We report on a source for producing size-selected nanoclusters based on the combination of radio frequency magnetron plasma sputtering and gas condensation. The use of plasma sputtering to vaporize a target is applicable to a large range of materials; Ag, Au, Cu, and Si have been attempted to date. The source, combined with a time-of-flight mass filter, can produce clusters in the size range from 2 up to at least 70 000 atoms, depending on the target material, with a constant mass (M) resolution (M∕ΔM∼25) at an intensity that produces atomic monolayer coverage in as little as a few minutes. The source is also attached to an ultrahigh vacuum analysis chamber, which allows in situ surface chemical and structural analysis. Examples of cluster deposition experiments with the source are also presented.

We have explored the deposition of size-selected AgN+ clusters (N=50–200) onto the graphite surface (at room temperature) over the impact energy range of 250–2500 eV, via a combination of scanning tunneling microscopy experiments and molecular dynamics simulations. We show that the clusters are pinned to the surface when the impact energy exceeds a critical value, which is proportional to the cluster size, N, via the formation of a point defect at the impact site. This prevents lateral diffusion of the clusters even at room temperature.

We have investigated the impact of size-selected metal cluster ions ͑Ag n 2 ͒ on a covalently bonded substrate (graphite) over the energy range 15 -1500 eV by a combination of scanning tunneling microscopy and molecular dynamics simulations. The key result is that the fate of the cluster (penetration into the surface versus diffusion and aggregation on the surface), at intermediate energies, depends on the lateral localization of the cluster kinetic energy at specific surface sites and thus, for small clusters, on the orientation of the cluster and the target substrate site. [S0031-9007(98)07519-X]

We have investigated the implantation of Ag N ͑N 20 200͒ clusters into a graphite substrate over the range of energies ͑E͒ 0.75 6 keV using molecular dynamics simulations. We find that after implantation the silver clusters remain coherent, albeit amorphous, and rest at the bottom of an open tunnel in the graphite created by the impact. It is found that the implantation depth of the clusters varies linearly as E͞N 2͞3 . We conclude that the cluster is decelerated by a constant force proportional to its cross-sectional area. We also identify a threshold energy for surface penetration associated with elastic compression of the graphite substrate.

We have used the scanning tunnelling microscope (STM) to investigate the deposition of size-selected (Ag n ) + clusters, in the size range 50-400 atoms, on highly oriented pyrolytic graphite (HOPG). The cluster morphology has been explored as a function of impact energy. For high deposition energies (∼10 eV/atom) the clusters are pinned to the surface and flattened on impact. For lower impact energies (∼1 eV/atom) the footprint of the cluster on the surface is significantly reduced. By controlling the deposition parameters clusters have also been selectively sited at surface steps. Larger particles, arising from diffusion and aggegation of deposited clusters, are visible with the scanning electron microscope (SEM) but not the STM; we find definite evidence that the interaction with the STM tip disturbs these particles.