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No description available.

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Super-resolution imaging, the ability to resolve structures well below the diffraction limit, has changed fluorescence microscopy as we know it. Diffraction-unlimited microscopy, termed nanoscopy, is able to image deep inside the cell with a resolution approaching that of electron microscopes. A variety of different methods currently exist, the first proposed and demonstrated technique being stimulated emission depletion (STED) microscopy. In STED microscopy, the resolution limit of a laser scanning microscope is overcome by transiently silencing fluorescence in parts of the focus. Practical implementations of this method are limited by optical aberrations and the characteristics of the fluorophores used. In this thesis, I present a fully working, custom built, 3D dual-color STED microscope. A super-continuum source is used to provide all spectral bands necessary for excitation and efficient depletion to achieve a lateral and axial resolution of ~ 35 nm and ~ 90 nm, respectively. I characterize the system’s performance by imaging colloidal particles and single fluorescent molecules. Its biological applicability is demonstrated by imaging of nuclear pore complexes in U2OS and yeast cells, replication complexes in C2C12 cells, C2 toxin component C2I in HeLa cells and scaffold proteins in chemical synapses of hippocampal neurons. Advice on how to build such a microscope is given in the appendix. In addition, a theory is presented which demonstrates that the resolution of STED microscopes can be further enhanced by using the arrival time of spontaneous emission by time-gating the detection.

The use of PbS quantum dots (QDs) acting as light absorbers in a range of nanostructured solar cell devices has been investigated. The impact of different QD deposition methods, of the nature and structure of different metal oxides serving as electrodes, as well as interface and surface effects on device performance has been explored. Chapter 3 describes the application of in situ grown PbS QDs as absorber layer for extremely thin absorber solar cells with the inorganic solid hole transporter CuSCN. A polystyrene-block-poly(ethylene oxide) block copolymer was employed as a structure-directing agent for the synthesis of mesoporous TiO2 metal oxide thin films with high surface area and ordered porous structure. Chapter 4 outlines further work in which water-solubilized ex situ grown QDs capped with L-glutathione ligands were employed in order to improve the loading of the PbS QDs onto the internal surface of the porous oxide. Successful sensitization was achieved by inducing opposite surface charges on the surfaces of the QDs and the oxide in order to attract and attach QDs onto the surface of the porous supporting oxide film. The sensitized TiO2 electrodes were used to make efficient liquid electrolyte quantum-dot-sensitized solar cells (QDSCs). Chapter 5 describes the use of SnO2, which has a lower lying conduction band than TiO2, to fabricate scaffolding electrodes that were sensitized with water-solubilized PbS QDs. Passivation of the SnO2 electrodes with a thin layer of MgO, TiO2 and a combination of both was utilized to investigate the effect of surface treatments on the performance of solid-state QDSCs, using Spiro-OMeTAD as organic hole transporter. Chapters 6 and 7 deal with different approaches towards interface tuning in solid-state QDSCs. This part of the work involved the study of solar cell devices utilizing in situ grown PbS QDs with and without organic and inorganic surface passivation, and ex situ grown PbS QDs anchored on mesoporous TiO2 via organic linker molecules. The performance of the fabricated solar cells was evaluated with standard current-voltage and incident-photon-to-collected-electron efficiency measurements, and physical parameters of the devices were characterised with frequency- and time-resolved techniques such as electrochemical impedance spectroscopy, intensity-modulated photovoltage/photocurrent spectroscopy, and open circuit voltage decay measurements, respectively. Overall, the work highlights the importance of surface passivation of QDs, loading of the QDs onto porous semiconducting oxide electrodes, as well as the significance of interfacial effects between QDs, oxides and hole transporter to achieve high-efficiency devices.

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A variety of electronic and optoelectronic devices based on carbon nanotubes (CNTs) has been implemented during the last two decades. For their optoelectronic characterization, diffraction-limited techniques such as photocurrent (PC) and electroluminescence (EL) microscopy were employed. However, for the full characterization of these nano-devices, novel techniques providing nanoscale spatial resolution are desired. This work presents antenna-enhanced optoelectronic probing as a new scanning probe technique for the investigation of nanoelectronic devices. Based on tip-enhanced near-field optical microscopy, sub-diffraction spatial resolution is achieved by employing an optical antenna for the focusing of light. It is applied to study PC and EL signals with a spatial resolution better than 40 nm for the first time. Complemented with antenna-enhanced Raman and topography images, new insights into the optoelectronic properties of CNT based devices are gained. In the first part of this thesis, an antenna-enhanced photocurrent microscopy study is demonstrated. The signal enhancement mechanism of PC signals is investigated and compared with expectations based on theory. While in spectroscopic applications both the excitation AND the emission rate is enhanced, in optoelectronic applications either the excitation OR the emission rate is affected by the antenna. Theory predicts therefore a weaker total signal enhancement and a lower spatial resolution of optoelectronic signals compared to Raman scattering by a factor of √2, which is experimentally confirmed. Then, two applications are presented. First, CNT-metal interfaces are studied and an exponential decay of the band bending at the contacts with a decay length of about 500 nm is revealed. Second, sub-diffraction potential modulations along the CNT channel of another device are probed that remain undetected using confocal microscopy. Combined with high-resolution spectroscopic images of the Raman signal, defects can be excluded as the cause for these modulations. Correlating the PC with the topographic profile reveals charges associated with a particle on the sample substrate as the possible origin. In the second part, antenna-enhanced electroluminescence microscopy is introduced. The EL emitted by a heterogeneous CNT network is studied with a resolution better than 40 nm. For the first time, pinning of the EL emission to a point-like region of smaller than 20 nm is observed. This strong localization occurs at a junction of at least one metallic and one semiconducting CNT. By probing the PC signal at this junction, the presence of a strong local electric field is revealed, probably caused by a Schottky contact. This allows to identify impact excitation as the most likely origin of the EL emission. A second device, based on a single CNT, was investigated and, in contrast to the network device, the size of the EL source is extended over a length of more than 100 nm.