Tip-enhanced near-field optical microscopy on the quasi 1D semiconductors carbon nanotubes and CdSe nanowires

Optical microscopy is widely used to visualize small structures
that can not be seen by the unaided human eye. On the nanoscale,
however, the diffraction limit prevents conventional microscopy
from studying materials with the required spatial resolution. This
work reports on tip-enhanced near-field optical microscopy (TENOM),
a technique that allows for nanoscale optical imaging with high
detection sensitivity. It exploits the locally enhanced optical
fields at a laser illuminated metal tip that acts as an optical
antenna. The main aims of this work are to develop a better
understanding of the signal enhancement mechanisms in TENOM and to
apply the technique to different 1D semiconducting nanostructures,
namely single-walled carbon nanotubes (SWCNTs) and cadmium selenide
nanowires (CdSe NWs). In the first part, the angular distribution
of photoluminescence (PL) emission from SWCNTs with and without the
optical antenna is studied by imaging the back focal plane of the
microscope objective. Using model calculations, it is shown that
the PL of SWCNTs on a dielectric substrate can be described as
emission from a single in-plane point dipole despite the quasi 1D
structure of the nanotubes. The signal enhancement due to the
antenna is connected to a substantial redistribution of the angular
emission. A procedure for the individual quantification of the
excitation and radiation rate enhancement factors is developed and
applied to the experimental data. In the second part, nanoscale
optical imaging of CdSe NWs using TENOM is presented for the first
time. Spectrally resolved imaging reveals different band gaps for
different NWs and variations of the PL energy and intensity along
single NWs with energy gradients up to 1 meV/nm. Even bundled NWs
can be spatially resolved by their PL and Raman signals. The third
part reports on the angular and spectral emission properties of
CdSe NWs and the tip-induced changes in a TENOM measurement. In
contrast to SWCNTs, two perpendicularly oriented point dipoles are
required to describe the angular intensity distribution of PL
emission from CdSe NWs sufficiently. Again, tip-induced signal
enhancement is accompanied by a spatial redistribution of the
emission. The theoretical description is more complex than in the
case of SWCNTs, because different radiating dipole orientations in
the NW have to be taken into account that can interact with the
tip. Finally, investigations of the tip-sample distance dependence
of PL and Raman scattering are presented and discussed.