AB-INITIO STUDY OF VARIOUS PROPERTIES OF SMALL DIAMETER ARMCHAIR CARBON NANOTUBES

Abstract

A comprehensive ab-initio investigation of the stability, structural, electronic, optical and Raman-active properties has been performed for the small diameter armchair carbon nanotubes. A number of new features not discussed earlier are observed in the present study. The binding energies (BEs) for the ultrathin nanotubes with respect to the graphine sheet are negative and the magnitude of the negative BE decreases with the diameter of the tube approaching zero for the graphine sheet. The separation between the two van Hove singularities (vHs) around the Fermi level increases with the diameter of the tube. The main absorption arises from the transitions between the states at nonzero values of kz lying in the range 0.38–0.50. There is a large variation in the magnitude of the optical matrix-element with the wave vector. The energy range of the strong optical absorption increases with the diameter of the tube. The presently predicted absorption and the RBM frequencies are in good agreement with the available experimental data. The variation of the radial breathing mode (RBM) frequency with diameter "d" of a tube obeys a relation which is very close to an experimentally determined relation obtained for a number of wide semiconducting nanotubes possessing a wide range of chiral angles.