New Spectroscopic Methods for Characterizing Single-Walled Carbon Nanotubes

Many important photophysical properties of SWCNTs remain inadequately determined, including (n,m)-specific absorption cross sections, emissive quantum yields, and fluorescence action cross sections. A major challenge has been quantifying structure-specific SWCNT concentrations in dispersed mixed samples. We have used short-wave IR fluorescence microscopy to image and count individual nanotubes within known volumes of structurally sorted samples, thereby directly finding (n,m)-specific particle concentrations. Separate measurements of the length distribution and bulk absorbance then give absolute absorption cross sections per SWCNT carbon atom. We find cross sections that are inversely related to nanotube diameter. E₁₁ cross sections are larger for mod 1 species than for mod 2, whereas the opposite pattern is found for E₂₂ transitions.

We also describe a new experimental method, Spectral Analysis of Fluorescence Fluctuations (SAFF). Here, long sequences of millisecond-scale emission spectra are captured from dispersed SWCNT samples that are dilute and thin enough to show clear intensity variations reflecting statistical variations in the numbers of nanotubes inside the small probed volume. Unlike normal fluorescence correlation spectroscopy, the SAFF method is spectrally resolved. The set of sequentially measured spectra is analyzed to reveal relative abundances of (n,m) species, isolate emission spectra of single species from complex mixtures, and sensitively monitor the presence of small SWCNT aggregates through spectral cross-correlations.

Reference

J. K. Streit, et al., Nano Letters 14, 1530 (2014).