Scanning tunneling microscopy in electrochemical environments

Citation

Heben, Michael Joseph (1990) Scanning tunneling microscopy in electrochemical environments. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8myg-1s98. https://resolver.caltech.edu/CaltechETD:etd-06122007-104233

Abstract

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Scanning tunneling microscopy (STM) has been demonstrated to be an important analytical tool for the in situ characterization of electrode surfaces. However, in an electrochemical environment, the tip/sample bias necessary for electron tunneling can also drive heterogeneous charge transfer reactions between the solution-contacted portions of the tip and sample. This faradaic current acts in parallel to the normally measured tunneling current and the measured sum may not exhibit the expected exponential dependence on tip/sample separation. As a result, the interpretation of STM-derived data is not straightforward. The faradaic current component can be selectively reduced by coating the shaft and apex of the STM tip with insulating layers. Two tip-insulating procedures were developed, employing thin layers of either glass or polymer, that effectively reduce tip/sample faradaic currents even in the presence of redox active ions. The amount of exposed metal remaining after insulation was characterized with cyclic voltammetry. Using these insulated STM tips in an instrument constructed specifically for electrochemical investigations, atomic resolution images of highly ordered pyrolytic graphite were obtained in deionized water and in aqueous 0.1 [...] NaCl solutions at tip biases of ± 1.5 V, and in aqueous solutions simultaneously containing 0.1 [...], 0.1 [...] and 1.0 [...] NaCl at biases of ± 0.8 V. Images of MoS2 were obtained in 0.5 [...] NaClO4 at tip potentials as large as + 0.5 V. The distance dependence of the tip/sample faradaic current was investigated for the two different kinds of coated tips. A reduction in the faradaic current at tip/sample spacings less than ca 0.3 [mu]m was observed for polymer-coated tips. No such reduction was observed for glass-coated tips. Thin-layer current enhancements were observed for these tips when the amount of exposed metal area was relatively large (ca > 50 nm2). The tip insulation techniques were also used to produce, with minor modifications, conical and hemispherical ultramicroelectrodes. These were characterized with cyclic voltammetry and chronoamperometry and were applied to the measurement of the heterogeneous rate constant for Ru[...] reduction.

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