The physics of cluster-substrate interactions studied by scanning tunneling microscopy

Shihcher Lin, Purdue University

Abstract

This thesis research focuses on the interactions between nanometer-sized clusters and atomically flat substrates. In order to study this phenomenon, we built an ultrahigh vacuum scanning tunneling microscope (UHV-STM) facility, which is connected to a multiple expansion cluster source (MECS). With this facility we have been able to study the morphology of preformed Au clusters soft-landed on various substrates (Au, HOPG, PbS, MoS$\sb2,$ and H-terminated Si(111)) in vacuum. A major difficulty in STM studies of supported clusters is that the STM tip tends to move the clusters because the binding energy between the cluster and the substrate is too small. It was found that Au clusters supported on MoS$\sb2$ and H-terminated Si could be imaged reliably and reproducibly by STM. Due to the fact that electronic artifacts, tip-sample interactions and tip shape can distort STM images of supported clusters, STM was combined with non-contact atomic force microscopy (AFM) and transmission electron microscopy (TEM) to acquire quantitative information about the morphology of Au clusters supported on MoS$\sb2.$ In the case of preformed, nanometer diameter Au clusters, we find that the ratio of cluster diameter to cluster height is $\sim$1.5. Thus, there is substantial cluster deformation induced by the MoS$\sb2$ substrate. A simple model is proposed to interpret this finding. We also find that the extent of cluster deformation on H-terminated Si(111) is greater than it is on MoS$\sb2.$ Scanning tunneling spectroscopy (STS) measurements were carried out on nanometer diameter Au clusters supported on H-terminated Si(111). These I-V curves are discussed in terms of current models for the metal/semiconductor interface.

Degree

Ph.D.

Advisors

Andres, Purdue University.

Subject Area

Chemical engineering

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