by engineering a phonon "bottleneck," a mismatch between the gaps in the well and barrier phonon structure. Such a mismatch leads to poor phonon transport and therefore prevents energy from leaving the system in the form of heat. In this paper, we perform a superlattice phonon calculation to verify the "bottleneck" effect and build on this a model to predict the steady state of the hot electrons under photoexcitation. We describe the electrons and phonons with a coupled Boltzmann equation system and numerically integrate it to get the steady state. We find that inhibited phonon relaxation does lead to a more out-of-equilibrium electron distribution and discuss how this might be enhanced. We examine the different behaviors obtained for various combinations of recombination and relaxation rates and their experimental signatures.

A generalized Drude–Smith model for ultrafast pump-and-probe experiments and its application for studying of electronic dynamic properties of polycrystalline diamond

In this work, we generalize the Drude–Smith model (DSM) proposed for optical conductivity in frequency-domain to the case of ultrafast pump-and-probe experiments, where the results are measured in time-domain. The generalized DSM (GDSM) considers the optically induced electronic backscattering or localization effect, and can be utilized for studying the electronic dynamics induced by pulsed radiation field in time-domain. By applying the optical-pump and terahertz-probe (OPTP) technique via normal reflection measurement, we study the electronic dynamic properties of polycrystalline diamond (PCD) on Si substrate and examine the validity of the proposed GDSM. We find that this model can lead to a better fitting between the experimental and theoretical results and can reasonably reflect the information about time resolution of the measurement system and the energy relaxation time for free carriers in PCD. For PCD subjected to 800 nm wavelength fs laser irradiation, the effect of carrier backscattering decreases and the electronic energy relaxation time increases with increasing pump radiation fluence. We hope that the GDSM from this work can be helpful for understanding the electronic dynamics in electronic and optoelectronic materials measured by, e.g., ultrafast pump-and-probe experiments.

Colloidal Quantum Dots in Very-Long-Wave Infrared Detection: Progress, Challenges, and Opportunities.

The very long wave infrared (VLWIR) is an electromagnetic wave with a wavelength range of 15-30 μm, which plays an important role in missile defense and weather monitoring. This paper briefly introduces the development of intraband absorption of colloidal quantum dots (CQDs) and investigates the possibility of using CQDs to produce VLWIR detectors. We calculated the detectivity of CQDs for VLWIR. The results show that the detectivity is affected by parameters such as quantum dot size, temperature, electron relaxation time, and distance between quantum dots. The theoretical derivation results, combined with the current development status, show that the detection of VLWIR by CQDs is still in the theoretical stage.

Multi-core/shell SiO2@Al2O3 nanostructures deposited on Ti3AlC2 to enhance high-temperature stability and microwave absorption properties

Abstract Poor high-temperature stability (HTS) and weak microwave absorption performance (MAP) are a major restriction for wave-absorbing materials in elevated temperature ambient. Consequently, the Stöber process and the sol–gel method are first devised and used to create multi-core/shell SiO2@Al2O3 nanostructures (MCSNs) on Ti3AlC2 (TAC). The MCSNs with a thickness of 135–215 nm raise the starting oxidation temperature of the matrix by 400°C. Furthermore, the weight gain drops from 17.44 to 2.32% within 1 h at 800°C. The effective absorption bandwidth with a reflection loss (RL) ≤ −10 dB of the MCSNs-coated TAC is 3.25 GHz (8.68–11.27 and 11.63–12.29 GHz) at a thickness of 2.0 mm, which is 4.7 times that of the matrix. The minimum RL is reduced by a factor of 2.77 from −10.68 to −29.55 dB. The enhanced MAP is due to the introduced multiple reflection events and scattering mechanism as well as the enhanced electronic polarization, interface polarization, and polarization relaxation. The growth of the MCSNs provides a reference for the design and preparation of bifunctional materials with good HTS and MAP.