First-Principles Investigation of Structural, Electronic, Thermoelectric, and Hydrogen Storage Properties of MgXH3 (X = Cr, Mn, Fe, Co, Ni, Cu) Perovskite Hydrides

Understanding the Temperature Dependence of the Seebeck Coefficient from First-Principles Band Structure Calculations for Organic Thermoelectric Materials

Studies of strongly correlated electron systems have been at the forefront of research in condensed matter physics ever since the discovery of the co-existence of strong Pauli-paramagnetism and superconductivity in the archetypal heavy-fermion compound CeCu2Si2 in 1979. The construct of correlated electron physics typifies the behavior of thermal and electronic properties of a material when the Coulomb interaction between conduction electrons exceeds the electron kinetic energy at a given thermal energy and redefines in remarkable ways our understanding of the behavior of a metal near its ground state. While correlated electron behavior has by now been demonstrated in a variety of different types of materials, Kondo systems in particular are arguably the most intensively studied among these. The Kondo interaction is used to describe the effect that a spin-magnetic ion has on its environment when immersed in the conduction electron sea of a metal. The localized spin of the Kondo ion polarizes nearby conduction electrons to form a so-called Kondo cloud, which acts to screen and magnetically (partially) neutralize the localized spin. In Kondo systems, the low-temperature behavior is prone to the formation of heavy fermions, which is the term given to quasiparticle excitations that define the emergence of effective electron masses that can be up to three orders of magnitude greater than that of a free electron. The Kondo effect presents itself in three guises: first, the single-ion Kondo state which is found in a metal having only a small amount of magnetic ions dissolved into it; second, the incoherent Kondo state in materials where there is a Kondo ion in every crystallographic unit cell of the material, but the Kondo ions remain incoherent or uncoupled from each other; and third, the coherent Kondo lattice state which manifests itself toward low temperatures where the interaction between Kondo ions becomes comparable to the thermal energy of conduction electrons that mediate magnetic exchange between Kondo ions. In a small number of cases, the outcome of a material condensing into the Kondo state turns out to be the peculiar formation of a very narrow energy band gap at the metallic Fermi energy. Such a band gap has significant consequences in practically all of the physical properties of a material that stem from the behavior of conduction electrons in proximity of the Fermi energy. This is most readily seen in electrical resistivity, heat capacity, and magnetic susceptibility. The band gapping gives cause to the term Kondo insulator (also referred to as Kondo semimetal or heavy-fermion semiconductor) that is used to describe this exceptional variety of Kondo systems. The term Kondo insulator is in general use although most Kondo insulators have a small but finite electrical conduction in the low-temperature limit where Kondo screening may be accomplished to its full extent. While the Kondo lattice ground state is exemplified by a very high density of electronic states at the Fermi energy, Kondo insulators, on the other hand, have, by virtue of narrow band gapping, a low density of electronic states. It remains a counter-intuitive observation, therefore, that despite their low density of states, Kondo insulators have curiously strong spin polarization energy scales and accompanying high values of their Kondo temperature, being the defining quantity which acts as an organizing principle in their temperature-dependent physical properties. In this article, we review the fundamentals of the Kondo insulating state, and we discuss the theoretical principles of what is presently understood about the formation of a Kondo insulator. The experimental results of a selected number of examples that have gained prominence in this class of materials are compared to each other in order to seek out similarities that may help deepen our understanding of the Kondo insulating state.

Segment of a periodic table depicting the elements that are molecular/atomic (blue) and that have extended network structures (gray) at STP.

Comparison of calculated photoionization cross-sections of diamond and silicon in the approximations of plane wave and orthogonalized plane wave

Polycrystalline silicon thin film transistors with thin body and thin gate oxide can realize high performance and relative low power consumption simultaneously. Considering their wide applications, a surface-potential-based drain current model suitable for devices with the above structure is derived on charge sheet approximation considering the double exponential trap state distribution, the interface charge and the effect of the back surface potential. According to the calculated areal density of the charges under the gate oxide, the ionized acceptors and the trapped charges, the areal density of the inversion charge is obtained. Under several mathematical treatments, a surface-potential-based drain current model suitable for devices with thin body and thin gate oxide is developed accompanying the quantitative model-validity conditions for low and high state densities respectively. Under high and low state densities respectively, this proposed surface-potential-based drain current model is verified by 2D-device simulation in devices’ transfer characteristics under various drain biases in the situations without or with interface charge.

A comprehensive first‐principles and machine learning study is conducted on 102 halide double perovskites to identify promising candidates for thermoelectric applications. The HSE06 hybrid functional within the Quantum ATK framework is used to accurately determine electronic structures, bandgaps, and total and partial densities of states. Boltzmann transport theory is applied to figure out important thermoelectric parameters, such as the Seebeck coefficient, electrical conductivity, and ZT values over a wide range of temperatures. Supervised machine learning models are trained on density functional theory (DFT)‐derived descriptors to speed up the discovery of new materials. These models demonstrate high predictive accuracy for thermoelectric performance across different chemical spaces. A detailed analysis of the electronic band structures and orbital contributions is carried out for Rb 2 GeI 6 , Rb 2 PbI 6 , Cs 2 SnBr 6 , and In 2 PtCl 6 , some of the best‐performing compounds. A wide range of behaviors is observed, including metallic, degenerate, and wide‐bandgap semiconducting, which correlate with distinct transport properties. This unified method shows how using accurate DFT, transport theory, and machine learning together can help find new materials with specific functions. This will lead to the development of next‐generation thermoelectric technologies based on environmentally friendly halide perovskites.

HfO2, as a gate dielectric material for the charge trapping memory, has been studied extensively due to its merits such as high k value, good thermal stability, and conduction band offset relative to Si, etc.. In order to understand the reason why the charge trapping efficiency is improved by high k capture layer with respect to charge trapping type memory, the variation of HfO2 crystal texture induced by oxygen vacancy and the influences of it are investigated using the first principle calculation based on density functional theory. Results show that the distance of the nearest neighbor oxygen atom from oxygen vacancy is markedly reduced after optimization, whereas the decrease of distances between the next nearest neighbor oxygen atom from oxygen vacancy and hafnium is less. The change of local crystal lattice is caused by optimized oxygen vacancy for it significantly changes the local lattice, but rarely influences the far lattice. Deep energy level and density of electron states in conduction band are contributed by Hf atoms, while the density of electron states in valence band is contributed by O atoms. The local density of electron states in each element and the total density of electron states in the optimization system are all larger than those in the system without optimization, and the sum of the local densities of electron states is less than the total density of electron states. The trapped charges are moving mainly around the oxygen vacancy and the adjacent atoms of oxygen in the optimization system, but the charges are without optimization throughout the system. The local energy of charge is increased in optimized defect system, while the local energy of charge is conspicuously reduced in the system without optimization, i.e. lattice variation without saturation characteristic has a large effect on the local energy of charge. Results further prove that the change of crystal lattice induced by oxygen vacancy has strong ability to capture charge, which helps improve the features of memory.

X-ray photoelectron spectroscopy and full potential studies of the electronic density of state of ternary oxyborate Na 3La 9O 3(BO 3) 8

Na<sub>2</sub>KSb photocathodes have many applications in vacuum optoelectronic devices, such as photomultiplier tubes, image intensifiers, and streak image tubes for high-speed detection and imaging in extremely weak light environments, due to their advantages of high temperature resistance, small dark current, low vacuum requirement, low fabrication cost and high fabrication flexibility. In addition, this type of photocathode has important application prospect in high brightness accelerator photoinjectors. To guide the fabrication of high-sensitivity Na<sub>2</sub>KSb photocathodes, Na<sub>2</sub>KSb surfaces with different surface orientations and atom terminations are investigated by the first-principles calculation method based on the density functional theory to obtain the most stable and most favorable surface for electron emission. From the perspectives of surface energy, adsorption energy, and work function before and after Cs adsorption, it is revealed that the Na<sub>2</sub>KSb (111) K surface exhibits superior surface stability and electron emission capability. Furthermore, the electronic structure and optical properties of Cs adsorption and Cs/O co-adsorption on the Na<sub>2</sub>KSb (111) K surface under different Cs coverages are analyzed, and the mechanism of Cs/O deposition on Na<sub>2</sub>KSb surface is studied. The adsorption energy of Cs in the Cs/O adsorption model is much larger than that in the single Cs adsorption model, indicating that the adsorption of O atoms on the Na<sub>2</sub>KSb surface can make the adsorption of Cs atoms on the surface stronger, and thus increasing the adhesion of Cs atoms on the surface. After adsorption of Cs on the Na<sub>2</sub>KSb (111)K surface, the surface work function only decreases by 0.02 eV, while the maximum work function decrease for the Cs/O adsorbed surface is 0.16 eV, with the Cs coverage of 2/4 ML and the O coverage of 1/4 ML. The adsorption of Cs/O atoms on the surface facilitates the charge transfer above the surface and results in charge accumulation, which can form the effective surface dipole moment. The magnitude of the surface dipole moment is directly related to the change of work function. Furthermore, through the analysis of the electronic band structure and density of states, it is found that the adsorbed Cs atoms have additional contribution to the band structure near the conduction band minimum. After the introduction of O atoms, the valence band moves up, also the bottom of the conduction band and the top of the valence band become flat. The Cs/O deposition is beneficial to increasing the absorption of near-infrared light on the Na<sub>2</sub>KSb surface, but it will reduce the absorption of ultraviolet light and visible light, and the refractive index will also decrease. This work has a certain reference significance for understanding the optimal emission surface of Na<sub>2</sub>KSb photocathode and the mechanism of surface Cs/O deposition.

We perform molecular dynamics (MD) simulations to investigate the effect of polar surfaces on the thermal transport in zinc oxide (ZnO) nanowires. We find that the thermal conductivity of nanowires with free polar (0001) surfaces is much higher than that of nanowires that have been stabilized with reduced charges on the polar (0001) surfaces, and also hexagonal nanowires without any transverse polar surface, where the reduced charge model has been proposed as a promising stabilization mechanism for the (0001) polar surfaces of ZnO nanowires. From normal mode analysis, we show that the higher thermal conductivity is due to the shell-like reconstruction that occurs for the free polar surfaces. This shell-like reconstruction suppresses twisting motion in the nanowires such that the bending phonon modes are not scattered by the other phonon modes, and this leads to substantially higher thermal conductivity of the ZnO nanowires with free polar surfaces. Furthermore, the auto-correlation function of the normal mode coordinate is utilized to extract the phonon lifetime, which leads to a concise explanation for the higher thermal conductivity of ZnO nanowires with free polar surfaces. Our work demonstrates that ZnO nanowires without polar surfaces, which exhibit low thermal conductivity, are more promising candidates for thermoelectric applications than nanowires with polar surfaces (and also high thermal conductivity).

First-principles screening of structural, electronic, optical and elastic properties of Cu-based hydrides-perovskites XCuH3 (X=Ca and Sr) for hydrogen storage applications

We report the electronic and thermal properties of scandium monoantimonide ScSb by means of the Seebeck coefficient, thermal conductivity, specific heat, and nuclear magnetic resonance measurements. The experimental Seebeck coefficient exhibits a strong temperature dependence, and the theoretical calculation based on the two-band model provides a realistic description of the observed feature. The analysis of the thermal conductivity reveals that the lattice thermal conductivity dominates at low temperatures while electronic thermal conductivity makes a major contribution at high temperatures. A small value of the Sommerfeld coefficient of $0.38\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\text{--}2}$ was extracted from the low-temperature specific heat measurement, indicative of a low electronic Fermi-level density of states (DOS) in ScSb. Furthermore, we have deduced the Sc $3d$ and Sb $5s$ partial Fermi-level DOSs based on the Korringa behavior in the $^{45}\mathrm{Sc}$ and $^{121}\mathrm{Sb}$ NMR spin-lattice relaxation rates. The determined values of the DOS are quite low, giving strong evidence for the semimetallic character in ScSb.

In recent years there has been considerable interest in electrode processes at metal surfaces with alkanethiol monolayers attached to them. One effect of the monolayer is that the electron transfer rate from the redox reagent in solution to the metal (gold or platinum are commonly used) becomes clearly nonadiabatic and the effect of the density of states of the metal on such a rate can be investigated. We develop a way of writing the wave function of a semi-infinite metal using tight-binding matrix elements and the 'Z-transform', a discrete Laplace transform. Using these k-dependent metal wave functions we calculate the coupling matrix element between the metal and the redox reagent and thus the electron transfer rate constant. We then study the effect of changing the density of electronic states at the Fermi level (DOS) of a metal on the rate of nonadiabatic electron transfer by changing the metal. The DOS of platinum is about 7.5 times that of gold, the difference being mainly due to the d-band of Pt. Inspite of this difference, the calculated electron transfer rate constant increases only by a factor of about 1.8. Bands which are weakly coupled (e.g., the d-band of Pt in the present case) contribute much less to the rate constant than is suggested by their density of states. Thereby, the rate constant is approximately independent of the density of states in two cases: adiabatic electron transfer and nonadiabatic electron transfer when the extra density of states is due to weakly coupled bands. Our results are in agreement with experiments performed with systems similar to those used in our calculations. We next employ our method to calculate the temperature dependence of the electronic contribution to the nonadiabatic electron transfer rate constant at metal and semiconductor electrodes. We find that the electronic contribution in metals is proportional to T, and under conditions for the maximum rate constant, that at semiconductor electrodes is also proportional to T, but for different reasons than in the case of metals (Boltzmann statistics and transfer at the conduction band edge for the semiconductor vs. Fermi-Dirac statistics and transfer at the Fermi level, which is far from the band edge, of the metal). On a different topic, we study the inverse photoemission spectra at metal electrode-liquid interfaces. In such experiments, an electron transfer redox agent was used to inject electrons or holes into a metal and create excited electronic states of the metal. Emission thus occurs in competition with energy loss and radiationless transitions. Some of the excited states decay radiatively and gave a frequency-dependent spectrum. The spectrum may be analysed to probe the electronic structure of the metal above and below its Fermi level. The experimental technique, known in the literature as charge transfer inverse photoemission spectroscopy (CTRIPS), is treated theoretically here. We give a possible explanation of the data using a model, experimental band structures (from vacuum inverse photemission) and surface states from solution electroreflectance (ER) experiments and propose experiments that could be performed to further clarify the mechanism of electron transfer.

The origin of second harmonic generation (SHG) of Li3VO4 was investigated from the viewpoint of the band structure by using the tight-binding method. The tight-binding parameters were optimized to reproduce the density of states (DOS) obtained from x ray photoelectron spectroscopy and the optical band gap. Although Li3PO4 has the same crystal structure as Li3VO4, it shows no SHG. To explain the difference in optical nonlinearity we compared the electronic structures of Li3VO4 and Li3PO4, in particular at the bottom of conduction band (CB) and the top of valence band (VB), since they are known to play a primary role in SHG. In Li3PO4, the bottom of CB consists of P 3s and O 2p orbitals and the top of VB is composed of O 2p orbitals. These electronic structures result in a relatively low DOS at the bottom of CB and a wide band gap in Li3PO4. On the other hand, in Li3VO4, both bottom of CB and top of VB are composed of V 3d and O 2p. The preferential contribution of V 3d orbitals to the band edge states leads to a high DOS at the bottom of CB, a narrow band gap and delocalization of electrons on V–O bonds. We conclude that these electronic structures are responsible for the high optical-nonlinearity of Li3VO4.

Metallic nanowires are widely used in energy conversion and storage, especially in the thermal management area, because of their high specific surface area, rich active sites, and high thermal conductivity. Metallic nanowires, such as copper or silver nanowires, are extensively applied to prepare the next-generation thermal interface materials with excellent thermal conductivity, light weight, high strength and ductility. Metallic hollow nanowires, which hold the typical one-dimension hollow nanostructures, have high axial thermal conductivity to prepare advanced thermal interface materials applied in thermal management and waste heat recovery of high-power microelectronic devices. Thermal conductivity is one of the most important indicators to assess the thermal performance of thermal interface materials. Over the past decades, many studies in both theory and experiment have been carried out to evaluate the thermal conductivity of solid nanowires. Molecular dynamics (MD) simulation has been applied to calculate the thermal conductivity of single nanowires, single core-shell nanowires and super-lattice nanowires. Meanwhile, advanced measuring techniques, including 3ω method, Raman spectroscopy and T-type method, have been invented and developed to measure the thermal conductivity of single nanowires. However, investigations on the thermal conductivity of metallic hollow nanowires are limited. Considering the difficulty in the fabrication and thermal conductivity measurement of single hollow metallic nanowires, creating a theoretical thermal conductivity model is urgently required. This work developed the electrical thermal conductivity model, phonon thermal conductivity model and phonon specific heat model of metallic nanowires to study the size effect on the mean free path, group velocity and specific heat capacity of the material. This study also proposed the effective thermal conductivity model of metallic hollow nanowire. These models have been used to study the effect of the both length and thickness of the metallic hollow nanowire on the effective thermal conductivity as well as the influence of the wall thickness on the electronic and phonon thermal conductivity. Finally, the mechanism of size effect on the thermal conductivity was discussed, and a reasonable interpretation based on the developed model was also proposed. Results show that an exact thermal conductivity model, validated by the experimental data from open-reported literature, was established with a correlation coefficient high than 90%. The size effect on the thermal conductivity of both hollow copper nanowire and solid copper nanowire was observed with the increased length and thickness. The thermal conductivity of solid copper nanowire was about 1.2 times higher than that of the hollow copper nanowire with the same length of 800 nm. In detail, the electronic thermal conductivity of solid copper nanowire was nearly 18.7% higher than that of hollow copper nanowire, while their phonon thermal conductivities almost remained unchanged. The size effect on the specific heat of hollow copper nanowire was also observed. The thermal conductivity of the hollow copper nanowire was 1.6 times higher than that of bulk copper and 1.2 times higher than that of a solid copper nanowire with the similar thickness.