Alkanes
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Chain molecules consisting of -CH2- groups with slightly bulky
end groups -CH3 are chemically simple objects, which have some attractive
features for AFM studies. First of all, alkane layers on graphite
can be used as standard and calibration samples for imaging in the
sub-100 nm scale. In addition, these objects can be also considered
as primitive heterogeneous system or primitive block copolymer in
which -CH3 groups and -CH2- represent "blocks" dissimilar
in their size, mobility and mechanical properties. This is a reason
that these groups exhibit different contrast in he AFM images.
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| Fig.1 Height image of C390H782 normal alkane
layer on graphite. DP18/HI'RES probe.Scan size 1 µm. Height 3 nm. Image courtesy of
D. Ivanov. |
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Other interesting feature is an alkane arrangement on different
substrates, which is a result of competition between alkane molecules
interaction with a substrate and their crystallization behavior.
Therefore, alkane molecules either align parallel to the surface
or they form crystals with chains oriented perpendicular to the
substrate. AFM is one of few techniques than can be used for characterization
of these objects and their various transformations. The alkanes
and, especially, those with ultra long chains with number of carbon
atoms over 100 are excellent models of polyethylene - one of mass-produced
polymer. AFM studies of chain folding, crystallization and melting
in ultra long alkanes offer unique information about similar processes
in this and other polymers.
Use of most sharp probes is always preferable for high-resolution
studies of alkane layers. Yet the sharp tips can be less subjective
to damage when softer cantilevers are used. Therefore probes with
sharp tip should have stiffness less than 5 N/m. Actually, because
sharp tips experience less adhesive interactions therefore probes
with such tips can have very soft cantilevers and still be applied
for tapping measurements. See high resolution imaging for details.
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