Biomedical Engineering Reference
In-Depth Information
Time = 0 s
Time = 450 s
Time = 570 s
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Time = 720 s
Time = 1530 s
66.9 nm
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Fig. 3.25. Electrochemical AFM example. Images showing the morphology of a CdTe film during
electrochemical deposition of Au, at various times (as shown in figure) at a potential of
0.35 V.
Reproduced with permission from [230].
3.2.7 Thermal modes
It is possible to use derivatives of AFM to measure thermal properties of the sample
[231]. Typically, this is done by using a resistive probe, which can locally heat the
sample or measure the temperature locally, i.e. act as a thermometer. The first such
probes were the so-called Wollaston wire probes, which consist of a very fine platinum
wire bent into a v-shape. The apex of the v formed the tip of the probe. Later, micro-
machined probes, developed from silicon nitride cantilevers, with a palladium layer
which thins greatly at the tip apex, to act as the resistor, were developed. One common
experiment involves applying a potential to the probe, which heats the resistance. As the
sample is scanned (in contact mode), heat from the probe will flow into the sample, the
amount depending on the thermal properties of the sample, and a feedback circuit adjusts
the current flowing through the resistor, to keep the resistance, and thus the temperature,
constant. Plotting the current applied to the probe gives the thermal image, and a
topographical image is collected simultaneously. An example of the sort of data that
may be collected with this technique is shown in Figure 3.26. This method is commonly
termed scanning thermal microscopy (SThM). The thermal image in SThM is therefore a
map of thermal conductivity, although it might be necessary to deconvolve topographic
contributions [231]. By using temperature modulation (i.e. by supplying an AC current to
the resistor rather than a DC current), the depth sensitivity may be changed, allowing for
