Exciton dynamics in polymer wrapped carbon nanotubes

• Main Author: Siddique, Sofia
• Other Authors: Allam, Jeremy
• Published: University of Surrey 2015
• Subjects: 530
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.640896

This thesis describes an experimental study of exciton recombination in isolated semiconducting single-wall carbon nanotubes synthesised by different methods and wrapped in different polymers PFO (9,9-dioctylfluorenyl-2,7-diyl), PFO-BPy (9,9-Dioctyfluorenyl-2,7-diyl-Bipyridine) and P3HT (poly 3-hexylthiophene). We present a comprehensive study using femtosecond transient absorption measurements of the kinetics of exciton recombination, where diffusion of excitons in the confined one-dimensional system significantly affects their optical and electronic properties. In all studied samples of isolated nanotubes, an exciton-exciton process dominated the recombination under high excitation, and exhibited a distinct crossover to a diffusion-limited regime with anomalous kinetics at late times. We attribute the reaction-diffusion crossover to a finite reaction probability per exciton-exciton encounter. We have demonstrated a methodology to determine the microscopic parameters controlling reaction and diffusion processes, based on measurements at high initial density where the optical absorption is fully saturated. In studies of the same nanotube species synthesized by both HiPCO (high pressure catalytic decomposition of carbon monoxide) and CoMoCAT (cobalt and molybdenum catalysts based chemical vapour deposition technique) methods and wrapped by different polymers, the exciton reaction probability was approximately constant, corresponding to on average one in five exciton-exciton interactions resulting in an exciton recombination. On the other hand, there was significant variation in the diffusive hopping time for samples synthesized by different processes or subject to different processing. This is consistent with the wide range of values for the diffusion coefficient reported in the literature. We have found that excitons in nanotubes synthesized by the HiPCO are more mobile than CoMoCAT nanotubes, by a factor of ~2 for nanotubes wrapped in P3HT. This may be associated with a lower defect concentration in HiPCO nanotubes compared to CoMoCAT. The nanomaterials studied in this thesis are promising for nanotube-organic hybrids based light emitting and harvesting devices. Our findings will not only facilitate the selection of materials in these applications but also represent experimental data that challenges existing theoretical models for kinetics of non-equilibrium stochastic systems.