Résumé
The exciton recombination has been monitored in an ensemble of micelle suspended semiconducting single-wall carbon nanotubes by means of time-resolved photoluminescence. Selective excitation and detection allow us to get insight into the specific response of the (9,4) and (8,6) chiral families. We show that the non exponential response can be reproduced quantitatively taking into account the presence of residual small bundles (2 or 3 tubes). The long-living monoexponential tail gives access to the exciton dynamics in genuine isolated nanotubes whereas the fast components are related to the coupling between tubes within a bundle. We show that the quantum yield in genuine isolated nanotubes may be as high as 1%. Typical recombination times for the (9,4) nanotubes are of the order of 50 ps at room temperature and 300 ps at 10 K. On the other hand, the integrated photoluminescence signal shows a clear maximum at 40 K with a sharp drop of the signal for lower temperatures and a soft decrease of the signal up to room temperature. Both the recombination time and photoluminescence signal evolutions as a function of the temperature will be discussed quantitatively in terms of excitonic bright state coupling to either excitonic dark states or defect states. An estimate of the dark/bright states splitting will be derived. We will show however that this coupling is not expected to play a critical role at room temperature and cannot account for the low quantum yield usually reported for photoluminescence measurements in ensembles.