Review—Photophysics of Trions in Single-Walled Carbon Nanotubes

Abstract

The photophysics of trions in single-walled carbon nanotubes (SWCNTs) is reviewed briefly. The trion state is observed energetically below the exciton state in hole-doped SWCNTs, and shows a simple single-exponential decay. The decay dynamics of trions depends on the photoexcitation condition. The optical responses of trions can be discussed by considering the energy relaxation of the optically accessible trion state. Detailed studies of excitons, trions, and biexcitons provide a deep understanding of the material properties of SWCNTs and are required for the design of photonic devices based on SWCNTs. 3,4 which has led to accelerated experimental studies on various optical properties of SWCNTs. Electrons and holes are strongly spatially confined to diameters on the order of 1 nm, resulting in room temperature stable excitons with large binding energies in the range of 200-400 meV.5-11 The exciton dynamics thus dominates the optical properties of semiconducting SWCNTs. 5,6 High-quality carbon nanomaterials provide an excellent experimental stage for studies on photophysics of excitons and exciton complexes.The exciton structures in SWCNTs are complicated due to the intrinsic properties of graphene, their unique helical structures, and coulomb interactions. 5 The origin of the optical absorption and PL is ascribed to the dipole-allowed (bright) exciton states, and their dynamics are affected by the interplay with the dipole-forbidden (dark) exciton states. Single nanotube spectroscopy has revealed details of the intrinsic exciton properties. For example, magneto-optical studies showed that the even-parity singlet dark exciton state is located energetically below the bright exciton state.12,13 Thermalization between the bright and dark exciton states determines the temperaturedependent PL properties 12,13 and the exciton diffusion length. 14,15 Furthermore, strong spatial confinement accentuates the exciton-exciton and exciton-electron interactions. Time-resolved transient absorption (TA) and PL spectroscopy measurements have revealed that multiple exciton states show rapid decays due to nonradiative Auger recombination. [16][17][18][19] Nonradiative Auger recombination, which competes with exciton diffusion, determines the PL efficiency of semiconducting SWCNTs, [20][21][22][23][24] and it ensures nonclassical PL by preventing the occupation of identical exciton states. 25Carrier doping is one of the most important techniques in semiconductors, which controls their electrical and optical properties. In SWCNTs, carrier doping has been actively studied using chemical modification [26][27][28][29][30][31][32][33][34][35][36][37] 47 Under strong photoexcitation conditions, dissociated carriers resulting from Auger recombination can create trion states with a subsequent absorbed photon. 43 Positive and negative trion states show almost the same binding energies. 46 Time-resolved TA and PL measurements have revealed fast formation of the trion state through the exciton-hole interaction. 48,49 Active experimental stud...

Giant Negative Terahertz Photoconductivity in Controllably Doped Carbon Nanotube Networks

“…A similar reduction in τ for increasing p-type doping was observed for the gated device. These numbers are close to the 2.7 ps single-exponential trion lifetime reported for hole-doped SWCNTs. , Trions in polymer-wrapped (6,5) semiconducting SWCNTs were shown to be formed within 500 fs of photoexcitation by rapid exciton diffusion to less-mobile holes, before subsequently decaying with a 2 ps lifetime . The recovery times observed here are therefore consistent with trion decay times, which are similar to exciton lifetimes.…”