Saleem Ayaz Khan

Electronic band structure and specific features of AA- and AB-stacking of carbon nitride (C3N4): DFT calculation

The state-of-the-art all-electron full potential linear augmented plane wave based on density functional theory was applied for calculating the structural, electronic band structure, effective mass, density of state, and valence electron charge density of AA- and AB-stacking of carbon nitride (C3N4). Calculations were performed using four schemes, namely local density approximation (LDA-CA) by Ceperley–Alder, generalized gradient approximation developed by Perdew Burke and Ernzerhof (GGA-PBE), Engel–Vosko generalized gradient approximation (EV-GGA) and the recently developed modified Becke–Johnson (mBJ) potentials. The calculated band structure and total density of states verify the semiconducting nature of the two configurations with band gap of about 2.589 eV and 2.990 eV for AA- and AB-stacking of C3N4. The effective mass ratio of electron, heavy hole and light hole were calculated for both AA- and AB-stacking. The upper valence band in AB-stacking is much flatter as compared to AA-stacking, and elucidates a dramatic increase in the effective mass ratio of heavy holes which cause a reduction in group velocity of the wave packet and consequently decrease the mobility of the charge carrier. The partial density of state shows the variation in the orbital state and its strength as one moves from AA- to AB-stacking. The strong hybridization among the orbitals of both stacking (AA- and AB-) shows covalent nature of the bonds. The calculated valence electron charge density contour shows the prevailing covalent nature of C and N bonds with negligible percentage (8.08%) of ionicity.

Thermoelectric properties of a single graphene sheet and its derivatives

The thermoelectric properties of pristine graphene and H2S adsorbed onto bridge, hollow and top sites of a graphene sheet are investigated using the semi-classical Boltzmann transport theory. The average values of electrical conductivity, thermal conductivity, Seebeck coefficient, figure of merit (ZT) and the average value of the power factor (Pav) are reported and discussed in detail. While pristine graphene is a zero band gap semiconductor, adsorption of H2S onto the bridge site opens up a direct energy gap of about 0.04 eV, adsorption of a H2S molecule onto the top site opens up a gap of 0.3 eV, and adsorption of H2S onto the hollow site makes it metallic. The investigation of ZT and power factor values suggests that a top-site configuration could be a potential candidate for thermoelectric applications in the range 300–600 K.

Linear and nonlinear optical properties for AA and AB stacking of carbon nitride polymorph (C3N4)

The linear and nonlinear optical susceptibilities of AA and AB stacking of the carbon nitride polymorph were calculated using the all electron full potential linear augmented plane wave method based on density functional theory. The complex part of the dielectric function is calculated using the recently modified Becke and Johnson (mBJ) approximation which gives a better optical gap in comparison to the Ceperley–Alder (CA) local density approximation, the Perdew–Burke–Ernzerhof generalized gradient approximation, and the Engel–Vosko generalized gradient approximation. The complex dielectric function and other optical constants like refractive index, absorption coefficient, reflectivity and energy loss function are calculated and discussed in detail. The calculated uniaxial anisotropy (−1.06 and −1.04) gives a maximum value of birefringence (−0.89 and −0.87) which increases the suitability of both AA and AB stacking for a large second harmonic generation. The calculated second order susceptibility tensor components |χ(2)333(ω)| at the static limit are 19.4 pm V−1 and 59.6 pm V−1 for AA and AB stacking which increases to 34.2 pm V−1 and 106.7 pm V−1 at λ = 1064 nm. The first hyperpolarizability β333(ω) for AA and AB stacking C3N4 for the dominant component |χ(2)333(ω)| at the static limit are calculated to be (1.6 × 10−30 esu and 9.6 × 10−30 esu) respectively.

Magnetocrystalline anisotropy of FePt: A detailed view

To get a reliable ab initio value for the magnetocrystalline anisotropy (MCA) energy of FePt, we employ the full-potential linearized augmented plane wave (FLAPW) method and the full-potential Korringa-Kohn-Rostoker (KKR) Green function method. The MCA energies calculated by both methods are in good agreement with each other. As the calculated MCA energy significantly differs from experiment, it is clear that many-body effects beyond the local density approximation are essential. It is not really important whether relativistic effects for FePt are accounted for by solving the full Dirac equation or whether the spin-orbit coupling (SOC) is treated as a correction to the scalar-relativistic Hamiltonian. From the analysis of the dependence of the MCA energy on the magnetization angle and on the SOC strength it follows that the main mechanism of MCA in FePt can be described within second order perturbation theory. However, a distinct contribution not accountable for by second order perturbation theory is present as well.

Coulomb interaction and spin-orbit coupling calculations of thermoelectric properties of the quaternary chalcogenides Tl2PbXY4 (X = Zr, Hf and Y = S, Se)

The increase in energy demands is leading to growing interest in new thermoelectric inorganic materials, such as the chalcogenides. The recently synthesized quaternary chalcogenide, Tl2PbXY4 (X = Zr, Hf and Y = S, Se), compounds were investigated using the full potential linear augmented plane wave method based on density functional theory. We used the generalized gradient approximation plus the optimized effective Hubbard parameter U to treat the exchange correlation. The existence of heavy metals (Tl, Pb and Hf) required the application of relativistic spin–orbit coupling via a second variational procedure. Tl2PbHfS4, Tl2PbHfSe4, Tl2PbZrS4 and Tl2PbZrSe4 compounds were found to be semiconductors with indirect band gaps of 0.911, 0.659, 0.983 and 0.529 eV, respectively. The types of carriers and electrical transport properties of Tl2PbXY4 (X = Zr, Hf and Y = S, Se) are attributed to the Tl-d and S/Se-s electronic states near the Fermi level. Optical properties were investigated via the calculation of dielectric function and reflectivity. Using Boltzmann theory, we compared the thermoelectric properties and we found that Tl2PbHfS4 could be a good candidate for thermoelectric devices.

Local environment effects in the magnetic properties and electronic structure of disordered FePt

Local aspects of magnetism of disordered FePt are investigated by ab initio fully relativistic full-potential calculations, employing the supercell approach and the coherent potential approximation (CPA). The focus is on trends of the spin and orbital magnetic moments with chemical composition and with bond lengths around the Fe and Pt atoms. A small but distinct difference between average magnetic moments obtained when using the supercells and when relying on the CPA is identified and linked to the neglect of the Madelung potential in the CPA.

Predicted Thermoelectric Properties of the Layered XBi4S7 (X = Mn, Fe) Based Materials: First Principles Calculations

We report a theoretical investigation of electronic structures, optical and thermoelectric properties of two ternary-layered chalcogenides, MnBi4S7 and FeBi4S7 , by combining the first principles density functional calculations and semi-local Boltzmann transport theory. The calculated electronic band structure have demonstrated that both compounds exhibit indirect band gaps. The optical transitions are explored via the dielectric function (real and imaginary parts) along with other related optical constants including refractive index, reflectivity, and energy loss spectrum. These chalcogenides have exhibited interesting thermoelectric properties such as Seebeck’s coefficient, electrical and thermal conductivity, and power factor as function of temperatures.

Optoelectronic Structure and Related Transport Properties of Ag2Sb2O6 and Cd2Sb2O7

Using the full-potential linearized augmented-plane wave method, the electronic structure and thermoelectric properties of Ag2Sb2O6 and Cd2Sb2O7 compounds have been explored. The modified Becke–Johnson potential was applied to treat the exchange–correlation energy term. The electronic band structures reveal that the valence-band maximum and conduction-band minimum occur at Γ point, indicating that Ag2Sb2O6 and Cd2Sb2O7 are direct energy bandgap semiconductors. Strong hybridization appeared between Ag (Cd)-s/p and O-s/p states. The optical properties, i.e., complex dielectric function, reflectivity, refractive index, and energy loss function, reveal high reflectivity in the ultraviolet energy range, indicating usefulness of these materials in shields from high-energy radiation. Combining transport theory and the outputs from the full-potential linearized augmented-plane wave calculations, the thermoelectric properties were analyzed as functions of temperature. Due to their high thermopower and narrow bandgap, Ag2Sb2O6 and Cd2Sb2O7 are suitable materials for application in optoelectronic and thermoelectric devices.

Tailoring the electronic structure and optical properties of cadmium-doped zinc oxides nanosheet

Abstract Cd-doped ZnO nanosheet (ZnO NS) were investigated using a full-potential linearized augmented plane wave method within the generalized gradient approximation (GGA) to calculate the electronic structure and its optical response. The calculated band structures have shown that the Cd-doped ZnO NS is a direct band gap semiconductor at Γ with 1.50 eV band gap. The contribution of each atom/orbital were commented in light of total and partial densities of states. We also derived the optical constants (mainly the dielectric constants ε1(0) and ε2(0)), the absorption coefficient I(ω), refractive index n(ω), extinction coefficient k(ω), and energy-loss function L(ω). The spectrum of absorption coefficient has revealed to increase rapidly for photon energies higher than 2.5 eV. The absorption spectrum was found to be limited in energy region due to different contributions electronic transitions that occurred within ZnO NS and effect of Cd doping. Reducing the band gap of ZnO NS to low values is suitable process for light-emitting devices and solar cells applications.

Optoelectronic and Magnetic Properties of Eu2Si5N8: An Ab-initio Study

Abstract Eu2Si5N8 is considered the most important compound in the development of inorganic materials with high potential and performance. Therefore, the electronic, magnetic and optical properties of Eu2Si5N8 are investigated here using density functional theory. The electronic interactions are described within the generalised gradient approximation, GGA+U (where U is the Hubbard Coulomb energy term). The calculated energy gap was 3.5 eV for the investigated compound, resulting in a direct band gap semiconductor. The optical constants, including the dielectric function, refractive index, absorption coefficient, reflectivity, and energy loss function were calculated for radiation up to 14 eV. The optical properties demonstrate that the main differences in absorption, reflectivity, energy-loss function and refractive index occur in the infrared and visible regions for the spin-up and spin-down states, which makes this material an excellent candidate for optical memory devices.