Geometric and electronic properties of size-selected metal clusters on surfaces

摘要

Within this thesis, we present a project regarding metallic clusters on surfaces. We use scanning tunneling microscopy to investigate their geometric structure and scanning tunneling spectroscopy and ultraviolet photoelectron spectroscopy to study their electronic properties. A new cluster deposition machine built within a priority program of the German Research Foundation (DFG) [1] allows the investigation of size-selected clusters on surfaces.In a first set of experiments, we produced clusters by metal island growth on rare-gas multi-layers and investigated several combinations of different cluster materials on vari-ous rare gases on miscellaneous substrates. On a rare gas film of 60-monolayer thickness, the clusters are electronically decoupled from the substrate. This leads to a change of the reference energy, which is no longer the FERMI energy but the vacuum energy of the substrate. In this sense, we use the term free clusters on substrate.In a second part, after the successful setup and initial operation of the cluster deposition machine, we use a magnetron sputter gas aggregation cluster source [2] and a semi-continuous time-of-flight mass selector [3] to deposit mass-selected silver clusters (number of atoms n = 40, 55, 80, ...,923, and 2130) onto different substrates.In particular, mass-selected silver clusters with closed icosahedral shells deposited on a gold surface functionalized with a C60 monolayer show very interesting results. Despite of using five different silver cluster sizes between 55 and 923 atoms, we observed for all samples clusters with almost the same average cluster height of 1.7 nm. In addition, for Ag561 and Ag923 clusters we observed also larger clusters that roughly agree with the expected cluster sizes. Our present hypothesis is that the clusters deposited at room temperature, exhibit rather a metastable than an equilibrium shape.[1] DFG priority program SPP 1153: Clusters in Contact with Surfaces - Electronic Structure and Magnetism, website: http://www.ieap.uni-kiel.de/surface/ag-berndt/spp/index.html.[2] H. HABERLAND, M. MALL, M. MOSELER, Y. QIANG, Th. REINERS, and Y. THURNER: Filling of micron-sized contact holes with copper by energetic cluster impact, J. Vac. Sci. Technol. A 12, 2925 (1994).[3] B. v. ISSENDORFF and R. E. PALMER: A new high transmission infinite range mass selector for cluster and nanoparticle beams, Review of Scientific Instruments 70, 4497 (1999).