Sorting Out Carbon Nanotube Conductivity

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Science  27 Apr 2001:
Vol. 292, Issue 5517, pp. 597
DOI: 10.1126/science.292.5517.597f

Carbon nanotubes, whether the multiwalled (MWNT) or the single-walled (SWNT) variety, are formed as mixtures of metallic and semiconducting tubes. Two reports examine how structure influences conductivity and how structures can be modified to select for metallic or semiconducting properties (see the Perspective by Dresselhaus). Both the “armchair” and “zigzag” configurations of SWNTs have been thought to be metallic, but recent theoretical work that has considered the distortion in the chemical bonds cast doubt on that perception. Ouyang et al. (p. 702) obtained low-temperature scanning tunneling spectra of individual and bundles of armchair and zigzag SWNTs, and clarify that the zigzag types are not true metals but display an energy gap at the Fermi level that depends on the diameter of the tube. Device applications would benefit from ways to control the conductivity of nanotubes that connect electrodes. Collins et al. (p. 706) show that an extreme approach—running high currents through the tubes in air—selectively removed the outermost layers of MWNTs or particular tubes from SWNT bundles to create either a metallic or semiconducting contact. Entire arrays of field-effect nanotube transistors were fashioned in this manner.


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