Electrodeposition
Electrodeposition is another example of an old synthesis
technique that has been recently applied towards the creation of
nanostructures. The premise of the
technique is quite simple. An applied
electric field draws precursor ions towards the substrate's surface. Once the ion reaches the surface, it
chemically bonds with certain bonding sites and stays there. This technique is great for creating
monolayers and thin films to substrates that are conductive.
For nanotechnology applications, some additional tweaking
must be done to the precursors or the substrate. The most common use of electroplating in nanotechnology comes in
the form of nanocrystalline metals.
When metals form in the solid state, they are not a perfect single
crystal. Instead, many identical crystals
lie side by side with random orientations.
These patches are known as grains.
Under traditional circumstances, these grains have sizes in the
micrometer scale.
Researchers have been able to alter the growth conditions to
produce grains with diameters in the nanometer scale. This is desirable because a metal's strength is almost entirely
dictated by its grain size. The scaling
that governs strength and grain size is called the Hall-Petch relationship. It states that a metal's hardness
varies inversely with the root of its grain diameter. The smaller the grain, the higher the hardness. There's a limit to how small a grain can get
before this relationship breaks down.
What happens beyond this breakdown is still under debate.
To create nanocrystalline metals with electrodeposition, it's
important to control the two major modes of grain evolution: nucleation and
growth. Nucleation is a term used to
describe how often a random arrangement of incoming atoms will spontaneously
order itself into a crystal. Growth is
the term used to describe how quickly these nucleated regions spread
outwards. To create nanocrystalline
metals, one needs to encourage nucleation and limit growth. This can be accomplished in two ways: pulse
plating and dopants. By pulsing the
electric field, the electrodeposition only happens in short bursts. This part encourages nucleation. Dopants are contaminating atoms that can be
added through a variety processes. They
act as an effective barrier for growth by tying up the growth fronts with
defects. The combination of the two has
successfully yielded many varieties of nanocrystalline metals.
In a spin on the process, researchers have found that
reverse electrodeposition, or electro-decomposition, can be used to create nanopores. If a substrate is treated with an array of physical
indentations, a reverse field will cause the indentations to begin sinking into
the material in a vertical column. The
result is a large grid of nanopores that can be used to grown nanowires.
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