A single
electron makes the difference between "on" and "off" for a new
transistor made from a single carbon nanotube, whose minute size and low energy requirements should make I an ideal device for
molecular computers. Dutch
researchers introduced this nanotube single electron transistor, the first to operate efficiently at room temperature. "We have added yet another important piece to the toolbox for molecular electronics," said author Cees Dicker of Delft University of Technology, in the Netherlands.
Used in all kinds of electronic devices,
transistors may be best known as the workhorses of the computer industry. Working together, million of transistors on a single silicon chip help perform logic functions or store information. In their "off" state, transistor block a small voltage is applied, they allow current to flow.
As researchers make computer chips ever-smaller, the idea of using a type if transistor called a "single electron transistor" or SET has become increasingly appealing. Like several other electronic devices, they can be made at a molecular scale, and would take up far less space than their conventional silicon counterparts. The particular advantage of SETs is that they only require one electron to toggle between on and off states. In contrast, transistors in conventional microelectronics use millions. Researchers currently foresee a limit to how densely they will be able to pack such conventional transistors together, because the abundance of
electrons whizzing around would ultimately produce too much heat for the chip to function. Sets might provide a means to avoid this problem.
A SET is like a one way bridge with tools at each end that control whether cars can cross, one by one. Specifically, it consists of a metallic "island" separated from "source" and "drain" electrodes by two barriers, through which electrons can tunnel. A gate attached to the island tunes the voltage of the whole system. Controlling the voltage on the gate regulates the number of electrons hopping on or off the island, one at a time.
But, there’s a catch: most previous SETs could only operate at super low temperatures, because heat can also provide the energy necessary to add electrons to the island. Now, Dekker’s group has made a device so tiny that heat fluctuations are irreverent, even at room temperatures. That’s because the smaller the space in which electrons are confined on the island the more energy it takes to add them.
Dekker and his colleagues started with a single carbon nanotube, and used the tip of an atomic force microscope to create sharp bends, or buckles in the tube. These buckles worked as the barriers, only allowing single electrons through under the right voltages. The whole device was only 1 nm wide & 20 nanometers long, altogether less than 1/500th the distance across a human hair. Researchers may someday assemble these transistors into the molecular versions of silicon chips, but there are still formidable hurdles to cross.
One basic challenge to any applications will be producing the
devices more efficiently. It now takes a student all afternoon in the lab to make just one of the buckled nanotubes. But, Dekker proposed that it might be possible to use a patterned substrate to physically induce buckles in many nanotubes at once, or to do this via chemical processes.
The authors also discovered some unusual physics when they investigated how exactly their single electron transistor was working. In most versions, the electrons hop on and off the island independently, but this wasn’t the case for Dekker’s group. Instead, the electrons seemed to have a type of quantum connection that has not been observed before, in which they hopped on and off in an intimately coupled way. "The present work shows that short metallic nanotubes can be applied as RTSETs (room temperature single –electron transistors). It also exemplifies that the search for functional molecular devices often yields interesting fundamental science," the authors wrote in their papers.
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