AUSTIN, Texas Shrinking the dimensions of polysilicon devices to four nanometers, researchers at the University of Texas have extruded nanowires that are 25 times smaller than the vias on current silicon chips. The technique starts by heating and pressurizing silicon until it becomes a supercritical fluid.
"We use gold quantum dots as seeds, which contain the silicon in its supercritical fluid state. When the silicon concentration becomes great enough, the gold ejects the silicon in the form of tiny nanowires," said Keith Johnston, who with fellow professor Brian Korgel recently reported the team's research results.
Nanowires are just one building block from the growing nanotechnology tool kit, but the ubiquitous need to wire nanodevices together makes them critical to the success of nanotechnology.
At nanotechnology's small scales, quantum effects come to dominate electronic interactions, but even quantum dots need wires to interconnect them. The researchers' basic technique for building nanowires consisted of heating silicon atoms embedded in an organic vehicle until the silicon molecules came apart to became free silicon atoms. The chamber in which the free silicon atoms circulated contained gold quantum dots consisting of 100 to 200 gold atoms. When they came into contact with the gold particles, the free silicon atoms dissolved inside them, forming nanocrystals.
As more silicon atoms became dissolved inside the gold particles, they congregated at the center, causing the internal pressures and temperature to rise until the silicon became a supercritical fluid. That occurs at about 5,000 pounds/square inch pressure and 500C. Many researchers have investigated the properties of supercritical fluids that is, fluids that would be vapors if not for the high pressure containing them but Johnston and Korgel claim to be the first researchers to have investigated supercritical fluids at nanoscales.
"The pressure and temperature become so high that the silicon should vaporize into a gas, but inside these tiny gold particles it instead becomes a supercritical fluid," said Johnston. When the concentration of silicon atoms reaches a critical point, the interior ejects the silicon atoms in a tiny but steady stream from between the gold atoms.
The researchers chemically installed tiny spigot molecules, called capping ligands, on the side of the gold particles to help the stream of silicon atoms to solidify into a wire of uniform width. The ligands look like little hairs on the gold particles. They keep the particles from sticking together and ensure that the silicon nanowires are extruded at the same diameter from all of the equally sized gold quantum dots.
Johnston and Korgel observed strange behaviors from the nanowires that break the rules of macroscopic silicon but seem to hold true for nanometer-sized silicon wires. For instance, silicon does not usually emit light, but Johnston and Korgel claim to have observed their nanowires fluorescing. Thus, said Johnston, "we may be able to build computer monitors from silicon by using nanotechnology."
He further claimed that it's "theoretically possible to tune the size of our quantum dots and nanowires to get nearly any other desired property." Such designer devices are purely theoretical, however, since researchers today are still having trouble building traditional devices with nanowires and quantum dots.
For their part, Johnston and Korgel are working on a quantum field-effect transistor by interconnecting quantum dots with nanowires. "We are putting devices at both ends of our nanowires, so that we can plug them into our quantum dots and make little circuits," said Johnston.
No one has yet demonstrated a traditional FET-like device working at nanoscales, Johnston and Korgel said, and that hurdle was challenging enough. "We want to prototype actual devices working together in real circuits," Johnston said. "That's what has never been done before with these materials."