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.