RESEARCH NEWS

June 200410

Peter J. Burke and colleagues at the

University of California, Irvine have

demonstrated the operation of single-

walled carbon nanotube (SWNT)

transistors at microwave frequencies

[Li et al., Nano Lett. (2004) 4 (4),

753]. Such devices may be useful in

radio frequency signal processing,

computing, cell phones, and wireless

communications.

“Since the invention of nanotube

transistors, there have been

theoretical predictions that they can

operate very fast,” says Burke.

“However, ours is the first

experimental result to show they can

operate at microwave (GHz) speeds.”

The researchers fabricated SWNT

transistors with a back-gated

geometry. SWNTs were grown on a Si

substrate, Ti/Au contacts added, and a

gate voltage applied to the substrate.

“Nanotubes are typically high

impedance devices,” explains Burke.

“What we did was to construct an

impedance matching circuit in order to

measure the source-drain impedance

at microwave frequencies.”

A resonance was observed in the

microwave reflection coefficient of a

semiconducting nanotube and

impedance matching circuit at

2.6 GHz. At this frequency, the

researchers measured a change in the

nanotube source-drain impedance

when applying a gate voltage, verifying

the transistor action.

Burke suggests that nanotube

transistors may be faster than

conventional semiconductor

technologies [Solid State Electron.

(2004), in press]. He predicts that the

cut-off frequency for nanotube

transistors (where transistor gain falls

to unity) could reach the terahertz

range, assuming that gate lengths are

comparable to current Si technology.

Jonathan Wood

Fast nanotubetransistorsNANOTECHNOLOGY

Two groups have shown the existence of an ultralowfriction state at the nanoscale using atomic forcemicroscopy (AFM) capable of measuring lateralforces in the piconewton range. In both studies, theenergy dissipated in atomic friction is measured asan AFM tip is dragged over a surface. Normally, thetip sticks to an atomic position on the surface then,when the force is sufficient, slips to the next atomicposition and so on, resulting in a ‘sawtooth’modulation of the lateral force. Using differentmaterial systems, the two groups in theNetherlands and Switzerland observed a transitionfrom this stick-slip motion to continuous sliding withimmeasurably small energy dissipation.Joost W. M. Frenken and colleagues at LeidenUniversity, FOM-Institute for Atomic and MolecularPhysics, and Delft University of Technology in theNetherlands used a tungsten tip on a graphitesurface [Dienwiebel et al., Phys. Rev. Lett. (2004)92 (12) 126101]. “In the experiment, the tip wasdecorated by a tiny flake of graphite,” says Frenken,“which slid over an extended graphite surface.” When the atomic lattice of the flake was alignedwith that of the surface, stick-slip behavior wasseen. But at all other orientations, the friction was

near zero. “When the two lattices are not inregistry,” says Frenken, “there is a high degree offorce cancellation.” Essentially, some of the atoms inthe graphite flake are sticking, while others areslipping. Frenken uses ‘superlubricity’ to describethe situation and suggests that it may explain thevery low interwall friction observed between thenested surfaces of multiwalled carbon nanotubes.Researchers at the University of Basel, Switzerlandmade similar observations on a NaCl crystal with aSi tip [Socoliuc et al., Phys. Rev. Lett. (2004) 92(13) 134301]. Roland Bennewitz, now at McGillUniversity in Canada, and colleagues measured thefrictional force while varying the normal forcepushing the tip into surface.”Below a certain load onthe contact, the dissipation becomes immeasurablylow, although we still clearly observe the atomicstructure in the lateral force signal,” he says.Understanding this friction behavior has relevance inareas from lubrication of micromachines toatomistic simulations of nanoscale systems. Theresults indicate that it may be possible to controlfriction at this scale and design devices withsurfaces that slide smoothly over each other.Jonathan Wood

Near-zero friction at the nanoscaleNANOTECHNOLOGY

Smallest possible CNT has 3 Å diameterNANOTECHNOLOGY

The smallest stable carbon nanotube (CNT)is 3 Å in diameter, according to researchersat Meijo University, Japan andForschungszentrum Jülich, Germany [Zhao et al., Phys. Rev. Lett. (2004) 92 (12),125502].Previously, the smallest possible diameter ofCNTs was believed to be 4 Å, based onenergetic considerations. Such nanotubescan be capped with a hemisphere of the

smallest fullerene, C20, which containscarbon atoms arranged in pentagons.Now, 3 Å CNTs have been observed insidemultiwalled CNTs by high-resolutiontransmission electron microscopy (HRTEM).The nanotubes are prepared by arc dischargein hydrogen gas at a pressure of 8 x 103 Pa.Xinluo Zhao and colleagues believe thediameter is consistent with a nanotubestructure having just four hexagons ofcarbon atoms around its circumference.Density functional calculations indicate thecaps on the end of the CNT are likely to behalf of a C12 hexagonal prism. Furthercalculations suggest that a 3 Å CNT wouldhave a characteristic radial breathing modefrequency of 787 cm-1, which it may bepossible to observe in Raman spectra. “We are working on the preparation of 3 ÅCNTs in single-wall carbon nanotubes withdiameters of approximately 10 Å,” saysZhao. “This might lead us to obtain thesmallest conducting coaxial nanocable.”Jonathan Wood

HRTEM image of a CNT inside an MWNT. The 3 Å diameter of the

CNT and the 3.4 Å interlayer space of the MWNT are marked.

(Copyright © 2004 American Physical Society.)