Imaging size-selected silicon clusters with a low-temperature scanning tunneling microscope

Size-selected Si₃₀ and Si₃₉ clusters produced by a laser vaporization cluster source are deposited on the Ag(111) surface at room temperature and at liquid-nitrogen temperature respectively. Subsequently, the sample is transferred at low temperature (120 K) in a separate mobile ultrahigh vacuum chamber (vacuum-suitcase) from the cluster source to a low-temperature scanning tunneling microscope (STM). Soft landing of the supported clusters is indicated by the following observations: (i) atomic-resolution images taken at low bias voltages show transparent Si clusters and an unperturbed Ag(111) substrate; (ii) manipulation experiments on the supported clusters and subsequently taken atomic-resolution images show a defect-free Ag(111) surface. In spite of the fact that the clusters are mass-selected in the gas phase, a statistical analysis of the STM images indicates a finite size-distribution on the support. This finding is attributed to the presence of different isomers and/or cluster orientations on the surface.