Role of covalent defects on phonon softening in metallic carbon nanotubes

We have examined how electrical characteristics and charging dependent Raman G-band phonon softening In Individual metallic carbon nanotubes are Influenced by covalent defects. In addition to decreasing electrical conductance with increasing on/off current ratio eventually leading to semiconducting behavior, adding covalent defects reduces the degree of softening and broadening of longitudinal optical (LO) phonon mode of the G-band near the charge neutrality point where the bands cross. On the other hand, the transverse optical (TO) mode softening Is enhanced by defects. Implications on the Interpretation of Raman G-band phonon softening and on utilizing Raman spectroscopy to examine covalent functlonallzatlon are discussed.