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, islolation reduces the intrusiveness of the probe on the surfaces themselves, so the isolating layer should be as thin as possible.

V. Results

Scans acquired during the process of electroplating of graphite with Au demonstrated that a graphite surface can be imaged under a commercial gold plating solution, then plated, and then imaged again without removing it from the plating solution.

The special feature of the experimental technique that was used was that the tip itself was not an electrode. It was placed in a tube-type electrode and isolated from the latter. During the process of plating the tip was removed into the tube, so that it couldn't influence the process. The STM used in in the experiment allowed to immerse both the sample and the tip 2 mm below the surface of the plating solution. It was found that the piezo electrodes develop a surface conductivity due to the humidity above the solution which can be great enough to allow coupling of substantial currents from the piezo electrodes to the tunneling tip. One of the possible solutions of this problem is use of the sealed sample cells.

Ag deposition on graphite was carried out in several steps, in attempt to see the initial phase of deposition. The relatively narrow field of view of the STM used didn't allow to do so, and the backward consequence of operations was undertaken. The silver was removed from the surface of graphite electrochemically in a series of oxidizing voltage pulses, and finally the image of an isolated Ag island was achieved. Still, the mechanism of the Ag deposition is not clear yet, since it is unknown, if it follows either island or layer plus island model. It was also showed that atomic resolution is not the function of the tip alone, but also of the sample.

In contrast to the study on graphite, a similar study on Au (111) surface did reveal the initial stages of electroplating. So, this study proved that STM can be applied to the studies of plating on a metal surfaces in situ.

Experiments that were carried out with DNA also were successful, and the ability of STM to provide images of thin insulating materials as DNA have important applications for both chemistry and biology.

Multiple research have been done with different types of materials. So, Itaya and Sugawara observed electrodeposition of Pt on graphite; Itaya et al. imaged Pt electrode in sulfuric acid solution; Morita et al. observed the topography of Au, Ag and Pt foils in water and aqueous solutions of potassium chlorate and sodium perchlorate. Drake et al. managed to image the corrosion of iron.

VI. Local surface modification

A very useful application of STM is the use of the STM tip to directly modify surfaces on the atomic and nanometer scale. A number of research groups performed experiments in this field. The Basel group fabricated nanometer-scale structures by local melting of glassy metal substrates. DeLozanne used STM tip as a microelectrode to induce localized organometallic vapor phase deposition of cadmium. Jaklevic and Elie produced nanometer depressions by touching a gold single crystal with tip and observed their subsequent diffusion. Clarke's group intended gold single crystals by tip touching and made hillocks by briefly increasing the tunneling current to 1 mA.

Authors found that if a critical voltage of 2.8 V is applied to the tip, a 10-nm diameter and 2-nm deep depression is created in the gold surface that is immersed into fluorocarbon grease (the nonpolar grease seems to be important to this process). Operations in air, water and aqueous solutions weren't successful at the voltage biases higher than 2 V due to the disruption of the sample surface.

One of the problems is that any modifications done to the surfaces in air do not last for long. Normally it takes about one day from thermal drifts and other effects to destroy them. Luckily, Foster and Frommer showed that the same modifications can be done to graphite surface as well, and if the precise voltage pulses are used, the modifications can be saved for much longer. This effect can lead in future to the new type of high-density data storage.

Bard et al. have demonstrated another technique for local surface modifications with an STM: the tunneling tip can be used to drive local photoelectrochemistry, resulting in the straight line or L-shaped line (see examples) that mirror the path taken by the tip with applied voltage.

VII. Summary

As it can be seen from this paper, STM can be extremely useful in electrochemical studies. It is capable of providing atomic resolution images of samples in water and many aqueous solutions. It is also capable of local surface modification by a variety of methods including local electrochemistry driven by the tunneling tip.