Photoluminescence Side Band Spectroscopy of Individual Single-Walled Carbon Nanotubes

Photoluminescence spectra of single-walled carbon nanotubes (SWCNTs) have been recorded and analyzed for selected individual nanotubes and structurally sorted bulk samples to clarify the nature of secondary emission features. Room temperature spectra show, in addition to the main peak arising from the E₁₁ bright exciton, three features at lower frequency, which are identified here (in descending order of energy difference from E₁₁ emission) as G₁, X₁, and Y₁. The weakest (G₁) is interpreted as a vibrational satellite of E₁₁ involving excitation of the ∼1600 cm^-1 G mode. The X₁ feature, although more intense than G₁, has a peak amplitude only ∼3% of E₁₁. X₁ emission was found to be polarized parallel to E₁₁ and to be separated from that peak by 1068 cm^-1 in SWCNTs with natural isotopic abundance. The separation remained unchanged for several (n,m) species, different nanotube environments, and various levels of induced axial strain. In ¹³C SWCNTs, the spectral separation decreased to 1023 cm^-1. The measured isotopic shift points to a phonon-assisted transition that excites the D vibration. This supports prior interpretations of the X₁ band as emission from the dark K-momentum exciton, whose energy we find to be ∼230 cm^-1 above E₁₁. The remaining sideband, Y₁, is red-shifted ∼300 cm^-1 from E₁₁ and varies in relative intensity among and within individual SWCNTs. We assign it as defect-induced emission, either from an extrinsic state or from a brightened triplet state. In contrast to single-nanotube spectra, bulk samples show asymmetric zero-phonon E₁₁ peaks, with widths inversely related to SWCNT diameter. An empirical expression for this dependence is presented to aid the simulation of overlapped emission spectra during quantitative fluorimetric analysis of bulk SWCNT samples.