Investigations at the Solid-Liquid Interface by Scanning Tunneling Microscopy.

Scanning tunneling microscopy (STM) has been used to investigate electrochemical processes at the solid-liquid interface. These investigations have been conducted in two specific areas, namely bioelectrochemistry and semiconductor electrochemistry. In the area of bioelectrochemistry, the STM was used to examine the morphology of the surface and determine the conditions necessary to electrochemically deposit a monolayer of nucleic acids. Parameters that were varied included: substrate material (graphite and gold), nucleic acid concentration (1.0-150 mug/ml), applied potential (between +2 and -2 V vs. Ag/AgCl) and potentiostatic pulse duration (10-180 s), and Tris buffer solution concentration (0.1-10 mM) and pH (7.3-8). Under most of the conditions studied, the deposited material was heterogeneously distributed over the surface as aggregates with small patches of isolated or loosely packed molecules. Conditions for repeatable homogeneous coverage were obtained by applying +1 V vs. Ag/AgCl to a gold substrate for one minute in a solution of 3 mM Tris buffer (pH 7.3) and a DNA concentration of 10 mug/ml. However, the homogeneous deposits consisted of a buffer salt complex with the nucleic acid. This salt complex prevented high resolution STM imaging of the nucleic acid. In the area of semiconductor electrochemistry, the STM was used to locally modify semiconducting surfaces on the nanometer scale. This was achieved by using the STM tip to either etch into the semiconductor surface or deposit gold onto the surface, while under solution. In the case of etching, the applied electric field existing between the tip and sample probably causes local oxidation of the surface. Subsequently, an etching solution removes the oxidized portion and leaves a depression in the sample. The oxide growth depended upon the length of time that the STM tip spent over a region. In the case of deposition, photoelectrochemical techniques were used to generate additional electrons in p-type GaAs(100). The STM tip bias was then used to bend the semiconductor bands to allow the photo -generated electrons to move toward the surface. This band bending occurred only beneath the STM tip and thus localized the gold deposition to this region. The size of the gold deposit depended upon the magnitude and duration of the potential applied to the STM tip.