Figure 21. Fluorescence excitation spectrum of the 875-nm bandgap emission feature in a 296 K sample of SWNTs suspended in SDS in H2O. The strong excitation feature at 581 nm is assigned as the second van Hove absorption of this nanotube species. Reprinted with permission from [152], M. J. O'Connell et al., Science 297, 593 (2002). © 2002, American Association for the Advancement of Science.

intensity distribution should closely reflect the distribution of nanotube abundance in the sample. In fact, the variation of quantum yields with diameter or chiral angle has not been clearly established yet, but, as a first approximation, this method provides a direct computation of the distribution of individual SWNT present in a sample. The particular SWNT sample studied by the authors was obtained by CO disproportionation at high pressure, using Fe(CO)5 as a catalyst. The diameter distribution computed for this material was centered at 0.93 nm, while the chiral angle distribution was centered at 30°. The values obtained for the diameter distribution were in good agreement with previous TEM and Raman studies. Of special interest is the result concerning the chiral angle distribution; this outcome is an agreement with previous reports that indicated that SWNT are formed closer to the armchair rather than zigzag structure.

A similar report of photoluminescence in SWNT has been recently reported by another group [154]. In this particular case the SWNT material under study was obtained by pulsed laser vaporization. The authors followed the same methodology described above and obtained a diameter distribution centered at large diameters (around 1.3 nm). Interestingly, the distribution of chiral angles was relatively similar to the one obtained by Bachilo et. al. [153] on SWNT samples produced by the HiPCO process; i.e. a chiral angle distribution with SWNT having a relatively large chiral angles. A new result that again suggests a relative higher stability and abundance of large chiral angle SWNT.

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