Effect of oxidation on electronic and geometric properties of carbon nanotubes

We have studied an oxidation effect of multiwalled carbon nanotubes (MWCNTs) grown by thermal chemical vapor deposition (CVD) using ultraviolet photoelectron spectroscopy (UPS) and transmission electron microscopy (TEM). With an increasing oxygen exposure, the carbon 2p-pi states at similar to3eV below the Fermi level in the UPS spectra almost disappear. whereas the 2p-sigma states around 6eV are significantly increased. Annealing above 1000 K results in an increase of the density of states (DOSs) near the Fermi level, although a complete recovery of the 2p-pi electron states is not observed. We also observe a mode around 23eV that is associated with the collective sigma + pi plasmon excitation and chemisorbed C-O bonds, which is confirmed by the density functional calculations.