Reversible hole engineering for single-wall carbon nanotubes

Experimental results are provided for reversible generation of holes on single-wall carbon nanotubes and their closing by temperature treatment. The generation of the holes was analyzed by checking the amount Of C-60 fullerenes that can be filled into the tubes and subsequently transformed to an inner-shell tube. The concentration of the latter was determined from the Raman response of the radial breathing mode. The tube opening process was performed by exposure of the tubes to air at elevated temperatures. This process was found to be independent from the tube diameters. In contrast, the tube closing process was found to depend strongly of the tube diameter. For large diameter tubes (d = 1.8 nm) the activation energy was 1.7 eV whereas for the small diameter tubes this energy was only 0.33 eV. Optimum conditions for tube closing were found to be one hour at 800 degrees C or 10 minutes at 1000 degrees C. From the almost identical Raman spectra for the tubes before and after engineering, a predominant generation of the holes at the tube ends is concluded.