Adsorption effects on radial breathing mode of single-walled carbon nanotubes

Abstract

For elucidation of the adsorption effects on the vibration properties of single-walled carbon nanotubes (SWNTs), photoluminescence and Raman scattering spectra from SWNTs at different vapor pressure of water were simultaneously measured and a molecular dynamics (MD) simulation was performed. The water vapor pressure dependence and its tube diameter $({d}_{\mathrm{tube}})$ dependence of the frequency of the radial breathing mode (RBM) peaks $({\ensuremath{\omega}}_{\mathrm{RBM}})$ and the optical transition energy $({E}_{ii})$ indicate that the physical adsorption is quite important, and both ${\ensuremath{\omega}}_{\mathrm{RBM}}$ and ${E}_{ii}$ clearly depend on the number density of adsorption molecules on the SWNT surface. A simple adsorption model, where the vibrational coupling between the surrounding adsorption layer and SWNTs via van der Waals interaction is considered for RBM, reproduces the experimental and MD simulation results of ${\ensuremath{\omega}}_{\mathrm{RBM}}$ in a wide ${d}_{\mathrm{tube}}$ range for various SWNTs, such as isolated SWNTs in vacuum, SWNTs with adsorption water layer, and even bundled SWNTs. On the basis of the model, the variation of the relationship between ${\ensuremath{\omega}}_{\mathrm{RBM}}$ and ${E}_{ii}$ in a Kataura plot for various SWNT samples can also be understood generally as the ``environmental effects.''