Size and Structure Dependence of Electronic States in Thiolate-Protected Gold Nanoclusters of Au25(SR)18, Au38(SR)24, and Au144(SR)60

The electronic states of three different sizes of compositionally precise thiolate-protected gold nanoclusters, Au-25(SR)(18), Au-38(SR)(24), and Au-144(SR)(60) (R = C12H25), have been evaluated by X-ray photoemission spectroscopy. The Au 4f core-levels of the nanoclusters are well reproduced by two spectral components derived from centered core-Au and positively charged shell-Au atoms, the numbers of which are determined based on the atomic structures of the nanoclusters. The spin-orbit splitting of Au 5d(5/2) and 5d(3/2) in the valence band becomes narrower than that for bulk Au, depending on the cluster size, which is quantitatively characterized by a reduction in the average coordination number of Au. The Au 5d valence-band spectra also show that the charge reorganization of 5d electrons induced by interaction with thiol molecules is more significant for the 5d(5/2) than the 5d(3/2) orbital.