Two University of Texas at Austin chemical engineers have made a scientific breakthrough in the production of far smaller silicon wires, using revolutionary methods that could lead to development of other new materials with exciting new properties. Silicon wires of this extremely small size will be needed in the construction of the computers of the future and for optoelectronic devices, such as lasers, sensors, computer screens and other flat panel displays.
Dr. Brian Korgel, 31, and Dr. Keith Johnston, 44, professors in the department of chemical engineering have produced silicon "nanowires" using tiny particles of gold suspended under pressure in a compressed fluid at a high temperature. Korgel and Johnston are members of the multi-disciplinary Texas Materials Institute that conducts research in metals, semiconductors, ceramics, polymers and composites. Their research in the world of nanotechnology has been published recently in the journal Science.
"They have no idea how they are going to be making the next generation of devices 10 years from now. That's what we're working on," Korgel said. There are one million nanometers in a millimeter. Today's designers are working toward production of computer components that are 100 nanometers long. "We have made components that are four nanometers long, so we are 25 times smaller," Korgel said. The researchers produce their nanowires by heating silicon atoms connected to organic molecules until the silicon atoms come loose and form free silicon atoms. This is done in the presence of small clusters of gold atoms referred to as nanocrystals or quantum dots. The quantum dots in this research consist of 100 to 200 atoms of gold. "The gold quantum dots are the seeds that start the growth of silicon nanowires," Johnston explained. The silicon atoms don't remain free for long, either congregating together or dissolving within the gold quantum dots. "Fortunately for us, the silicon prefers to dissolve into the gold nanocrystals," said Korgel.
When the silicon dissolves inside the gold particles and the silicon concentration inside the gold becomes great enough, the gold ejects the silicon in the form of a wire. Molecules called "capping ligands" can be attached chemically to the gold quantum dots during their formation to keep them uniform in size. Ability to produce a uniform size is a crucial factor when the goal is mass production of components.
"Ligands extend like hairs on the outside of the particles to keep the particles from sticking together," Johnston said. "We're starting with uniform gold particles that produce silicon wires with basically the same size."
The researchers' new method of making nanowires is revolutionary in its use of supercritical fluids -- fluids that are put under high pressure and high temperatures, in this case 5000 pounds per square inch and 500 degrees Celsius. "We have used supercritical fluids to control chemical reactions for the last 15 years, but never for the nanoscale materials," Johnston said.
Korgel added: "At that temperature we would expect the molecules to form a gas, but the pressure squeezes the molecules back into a fluid. Although this fluid is not a liquid in the sense that we think of liquids, it is, in fact, a supercritical fluid. These supercritical fluids have a variety of very interesting properties in their own right, and we are starting to exploit this unique medium to make new materials that cannot be made any other way."
The properties, or behavior, of the nanowires are affected by quantum rules that only apply in the nanoworld. Learning to manipulate materials in this microscopic world could open the door to discoveries of what are, in effect, entirely new materials. "When we make things as small as this, it affects the material properties so that silicon no longer really behaves like silicon," Korgel said. For example, silicon normally does not emit light. But in the nanoworld, silicon can emit light. It can be used in the construction of extremely high resolution light emitting devices that can, for example, be used as computer monitors and TV screens.
"Instead of mining the Earth for a material with the appropriate material properties, we can just tune the size of the quantum wire or quantum dot to engineer materials with the desired properties," Korgel said.
In the future, Johnston said that nanowires may be used as connectors for quantum dots. "As nanoparticles (quantum dots) are used as optoelectronic devices, nanowires will be a natural way to connect them," Johnston said. "As quantum dot technology advances, nanowires will be very useful."
Korgel says that the researchers now are testing what happens when prototype devices are created out of such small materials, by putting electrodes at both ends of the nanowires to "plug" them in and make little circuits."
We are now trying to make a field effect transistor, a type of electronic device, using these nanowires as a conduit for electrons," Korgel
said. "It hasn't been done before, so we want to see if it will work. We're
trying to take these new materials and actually make prototype devices."