Showing papers by "Abhishek K. Singh published in 2005"
TL;DR: In this article, Thorium is found to be the only element in the Periodic Table that stabilizes this dodecahedral fullerene with icosahedral symmetry in the neutral state.
Abstract: We show using ab initio electronic structure calculations that the dodecahedral fullerene of silicon ${\mathrm{Si}}_{20}$ is stabilized by thorium encapsulation. Thorium is found to be the only element in the Periodic Table that stabilizes this fullerene with icosahedral symmetry in the neutral state. The preference for $s{p}^{3}$ bonding in silicon makes it an optimal cage with all pentagonal faces in contrast to carbon for which ${\mathrm{C}}_{20}$ is difficult to stabilize. Similar to ${\mathrm{C}}_{60}$, this is the highest symmetry cluster of silicon and should be abundant. It could lead to the possibilities of novel new phases and derivatives of silicon.
54 citations
TL;DR: Results of ab initio calculations on silicon nanowires oriented along the [110] direction are reported and it is shown for the first time that these pristine silicon Nanowires are indirect band gap semiconductors.
Abstract: We report results of ab initio calculations on silicon nanowires oriented along the [110] direction and show for the first time that these pristine silicon nanowires are indirect band gap semiconductors. The nanowires have bulk Si core and are bounded by two (100) and two (110) planes in lateral directions. The (100) planes are atomically reconstructed with dimerization in a manner similar to the (100) surface of bulk Si but the dimer arrays are perpendicular to each other on the two (100) planes. An interesting consequence of surface reconstruction is the possibility of polytypism in thicker nanowires. We discuss its effects on the electronic structure. These findings could have important implications for the use of silicon nanowires in nanoscale devices as experimentally [110] nanowires have been found to grow preferentially in the small diameter range.
41 citations
TL;DR: It is reported from ab initio calculations that thorium encapsulation can be used to stabilize highly symmetric cages of germanium with 16 and 20 atoms, and the lowest energy structures of these clusters are different from the recently found silicon fullerenes and are similar to clusters found in bulk metallic alloys.
Abstract: We report from ab initio calculations that thorium encapsulation can be used to stabilize highly symmetric cages of germanium with 16 and 20 atoms. The lowest energy structures of these clusters are different from the recently found silicon fullerenes and are similar to clusters found in bulk metallic alloys. The binding energies of these clusters are higher compared with the values for the elemental germanium clusters of comparable sizes, and this suggests a strong possibility of their experimental realization in large quantities. Also, Th@Ge16 has a large highest occupied−lowest unoccupied molecular orbital (HOMO−LUMO) gap of 1.72 eV that makes it interesting for optoelectronic applications.
31 citations
Abstract: Using ab initio total energy calculations we design a very thin semiconducting nanotube of germanium with a direct band gap by encapsulation of $\mathrm{Mo}$ or $\mathrm{W}$. This finding is an outcome of studies of assemblies of ${\mathrm{Ge}}_{18}{\mathrm{Nb}}_{2}$ clusters into nanotubes. The infinite $\mathrm{Nb}$-doped nanotube is metallic. However, the electronic structure has a significant gap above the Fermi level. When $\mathrm{Nb}$ is replaced by a $Z+1$ element such as $\mathrm{Mo}$ or $\mathrm{W}$, it leads to the formation of a semiconducting nanotube. The atomic structure of these nanotubes is based on a novel alternate prism and antiprism stacking of hexagonal rings of germanium. Such an arrangement is optimal for ${\mathrm{Ge}}_{18}{M}_{2}$ ($M=\mathrm{Nb}$, $\mathrm{Mo}$, and $\mathrm{W}$) clusters that serve as the building blocks of nanotubes. These results demonstrate that by just changing the $M$ atom in the growth process, we can form metallic, semiconducting, and $n$ or $p$ types of nanotubes, opening new possibilities for nanoscale devices.
20 citations
TL;DR: In this article, the authors explore the stability of thin nanotubes using ab initio calculations with the generalized gradient approximation for the exchange-correlation energy and find that the pentagonal SiO nanotube is the most stable.
Abstract: The ability of silica to form different phases can be used to stabilize its nanostructures. Here we explore the stability of thin nanotubes of ${\mathrm{SiO}}_{x}$ ($x=1$ and 2) using ab initio calculations with the generalized gradient approximation for the exchange-correlation energy. We find that the pentagonal nanotubes are energetically most stable. The pentagonal SiO nanotube is a semiconductor with the largest calculated band gap of 0.90 eV, which is close to the value for bulk Si. The ${\mathrm{SiO}}_{2}$ nanotubes are, however, insulating similar to bulk silica and could be promising as the thinnest insulating layers for nanodevices. Our results demonstrate that we can get the most important circuit elements for nanoelectronics, namely semiconducting, as well as insulating nanotubes based on silicon in the subnanometer regime.
17 citations
TL;DR: In this article, first-principles density functional calculations were used to study the possible phases of CeMnNi$4$ and show that the ground state is ferromagnetic.
Abstract: Using first-principles density functional calculations, we study the possible phases of CeMnNi$_{4}$ and show that the ground state is ferromagnetic. We observed the hexagonal phase to be lowest in energy whereas experimentally observed cubic phase lies slightly higher in energy. We optimized the structure in both phases and in all different magnetic states to explore the possibility of the structural and magnetic phase transitions at ground state. We do not find any phase transitions between the magnetic and non-magnetic phases. The calculated structural, magnetic properties of cubic phase are in excellent agreement with experiments. Further, we do not observe half metallic behavior in any of the phases. However, the cubic phase does have fewer density of states for down-spin component giving a possibility of forming half metallic phase artificially, introducing vacancies, and disorder in lattice.
7 citations
TL;DR: In this article, thorium encapsulation can be used to stabilize highly symmetric cages of germanium with 16 and 20 atoms, and the lowest energy structures of these clusters are different from the recently found silicon fullerenes and are similar to clusters found in bulk metallic alloys.
Abstract: We report from ab initio calculations that thorium encapsulation can be used to stabilize highly symmetric cages of germanium with 16 and 20 atoms. The lowest energy structures of these clusters are different from the recently found silicon fullerenes and are similar to clusters found in bulk metallic alloys. The binding energies of these clusters are higher compared with the values for the elemental germanium clusters of comparable sizes, and this suggests a strong possibility of their experimental realization in large quantities. Also, Th@Ge16 has a large highest occupied−lowest unoccupied molecular orbital (HOMO−LUMO) gap of 1.72 eV that makes it interesting for optoelectronic applications.
3 citations