Electronic Density Of States Research Articles

Thermoelectric figure-of-merit of metastable crystalline ST12 germanium allotrope

Computational science has facilitated significant improvement in identifying stable crystal systems that are good thermoelectric materials with a high figure-of-merit (ZT). One approach for further discontinuous improvement is to explore the metastable phase of the crystals, particularly because there is an increasing need for thermoelectric materials operating at room temperature in energy harvesting applications. Here, using first-principles calculations, we find that the single-crystalline metastable ST12 germanium allotrope has an much higher ZT than DC8 stable phase. The optimal ZT for p- and n-type doping is 0.22 and 0.27 at room temperature, respectively, and the values reach 0.56 and 0.87 at 500 K. The electrical and thermal conductivities are higher and significantly lower than those of stable germanium, respectively. This is attributed to the smaller carrier effective mass and denser phonon bands with a lower group velocity and larger scattering phase space. The Seebeck coefficient remains high owing to the adequate bandgap and large derivative of the electron density of states with respect to the Fermi energy at the band edges. This work demonstrates that the metastable ST12 germanium allotrope, which has been recently synthesized, is a promising candidate to realize improvement in thermoelectric materials, and this case study encourages future exploration of metastable materials.

Structure, stability and electronic properties of zirconium nitride nanoclusters

Zirconium nitride (ZrN) is an industrial material having very high melting point, hardness and chemical stability. Compared to the bulk ZrN, its nanoclusters (NCs) are rarely explored. In this report, we generate a few ZrN NCs using ab initio molecular dynamics simulation. We investigate their electronic properties in terms of Bader charge, electron localization function and density of states. The obtained results are compared with their bulk counterpart. We also study the static and dynamical stability of the NCs with the help of binding energy, density of states and phonon dispersion spectra. The electron localization function of bulk ZrN is reported here for the first time to better understand its often confusing bonding behaviour.

Electrophysical properties of laser-structured carbon nanomaterials functionalized with LaB6 nanoparticles

In this paper, we perform density functional theory (DFT)-calculations to study the effect of LaB6 nanoparticles on the electrophysical properties of carbon nanomaterial based on laser-structured multi-walled carbon nanotubes (MWCNTs). Based on the calculation results, the most energetically favorable landing sites for LaB6 nanoparticles on the surface of the carbon framework are determined. It is found that during the functionalization with LaB6 nanoparticles, the carbon framework takes on a charge from the atoms of lanthanum and boron, which leads to an increase in the density of electronic states of the MWCNT nanostructure. It is shown that even at low concentrations of LaB6 nanoparticles (4.5 %), the work function of the carbon nanomaterial decreases by 0.5 eV. The results of the calculations are verified in the course of a real experiment. To carry it out, using the original technology of laser irradiation, a hybrid nanomaterial was created from a vertical array of MWCNTs functionalized with LaB6 nanoparticles. Pulsed laser action on an array of MWCNTs with an adjusted energy density of 0.15 J/cm2 made it possible to shorten, align, and orient the upper ends of the nanotubes perpendicular to the substrate. The effect of binding of LaB6 nanoparticles to a carbon surface has been experimentally established. Laser exposure of the MWCNT array coated with LaB6 nanoparticles resulted in a ∼ 5 by times increase in electrical conductivity compared to the original MWCNT array. Recording the emission current-voltage characteristics of hybrid nanomaterials demonstrated a decrease in the total work function of the carbon nanomaterial after functionalization with LaB6 nanoparticles by 16 %. Field emission current density values were calculated for all samples based on their measured field emission CVC characteristics. The MWCNT array exhibited the highest current density of 0.37 A/cm2 after LaB6 deposition and laser exposure. It is predicted that the carbon nanomaterial based on laser-structured MWCNTs coated with LaB6 nanoparticles will be an excellent nanomaterial for field emission electronics.

The Adsorption Mechanisms of SF6-Decomposed Species on Tc- and Ru-Embedded Phthalocyanine Surfaces: A Density Functional Theory Study.

Designing high-performance materials for the detection or removal of toxic decomposition gases of sulfur hexafluoride is crucial for both environmental monitoring and human health preservation. Based on first-principles calculations, the adsorption performance and gas-sensing properties of unsubstituted phthalocyanine (H2Pc) and H2Pc doped with 4d transition metal atoms (TM = Tc and Ru) towards five characteristic decomposition components (HF, H2S, SO2, SOF2, and SO2F2) were simulated. The findings indicate that both the TcPc and RuPc monolayers are thermodynamically and dynamically stable. The analysis of the adsorption energy indicates that H2S, SO2, SOF2, and SO2F2 underwent chemisorption on the TcPc monolayer. Conversely, the HF molecules were physisorbed through interactions with H atoms. The chemical adsorption of H2S, SO2, and SOF2 occurred on the RuPc monolayer, while the physical adsorption of HF and SO2F2 molecules was observed. Moreover, the microcosmic mechanism of the gas-adsorbent interaction was elucidated by analyzing the charge density differences, electron density distributions, Hirshfeld charges, and density of states. The TcPc and RuPc monolayers exhibited excellent sensitivity towards H2S, SO2, and SOF2, as evidenced by the substantial alterations in the band gaps and work functions of the TcPc and RuPc nanosheets. Our calculations hold significant value for exploring the potential chemical sensing applications of TcPc and RuPc monolayers in gas sensing, with a specific focus on detecting sulfur hexafluoride.

Electron-to-nuclear spectral mapping via dynamic nuclear polarization.

We report on a strategy to indirectly read out the spectrum of an electronic spin via polarization transfer to nuclear spins in its local environment. The nuclear spins are far more abundant and have longer lifetimes, allowing for repeated polarization accumulation in them. Subsequent nuclear interrogation can reveal information about the electronic spectral density of states. We experimentally demonstrate the method by reading out the ESR spectrum of nitrogen vacancy center electrons in diamond via readout of lattice 13C nuclei. Spin-lock control on the 13C nuclei yields a significantly enhanced signal-to-noise ratio for the nuclear readout. Spectrally mapped readout presents operational advantages in being background-free and immune to crystal orientation and optical scattering. We harness these advantages to demonstrate applications in underwater magnetometry. The physical basis for the "one-to-many" spectral map is itself intriguing. To uncover its origin, we develop a theoretical model that maps the system dynamics, involving traversal of a cascaded structure of Landau-Zener anti-crossings, to the operation of a tilted "Galton board." This work points to new opportunities for "ESR-via-NMR" in dilute electronic systems and in hybrid electron-nuclear quantum memories and sensors.

Bilayer Kagome Borophene with Multiple van Hove Singularities.

The appearance of van Hove singularities near the Fermi level leads to prominent phenomena, including superconductivity, charge density wave, and ferromagnetism. Here a bilayer Kagome lattice with multiple van Hove singularities is designed and a novel borophene with such lattice (BK-borophene) is proposed by the first-principles calculations. BK-borophene, which is formed via three-center two-electron (3c-2e) σ-type bonds, is predicted to be energetically, dynamically, thermodynamically, and mechanically stable. The electronic structure hosts both conventional and high-order van Hove singularities in one band. The conventional van Hove singularity resulting from the horse saddle is 0.065eV lower than the Fermi level, while the high-order one resulting from the monkey saddle is 0.385eV below the Fermi level. Both the singularities lead to the divergence of electronic density of states. Besides, the high-order singularity is just intersected to a Dirac-like cone, where the Fermi velocity can reach 1.34 × 106m s-1. The interaction between the two Kagome lattices is critical for the appearance of high-order van Hove singularities. The novel bilayer Kagome borophene with rich and intriguing electronic structure offers an unprecedented platform for studying correlation phenomena in quantum material systems and beyond.

Electronic and optical characteristics of long-period stacking ordered phases in Mg-Y-Zn alloys: A first-principles study

Electronic and optical properties of long-period stacking order (LPSO) phase in Mg-Y-Zn alloy are calculated and analyzed by first principles in this paper. The electronic properties and thermodynamic stability of 14 H and 18 R LPSO phases were studied by means of electron density of state (DOS), COHP analysis, electron localization function (ELF) and charge density distribution, and their optical properties, including absorption coefficient, reflectivity, refractive index, energy loss function and extinction coefficient, were discussed, and their variation trends with photon energy and related factors were analyzed. The findings indicate that both phases possess metallic characteristics and exhibit no energy gap. Furthermore, the thermodynamic stability of the 14 H phase surpasses that of the 18 R phase, owing to its stronger covalent bonds metal mixed bonds. In terms of absorption coefficients, both phases display high values within the energy range of 0–19 eV, with their most potent absorption peaks occurring at photon energies of 4.48 eV and 4.8 eV for the 14 H and 18 R phases, respectively. Moreover, the photon absorption rate is inversely proportional to the density of L12 clusters in both phases.

Impact of Mn Alloying on Phase Stabilities, Magnetic Properties and Electronic Structures in Fe.

Impacts of Mn alloying on lattice stabilities, magnetic properties, electronic structures of the bcc and fcc phases and the fcc→bcc phase transition in Fe16-xMnx (x = 0, 1 and 2) alloys are studied by first-principles calculations. Results show that the doped Mn atom prefers ferromagnetic and antiferromagnetic interaction with the host Fe atoms in the bcc and fcc phases, respectively. In these two phases, the magnetic moment of Mn is smaller and larger than Fe, respectively. The local moment of Fe is decided by the Fe-Mn distance in the bcc phase, whereas in the fcc phase, it is determined by spatial orientation with Mn. In the different phases, Mn prefers different site occupations, which can be understood from the electronic density of states near Fermi energy, implying a possibility of element redistribution during phase transition. The driving force of phase transition decreases with Mn alloying. Both destabilized bcc phase and stabilized fcc phase contribute to the inhibited phase transition, but the latter plays a dominant role. Antiferromagnetism is recognized as the key reason for the enhanced stability of the fcc phase by Mn alloying.

Electronic structure of germanium dioxide with rutile structure according to ab initio computer simulation data

The article focuses on the electronic structure of the tetragonal crystalline modification of germanium dioxide. The electronic structure was theoretically studied by means of the full-potential linearized augmented plane wave method using the Wien2k software. Total and partial densities of electronic states were calculated. The spectra of the X-ray absorption near edge structure were simulated for various absorption edges of germanium and oxygen atoms. The Z+1 approximation method was used to calculate Ge K-, Ge L3- and O K absorption edges for the tetragonal modification of GeO2. The result obtained for the Ge K absorption edge is in good agreement with the experimental data. The Ge L3 spectrum was calculated for the first time, and the result is of predictive nature. In order to obtain a better agreement with the experimental calculations of the oxygen K absorption edge, besides the Z+1 approximation method, we also used the core hole method, including the simulation of a partial core hole. The study demonstrated that the use of a core hole with an electron charge of 0.7 results in a better agreement between the calculations and the experiment

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.

Influence of chalcogenides on the optoelectrical properties of semiconductor ternary alloys Zn₀.₉₅Cd₀.₀₅X (X=S, Se and Te): A DFT study

In this article comprehensive study of the thermoelectric and optical properties of the Zn₀.₉₅Cd₀.₀₅X (X = S, Se and Te) by using first principle DFT calculation incorporating the Wien2k code, has been considered. Firstly, we determined the optimized structural parameters, comprising of the atomic coordinates. All the properties like the energy band structure, densities of electronic states and energy dependence of the optical functions were figured out. We found the decrease in their band gaps (3.5 eV, 3.0 eV and 1.9 eV respectively) as we replace S by Se and Te, respectively. We found that they are having direct band gap having transition along Γ- Γ symmetry point. The electronic interband transitions accountable in the optical spectra were specified. In parallel the standard Boltzmann transport theory were used to calculate the thermoelectric parameters, including Seebeck coefficient, electrical and thermal conductivities and figure of merit. In response we found that the investigated compounds are potential candidates for thermoelectric applications.

The magnetocaloric, magnetic, optical, and thermoelectric properties of EuB6 compound: DFT and Monte Carlo study

AbstractThe hexaboride EuB6 has been proven to have qualities that theoretically travel from being electronic to being magneto‐caloric. Under GGA‐PBE and GGA + SOC + U schemes, first‐principles calculations have been used to examine the electronic band structure and density of states. The magnetic moment and the interaction constant have each been calculated using DFT. Additionally, the thermoelectric properties are considered within the particular transport restrictions in order to determine the dimensionless figure of merit (ZT). The extremum of the ZT for EuB6 is determined by using the Seebeck, electrical, thermal, and lattice conductivity coefficients, which are computed. At 1800 K, a maximum ZT of 0.22 is discovered. The computed values of optical conductivity, electron energy loss, absorption coefficient, dielectric tensor, refractive index, and extinction coefficient throughout the range of 0–13 eV anticipate the optical considerations of such an alloy. The infrared spectrum is that where this chemical is active. The critical temperature (Tc) has been determined using the Metropolis algorithm and the Monte Carlo simulation. The material exhibits a ferromagnetic to paramagnetic phase transition, as indicated by temperature‐dependent magnetization. The magnetocaloric efficiency of this material can be measured using the magnetic entropy change (−ΔSm) and the relative cooling power.

LVC/MM: A Hybrid Linear Vibronic Coupling/Molecular Mechanics Model with Distributed Multipole-Based Electrostatic Embedding for Highly Efficient Surface Hopping Dynamics in Solution.

We present a theoretical framework for a hybrid linear vibronic coupling model electrostatically embedded into a molecular mechanics environment, termed the linear vibronic coupling/molecular mechanics (LVC/MM) method, for the surface hopping including arbitrary coupling (SHARC) molecular dynamics package. Electrostatic embedding is realized through the computation of interactions between environment point charges and distributed multipole expansions (DMEs, up to quadrupoles) that represent each electronic state and transition densities in the diabatic basis. The DME parameters are obtained through a restrained electrostatic potential (RESP) fit, which we extended to yield higher-order multipoles. We also implemented in SHARC a scheme for achieving roto-translational invariance of LVC models as well as a general quantum mechanics/molecular mechanics (QM/MM) interface, an OpenMM interface, and restraining potentials for simulating liquid droplets. Using thioformaldehyde in water as a test case, we demonstrate that LVC/MM can accurately reproduce the solvation structure and energetics of rigid solutes, with errors on the order of 1-2 kcal/mol compared to a BP86/MM reference. The implementation in SHARC is shown to be very efficient, enabling the simulation of trajectories on the nanosecond time scale in a matter of days.

Electronic Structure and Transport Properties of Bi2Te3 and Bi2Se3 Single Crystals.

The electrical resistivity and the Hall effect of topological insulator Bi2Te3 and Bi2Se3 single crystals were studied in the temperature range from 4.2 to 300 K and in magnetic fields up to 10 T. Theoretical calculations of the electronic structure of these compounds were carried out in density functional approach, taking into account spin-orbit coupling and crystal structure data for temperatures of 5, 50 and 300 K. A clear correlation was found between the density of electronic states at the Fermi level and the current carrier concentration. In the case of Bi2Te3, the density of states at the Fermi level and the current carrier concentration increase with increasing temperature, from 0.296 states eV-1 cell-1 (5 K) to 0.307 states eV-1 cell-1 (300 K) and from 0.9 × 1019 cm-3 (5 K) to 2.6 × 1019 cm-3 (300 K), respectively. On the contrary, in the case of Bi2Se3, the density of states decreases with increasing temperature, from 0.201 states eV-1 cell-1 (5 K) to 0.198 states eV-1 cell-1 (300 K), and, as a consequence, the charge carrier concentration also decreases from 2.94 × 1019 cm-3 (5 K) to 2.81 × 1019 cm-3 (300 K).

Features of hole transport and density of localized states in CuCr1-xMgxO2 and CuCr1–yMgyO2/(MgCr2O4)x–y polycrystalline ceramics

Magnesium doped polycrystalline ceramic samples of cooper chromite (I) with 0.6-4.0 at % Mg content have been synthesized. Phase composition of ceramics has been investigated by X-ray diffraction. Temperature dependencies of electrical resistivity and Seebeck coefficient have been measured by four probe method and analyzed in frame of variable range hopping conductivity. The density of localized electronic states and characteristic energy of its variation near Fermi energy have been estimated. It was obtained that the density of localized states at Fermi energy increases with an increase of Mg content, while characteristic energy of variation of localized state density near Fermi energy decreases. Obtained results show that relatively large values of Seebeck coefficient in Mg doped copper chromite (I) can be understood within variable range hopping transport of holes with rapidly increasing density toward valence band maximum.

Compressibility behavior of CoxB: Experiment and computation

Structural stability studies on Co2B and Co3B are performed using Mao-Bell type diamond anvil cell. The tetragonal structure of Co2B and the orthorhombic structure of Co3B are stable up to 41 and 43 GPa, respectively. The bulk modulus obtained from high-pressure XRD experiments is 282 ± 21 GPa and 254 ± 10 GPa for Co2B and Co3B, respectively, which reasonably agrees with the computed value. The first principle calculations are carried out to understand the elastic and mechanical properties of CoxB (x: 1, 2, and 3). Adding B into Co enhances the elastic and mechanical properties marginally up to Co2B. Whereas in CoB, a significant increase in the elastic and mechanical properties was observed. The electronic density of states of CoB reveals the presence of a pseudo gap at Fermi energy leading to higher bulk modulus among the cobalt borides. Bader charge analysis shows a transfer of charge from Co to B. The valence charge variation indicates the strengthening of B–B bonds at the expense of Co–Co bonds leading to enhanced elastic properties.

Different look at S-p bonding orbital in CdS, with Zn and Se series gap investigations

The electronic characteristics of Cadmium sulfide (CdS) series Zn x Cd16−x S8Se8 and Cd16Se16−x S x are investigated. According to the calculations, barely 12% Zn (X = 2) in the form Zn2Cd14Se16 is required to maximize the gap. To properly evaluate the excited states, the generalized gradient approximation (GGA) and modified Becke-Johnson (mBJ) potentials are utilized in the computations. The parent compound CdS exhibits three distinct forms of crystal structures, with the hexagonal structure being remarkably stable under the GGA and mBJ functionals. The partial density of states around the Fermi level demonstrates that the system’s electronic characteristics are determined by the interaction of S-p and Cd-s,p states. A substantial splitting of the valence band, shown by band structure analysis, suggests a greater polarization crystal field in the hexagonal CdS structure. The stereochemical activity of the S atom stimulated the CdS compound for extremely tiny displacements of 0.0014 Å and 0.043 Å for the Cd and S atoms in the [001] direction, respectively, as well as tilting of the Cd-S-Cd link. This compound exhibits lone pair properties, with three valance electrons of the p-orbital participating in covalent bonding and S-s electrons fully occupied. Furthermore, Van Hove singularities are discovered in the electron density of states due to the Cd-d atoms that support in the analysis of optical absorption spectra. Finally, the results suggest that CdS has potential for solar cells applications due to its direct wide band gap semiconductor features,

Electronic correlations in epitaxial CrN thin film

Chromium nitride (CrN) spurred enormous interest due to its coupled magnetostructural and unique metal-insulator transition. The underneath electronic structure of CrN remains elusive. Herein, the electronic structure of epitaxial CrN thin film has been explored by employing resonant photoemission spectroscopy (RPES) and X-ray absorption near edge spectroscopy study in combination with the first-principles calculations. The RPES study indicates the presence of a charge-transfer screened 3d ^n{underline{L}} (L: hole in the N-2p) and 3d ^{n-1} final-states in the valence band regime. The combined experimental electronic structure along with the orbital resolved electronic density of states from the first-principles calculations reveals the presence of Cr(3d)-N(2p) hybridized (3d ^n{underline{L}}) states between lower Hubbard (3d ^{n-1}) and upper Hubbard (3d ^{n+1}) bands with onsite Coulomb repulsion energy (U) and charge-transfer energy (Delta) estimated as approx 4.5 and 3.6 eV, respectively. It verifies the participation of ligand (N-2p) states in low energy charge fluctuations and provides concrete evidence for the charge-transfer (Delta<U) insulating nature of CrN thin film.

Tailoring magnesium-based hydrides as potential and reversible materials for solid-state hydrogen storage: A first-principles study

Hydrogen is a promising candidate for green energy sources for future endeavors because of its abundance on Earth. Although its storage is a major challenge for the researchers of this era because of its unsafe and highly explosive nature. The structural, optoelectronic, thermoelectric, vibrational, thermodynamic properties and hydrogen storage capacity of XMgH3 ([Formula: see text], Ba) are carried out by using the full potential linearized augmented plane wave (FP-LAPW) method in the DFT framework. The theoretical study about these magnesium-based metal hydride perovskites, i.e., SrMgH3 and BaMgH3, declares them structurally stable compounds in space group Pm-3m. The optimization graph for SrMgH3 and BaMgH3 reflects the lowest ground state energy, i.e., −6759[Formula: see text]Ry and −16683[Formula: see text]Ry, respectively. Comparatively, BaMgH3 seems to be more stable. The electronic band structures and density of states declare them pure metallic due to zero band gap and overlapping of electronic states of the valence and the conduction bands. The electrical conductivity of BaMgH3 increases up to [Formula: see text] and thermal conductivity [Formula: see text] in the temperature range 100[Formula: see text]K to 1000[Formula: see text]K revealing the good metallic character of BaMgH3. The optical analysis portrays the absorption of compounds in the visible range along with valance shell electrons to the weak bond of hydrogen and dissociates hydrogen molecules at a certain intensity of light. BaMgH3 compound shows minimum scattering and maximum absorption of light in the visible region up to 3[Formula: see text]eV. The reflectivity peaks in the visible region 3.0[Formula: see text]eV show that 40% of light energy is absorbed due to the opaque nature of BaMgH3. Both these compounds are declared thermodynamically stable due to negative free energy such as −1.20[Formula: see text]eV for SrMgH3 and −1.50[Formula: see text]eV energy for BaMgH3 at 1000[Formula: see text]K, respectively. Moreover, the three acoustic modes showing zero imaginary phonon frequencies at [Formula: see text] symmetry points predict these compounds’ structural and thermodynamical stability. The gravimetric hydrogen storage concentration of SrMgH3 and BaMgH3 is determined as 2.637% and 1.836%, respectively.

Twists and Puckers: Tuning Crystal Chemistry in the La(AuxGe1-x)2 Compositional Series.

The physical properties of solid-state materials are closely tied to their crystal structure, yet our understanding of how competing structural arrangements energetically compare is limited. In this work, we explore how small differences in composition affect the structure in the La(AuxGe1-x)2 series of compounds, which comprises four unique structure types between LaGe2 and LaAu2. This family includes the previously unknown AlB2-type compound with stoichiometry La(Au0.375Ge0.625)2 as well as La(Au0.25Ge0.75)2, an intergrowth of the AlB2 and ThSi2 structure types. We then study the chemical forces driving the structure changes and use phonon band structure calculations and DFT-Chemical Pressure to evaluate atomic-size effects. These calculations show that the parent AlB2 structure type is disfavored in Au-rich compounds due to soft atomic motions along the c axis. The instability of AlB2-type LaAuGe is confirmed by the presence of imaginary modes in the phonon band structure that correspond to a "puckering" of the hexagonal AlB2-type lattice, resulting in the experimentally observed LiGaGe structure type. The impact of size effects is less clear for Au-poor compositions; instead, twisting the AlB2 structure type to form the ThSi2 type opens a pseudogap at the Fermi level in the electronic density of states. This investigation demonstrates how crystal structure in solid-state materials can be compositionally tuned based on balancing size and electronics when multiple structure types are in close thermodynamic competition.