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Unprocessed single-walled carbon nanotubes suspended in air at room temperature emit bright, sharply peaked band gap photoluminescence. This is in contrast with measurements taken from nanotubes lying on the flat surface for which no luminescence was detected. Each individual nanotube has a luminescence peak of similar linewidth ( approximately 13 meV), with different species emitting at various different wavelengths spanning at least 1.0 to 1.6 microm. A strong enhancement of photoluminescence intensity is observed when the excitation wavelength is resonant with the second Van Hove singularity, unambiguously confirming the origin of the photoluminescence.
Lefebvre et al. (Tue,) studied this question.