Optical characterization of single walled carbon nanotubes dispersed in sodium cholate and sodium dodecyl sulfate

The absorption spectrum of the commercially procured SWNTs dispersed in 2 wt % aqueous solution of Sodium cholate and also in 1 wt % of Sodium dodecyl sulfate was studied in the wavelength range 240-2400 nm. Two distinct bands each containing three peaks in the nIR range were observed for both the suspensions. These two bands were interpreted to be due to transitions between van Hove singularities E
₁₁
and E
₂₂
, which are symmetrically located at the opposite sides of the Fermi level in the density of states of the semiconducting SWNTs. These observed peaks were compared with the empirical Kataura plot in order to estimate the diameters and chiralities of the nanotubes. The diameters of the nanotubes were also calculated theoretically using tight binding approximation. It was found that the values of diameters estimated theoretically are lower as compared to those calculated from the experimentally observed E
₁₁
and E
₂₂
peaks using Kataura plot. This discrepancy is found to be higher for E
₁₁
peaks than for E
₂₂
peaks. It has been suggested that the reason for this is that E
₁₁
peaks are blue shifted due to coulomb interactions and exciton formation and these effects becomes weaker as we move towards higher order singularities. The excitons thus play a significant role in determining the band properties of nanotubes. The intensity of E
₁₁
peaks is also found to be higher than that of E
₂₂
peaks and this is attributed to the lifetime of the excitons in the transitions. The excitons have a very low lifetime for V
₂
-C
₂
transition as compared to those associated with V
₁
-C
₁
transitions causing electron and hole pair to relax very quickly for the second order van Hove singularity. The TEM images of the sample were also recorded and the tubes were found to be present in a bundle of size 50-100 nm in diameter. Due to bundlingintertube coupling occurs causing a red shift in the transition energies.Therefore the peaks suffer from blue shift and red shift both but effect of blue shift is found to be more predominant.