Electronic structures of capped carbon nanotubes under electric fields

We have investigated the electronic structure of capped carbon nanotubes under electric fields by density functional calculations. The calculated effective work function of the capped armchair nanotubes decreases linearly with increasing electric fields, whereas that of the metal tip decreases quadratically. We analyzed the density of states, highest occupied molecular orbitals (HOMO's) and lowest unoccupied molecular orbitals (LUMO's) for various charged states under electric fields. While the HOMO and the LUMO are not localized at the cap for a neutral state, the HOMO and LUMO of a negatively charged state are localized at the cap, particularly under electric fields. This localization plays an important role in the field emission. The Mulliken charge population shows that the charge accumulation is not dependent on the local atomic geometry but on the sharpness of the tip.