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

Size-selected Si-30 and Si-39 clusters produced by a laser vaporization clu ster source are deposited on the Ag(111) surface at room temperature and at liquid-nitrogen temperature respectively. Subsequently, the sample is tran sferred at low temperature (120 K) in a separate mobile ultrahigh vacuum ch amber (vacuum-suitcase) from the cluster source to a low-temperature scanni ng tunneling microscope (STM). Soft landing of the supported clusters is in dicated by the following observations: (i) atomic-resolution images taken a t low bias voltages show transparent Si clusters and an unperturbed Ag(111) substrate; (ii) manipulation experiments on the supported clusters and sub sequently taken atomic-resolution images show a defect-free Ag(lll) 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-distribu tion on the support. This finding is attributed to the presence of differen t isomers and/or cluster orientations on the surface. (C) 2000 Elsevier Sci ence B.V. All rights reserved.