Noble metal clusters and C60-buckminsterfullerenes on carbon surfaces : a photoelectron spectroscopy study

Photoelectron spectroscopy (MXPS) studies on chemically synthesized thiol-passivated gold nanoparticles revealed a thiol-chain length dependence on the thermal desorption behavior of the passivating shell, on particle self-assembly properties and on X-ray induced thiol-gold bond breaking. Observed changes in the Au 4f core-level binding energies motivated to study thiolgold interaction in more detail. Adsorption of thiol molecules on gold clusters should yield insight to the nature of the still not fully understood Au-S bond. To avoid exposure to ambient atmosphere cluster deposition and thiol adsorption had to be performed in vacuo. Therefore, the growth of clusters on defect-rich carbon surfaces by means of metal evaporation was studied at first. Investigation of the early stages of cluster nucleation and growth showed a considerable interaction of deposited gold clusters with carbon defects, heavily shifted (≈eV) second feature in the Au 4f core-level spectra. This was attributed to the existence of a bimodal cluster size distribution. It was found that deposition of heated substrates (400-600 ◦ C) represent a possibility to preferentially grow nanoparticles on defect-sites. Having studied nanoparticle growth gold cluster samples were prepared to be exposed to a thiol atmosphere. MXPS spectra were taken before and after exposure to monitor changes in the clusters’ electronic configuration. It could be shown that thiol adsorption induces positive binding energy shift in gold core-level and valence band spectra. This was interpreted as due to electron charge localization by Au-S bonds. Changes in the shape of MXPS valence band spectra where attributed to re-hybridization of Au 5d electrons (creation of Au-S bonds) and a close examination of the Fermi edge region hinted to a thiol-induced metal-to-insulator transition. These results are of high interest concerning the electrical properties of metal-organic contacts. Motivated by the results obtained from cluster deposition on defect-rich carbon surfaces the work was extended to buckminsterfullerene molecules. The interaction of C60 with defects and the thermal stability was investigated by means of UV Photoemission Spectroscopy (UPS). It was demonstrated that fullerenes chemisorb at defects indicated by a higher desorption temperature as on pristine HOPG. This was reflected by significantly shifted and broadened VB features. The higher desorption temperature for C60 attached to defects than for species adsorbed on undamaged HOPG might allow in combination with Focused Ion Beam techniques to prepare fullerene-decorated nanostructures. In conclusion, the different experiments and results presented here have shown that artificially created defects on carbon (esp. HOPG) substrates might be promising way of surface nanostructuring. The choice of carbon is interesting with regard to applications where biocompatibility1 of the substrates is demanded.