Implantation and electron emission in cluster-surface collisions

Abstract

Microscopic processes induced by the controlled deposition of mass selected silver clusters on graphite (HOPG) and Pt(111) are investigated. The implantation of silver clusters into the HOPG surface is analyzed. The first step consists in a systematic study of the implantation depths of AgN+ (N=1,3,7,9,13) clusters into HOPG, as a function of the cluster size and of the incoming energy. This is achieved by controlling the thermal oxidation of the bombarded graphite surface. This process results in etching of the cluster-induced defects to form pits which grow laterally while maintaining the depth of the implanted cluster. The morphology of the surface is characterized by the STM method, which provides information on the microscopic structure of the examined sample. We observe a scaling of the implantation depth with the momentum of the cluster, in agreement with recent results reported in the literature. In particular, a universal behavior is recognized when scaling the momentum with the cluster projected surface. Within this model, we find that the real geometry of the cluster plays a dominant role. It is also explored whether the single cluster behaves as a sum of independent atoms, or if molecular phenomena are present. In particular, we find molecular effects in the stopping power that the cluster experiences in penetrating the substrate. The electron emission induced by cluster-surface collisions is presented as a function of the two different employed substrates (HOPG and Pt(111)), and of size and energy of the incoming AgN+ (N=1,2,3,4,5,7,8,9) clusters. In order to understand the origin of emitted electrons, we investigate the different electron emission and charge transfer processes during the collision. Emission is observed below the classical threshold and results are interpreted within a recent model based on the semi-localization of valence electrons. The substrate itself plays a role in the electron emission processes, and higher emission yields are measured for clusters impact on the Pt(111) target. Molecular effects are also investigated. For both substrates we find a size-dependent sublinear effect at low velocities and a superlinear effect at higher velocities, similar to the case of hydrogen projectiles reported in the literature. We try to find oscillations in the electron emission yield, which would bear information on the charge-exchange processes during the collision as well as on the electronic structure of the cluster and the substrate. Such oscillations - recently suggested in Meiwes-Broer's work - are not clearly identified in our data.