An ab initio study of optical and Raman spectra of heavily Li-doped 4 Å carbon nanotubes

An extended and systematic ab initio investigation of the energetics, structural, electronic, optical and Raman-active properties of heavily Li-doped ultrathin 4 Å diameter carbon nanotubes of different chiralities has been performed. A number of new features not discussed earlier are observed in the present study. Our values for the binding energies of the Li-doped tubes are higher than the ones reported earlier. We find the Li-doped (3, 3) tubes stable in contrast to an earlier prediction. For each type of the tube, the saturation intake of Li atoms inside the tube is 8–10% only, whereas on the surfaces of the (5, 0) and (4, 2) tubes, this intake is as high as 100%, and it forms a circular tube of Li+ ions around the tube. Li atoms residing on the axis inside the tube do not modify the symmetry of the host tube and thus the electronic structure (except the upward shifting of the Fermi surface, EF) in contrast to surface Li atoms which (apart from the (4, 2) tube, which possesses no symmetry) destroy the symmetry and drastically alter the electronic structure. In general, Li doping inside or outside the tube increases the electron density of states (DOS) several times except in the (5, 0) tube doped with a small concentration of Li atoms, where it decreases. For one inner Li atom in each tube, the peak structure in the optical absorption remains essentially the same as that of the pristine tube. In general, the optical absorption is different for the various nanotube–Li configurations studied. For each type of tube, in general, the RBM frequency reduces with the dilation of the diameter of the optimized tube. The high values of DOS obtained for certain Li concentrations may lead to high values of electrical conductivity and the superconducting transition temperatures in Li-doped tubes as has already been reported in pristine tubes and ropes.