State Density Values Research Articles

Group VIII Transition Metal (Fe, Ru & Os) Embedded Graphitic Carbon Nitride as an Acetone Sensor: A First Principle Investigation

This study investigates the acetone sensing behavior of pristine graphitic carbon nitride (gCN) and group VIII transition metal (Fe, Ru & Os) embedded gCN monolayer using DFT calculations. Key structural and electronic properties such as adsorption energy, band structure, and density of states (DOS) have been examined. The calculated adsorption energy of pristine gCN is −1.32 eV, which improves to –3.52, −2.75, and −1.73 eV for Fe, Ru, and Os embedded gCN, respectively. The band structure also indicates a reduction in the band gap of gCN after acetone adsorption due to the introduction of Fe, Ru, and Os. Furthermore, after the adsorption of acetone, the DOS values exhibit a significant increase from 13.48 eV−1 for pristine gCN to 439, 423, and 332 eV−1 for Fe, Ru, and Os embedded gCN, respectively.

High- Tc superconductivity in doped molecular superconductors of K4B8−xMxH32 (M = C, N) under high pressure

Stabilized and metallic light elements hydrides have provided a potential route to achieve the goal of room-temperature superconductors at moderate or ambient pressures. Here, we have performed systematic DFT theoretical calculations to examine the effects of different light elements C and N atoms doped in cubic K4B8H32 hydrides on the superconductivity at low pressures. As a result of various atoms substituting, we have found that metallic K4B _{8-x} M x H32 (M = C, N) hydrides are dynamically stable at 50 GPa, band structures and density of states (DOS) indicate that sizeable Tc correlates with a high B–H DOS at the Fermi level. With the increasing of B atoms in K4B _{8-x} M x H32 hydrides, the DOS values at Fermi level have been improved due to the delocalized electrons in B–H bonds, which result in strong electron–phonon coupling (EPC) interaction and increase the Tc from 19.04 to 77.07 K for KC2H8 and KB2H8 at 50 GPa. The NH4 unit in stable K4B7NH32 hydrides has weakened the EPC and led to low Tc value of 21.47 K. Our results suggest the light elements hydrides KB2H8 and K4B7CH32 could estimate high Tc values at 50 GPa, and the boron hydrides would be potential candidates to design or modulate hydrides superconductors with high Tc at moderate or ambient pressures.

Temperature Evolution of Composition, Thermal, Electrical and Magnetic Properties of Ti3C2Tx-MXene.

MXenes are a family of two-dimensional nanomaterials. Titanium carbide MXene (Ti3C2Tx-MXene), reported in 2011, is the first inorganic compound reported among the MXene family. In the present work, we report on the study of the composition and various physical properties of Ti3C2Tx-MXene nanomaterial, as well as their temperature evolution, to consider MXenes for space applications. X-ray diffraction, thermal analysis and mass spectroscopy measurements confirmed the structure and terminating groups of the MXene surface, revealing a predominant single OH layer character. The temperature dependence of the specific heat shows a Debye-like character in the measured range of 2 K-300 K with a linear part below 10 K, characteristic of conduction electrons of metallic materials. The electron density of states (DOS) calculations for Ti3C2OH-MXene reveal a significant DOS value at the Fermi level, with a large slope, confirming its metallic character, which is consistent with the experimental findings. The temperature dependence of electrical resistivity of the MXene samples was tested for a wide temperature range (3 K-350 K) and shows a decrease on lowering temperature with an upturn at low temperatures, where negative magnetoresistance is observed. The magnetoresistance versus field is approximately linear and increases its magnitude with decreasing temperature. The magnetization curves are straight lines with temperature-independent positive slopes, indicating Pauli paramagnetism due to conduction electrons.

DFT studies of BaTiO3

The structure of stable phases is investigated using first-principle calculations based on the functionals: LDA, GGA and newly developed general-purpose heavily constrained and appropriately normalized meta GGAfunctional (SCAN). The purpose of this study is to theoretically study the atomic displacements and band gap of the cubic, tetragonal, orthorhombic, rhombohedral perovskite phases for the comparison LDA, GGA-PBE and meta-GGA functionals using the density functional theory method. The obtained data of the density of states (DOS) showed that the values of the band gap energies are underestimated, and the DOS values show that the upper part (valence band) mainly consists of O 2p orbitals, the lower part (conduction band) consists of Ti 3d orbitals. The rhombohedral phase has a mixed composition of Ti states in the conduction band with a greater degree of 3dz2 than 3dxy. The values of the energies of the band gap (Egap) and the density of states show reasonable agreement with experimental and theoretical data. The LDA functional and, to a lesser extent, the GGA - PBE functional can also provide fairly accurate information about atomic displacements in these crystals. The values calculated by the SCAN functional do not differ much from the GGA and LDA functionals.

Computational investigation of Zn doped graphitic carbon nitride (gCN) monolayer for CO2 sensing

In the present work, we have investigated the sensing behavior of pristine graphitic carbon nitride (gCN) and Zn doped gCN monolayer (Zn-gCN) for CO2 gas sensing using DFT calculations. Structural and electronic properties such as adsorption energy, band structure, and density of states (DOS) have been investigated. The calculated adsorption energy of pristine gCN is −1.33 eV. To improve this value, doping is patterned at three sites (i.e., terminal N site, edge N site, and ring N site). The adsorption is maximum at the terminal N site followed by ring N site and edge N site with a value of −8,29 eV, −2.48 eV, and −2.16 eV, respectively. Band gap structure also reflects that the doping at terminal N site is most favorable with a 52.7% shrinking of band gap subsequent to adsorption of CO2 gas compared to pristine gCN. Further, the DOS values also exhibit noticeable change after doping Zn atom at different sites in gCN towards adsorption of CO2 with a maximum increase at the terminal N site. The present study reveals that the sensing performance of the Zn-gCN is significantly enhanced compared to pristine gCN and can be a promising material for CO2 gas sensing.

Charge transport studies in flexible and rollable Polypyrrole-PVDF composite films

Polypyrrole has a unique place among conducting polymers discovered so far, due to its advantageous properties such as low-cost synthesis, stable electrical conductivity, non-toxicity, environmentally benign nature, etc. In this study, the polymerization of polypyrrole is carried out inside the porous structure of the polyvinylidene fluoride (PVDF) matrix using the chemical oxidation method to prepare flexible and rollable conducting films. The resistivity measurement of these flexible and rollable Polypyrrole-PVDF films were carried out using a standard four-point probe method in the temperature range of 320–10 K. The room temperature dc electrical conductivity of the film was found to be ∼ 50 S/m. To understand the charge transport mechanism in these films the experimental data were analyzed in the light of Mott’s variable range hopping model and Fluctuation assisted tunneling model. It was found that experimental data follows the three-dimensional variable range hopping behavior from 320 K to 90 K and below 90 K a crossover from hopping to fluctuation-assisted tunneling was observed. Mott’s parameters were measured in the temperature range of 320–90 K and the corresponding values for density of states at the Fermi level N(EF), average hopping distance (R), and average hopping energy (W) were found to be 5.54 × 1029m-3eV−1, 2.94 Å and 17.7 meV respectively.

DFT based study of transition metals (Au,Ag,Pd& Pt) doped graphitic carbon nitride (gCN) monolayer as a CO gas sensor

In the present study, we explored the sensing behavior of pristine gCN and transition metal (Au, Ag, Pd and Pt) doped gCN monolayer for CO gas molecule using DFT calculations. Structural and electronic properties such as adsorption energy, band structure and density of states (DOS) have been investigated. An increase of 17.03%, 15.08%, 2.24% and 4.99% is observed for Au, Ag, Pd and Pt doped gCN as compared to pristine gCN towards CO gases. Moreover the band gap also decreases considerably after doping transition metals in it which futher reduces after introduction of CO gas. DOS value also increases. The study revealed that the sensing performance of gCN is enhanced by doping the pristine form with the transition metals (Au, Ag and Pt) and hence doped gCN can be a favorable material for CO gas sensing.

Characterization of optimized properties for InGaZnO semiconductor and thin film transistor by oxygen partial pressure

The InGaZnO thin films with varied the O2 flow ratios were deposited by a magnetron sputtering system. The effect of O2 content on the structure, surface morphology, optical and electrical properties of InGaZnO thin films is studied. Then, we investigate the relationship between bias stress stability and density of states (DOS) of thin film transistors (TFTs) with various oxygen partial pressure InGaZnO (IGZO) as the channel layer. The temperature-dependent field-effect measurements are used to calculate DOS of IGZO TFTs. With the increase of the oxygen partial pressure, the value of DOS decreases from 2.2 × 1016 to 5.5 × 1015 cm-3. Moreover, the positive gate bias stress of IGZO TFTs are investigated. Under positive gate bias stress, the transfer curves of IGZO TFTs with various oxygen partial pressure show an obvious shift, the value of threshold voltage shift () is 5.1, 2.94, 2 and 0.8 V, respectively. The results suggest that the device with a smaller DOS is more stable than others. Therefore, this study offers a brief and accurate method to demonstrate instability mechanism of IGZO TFTs.

Study of a new layered ternary chalcogenide CuZnTe2 and its potassium intercalation effect

A new layered ternary chalcogenide CuZnTe2 and its effect due to potassium (K) intercalation have been investigated using ab-initio method under the framework of density functional theory (DFT). Here, we report the structural, electronic and elastic properties of both proposed parent compound CuZnTe2 and intercalated KCuZnTe2. The electronic band structures and the density of states (DOS) of both these chalcogenides have also been studied. The parent compound demonstrates p-type conductivity with the energy band gap of 0.7 eV but surprisingly, the increase of energy gap (1.5 eV) is found in the intercalated KCuZnTe2, a direct-transition type semiconductor. The optical absorption result in KCuZnTe2 also shows the identical value of gap energy calculated by Wood-Tauc theory. The density of states (DOS) in the valence band for both compounds is dominated by the partial contribution of Cu/Zn 3d and Te 5p orbitals but the prime contribution of Cu/Zn 4s and Te 5s mainly in the conduction band DOS. The DOS value at around Fermi level in these chalcogenides is indicating the degeneracy behavior of a semiconductor. Both compounds are mechanically stable and also malleable. We also calculated the thermal properties in the intercalated KCuZnTe2 using quasi-harmonic Debye model. The observed values of Debye temperature, specific heat capacities and volume expansion coefficient using this model is almost consistent with the estimated values given in theory.

Electronic, Structural, and Magnetic Properties of Hole-Doped Iron-Based Superconductors Using First Principle Study

Electronic, structural, and magnetic properties of Li0.5FeAs, Na0.5FeAs, and K0.5FeAs were investigated using ab initio method. The crystal structure of undoped compounds, LiFeAs, NaFeAs, and KFeAs, is found to be tetragonal with P4/nmm space group, while those of hole-doped compounds is body centered tetragonal with I4/mmm space group. Based on the total energy values obtained in the case of undoped compounds, it has been inferred the presence of antiferromagnetic ordering, while those in the case of hole-doped compounds, the ferromagnetic (FM) ground state may be present. As the density of states at the Fermi level increases with hole doping, it has been concluded that superconductivity might have arisen due to the enhancement in the density of state values. Finally, from the Fermi surface studies of hole-doped compounds, it has been concluded that due to nesting across the Brillouin zone boundary, the ferromagnetism and superconductivity might be coexisting.

Boron doped graphene oxide with enhanced photocatalytic activity for organic pollutants

In present study, boron doped reduced graphene oxide (BR-GO) is synthesized by reducing graphene oxide (GO) in the presence of boric acid, and employed as a photocatalyst for the degradation of organic pollutants methyl orange (MO) and methylene blue (MB). GO and BR-GO are systematically characterized by X-ray diffraction, Raman spectroscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and UV–vis spectroscopy. The modifications in the structure of GO are confirmed. The BR-GO has sheet structure with various graphene islands and disordered regions. The electrical conductivity has been measured using two-probe method. The conductance is explained by two-dimensional variable range hopping model. The photocatalytic activity of GO is enhanced on doping with boron due to increase in the density of states value near Fermi level. The adsorption mechanism and kinetic studies for the degradation of MB and MO dyes suggest that the photocatalytic degradation follows a pseudo-second order mechanism.

Optimization of Ca14MgSb11 through Chemical Substitutions on Sb Sites: Optimizing Seebeck Coefficient and Resistivity Simultaneously

In thermoelectric materials, chemical substitutions are widely used to optimize thermoelectric properties. The Zintl phase compound, Yb14MgSb11, has been demonstrated as a promising thermoelectric material at high temperatures. It is iso-structural with Ca14AlSb11 with space group I41/acd. Its iso-structural analog, Ca14MgSb11, was discovered to be a semiconductor and have vacancies on the Sb(3) sites, although in its nominal composition it can be described as consisting of fourteen Ca2+ cations with one [MgSb4]9− tetrahedron, one Sb37− linear anion and four isolated Sb3− anions (Sb(3) site) in one formula unit. When Sn substitutes Sb in Ca14MgSb11, optimized Seebeck coefficient and resistivity were achieved simultaneously although the Sn amount is small (<2%). This is difficult to achieve in thermoelectric materials as the Seebeck coefficient and resistivity are inversely related with respect to carrier concentration. Thermal conductivity of Ca14MgSb11-xSnx remains almost the same as Ca14MgSb11. The calculated zT value of Ca14MgSb10.80Sn0.20 reaches 0.49 at 1075 K, which is 53% higher than that of Ca14MgSb11 at the same temperature. The band structure of Ca14MgSb7Sn4 is calculated to simulate the effect of Sn substitutions. Compared to the band structure of Ca14MgSb11, the band gap of Ca14MgSb7Sn4 is smaller (0.2 eV) and the Fermi-level shifts into the valence band. The absolute values for density of states (DOS) of Ca14MgSb7Sn4 are smaller near the Fermi-level at the top of valence band and 5p-orbitals of Sn contribute most to the valence bands near the Fermi-level.

Comparative study of Mo2Ga2C with superconducting MAX phase Mo2GaC: First-principles calculations

The structural, electronic, optical and thermodynamic properties of Mo2Ga2C are investigated using density functional theory (DFT) within the generalized gradient approximation (GGA). The optimized crystal structure is obtained and the lattice parameters are compared with available experimental data. The electronic density of states (DOS) is calculated and analyzed. The metallic behavior for the compound is confirmed and the value of DOS at Fermi level is 4.2 states per unit cell per eV. Technologically important optical parameters (e.g., dielectric function, refractive index, absorption coefficient, photo conductivity, reflectivity, and loss function) are calculated for the first time. The study of dielectric constant (ɛ1) indicates the Drude-like behavior. The absorption and conductivity spectra suggest that the compound is metallic. The reflectance spectrum shows that this compound has the potential to be used as a solar reflector. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats, and thermal expansion coefficient of Mo2Ga2C MAX phase are derived from the quasi-harmonic Debye model with phononic effect also for the first time. Analysis of Tc expression using available parameter values (DOS, Debye temperature, atomic mass, etc.) suggests that the compound is less likely to be superconductor.

Nuclear Level Density Calculation of Astrophysical Reactions

Nuclear state density was calculated for the nuclear reaction inside the core of three different main- sequence stars namely, Sun, Sirius and Vega. The selected nuclei for present calculation were H 4 , C 12 , N 14 , and O 16 ; and the state density of Fe 54 nucleus was also included for comparison. The state density was calculated based on the exciton theory of nuclear reaction from Ericson, Williams and pairing formulae. The results confirmed that any change in the configuration will result different values of state density and the results showed that the dependence on energy is not linear, and higher densities were found at higher energies, but the change was less.

Energy resolved electrochemical impedance spectroscopy for electronic structure mapping in organic semiconductors

We introduce an energy resolved electrochemical impedance spectroscopy method to map the electronic density of states (DOS) in organic semiconductor materials. The method consists in measurement of the charge transfer resistance of a semiconductor/electrolyte interface at a frequency where the redox reactions determine the real component of the impedance. The charge transfer resistance value provides direct information about the electronic DOS at the energy given by the electrochemical potential of the electrolyte, which can be adjusted using an external voltage. A simple theory for experimental data evaluation is proposed, along with an explanation of the corresponding experimental conditions. The method allows mapping over unprecedentedly wide energy and DOS ranges. Also, important DOS parameters can be determined directly from the raw experimental data without the lengthy analysis required in other techniques. The potential of the proposed method is illustrated by tracing weak bond defect states induced by ultraviolet treatment above the highest occupied molecular orbital in a prototypical σ-conjugated polymer, poly[methyl(phenyl)silylene]. The results agree well with those of our previous DOS reconstruction by post-transient space-charge-limited-current spectroscopy, which was, however, limited to a narrow energy range. In addition, good agreement of the DOS values measured on two common π-conjugated organic polymer semiconductors, polyphenylene vinylene and poly(3-hexylthiophene), with the rather rare previously published data demonstrate the accuracy of the proposed method.

Rendering High Charge Density of States in Ionic Liquid-Gated MoS2 Transistors

We investigated high charge density of states (DOS) in the bandgap of MoS2 nanosheets with variable temperature measurements on ionic liquid-gated MoS2 transistors. The thermally activated charge transport indicates that the electrical current in the two-dimensional MoS2 nanosheets under high charge density state follows the Meyer-Neldel rule. The achieved high charge density allows the surface DOS estimation of MoS2 nanosheets, which have distinct peaks at 0.135 and 0.145 eV below the conduction band, with the largest DOS values of 9.75 × 1014 and 4.33 × 1014 cm–2 eV–1, respectively. This may represent the monolayer MoS2 nanosheets that coexist with the MoS2 multilayer in the channel area of the ionic liquid-gated MoS2 transistors.

Extraction of the sub-bandgap density-of-states in polymer thin-film transistors with the multi-frequency capacitance-voltage spectroscopy

The multi-frequency capacitance-voltage (C-V) spectroscopy is proposed for extracting the sub-bandgap density-of-states (DOS) of polymer semiconductors and demonstrated in three different thiophene-based organic thin-film transistors including poly(3-hexylthiophene), poly(3,3′′′-didodecylquaterthiophene), and poly(didodecylquaterthiophene-alt-didodecylbithiazole). The density of exponential tail and exponential deep states are extracted to be in the range of 3.0 × 1018 ∼ 1.5 × 1019 cm−3 eV−1 and 3.0 × 1016 ∼ 3.0 × 1017 cm−3 eV−1, respectively. The extracted DOS correspond to the polymer semiconductor-dependence of the measured crystallinity and mobility. In addition, the extracted DOS values are verified by comparing the measured I-V characteristics with the simulated results through a technology computer-aided design tool.

First-Principles Study of Electronic Structure of V-Si-N Nanocomposite Thin Film

In this paper, the method for plane wave ultrasoft pseudopotentials of first-principles is adopted to calculate electronic structures of three models including VN crystal, the absence of V atoms and the replacement of V atoms by Si atoms by the VASP software package. On the basis of the optimized VN’s lattice constant, the energy band structures and density of states(DOS) curves of those three models are analyzed. The results show that the 3d electrons of V atoms determine that VN crystal is a conductor. The crystal lack of V atoms forms a peak caused by the empty position near the Fermi level. Its valence band energy level splits and the ability to form bonds reduces to be a metastable phase structure. The formation of solid solution interface due to Si atoms replacing V atoms leads to the peak value of total DOS to decrease, the distribution of electrons to be more diffuse and the ability to form bonds to strengthen.

The Effect of Vacancy on the Phase Stability of TiNi Shape Memory Alloy from First-Principle Calculation

The effect of vacancy on the phase stability of TiNi shape memory alloy has been investigated by the first-principle method based on the density functional theory with generalized gradient approximation. The formation energy, formation heat, formation enthalpy per atom and density of states (DOS) of TiNi alloy with and without vacancy are calculated. The results indicate that the favorable point defect is Ni vacancy in both B2 and B19’ phases of TiNi alloy. The existence of vacancy increases the formation enthalpies per atom difference and thus decreases the phase stability. The DOS values at Fermi level of martensitic phases are lower than that of austenite phases in both perfect TiNi and TiNi with Ni vacancy, indicating the natural transformation from austenite to martensite upon cooling.

Improved Electrical Properties and Thermal Stability of GeON Gate Dielectrics Formed by Plasma Nitridation of Ultrathin Oxides on Ge(100)

We demonstrated the impact of plasma nitridation on thermally grown GeO2 for the purposes of obtaining high-quality germanium oxynitride (GeON) gate dielectrics. Physical characterizations revealed the formation of a nitrogen-rich surface layer on the ultrathin oxide, while keeping an abrupt GeO2/Ge interface without a transition layer. The thermal stability of the GeON layer was significantly improved over that of the pure oxide. We also found that although the GeO2 layer is vulnerable to air exposure, a nitrogen-rich layer suppresses electrical degradation and provides excellent insulating properties. Consequently, we were able to obtain Ge-MOS capacitors with GeON dielectrics of an equivalent oxide thickness (EOT) as small as 1.7 nm. Minimum interface state density (Dit) values of GeON/Ge structures, i.e., as low as 3 x 1011 cm-2eV-1, were successfully obtained for both the lower and upper halves of the bandgap.