Bhalchandra S. Pujari

A systematic study of electronic structure from graphene to graphane

While graphene is a semi-metal, a recently synthesized hydrogenated graphene called graphane is an insulator. We have probed the transformation of graphene upon hydrogenation to graphane within the framework of density functional theory. By analysing the electronic structure for 18 different hydrogen concentrations, we bring out some novel features of this transition. Our results show that the hydrogenation favours clustered configurations leading to the formation of compact islands. The analysis of the charge density and electron localization function (ELF) indicates that, as hydrogen coverage increases, the semi-metal turns into a metal, showing a delocalized charge density, then transforms into an insulator. The metallic phase is spatially inhomogeneous in the sense it contains islands of insulating regions formed by hydrogenated carbon atoms and metallic channels formed by contiguous bare carbon atoms. It turns out that it is possible to pattern the graphene sheet to tune the electronic structure. For example, removal of hydrogen atoms along the diagonal of the unit cell, yielding an armchair pattern at the edge, gives rise to a bandgap of 1.4 eV. We also show that a weak ferromagnetic state exists even for a large hydrogen coverage whenever there is a sublattice imbalance in the presence of an odd number of hydrogen atoms.

Density Functional Investigations of Defect-Induced Mid-Gap States in Graphane

We have carried out ab initio electronic structure calculations on graphane (hydrogenated graphene) having single and double vacancy defects. Our analysis of the density of states reveals that such vacancies induce the mid-gap states and modify the band gap. The induced states are due to the unpaired electrons on carbon atoms surrounding the vacancy. Interestingly, the placement and the number of such states is found to be sensitive to the distance between the vacancies. It turns out that such vacancies also induce a local magnetic moment.

Fen (n=1–6) clusters chemisorbed on vacancy defects in graphene : Stability, spin-dipole moment, and magnetic anisotropy

In this work, we have studied the chemical and magnetic interactions of Fen (n = 1–6) clusters with vacancy defects (monovacancy to correlated vacancies with six missing C atoms) in a graphene shee ...

Electronic structure of spherical quantum dots using coupled cluster method

2, 6, 12, and 20 electron quantum dots have been studied using coupled cluster at singles and doubles level and extensive multireference coupled cluster (MRCC) method. A Fock-space version of MRCC (FSMRCC) containing single hole-particle excited determinants has been used to calculate low-lying excited states of the above system. The ionization potential and electron affinity are also calculated. The effect of correlation energy on excitation energy and charge density is shown by calculating them at the high density region (low value of density parameter rs) and at the low density region (high value of density parameter rs).

Spectral signatures of thermal spin disorder and excess Mn in half-metallic NiMnSb

Effects of thermal spin disorder and excess Mn on the electronic spectrum of half-metallic NiMnSb are studied using first-principles calculations. Temperature-dependent spin disorder, introduced within the vector disordered local moment model, causes the valence band at thepoint to broaden and shift upwards, crossing the Fermi level and thereby closing the half-metallic gap above room temperature. The spectroscopic signatures of excess Mn on the Ni, Sb, and empty sites (MnNi ,M nSb ,a nd MnE) are analyzed. MnNi is spectroscopically invisible. The relatively weak coupling of MnSb and MnE spins to the host strongly deviates from the Heisenberg model, and the spin of MnE is canted in the ground state. While the half-metallic gap is preserved in the collinear ground state of MnSb, thermal spin disorder of the weakly coupled MnSb spins destroys it at low temperatures. This property of MnSb may be the source of the observed low-temperature transport anomalies.

Ab Initio Studies on the Hydrogenation at the Edges and Bulk of Graphene

The opening of a band gap in graphene through chemical functionalization and realization of nanostructures, is an important issue for technological applications. Using first principles density functional theory, we show that how one can modify the electronic structure of bulk and nanoribbons of graphene by hydrogenation. It is shown that the hydrogenation of bulk graphene occurs through the formation of compact hydrogenated C islands. This also paves a unique way to realize zigzag and armchair nanoribbons at the interfaces between hydrogenated and bare C atoms and opens up the possibility to tune the band gap by controlling the width of the graphene-graphane interface. Moreover, we have studied the stability of hydrogenated edges of nanoribbons at finite temperature and pressure of hydrogen gas. It is shown that a dihydrogenated edge, which opens up a gap, can be stabilized under certain thermodynamic conditions.