Tunnelling spectroscopy on silver islands and large deposited silver clusters on Ge(001)

Morphology and electronic properties of silver islands and deposited silver clusters on Ge(001) have been studied using scanning tunnelling microscopy (STM) and spectroscopy (STS) at low temperatures. Already the clean surface bears an interesting electronic structure, which is obvious from the STS. The tunnelling spectra exhibit strong peaks associated with dangling bond-derived surface states and an antibonding σ-state of the surface dimer. For silver islands of only few monolayers in height, complex spectra are interpreted to be dominated by metal–semiconductor interface states. These islands show energy gaps which are not observed for larger ones beyond 1 nm in height. Spectra of the larger islands contain a series of distinct peaks originating from lateral and three-dimensional electron confinement, respectively. Silver clusters – preformed in the gas phase using a cluster source – have been fabricated, size selected and deposited onto germanium(001). In tunnelling spectra dips at the Fermi level are accompanied by two maxima. These characteristics seem to be almost independent from the cluster size. Additional weak structures are found at higher bias voltages, which are understood in terms of quantized states. PACS 36.40.Cg; 61.46.+w; 73.20.At