Customized Y-Shaped Carbon Nanotubes Can Compute

Researchers at UCSD and Clemson University have discovered that specially synthesized carbon nanotube structures exhibit electronic properties that are improved over conventional transistors used in computers. The Y-shaped nanotubes behave as electronic switches similar to conventional MOS (metal oxide semiconductor) transistors, the workhorses of modern microprocessors, digital memory, and application-specific integrated circuits.

The increase in the speed and power efficiency of electronics over the past two decades was primarily due to the steady shrinkage in size of conventional transistors. Chipmakers have reduced the minimum feature size of transistors to about 100 nanometers, and that dimension is expected to shrink by the end of this decade. However, industry experts predict that fundamental technological and financial limits will prevent the makers of conventional MOS transistors to reduce their size much further. However, the Y-shaped nanotubes are only a few tens of nanometers thick and can be made as thin as a few nanometers.

The new transistors were initially grown as straight nanotube elements. Titanium-modified iron catalyst particles added to the synthesis mixture are then attached to the straight nanotubes, nucleating additional growth, which continued like branches growing from a tree trunk. Consequently, the nascent nanotubes assumed a Y-shape with the catalyst particle gradually becoming absorbed at the junction of the stem and two branches.

When electrical contacts are attached to the nanotube structures, electrons travel into one arm of the Y, hop onto the catalyst particle, and then hop to the other arm and flow outward. Experiments conducted at UCSD’s Jacobs School of Engineering showed that the movement of electrons through the Y-junction can be finely controlled, or gated, by applying a voltage to the stem. The researchers hypothesized that positive charge applied to the stem enhances the flow of electrons through the two arms, producing a strong “on” signal. However, when the polarity of the charge is reversed, the movement of electrons through the arms essentially stops, creating an “off” signal. Such binary logic is the basis of nearly all transistors.

Visit www.ucsd.edu.