NEWS OF THE WEEK
SCIENCE
April 30, 2001
Volume 79, Number 18
CENEAR 79 18 pp. 13
ISSN 0009-2347
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NANOTUBE CONDUCTION
Understanding, control of electrical properties bring uses closer to reality

MITCH JACOBY

Carbon nanotubes have captured the imagination of scientists with their promise of smaller, faster, and better electronic devices. But applications of nanotubes in molecular electronics, nanotechnology, or other areas cannot be said to lie just around the corner--at least not yet--because scientists still do not fully understand the tiny tubes' electrical conductivity properties nor have they gained sufficient control over them.

ROLLED UP STM studies at Harvard show that certain "zigzag" nanotubes (top), originally predicted to be metallic, are semiconducting. "Armchair" nanotubes (bottom) behave like metals only when isolated from other tubes.
Now, researchers at two institutions report separately on nanotube advances that may help device designers put nanotubes to use. In one study, researchers at Harvard University answer a key question about theoretically predicted properties: Are nanotubes that have long been thought to be metallic really metals The answer, in most cases, is no [Science,
292, 702 (2001)].

In the other investigation, scientists at IBM report that they have constructed the first array of transistors made of nanotubes. The group has devised procedures for solving the problem of separating metallic nanotubes from those that are semiconductors [Science, 292, 706 (2001)].

The new findings substantially advance the researchers' knowledge of conduction processes in carbon nanotubes, writes Mildred S. Dresselhaus in a commentary in the same issue of Science. Dresselhaus, a professor of physics and electrical engineering at MIT, adds that "realization of practical nanotube devices is brought substantially closer to reality" by these recent developments.

Based on symmetry, scientists argued a decade ago that electrical conduction in nanotubes is partly governed by geometry. Nanotubes that combine certain tube diameters and orientation of carbon hexagons relative to the tube axis were predicted to be metallic, while tubes exhibiting certain other geometries were expected to be semiconducting.

Testing the predictions, Harvard chemistry professor Charles M. Lieber and coworkers Min Ouyang, Jin-Lin Huang, and Chin Li Cheung used a low-temperature scanning tunneling microscopy procedure that allows them to directly probe the electronic structure of single-walled nanotubes. The group finds that so-called metallic nanotubes with the "zigzag" configuration are characterized by energy band gaps--meaning the "metals" are really semiconductors.

"Armchair" nanotubes, also predicted to be metallic, indeed behave like metals when the tubes are isolated from one another, the Harvard group reports. But when they're grouped in bundles, armchair tubes act more like semiconductors, in that their band structure is marked with pseudogaps.

Lieber notes that the findings indicate which type of nanotubes to experiment with and which to avoid when trying to construct microscopic circuits.

But even with a more detailed understanding of nanotube band gaps, scientists have been bogged down by another metallic versus semiconductor problem. Invariably, synthesis procedures produce spaghetti-like mixtures of the two types of tubes. That means that semiconducting applications like transistors are doomed from the outset because of the metallic tubes.

That's where the IBM work comes in. Phaedon Avouris, manager of nanometer-scale science and technology at IBM's T. J. Watson Research Center in Yorktown Heights, N.Y., reports that ropes of mixed single-walled nanotubes can be picked clean of the metallic tubes in a simple procedure, leaving an intact bundle of semiconducting tubes.

To sort through the bundles, Avouris and postdoctoral researchers Philip G. Collins and Michael S. Arnold deposit ropes of mixed nanotubes on a silicon oxide support that sits on a silicon wafer. Next, the group uses lithography techniques to fashion a set of electrodes around the bundles. Then, by applying appropriate voltages to certain electrodes, the researchers selectively switch off and insulate the semiconducting tubes. Finally, by applying high voltage to the circuit, the researchers oxidize just the metallic tubes, causing breakdown.

Using their procedure, the IBM group members fashioned an array of field-effect transistors from single-walled nanotube ropes and have demonstrated that multiwalled nanotubes can be peeled controllably shell-by-shell. Because a nanotube's band gap depends on tube diameter, the procedure enables researchers to prepare nanotubes with custom band gaps--opening the door even wider to electronic applications.

TUBES APEELING Electrical breakdown procedure developed at IBM thins nanotubes. One segment of a multiwalled tube is stripped of three shells (green) while another portion (purple) is 10 shells thinner than the original thickness (red).

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