Scanning tunneling microscopy: a natural for electrochemistry
Scanning tunneling microscopy: a natural for electrochemistry
microscopy: a natural for electrochemistry
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.
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
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.
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.
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
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
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
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
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