Indexing the Quality of Single-Wall Carbon Nanotube Dispersions Using Absorption Spectra

We have found that absorption spectra of single-wall carbon nanotube (SWCNT) dispersions can be accurately represented as linear combinations of two underlying spectra. One, assigned to purified well-dispersed SWCNTs, is structured; the other, assigned to impurities and aggregates, is diffuse. To assess the quality of SWCNT dispersions, a small set of visible and short-wave IR spectra are first measured for a dispersed sample as it is purified by centrifugation. Those data are analyzed using a simple arithmetic process to estimate the pair of underlying component spectra. Then the spectra of other sample dispersions from the same SWCNT source are fit as linear combinations of the two component spectra, giving coefficients that provide a simple metric for sample quality (the S-index). We have validated this approach with measurements of fluorescence intensities, particle abundances, length distributions, and emissive aggregates in samples containing SWCNTs from three growth sources dispersed by high-shear mixing or tip sonication and then purified by moderate centrifugation, ultracentrifugation, or magnetic processing. Our absorption spectral analysis correlates closely with results from other characterization techniques. One unexpected outcome of our analyses is the estimate that no more than 7% by mass of raw SWCNT material used to prepare aqueous sodium deoxycholate suspensions becomes well-dispersed SWCNTs