Number 363, March 23, 1998 by Phillip F. Schewe and Ben Stein
NANO-ELECTROMECHANICAL SYSTEMS (NEMS) will be 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, the Caltech researchers (which include Axel Scherer, email@example.com) has fabricated a 10x10x100-nm suspended beam of gallium arsenide 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.
NEW METHODS OF STUDYING TURBULENCE, reported at the APS meeting, have enabled physicists to track in detail for the first time the accelerations of a particle moving through flows with atmospheric-level turbulence (Eberhard Bodenschatz, Cornell, 607- 255-0794), and to cause magnetically trapped electrons to act like fluid particles on a flat surface (Fred Driscoll, UC-San Diego, 619- 534-2498). Bodenschatz described how a light-sensitive diode measured the movements of a particle jiggling through a fluid at up to 200 times the acceleration of gravity. For upcoming experiments, the group has installed a "silicon-strip detector" used in high-energy physics to make up to 100,000 measurements per second of multiple particles in the fluid, the better to study how particles that are initially close together move apart in a very turbulent flow such as a volcanic eruption. Meanwhile, Driscoll investigated turbulence by using a strong magnetic field to trap a cigar-shaped column of a billion electrons. Viewed from the end of the column, the electrons moved like fluid particles on a 2D surface. Intriguingly, turbulent flows of these electrons spontaneously settled into "vortex crystals," geometric patterns of whirlpool-like eddies that stayed frozen in place. (Also see lay language papers by Bodenschatz and Driscoll.)
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.)