Biomedical Engineering Reference
In-Depth Information
Chapter 3
The range of available AFM modes and experiments are at the heart of modern AFM. The
wide variety of experiments that may be performed with an AFM make it a versatile,
powerful tool. Initially the only mode available for imaging was contact mode, and this
limited the types of samples that could be examined, the types of experiments that could be
performed, and the type of data that could be produced. Now there are a very large number
of possible modes of operation of AFM. For example, in 1999, Friedbacher
et al
.
attempted to list the names of all the SPM modes described, and arrived at more than 50
terms [94]. Some of these SPM modes were STM or SNOM (Scanning Nearfield Optical
Microscopy), but there are
at least
20 different modes of AFM. SNOM is an example of
using the close contact and position control of the AFM to measure properties of the
surface other than topography (in this case, optical properties). As shown in this chapter,
many of newer modes in AFM are along these lines: techniques that use the incredible
resolution provided by scanning a probe close to the surface with an AFM to measure
different properties of the sample surface on the nanoscale. SNOM (also sometimes called
NSOM, near-field scanning optical microscopy), is a very powerful technique combining
near-AFM resolution with the spectroscopic information that is available by using light-
based techniques. However, SNOM is not covered in this topic because although some
early SNOMs instruments were developed by modification of AFMs, experiments are now
generally carried out with specialized SNOM instruments, which are rather different from
a normal AFM. For reviews of SNOM, see [95, 96]. A table categorizing major techniques
in Scanning Probe Microscopy is shown in Figure 3.1.
For the purposes of making this a practical guide to AFM, in this chapter we concentrate
on techniques likely to be accessible and of interest to the reader. This means that we will
not describe in detail techniques which are not attainable with commercial AFMs without
significant modification, nor cover modes that have been described but not widely
adopted. Some more advanced techniques are covered in the applications section,
Chapter 7.
The basis of AFM as a microscopic technique is that it measures the topography of the
sample. As described in Chapter 1, the datasets generated in this way are not conventional
images, as produced by optical microscopy, but rather a map of height measurements.
These may be later transformed into a more naturalistic image with light shading,
perspective, etc. to help us picture the shape of the sample (this process is covered in
Chapter 5). In order to make these height measurements, a variety of modes have
developed, which can be divided into those modes which measure the static deflection
