Biomedical Engineering Reference
In-Depth Information
Typically one of two methods is used for preparing nanotube samples for AFM imaging:
catalyst growth or deposition. Catalyst growth is the best method for creating a clean
sample for studying the unique properties of single-wall nanotubes. When preparing
carbon nanotube samples for AFM imaging with deposition, it is important to use a
dispersant. Very diluted dispersant suspensions of carbon nanotubes are spin coated on a
silicon wafer or other flat substrate, rinsed thoroughly with water, and then dried in air.
Any commercial spin-coater may be used.
Other solid samples
Metals or other solid samples may be imaged with little or no sample preparation.
Cleaning (especially degreasing) can be required for some samples, and large samples
may need to be cut to size. The lack of sample preparation for most solid samples is a great
advantage of AFM, and means overall imaging speed with such samples can be higher
than for electron microscopy. As with other microscopy techniques, polishing is required
in order to observe metal grains [318].
4.1.1 Substrates for AFM
When preparing samples for AFM, especially particulate samples, a substrate must be
chosen on which to mount the samples. In the case of very large samples, or very concen-
trated preparations, the nature of the substrate can be unimportant, but for many cases, it is
crucial for correct sample preparation and good results. This is particularly important for
high-resolution work, and looking at individual molecules in particular, for which an
atomically flat substrate is usually required. For imaging of larger features, a substrate
with a higher roughness can be adequate. As well as the roughness, the chemical nature of
the substrate can be important. The intrinsic nature of the substrate is important in deter-
mining whether particular samples adhere well, and in addition, if substrate treatment is
required some substrates are easier to modify than others. For example, highly oriented
pyrolitic graphite (HOPG) is a commonly used substrate that is very simple to obtain in
atomic flatness. This is because, along with mica, it is a layered material that is easily
cleaved. Cleaving such materials exposes atomically flat faces, completely free of contam-
ination. However, chemical modification of HOPG is not simple. On the other hand, gold is a
highly stable material that is extremely simple to modify chemically, but while it is possible
to produce extremely flat surfaces with it, it is considerably more difficult than for HOPG or
mica. Table 4.1 summarizes some properties of commonly used substrates for AFM.
As shown in Figure 4.1, after sample preparation and placing the sample in the instrument,
the next step is to insert a probe into the AFM. It is possible that a probe will already be
inserted, but when beginning with a new set of experiments, a new probe is usually
inserted. Great care must be taken when handling the cantilever chips as they are very
small and very delicate. A clean pair of tweezers with flat tips helps. Usually the probe
holder will have a slot for the chip and have some sort of small spring or clip to hold it in
