Analyzing Absorption Backgrounds in Single-Walled Carbon Nanotube Spectra

Abstract

The sources of broad backgrounds in visible-near-IR absorption spectra of single-walled carbon nanotube (SWCNT) dispersions are studied through a series of controlled experiments. Chemical functionalization of nanotube sidewalls generates background absorption while broadening and red-shifting the resonant transitions. Extensive ultrasonic agitation induces a similar background component that may reflect unintended chemical changes to the SWCNTs. No major differences are found between spectral backgrounds in sample fractions with average lengths between 120 and 650 nm. Broad background absorption from amorphous carbon is observed and quantified. Overlapping resonant absorption bands lead to elevated backgrounds from spectral congestion in samples containing many SWCNT structural species. A spectral modeling method is described for separating the background contributions from spectral congestion and other sources. Nanotube aggregation increases congestion backgrounds by broadening the resonant peaks. Essentially no background is seen in sorted pristine samples enriched in a single semiconducting (n,m) species. By contrast, samples enriched in mixed metallic SWCNTs show broad intrinsic absorption backgrounds far from the resonant transitions. The shape of this metallic background component and its absorptivity coefficient are quantitatively assessed. The results obtained here suggest procedures for preparing SWCNT dispersions with minimal extrinsic background absorptions and for quantifying the remaining intrinsic components. These findings should allow improved characterization of SWCNT samples by absorption spectroscopy.

Unique Origin of Colors of Armchair Carbon Nanotubes

“…The fitting function consisted of a sum of six Lorentzian peaks, one for each armchair (10,10) through (5,5), along with a polynomial function used to fit the absorption baseline/background. While the absorption background has multiple possible intrinsic and extrinsic origins, 11,12 it is evident that the fit produces excellent agreement with the experimental data (adjusted R 2 goodness-of-fit value = 0.996). Similar Lorentizian fits for the absorption spectra of samples #1, 3, 4, 5 and 6 result in adjusted R 2 goodness-of-fit values = 0.984, 0.998, 0.997, 0.998 and 0.998, respectively (see supplementary Fig.…”

“…Armchairenriched samples #3 and #4, which are produced from HiPco material, absorb mainly in the green region, producing suspensions that are colored magenta to purple, depending on the varying amounts of (7,7), (8,8), and (9,9) absorbed. Finally, armchair-enriched arc-discharge material (sample #6) appears green in color due to dual absorption in both the red, due to the first optical transitions of (10,10) and (11,11), and in the blue, due to the second optical transitions of (11,11) and (12,12). Hence, all observed colors for these suspensions obey the rules of subtractive color theory and are a result of the highly spectrally-concentrated absorption bands of armchair SWCNTs, a direct and macroscopic consequence of their excitonic optical properties.…”

Optimizing sonication parameters for dispersion of single-walled carbon nanotubes

“…Besides, there are another two absorption peaks for (9, 1) and (7, 5) SWCNTs, which can be seen at 915 nm and 1026 nm, respectively [28,30]. As a result, the aggregation of specific spectra features led to the broadening of the absorption peaks [30][31].…”

“…For a given absorption spectrum of SWCNTs, it has been shown that well-resolved features mean the fraction of individual SWCNTs is higher [29]. The I MAX -to-FWHM ratio has been proven to be related to the degree of SWCNT debundling and aggregation, which is used to evaluate the purity of SWCNT samples [31][32]. As shown in Fig.…”

Influence of Acoustic Cavitation on the Controlled Ultrasonic Dispersion of Carbon Nanotubes

“…This is illustrated in Figure 7a, which depicts 30% cmc (right vial) and 300% cmc (left vial) solution left standing for 14 days after ultrasonication for 60 minutes at 200 W. Whereas a uniform dispersion was maintained in the 300% cmc surfactant solution, significant sedimentation was observed at 30% cmc, indicating the critical role of the surfactant concentration. UV-Vis absorption spectroscopy may be used to characterize the quality of the CNT dispersion 29 and typically via the analysis of the resonance peak ratio 30 . This is depicted in Figure 7b for the two different surfactant concentrations treated at 200 W.…”