g/cm$^3$) than to that of diamond ($\rho_{\rm dia} = 3.515$ g/cm$^3$) suggesting a structure that contains regions with trivalent coordination.[@Robe86] The similarities in the electronic structure between a-C and graphite are supported by electron energy loss spectroscopy (EELS) measurements,[@RFG04] which show spectra that differ significantly from those of diamond. Amorphous-carbon films produced with similar methods and of similar density have been found to be semi-conducting.[@Robe86; @TL04]

Note that, since $\rho_{\rm subs} < \rho_{\rm graph}$ and since in the amorphous material some tetravalent C with higher local density must be present, there must exist areas with very low density or even voids for compensation. This assumption is supported by the results presented in Sec. \[sectionsurface\].

The Au clusters were collected from the primary beam of a laser vaporization cluster source, which has been described elsewhere in detail.[@WWVK04; @WGGK02] In brief, the laser vaporization cluster source is a variant of the Smalley-deHeer-type [@MdH90; @DDPS81] setup optimized by Heiz [@HVTS97] for high yield. The source is equipped with a rotating gold disc target with a diameter of 50 mm which is sealed with a Teflon gasket against the source block. A pulsed laser (Neodym-YAG, Continuum, 532 nm, 30 Hz repetition rate) is focused 