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."
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.