Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--Michigan State University. Dept. of Electrical & Computer Engineering, 2007. / Title from PDF t.p. (viewed on Apr. 21, 2009) Includes bibliographical references (p. 123-129). Also issued in print.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 177-184). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 109-126).

In this dissertation, cross-sectional potential imaging of GaAs-based homoepitaxial, heteroepitaxial and quantum well structures, all grown by atmospheric pressure Metal-organic Vapor Phase Epitaxy (MOVPE) is investigated. Kelvin probe force microscopy (KPFM), using amplitude modulation (AM) and frequency modulation (FM) modes in air and at room temperature, is used for the potential imaging. Studies performed on n-type GaAs homoepitaxial structures have shown two different potential profiles, related to the difference in electron density between the semi-insulating (SI) substrate and the epilayers. It is shown that the contact potential difference (CPD) between the tip and sample is higher on the semi-insulating substrate side than on the n-type epilayer side. This change in CPD across the interface has been explained by means of energy band diagrams indicating the relative Fermi level positions. In addition, it has also been found that the CPD across the interface increases with electron density. This result is in qualitative agreement with theory. In addition, as known from literature, even under ambient conditions FM mode KPFM provides better lateral resolution and more realistic CPD values than AM mode KPFM. Compared to the case of AM mode analysis, where the experimental CPD values were on average of the theoretical values, the CPD values from FM mode analysis are on average of the theoretical ones. Furthermore, by using FM mode, the transition across the interface is sharper and the surface potential flattens/saturates as expected when scanning sufficiently far away from the junction. The non-neutral space charge region of the sample with an electron density of for example, is as measured by FM-KPFM, whereas for AM-KPFM, the width is even more than and the potential profiles do not saturate. For the p-type GaAs homoepitaxial structures, FM mode measurements from a sample with a dopant density of are presented. As in the case of n-type GaAs,a similar potential profile showing two main domains has been obtained. However, unlike the case of type GaAs where the potential measured on the epilayer side is higher than that on the substrate side, the potential on the epilayer side of the junction is lower in this case due to the fact that the Fermi level of p-type GaAs is below that of the substrate.

No description available.

Xue Kun. / "July 2005." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract in English and Chinese.

Traditional light microscopy suffers from the diffraction limit, which limits the spatial resolution to λ/2. The current trend in optical microscopy is the development of techniques to bypass the diffraction limit. Resolutions below 40 nm will make it possible to probe biological systems by imaging the interactions between single molecules and cell membranes. These resolutions will allow for the development of improved drug delivery mechanisms by increasing our understanding of how chemical communication within a cell occurs. The materials sciences would also benefit from these high resolutions. Nanomaterials can be analyzed with Raman spectroscopy for molecular and atomic bond information, or with fluorescence response to determine bulk optical properties with tens of nanometer resolution. Near-field optical microscopy is one of the current techniques, which allows for imaging at resolutions beyond the diffraction limit. Using a combination of a shear force microscope (SFM) and an inverted optical microscope, spectroscopic resolutions below 20 nm have been demonstrated. One technique, in particular, has been named tip enhanced near-field optical microscopy (TENOM). The key to this technique is the use of solid metal probes, which are illuminated in the far field by the excitation wavelength of interest. These probes are custom-designed using finite difference time domain (FDTD) modeling techniques, then fabricated with the use of a focused ion beam (FIB) microscope. The measure of the quality of probe design is based directly on the field enhancement obtainable. The greater the field enhancement of the probe, the more the ratio of near-field to far-field background contribution will increase. The elimination of the far-field signal by a decrease of illumination power will provide the best signal-to-noise ratio in the near-field images. Furthermore, a design that facilitates the delocalization of the near-field imaging from the far-field will be beneficial. Developed is a novel microscope design that employs two-photon non-linear excitation to allow the imaging of the fluorescence from almost any visible fluorophore at resolutions below 30 nm without changing filters or excitation wavelength. The ability of the microscope to image samples at atmospheric pressure, room temperature, and in solution makes it a very promising tool for the biological and materials science communities. The microscope demonstrates the ability to image topographical, optical, and electronic state information for single-molecule identification. A single computer, simple custom control circuits, field programmable gate array (FPGA) data acquisition, and a simplified custom optical system controls the microscope are thoroughly outlined and documented. This versatility enables the end user to custom-design experiments from confocal far-field single molecule imaging to high resolution scanning probe microscopy imaging. Presented are the current capabilities of the microscope, most importantly, high-resolution near-field images of J-aggregates with PIC dye. Single molecules of Rhodamine 6G dye and quantum dots imaged in the far-field are presented to demonstrate the sensitivity of the microscope. A comparison is made with the use of a mode-locked 50 fs pulsed laser source verses a continuous wave laser source on single molecules and J-aggregates in the near-field and far-field. Integration of an intensified CCD camera with a high-resolution monochromator allows for spectral information about the sample. The system will be disseminated as an open system design.

The goal of this thesis was the advanced design, fabrication, and application of combined atomic force microscopy - scanning electrochemical microscopy (AFMSECM) probes for high-resolution topographical and electrochemical imaging. The first part of the thesis describes innovative approaches for the optimization of AFM-SECM probe fabrication with recessed frame electrodes. For this purpose, commercial silicon nitride AFM cantilevers were modified using optimized critical fabrication processes including improved metallization for the deposition of the electrode layer, and novel insulation strategies for ensuring localized electrochemical signals. As a novel approach for the insulation of AFM-SECM probes, sandwiched layers of PECVD SixNy and SiO2, and plasma-deposited PFE films were applied and tested. Using sandwiched PECVD SixNy and SiO2 layers, AFM-SECM probes providing straight (unbent) cantilevers along with excellent insulation characteristics facilitating the functionality of the integrated electrode were reproducibly obtained. Alternatively, PFE thin films were tested according to their utility for serving as a mechanically flexible insulating layer for AFM-SECM probes. The electrochemical characterization of PFEinsulated AFM-SECM probes revealed excellent insulating properties at an insulation thickness of only approx. 400 nm. Finally, AFM-SECM cantilevers prepared via both insulation strategies were successfully tested during AFM-SECM imaging experiments. In the second part of this thesis, disk-shaped nanoelectrodes were for the first time integrated into AFM probes for enabling high-resolution AFM-SECM measurements. Disk electrodes with an electrode radius < 100 nm were realized, which provides a significantly improved lateral resolution for SECM experiments performed in synchronicity with AFM imaging. Furthermore, the developed fabrication scheme enables producing AFM-SECM probes with integrated disk nanoelectrodes at significantly reduced time and cost based on a highly reproducible semi-batch fabrication process providing bifunctional probes at a wafer scale. The development of a detailed processing strategy was accompanied by extensive simulation results for developing a fundamental understanding on the electrochemical properties of AFM-SECM probes with nanoscale electrodes, and for optimizing the associated processing parameters. Thus fabricated probes were electrochemically characterized, and their performance was demonstrated via bifunctional imaging at model samples. The third part of this thesis describes the development and characterization of the first AFM tip-integrated potentiometric sensors based on solid-state electrodes with submicrometer dimensions enabling laterally resolved pH imaging. Antimony and iridium oxides were applied as the pH sensitive electrode material, and have been integrated into the AFM probes via conventional microfabrication strategies. The pH response of such AFM tip-integrated integrated pH microsensors was tested for both material systems, and first studies were performed demonstrating localized pH measurements at a model system.