Atomic Structure and Electronic Properties of Single Walled Carbon Nanotubes

Atomically resolved images of in situ vapor-deposited carbon structures believed to be Multi-Walled Carbon Nanotubes (MWNTs) were first reported by Sattler and Ge [16]. Bias-dependent imaging [17] and STS investigations [18] of independently characterized arc generated MWNTs suggested that some fraction of MWNTs produced by the arc method were semiconducting, and in these semiconducting nanotubes, the STS data suggested that the energy gap depended inversely on diameter. Subsequent STM and STS studies of MWNTs and SWNTs have provided indications of different structures and structure-dependent electronic properties, but have not revealed an explicit relationship between structure and electronic properties. The failure of these previous studies to elucidate clearly the expected diameter and he-licity dependent electronic properties of nanotubes can be attributed in part to the lack of pure SWNT samples, since (1) the electronic band structure of MWNTs is considerably more complex than SWNTs, and (2) relatively pure samples are required to carry out unambiguous STM and STS measurements.

The development of techniques to produce and purify relatively large quantities of SWNTs has made possible definitive testing of the remarkable predicted electronic properties of nanotubes [19,20,21]. Indeed, seminal STM and STS measurements of the atomic structure and electronic properties of purified SWNTs by Wildoer et al. [10] and Odom et al. [11] have shown that the electronic properties do indeed depend sensitively on diameter and helic-ity. In both of these studies, the SWNTs were grown by laser vaporization, ultrasonically suspended in organic solvents, and then deposited by spin coating onto Au (111) substrates. Subsequent STM imaging studies were carried out at low-temperature, in ultra-high vacuum STMs.

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