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Glückert, Jan Tibor (2014): Optical spectroscopy of individual single-walled carbon nanotubes in an electric gate structure: tuning the photoluminescence with electric fields. Dissertation, LMU München: Fakultät für Physik



Semiconducting single-walled carbon nanotubes (CNTs) exhibit a chirality depended band structure of a one-dimensional lattice. Due to the radiative recombination of excitons CNTs emit photoluminescence in the near and mid infrared ranges depending on the tube diameter. Excitons are subject to diffusion along the tube before radiative recombination. Thereby they probe sites that give rise to spin-flips or non-radiative decay, or, at cryogenic temperatures, they localize in zero-dimensional quantum dots at the minima of the local energy potential landscape. Thus, the optical spectroscopy of individual CNTs probes not only the intrinsic exciton dynamics, like diffusion and intrinsic life-time, but also disorder of the CNT lattice and its environment. Intrinsic and extrinsic inhomogeneities and impurities may give rise to photoluminescence quenching, brightening of dark exciton states or generation of charged exciton complexes. In the framework of this thesis the physics of excitons in CNTs was investigated in two ways: On the one hand their environment was varied with an static electric field, on the other hand the CNTs were isolated from their environment. A comprehensive set of optical spectroscopy techniques was used to study individual CNTs at low temperatures. This included photoluminescence excitation, (time-resolved) photoluminescence, and photon correlation spectroscopy. This work identified exciton localization as predominant feature of individual CNTs at cryogenic temperatures. CNTs on substrate exhibited asymmetric line shapes at low temperature and temperature dependent shifts on the PL energy. Moreover for constant temperature, PL energies were subject to spectral diffusion, which arose - in analogy to compound semiconductor quantum dots - from interaction with a few close charge fluctuators in the dielectric environment. In addition, evidence for exciton localization was provided by the non-classical photon emission statistics of cryogenic CNTs. The main focus of this thesis was the study of individual CNTs in a static electric field. A metal-oxide-semiconductor device was used to probe for the transverse polarizability of excitons. In consequence, the PL energy of CNTs exhibited red-shifts as a quadratic function of the perpendicular electric field. However, a subclass of CNTs was characterized by satellite peaks in the emission profile. By their energy splitting they were assigned to PL emission from dark exciton states, e.g. triplet and k-momentum excitons, and resulted presumably from impurity induced symmetry breaking. As a function of the electric field, CNTs with a broken symmetry featured linear shifts of the PL energy of bright and triplet excitons. A third energy scale in the exciton fine structure was manifested by CNTs that exhibited the emergence of a satellite peak as a function of the electric field. These satellites were assigned to the PL of trions generated by doping of individual CNTs with charges from close oxide states. Presumably such close charge states played also an important role in the variation of the excitation spectra of individual CNTs, which was observed as a function of the applied electric field. This variation could be mediated by switching of charge states, which varied the localization potential of excitons. Finally, the extrinsic effects of the surrounding dielectric medium were contrasted by the remarkable optical properties of as-grown suspended CNTs. Freely suspended CNTs featured isolated localized excitons with narrow linewidths, intrinsic exciton lifetime and a significantly increased quantum yield. Moreover, they lack signatures of spectral diffusion or intermittency even on the shortest timescales.