, thin tubes of pure carbon, conduct electricity faster than most other materials and do not lose electrons, a property called ballistic conductivity. Unfortunately, not all nanotubes are equal: Some act like metals and conduct electricity spontaneously, while others conduct electricity only with a jump start. To build a chip out of nanotube transistors, manufacturers have to filter out the metallic ones.
The second problem comes in placing the nanotube transistor onto a silicon chip. NEC and Intel have developed experimental methods to grow nanotubes onto the surface of a silicon wafer. The metallic nanotubes are then singed off, leaving only those that act like a semiconductor.
Under the technique devised at Penn, nanotubes are made in a chemical foundry, the, and then placed in moist, slightly acidic air. A magnetic field is applied that separates the metallic products from the semiconductor nanotubes.
The harvested semiconducting nanotubes are then poured into a solution. A silicon wafer with dots of a gluelike substance is then dipped in. Nanotubes stick to the glue and create connections between two points on the silicon water. Excess glue, and stray nanotubes, are subsequently washed off.
Using this or a similar process to produce chips remains years away, and several technical hurdles remain. Still, in theory the technique could be used to create intricate circuit patterns.
"We dip the chips into nanotubes much like dipping an ice cream cone into candy," Danvers Johnston, a graduate student in Johnson's laboratory and lead author of the study, said in a statement. "Ultimately we can make it so that the nanotubes only stick where want them to in order to form a circuit."