Optical spectroscopy of individual single-walled carbon nanotubes in an electric gate structure

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.