Biomedical Engineering Reference
In-Depth Information
Phase imaging is also sometimes particularly useful for AFM imaging where the sample
is dynamic, i.e. for imaging moving systems [472, 479].
Without a doubt, AFM is a vital technique for nanotechnology and the nanosciences. The
combination of extremely high-resolution, three-dimensional information, and local prop-
erty measurement means that AFM is considered one of the most important tools in
nanotechnology. AFM is used to make important discoveries in all areas of nanoscience.
For many systems with dimensions of the order of 10 nm, AFM-based techniques are the
only solution to make dimensional, electrical, magnetic and mechanical measurements
with the accuracy required [395, 480-482]. In addition, AFM can be used to alter and even
to build nanostructures [249, 266]. The examples chosen for this chapter are just a few
important examples of the areas where AFM has been shown to be particularly useful.
7.2.1 Nanoparticle measurement
Nanoparticles are probably the most widespread nanostructures, and over the last 10 years
or so there has been an enormous increase in the interest in their production due to their
relatively simple preparation combined with unique properties. Many of the unique
properties of nanoparticles are directly related to their size, for example the photolumi-
nescence of quantum dots which changes significantly if the dots grow a few angstroms in
size [483]. For this reason, it is very important to have a tool to characterize with extreme
accuracy the size of such particles. Of course, the AFM is also capable of providing more
information than just topography, and many other properties of nanoparticles have been
probed as well [221, 278, 484-486].
AFM can be used to measure an extremely wide range of nanoparticles including
different metal nanoparticles [279, 290, 487-490], metal oxide particles [491], many
types of composite metal/organic particles [123, 280, 492-494], synthetic polymer par-
ticles [480, 495, 496], biopolymer nanoparticles [283], nanorods [497, 498], quantum dots
[499] and others [79, 500]. Some AFM images of nanoparticles with different morphology
are shown in Figure 7.11. For pure metallic nanoparticles, TEM is often considered the
technique of choice, because metallic particles have high contrast in TEM, don't require
coating, and are not affected by vacuum. For such particles, TEM will be just as quick, if
not quicker than AFM for measurement of many particles. In Figure 7.11 it is shown how,
with care, TEM diameters and AFM particle heights correlate very closely for metallic
particles. For hybrid particles with metallic cores and organic coatings, TEM will only
show the metallic core whereas AFM will measure both. In these circumstances, therefore,
it can be useful to combine the two techniques [280]. However, some particles, such as
polymer particles will require extensive sample preparation for TEM imaging (and even
then, imaging is indirect), so such particles are better suited to AFM analysis. AFM
analysis of coated nanoparticles can even distinguish the thickness of subtly different
coatings [493].
Carrying out imaging of nanoparticles for size measurements is exceptionally simple in
AFM. Typically, all the analyst needs to do is to deposit a droplet of nanoparticle
