Nanowire Process Leaves Lab for Industry

Alexander E. Braun, Senior Editor -- Semiconductor International, 11/28/2007 8:33:00 AM

Although there are a variety of approaches to producing nanowires, the capability to scale to comparatively large volumes of freestanding nanowires — while at the same time controlling and tuning their properties — is not something simply accomplished. Nanowires are far from being a commodity product; someone wanting them cannot just order them from a chemical company catalog. This may be changed by a process originating from research carried out by Professor Brian Korgel in the Chemical Engineering Department of the University of Texas at Austin, which is now being commercialized by Piñon Technologies (Austin, Texas).

The company, founded by Korgel last June, has concentrated on producing nanowires and controlling their diameters, as opposed to putting them down or using them in a pilot process. “We’ve been focused on using nanowires for different types of devices and applications,” Korgel said. “Now, we’re evolving the nanowire process technologies that we originally developed at the university. My research group had gotten to the point at which we were successfully making nanowires and being contacted daily by companies asking for them.”

The process developed at the University of Texas enables the production of grams of nanowires per day. This makes it unique when compared with more traditional processes, such as chemical vapor deposition (CVD), in which nanowires are grown on a substrate and then the reactor is shut down and the nanowires are scraped off.

SEM image of rows of horizontal zinc-oxide nanowires grown on a sapphire surface. Gold nanoparticles are visible on the ends of each row. The nanowires are 3 nm in diameter. (Source: NIST)

In the method that Piñon owns through a number of patents, the nanowires are made in a solvent, using a procedure similar to the chemical processes for making polymers, where reactant solutions are fed into a reactor that heats them up and produces the polymer. “If we’re making silicon wires, we feed in a reactant for silicon, a small molecule, and a seed of metal particles, then there is a higher-temperature process that spits out nanowires in the back,” Korgel said, comparing it with a mass production process. “What we’re doing is unique compared to using CVD to make silicon wires, where you feed in a reactant in a typical gas phase reactor, but then have to grow the wire from a substrate surface where the metal seed particles go. Our way permits you to use the reactor’s entire volume and essentially maintain a continuous process. This makes it much more scalable.”

At present, production is limited to silicon and germanium nanowires; these are single crystals 10 nm in diameter or less, and can be micrometers in length. However, this process, which fabricates nanowires dispersible in solvents with tunable size and composition, has also produced GaAs, InAs, GaP, InP, ZnS, ZnSe and SnTe versions.

Korgel’s company has just received funding in the form of a National Science Foundation (NSF) STTR R&D grant with a Jan. 1 start date, for work to scale up the synthesis process. Presently, the company is attempting to make very inexpensive nanowire transistors. Primary applications for these “mass-produced” nanowire transistors are expected to be in the area of chemical sensing and for disposable devices. “We are considering a broad spectrum of other application possibilities,” Korgel said, adding that they are working with at least two large companies he declined to name because partnering is being discussed.