Chemical vapor deposition (CVD) is an example of a bottom-up approach. In general, CVD refers to a group of processes where solids form out of a gaseous phase. Scientists deposit catalysts (such as gold nanoparticles) on a base, called a substrate. The catalysts act as an attraction site for nanowire formation. Scientists put the substrate in a chamber with a gas containing the appropriate element, such as silicon, and the atoms in the gas do all the work. First, atoms in the gas attach to atoms in the catalysts, then additional gas atoms attach to those atoms, and so on, creating a chain or wire. In other words, the nanowires assemble themselves.
A new way to build nanowires is to print them directly to the appropriate substrate. A team of researchers in Zurich pioneered this method. First, they carved a silicon wafer so that the raised portions on the wafer coincided with the way they wanted the nanowires arranged. They used the wafer like a stamp, pressing it against a synthetic rubber called PDMS. They then drew a liquid filled with gold nanoparticles, called a colloidal suspension, across the PDMS. The gold particles settled into the channels created by the silicon wafer stamp. Now the PDMS became a mold capable of transferring a "print" of gold nanowires onto another surface. PDMS molds can be used repeatedly and may play a role in the mass production of nanowire circuitry in the future [source: Nature Nanotechnology].
Several laboratories have created transistors using nanowires, but their creation requires a lot of time and manpower. Nanowire transistors perform as well or better than current transistors. If scientists can find a way to design a way to produce and connect nanowire transistors together efficiently, it will pave the way to smaller, faster microprocessors, which will allow the computer industry to keep up with Moore's Law. Computer chips will continue to get smaller and more powerful.
Research in nanowire production continues across the world. Many scientists believe it's just a matter of time before someone comes up with a viable way to mass produce nanowires and nanowire transistors. Hopefully, if and when we reach that point, we'll also have a way to arrange nanowires the way we want so that we can use them to their full potential.
In the next section, we'll learn about the potential applications of nanowire technology.