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Commercially procured single-walled carbon nanotubes were dispersed in 2 wt% solution of sodium cholate and also in 1 wt% solution of sodium dodecyl sulfate. The absorption spectrum of the suspensions was studied in ultraviolet-visible-near-infrared (UV-vis-NIR) range. Two distinct bands, each containing three peaks, were observed in NIR range for both the suspensions. These peaks correspond to transitions between van Hove singularities E 11 and E 22 in the density of states of the semiconducting nanotubes. Comparing positions of the observed peaks with the empirical Kataura plot, the diameters and chiralities of the nanotubes were estimated. Using tight binding approximations, the diameter of the nanotubes was also estimated theoretically. Discrepancies between the theoretically calculated diameters and those obtained by empirical Kataura plots are found to be higher for E 11 peaks. It has been suggested that the reason for this discrepancy is that the observed E 11 peaks are blue-shifted due to Coulomb interactions and exciton formation.

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