Carbon Nanotube Atomic Structure

A large scale STM image of individual tubes and small ropes containing a number of individual SWNTs is shown in Fig. 2a. A high-resolution image of a SWNT (Fig. 2b) exhibits a graphite-like honeycomb lattice, thus enabling the determination of the (n, m) indices from the image. The (n, m) indices were obtained from the experimentally measured values of the chiral angle and diameter. The chiral angle was measured between the zigzag (n, 0) direction (the dashed line connecting sites separated by 0.426 nm) and the tube axis.

The angle measurements were confined to the tops of the atomically resolved nanotubes, which minimizes contributions from the sides of the highly curved tubes, and over distances at least 20 nm to eliminate possible twistdeformation contributions. The SWNT diameters were determined from the projected widths of the nanotube images after deconvoluting the tip contribution to the image. This approach yields a more robust diameter than that determined from the cross-sectional height, since the apparent height is highly dependent upon imaging conditions.

Atomically resolved images of isolated SWNTs on a Au (111) substrate are shown in Fig. 3a,b. The measured chiral angle and diameter of the tube in Fig. 3a constrain the (n,m) indices to either (12,3) or (13,3). Note that a (12,3) tube is expected to be metallic, while a (13,3) tube should be semiconducting. On the other hand, the chiral angle and diameter of the SWNT in Fig. 3b constrain the indices to (14, —3). This tube has helicity opposite to the SWNT in Fig. 3a.

Fig. 2. STM images of nanotubes. (a) Large area showing several small bundles and isolated SWNTs on a stepped Au (111) surface. The white arrows indicate individual SWNTs and the black arrows point to small ropes of SWNTs. (inset) Schematic diagram of the STM experiment [13]. (b) SWNT on the surface of a rope. The solid, black arrow highlights the tube axis and the dashed line indicates the zigzag direction [11]

Fig. 2. STM images of nanotubes. (a) Large area showing several small bundles and isolated SWNTs on a stepped Au (111) surface. The white arrows indicate individual SWNTs and the black arrows point to small ropes of SWNTs. (inset) Schematic diagram of the STM experiment [13]. (b) SWNT on the surface of a rope. The solid, black arrow highlights the tube axis and the dashed line indicates the zigzag direction [11]

Bias Voltage (V)

Fig. 3. STM imaging and spectroscopy of individual nanotubes. (a, b) Constant current images of isolated nanotubes. The Au (111) lattice is clearly seen in (a). (c, d) Calculated normalized conductance, (V/I)dI/dV and measured I-V (inset) recorded on the nanotubes in (a, b) [11,29]

Bias Voltage (V)

Fig. 3. STM imaging and spectroscopy of individual nanotubes. (a, b) Constant current images of isolated nanotubes. The Au (111) lattice is clearly seen in (a). (c, d) Calculated normalized conductance, (V/I)dI/dV and measured I-V (inset) recorded on the nanotubes in (a, b) [11,29]

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