Tunneling Microscopy of Surfaces Immersed in Aqueous Solutions.

An understanding of the solid-liquid interface, specifically the solid-water interface, is critical to the understanding of important processes in chemistry. Fewer instruments are available to study processes occurring at surfaces in water than processes occurring at surfaces in vacuum. The scanning tunneling microscope (STM), which has revolutionized the study of surfaces in vacuum with its atomic-resolution images, now can also provide atomic -resolution images of surfaces in water. Steps taken by the tunneling microscopy group at UCSB which led to this new knowledge include the following: (1) measuring current vs. voltage (I-V) curves of metal-insulator-semiconductor tunnel junctions that featured a mechanically variable insulating gap or "SET junction" (at 4.2 K the superconducting energy gap of lead could be observed); (2) building a tunneling microscope using design principles learned from these mechanically variable tunnel junctions; (3) operating a tunneling microscope in electrically insulating liquids such as paraffin oil and liquid nitrogen; (4) obtaining atomic-resolution images of the surface of highly oriented-pyrolytic graphite (HOPG) in air; (5) observing that partially insulated tunneling tips can minimize the current that is due to ions in an aqueous solution; (6) obtaining atomic resolution on graphite in de-ionized water; and (7) obtaining atomic resolution on graphite in 0.05 molar silver perchlorate solution. We use this new capability of tunneling microscopes to study (1) the corrosion of iron in saline solution; (2) the electrochemical deposition of silver onto graphite, yielding an island of deposited silver on an otherwise bare graphite surface; and (3) the surface of GaAs (100), immersed in a stabilizing potassium hydroxide solution.