Scanning tunneling microscopy: a natural for electrochemistry I. Introduction In a few years since the pioneering work of G. Binnig and H. Rohrer, the scanning tunneling microscope (STM) has evolved into a powerful analytical instrument. STMs operating in vacuum have yielded useful detailed information on conductor and semiconductor surface reconstructions and even molecular and atomic adsorbates.
It is clear now that STMs can operate not only in vacuum, but also with the samples covered with electrolytes. Electrolytes, though ionic conductors, are insulators as far as electron flow is concerned. In means, that electron tunneling can also occur in electrolytes. The basic principles of scanning tunneling microscopy are simple. A very sharp tip, mounted on a piezoelectric 3-dimensional
XYZ scanner, is positioned close enough to the surface of a sample for an electron tunneling current to flow between the tip and the surface. The tunneling current is the function of the gap between the tip and the surface. The whole system is controlled with a special computer program. As the tip scans over the surface, applying voltage to the XY parts of the scanner, it traces the contours as small as a fraction of an atomic diameter.
The feedback system applies voltage pulses to the Z part to keep the tunneling current constant. Thus, one scan of STM is just a plot of the voltage the feedback system applies to the Z part versus the voltage the scanning system applies to the x part. II. Theory STM is capable of giving images that appear to be simply topographs of surfaces.
This view is adequate in many cases, especially when the variations of Z height are large compared to the so called “characteristic height” which is the height of electronic “atmospheres” surrounding the tip and the sample. The key to the high resolution provided by STM is the rapid change of the tunneling current with distance between the tip and the surface. According to it, if the feedback system keeps the tunneling current constant within 10%, the distance