"Fabrication of sub-50 nm nanotube devices has been a challenge due to the resolution limits of the lithography equipment," Hongjie Dai told nanotechweb.org. "We decided to explore new fabrication schemes to overcome this. It's desirable to build such ultrashort nanotube devices to study the performance limits of nanotubes and to make truly ballistic devices at room temperatures in both the low bias and high bias operation regimes."

To make devices containing nanotubes between 10 and 50 nm long, Dai and colleagues grew single-walled carbon nanotubes by chemical vapour deposition onto an array of catalyst sites on a silicon substrate. Then they used photolithography to deposit an array of palladium contacts situated 3 µm apart - two for each device.

Looking at the devices' electrical transport properties enabled the scientists to find those devices in which just one metallic or semiconducting nanotube bridged the pair of electrodes. Typically, an array of around 100 devices on a 4 x 4 mm chip produced tens of devices containing individual nanotubes.

Next, the team took these 3 µm nanotube devices and carried out another photolithography step to open up 10 µm square windows in a layer of photo-resist over the nanotubes and electrodes. They deposited another layer of palladium by angle electron-beam evaporation, holding the substrate at an angle to the deposition direction. As a result, the existing palladium contacts acted as shadow masks and the process created gaps of between 10 and 50 nm between two electrodes.

"This method can potentially allow for wafer scale fabrication of such devices without relying on electron-beam lithography," said Dai. "By fabricating metallic nanotube devices with a tube length of about 10 nm, we have also observed current density as high as 4 x 109 A/sq. cm, which is among the highest tolerable by any conductor at room temperature. This demonstrates the potential of using nanotubes as interconnect elements."

The team also used a device containing a 50 nm semiconducting nanotube as a nanotube field-effect transistor. The transistor reached a current of about 20 µA at a low bias voltage of about 0.4 V. In contrast, devices with 3 µm long channels only reached similar currents at a bias voltage of 2 V. The researchers say this suggests that the 50 nm nanotube field-effect transistors are significantly more ballistic than long channel devices under high bias and current conditions. This could be valuable for ultra-fast electronics as the on-state current of a transistor is directly proportional to its speed.

"This fabrication scheme can be used for making ballistic nanotube electronic devices and ultra-sensitive molecular sensors," said Dai. "Furthermore this method can be applied to other materials such as nanowires and nanorods."

The researchers, who reported their work in the Proceedings of the National Academy of Sciences, say they now plan to explore the transport physics and applications of such short nanotube devices.