Density-functional-theory calculations of charged single-walled carbon nanotubes

The electronic structures of charged carbon nanotube cluster models were calculated with the density-functional-theory (DFT) method. With the increasing number of extra electrons, (a) a potential barrier is formed near the tube wall; its height increases and the width at the highest occupied molecular-orbital (HOMO) level decreases; (b) the HOMO energy increases linearly, giving chances to electrons to tunnel through the barrier and escape; (c) the total energy first decreases and then increases, presenting a perfect parabolic curve and indicating an optimum number of extra electrons; and (d) electrical field near the tube was worked out. A comparison between the tube and a perfect conductor demonstrates some of their resemblances, but the exact properties such as the tube capacitance depend on the quantum theory calculations. The total-energy variation obeys Koopmans' theorem and although calculated with DFT method, it can be expressed in terms of the tube's classical parameters such as capacitance and work function. The electronic charge rearrangement after the addition of extra electrons is presented and this might be the determinant of the electronic structures of charged carbon nanotubes.