Scanning tunneling microscopy and spectroscopy studies of small metallic clusters
Abstract
Experimental studies on small clusters of atoms are a part of the growing field of nanometer-scale science. In this thesis work, small nanometer-sized clusters of metal atoms have been studied with the newly developed technique of scanning tunneling microscopy (STM) and spectroscopy (STS). Physics and chemistry of small metallic clusters are interesting because clusters have novel properties due to their extremely small size compared to bulk samples. To focus on and study a cluster that is one to ten nanometers in diameter require a surface sensitive and highly localized experimental technique. The technique we used for this study is the STM, which is a powerful new technique for surface science studies. Not only topographic pictures of various kinds of surfaces can be routinely obtained, but also electronic properties of surfaces can be studied. We will discuss in this thesis the construction of a "pocket-sized" STM and its performances through a series of examples. A multiple expansion clusters source (MECS) was used to form a molecular beam of clusters (mostly gold clusters) and deposit them on atomically flat substrates of gold, platinum, or graphite. Experimental results show that the supported clusters have shapes that are deformed from the preformed spherical ones. The flat spherical cap shapes of clusters are explained by a model which emphasizes the internal stress inside a nanometer-sized cluster. The agreement between our experimental results and the model is excellent. A very useful aspect of the STM is its capability to study a cluster with tunneling current-bias voltage (I-V) measurements. We have performed I-V measurements on cluster samples and observed significant differences between I-V curves on the bare substrates and on supported clusters. Such information reveals electronic properties of supported metallic clusters. Another interesting area in nanometer-scale science is surface modification on nanometer scales. We have successfully used our STM to write complex symbols in atomically flat gold surfaces. The conditions require to electroetch nanometer-sized craters in gold substrates are identified. Reproducible modifications are demonstrated. Letters and complex symbols with line widths as small as 2 nm have been written. Experiments show that a good tunneling tip is not destroyed by the writing process.
Degree
Ph.D.
Advisors
Reifenberger, Purdue University.
Subject Area
Condensation
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