Conductive diamond/nanotubes promise ice chips

Portland, Ore. — Argonne National Laboratories has found a way to make diamond a conductor as well as an insulator and semiconductor, opening the door to a new era of all-diamond chips. A spin-off company, Advanced Diamond Technologies Inc., has licensed the technology and material for development.

In general, diamond deposition yields high-performance, long-lasting, radiation-hard dielectric films that can be thin or thick, can be etched alongside silicon components and can be doped either as n- or p-type semiconductors (see www.eetimes.com, article ID: 164900968). Diamond's stiffness yields faster resonators, its smoothness yields friction-free microelectromechanical systems and its chemical inertness makes it ideal for bioengineered devices such as human implants.

Argonne's patented ultrananocrystalline-diamond deposition taps a plasma-enhanced chemical-vapor process that is seeded with 2- to 5-nanometer grains of diamond. Instead of growing layers of single-crystal diamond one atom at a time, Argonne's process grows the material from seeds to islands to film. By adjusting the ultrananocrystalline process, the lab's researchers have managed to grow nanotubes between the diamond islands, turning what would ordinarily be a dielectric that insulates as well as silicon dioxide into a conductor that conducts as well as aluminum or copper.

"We have integrated the hardest and the strongest substances on Earth: diamond and nanotubes," said staff scientist John Carlisle. "Other people have grown diamond on nanotubes or have grown nanotubes on diamond, but we are the first group to succeed in growing the two different allotropes of carbon simultaneously. The nanotubes are covalently bonded to the diamond at the nanoscale, yielding infinite possibilities."

The self-assembling-hybrid process grows the ultrananocrystalline diamond and carbon nanotubes in an Ar/CH4 plasma that controls the relative fraction and configuration of the diamond and the nanotubes, yielding a structure that is said to have unique mechanical, tribological and electrochemical properties. "For instance, diamond by itself does not have any photovoltaic properties, but when carbon nanotubes are added, we may find clever ways to process light with diamond," said Neil Kane, president of Advanced Diamond Technologies (Champaign, Ill.; www.thindiamond.com).

Argonne discovered the material "the way science usually does: by accident," said Carlisle, who is also chief technology officer for Advanced Diamond Technologies. Carlisle's team at the lab had been trying to grow diamond on high-performance pump seals for a contract with the U.S. Department of Energy, he said, "but what we got was a bunch of carbon nanotubes integrated inside our diamond film. They looked like a neural network growing between the supergrains of ultrananocrystalline diamond."

Carlisle and Argonne colleagues Orlando Auciello, Jeffrey Elam and Xingcheng Xiao discovered that iron contamination had seeded their material and thereby induced it to grow nanotubes. "Iron catalyzes the growth of nanotubes, so the idea came that if we put iron on a surface along with diamond seeds, could we grow diamond and nanotubes together," said Carlisle.