Energy gap of silicon clusters studied by scanning tunneling spectroscopy

Scanning tunneling spectroscopy was performed on small pristine silicon clusters on graphite. For the investigated clusters with sizes between 2.5 and 40 {}, two distinct size regimes, were found: below 15 {}, Si clusters show energy gaps up to 450 meV, above that size only zero gaps are observed. We conclude that a structural transition occurs at the size of 15 {}. Larger clusters assume the diamond structure, and the zero gaps originate from the dangling bonds on the cluster surface. Smaller clusters appear in high-coordination structures, which have eliminated the dangling bonds.