Biomedical Engineering Reference
In-Depth Information
Particular emphasis is given to the patterning of quasi 1D nanowire
and bicontinuous gyroid arrays, both of which are of particular
interest for device applications. The first study of these highly
ordered electrochemically patterned semiconductor arrays used
in a real device application is described in a dye-sensitized bulk
heterojunction solar cell.
2.1 INTRODUCTION
The engineering of material architectures and composite structures
on the nanometer length scale presents an extremely versatile route
to creating enhanced and novel functionalities. Spontaneous self-
assembly of constituent parts offers a rather attractive way around
the ever-increasing demands of patterning from the top down on
ever-smaller length scales. Block copolymers (BCPs), consisting of
block sequences of two or more distinct monomer units, are a class
of macromolecule that exhibit such self-assembly on the 10 nm
scale. The assembly is readily tunable; depending on molecular
characteristics (chemical composition and molecular weights) and
processing parameters (temperature or application of external
fields), the copolymer molecules organize themselves into a rich
spectrum of highly ordered, chemically heterogeneous phases on the
scale of a single polymer chain. The topology of these microphases
ranges from close-packed spheres, hexagonally packed cylinders and
bicontinuous gyroids to sheet-like lamellae and perforated lamellae.
Many proposed applications exploiting BCP patterns, for
example high-density memory, photovoltaics, battery electrodes,
and sensor technologies, are surface-supported, that is, in a film
geometry. Low-cost, large-area solution processing of films is no
problem for many polymers. We must, however, contend with the
additional complication of interactions with external interfaces,
which strongly influence the behavior of the molecular assembly. The
grand challenge to the application of BCP patterning in novel film
devices lies in simultaneous control of the microphase, coupled with
a way of transferring such model structures into active materials and
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