Electronic structures of size-selected single-layered platinum clusters on silicon(111)-7x7 surface at a single cluster level by tunneling spectroscopy.

Tunneling spectra of size-selected single-layered platinum clusters (size range of 5-40) deposited on a silicon(111)-7x7 surface were measured individually at a temperature of 77 K by means of a scanning tunneling microscope (STM), and the local electronic densities of states of individual clusters were derived from their tunneling spectra measured by placing an STM tip on the clusters. In a bias-voltage (V(s)) range from -3 to 3 V, each tunneling spectrum exhibits several peaks assignable to electronic states associated with 5d states of a constituent platinum atom and an energy gap of 0.1-0.6 eV in the vicinity of V(s)=0. Even when platinum cluster ions having the same size were deposited on the silicon(111)-7x7 surface, the tunneling spectra and the energy gaps of the deposited clusters are not all the same but can be classified in shape into several different groups; this finding is consistent with the observation of the geometrical structures of platinum clusters on the silicon(111)-7x7 surface. The mean energy gap of approximately 0.4 eV drops to approximately 0.25 eV at the size of 20 and then decreases gradually as the size increases, consistent with our previous finding that the cluster diameter remains unchanged, but the number density of Pt atoms increases below the size of 20 while the diameter increases, but the density does not change above it. It is concluded that the mean energy gap tends to decrease gradually with the mean cluster diameter. The dependence of the mean energy gap on the mean Pt-Pt distance shows that the mean energy gap decreases sharply when the mean Pt-Pt distance exceeds that of a platinum metal (0.28 nm).