Density functional theory studies on covalent functionalization of single-walled carbon nanotubes with benzenesulfonic acid

The calculated Raman spectra of the covalently functionalized zigzag single-walled carbon nanotubes, f-(n,0)-SWCNTs (n = 6–10), with benzenesulfonic acid showed that the radial breathing modes (RBMs) frequencies (ωRBM) of the sidewall functionalized tubes are red shifted in comparison with the frequencies of their non-functionalized counterparts, (n,0)-SWCNTs. It is also observed that this ωRBM red-shift is tube-diameter (dt) dependent, where almost no redshifts are observed for the (10,0)-SWCNT. Moreover, many new Raman bands, which results from the functional group, appear in the low energy region, ranging from 100 to 600 cm−1. In the high frequency region, resulting from the functional group, the Raman spectra of the f-(n,0)-SWCNTs exhibited a few new peaks above the tangential bands of the SWCNTs (ranging from 1580 to 1650 cm−1), which entirely belongs to the CC stretching, including bending deformation of the CCC bonds, and rock of CH in phenyl group of the benzenesulfonic acid as well as many new Raman peaks dispersed through spectrum. The calculated IR spectra also exhibited many new peaks through spectra of the f-(n,0)-SWCNT when comparing these with corresponding IR spectrum of the isolated nanotube. Furthermore, the calculated vertical singlet-singlet electronic transitions of the f-(n,0)-SWCNTs significantly altered in the optical structure of the nanotube and exhibited charge transfer states for both the functionalized (8,0)- and (12,0)-SWCNTs. The small distances among the calculated dipole-allowed and forbidden electronic energy levels also suggested an internal crossing (IC) processes.