Berkeley Lab scientists have created insulated electrical wires that are about 100,000 times narrower in diameter than a human hair. These insulated wires are single-walled carbon nanotubes encased within an outer sheath of boron nitride nanotubes. The ultra-high-strength wires were reported in the April 18, 2003, issue of the journal Science.

"The ability to insulate nanowires opens up new possibilities for nanoelectronics," says Alex Zettl, a physicist with Berkeley Lab's Materials Sciences Division (MSD) and UC Berkeley's Physics Department, who led the research. "Insulation keeps different wires from shorting to each other or to nearby conductors, and will allow the wires to serve as the basis of coaxial cables or a simple gating configuration for the production of electronic devices such as transistors."

Coauthoring the Science paper with Zettl were William Mickelson, Shaul Aloni, Wei-Qiang Han and John Cumings.

Nanoscience marches on

	The inside diameter of a boron nitride tube determines the configuration of the carbon buckyballs packed inside it: (a) in a 2-nanometer tube, the buckyballs are staggered; (b) in a 2.8-nanometer tube, the buckyballs assemble as rotating triangles; (c) at a diameter of 3.3 nanometers, the buckyball nanowire is shaped like a corkscrew

First came the discovery in 1985 of fullerenes, the cage like structures of carbon atoms, the most famous of which is carbon-60, the buckyball. Then came the discovery in 1991 of the long, thin, hollow cylinders of carbon atoms called nanotubes. In 1998, scientists created the first carbon "peapods," carbon nanotubes that were packed with a linear chain of buckyballs.

Now, from Zettl's group, which in 1997 made the first nanotubes out of pure boron nitride, comes the "boron nitride silo." When a boron nitride silo is packed with buckyballs, then subjected to a ten-minute blast from an intense beam of electrons, the result is a carbon nanowire conductor enclosed within the world's strongest wide gap insulating fiber.

Zettl and his group made their boron nitride nanotubes using a plasma arc technique, developed earlier in Zettl's laboratory, in which a hot electrical discharge is sent between two boron-rich electrodes in a chamber filled with pure nitrogen gas. This yields an abundance of boron nitride nanotubes in the soot that forms along the chamber walls.

The soot can then be heat-treated to open the tips of the tubes, creating a boron nitride silo. The silos were packed with buckyballs by sealing the soot in vacuum inside a quartz ampoule along with carbon-60 powder, then heating the ampoule to between 550 and 630 degrees Celsius for 24 to 48 hours.

A slew of possibilities

In addition to creating insulated carbon nanowires, Zettl says that boron nitride silos can serve as model systems for studying the mechanical, electronic, thermal, and magnetic properties of "dimensionally-constrained" configurations of densely packed molecules such as buckyballs, a critical need for the development of nanotechnology.

"When individual carbon-60 spheres just barely fit inside the boron nitride cylinder, we got the linear-chain or classic peapod configuration," Zettl says. "However, at a slightly larger cylinder diameter, the spheres began to form a staggered chain. At still larger diameters, the staggered chain became close-packed, followed by a corkscrew-like formation."

Electron microscope images show boron nitride nanotube silos housing buckyballs packed in different configurations: (a) staggered, (b) rotated triangles, (c) corkscrew. In (d), the stacking is disordered.

Additional information

• "Packing C60 in Boron Nitride Nanotubes," by W. Mickelson, S. Aloni, Wei-Qiang Han, John Cumings, and A. Zettl, Science, 18 April 2003

• More on Alex Zettl's research group