Nano-electromechanical systems; DNA wires

Max More (
Tue, 31 Mar 1998 10:12:56 -0800

The American Institute of Physics Bulletin of Physics News
Number 363 March 23, 1998 by Phillip F. Schewe and Ben Stein

faster, smaller, and more energy efficient than the present day
micro-electromechanical systems (MEMS), an example of which is
the accelerometer that triggers airbags in cars. At last week's
American Physical Society meeting in Los Angeles, Michael
Roukes of Caltech (626-395-2916) described the leading edge of
NEMS research. Using lithography and etching techniques, he has
fabricated a 10x10x100-nm suspended beam of silicon which
oscillates at an estimated frequency of 7 GHz (although no detector
can yet "hear" the vibrations). Such a resonator will eventually be
used in microwave signal processing (for modulating or filtering
signals). The speed and stability of nanoscopic silicon arms might
even facilitate the advent of some new kind of Babbage-type
computer in which mechanical levers once again serve as processing
or memory elements.(In other words, a machine with "moving
parts" may not be so bad.) Silicon structures in this size regime will
also be used as cantilever probes in magnetic resonance force
microscopy (the goal being atomic-resolution NMR imaging; see
Update 313) and as calorimeters for the study of quantized heat
pulses (Update 320). Roukes' colleague, Andrew Cleland of UC
Santa Barbara, described a paddle-shaped silicon structure (whose
smallest lateral feature was 200 nm) for detecting very small
amounts of electrical charge, with a potential application in high-
sensitivity photodetection (see also Nature, 12 March 1998). At the
same APS session, Rex Beck of Harvard reported a NEMS force
sensor which integrates a field effect transistor into a scanned probe
microscope. The present sensitivities are about 10 angstroms for
displacement and 5 pico-Newtons for force (per square root of the
frequency), but Beck expects improvements as the size of the device
shrinks. The smallest transistor-probe structure Beck reported had
dimensions of 3x2 microns x 140 nm.

DNA WIRES, only 12 microns long and 100 nm wide, have been
strung between gold electrodes. DNA is attractive as a potential
component in nano-design applications because of its molecular-
recognition, self-assembly, and mechanical properties. DNA does
not, however, conduct electricity. In an experiment at the Technion-
Israel Institute of Technology, researchers first spanned the gap
between two electrodes with a tiny DNA causeway and then
exposed the structure to silver ions, which made a conducting path.
(Nature, 19 Feb. 1998.)