Dateline: May 14, 2001

Researchers have carried out STM measurements on carbon nanotubes that greatly increase our understanding of which types of tubes conduct electricity [1]. Nanotube models can be created by rolling a model of a graphite sheet into a cylinder. The direction which is chosen for the rolling defines the "chiral vector" of the tube and thus the properties. Certain types of tubes previously thought to be conducting have been found to contain energy gaps not present in planar graphite, which have been attributed to the curved surface of the cylinders. Furthermore, tubes that are mirror images of each other (stereoisomers) have been found to demonstrate differences in conductive properties (symmetry breaking), as a result of local interactions, for instance with a surface or neighboring tubes. While these measurements are clearly important for nanotube based electronics, they also provide insights into the mechanisms involved in other low dimensional wires.

A major problem in the use of nanotubes for electronic applications has been our inability to purify them, since chemically they are all quite similar even if their conductive properties differ significantly. Researchers at IBM have found a way to use those differences in conductive properties to destroy conducting tubes leaving behind only the semiconducting tubes, a method they have called constructive deconstruction [2]. By attaching electrodes to each side of a bundle of nanotubes (or a multi-walled nanotube), the researchers can pass a current through the tubes. Using a large current will result in localized heating and subsequent destruction of the conductive tubes (or shells in the case of multi-walled tubes), thus leaving behind only the tubes that are less conducting. As the tubes or shells are destroyed, the conductivity of the nanotube field effect transistor (see figure) drops in a stepwise fashion, with each step corresponding to the destruction of a particular tube or shell. While it is not economical to process nanotubes or bundles one by one by laying down electrodes for each molecule or bundle, the method demonstrates that tubes can be purified based on their conductivity. Thus, the search for a more parallel method of constructive deconstruction is on.

References:

[1] M. Ouyang, J.-L. Huang, C. L. Cheung, C. M. Lieber "Energy Gaps in 'Metallic' Single-Walled Carbon Nanotubes" (2001) Science 5517, 702-705. Abstract