Biomedical Engineering Reference
In-Depth Information
Fig. 3.24. Example of Kelvin probe and electric force microscopy. AFM height image (A, shaded
image), Kelvin probe (B), and EFM (C) images of carbon nanotubes on a gold surface. The images
are not all in exactly the same place; the red arrow highlights a connection between two nanotubes in
each image. Reproduced from [227], with permission.
V bias ¼
V DC þ
V AC sin t
ð
3
:
4
Þ
In other words, the AC voltage is oscillating at the resonant frequency of the cantilever
[224]. Thus, the probe's electric potential is varying at frequency ø . If the sample's
potential is not the same, the difference in electrical potential will cause the cantilever
to mechanically vibrate at the frequency
ø
, and which means that the electrical signal from
the photodetector will be modulated at
ø
. A feedback circuit then compares
ø
with
ø mod ,
and outputs a DC voltage to the sample that minimizes the oscillation at
ø mod . This occurs
when the applied potential V DC is equivalent to the surface potential V s . So the voltage V DC
that is require to minimize
ø mod is digitized with the A/D converter and displayed on the
PC as the potential image [225, 226]. By SKPM, absolute values of the sample work
function can be obtained if the tip is first calibrated against a reference sample of known
work function.
3.2.6 Electrochemical AFM
Although not really a separate mode, it is worth mentioning that it is rather simple to
study a surface as a function of applied potential using the AFM [228]. Changes in
sample topography with applied potential are the results of electrochemical reactions,
and so this technique is known as electrochemical force microscopy. In situ imaging of
such processes is achieved with an electrochemical cell which is a modified liquid cell
with the addition of electrodes to bias the sample and a potentiostat. By ramping the
applied potential to the oxidation or reduction potential of the surface during scanning,
or between scans, it is possible to directly observe oxidation or reduction processes on
the sample surface. Such processes tend to give rise to small (or slow) changes in
sample topography, hence the usefulness of electrochemical AFM. Furthermore, it
is possible, using more modifications of the instrument, to combine imaging with
electrochemical measurements at the nanoscale, a technique referred to as scanning
electrochemical AFM [229]. An example image showing results from electrochemical
AFM is shown in Figure 3.25.
 
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