* 1996 Highlight * Molecular Design Institute (MDI) * Materials Sciences Division, Lawrence Berkeley National Laboratory *

Design for "Nanotube Diodes" Proposed
Theory Predicts How Structure of Tubes Affects Electronic Properties
Marvin Cohen and Steven Louie

  View a pdf file of the figure

The theory group at Berkeley Lab's Molecular Design Institute (MDI) under the direction of Marvin Cohen and Steven Louie has developed a design for a tiny diode based on carbon "nanotubes" about, 1/3,000 the diameter of a human hair. Their work with postdoctoral researchers Vincent Crespi and Leonor Chico and graduate student Lorin Benedict, suggests the feasibility of ultrasmall transistors and other electronic devices based on "Buckyballs" and "Buckytubes," the new forms of carbon discovered by R. Smalley at Rice University in 1985 (Nobel Prize, 1996) and S. Iigimia at NEC in 1991, respectively.

The carbon nanotubes of these devices are constructed of one molecule thick sheets of graphite rolled into cylinders. The theorists found that when the sheets are rolled evenly, and the rows of carbon hexagons in graphite meet end-to-end (see figure), quantum mechanical restrictions force the [[pi]] electrons to occupy separate energy levels, giving the structure semiconductor properties. However, the theory predicted for the first time that if the carbon sheets are rolled in a specific spiral pattern, with the carbon hexagons of each row offset from one another, the electrons are not confined to discrete energy bands and the structure behaves like a metal.

Cohen, Louie, and collaborators designed their diode to be a chimeric carbon nanotube with functionally different ends (see figure). One end of the tube has the carbon hexagons aligned in bands, and thus is semiconducting, while the other end has the spiral structure and thus is metallic. The key to constructing such a tube was finding a way to join the two types of cylinders. The team solved this problem by aligning the two types of tubes at an angle, and by incorporating 7 and 5 membered rings at the joint. In this structure, higher energy electrons from the semiconductor half can flow "downhill" into the metallic half, but they cannot travel the other way without an extra electrical "shove" in the form of an applied voltage. In other words, the tube functions as an atomic-scale diode.

The MDI experimental group has begun work to synthesize these novel structures; there are reports from other investigators of somewhat larger tubes with bends somewhat like those required by the MSD design. Most carbon nanotube synthesis methods actually make multiwall tubes and it is unknown how this feature will affect the electronic properties, which are predicted to be very sensitive to the structure of the tubes involved. Other researchers have pointed out the difficulties in performing electrical tests on such tiny devices -- the new "nanoelectrode" method developed recently by other MSD researchers may be useful in this regard (see highlight: Nanoelectrodes Enable Electronic Studies of Single Nanocrystals (1996)).

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Marvin Cohen (510.642-4753) and Steven Louie (510.642-1709), Materials Sciences Division (510.486-4755),
see websites: http://tiger.berkeley.edu/cohen.index.html and http://tiger.berkeley.edu/louie/index.html.

L. Chico, Vincent H. Crespi, L. X. Benedict, Marvin L. Cohen, and Steven G. Louie, "Pure-carbon nanoscale devices: nanotube heterojunctions," Phys. Rev. Lett. 76, 971 (1996).

Research funding from the Office of Naval Research (ONR).

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