purity counts when it comes to carbon nanotubes: conference report

RESEARCH NEWS

May 2004 15

Finding applications for nanotubes

At a second symposium, Purification,

Separation, and Characterization of

Single-Walled Nanotubes, Benoit

Simard of the National Research

Council of Canada and Duke University’s

Jie Liu stated that large-scale, low-cost

synthesis of SWNTs is still needed

before their unique properties can be

utilized commercially. M. Yudasaka of

NEC Corporation, Japan added that, for

some applications, site- and direction-

selective large-scale processes with

SWNT diameter and chirality control

able to produce meter-length, single-

crystal SWNTs are required. Currently,

the two main synthetic methods for

SWNTs are laser/oven and chemical

vapor deposition (CVD). Liu described

an improved CVD method that he said

could be readily scaled up at low cost.

Yoichi Murakami of the University of

Tokyo noted that controlled positioning

of SWNTs on substrates is required for

efficient production of high-purity

SWNTs in CVD processes. He has

achieved the vertical growth of SWNTs

in a thermal CVD process on a quartz

surface containing a densely

monodispersed Co-Mb catalyst coating.

Wall defects, which can form during

production, could have a significant

effect on properties, however,

according to David Tománek of Michigan

State University. He reported computer

simulations that indicate a self-healing

mechanism occurs at high temperature

and under electronic excitation.

SWNTs synthesized by current methods

include impurities, such as fullerenes,

organics, metal clusters, and graphitic

shells, which may comprise as much as

40 wt.%. SWNT purification without

damaging the tubes or adding chemical

functional groups is difficult, said

Simard. But the efficient removal of

catalyst residues and carbonaceous

impurities is necessary, according to

Katherine Gilbert of Colorado School of

Mines. Treatment of nanotube material

in flowing CO2 at 800-1000°C oxidizes

the carbon coating the metal particles,

reducing the metal content to <1 wt.%.

Liu described a purification process

that removes catalysts, their supports,

and amorphous carbon.

John K. Borchardt

Purity counts when it comes to carbon nanotubes

The potential applications of carbon and inorganic nanotubesin nanoscale electronics, optoelectronics, and biochemicalsensors were outlined during the Nanocrystals andNanotubes Symposium at the American Chemical SocietySpring Meeting in Anaheim. Peidong Yang of the University of California, Berkeleydescribed the synthesis of GaN nanotubes with innerdiameters of 30-200 nm and wall thicknesses of 5-50 nm.Using a templating method, Yang’s group has also preparedsilica nanotubes. Unlike carbon nanotubes, these inorganicnanotubes are hydrophilic, easily dispersed to form colloidalsuspensions, and are readily functionalized because of theirpolar surfaces. Yang suggests that these nanotubes could beused to interconnect fluid reservoirs in nanofluidic devices toserve as a single molecule sensor. One of Yang’s group, Peter Pauzauski, is using metal-organicchemical vapor deposition and careful selection of the

substrate to control crystallographic growth directions andproduce massive, high-density arrays of GaN nanowires withdistinct geometric and physical properties. Substrate-inducedconstraints of lattice parameter matching and symmetryresult in nanowires with triangular and hexagonal cross-sections. The shape and cavity size of the nanowires appearsto affect lasing behavior. According to R. Bruce Weisman of Rice University, thediscovery of band gap fluorescence from single-walled carbonnanotubes (SWNTs) isolated in aqueous surfactantsuspensions offers many new opportunities. Aspectrofluorimetric study indicates distinct visible absorptionand near-infrared band gap emission transitions for morethan thirty different structural forms of the semiconductingnanotubes. Group member Dmitry Tsyboulski suggests thatnear-infrared band gap fluorescence provides a useful newtool for studying SWNT structure-dependent chemical andphysical processes. One such process is reversible quenchingof nanotube fluorescence in acidic environments. Fluorimetry-based tuned excitation and emission wavelengths can be usedto monitor the fluorescence of specific SWNT species inaqueous surfactant suspension as a function of pH. For SWNTs to compete with Si-based technology in nanoscaleelectronics, the properties, location, and orientation of thenanotubes must be controlled. Jie Liu of Duke Universityreported a rapid-heating chemical vapor deposition methodthat provides more control of the structure and properties ofthe synthesized nanotubes than other synthetic methods. Thediameter of SWNTs can be controlled using preformednanoparticles and nanoclusters as catalysts, the locations ofwhich are determined by photolithographic patterning of thecatalyst on the substrate. The orientation is controlled by thefeed gas flow.John K. Borchardt

CONFERENCE REPORT

Surface plot of fluorescence intensity as a function of excitation and emission wavelengths

for an aqueous surfactant suspension of SWNTs. (Image courtesy of Rice University.)

CONFERENCE REPORT