Exciton relaxation dynamics in a one-dimensional semiconductor
Xiao, Yee Fang
MetadataShow full item record
Carbon nanotubes are intriguing materials and extensively studied for both their fundamental properties and extraordinary performance in various applications during the last 20 years. They are extremely small in diameter, light in weight, sensitive to the environment, strong, and chemically stable. They can be either metallic or semiconducting depending on their species. The semiconducting species can absorb and emit light in a wide range of wavelengths. These outstanding properties of carbon nanotubes promise abundant applications that may be revolutionary. The opto-electronic behaviour of a single-walled carbon nanotube (SWCNT) is extremely sensitive to its physical structure and ambient environment. Structural defects and surrounding environment are extrinsic influential factors that often obscure the understanding of the intrinsic behaviour. Progress on SWCNT synthesis has been made continuously but not until the last 10 years, have single SWCNTs been isolated individually and from substrates so that their fluorescence can be detected. The fundamental science of an optically generated exciton (an electron-hole pair) in an ideal semiconducting SWCNT is not fully understood despite many studies of exciton behaviour using various optical approaches. The major challenge is controlling SWCNT sample qualities. SWCNT's fundamental properties, such as the absorption cross section, quantum efficiency, radiative and nonradiative lifetimes, remain under debate. Knowing the intrinsic SWCNT properties is essential to understand exciton transport and relaxation mechanisms. To minimize the extrinsic effects, we have selected high-quality unprocessed SWCNTs for investigation. Collaboration with Dr. P. Finnie and Dr. J. Lefebvre at National Research Council Canada, allow us to access pristine SWCNTs individually. Since the emission from a single SWCNT is low, it requires unconventional methods to measure the PL dynamics. Suggested by the results, exciton transport in a semiconducting SWCNT is diffusional at room temperature, with high diffusivity (130 -350 cm^2/s) and long diffusion length (1 - 5 µm). At lower temperatures, we observed a more efficient exciton-exciton interaction that suggests the contribution from hot excitons or a longer existence of delocalized excitons. Highly efficient exciton-exciton annihilation and long coherence time in a SWCNT are promising for making a single-photon source at near-infrared wavelength range and developing quantum computers.