Tiny Machines: Research

Smaller than MEMS

In 2003, Berkeley physicist Alex Zettl built an electric motor that measures only 500 nanometers across, two orders of magnitude smaller than the electrostatic comb devices. At this scale, conventional MEMS design won’t work, because friction would overwhelm the applied electrostatic forces, so the rotor—a gold plate—is mounted on a shaft of nested carbon nanotubes, with one fastened to the rotor and the others to the silicon base. These nanotubes provide nearly frictionless bearings and also serve as the electrical connection to the rotor. (See Tubular Peas.)

The nanotube motor shaft of the 500 nanometer motor (image courtesy of Zettl research group, UC Berkeley).

The nanotube motor shaft of the 500 nanometer motor (image courtesy of Zettl research group, UC Berkeley).

Simulation of motor shaft and gears built up from individual atoms. (image courtesy of NASA).

When time-varying voltages are applied to the rotor and to other stator electrodes around it, the rotor turns. When the voltages are shut off, the rotor stops promptly, since it has little inertia, and stray charges produce electrical forces that quickly bring it to rest. See the movie to view the motor in action.

The scanning electron microscope used to create the image can run only as fast as 30 frames per second (fps), whereas the motor is predicted to spin at hundreds of millions of cycles per second. Consequently, above 30 revolutions per second the researchers currently can’t tell if the motor is actually spinning or only oscillating back and forth, and measuring the top speed of the motor will require improvement in the imaging technology.

Like MEMS devices, the rotor is driven electrostatically and is mounted in a support prepared by masking and etching silicon. But unlike MEMS, which takes advantage of batch processing for low-cost fabrication, Zettl’s motor is a painstakingly assembled proof of concept device, but nevertheless one with possible applications ranging from routing optical signals to carrying out procedures inside cells. For now, the motor enables fundamental studies of nanotube’s electrical and mechanical properties.

Someday, motors may be built up from individual atoms, as simulated in the NASA image of meshing gears. [For the technology of moving individual atoms, visit Seeing Atoms] Also, a brand new technology has emerged that spins a motor by shining light on it, doing away with the wiring and even the bearings. MEMS and NEMS promise all sorts of interesting designs and applications—stay tuned for more.

Scanning electron microscope video of the 500 nanometer motor in operation. The nanotube supports are visible. (video courtesy of Zettl research group, UC Berkeley).