Theses and Dissertations at Montana State University (MSU)
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Item Temperature dependent second harmonic generation studies of materials used in energy conversion applications(Montana State University - Bozeman, College of Engineering, 2022) McNally, Marshall Traver; Chairperson, Graduate Committee: Robert Walker; This is a manuscript style paper that includes co-authored chapters.Materials in energy conversion devices often undergo a variety of degradation mechanisms. Solid oxide fuel cell cathodes materials, for example, are subject to surface compositional changes due to material segregation. The extreme operating conditions in these energy conversion devices requires the development of an operando technique that is surface and material specific to accurately probe these degradation mechanisms. Second harmonic generation (SHG) is a surface specific technique that probes the electronic structure of a material using the 2nd order polarizability. Using well characterized materials like Au, Si and NiO, we began investigating how high temperatures (260 °C) and atmospheric composition affected the surface electronic structures. To do this, a custom sample chamber dubbed TROPICS was designed and built to achieve temperature, atmospheric compositional and eventually, electrochemical control. We found that gold's SH intensity was enhanced (3.5 times) when O 2 was present in the atmosphere but this enhancement disappeared at high temperatures. Using data from titrating O 2 into a N 2 atmosphere, we concluded that a monolayer of O 2 was forming on the gold surface, providing backbonding opportunities for gold's free electrons into the partially filled O 2 pi* orbitals. Similar behavior was seen in N-type Si which also showed SH enhancement at room temperature. However, P-type and undoped Si showed no such atmospheric dependent behavior. SHG experiments done with NiO showed decoupled behavior in the electronic structure recovery between the bulk and surface. After heating to 260 °C, the SH signal did not return to pre-heating intensities but required ~60 and ~90 minutes in N 2 and air respectively. The difference in recovery time between N 2 and air could be attributed to interactions between the still paramagnetic NiO electrons and the partially filled O 2 pi* orbitals.Item Unusual isomerization behavior of organic solutes at the aqueous-silica interface(Montana State University - Bozeman, College of Letters & Science, 2019) Purnell, Grace Elizabeth; Chairperson, Graduate Committee: Robert Walker; Robert A. Walker was a co-author of the article, 'Hindered isomerization at the silica/aqueous interface: surface polarity or restricted solvation?' in the journal 'Langmuir' which is contained within this dissertation.; Robert A. Walker was a co-author of the article, 'Surface solvation and hindered isomerization at the water/silica interface explored with second harmonic generation' in the journal 'The journal of chemical physics' which is contained within this dissertation.; Marshall T. McNally, Patrik R. Callis and Robert A. Walker were co-authors of the article, 'Buried liquid interfaces as a form of chemistry in confinement: the case of 4-dimethylaminobenzonitrile at the silica-aqueous interface' submitted to the journal 'The journal of the American Chemical Society' which is contained within this dissertation.; Marshall T. McNally, and Robert A. Walker were co-authors of the article, 'Isomerization at aqueous-silica interfaces and the role of solute structure' submitted to the journal 'Chemical physics letters' which is contained within this dissertation.Experiments described in this thesis address the question of how strong association between water molecules and the silica surface alter the solvation and isomerization behavior of adsorbed organic molecules from bulk solution limits. The work was motivated by the hypothesis that the ice-like structure induced by strong hydrogen bonding with the surface silanol groups would restrict solute isomerization. This hypothesis was tested using 2 surface-specific spectroscopic techniques: second harmonic generation (SHG) and time-correlated single photon counting in a total internal reflection geometry (TIR-TCSPC). This work examined two different 7-aminocoumarin dyes (Coumarin 151 and Coumarin 152) and dimethylaminobenzonitrile (DMABN). Coumarin 152 and DMABN both isomerize to form a twisted intramolecular charge transfer (TICT) state upon photoexcitation, whereas Coumarin 151 forms a simple (planar) intramolecular charge transfer state. SHG studies characterized the local solvation environment surrounding adsorbed molecules by providing electronic excitation energies that were compared to bulk excitation energies in different representative solvents. TIR-TCSPC measured the time-resolved emission of adsorbed molecules and quantified a solute's tendency to form TICT (or ICT) isomers at the aqueous-silica interface. Together, SHG and TIR-TCSPC provide a cohesive description of the local polarity across an aqueous-silica interface and how restricted solvent dynamics change a solute's photophysical chemistry. TIR-TCSPC studies reported that both C152 and DMABN are unable to isomerize to TICT states at the aqueous-silica interface, acting as if they were solvated in a nonpolar solvent or in a confined geometry. SHG studies confirm that the aqueous-silica interface is, in fact, more polar than the bulk aqueous limit, strongly implying that the observed effects are dynamic in origin rather than polarity driven. In contrast, studies of C151 show that this solute is largely insensitive to anisotropic, restrictive surface effects. Together results from these three molecules lead us to conclude that adsorption to the strongly associating aqueous-silica interface restricts large amplitude isomerization in organic molecules. Adsorption to less strongly associating interfaces does not cause this restriction. In the event that photo-induced isomerization does not require large amplitude motion, interfacial solvation has little effect on adsorbed solute behavior.Item Investigation of nanoscale etching and poling of lithium niobate(Montana State University - Bozeman, College of Engineering, 2014) Smith, Stacie Elizabeth; Chairperson, Graduate Committee: Wataru NakagawaThe capabilities of some nonlinear optical devices can be improved through approaches such as nano-optics. Two methods, in particular, that can enhance the wavelength conversion efficiency and versatility of current second harmonic generation (SHG) devices are creating nanoscale domain inversions (to make for efficient quasi-phase matched SHG devices at various wavelengths) and gratings in lithium niobate (to potentially achieve exact-phase matching). This thesis explores these options, creating nanoscale domain inversions and nanostructuring lithium niobate, in order to enhance current SHG devices. First, an in-depth literature survey is provided detailing the current research regarding structuring lithium niobate. Next, a description and analysis of the inductively coupled plasma reactive ion etch (ICP-RIE) etching procedures used are provided, followed by a discussion of the poling of lithium niobate using an all optical poling technique. Suggestions for continued development are presented based on the successes and failures of the procedures used for this work. The goal of this thesis is to show that lithium niobate can be nanostructured using ICP-RIE etching techniques and optical poling methods. This goal is a foundation towards the long-term goal of building more efficient nonlinear optical devices. Nanostructuring lithium niobate suggests that improved nonlinear optical devices can be made in the future, by means of nanoscale domain inversions for quasi-phase matching or nanoengineered gratings intended for exact-phase matching.