Exciton Mobility and Localized Defects in Single Carbon Nanotubes Studied with Tip-Enhanced Near-Field Optical Microscopy

In this work, single-walled carbon nanotubes (SWNTs) have been
studied using tip-enhanced near-field optical microscopy (TENOM).
This technique provides a sub-diffraction spatial resolution of 15
nm on the basis of strong local signal enhancement, which allows
for nanoscale imaging of the photoluminescence (PL) intensity and
energy along single semiconducting SWNTs. Thereby, the mobility of
excitons and their interaction with defects and spatial exciton
energy variations can be directly visualized. Similarly, the local
Raman scattering properties of metallic SWNTs have been
investigated, revealing the microscopic relation of localized
defects and the resulting Raman D-band intensity. The first part of
the thesis presents a newly developed numerical description of
exciton mobility and local quenching at defect sites, accounting
also for the TENOM imaging process. This highly flexible model is
used to quantitatively evaluate experimental observations such as
photo-induced PL blinking and strong spatial PL intensity
variations of single semiconducting SWNTs. The main finding is that
exciton propagation can be described as ne-dimensional diffusion
with a diffusion length of 100 nm for the studied nanotubes,
determined independently from both the PL blinking characteristics
and the direct visualization using high-resolution TENOM. The
temporal and spatial PL variations result from efficient exciton
quenching at localized defects and the nanotube ends. The second
part reports on the first observation of exciton localization in
SWNTs at room temperature, leading to strongly confined and bright
PL emission. Localization results from narrow exciton energy minima
with depths of more than 15 meV, evidenced by energy-resolved
near-field PL imaging. Complementary simulations using a modified
numerical model accounting for energy gradients are in good
agreement, predicting a significant directed diffusion towards
energy minima yielding locally enhanced exciton densities. The
energy variations are attributed to inhomogeneous DNA-wrapping of
the nanotubes, used for their separation during sample preparation.
In the last part, the microscopic relation between the
defect-induced Raman D-band and the defect density has been
investigated for metallic SWNTs. The length scale of the D-band
scattering process in the vicinity of defects was imaged with TENOM
for the first time and found to be about 2 nm. Furthermore,
localized defects have been photo-generated intentionally by the
strong fields at the tip while recording the evolution of the local
Raman spectrum. Based on this data, a quantitative relation could
be determined, that is highly relevant for the characterization of
carbon nanotubes via Raman spectroscopy.