6 July 2001
One Electron Transistor For Molecular Computers
by Kate Melville
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 it an ideal device for molecular computers. Dutch researchers introduce this nanotube single electron transistor, the first to operate efficiently at room temperature, in the 29 June issue of the international journal, Science.
"We've added yet another important piece to the toolbox for molecular electronics," said author Cees Dekker 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, transistors block the flow of current, but when a small voltage is applied, they allow current to flow.
As researchers make computer chips ever-smaller, the idea of using a type of 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'll 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.
An SET is like a one-way bridge with tolls 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 irrelevant, even at room temperature.
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 and 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.
"Only four years ago did we measure for the first time any electronic transport through a nanotube," said Dekker. "Now, we are exploring what can be done and what cannot in terms of single-molecule devices. The next step will be to think about how to combine these elements into complex circuits."
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 paper.