C. Kamal

Arsenene: Two-dimensional buckled and puckered honeycomb arsenic systems

Recently, phosphorene, a monolayer honeycomb structure of black phosphorus, was experimentally manufactured and has attracted rapidly growing interest. Motivated by phosphorene, here we investigate the stability and electronic properties of the honeycomb structure of the arsenic system based on first-principles calculations. Two types of honeycomb structures, buckled and puckered, are found to be stable. We call them arsenenes, as in the case of phosphorene. We find that both buckled and puckered arsenenes possess indirect gaps. We show that the band gap of puckered and buckled arsenenes can be tuned by applying strain. The gap closing occurs at 6% strain for puckered arsenene, where the bond angles between the nearest neighbors become nearly equal. An indirect-to-direct gap transition occurs by applying strain. Specifically, 1% strain is enough to transform puckered arsenene into a direct-gap semiconductor. We note that a bulk form of arsenic called gray arsenic exists which can be used as a precursor for buckled arsenene. Our results will pave the way for applications to light-emitting diodes and solar cells.

Martensitic transition, ferrimagnetism and Fermi surface nesting in Mn2NiGa

The electronic structure of Mn2NiGa has been studied using density functional theory and photoemission spectroscopy. The lower-temperature tetragonal martensitic phase with c/a= 1.25 is more stable compared to the higher-temperature austenitic phase. Mn2NiGa is ferrimagnetic in both phases. The calculated valence band spectrum, the optimized lattice constants and the magnetic moments are in good agreement with experiment. The majority-spin Fermi surface (FS) expands in the martensitic phase, while the minority-spin FS shrinks. FS nesting indicates occurrence of phonon softening and modulation in the martensitic phase.

Direct band gaps in group IV-VI monolayer materials: Binary counterparts of phosphorene

We perform systematic investigation on the geometric, energetic and electronic properties of group IV-VI binary monolayers (XY), which are the counterparts of phosphorene, by employing density functional theory based electronic structure calculations. For this purpose, we choose the binary systems XY consisting of equal numbers of group IV (X = C, Si, Ge, Sn) and group VI elements (Y = O, S, Se, Te) in three geometrical configurations, the puckered, buckled and planar structures. The results of binding energy calculations show that all the binary systems studied are energetically stable. It is observed that, the puckered structure, similar to that of phosphorene, is the energetically most stable geometric configuration. Our results of electronic band structure predict that puckered SiO and CSe are direct band semiconductors with gaps of 1.449 and 0.905 eV, respectively. Band structure of CSe closely resembles that of phosphorene. Remaining group IV-VI binary monolayers in the puckered configuration and all the buckled monolayers are also semiconductors, but with indirect band gaps. Importantly, we find that the difference between indirect and direct band gaps is very small for many puckered monolayers. Thus, there is a possibility of making these systems undergo transition from indirect to direct band gap semiconducting state by a suitable external influence. Indeed, we show in the present work that seven binary monolayers namely SnS, SiSe, GeSe, SnSe, SiTe, GeTe and SnTe become direct band gap semiconductors when they are subjected to a small mechanical strain (<= 3 %). This makes nine out of sixteen binary monolayers studied in the present work direct band gap semiconductors. Thus, there is a possibility of utilizing these binary counterparts of phosphorene in future light-emitting diodes and solar cells.

Calculation of ground- and excited-state energies of confined helium atom

We calculate the energies of ground and three low lying excited states of confined helium atom centered in an impenetrable spherical box. We perform the calculation by employing variational method with two-parameter variational forms for the correlated two-particle wave function. With just two variational parameters we get quite accurate results for both ground and excited state energies.

The van der Waals coefficients between carbon nanostructures and small molecules: A time-dependent density functional theory study

We employ all-electron ab initio time-dependent density functional theory based method to calculate the long-range dipole-dipole dispersion coefficient, namely, the van der Waals (vdW) coefficient (C(6)) between fullerenes and finite-length carbon nanotubes as well as between these structures and different small molecules. Our aim is to accurately estimate the strength of the long-range vdW interaction in terms of the C(6) coefficients between these systems and also compare these values as a function of shape and size. The dispersion coefficients are obtained via Casimir-Polder relation. The calculations are carried out with the asymptotically correct exchange-correlation potential-the statistical average of orbital potential. It is observed from our calculations that the C(6) coefficients of the carbon nanotubes increase nonlinearly with length, which implies a much stronger vdW interaction between the longer carbon nanostructures compared with the shorter ones. Additionally, it is found that the values of C(6) and polarizability are about 40%-50% lower for the carbon cages when compared with the results corresponding to the quasi-one-dimensional nanotubes with equivalent number of atoms. From our calculations of the vdW coefficients between the small molecules and the carbon nanostructures, it is observed that for H(2), the C(6) value is much larger compared with that of He. It is found that the rare gas atoms have very low values of vdW coefficient with the carbon nanostructures. In contrast, it is found that other gas molecules, including the ones that are environmentally important, possess much higher C(6) values. Carbon tetrachloride as well as chlorine molecule show very high C(6) values with themselves as well as with the carbon nanostructures. This is due to the presence of the weakly bound seven electrons in the valence state for the halogen atoms, which makes these compounds much more polarizable compared with the others.

Density functional investigation on the structures and properties of Li atom doped Au20 cluster

Structures and properties of an Au20 cluster doped with two Li atoms, Au18Li2, have been investigated using relativistic density functional theory within the framework of the zeroth-order regular approximation. Various initial structures have been generated and employed for geometry optimization followed by vibration analysis to check the stability of the final optimized structures. We have calculated various properties like binding energy, ionization potential, electron affinity and the HOMO–LUMO gap of these structures. It has been found that two dopant Li atoms favour occupying two different surface positions of the pyramidal Au20 cluster. The binding energy of the surface-doped Au18Li2 cluster is 1.017 eV higher than that of the pure Au20 cluster and the HOMO–LUMO gap (1.742 eV) is as high as a pure Au20 cluster (1.786 eV). Interestingly, we observe that the HOMO–LUMO gap as well as the binding energy can be increased beyond those of the Au18Li2 cluster with the help of further Li atom doping. In fact, a doped tetrahedral Au16Li4 cluster, where all the dopants are at the surface sites, possesses a very high HOMO–LUMO gap of 2.117 eV. Geometric and energetic parameters indicate that the Au16Li4 cluster might be considered as a possible ‘superatom’ in the design of novel cluster-assembled materials.

Ab initio investigation on hybrid graphite-like structure made up of silicene and boron nitride

Abstract In this work, we report our results on the geometric and electronic properties of hybrid graphite-like structure made up of silicene and boron nitride (BN) layers. We predict from our calculations that this hybrid bulk system, with alternate layers of honeycomb silicene and BN, possesses physical properties similar to those of bulk graphite. We observe that there exists a weak van der Waals interaction between the layers of this hybrid system in contrast to the strong inter-layer covalent bonds present in multi-layers of silicene. Furthermore, our results for the electronic band structure and the density of states show that it is a semi-metal and the dispersion around the Fermi level ( E F ) is parabolic in nature and thus the charge carriers in this system behave as nearly-free-particle-like. These results indicate that the electronic properties of the hybrid bulk system resemble closely those of bulk graphite. Around E F the electronic band structures have contributions only from silicene layers and the BN layer acts only as a buffer layer in this hybrid system since it does not contribute to the electronic properties near E F . In case of bi-layers of silicene with a single BN layer kept in-between, we observe a linear dispersion around E F similar to that of graphene. However, the characteristic linear dispersion becomes parabola-like when the system is subjected to a compression along the transverse direction. Our present calculations show that the hybrid system based on silicon and BN can be a possible candidate for two-dimensional layered system, akin to graphite and multi-layers of graphene.

Density functional study of α-amino acids: structural, energetic and vibrational properties

The structural, energetic and vibrational properties of the 20 standard α-amino acids, in each of several different low-energy conformations, have been investigated using all-electron density-functional theory. The Becke–Perdew exchange-correlation potential within the generalized gradient corrections to the local density approximation exchange and correlation energy was used, along with a Slater-type expansion of the Kohn–Sham orbitals. The structures and bond lengths, conformation energies, and infrared vibrational spectra of the stable conformers of these molecules have been predicted and various features, including those arising due to intramolecular hydrogen bonding, identified. The results of our accurate ab initio study, the most comprehensive to date, are in good agreement with the few earlier gas-phase experimental and theoretical results available from the literature, and provide a benchmark for further experiments and for obtaining a deeper understanding of this vital class of biomolecules.

Ab initio study of stoichiometric gallium phosphide clusters

We have studied the static dipole polarizability of stoichiometric gallium phosphide clusters (Ga(n)P(n) with n=2-5) by employing various ab initio wave function based methods as well as density functional theory/time dependent density functional theory (DFT/TDDFT). The calculation of polarizability within DFT/TDDFT has been carried out by employing different exchange-correlation functionals, ranging from simple local density approximation to an asymptotically correct model potential-statistical average of orbital potential (SAOP) in order to study their influences. The values obtained by using the model potential-SAOP are lower than those obtained by local density approximation and generalized gradient approximation. A systematic analysis of our results obtained using the DFT/TDDFT with several exchange-correlation functionals shows that the values of polarizability obtained within generalized gradient approximation by using Perdew-Burke-Ernzerhof exchange with Lee-Yang-Parr correlation functional and Perdew-Burke-Ernzerhof exchange-correlation functionals are the closest to the corresponding results from Møller-Plesset perturbation theory. We have found that the value of average static dipole polarizability per atom reaches the bulk limit from the above as the size of the clusters increases.

High-pressure studies on the properties of FeGa3 : Role of on-site Coulomb correlation

High pressure X-ray diffraction measurements have been carried out on the intermetallic semiconductor FeGa