X-ray emission studies of the valence band of nanodiamonds annealed at different temperatures

X-ray emission spectroscopy has been applied to examine the electronic structure of onion-like carbon (OLC) generated by the annealing treatment of nanodiamonds (ND). The C Kα spectra of OLC produced in the temperature range of 1600-1900 K were found to be markedly different from the spectrum of particles formed at 2140 K and to be characterized by better ordering of graphitic shells. The latter spectrum was shown to be very similar to the C Kα of polycrystalline graphite, while the former ones exhibited a significant increase of the high-energy maximum that might be caused by the defect structure of graphitic networks forming at the intermediate temperatures. The experimental spectra were compared with the theoretical spectra from quantum-chemical semiempirical AM1 calculation of several models: a fullerene molecule, C240, having icosahedral structure, a C240 molecule incorporating a greater number of nonhexagonal rings, and a holed structure formed by removing pentagons from the icosahedral molecule. The density of high-energy electronic states in the valence band of the graphitic cage was found to be practically invariant to a change in ring statistics but to significantly increase because of localization of electrons on the zigzag sites of a hole boundary