Excitation energy transfer is a long-lasting issue in the fields of photoscience and materials science encompassing physics, chemistry, and biology. We report femtosecond energy transfer of quasi-one-dimensional excitons in single-walled carbon nanotube bundles, which we investigate using time-resolved luminescence spectroscopy. Luminescence decay times are found to increase with decreasing photon energy from 1.2 to 0.6 eV. The energy-dependent decay behavior is analyzed using a simple rate equation based on the measured diameter distribution of the tubes. The rate of exciton energy transfer per nanotube from an excited semiconducting tube to adjacent semiconducting tubes [1.8-1.9((0.2) Â 10 12 s -1 ] is 1.6 times that of transfer to adjacent metallic tubes [1.1((0.2) Â 10 12 s -1 ]. The observed transfer rates are much lower than those predicted by the F€ orster model. This finding provides insight into the energy-transfer mechanisms of one-dimensional excitons. SECTION Nanoparticles and Nanostructures

We study exciton energy transfer in double-walled carbon nanotubes using femtosecond time-resolved luminescence measurements. From direct correspondence between decay of the innertube luminescence and the rise behavior in outertube luminescence, it is found that the time constant of exciton energy transfer from the inner to the outer semiconducting tubes is ∼150 fs. This ultrafast transfer indicates that the relative intensity of steady-state luminescence from the innertubes is ∼700 times weaker than that from single-walled carbon nanotubes.

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