We compare Raman spectra from aqueous suspensions of length-separated single-walled carbon nanotubes (SWCNTs) dispersed using either polymer adsorption of single-stranded DNA or miscelle encapsulation with sodium deoxycholate surfactant. The Raman sp
ectral features, other than the D-band, increase monotonically with nanotube length in both dispersion schemes. The intensity ratio of the disorder-induced D to G Raman bands decays as a function of SWCNT length, proportional to 1/L, as expected for endcap defects. While the UV-vis absorption and fluorescence also increase with length for both dispersants, the fluorescence intensity is dramatically lower for DNA-wrapped SWCNTs of equal length. The similarities in the length-dependent D/G ratios exclude defects as an explanation for the fluorescence decrease in DNA versus deoxycholate dispersions.
We have investigated excitons in highly-aligned single-walled carbon nanotubes (SWCNTs) through optical spectroscopy at low temperature (1.5 K) and high magnetic fields ($textbf{textit{B}}$) up to 55 T. SWCNT/polyacrylic acid films were stretched, gi
ving SWCNTs that are highly aligned along the direction of stretch ($hat{n}$). Utilizing two well-defined measurement geometries, $hat{n}paralleltextbf{textit{B}}$ and $hat{n}perptextbf{textit{B}}$, we provide unambiguous evidence that the photoluminescence energy and intensity are only sensitive to the $textbf{textit{B}}$-component parallel to the tube axis. A theoretical model of one-dimensional magneto-excitons, based on exchange-split `bright and `dark exciton bands with Aharonov-Bohm-phase-dependent energies, masses, and oscillator strengths, successfully reproduces our observations and allows determination of the splitting between the two bands as $sim4.8$ meV for (6,5) SWCNTs.
