Electronic structures of carbon nanotubes under electric fields

Abstract

Field Emission from Capped Carbon Nanotubes The electronic structures of capped carbon nanotubes were investigated using density functional calculations. We analyzed the density of states (DOSs), highest occupied molecular orbitals (HOMOs), and lowest unoccupied molecular orbitals (LUMOs) for various charged states with electric fields. When the electric field is applied along the tube axis, charges are transferred from the occupied levels localized at the top pentagon of the cap, and not from the HOMO localized at the side pentagon, to the unoccupied levels. We found that the charge densities at the top of the armchair cap show two- or five-lobed patterns depending on the field strength, whereas those of the zigzag cap show a three-lobed pattern. Adsorption of Molecules on Carbon Nanotube Edges We have investigated systematically the effects of various gas adsorbates $(H_2, N_2, O_2, and H_2O)$ on the electronic structures and the field emission properties of open edges of carbon nanotubes by density functional calculations. All the molecules, except $N_2$, dissociate and chemisorb on open nanotube edges with large adsorption energies. The Fermi levels are moved towards the valence (conduction) bands for $O_2(H_2, H_2O)$ adsorption induced by the Mulliken charge transfer on the tube edge. The Fermi level shift for $N_2$ adsorption is negligible. Adsorption of $H_2O$ enhances the field emission current, whereas $H_2$ adsorption does not affect the field emission current much due to the absence of the density of states near the Fermi level. Electronic Structures at Carbon Nanotube Tips We have investigated the electronic structures of chemically modified carbon nanotube tips under electric fields using density functional calculations. Hydrogen, oxygen, hydroxyl group-terminated nanotubes, including open-ended and capped ones, has been considered as the field emitters or probe tips. In case of the open-ended tubes, the field emission originates primarily from...