Share this:
03/12/1998 08:00:00

Physicists create first nanometer-scale mechanical charge detector

PASADENA—Wristwatch cellular phones and space probes the size of baseballs would certainly have some eager customers, but both are still the stuff of science fiction.

Nonetheless, physicists are making strides these days in the sort of miniaturization that could someday make tiny electromechanical devices a reality. One such milestone, the first nanometer-scale mechanical charge detector, is reported in the current issue of Nature.

According to Michael Roukes, professor of physics at Caltech and coinventor of the device, the new electrometer is among the most sensitive charge detectors in existence, and definitely the first based upon nanomechanical principles.

"One compelling reason for doing this sort of thing is to explore entirely new avenues for making small, ultralow power electronic devices," says Roukes.

"Making new types of electronic devices that involve moving elements, which we call nanoelectromechanical systems, will open up a huge variety of new technological applications in areas such as telecommunications, computation, magnetic resonance imaging, and space exploration. And the physics is exciting, besides."

The device fabricated at Caltech by Roukes and his former postdoctoral collaborator, Andrew Cleland (now an assistant professor at UC Santa Barbara), is a good example of the type of advances in solid-state devices that currently are loosely gathered these days under the rubric "nanotechnology." Roukes says he generally avoids using the term. "Rather, this is the kind of science that is building the foundation for real nanotechnology, not the stuff of fiction. Right now Mother Nature is really the only true nanotechnologist."

A nanometer is one-billionth of a meter, which is about a hundred-thousandth the width of a human hair. A few atoms stacked side-by-side span about a nanometer.

To give an idea of the scale, Roukes points out that the devices are far smaller than cellular organisms; a clear picture of the device's inner workings can only be taken with an electron microscope.

The scale is especially noteworthy when one considers that the electrometer is actually a mechanical device, in the same manner as an old-fashioned clock. In other words, there are moving parts at its heart. In the Caltech devices, movement is induced by tiny wires that exert forces on the nanomechanical elements when a minute external electrical current is applied to them.

"The simplest kinds of mechanical structures are resonators, for example, cantilevers—in other words, a structure like a diving board—or thin clamped beams, something like a thick guitar string attached at both ends," Roukes explains. "They really are mechanical structures—you 'pluck' them to get them to vibrate."

"What's fascinating is that, if you can get these things small enough, they'll vibrate billions of times per second—which gives them the same frequency as the microwaves used in telecommunications," he says. "That's because their mass is very small, which means there's less inertia for internal forces to overcome.

There is a second important aspect to nanomechanical systems, Roukes adds. "Because the distances involved are very small, the amplitudes of their vibrations are very small. For this reason, the amount of energy you would have to put into such devices to get them going is extremely minute.

"This means that for certain critical applications—like small communicators and miniaturized satellites—you would not have to carry along nearly as much energy to run the device."

The latter would be fortuitous in any circumstances where carrying along power is difficult. Transistors in the best receiving devices today can run on a few thousandths of a watt, but with nanotechnology, they could run on a few billionths of a watt, or less. Thus, planetary space probes (which employ such devices in spades) could be much smaller, since they could get by with a much smaller energy source.

At the center of the Caltech nanoelectromechanical charge detection device are small rods that vibrate something like a nanoscale tuning fork. In their ultimate incarnation, which Roukes believes his lab can achieve in the next few years, these rods will be about 100 nanometers long, 10 nanometers wide, and 10 nanometers thick.

Roukes indicates that a silicon beam of such small dimensions would vibrate at about 7 gigahertz (or 7 billion times per second) if it is clamped down at both ends. When one considers that a top-of-the-line personal computer these days has a clock speed about twenty times slower, the advantages become apparent.

But it's not necessarily the replacement of conventional computer components that Roukes is after. It turns out that the small resonators his group is currently able to manufacture on campus—if cooled to temperatures a few tenths of a degree above absolute zero—sit right at the border where the quantum effects governing individual atoms and particles take over.

Working with these quantum effects is a daunting technological challenge, but success could lead to devices such as quantum computers.

"There is a natural dividing line that depends on the temperature and the frequency. Basically, if you can get the temperature low enough and the frequency is high enough, then you can operate at the quantum level.

"We could do this today," Roukes says. "In my laboratories we can get to temperatures a few thousandths of a degree above absolute zero. We also have the sizes small enough to give us sufficiently high frequencies.

"But what we don't yet know how to do is to probe these structures optimally."

In fact, one of the main themes of work in Roukes's group on nanoscale electromechanical devices is pretty much "how to talk to the devices and how to listen to them," he says. To measure a system is to probe it somehow, and to probe it is to interact with it.

The problem is that interacting with the system is, in essence, to alter its properties. In the worst case, which is easy to do, one could actually heat it sufficiently to raise its energy above the point at which it would cease functioning as a quantum-limited mechanical device.

"But there are lots of different physical processes on which we can base signal transducers. We are looking for the right approach that will allow us to listen to and hear from these devices at the scale of the quantum limit," he says.

"There's lots of interesting physics, and practical applications that we are learning about in the process."

As far as the device reported in Nature is concerned, Roukes says that the scales involved set a milestone—that of submicron mechanical structures—that is encouraging for scientists and technologists in the field. In addition to possibilities for telecommunications, techniques on which the experimental prototype is based should also lead to significant improvements in magnetic resonance detection.

These, in turn, could lead to imaging with a thousand times better resolution than that currently available.

Roukes's group, in close collaboration with P. Chris Hammel's group at Los Alamos National Laboratory, is already hard at work on these possibilities.

Written by Robert Tindol