The relatively low intensity of the screened component compared to the unscreened component could be explained by the presence of a depletion layer devoid of free charge carriers near the oxide surface, which for other metal oxides has been shown to be of similar nanometer length scales as the probe depth of XPS. The filling of a depletion layer near the NC surface explains the relative increase in intensity of the small band near the Fermi level compared to the valence band as the material is reduced (Figure ).