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A systematic study of a Li atom and four types of SiGe (6,6) nanotubes (NTs) of length ~20 Å has been performed to explore SiGeNTs' viability as anode material in Li-ion batteries. For all structures, full geometry and spin optimizations were performed without any symmetry constraints. We used the hybrid functional B3LYP and an all electron 6-311G**//3-21G* basis set implemented in GAUSSIAN 09 suite of software. All possible internal and external adsorption sites were considered, and it was found that some tubes deformed as a result of the Jahn-Teller effect. Out of all four types, type III NTs were found to retain the tubular shape for most cases of adsorption. Our calculations predict that SiGeNTs attract Li atom on top of Si, on top of Ge, and on top of hollow sites with adsorption energies ranging from 0.888 to 1.657 eV. Most probable sites for external and internal adsorptions were quasi on top of Ge site and quasi hollow site, respectively. After adsorption of the Li atom, Jahn-Teller active orbitals of the systems vanished from the frontier orbital region, indicating that more Li atoms can be adsorbed while keeping the desired tubular shape. In general, the difference between the highest occupied molecular orbital (HOMO) energy level and the lowest unoccupied molecular orbital (LUMO) energy level decreased with respect to the bare SiGeNT values. The decreased HOMO-LUMO gaps vary between 0.685 and 0.930 eV. Charge population analysis indicated that charge generally accumulated around the nearest neighbor atoms to the Li atom. We conclude that with the careful placement of Li atoms, SiGeNTs can be very successful as anode material in Li-ion battery technology. [ABSTRACT FROM AUTHOR]

Ab initio calculations have been performed to study the electronic and geometric structure properties of zigzag silicon nanotubes. Full geometry and spin optimizations have been performed without any symmetry constraints with an all electron 3-21G* basis set and the B3LYP hybrid functional. The largest zigzag SiNT studied here, (12, 0), has a binding energy per atom of 3.584 eV. Atomic hydrogen and oxygen adsorptions on (9, 0) and (10, 0) nanotubes have also been studied by optimizing the distances of the adatoms from both inside and outside the tube. The adatom is initially placed in four adsorption sites-parallel bridge (PB), zigzag bridge (ZB), hollow, and on-top site. The on-top site is the most preferred site for hydrogen atom adsorbed on (9, 0), with an adsorption energy of 3.0 eV and an optimized distance of 1.49 Å from the adatom to the nearest silicon atom. For oxygen adsorption on (9, 0), the most preferred site is the ZB site, with an adsorption energy of 5.987 eV and an optimized distance of 1.72 Å. For atomic hydrogen adsorption on (10, 0), the most preferred site is also the on-top site with an adsorption energy of 3.174 eV and an optimized distance of 1.49 Å. For adsorption of atomic oxygen on (10, 0), the most preferred site is PB site, with an adsorption energy of 6.306 eV and an optimized distance of 1.71 Å. The HOMO-LUMO gaps of (9, 0) after adsorptions of hydrogen and oxygen atoms decrease while the HOMO-LUMO gaps of (10, 0) increase after adsorption of hydrogen and oxygen. [ABSTRACT FROM AUTHOR]

A systematic study of silicon carbide nanocones of different disclination angles and different tip geometries using the finite cluster approximation is presented. The geometries of the nanocones have been spin optimized using the hybrid functional B3LYP (Becke's three-parameter exchange functional and the Lee-Yang-Parr correlation functional) and the all electron 3-21G* basis set. The study indicates that the binding energy per atom or the cohesive energy of the nanocones depends not only on the size of the nanocones but also on the disclination angle of the nanocones. The study also shows that the electronic properties of nanocones depend on disclination angles, size of the nanocone clusters, and the structure of edge of the nanocones. A study of binding energies, natural bond orbital (NBO) charge, density of states, and the gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) has been performed for all nanocones from disclination angles of 60° to 300°. [ABSTRACT FROM AUTHOR]