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fundamental aspects is the precise analysis of as-produced SWCNTs, as a widely applicable, readily and quickly available standard protocol for the determination of the absolute nanotube purity (with regard to structurally perfect SWCNTs in the bulk sample) is still lacking so that evidence from multiple spectroscopic and microscopic techniques usually needs to be cumulated in time-consuming studies. In 2003 the group of Haddon and coworkers started to follow the approach of determining relative nanotube purities by the aid of solution phase absorption spectroscopy. [13] The methodology relies on calculating the resonant ratio (RR) in the absorption spectra of nanotubes which they defined as the ratio of the integrated resonant SWCNT peaks arising from the excitonic interband transitions [14–16] [A(S)] to the total integrated area of the same region including the non-resonant background [A(T)]. In a variety of subsequent investigations, they were able to establish the technique as quick and readily available relative purity evaluation of bulk samples. [17] However, a bottleneck is formed, as no absolute purity can be determined due to the lack of a perfectly pure SWCNT sample that can be used as a reference system. Herein, we present that such a perfect SWCNT reference can be obtained from the as-produced bulk material by the aid of density gradient ultracentrifugation (DGU). In the past five years, DGU has evolved to a highly potent and versatile nanotube sorting technique where SWCNT samples can be separated according to diameter, electronic properties, or even down to single chiralities or helicities. [18–24] In principle, surfactant-dispersed SWCNTs are separated in a centrifugal field according to their buoyant densities by the aid of a density gradient medium. As the buoyant density is dependent on the surfactant adsorption, the fractionation process can be fine-tuned by using different combinations of surfactants. Additional to the “classical” DGU nanotube sorting approach, evidence has been provided that the buoyant density of covalently functionalized SWCNTs is significantly higher compared to the non-functionalized counterparts with differences in the range of 100 kg m 3 in the case of sodium-cholate-dispersed hydroxyphenylated SWCNTs. [25] We now applied this technique to separate the perfect, defect-free, nonfunctionalized SWCNTs of a bulk HiPco nanotube sample [26] from the defect material and amorphous carbon in DGU separation according to structural integrity. This enabled us to provide a reference spectrum for the determination of the absolute nanotube purity following Haddon’s approach of solution phase absorption spectroscopy. As the nanotube absorption spectra are also a fingerprint of the aggregation state, [27] we furthermore revealed that the resonant ratio in the absorption spectra is independent on the degree of SWCNT debundling by comparing the RR in different solvent systems (N,N’-dimethylformamide, N-cyclohexyl-2-pyrrolidone and 2 wt % sodium deoxycholate (SDC) aqueous surfactant solution), where it is known from literature that nanotube individualization strongly varies. [28–32] Accordingly, we provide an |
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