Biomedical Engineering Reference
In-Depth Information
The EFM image after this process is shown in the third image. At this time, both the tip and
CNT are negatively charged, and thus a repulsive interaction occurs, leading to a positive
frequency shift. Line scans over the CNT in the EFM images are shown on the right. It is
interesting to observe that although the charge was injected at one point on the CNT
(indicated by the arrow in the left-most image), the nanotubes is homogeneously bright in
the EFM image, except for a small defect at the end, showing the spreading of the charge
through the nanotube. This effect can be used to detect inhomogeneities and defects in
CNTs [550].
Another advantage of AFM for this sort of study is that with AFM it is possible to
mechanically challenge the nanostructures in addition to electrically changing and probing
them. The relation between the electrical properties of CNTs and their mechanical
deformation is a topic of intense interest due to potential applications [573]. There are
several different ways in which AFM can be used to alter CNTs mechanically, and then
measure the effect on their electrical properties [550, 556, 558, 574]. In the example
shown in the lower part of Figure 7.17, two types of SWNTs are considered, semicon-
ducting and metallic nanotubes. The semiconducting nanotubes are able to maintain only a
small charge in EFM experiments similar to those shown at the top of Figure 7.17.
However, upon compressing the semiconducting nanotubes mechanically by the AFM
probe (shown in the figure by the green circles, which is the measured diameter of the
tubes), the behaviour of the semiconducting CNTs change, and approaches that of the
metallic CNTs (shown by the red triangles approaching the black squares). In this
example, AFM was used to change the sample, as well as to characterize the effects
both in terms of topographical and electrical changes. This allowed the direct observation
of mechanically induced semiconducting-metallic behaviour crossover which had been
previously only indirectly observed [558, 575].
The biological or life sciences constitute without a doubt, one of the most important
application areas for AFM. This is evident from the fact that nearly all AFMmanufacturers
build specialist models of AFM for biological sciences, and there even exists at least one
company that
only
makes AFMs designed specifically for applications in the life sciences.
This is despite the fact that
any
AFM instrument can be used for biological applications. In
fact, AFM itself came about partly in order to extend the possibilities of STM to biological
samples. Many innovations in AFM technique and instrumentation which are now used in
other application areas also came about due to the interests of biologists in the use of AFM,
such as IC-AFM, and later the extension of IC-AFM mode to use in liquid [576], and low-
noise force spectroscopy.
As a microscopy technique, AFM has several key advantages for biological application.
Probably the most important of these is the ability to work under physiological-like
conditions. Almost all biological processes occur in liquid, and often depend strongly on
the presence of certain salts, and the temperature of the solution. Many biological samples
also change their structures dramatically when dried. Therefore the ability of AFM to
image and measure samples in buffer solution, at 37
8
C, at any ionic strength or pH is of
vital importance to many biological experiments. Furthermore, AFM is particularly simple
