Effect of doping on the radial breathing mode of carbon nanotubes
I.C. Gerber, P. Puech, A. Gannouni, W.S. Bacsa
CEMES / CNRS, FR
Keywords: nanotubes, doping, DFT, Raman
Abstract:Doping CNTs is an attractive alternative to control the electronic conductivity of CNTs independent of their diameter. Highly conducting nanotubes are of interest for applications in transparent thin film electrodes. When doping CNTs through substitution quasi bound states are formed which can induce significant modifications of the density of states (DOS) in the vicinity of the Fermi level. Raman spectroscopy is one of the most frequently used non-invasive diagnostic tools for CNTs. Resonant excitations make Raman spectroscopy sensitive to individual tubes. The low frequency radial breathing mode (RBM) corresponds to an in-phase motion of the atoms in radial direction and the frequency is inversely proportional to the tube diameter apart of a constant shift which depends on its interaction with the environment. Using density functional theory and a spring constant model we have calculated the changes of the RBM frequency as a function of different substitution configurations and concentrations. Our calculations show that the stable substitution configurations do not favor the formation of nitrogen-nitrogen bonds in zig-zag tubes and that the RBM frequency depends strongly on the substitution site within the tube wall. Considering the local symmetry of the RBM and optical mode we find a unique relationship between the ratio of the two frequencies and the ratio of the carbon-carbon bond length and tube diameter.