Raman spectra variation of partially suspended individual single-walled carbon nanotubes

The electrical and optical properties of single-walled carbon nanotubes (SWNTs) have been shown to be sensitive to their environment. Therefore it is very important to understand and exploit the environmental effect on the properties of nanotubes, especially for individual SWNTs. We report herein a systematic investigation of the Raman spectra variation of 15 individual SWNTs partially suspended on trench-contained substrates. Our experiments are conducted with low laser power to exclude possible heating effects. Most SWNTs show enhanced Raman signals for their suspended segment compared with their segment sitting on the SiO2 substrate, with several exceptions exhibiting either similar Raman intensity or a reverse result. Apart from this distinct intensity contrast, moderate radial breathing mode (RBM) frequency variations are observed for some nanotubes, which can be attributed to nanotube-substrate interactions. By analyzing the behaviors of the RBM full width at half-maximum (fwhm) and the intensity ratio between the anti-Stokes and Stokes spectra (I-AS/I-S), we can infer the shift of the nanotube transition energy E-ii for the segment of nanotube sitting on the substrate relative to the freely suspended segment. These Raman spectra variations can be attributed to the van der Waals interactions between the nanotube and the substrate, which give rise to both a structural modification (as a radial deformation) and an electronic modification (as a change in the electronic density of states) for the nanotubes on substrate.