Measurements of solvent effects on local barrier heights in scanning tunneling microscopy

Citation

Longin, Teresa L. (1995) Measurements of solvent effects on local barrier heights in scanning tunneling microscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/nyj2-gw70. https://resolver.caltech.edu/CaltechETD:etd-10172007-085145

Abstract

Scanning tunneling microscopy (STM) has proven to be a powerful tool for obtaining atomic resolution images of surfaces and adsorbates on surfaces. In addition, the technique can be used to make highly localized electronic measurements of surfaces, adsorbates, and molecules in the tunnel gap. Such measurements, generally known as scanning tunneling spectroscopy (STS), can probe either the electronic density of states or the effective barrier height. Studying the effects of solvent on the effective barrier height is particularly interesting, since that is a direct measure of the bridging capacity of the solvent and has analogs in heterogeneous electron transfer and electron transfer in proteins and frozen glasses. Unfortunately, the effective barrier height as measured in STM experiments is very sensitive to contamination on the sample surface and on the tip, and it is difficult to differentiate between solvent effects and contamination effects. We attempted to measure effective barrier heights in air and in hexadecane using a solvent compatible STM and a sample whose surface is resistant to contamination. The cleanliness and order of the sample surface was determined before and after STM experiments using LEED and XPS, and estimates were made of the degree of contamination. Unfortunately, the barrier heights in air were much lower than expected (<1 eV) and indistinguishable from those measured in hexadecane.

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