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.

Previously, the main deterrent to growing nanotubes and diamond simultaneously was that nanotubes grow at microns-per-minute rates, while a single micron of diamond takes hours to grow. The team discovered it could equalize the growth rate by using an argon-rich plasma chemistry that limits the amount of hydrogen in the reaction chamber.

"Since there is not a lot of hydrogen around, the nanotubes' growth slows down so they can form simultaneously with the diamond into a dense, integrated film," said Carlisle. "Next we want to use lithographic patterning techniques to control the growth even better, so we can grow arrays of integrated nanotubes separated by islands of diamond."

Argonne also hopes to enhance the structural strength of its diamond films. "Our diamond films are very hard, low-friction and wear-resistant, but they are brittle," said Carlisle. "With the integrated nanotubes, our films should be much stronger than before."

Electronics potential
For electronics applications, the team hopes to harness the natural tendency of the nanotubes to grow perpendicularly to the diamond surface. "There are electrostatic forces that also attract the nanotubes to grow between the supergrains of diamond, after which the diamond 'fills in,' " said Carlisle. "We are looking to let the nanotubes grow through the diamond so that they conduct electricity in one direction and heat in the other direction. We think we can transport these two types of energy in opposite directions, which would have applications for thermoelectric materials."

The team is also exploring ways to use the diamond to stabilize the nanotube. For instance, for cold-cathode emitters, nanotubes can be grown so that the surrounding diamond stabilizes the tubes, preventing them from unraveling during electron emission.

The hybrid may also prove useful in converting sunlight into electricity, Carlisle said. "Researchers envision using this material for geosynchronous-orbit solar cells, because our material is radiation-hard and may be able to harvest the whole spectrum of direct sunlight," whereas conventional solar cells are narrowband.

The team also hopes to enable all-diamond chips for bioengineering apps.