Biomedical Engineering Reference
In-Depth Information
were approximately 100-200 nm thick, and this highlights a strength of AFM-based
nanoindentation for analysis of polymer composites; namely the direct measurement of
mechanical property variation at the nanoscale.
The analysis shown in Figure 7.4 was very simple; the distance the AFM indented into
the sample was calculated by comparing the cantilever deflection with that obtained on a
stiff surface (the steel curve shown at the top of Figure 7.4). Further details of the
mechanical properties of polymers can be obtained, however. Thus, it's possible to obtain
parameters of the surface such as Young's modulus (
E
) or sample spring constant, which
can be compared to values obtained for bulk materials, or other materials at the nanoscale.
However, to obtain such parameters from AFM data, it is necessary to know or to measure
the shape of the probe, as well as the spring constant of the cantilever [168]. Methods to
measure these parameters were covered in Chapter 2. Furthermore, knowing the shape of
the probe, one must model it with an appropriate shape using the measured dimensions,
thus the indentation into the surface may be modelled such that
E
or the sample spring
constant can be obtained [419]. AFM-based nanoindentation is a very powerful technique
for the
in situ
characterization of the interfaces of heterogeneous polymer systems, and the
nature and extent of polymer mixing, as well as other surface and interface effects in
technologically important systems can be probed by this technique [168, 181, 422, 423].
7.1.3 Atomic-resolution imaging of crystal structures
One of the most exciting capabilities of AFM is the extremely high resolution that is
possible. Achieving atomic resolution, however, is only possible or even useful under
certain circumstances. Most solid materials are made up of a diverse collection of poorly
organized molecules, meaning that atomic resolution is almost meaningless. We can really
only interpret atomic-resolution images in extremely pure samples. For this reason,
atomic-resolution AFM is almost entirely limited to application in the physical sciences.
However, despite these limitations, some of the results available from such efforts are truly
astonishing, such as the work shown in Chapter 3 from Morita and co-authors enabling
discrimination of individual atoms using spectroscopic non-contact-mode AFM [8, 424].
Many atomic-resolution images produced by AFM are produced by non-contact-mode
AFM (using FM detection), and often carried out in high-vacuum conditions [425]. Thus,
these studies are out of the scope of most AFM systems, which need to be specially
adapted for FM detection or vacuum work. However, it should be remembered that with
great care, ultra-high resolution is also achievable in ambient/liquid conditions [372]. In
order to discuss atomic-resolution imaging, it's important to define what is meant by
atomic resolution. True atomic-resolution images allow the discrimination of individual
atoms. However, some images have been described as 'atomic resolution' that do not
allow this, but instead show the
average
arrangement of atoms on a surface. Thus, this
pseudo
-atomic resolution or 'atomic lattice resolution', as we shall refer to it here, does not
allow imaging of individual adatoms, or of atomic vacancies. This atomic lattice reso-
lution is rather simple to achieve with a normal AFM under ambient conditions. The main
requirements are a sharp, flexible probe, very fast scanning (
20 Hz), and a flat, very
clean surface (which can be easily obtained by cleaving mica or HOPG). Two examples of
atomic lattice resolution measured with AFM are shown in Figure 7.5. Both of these
images were obtained in contact-mode AFM in liquid (water and ethanol, respectively).
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