Time- and Polarization-Resolved Photoluminescence Decay from Isolated Polythiophene (P3HT) Nanofibers

Isolated nanofibers present unique opportunities to investigate excitonic processes and dynamics in a confined crystalline geometry, where structural order in the tranverse (intrachain) and longitudinal (interchain) directions can be tuned by polymer molecular weight, regioregularity, and solvent processing conditions. We report on time- and polarization-resolved photoluminescence (TRPL) studies from isolated crystalline P3HT nanofibers (also known as nanowires), which reveal a highly reproducible short-time decay behavior, appearing as an amplified spontaneous emission process (biexciton annihilation) as signaled by a quadratic excitation power dependence in amplitude and decay rate. In the long-time (5-100 ns) regime, we observed a power-law decay in the photoluminescence similar to that seen in thin films and nanoparticles; however, for certain nanofiber families (prepared from p-xylene) we observe an extremely long-lived PL component which we postulate arises from deeply trapped carriers in chain packing faults within the NF. Finally, we probe depolarization dynamics in individual nanofibers using polarization-resolved TRPL measurements in which both arrival time and (parallel/perpendicular) polarization state relative to the nanofiber axis are resolved, delineating the different dynamics associated with intra- and interchain excitons.