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X-ray diffraction (XRD) studies on single-walled carbon nanotube (SWNT) samples prepared by the arc-discharge method were reported. The XRD profile was basically explained to be a result of triangular packing of SWNTs with a lattice constant of 17.1 Å and an average nanotube radius of 7.1 Å. We found an anomalous change in XRD profiles before and after heat-treatment of the SWNT samples in air at ∼350°C. Combined with gravimetric measurements and resistivity measurements, a detailed simulation of the XRD profiles showed that air (oxygen, and/or nitrogen and/or water) can be condensed inside the SWNTs.

A detailed three-dimensional structural analysis of single-walled carbon nanotubes was carried out using a scanning tunneling microscope (STM) operated at room temperature in ambient conditions. On a microscopic scale, the images show tubes condensed in ropes as well as tubes which are separated from each other. For a single-wall nanotube rope, the outer portion is composed of highly oriented nanotubes with nearly uniform diameter and chirality. On separated nanotubes, atomically resolved images show variable chirality ranges between 0°–30°, and variable diameter (1–3 nm), with no one type dominant. From STM and scanning tunneling spectroscopy measurements we confirmed the correlation between chirality and the electronic properties, namely the tuning from metallic to semiconducting. We also observed a rectifying behavior correlated with the chiral angle of 25°, an important observation for nanodevices application.

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