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Figure 20. Emission spectrum of individual SWNTs suspended in SDS excited by 8-ns, 532-nm laser pulses, overlaid with the absorption spectrum of the sample in this region of the first van Hove bandgap transitions. Reprinted with permission from [152], M. J. O'Connell et al., Science 297, 593 (2002). © 2002, American Association for the Advancement of Science.

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Figure 20. Emission spectrum of individual SWNTs suspended in SDS excited by 8-ns, 532-nm laser pulses, overlaid with the absorption spectrum of the sample in this region of the first van Hove bandgap transitions. Reprinted with permission from [152], M. J. O'Connell et al., Science 297, 593 (2002). © 2002, American Association for the Advancement of Science.

the first allowed electronic transition (Eu) was also present in the emission spectrum. This correspondence lead the authors to assign the emission spectra to semiconducting SWNTs.

The authors also observed that the photoluminescence intensity was dramatically reduced by aggregation of the SWNT in bundles or by changes on the pH of the SDS solution. This intensity reduction can explain the absence of fine structure on SWNT fluorescence spectra reported in earlier studies.

Another remarkable consequence of this work is that by evaluating the complete excitation-emission matrix in the near infrared to the near ultraviolet range, structural information of individual SWNTs can be obtained. For instance, optical excitation of a SWNT to a second van Hove transition (E22) will be immediately followed by fast relaxation before emission in the first branch transition (Eu) is observed. Therefore by monitoring the intensity of the E11 transition while varying the excitation energy, the second branch transition E22 that gave rise to that particular first branch peak E11 can be identified. Figure 21 shows that excitation spectrum for a E11 transition of 875 nm. The excitation spectrum showed a distinct second branch (E22) feature centered at 581 nm. The authors were the first to point out that this information could be used to identify the specific indexes (n, m) for every SWNT present in the sample [152]. Soon after, they reported that this goal had indeed been accomplished [153]. They were able to build the entire excitation/emission experimental matrix in the UV-NIR range and each excitation/emission pair was associated with specific E11 and E22 optical transitions for each individual SWNT. By combining these fluorescence results with the values for SWNT diameter obtained by Raman spec-troscopy on the same set of samples and with the results of numerical simulations, a direct assignment of the specific (n, m) nanotube structure for each one of the fluorescence data points peaks was achieved. The measured fluorescence intensity as a function of the nanotube diameter and chi-ral angle was also obtained. They further assumed that the

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