Scanning tunneling microscopy: a natural for electrochemistry

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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 remains constant to within a fraction of an atomic diameter. III. Instrumentation There has been a tendency to simplification of STMs since the time of their initial development. Nowdays an average STM does not require a high vacuum conditions and cryogenic operational temperatures. There is a number of

commercial STM manufacturers, and a commercial STM is considered to be more convenient than the home-built one. Two points are vital for successful application of an STM for research, and one should pay attention to them before purchasing or designing a microscope. The first one is vibration isolation. It is impossible to achieve atomic resolution images of good quality, if no vibration protection is provided. There is a wide variety of vibration isolation platforms available, but none of them looks as good as a piece of concrete suspended by rubber cords. The second point is STM’s ability to aquire images rapidly. An STM that can not aquire an entire image in less than 10 seconds can be useful for ultra-high-vacuum applications only, not for electrochemistry. The reason are

thermal drifts caused by various reasons. IV. Tips The ideal tip for use in solutions would have its entire surface insulated except for the terminal atom of the tunneling probe. It is known that a voltage applied between any two electrodes in solution drives electrochemical process at the electrode surface and result in a current whose amplitude depends on the solution, the electrode surfaces, and the applied voltage. For a given set of these three parameters, the total current can be minimized by minimizing the uninsulated surface of the tip. In principle, only the last atom of the tip needs to be conductive for tunneling, the rest of the exposed tip only serves to increase the unwanted faradaic currents. Tip isolation can be done with glass and, furthermore, with SiO2. Still,