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

Abstract

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.

Designing the Interface of Carbon Nanotube/Biomaterials for High-Performance Ultra-Broadband Photodetection

“…The direct contact of each SWCNT with multiple Cyt c molecules is critical for effective charge separation because of the short exciton lifetime in the SWCNT film. 35 It should be noted that the broadband photodetection demonstrated in this work using s-SWCNT/Cyt c contains contributions from both Cty c (bandgap in visible) and s-SWCNT (bandgap in NIR). In fact, this s-SWCNT/Cyt c nanocomposite photosensitizer takes advantage of the complementary morphologies, electronic structures, and optical properties of the constituent optical molecules of Cyt c and CNT.…”

“…Each SWCNT may have multiple Cyt c molecules attached, which results in multiple heterojunctions. The direct contact of each SWCNT with multiple Cyt c molecules is critical for effective charge separation because of the short exciton lifetime in the SWCNT film . It should be noted that the broadband photodetection demonstrated in this work using s-SWCNT/Cyt c contains contributions from both Cty c (bandgap in visible) and s-SWCNT (bandgap in NIR).…”

Formation of Hot Excitons, Annealing, and Relaxation in Conjugated Polymers under an External Optical Pulse

“…When the conjugated polymer macromolecule is excited by the external laser pulse as mentioned, the localization of the electron occurs within 80 fs, as shown in Figure , which is in agreement with the previous experimental research. Once the hot exciton is formed in the polymer molecule, the localization of electron cloud of hot exciton just appears within 100 fs after the excitation. ,, With the formation of the hot exciton, the electron in the HOMO will transit into the LUMO, thereby leading to the change of the electron occupation number in the HOMO and the LUMO. Thus it becomes necessary to move our focus to the evolution of the electron population in the LUMO and HOMO.…”

“…Related experimental research reveals the details of the evolution for lasers with different wavelengths and provides the time scale of the exciton formation, relaxation, the radiative decay. 27,32,33 3.1. External Laser Pulse.…”

“…Within the π-electron system of low dimension, such as a semiconducting single-walled carbon nanotube, time-resolved two-photon photoemission spectroscopy (TR-TPPE) illustrates that due to the photons with energies above the band gap the hot excitons are not only populated but also lose most of their excess energy within the first 100 fs after photoexcitation. After undergoing relaxation, the hot excitons are either annihilated or trapped within a few picoseconds . What is the core factor that drives the relaxation during of the hot exciton?…”