Infrared properties of single-walled carbon nanotubes calculated from first principles

The vibration modes and the infrared (IR) spectra of the zigzag and chiral single-walled carbon nanotubes (SWNTs) have been calculated from the density functional theory in the local density approximation. Three IR-active modes are found to exist in the zigzag SWNTs in contrast to six in the chiral one, which agrees well with the prediction of the rod-group theory. Also, the frequency of low-frequency E-1u mode of zigzag tubes is found to be strongly diameter dependent, which could be used for measurement of tube diameter in experiment. It is very interesting to find that the (n-m) mod 3=-1 or 0 tubes exhibit anomalous big IR intensities of their A(2u) (or A(2)) modes, much larger than those of the (n-m) mod 3=+1 ones, showing a clear family behavior, which is expected to be easily observed in future experiments.