Biomedical Engineering Reference
In-Depth Information
electrolytes. In some cases, this problem can be avoided by cross-
linking the template, for example by UV-cross-linking of polystyrene.
Aqueous electrolytes normally present no such solubility problem
but must often contain additives such as alcohols to reduce their
surface tension and ensure wetting of the internal pore structure
[55]. Water-wettable pores can be achieved by chemical modification,
for example using triblock copolymers with a hydrophobic central
block that is left lining the pore surface [75].
The temperature stability of the template is dictated largely by
the polymer glass transition temperature, which for polystyrene
is found around 100
°
C (again further stability can be achieved
using cross-linking). Polymer substrates are less robust than
many inorganic nanostructures (and therefore more sensitive to
extreme electrochemical deposition conditions). This apparent
drawback becomes a significant advantage for applications in
which freestanding electrodeposited nanostructures are desirable,
for example, the patterning of high-surface-area capacitive or
catalytic coatings, batteries, and photovoltaics [76, 77]. Non-cross-
linked polymers can simply be rinsed away in an organic solvent.
Alternatively, dry etching by exposure to UV-ozone or moderately
high temperature (300-400
°
C in air) ensures that the polymer is
degraded without exposing the templated nanostructures to a drying
liquid phase. Capillary bridges formed by the receding liquid exert a
powerful attractive force between adjacent objects, and often lead
to collapse of freestanding nanostructures standing array. Relatively
elaborate processing, such as supercritical drying, is required to
keep these structures intact.
2.4.4 Nanowire Replicaion: Cylinder-Forming BCP
Templates
2.4.4.1 In-plane nanowires
The extremely high aspect ratio of the cylindrical microphase makes
it a natural candidate for templating arrays of nanowires. In-plane
wires can be patterned from very thin films (comparable to domain
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