Top left, schematic rendering of process. Bottom left, MgO nanowires ready for coating. Right, completed Fe3O4 nanocable.
Chongwu Zhou, an assistant professor in the USC Viterbi
School 's Department of Electrical Engineering, is creating dense arrays of ultrafine wires made of magnesium oxide (MgO), each coated with uniform, precisely controlled layers of TMO.
"But now we can supply a group of previously unavailable materials to the nanotechnology community,” Zhou says.
The Zhou team demonstrated their technique with four different TMOs: YBCO, a well-known superconductor with a high transition temperature; LCMO, a material showing "colossal" magnetoresistance; PZT, an important ferroelectric material; and Fe 3O 4, known as magnetite in its strongly magnetic mineral form.
The new structures all start with a new technique Zhou and his co-workers developed to create arrays of nanowires by condensing magnesium oxide vapor onto magesium oxide plates using gold as catalyst. This produces a forest of magnesium oxide nanowires, each 30-100 nanometers in diameter and 3 microns (100 millionth of an inch) long, all growing parallel at a constant angle to the substrate plate.
"Now the magic starts,” Zhou says. A laser vaporizes the TMO, which then condenses directly out of the gaseous state onto the waiting magnesium oxide cores in very precise fashion. This process is called "pulsed laser deposition."
The final product looks like nano-sized coaxial cable, with an magnesium oxide core and TMO sheath. "The trick is we can preserve the TMO composition using this technique,” says Zhou, “while other techniques cannot.”
Zhou wrote in a paper recently accepted for publication in Nano Letters and currently circulating on the Internet, that the assemblies "can be tailored for a wide variety of applications, including low-loss power delivery, quantum computing, ultrahigh density magnetic data storage, and more recently, spintronic applications.”
"We expect that these TMO nanowires may offer enormous opportunities to explore intriguing physics at the nanoscale dimensions,” says Zhou.
Zhou, the winner of the USC Viterbi School’s 2004 Junior Faculty Research Award, believes that the four new nanowires are only the beginning. "Our synthetic approach will lead to other new nanostructures," he says.
Working with Zhou were Song Han, Chao Li, Zuqin Liu, Bo Lei, Daihua Zhang, Wu Jin, Xiaoleiu Liu, and Tao Tang. A National Science Foundation CAREER award and DARPA supported the research.
-- Eric Mankin