Selective Synthesis of Subnanometer Diameter Semiconducting Single-Walled Carbon Nanotubes

Subnanometer single-walled carbon nanotubes (sub-nm SWNTs) were synthesized at different temperatures (600, 700, and 800 degrees C) using CoMn bimetallic catalysts supported on MCM-41 silica templates. The state of the catalyst was investigated using X-ray absorption, and the (n,m) indices of the sub-nm SWNTs were determined from Raman spectroscopy and photoluminescence measurements. We find that the size of the metallic particles that seed the growth of sub-nm SWNTs (diameter approximately 0.5-1.0 nm) is highly sensitive to the reaction temperature. Low reaction temperature (600 degrees C) favors the growth of semiconducting tubes whose diameters range from 0.5 to 0.7 nm. These results were also confirmed by electrical transport measurements. Interestingly, dominant intermediate frequency modes on the same intensity scale as the Raman breathing modes were observed. An unusual "S-like" dispersion of the G-band was present in the Raman spectra of sub-nm SWNTs with diameters <0.7 nm.