Characterization of carbon nanotubes suspended between nanostructures using micro-Raman spectroscopy

We characterized suspended single-wall carbon nanotubes (SWNTs) bridging ultra-fine nano-pillars using microscopic Raman spectroscopy, mainly examining the structural dependence on pillar materials (Si or SiO2) and pillar spacing. The SWNTs were synthesized by thermal CVD, in which CH4 and Fe were the carbon feedstock and catalyst. The frequencies of G bands, D bands and RBM signals show very little dependence on the pillar composition and spacing. The growth yield of the suspended SWNTs evaluated from G band intensity decreases for both Si and SiO2 pillars with increasing pillar spacing, and the rate of the decrease is more rapid for the SiO2 pillar. Defect density evaluated from the intensity ratio of the D and G bands hardly depends on the pillar spacing but significantly on the pillar materials, indicating that the SWNTs grown on SiO2 pillars contain fewer defects than on those of Si pillars. The SWNT yield and the defect formation during their bridging growth may be related to the growth mechanism and is consistent with the vibratory growth model.