Focus: Nanotube Electronics
Microscopic wires of the future could be made from carbon nanotubes–rolled-up sheets of graphite only angstroms in diameter. Nanotubes could also be made into electronic devices like diodes and transistors, which are traditionally made from junctions of two or more semiconductors having different electrical properties. In the 21 June PRL a team reports their calculations of the basic current and voltage relationships for nanotube junctions, showing that nanotubes should indeed be useable for diodes and other electronic components, once the fabrication techniques improve.
David Tománek of Michigan State University in East Lansing calls nanotubes “dream materials” for building tiny circuits: They’re strong, nonreactive, tolerant of extreme temperatures, and pass current essentially without resistance. They’re also much smaller than any wires in today’s electronics. Surprisingly, nanotubes can have either metallic or semiconducting properties, depending on their geometry: Starting with the all-carbon honeycomb lattice of graphite, you can roll either type of material depending on the direction of the cylinder’s axis compared with the lattice.
Electronic amplifiers, switches, and computer logic elements are all made from combinations of semiconductor junctions–interfaces between pairs of materials with differing concentrations of the current carrying electrons and holes. In semiconductors, a simple junction makes a diode, which carries current in only one direction. Researchers have already manipulated the carrier concentrations in nanotubes, so Keivan Esfarjani and his colleagues at Tohoku University in Japan decided to investigate the most basic properties of simple nanotube junctions, to see how they might be suitable for use in circuits.
The team calculated the current transmitted for a range of applied voltages for nanotube junctions with several different geometries and found two useful properties. First, for semiconducting junctions, they found a range of voltages where no current would flow, and the range was not symmetric about zero voltage–exactly the property needed to make a diode. Second, for metallic junctions, they found regions of “negative differential resistance,” where increasing the voltage led to lower current–a property useful for other types of electronic components.
Team member Amir Farajian explains that there are two main obstacles to observing these effects in the lab. Although researchers know of ways to “dope” nanotubes to change the electron and hole concentrations, no one has yet made a junction of this type, which requires different doping on two parts of the same tube. The other problem is isolating nanotubes small enough to work. The calculations assumed nanotube diameters of 5.5 Å, and they showed that the effects become weaker with larger diameters.
Tománek says the work is another step toward making real nanotube-based electronics, an idea that “people have been discussing for a long time.” Only within the last two years have researchers begun to investigate the transport properties of nanotubes, he says, but since then “the progress has been astonishing.”