Scanning tunneling spectroscopy of carbon nanotubes
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Calculations predict that carbon nanotubes may exist as either semimetals or semiconductors, depending on diameter and degree of helicity. This communication presents experimental evidence supporting the calculations. Scanning tunneling microscopy and spectroscopy (STM-S) data taken in air on nanotubes with outer diameters from 17 to 90 A show evidence of one-dimensional behavior; the current-voltage (I-V) characteristics are consistent with a density of states containing Van Hove type singularities for which the energies vary linearly with inverse nanotube diameter.
Iijima et al.1'2 have described the existence of carbon nanotubes that possess diameters so small that quantum size effects should be apparent. A carbon arc technique to synthesize macroscopic amounts of this material has also been described.3 The majority of these tubes have chirality and screw axes. Topographic features of individual nanotubes4 and nanotube bundles,5 measured by STM and atomic force microscopy, were recently reported. Since the diameter of the tubules can be made smaller than the extent of the electronic de Broglie wavelength, they provide a prototypical onedimensional (ID) system. The ID energy bands for tubules have been calculated by a number of authors.6"12 In summary, the tubules can be either semimetallic or semiconducting, depending on the tubule diameter and chirality.6'7 The calculated density of states6 shows l/\l{E — Eo) singularities characteristic of ID systems. The separation between the singularities around the Fermi energy is the energy gap for the tubes that are semiconducting, and scales linearly with the inverse of the tube outer diameter.7-12 This contrasts with the case of a rod-shaped quantum wire, for which the gap is expected to scale with the inverse square of the diameter. The relevant energy scale for the gap in carbon nanotubes is y0 [3.14 eV (Ref. 6)], the nearest-neighbor overlap integral in graphite. Sublevel energy separations on the order of 1 eV are, therefore, expected [see Eq. (6) in Ref. 12] in tubes of 10 A diameter. Such separations are, in principle, observable in a room temperature experiment. We have grown carbon nanotubes using two graphite electrodes,3 and observed three distinct concentric regions of growth on the negative electrode: an inner black core containing the nanotubes surrounded by a ring of material with a metallic appearance and a dark gray material that covers the remainder of the electrode face. Figure 1 shows a field emission scanning micrograph of material taken from the center of the negative electrode. Samples for examination in the STM were prepared by J. Mater. Res., Vol. 9, No. 2, Feb 1994
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transferring electrode core material into a small vial of ethanol containing a polished single crystal of gold at the bottom. The vial was then placed in an ultrasonic bath until the solvent evaporated. This process disperses the tubes on the gold substrate, though the coverage is not uniform. STM and STS measurements were per
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