Nanotubes and Nanowires
Though not as well known as their little brother, the
quantum dot, nanowires and nanotubes have their own unique uses that will get
any nano-scientist excited.
Nanowires
Like the quantum dot, a nanowire exhibits quantum
confinement. Instead of confinement in
three directions, a nanowire has confinement in two directions (or free
electron movement in 1-D). Instead of
looking like a sphere, it naturally looks like a wire or cylinder. Nanowires can be metal, semiconductor, or
some compound between the two.
Synthesis techniques are mainly focused on a CVD process
with a gold monolayer on the substrate.
When the substrate is heated up, the gold monolayer dewets (unbinds)
from the substrate in liquid form. From
there, the liquid gold coalesces into nanoclusters and will naturally
self-assemble into a grid of gold nanoclusters. The gold acts as the catalyst in the VLS model of nanowire
growth. VLS stands for
vapor-liquid-solid. As researchers
understand it, the source material enters the CVD chamber as a gas
(vapor!). From there, it enters the
gold nanoclusters as liquid form. Once
the percentage of source material has supersaturated the nanoclusters, it
begins to solidify and grow outward in a perfect single crystal nanowire! The diameter of the nanowire can be roughly
controlled by controlling the diameter of the nanoclusters. The length of a nanowire can be controlled
by leaving the source on until the desired length is reached.
Semiconductor nanowires are supremely important for the
future of nanoelectronics. They can be
doped and physically crossed to form p-n junctions that are the basis of all of
our modern electronics. We'll discuss
their applications in the applications section.
Nanotubes
Nanotubes are a bit different. They are an entirely new class of material in their own
right. Carbon nanotubes have generated
a lot of interest in the media because of their unique properties.
The structure of a nanotube is essentially a sheet of
graphite (carbon) that has folded over to form a tube. Carbon nanotubes, in particular, have
demonstrated unheard of electronic conduction properties. On the physical side, a carbon nanotube is
stronger, more ductile, and more elastic than any other manmade material. Unfortunately, they're so small that no bulk
material can be made right now that can take advantage of their strengths. They have, however, found their way into
many proof of concept demonstrations of nanoelectronics devices.
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