• Volume 89, Issue 1

July 2017

• Preface

• The effects of different possible modes of uniaxial strain on the tunability of electronic and band structures in $\rm{MoS_2}$ monolayer nanosheet via first-principles density functional theory

$\it{Ab-initio}$ density functional theory-based calculations have been performed on monolayer (ML) $\rm{MoS_2}$ nanosheet to study the variation of its electronic properties with the application of uniaxial tensile and compressive strain along its two non-equivalent lattice directions, namely, the zig-zag and the arm-chair directions. Among all the strain types considered in this study, uniaxial tensile strain applied along the zig-zag direction is found to be the most efficacious, inducing a greater tunability in the band gap over a large energy range (from 1.689 to 0.772 eV corresponding to 0–9% of applied strain), followed by uniaxial tensile strain along arm-chair direction. In contrast, the $\rm{ML–MoS_2}$ nanosheet is found to be less sensitive to the compressive strain applied uniaxially along both the arm-chair as well as zig-zag directions.Moreover, the charges on Mo and S atoms are not found to undergo considerable changes under the application of uniaxial strain, as the atomic motion along the other direction is free from any constraint.

• Structural characterization of Mg substituted on A/B sites in $\rm{NiFe_{2}O_{4}}$ nanoparticles using autocombustion method

In the present paper, we are reporting the synthesis of pure nickel and magnesium ferrite $\rm[{NiFe_{2}O_{4}, MgFe_{2}O_{4}}]$ and magnesium-substituted nickel ferrite $\rm{(Ni_{1−x}Mg_{x/y}Fe_{2−y}O_{4}; x = y = 0.60)}$ on A/B sites withparticles size in nanometer range using autocombustion technique. In this study, it has been observed that with increase in sintering temperature, the estimated bulk density of the materials increases. The XRD patterns of the samples show the formation of single-phase materials and the lattice parameters are estimated from XRD patterns. From Raman spectra, the Raman shift of pure $\rm{NiFe_{2}O_{4}}$ and $\rm{MgFe_{2}O_{4}}$ are comparable with the experimental values reported in literature. The Raman spectra give five Raman active modes $\rm{(A_{1g} + E_{g} +3F_{2g})}$ which are expected inthe spinel structure.

• Structural and optical characteristics of Ce, Nd, Gd, and Dy-doped $\rm{Al_{2}O_{3}}$ thin films

We present the optical properties of rare earth (RE)-doped $\rm{Al_{2}O_{3}}$ thin films and discuss their possible use in applications like gate dielectric material and in coating industry. Aluminum oxide films doped with RE elements such as Ce, Nd, Gd, and Dy are synthesized on glass substrate using ultrasonic spray pyrolysis technique at 400$^{\circ}$C. The concentration of rare earth element is varied from 0.5 to 5 mol% in 0.1 M solution of $\rm{Al_{2}O_{3}}$. The X-ray diffraction analysis indicates that the thin films deposited with and without rare earth doping have an amorphous structure. Further, the optical properties of RE-doped $\rm{Al_{2}O_{3}}$ thin films are studied by using UV–visible spectroscopy and photoluminescence measurement. The band gap is found to be 4.06eV for $\rm{Al_[2}O_{3}}$ thin film. A small blue shift is seen in the optical spectra of RE-doped samples as compared to undoped $\rm{Al_[2}O_{3}}$ film. Dielectric constant of alumina thin film increases with doping of Gd and Dy while it decreases with Ce and Nd doping. Concentration quenching effects are observed in the photoluminescence spectra of Ce, Nd, Gd, and Dy-doped $\rm{Al_[2}O_{3}}$ films. Among all these RE-doped $\rm{Al_[2}O_{3}}$ thin films, Gd and Dy-doped $\rm{Al_[2}O_{3}}$ films exhibit a potential for the construction of dielectric gate in transistors or as a coating material in the semiconductor industry.

• Electronic structure and optical properties of prominent phases of $\rm{TiO_2}$: First-principles study

First-principles study based on density functional theory $\rm{(DFT)}$ of two prominent phases, the rutile and the anatase phases, of titanium dioxide $\rm{(TiO_2)}$ are reported within the generalized gradient approximation $\rm{(GGA)}$. Our calculated band structure shows that there is a significant presence of O-2p and Ti-3d hybridization in the valence bands. These bands are well separated from the conduction bands by a direct band gap value of 1.73 eV in the rutile phase and an indirect band gap value of 2.03 eV in the anatase phase, from $\Gamma$ to $\rm{X}$. Our calculations reproduced the peaks in the conduction and valence band, are in good agreement with experimental observations.Our structural optimization for the rutile and anatase phase led to lattice parameter values of 4.62 Å and 2.99 Å rutile and 3.80 Å and 9.55 Å for anatase for $a$ and $c$. The static dielectric values 7.0 and 5.1 for the rutile and anatase phases respectively are in excellent agreement with experimental results. Our calculation of optical properties reveals that maximum value of the transmittance in anatase phase of $\rm{(TiO_2)}$ may be achieved by considering the anisotropic behaviour of the optical spectra in the optical region for transparent conducting application.

• Magnetic interactions and electronic structure of $\rm{Pt_{2}Mn_{1−x}Y_{x}Ga (Y = Cr and Fe)$ system: An $\it{ab-initio}$ calculation

First-principles density functional theory-based calculations have been carried out to predict the effects of Mn replacement by Fe and Cr on electronic as well as magnetic properties of $\rm{Pt_{2}MnGa}$ and $\rm{Ni_{2}MnGa}$. All the materials studied here are predicted to have conventional Heusler alloy structure in their ground state and they are found to be electronically stable on the basis of their respective formation energy. The replacement of Mn by Fe leads to a ferromagnetic ground state whereas in case of Mn replacement by Cr an intrasublattice antiferromagnetic configuration has been observed to have lower energy. We study the magnetic exchange interaction between the atoms for the materials with ferromagnetic and antiferromagnetic configurations to show the effects of Fe and Cr substitution at Mn site on the magnetic interactions of these systems. Detailed analysis of electronic structure in terms of density of states has been carried out to study the effect of substitution.

• Computational studies of third-order nonlinear optical properties of pyridine derivative 2-aminopyridinium p-toluenesulphonate crystal

We have reported a theoretical investigation on nonlinear optical behaviour, electronic and optical properties and other molecular properties of the organic nonlinear optical crystal 2-aminopyridinium ptoluenesulphonate(APPTS). The computation has been done using density functional theory (DFT) methodemploying 6-31G(d) basis set and Becke’s three-parameter hybrid functional (B3LYP). Calculated values of static hyperpolarizability confirm the good nonlinear behaviour of the molecule. Electronic behaviour and global reactivity descriptor parameters are calculated and analysed using HOMO–LUMO analysis. Energy band gap and simulated UV–visible spectrum show good agreement with experimental results. Other important molecular properties like rotational constant, zero-point vibrational energy, total energy at room temperature and pressure have also beencalculated in the ground state.

• A fragment-based approach towards $\it{ab-initio}$ treatment of polymeric materials

The broad range of applications of $\pi$-conjugated polymeric materials in industries such as automobiles, textiles, packaging, medical etc. have led to their extensive studies in both academic and industrial fields. Predicting the structure of these polymers is important for the study of their properties. The present work uses a ‘divide and conquer’-type approach for the $\it{ab-initio}$ studies of these polymeric systems. The method employs a fragmentation technique with independent fragment optimization for obtaining optimized geometries of the oligomers of various polymeric materials such as polyfuran, polypyrrole, polythiophene and other such $\pi$-conjugated polymers. A few test calculations performed in the study provide fair concurrence between the energies and the HOMO–LUMO energy gaps obtained using the fragmentation-based approach with those obtained using the full optimization of the whole oligomer. Also, a significant reduction in time complexity occurs for the present fragment-based approach compared to the parent system optimization. The results are encouraging and prompt for studies of large polymeric materials.

• Electronic and structural investigation of buckled antimonene using density functional theory calculation

Electronic and structural analysis of buckled antimonene has been performed using density functional theory-based $\it{ab-initio}$ approach. Geometrical parameters such as bond length and bond angle are very close to the single ruffle layer of rhombohedral antimony. Phonon dispersion along the high symmetry point of the Brillouin zone does not signify any soft mode. Electronic indirect band gap of 1.61 eV is observed for the single-layer antimonene. However, the occurrence of bilayered quasi-2D sheet consequent to metallic behaviour is due to significant electronic charge dispersion between interlayer region.

• Design and analysis of MEMS MWCNT/epoxy strain sensor using COMSOL

The design and performance of piezoresistive MEMS-based MWCNT/epoxy composite strain sensor using COMSOL Multiphysics Toolbox has been investigated. The proposed sensor design comprises su-8 based U-shaped cantilever beam with MWCNT/epoxy composite film as an active sensing element. A point load in microscale has been applied at the tip of the cantilever beam to observe its deflection in the proposed design. Analytical simulations have been performed to optimize various design parameters of the proposed sensor, which will be helpful at the time of fabrication.

• Density functional study of $\rm{AgScO_2}$: Electronic and optical properties

This paper focusses on the electronic and optical properties of scandium-based silver delafossite $\rm{(AgScO_2)}$ semiconductor. The density functional theory (DFT) in the framework of full potential linearized augmented plane wave (FP-LAPW) scheme has been used for the present calculations with local densityapproximation (LDA) and generalized gradient approximation (GGA). Electronic properties deal with energy bands and density of states (DOSs), while optical properties describe refractive index and absorption coefficient.The energy bands are interpreted in terms of DOSs. The computed value of band gap is in agreement with that reported in the literature. Our results predict $\rm{AgScO_2}$ as indirect band-gap semiconductor. Our calculated value of the refractive index in zero frequency limits is 2.42. The absorption coefficient predicts the applicability of $\rm{AgScO_2}$ in solar cells and flat panel liquid crystal display as a transparent top window layer.

• Spectroscopic and chemical reactivity analysis of D-Myo-Inositol using quantum chemical approach and its experimental verification

This paper describes the spectroscopic ($_{1}\rm{H}$ and $_{13}\rm{C NMR}$, FT-IR and UV–Visible), chemical, nonlinear optical and thermodynamic properties of D-Myo-Inositol using quantum chemical technique and its experimental verification. The structural parameters of the compound are determined from the optimized geometry by B3LYP method with $6-311++G(d,p)$ basis set. It was found that the optimized parameters thus obtained are almost in agreement with the experimental ones. A detailed interpretation of the infrared spectra of D-Myo-Inositol is also reported in the present work. After optimization, the proton and carbon NMR chemical shifts of the studied compound are calculated using GIAO and 6-311++G(d,p) basis set. The search of organic materials with improved charge transfer properties requires precise quantum chemical calculations of space-charge density distribution, state and transition dipole moments and HOMO–LUMO states. The nature of the transitions in the observed UV–Visible spectrum of the compound has been studied by the time-dependent density functional theory (TD-DFT). The global reactivity descriptors like chemical potential, electronegativity, hardness, softness and electrophilicity index, have been calculated using DFT. The thermodynamic calculation related to the title compound was also performed at $B3LYP/6-311++G(d,p)$ level of theory. The standard statistical thermodynamic functions like heat capacity at constant pressure, entropy and enthalpy change were obtained from the theoretical harmonic frequencies of the optimized molecule. It is observed that the values of heat capacity, entropy and enthalpy increase with increase intemperature from 100 to 1000 K, which is attributed to the enhancement of molecular vibration with the increase in temperature.

• A theoretical study on the B3 phases of ZnSe: Structural and electronic properties

A theoretical study on the structural stability and electronic properties of ZnSe is performed using the localized density approximation (LDA), generalized gradient approximation (GGA) and modified Becke– Johnson (mBJ)with Purdew–Burke–Ernzerhof (PBE-GGA) as the exchange correlation potential using full potentiallinearized augmented plane-wave method of density functional theory (DFT). The electronic structure calculation using the three approximations show that the LDA and the GGA methods underestimated the band gap while the band gap predicted by the mBJ is closer to the experimental result. The mBJ-GGA calculation shows a direct band-gap semiconductor of 2.5 eV. The total and partial densities of states of ZnSe are determined to study the energy band diagram.

• Optical properties of boron-group (V) hexagonal nanowires: DFT investigation

The paper presents structural, electronic and optical properties of boron-group V hexagonal nanowires (h-NW) within the framework of density functional theory. The h-NW of boron-group V compounds with an analogous diameter of 12 Å have been designed in (1 1 1) plane. Stability analysis performed through formation energies reveal that, the stability of these structures decreases with increasing atomic number of the group Velement. The band nature predicts that these nanowires are good electrical conductors. Optical behaviour of the nanowires has been analysed through absorption coefficient, reflectivity, refractive index, optical conductivity and electron energy loss spectrum (EELS), that are computed from the frequency-dependent complex dielectric function. The analysis reveals high reactivity of BP and BAs h-NWs to the incident light especially in the IR and visible ranges, and the optical transparency of BN h-NW in the visible and UV ranges.

• Structural and morphological characterization of CdSe:Mn thin films

CdSe:Mn thin films were grown by chemical bath deposition. The pH of the solution was maintained at 11. Dry films so obtained were annealed in vacuum ($10^{−1}$ Torr) for about 2 h at 400$^{\circ}$C. The annealed samples were subjected to morphological and structural characterization using scanning electron microscope and XRD. XRD was used for structural characterization whereas scanning electron microscope shows the surface morphology of the films. XRD spectra reveal that the grown CdSe films are polycrystalline in nature and have cubic structure. The average particle size decreases on doping CdSe with Mn ions. The FE-SEM images show spherical particles having uniform distribution. Optical characterization was done using PL studies and UV–Visible spectrophotometer. PL spectra show an increase in PL intensity on doping. Optical band gap also decreases on doping.

• Microscopic theory of substrate-induced gap effect on real AFM susceptibility in graphene

We address here a tight-binding model study of frequency-dependent real part of antiferromagnetic susceptibility for the graphene systems. TheHamiltonian consists of electron hopping upto third nearest-neighbours,substrate and impurity effects in the presence of electron–electron interactions at A and B sublattices. To calculate susceptibility, we evaluate the two-particle electron Green’s function by using Zubarev’s Green’s functiontechnique. The frequency-dependent real part of antiferromagnetic susceptibility of the system is computed numerically by taking 1000 × 1000 grid points of the electron momentum. The susceptibility displays a sharp peak at the neutron momentum transfer energy at low energies and another higher energy peak appearing at substrate-induced gap. The evolution of these two peaks is investigated by varying neutron wave vector, Coulomb correlation energy, substrate-induced gap, electron hopping integrals and A- and B-site electron doping concentrations.

• Dielectric constant of graphene-on-polarized substrate: A tight-binding model study

We report here a microscopic tight-binding theoretical study of the dynamic dielectric response of graphene-on-polarizable substrate with impurity. The Hamiltonian consists of first, second and third nearest neighbour electron hopping interactions besides doping and substrate-induced effects on graphene. We have introduced electron–electron correlation effect at A and B sublattices of graphene which is considered within Hartree–Fock mean-field approximation. The electron occupancies at both sublattices are calculated and solvedself-consistently and numerically for both up- and down-spin orientations. The polarization function appearing in the dielectric function is a two-particle Green’s function which is calculated by using Zubarev’s Green’s function technique. The temperature and optical frequency-dependent dielectric function is evaluated and compared with experimental data by varying Coulomb correlation energy, substrate-induced gap and impurity concentrations.

• # Pramana – Journal of Physics

Current Issue
Volume 89 | Issue 1
July 2017

• # Proceedings of the International Workshop/Conference on Computational Condensed Matter Physics and Materials Science (IWCCMP-2015)

Posted on November 27, 2015

Guest Editors: Anurag Srivastava, C. S. Praveen,
H. S. Tewari