Metal-semiconductor and semiconductor-semiconductor transitions in carbon nanotubes induced by intercalating alkali atoms

We show that the intercalating of alkali atoms with adequate concentration can transform a metallic single-walled carbon nanotube (SWCNT) into a semiconductor tube or a semiconductor SWCNT into a semiconductor tube with different band gaps due to a unique feature of the band structures in some SWCNTs as quasi-one-dimensional materials. As examples, we predict the intercalated structure Li2C60 in the (5,0) tube to be a semiconductor while the pure (5,0) tube is a metal and the structure Li2C112 in the (4,2) tube to be a semiconductor with a direct gap while the pure (4,2) tube has an indirect gap. Results of other alkali atoms intercalating in various carbon nanotubes show that this transition mechanism is quite general.