Spectroscopy Metals and Semiconductors

Subsequent characterization of the electronic properties of the atomically resolved tubes by tunneling spectroscopy can determine whether the electronic properties depend on structure. Tunneling current versus voltage data recorded along the two tubes discussed above exhibit very different characteristics (Fig. 3c and 3d), and the LDOS that is determined from these I-V data sets are quite distinct. For the tube assigned as (12,3) or (13,3), the LDOS is finite and constant between —0.6 and +0.6 V. This behavior is characteristic of a metal, and thus shows that the (12,3) indices provide the best description for the tube. Moreover, the normalized conductance data determined for the (14, —3) tube exhibit an absence of electronic states at low energies but sharp increases in the LDOS at —0.325 and +0.425 V. These sharp increases are characteristic of the conduction and valence bands of a semiconductor, and thus confirm our expectation that (14, —3) indices correspond to a semiconducting SWNT. These key measurements first verified the unique ability of SWNTs to exhibit fundamentally different electronic properties with only subtle variations in structure [7,8,9].

In addition, the semiconducting energy gaps (Eg) of SWNTs are predicted to depend inversely on the tube diameter, d, and to be independent of helicity. A summary of the energy gaps obtained by Odom et al. [11] for tubes with diameters between 0.7 and 1.1 nm is shown in Fig. 4. Significantly, these results and those obtained by Wildoer et al. [10] for tubes with larger diameters between 1 and 2 nm show the expected 1/d dependence. Moreover, these results can be used to obtain a value for the nearest neighbor overlap integral (y0) used in tight-binding calculations of the electronic properties by fitting to Eg = 2^oaC-C/d, where aC-C is 0.142 nm. The values obtained from the one parameter fit by [11] and [10] respectively, 2.5 eV and 2.7 eV, are in good agreement with the reported values in the literature that range from 2.4-2.9 eV [1,22,23].

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