Wong's group found that because the high-temperature step of growing the nanotubes can be separated from their subsequent transfer to a low-temperature substrate, even low-cost temperature-sensitive substrates can be used. Practical applications can now utilize the ballistic transport of electrons through carbon nanotubes to create field emitters for displays, electrical interconnects and thermal management inside chip packages.
The dangling ends of the demo nanotubes were left open to serve as emitters. The CVD growth technique used water-assisted etching to keep the nanotubes from closing off their ends during growth. Wong has demonstrated this ability to create thin films of open-ended carbon nanotubes, achieving multilayered structures with up to 10 layers of carbon nanotube films.
For the field-emitter demonstration, Wong grew a thin film with 1 million carbon nanotubes in a 20 x 20-µm square. The film was about 325 µm thick, and the diameter of the multiwalled nanotubes ranged from 10 to 20 nanometers, with 5-nm cores. By changing the catalyst thickness, reaction temperature and time, Wong demonstrated the ability to control nanotube film thickness. "We are using a diffusion-control growth mechanism that is very quick and easy to use," he said. "It takes only five to 20 minutes to grow a layer."
The nanotubes grew perpendicular to the substrate. They were successfully tested as a field emitter by using as a cathode from which electrons were emitted across a 180-µm gap to an anode in the form of a phosphor-coated glass display. The display lit up uniformly when driven by the carbon nanotube film functioning as a field emitter, Wong said.
Wong also plans to fill the open ends of the nanotubes with solder to enable high-conductivity interconnects. "Carbon nanotubes are less susceptible than copper or aluminum to electromigration [forming hairline open circuits] at high current densities," he said. "We also think it should be possible to wick [fill the insides] of nanotubes by capillary action."
By separating the growth of nanotubes from their assembly into devices, Wong aims to enable easy integration with existing IC fabrication equipment. Whereas drop tests of assembled devices have shown that closed-ended nanotube films detach, Wong's open-ended, soldered-down nanotube films were ruggedized and stayed attached even after repeated drops.