Nanotubes: Small but perfectly formed

Nanotubes: Small but perfectly formedRoy Rubinstein  
For an industry where small is beautiful, single-wall carbon nanotubes fit the bill nicely.
The attraction of the single molecule nanotube is that it is “an inherently small structure that you can use for a whole host of things”, Alan Johnson, Assistant Professor at the University of Pennsylvania told Electronics Weekly.
The nanotube is made from a graphite plane of carbon atoms rolled up and sealed at the two edges. The result can exhibit either semiconductor or metal properties depending on how it is rolled:
“If it is rolled along the bond or at a funny angle,” explained Johnson.  
  TUBEFET… A single-wall nanotube heterojunction. The transition from semiconductor to metal is at the bend.
Performance is another advantage promised by the nanotube structure. “Ballistic electron transport over nanometre distances promises terahertz (THz) switching speeds,” said Johnson. Ballistic transport refers to fast travel (without scatter) of an electron when injected into the tube. “It is not a demonstrated fact but the time constant is about 100fs,” said Johnson. This equates to a potential device switching speed of 10THz.
What is exciting researchers such as Johnson is that the nanotube has moved beyond the theoretical stage. A 1.4nm diameter, 10?m long nanotube has already been shown to have FET-like behaviour at room temperature.
According to Johnson, the TUBEFET turns out to be slightly more complicated than a MOSFET but has similar behaviour. The nanotube – behaving as a semiconductor – acts as the channel connecting the source and the drain, while a gate is used to turn it on and off.
The nanotube can also be used to demonstrate quantum electronics effects such as single electron devices. “The structure is the same except that the nanotube is acting as a metal not a semiconductor,” said Johnson. However, such effects have only been seen at temperatures of 10K.
Two key challenges remain before the potential of nanotubes starts to be realised. Separating the nanotubes into metal and semiconductor categories when they are made is one major obstacle.
The other is tackling the issue of self assembly, getting the single molecule nanotubes to assemble to form complicated circuit elements.
“It will probably be one to two decades before they are used in practical applications,” said Johnson.

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