Hot Exciton Relaxation and Exciton Trapping in Single-Walled Carbon Nanotube Thin Films

Time-resolved two-photon photoemission spectroscopy (TR-TPPE) was employed to investigate the hot exciton relaxation and exciton trapping dynamics in semiconductive, single-walled carbon nanotube thin films. Compared to other conventional optical ultrafast spectroscopy techniques, the TR-TPPE can temporally resolve both the energy and the population of optically excited excitons, which enables unambiguous identification of hot and relaxed band-edge exciton states. It is found that hot excitons populated by photons with energies above the band gap lose most of their excess energy within the first 100 fs after photoexcitation. Unlike isolated nanotubes, the generation of multiple excitons per absorbed photon is not observed for an excitation energy as high as five times the optical band gap. This difference is attributed to the different dielectric environment and the presence of intertube interaction in nanotube thin films. The subsequent population decay and energy relaxation dynamics of the band-edge excitons are measured. The excitons are either annihilated or trapped within a few picoseconds after photoexcitation. The rapid exciton annihilation and trapping implies that a device structure that can facilitate ultrafast exciton dissociation is critically needed for high performance nanotube optoelectronic devices.