IR Characterization of Tip-Functionalized Single-Wall Carbon Nanotubes

Vibrational frequencies of functionalized groups (-COOH, CONH2, and COOCH3) of tip-modified single-wall carbon nanotubes are estimated using density functional theory. Both metallic (5,5) and semiconducting (10,0) nanotubes are considered with single and multiple functional groups at their tip. Several differences in frequency and intensity of the characteristic C=O band between (5,5) and (10,0) tubes are observed, which might help experimentalists to identify different tubes. For example, (5,5) tubes exhibit higher C=O frequencies than (10,0) tubes for all groups, and these bands are more intense in the latter tubes. These differences persist within a narrow range of diameter. To understand the effect of nanotubes on the spectra, fragment models containing parts of tube attached to functional groups are also studied. Such a computationally inexpensive model (compared to full tube) faithfully reproduces IR spectra and may be used for a wide range of end-modified tubes.