Spectroscopy of localized excitons in carbon nanotubes

Semiconducting single-walled carbon nanotubes are nanoscale, hollow cylinders with photoluminescence of strongly bound electron-hole pairs (excitons). The optical emission shows diameter-dependent wavelengths that vary between 800 nm and the telecom wavelengths, rendering carbon nanotubes as promising building blocks for photonic and optoelectronic devices. Moreover, the recent discovery of antibunching in the photon emission statistics—the hallmark of a single-photon source—reveals significant potential for application in quantum communication and cryptography. Photoluminescence blinking, spectral diffusion, and rapid exciton decoherence and dephasing, however, challenge the practical implementation of related schemes. In this context, it is essential to investigate both the extrinsic effects and the intrinsic properties of the nanotube photoluminesence. In the framework of this thesis, a chemical vapor deposition setup was employed to fabricate pristine carbon nanotubes detached from the substrate for photophysical studies. To this end, a systematic study of several growth parameters was conducted. By this means, conditions that favor the synthesis of isolated carbon nanotubes with subnanometer diameters and wide band gaps for optical investigations were identified. Environmentally isolated carbon nanotubes grown with this method revealed optical properties clearly different from tubes supported by silica: The optical emission exhibited symmetric spectral profiles with full width at half maximum linewidths on the scale of millielectronvolts as well as stable emission energies and intensities. In contrast, the photoluminescence of tubes pinned to the dielectric was subjected to spectral diffusion and intensity fluctuations caused by charge fluctuations in the substrate. Furthermore, a remarkably narrow emission line with a low-energy side peak appeared at low temperatures. These spectral peculiarities were well reproduced by a model that implies localized excitons and substrate-induced modifications of their coupling to acoustic phonons. The analysis of spectral features and the spatially resolved photoluminescence along the nanotube axis identified characteristic signatures of unintentional exciton localization in substrate-supported carbon nanotubes. Exciton localization was found to be induced by extrinsic disorder and covalent sidewall defects with exciton trapping potentials ranging from a few to tens of millielectronvolts. Moreover, the results establish disorder-induced crossover from the diffusive to the localized regime of carbon nanotube excitons at cryogenic temperatures as a general feature for widely used micelle-encapsulated tubes and as-grown carbon nanotubes by the in-house fabrication method. Ultimately, it is demonstrated that the challenges for quantum photonic applications may be overcome by localized excitons in suspended carbon nanotubes: The photoluminescence of these tubes exhibited narrow optical lines free of spectral wandering and intensity intermittence, flourescence lifetimes of several nanoseconds as predicted by theory, photon antibunching as well as strong indication of a significant increase in the quantum yield.