Single-walled nanotubes are widely studied nanomaterials because of their unusual properties and potential applications. Unfortunately all practical current growth methods produce nanotubes with a variety of (n,m) structures and therefore physical properties. This inhomogeneity hinders some studies and applications in which samples with a single structure or a narrow range of structures are needed. To date, a number of sorting methods have been introduced to separate single-walled from multi-walled nanotubes, individualized from aggregated, metallic from semiconducting, water-filled from empty, and even right-handed from left-handed forms of the same (n,m) SWCNT species. Nonlinear density gradient ultracentrifugation (nDGU) separation has been shown to be one of the most selective tools for preparing high purity SWCNT samples. However, there has been little study of whether nDGU fractions actually are highly homogeneous. We have analyzed such fractions and find that layers containing enantiomers of individual (n,m) species show unexpected spectral inhomogeneities.

Each enantiomeric layer of (6,5) SWCNTs was extracted into three or four 48 µL fractions using a custom-built high precision automated fractionator. These extracted fractions were analyzed for their fluorescence and absorption spectra. They were then diluted into sodium cholate solution to provide uniform coatings, and the spectral measurements were repeated along with determinations of their absolute fluorescence quantum yields and measurements of variance spectra. The undiluted fractions within a single layer showed significant differences in spectral peak position and line width. Dilution reduced these inhomogeneities and caused ca. 50 cm^-1 blue shifts in E₁₁ peak positions for the upper enantiomer band but only 5 cm^-1 shifts for the lower one. Emission line widths were also decreased substantially and quantum yields increased by a factor of ca. 2. Nevertheless, the diluted fractions gave quantum yields of 0.8 ± 0.1% and varied by a factor of 1.5 within the lower layer. They reached a maximum of approximately 1%, which is similar to that for SWCNTs wrapped by PFO in toluene. Further insight was obtained from variance spectral data, which confirmed the differences in emission per nanotube within layers and revealed that the low frequency wing of the E₁₁ emission band of each fraction results from inhomogeneous broadening. Our results show that even the use of a highly selective SWCNT sorting method may not ensure spectroscopic homogeneity within the separated samples.