Valence Band Width Research Articles

Elevating Operation Voltage and Suppressing Phase Transition for Honeycomb‐Layered Cathodes by a Dual‐Honeycomb Structure Strategy

Honeycomb‐layered oxides are a class of cathode materials for sodium‐ion batteries with great potential due to their high voltage and high capacity. However, the structural instability and voltage fading during cycling limit their practical application. Herein, it is revealed that Te substitution into Na3Ni2SbO6 induces a new dual‐honeycomb structure, which can elevate the average discharge voltage of the cathode materials from 3.2 to 3.8 V with improved cycle stability and alleviated voltage decay. Synchrotron operando X‐ray diffraction demonstrates that Te substitution can suppress the O3−P3−O1‐phase transition during charge and discharge processes effectively, benefited from the strong TeO covalent bonds. The resulted Na2.2Ni2Sb0.2Te0.8O6 cathode exhibits a high capacity retention of 70.9% after 1000 cycles at 1C, with an elevated operating voltage of ≈3.8 V. Theoretical calculations reveal that the introduced TeO bonds break the symmetric distribution of charge in Ni/Sb honeycomb structure and elevate the operation voltage by increased valence band width. Proper Te substitution can promote the rate and cycle capability of the cathode by suppressing phase transition and decreasing the bandgap.

Energy Gaps, Optical Transitions, and Exciton Properties of ZnSe at High Pressures

Lattice parameters, band-gap energies, optical transitions and exciton properties of ZnSe at high-pressures up to 100 kbar have been studied using a pseudo-potential method. Results are generally in good agreement with experiment at zero pressure. Adachi’s expression formula for exciton binding energy and Bohr radius are adjusted giving a significant accordance with experiments. A very good accord is acquired between our obtained consequences concerning the refractive index and the high-frequency dielectric constant when using Hervé and Vandamme model. Upon compression up to 100 kbar, ZnSe remains a direct (Γ-Γ) semiconductor. The lattice parameter decreases from 5.6692 to 4.9075 Å, whereas the valence band width increases from 11.47 to 15.35 eV. A monotonic behavior has been found for all parameters of interest under hydrostatic pressure.

Water-air interface revisited by means of path-integral ab initio molecular dynamics.

Although nuclear quantum effects (NQEs) have been considered on bulk liquid water, the impact of these latter on the air-water interface has not yet been reported. Herein, by performing and comparing ab initio molecular dynamics (AIMD) and path integral AIMD (PI-AIMD) simulations, we reveal the impact of NQEs on structural, dynamical and electronic properties as well as IR spectra of the air-water interface at room temperature. NQEs, being able to describe a more accurate proton delocalization in H-bonded system than AIMD, reveal a different structural arrangement and dynamical behaviour of both bulk and interfacial water molecules in comparison to AIMD results. A more de-structured and de-bound water arrangement and coordination are identified when the quantum nature of nuclei are considered for both bulk and interfacial water molecules. Structural properties, such as inter-/intra-molecular bond lengths, coordination numbers and H-bonding angles of bulk and interfacial water molecules here calculated, are affected by NQEs mitigating the overstructured description given by AIMD. Further evidences of an AIMD overstructured description of bulk water are in the computed IR spectra, where an increased absorption peak intensity and an increased strength of the hydrogen-bond network are alleviated by NQEs. In addition, NQEs show a valuable impact on the electronic structure of the air-water interface, reducing the total valence bandwidth and the electronic energy band-gap when passing from bulk to interfacial water. This work proves how NQEs significantly affect properties and features of the air-water interface, that are essential to accurately describe H-bonded interfacial systems.

Effect of annealing temperature on the formation of oxygen vacancies on the surface of flower-like SnSe/SnO2 heterostructure and visible light catalytic performance

The morphology, structure, and oxygen vacancies in principle determine the light absorption, charge transfer, and separation of photocatalysts, thereby determining their photocatalytic performance. In this study, flower-like SnSe obtained from hydrothermal reactions was oxidized to obtain heterojunctions with SnSe/SnO2. Flower-like SnSe/SnO2 heterojunctions with different concentrations of oxygen vacancies were prepared by annealing them in an argon atmosphere at different temperatures. The flower-like SnSe/SnO2 series with different oxygen vacancies were systematically characterized by TEM, XRD, XPS, PL, and EPR. The research results demonstrate the presence of oxygen vacancies in the flower-like SnSe/SnO2. As the annealing temperature increases, the surface oxygen vacancy concentration shows a trend of first increasing and then decreasing. When the annealing temperature reaches 600 °C, the oxygen vacancy concentration of flower-like SnSe/SnO2 is the highest. Meanwhile, research has shown that surface oxygen vacancies help expand the light absorption range, and the increased valence bandwidth leads to effective charge transfer and separation, thereby promoting visible light photoactivity. SnSe/SnO2-600 °C exhibited excellent visible light photocatalytic activity. The photodegradation of methylene blue (MB) at ≥ 400 nm can reach ∼70% within 120 min. This study verified the route for the introduction of oxygen vacancies via facile calcination and constructed SnSe/SnO2 with surface oxygen vacancies, providing a reference with deep insights for improving photocatalytic activity.

Ab initio study of elastic, electronic, and vibrational properties of SnTe and PbTe.

The elastic, electronic, and vibrational properties of the ground state of the rocksalt SnTe and PbTe are investigated. The deduced elastic constants, namely, shear modulus, Young's modulus, and Poisson's ratio are in very good agreement with the experimental and other theoretical data. The electronic band structure and density of states are obtained with and without considering the spin-orbit coupling. The bandgaps of SnTe and PbTe with (without) spin-orbit coupling are 0.11 (0.05) eV and 0.01 (0.78) eV, respectively. The bandgaps with spin-orbit interactions are nearer to experimental data. The hybrid functionals give higher values of bandgaps for both the SnTe and PbTe. In both compounds, the bandgap increases with volume. The valence bandwidths, however, decrease with increasing volume. The vibrational frequencies are found in reasonable agreement with the experiment. The frequencies increase with pressure. In this work, the ab initio calculations of SnTe and PbTe crystals are carried out applying plane wave pseudopotential method using the QUANTUM ESPRESSO package. The PBE exchange and correlation functional based on GGA is considered. The fully relativistic norm-conserving pseudopotentials for Sn, Pb, and Te are used. The self-consistent field calculations are performed over a dense MP net of 18 × 18 × 18k-points. The energy cut-off of 70 Ryd was found sufficient to achieve convergence of 10-6 Ryd in total energy of the crystals.

Realizing the Lowest Bandgap and Exciton Binding Energy in a Two-Dimensional Lead Halide System.

Finding stable analogues of three-dimensional (3D) lead halide perovskites has motivated the exploration of an ever-expanding repertoire of two-dimensional (2D) counterparts. However, the bandgap and exciton binding energy in these 2D systems are generally considerably higher than those in 3D analogues due to size and dielectric confinement. Such quantum confinements are most prominently manifested in the extreme 2D realization in (A)mPbI4 (m = 1 or 2) series of compounds with a single inorganic layer repeat unit. Here, we explore a new A-site cation, 4,4'-azopyridine (APD), whose size and hydrogen bonding properties endow the corresponding (APD)PbI4 2D compound with the lowest bandgap and exciton binding energy of all such compounds, 2.19 eV and 48 meV, respectively. (APD)PbI4 presents the first example of the ideal Pb-I-Pb bond angle of 180°, maximizing the valence and conduction bandwidths and minimizing the electron and hole effective masses. These effects coupled with a significant increase in the dielectric constant provide an explanation for the unique bandgap and exciton binding energies in this system. Our theoretical results further reveal that the requirement of optimizing the hydrogen bonding interactions between the organic and the inorganic units provides the driving force for achieving the structural uniqueness and the associated optoelectronic properties in this system. Our preliminary investigations in characterizing photovoltaic solar cells in the presence of APD show encouraging improvements in performances and stability.

First principles investigation of the substitutional doping of rare-earth elements and Co in La4MgNi19 phase

RE-Mg-Ni alloys, including AB3, A2B7, A5B19 and AB4 types, have received extensive attention due to their excellent hydrogen storage properties. La4MgNi19 is the one of the outstanding candidate for hydrogen storage. In this work, the structure, phase stability and electronic structure of the different La-site partial substituted by Pr, Sm, Gd, Nd and NI-site substituted by Co have been investigated by means of the density functional theory.The calculation results show that La4MgNi19 alloy shows the negative enthalpy of formation, indicating the more stable in the thermodynamic. When the substitution of La occurs, among the two sites La(2c) and La(4f), Pr, Nd, Sm and Gd preferentially occupy the La(4f) site. And the addition of the four doping elements will reduce the stability of the phase. Among them, Pr substituted La4MgNi19 has the highest structural stability. When Co substituting Ni, a single Co atom occupies the Ni(12k) preferentially among the seven different Ni positions. During this process, the crystal structure will be destabilized. The DOS results show that the system still puts up the metallic character after substitution. Sm(La(4f)) has the maximum valence band width. The stability of four doped alloys from high to low is: Pr, Nd, Sm, Gd, which is consistent with the enthalpy of formation results. The enthalpy of formation of hydrides shows that, the bounding hydrogen capacity of the system can be obtained as Nb > Sm > Pr > Gd.

Bulk electronic structure of Mn2NiGa using hard x-ray photoelectron spectroscopy and density functional theory

Mn2NiGa is a potential magnetic shape memory alloy with austenite to the martensite phase transition. Here, we have investigated the bulk electronic structure of Mn2NiGa in the austenite and martensite phases studying its valence band using hard x-ray photoelectron spectroscopy (HAXPES). In the austenite phase, we observe a wide (≈10 eV) valence band (VB) spectrum with several prominent features. In order to explain the HAXPES VB spectra, we have compared our experimental VB spectra with the theoretical VB calculated using the partial density of states from our existing density functional theory (DFT) calculations. The shape of the experimental VB and energy positions of all features are in excellent agreement with the calculated VB and we find that the former is dominated by Ni 3d as well as 4s states of Mn, Ni, and Ga. An important observation is that experimental VB combined with the DFT-based VB calculation establishes the prevalence of the anti-site disorder in Mn2NiGa. Compared to the austenite phase, in the martensite phase, the VB shows a marginal decrease in the density of states around −0.5 eV below the Fermi level (E F ), and the main peak is slightly shifted towards E F . These experimental observations have been explained by considering the tetragonally distorted structure with the anti-site disorder in the martensite phase.

Satellite-Dominated Sulfur L2,3 X-ray Emission of Alkaline Earth Metal Sulfides.

The sulfur L2,3 X-ray emission spectra of the alkaline earth metal sulfides BeS, MgS, CaS, SrS, and BaS are investigated and compared with spectra calculations based on density functional theory. Very distinct spectral shapes are found for the different compounds. With decreasing electronegativity of the cation, that is, increasing ionic bonding character, the upper valence band width and its relative spectral intensity decrease. These general trends are qualitatively reproduced by the spectra calculations, which give quite an accurate description of the spectral shapes in the upper valence band region. On the low energy side of the sulfur 3s → 2p transition dominating the spectra, we find strong satellites caused by "semi-Auger" decays involving configuration interaction. These satellites, previously believed to be energetically forbidden for sulfur L2,3 emission and only observed for the L2,3 emission of Cl to Cr, increase in intensity as the bonding character becomes more ionic and dominate the spectra for SrS and BaS. The intensities, energies, and widths of the satellites vary strongly between the investigated compounds, giving a very specific spectral fingerprint that can be used for speciation analysis.

First-principle studies of electronic, elastic and vibrational properties of barytocalcite and paralstonite

The crystal structure, electronic, elastic and vibrational properties of two natural BaCa(CO3)2 polymorphs (barytocalcite and paralstonitis) were studied by the density functional theory methods using the hybrid B3LYP and gradient PBE functionals, taking into account the dispersion correction D3 to the total energy in the basis of localized orbitals of the CRYSTAL package, under the pressure about 3 GPa. It is shown that the cell volume of hexagonal paralstonite obtained by the PBE-D3 method per formula unit is 0.065 Å3 larger, and the total energy is 0.011 eV lower than that of a monoclinic barytocalcite. The parameters of the equation of state in the Birch-Murnaghan form are determined. The elastic constants of single crystals and, in the Voigt-Royce-Hill approximation, the elastic moduli and elastic wave velocities of polycrystals are calculated. In the electron energy spectrum, the width of the upper valence band is 0.85 eV in barytocalcite, and 0.94 eV in paralstonite, while the band gap is 4.85 eV and 4.36 eV, respectively, and it decreases with increasing pressure at rates of − 0.01 and − 0.02 eV/GPa. The frequencies and intensities of normal long-wave oscillations are calculated with the B3LYP functional. It is shown that qualitative differences in the spectra of infrared absorption and Raman scattering of light for two phases should be sought in the range of 1300÷1580 cm−1 in the region of intramolecular vibrations, and for the spectra of infrared absorption in the region of 260÷310 cm−1, and for light scattering in the range of 130÷180 cm−1 in the region of lattice vibrations.

Ab initio исследования влияния давления на структуру, электронные и упругие свойства карбонатов щелочных--щелочно-земельных металлов

Density functional theory with a gradient PBE functional and dispersion correction D3(BJ) in the basis of localized orbitals of the CRYSTAL17 package was used to study the effect of pressure on the structural, electronic, and elastic properties of K2Ca(CO3)2, K2Mg(CO3)2, Na2Ca2(CO3)3, Na2Mg(CO3)2. The parameters of the third-order Birch-Murnaghan equation of state are determined, and it is shown that the equilibrium volume and compressibility modulus depend linearly on the average cation radius. Under pressure, the widths of the upper valence bands increase maximally in K2Ca(CO3)2 and minimally in Na2Ca2(CO3)3, while the centers of gravity of the cationic states shift by ~0.2 eV. Elastic constants and polycrystalline moduli are calculated, which increase with decreasing average cation radius. The shear modulus for K2Ca(CO3)2 and K2Mg(CO3)2 decreases with increasing pressure, and this leads to anomalous behavior of the longitudinal and transverse acoustic wave velocities.

Ab initio studies of the effect of pressure on the structure, electronic and elastic properties of carbonates of alkali-alkaline earth metals

Density functional theory with a gradient PBE functional and dispersion correction D3(BJ) in the basis of localized orbitals of the CRYSTAL17 package was used to study the effect of pressure on the structural, electronic, and elastic properties of K2Ca(CO3)2, K2Mg(CO3)2, Na2Ca2(CO3)3, Na2Mg(CO3)2. The parameters of the third-order Birch--Murnaghan equation of state are determined, and it is shown that the equilibrium volume and compressibility modulus depend linearly on the average cation radius. Under pressure, the widths of the upper valence bands increase maximally in K2Ca(CO3)2 and minimally in Na2Ca2(CO3)3, while the centers of gravity of the cationic states shift by ~0.2 eV. Elastic constants and polycrystalline moduli are calculated, which increase with decreasing average cation radius. The shear modulus for K2Ca(CO3)2 and K2Mg(CO3)2 decreases with increasing pressure, and this leads to anomalous behavior of the longitudinal and transverse acoustic wave velocities. Keywords: ab initio calculations, buchliite, eithelite, shortite, equation of state, elastic moduli, density of states, pressure.

Interplay of crystal thickness and in-plane anisotropy and evolution of quasi-one-dimensional electronic character inReSe2

We study the valence band structure of ReSe$_{2}$ crystals with varying thickness down to a single layer using nanoscale angle-resolved photoemission spectroscopy and density functional theory. The width of the top valence band in the direction perpendicular to the rhenium chains decreases with decreasing number of layers, from 280 meV for the bulk to 61 meV for monolayer. This demonstrates increase of in-plane anisotropy induced by changes in the interlayer coupling and suggests progressively more one-dimensional character of electronic states in few-layer rhenium dichalcogenides.

Renormalization of the valence and conduction bands of (C6H5C2H4NH3)2PbI4 hybrid perovskite

Quasi two-dimensional hybrid organic-inorganic perovskites (HOIPs) have been rediscovered recently for photovoltaics due to a higher stability than other HOIPs. We focus here on the electronic structure of the 2D perovskite (C6H5C2H4NH3)2PbI4. We perform an experimental k-resolved determination of the valence and conduction bands by angle-resolved photoemission spectroscopy (ARPES) and inverse photoemission spectroscopy (IPES). The experimental and theoretical dispersions are compared. The valence band width is in agreement with that of renormalized theoretical bands, while no significative renormalization is observed for the conduction band. The effect of the spin–orbit coupling in the conduction band is also experimentally observed.

Effect of Noninteracting Intercalants on Layer Exfoliation in Transition-Metal Dichalcogenides

The control of friction at the atomic scale is fundamental to optimize the exfoliation of layered materials. To this aim, we report a density-functional investigation of how intercalated molecules affect the nanoscale friction of van der Waals transition-metal dichalcogenides. We find that the molecule does not interact with the electronic density of the layers directly; nonetheless it determines the features of the valence band of the system. In particular, the valence-band width appears to be a promising parameter to correlate the electronic properties with the nanofrictional response; it then constitutes a guide for the automatic search of intercalation molecules suitable for layer exfoliation. The present outcomes also constitute a theoretical tool for future investigations of the effect that intercalated species have on the nanoscale friction in layered materials.

GFCCLib: Scalable and efficient coupled-cluster Green's function library for accurately tackling many-body electronic structure problems

Coupled-cluster Green's function (GFCC) calculation has drawn much attention in the recent years for targeting the molecular and material electronic structure problems from a many-body perspective in a systematically improvable way. However, GFCC calculations on scientific computing clusters usually suffer from expensive higher dimensional tensor contractions in the complex space, expensive inter-process communication, and severe load imbalance, which limits it's use for tackling electronic structure problems. Here we present a numerical library prototype that is specifically designed for large-scale GFCC calculations. The design of the library is focused on a systematically optimal computing strategy to improve its scalability and efficiency. The performance of the library is demonstrated by the relevant profiling analysis of running GFCC calculations on remote giant computing clusters. The capability of the library is highlighted by computing a wide near valence band of a fullerene C60 molecule for the first time at the GFCCSD level that shows excellent agreement with the experimental spectrum. Program summaryProgram Title: GFCCLibCPC Library link to program files:https://doi.org/10.17632/j594wydctd.1Code Ocean capsule:https://doi.org/10.24433/CO.5131827.v1Licensing provisions: MIT LicenseProgramming language: C++Nature of problem: The applications of coupled cluster Green's function on large scale molecular electronic structure problems suffer from expensive higher dimensional tensor contractions in the complex space, expensive inter-process communication, and severe load imbalance. Tackling these issues are a key step in building high-performance coupled cluster Green's function library for its routine use in large scale molecular science.Solution method: We have developed a C++ library for large scale molecular GFCC calculations on high-performance computing clusters. We provide implementations for high dimensional tensor algebra for many-body methods (TAMM), Cholesky decomposition of high dimensional electron repulsion integral tensors, process group technique for mitigating load imbalance. The library is written in C++. The source code, tutorials and documentation are provided online. A continuous integration mechanism is set up to automatically run a series of regression tests and check code coverage when the codebase is updated.

Hf1−xZrxO2 and HfO2/ZrO2 gate dielectrics with extremely low density of interfacial defects using low temperature atomic layer deposition on GaN and InP

Achieving a negative capacitance field effect transistor with a subthreshold swing beyond the Boltzmann limit requires a “defect-free” dielectric-semiconductor interface. We grew alloyed (Hf1−xZrxO2) and stacked (HfO2/ZrO2) gate dielectrics on GaN and InP substrates using low temperature plasma enhanced atomic layer deposition. In situ ellipsometry data show that alloying hafnia with zirconia reduces the refractive index and widens the bandgap. The stacked and alloyed structures reveal very low capacitance-voltage hysteresis of 35 and 45 mV, respectively, on GaN. The density of interfacial traps as low as 1.12 × 1010 cm−2 eV−1 was achieved on GaN mainly due to the combination of very low dielectric growth temperature (100 °C) and high postfabrication heat treatment temperature (510 °C). The conduction and valence band offsets of the alloyed gate dielectrics on InP were measured and compared to pure zirconia using a combination of x-ray photoelectron spectroscopy and ellipsometry. The alloyed structures show a wider bandgap, larger conduction band offset, and smaller valence band offset compared to pure zirconia. This was attributed to the increase in the valence band width with hafnia addition, which reduces the alloyed gate dielectric’s valence band offset. We resolved the band structure alignement to be type I with band offsets of 3.53 eV for electrons and 1.03 eV for holes in Hf0.25Zr0.75O2/InP heterojunctions. The results allow for a clear and detailed picture of two distinct growth procedure that affect the interfacial defect concentration.

Simulation of Electronic Structure and some Properties of CdTe Crystals Using DFT

The current study investigated the electrical properties of Cadmium Telluride (CdTe)by using the first principle of density functional theory (DFT). The nanocrystals suggested being varied constantly over the network systematically so that the lowest value for energy is obtained, through which stability is obtained and through this exceptionality, the measurements of the properties are in their exact state. The conduction and the valence bandwidths were also studied. The investigations targeted the “highest occupied molecular orbital” (HOMO) [Ionization Potential], and the “lowest unoccupied molecular orbital” (LUMO) [Electron Affinity]. Total and cohesive energies, the atomic iconicity, electron affinity, energy gap (Eg), and the density of states (DOS) for 8, 16, 54, and 64 atoms. The results showed that the shape of the conduction and valence affect the crystal groups significantly, and the energy gap exhibited very close results to their practical counterparts that were previously conducted. When the lattice constant decreases the modulus of bulk and the waves of sound speed increase with the increase of the core atoms number. Subsequently, the applied pressure increases the Plasmon energy and bulk modulus. The key of study is to inspect if using materials in their nanoscale state gives special physical, electronic and optical properties through which devices are manufactured with high efficiency in the solar cell industry. Where the compound becomes a point of a sleeve, and the fluorescent peak shifts across the visible field to the UV field. This was obtained by controlling the size of the compound in 54 and 64, at which the energy gap showed an increase, which would make it more preferred to stimulate the electron from the valence band to the conduction band.

Study on the Physical Properties of Pulsed Laser Deposited Doped Copper Oxide (Cu2 O) Thin Films for Optical Device Applications

Cuprous oxide, (Cu2O) is a promising p-type semiconductor, finds practical applications in a wide range of optoelectronic devices. In this paper, pulsed laser deposition technique is employed to deposit doped Cu2O thin films. The influence of doping of silver (Ag), aluminium (Al) and co-doping of (Ag+Al) in Cu2O thin films is illustrated. X-ray diffraction pattern depicts cubic crystal structure and polycrystalline nature of grown thin films, having small crystallite size (~50 nm). Atomic force microscopy (AFM) obtained surface images of the films portrait uniform grain morphology with low surface roughness. The room temperature optical characterizations of the thin films, the transmittance versus wavelength in the UV-Visible region exhibits low transmission values upto 10–20%, illustrates the large absorption coefficient (α), numerical values varying from 104 to 105cm-1 for doped Cu2O films. The large values of absorption coefficient facilitate the optical and photovoltaic applications of the doped Cu2O films. The addition of dopant species Ag and Al, the optical band gap is increased and it varies in the range of 2.65−2.84 eV. The increased energy gap is attributed to the substitution of Al and Ag ions for the oxygen ions reduce the width of valence band to widen the energy gap. The I−V characteristics plot obtained at room temperature indicates low electrical resistivity (ρ ~ 10-2Ω- cm) of the films. The obtained results are of high relevance and indicate potential applications of the grown thin films in semiconductor devices such as solar cells, photodetectors and optical sources

High refractive index and extreme biaxial optical anisotropy of rhenium diselenide for applications in all-dielectric nanophotonics

Abstract We establish a simple quantitative criterium for the search of new dielectric materials with high values of refractive index in the visible range. It is demonstrated, that for light frequencies below the bandgap, the latter is determined by the dimensionless parameter η calculated as the ratio of the sum of the widths of conduction and valence bands and the bandgap. Small values of this parameter, which can be achieved in materials with almost flat bands, lead to dramatic increase of the refractive index. We illustrate this rule with a particular example of rhenium dichalcogenides, for which we perform ab initio calculations of the band structure and optical susceptibility and predict the values of the refractive index n > 5 $n{ >}5$ in a wide frequency range around 1 eV together with comparatively low losses. Our findings open new perspectives in search for the new high-index/low-loss materials for all-dielectric nanophotonics.