Biomedical Engineering Reference
In-Depth Information
the AFM cantilever, as the side of the probe tip pushed on the tubes laterally. Before this,
the tubes were also imaged by normal AFM, so that both the imaging and mechanical
testing capabilities of the AFM were used. One of the major difficulties in this sort of
experiment is the problem of how to immobilize the nanotubes while leaving a potion free
for mechanical testing [482]. Once testing occurs, the data must be further analysed in
order to obtain mechanical parameters of interest, and how the fixed end of the rod is
immobilized is important here, too. In the paper by Wong
et al
., tubes were randomly
immobilized above gaps in a substrate, and SiO
2
was used to fix one end of the rod to the
substrate. This process is inherently random, so AFM imaging is required to find suitable
tubes. The LFM experiments were carried out along the length of the tubes, and it was
noted that the carbon nanotubes appeared to buckle at a certain applied force, which was
the same wherever along the tube the force was applied. This is shown in Figure 7.12.
Another way to test nanotube structures is by axial compression [505]. This may be
carried out in one of two ways; the first method is via attachment to or growing the tube on,
the AFM probe, and then pushing it against a solid surface [511]. The second method
involves growing a sparse 'forest' of nanotube structures which are grown on or mounted
in a substrate, and then uses an AFM probe to compress them by pushing towards the
surface [512]. An illustration of the force curves recorder by this technique is shown in
Figure 7.13. Application of forces large enough to bend the fibres through almost 180
can
be carried out cyclically, as it does not result in bond breakage, due to the remarkable
flexibility of both single- and double-walled CNTs [502, 513]. Finally, it should be pointed
out that the related two-dimensional material graphene can also be probed by the same and
similar mechanical AFM-based techniques, which has produced some spectacular results,
notably showing graphene to be the strongest material yet tested [514-516].
8
7.2.3 Nanodevice construction with the AFM
Ever since Richard Feynman's famous speech 'There's plenty of room at the bottom', a
major goal of nanoscience has been the bottom-up assembly of useful devices [517].
Bottom-up assembly means fabricating nanodevices from small components (e.g. atoms or
molecules) rather than the traditional ('top-down') approaches of assembling nanostruc-
tures by assembly and removal of parts of large components, i.e. lithography.
There are several possible approaches to bottom-up assembly, but these can be broadly
grouped into two categories: bulk techniques and nanomanipulation techniques. Bulk
techniques are mostly based on careful manipulation of the chemical or biochemical
properties of building blocks such that they self-assemble into the desired structures.
These sorts of techniques can produce amazing structures, and can produce them in
large quantities but the complexity of the devices produced is limited, the manipulator's
control over the devices is very limited, and their structures are usually determined
indirectly. Nanomanipulation, on the other hand, involves assembling the tiny building
blocks directly, one at a time. This approach has the advantage of finer control over the
structures formed, and highly complex structures can be formed; on the other hand, it is
highly laborious, and is better suited to experimentation rather than mass-production.
AFM-based systems are well suited to this sort of task, due to the possibility to move the
probe with sub-nanometre accuracy. An additional advantage of AFM for this task is the
ability to use the instrument as a sensor as well as the manipulator.
