While prices of solar energy are becoming cost competitive with traditional fossil fuel resources, large scale deployment of solar energy has been limited by the inability to store excess electrical energy efficiently. One promising route towards both the capture and storage of solar energy is through photoelectrochemical water splitting, a process by which a semiconducting material can collect energy from the sun and use it to directly split water (H2O) into hydrogen fuel and oxygen. Unfortunately, photoelectrochemical water splitting devices are limited by the low efficiencies and high overpotentials of the oxygen evolution reaction (OER). To improve kinetics of OER, different electrocatalyst are often coated on the semiconductor. However, the role of the catalyst and the mechanism of charge transfer at the semiconductor|catalyst interface is not clear. It is important to understand this interface if we are to rationally design high performance water splitting cells. The research presented in this dissertation takes on two aims: 1) obtaining a fundamental knowledge of the charge transfer processes that take place at the semiconductor catalyst interface of photoanodes and 2) developing new experimental approaches that can be applied towards achieving the first aim. Specifically, this dissertation begins with a prospectus that outlines the state of the field, and the what was known about the semiconductor|electrocatalyst interface at the outset of the presented work (Chapter II). Next, the testing and application of new nanoelectrode AFM probes to study an array of electrochemical phenomena will be discussed (Chapter III). These probes will then be applied towards the study of hematite (Fe2O3) semiconductors coated with cobalt phosphate (oxy)hydroxide (CoPi) electrocatalyst (Chapter IV) and bismuth vanadate (BiVO4) semiconductors coated with CoPi electrocatalyst (Chapter 5). This dissertation includes previously published and unpublished co-authored material. / 2020-01-11

Falling technology costs are allowing renewable sources of energy to become increasingly more competitive with fossil fuel-based sources. However, challenges still remain in the widespread deployment of sources like wind and solar due to their intermittent nature and cost-prohibitive storage options. An attractive solution to address these issues is by using renewably derived energy to drive electrolysis reactions that generate useful chemicals and fuels. In order to do this effectively and economically, efficient and durable electrocatalysts are needed for the reactions of interest, such as hydrogen production from water electrolysis. Presently, the best catalysts for this process are noble metals such as platinum, which are expensive and in limited supply. The discovery and mechanistic understanding of earth abundant materials that can also efficiently catalyze these reactions remains a current research focus. Scanning probe microscopy (SPM) techniques can be used to aid in the discovery of these materials, as they are able to investigate catalyst surfaces in situ and at a higher resolution than conventional 3-electrode electroanalytical methods. This dissertation explores the use of two in situ SPM techniques, scanning electrochemical microscopy (SECM) and scanning photocurrent microscopy (SPCM), for evaluating both photocatalytic and electrocatalytic electrochemical systems. Three different studies that use these two techniques were carried out over the duration of my thesis work and are presented in Chapters 2 through 4. After providing an overview of solar fuels and SPM techniques in Chapter 1, Chapter 2 describes the design considerations, implementation and demonstration of a home-built SECM instrument for use with nonlocal continuous line probes (CLPs) that can achieve high areal imaging rates with compressed sensing (CS) image reconstruction. The CLP consists of an electroactive band electrode sandwiched between two insulating layers, where one of the insulating layers needs to be on the same length scale as the band electrode because it determines the average separation distance from the band electrode to the substrate. Similarly, the spatial resolution of the CLP is determined by the thickness of the band and the realizable imaging rate is determined by its width and linear scan rate. Like conventional SECM systems, a combination of linear motors and a bipotentiostat is needed. However, for the CLP-SECM system both linear and rotational motors are needed to scan at different substrate angles to obtain the necessary raw signal to reconstruct the target electrochemical image with CS algorithms. Detailed descriptions of the microscope design, CLP fabrication, and the procedures necessary to carry out the CLP-SECM imaging are given in this chapter. Measurements with this novel CLP-SECM microscope are done with flat platinum disk electrode samples of varying sizes. A substrate-generation-probe-collection mode is used during the SECM linescan measurements to illustrate procedures for position calibration of the system, CLP and substrate cleaning, as well as verifying the sensitivity along the length of the CLP. Finally, linescans over a three disk platinum sample were taken and CS image reconstruction was done, with as few as three linescans, to demonstrate the order of magnitude time advantage of this approach over conventional SECM scanning methods. In Chapter 3, colorimetric imaging studies are done using a pH dye indicator to visualize the plume of electroactive species that is generated during in situ SECM measurements for both conventional and CLP-SECM systems. In SECM, the signal recorded by the probe is facilitated by transport of electroactive species and not by direct contact between the probe and the substrate, which is typical of many scanning probe microscopy (SPM) techniques. One of the complexities with SECM is being able to fully understand the interaction between the electroactive species generated at the substrate and the probe. Thus in order to understand this further, a pH indicator dye is used to visualize pH gradients associated with the hydrogen product plume generated by water electrolysis during in situ SECM measurements. The in situ colorimetric experiments are then used to inform assumptions about the system and validate simulations using finite element modeling software. From this study, we are able to develop quantitative relationships to describe how the plume of electroactive species influences the recorded current at the probe for different probe geometries. Finally, we use this initial study as groundwork for investigating the influence of higher probe scan speeds where convection starts to play a role on the distortion of the signal and plume dynamics, and how it can be corrected using CS post-processing methods. Lastly, SPCM is employed in Chapter 4 to study the optical efficiency losses due to varying size bubbles on a photoelectrode surface. Individual single hydrogen bubbles ranging from 100 µm to 1000 µm were generated on a photoelectrode surface and a laser was used to scan over single isolated bubbles to create localized optical efficiency maps based on photocurrent and external quantum efficiency (EQE). Moreover, a ray-tracing model based on Snell’s law was also constructed to compare to experimental SPCM linescans. This model showed very good agreement to the experimental SPCM linescan results. This investigation showed that larger bubbles lead to higher optical efficiency losses, not only due to higher inactive electrochemically active surface areas (ECSAs) but also due to a larger region of total internal reflection of light from the edge regions of bubbles. A macroscale study over a large photoelectrode surface was also done where the images of the surface were taken while the “sawtooth” was measured under AM1.5 illumination. Consequently, a predictive current−time profile was generated from the single bubble SPCM empirical relationship between bubble size and optical losses and was compared to the experimental measurement. Understanding how bubbles can impact the efficiency of the overall system is important, as bubbles in a system and on an electrode surface increases ohmic resistances, optical losses, and kinetic losses. Overall, this study can be used as a starting point for designing systems, electrolyte, and catalyst surfaces to improve one or more of the aforementioned losses.

Fang, Qianying = 基於掃描探針顯微鏡(SPM)的關於鋁摻雜氧化鋅(ZnO:Al)局域電學性質之研究 / 方倩莹. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references (leaves 97-100). / Text in English; abstracts in English and Chinese. / Fang, Qianying = Ji yu sao miao tan zhen xian wei jing (SPM) de guan yu lü shan za yang hua xin (ZnO:Al) ju yu dian xue xing zhi zhi yan jiu / Fang Qianying. / Chapter I. --- Abstract / Chapter II. --- Acknowledgement / Chapter III. --- Table of contents / Chapter IV. --- List of figures / Chapter V. --- List of tables / Chapter 1 --- Introduction / Chapter 1.1 --- Motivations / Chapter 1.2 --- Outline of thesis / Chapter 2 --- Experimental Conditions and Techniques Used / Chapter 2.1 --- Sample preparation / Chapter 2.1.1 --- Radio frequency magnetic sputtering / Chapter 2.1.2 --- Substrates / Chapter 2.1.3 --- Thermal evaporation / Chapter 2.1.4 --- Thermal annealing / Chapter 2.2 --- Microscopic electrical measurement / Chapter 2.2.1 --- Conductive atomic force microscope (c-AFM) / Chapter 2.2.2 --- Scanning capacitance microscope (SCM) / Chapter 2.2.3 --- Surface Potential (SP) / Chapter 2.3 --- SEM and cathodoluminescence spectroscopy / Chapter 3 --- Calibrations / Chapter 3.1 --- Calibrations of c-AFM measurements / Chapter 3.1.1 --- Reproducible images / Chapter 3.1.2 --- Further statistical analysis / Chapter 3.1.3 --- Sample thickness effect / Chapter 3.1.4 --- Conclusions / Chapter 3.2 --- Calibrations of cathodeluminescence (CL) measurements / Chapter 3.2.1 --- Effect of removing residual magnetic field / Chapter 3.2.2 --- Effect of Faraday cup moving / Chapter 3.2.3 --- Time effect / Chapter 3.2.4 --- Effect of mirror shift / Chapter 3.2.5 --- Effect of electron beam shift / Chapter 3.2.6 --- Conclusions / Chapter 3.3 --- Calibrations of scanning capacitance microscope (SCM) measurements / Chapter 3.3.1 --- SCM images and morphological dependence of as-deposited AlOx/ZnO thin film / Chapter 3.3.2 --- Comparison between as-deposited and e-beam irradiated AlOx/ZnO thin film / Chapter 3.3.3 --- SCM images and morphological dependence of e-beam irradiated AlOx/ZnO thin film / Chapter 3.3.4 --- Conclusions / Chapter 4 --- Experimental Results and Data Analysis / Chapter 4.1 --- Conductive Atomic Force Microscope (c-AFM) / Chapter 4.1.1 --- Effect of scan rate / Chapter 4.1.2 --- Dual images and morphological dependence / Chapter 4.1.3 --- Statistic microscopic current-voltage (I-V) / Chapter 4.1.4 --- Schottky barrier at Pt-ZnO contact / Chapter 4.1.5 --- C-AFM artifact / Chapter 4.2 --- Scanning Capacitance Microscope (SCM) / Chapter 4.2.1 --- Dual images and morphological dependence / Chapter 4.2.2 --- Statistic microscopic SCM data-voltage (dC/dV-V) / Chapter 4.3 --- Surface Potential (SP) / Chapter 5 --- Discussions and Conclusion / Chapter 5.1 --- Mechanism / Chapter 5.2 --- Conclusions / Chapter 5.3 --- Future plan / Chapter 6 --- References

Tip Enhanced Near-field Optical Microscopy (TENOM) is a method for optically imaging at resolutions far below the diffraction limit. This technique requires optical nano-probes with very specialized geometries, in order to obtain large, localized enhancements of the electromagnetic field, which is the driver behind this imaging method. Traditional methods for the fabrication of these nano-probes involve electrochemical etching and subsequent FIB milling. However, this milling process is non-trivial, requiring multiple cuts on each probe. This requires multiple rotations of the probe within the FIB system, which may not be possible in all systems, meaning the sample must be removed from vacuum, rotated by hand and placed back under vacuum. This is time consuming and costly and presents a problem with reproducibility. The method presented here is to replace multiple cuts from a side profile with a small number of cuts from a top down profile. This method uses the inherent imaging characteristics of the FIB, by assigning beam dwell times to specific locations on the sample, through the use of bitmap images. These bitmaps are placed over the sample while imaging and provide a lookup table for the beam while milling. These images are grayscale with the color of each pixel representing the dwell time at that pixel. This technique, combined with grayscale gradients, can provide probes with a symmetric geometry, making the system polarization independent.

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

The primary goal of my dissertation was to build a Scanning Hall Probe Microscope (SHPM) for studying the domain structure of a ferromagnetic semiconductor (Ga[subscript 0.94]Mn[subscript 0.06]). This new semiconductor may be used in the emerging field of spintronics, where both the charge and spin of an electron are utilized. The first part of this dissertation introduces the scanning probe microscopy techniques that are used for our homemade SHPM performance test and images. In chapter 2, general spintronics and ferromagnetic semiconductor are introduced. A compact design of our LT-SHPM is introduced in chapter 3. A unique taper seal based on stainless steel and Cu for opening/closing the vacuum chamber is used for our homemade SHPM. In chapter 4, Hall probes are discussed. In this chapter, ESD (Electrostatic discharge) and its repair work are discussed. Finally, in Chapter 5, SHPM imaging results of Ga[subscript 0.94]Mn[subscript 0.06]As are discussed. We observed stripe domain patterns. We also observed the domain patterns as a function of magnetic field and temperature. / text

This dissertation presents experimental studies on Brownian motion at fast time scales, as well as our recent developments in Thermal Noise Imaging which uses thermal motions of microscopic particles for spatial imaging. As thermal motions become increasingly important in the studies of soft condensed matters, the study of Brownian motion is not only of fundamental scientific interest but also has practical applications. Optical tweezers with a fast position-sensitive detector provide high spatial and temporal resolution to study Brownian motion at fast time scales. A novel high bandwidth detector was developed with a temporal resolution of 30 ns and a spatial resolution of 1 °A. With this high bandwidth detector, Brownian motion of a single particle confined in an optical trap was observed at the time scale of the ballistic regime. The hydrodynamic memory effect was fully studied with polystyrene particles of different sizes. We found that the mean square displacements of different sized polystyrene particles collapse into one master curve which is determined by the characteristic time scale of the fluid inertia effect. The particle’s inertia effect was shown for particles of the same size but different densities. For the first time the velocity autocorrelation function for a single particle was shown. We found excellent agreement between our experiments and the hydrodynamic theories that take into account the fluid inertia effect. Brownian motion of a colloidal particle can be used to probe three-dimensional nano structures. This so-called thermal noise imaging (TNI) has been very successful in imaging polymer networks with a resolution of 10 nm. However, TNI is not efficient at micrometer scale scanning since a great portion of image acquisition time is wasted on large vacant volume within polymer networks. Therefore, we invented a method to improve the efficiency of large scale scanning by combining traditional point-to-point scanning to explore large vacant space with thermal noise imaging at the proximity of the object. This method increased the efficiency of thermal noise imaging by more than 40 times. This development should promote wider applications of thermal noise imaging in the studies of soft materials and biological systems. / text

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