As the name implies, nanowires are one-dimensional wires measured in
nanometers; that is, their diameter is no more than just a few
0.000000001 meters (0.000000001 meter corresponds to 1 nanometer which
is around a thousandth of a single strand of human hair). At the
nano-dimension, quantum mechanics applies since the nanowires are so
fine that normal (or Newtonian-scale) physics could not be relied upon
to explain their behavior.
Manufacturing the Nanowire:
Nanowires are manufactured from different materials. Thus, there are
wires with different properties that have been acquired from the source
material(s). Metallic nanowires are made from metals like nickel,
platinum or gold. Semi-conducting nanowires are made from the
semi-conducting silicon or indium phosphide. Insulating nanowires are
made from materials like silicon dioxide or titanium dioxide. Finally,
molecular nanowires are made from molecules of organic or inorganic
substances.
There are several approaches to manufacturing nanowires, all of which
can only be undertaken at the present time under laboratory conditions.
They are the following:
Suspension:
One suspension method is heating the source metal to its melting
point, touching the tip of a scanning tunneling microscope (STM) on its
surface, and then pulling the STM away. Some of the source material will
naturally cling to the STM and a strand of ultra-fine wire will
consequently be formed. This process of nanowire manufacture is somewhat
akin to touching one of your fork's tines to melting cheese and then
pulling the fork away; there will be a thin strand of cheese from the
dish to your fork.
One other suspension method is whittling down the source material
until it becomes nano-dimensional. This process of reduction is done
through chemical etching (where chemical reactions are used to reduce
the size of the source material) or though particle bombardment (where
molecules or atoms traveling at extremely high speeds are directed
towards the source material and the impact gradually reduces the
material's size).
Deposition
In deposition, particles of the source material are deposited on a
host material or substrate. For instance, a deposited metallic nanowire
may be made by taking a piece of plastic and then depositing metallic
atoms on its surface. The particles that have been deposited comprise
the nanowire while the plastic merely plays 'host' to the wire.
Synthesis
The most popular approach at the moment is a process called
Vapor-Liquid-Solid (VLS) synthesis which uses a source material like
silane gas (the 'vapor' – this is a series of silicon atoms bound to
hydrogen atoms) and a catalyst of liquid metal (the 'liquid' – liquid
gold is often used) which have been manufactured or self assembled to
become nano-dimensional.
The gas is exposed to the catalyst and this begins to soak into and
saturate the latter. After supersaturation is attained, the gas
solidifies and the wire begins to grow. Once the target length has been
achieved, the gas feed is simply turned off. The synthesis part of the
process comes when the original source material is, at certain
intervals, alternated with other materials to form compound nanowires.
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