Date of Award
Doctor of Philosophy in Chemistry
William B. Euler
With modern technological advances, we are seeing an increasing amount of electronic devices and materials going the route of smaller and faster. It has become of great importance to understand how these devices and materials work on the nanometer scale. Many of these new materials are coming to fruition in the form of thin films, and the topic of this dissertation focuses on the application of these materials to the field of chemical sensing. Chapter 1 of this dissertation investigates the photophysics of a thin film of the fluorescent dye, rhodamine 6G (Rh6G), on the surface of poly(vinylidene fluoride) (PVDF) coated glass, to determine the origin of a previously reported fluorescence enhancement. Three factors were identified that contribute to the increase in fluorescence seen in this system when compared to a thin film of Rh6G on a bare glass surface. First, the surface roughness of the underlying PVDF films provides a larger area for an excitation beam to interact with, which leads to the excitation of more Rh6G molecules. Second, the PVDF layer reduces the amount of aggregation between Rh6G molecules as the film thickness of the dye layer increase, which preserves the emissive monomeric form of Rh6G. Last, the PVDF thin films acts as a light trapping layer which leads to more Rh6G absorption events and more efficient use of the incident light.
Chapter 2 of this dissertation shows how the surface morphology of polystyrene (PS) thin films on a glass substrate evolves as the molecular weight (Mw) of the PS used to cast the film is changed. Optical profilometry was used to collect images of the different films which revealed that PS pillars are formed in the center which transition into wrinkles that extend out from the center. The periodicity and amplitudes of these features changed with the Mw, which was found to be a result of the direct relationship between the glass transition temperature of PS and its Mw. Furthermore, this study showed a method of producing micrometer sized wrinkled interfaces spontaneously, which before required a more complicated process to fabricate.
The last chapter of this dissertation, chapter 3, contains a spectroscopic study on thin films of differently charged xanthene dyes on the surface of PS coated glass to investigate potential ion-π interactions. Coupling deconvolution of absorbance and emission spectra with measurement of excited state lifetimes of the dyes on the PS surface revealed that a large portion of monomer emission from the cationic dye (Rh6G) and the anionic dye (disodium fluorescein, DSF) was quenched by formation of a weakly emissive exciplex. This is in stark contrast to the neutral dye (fluorescein 27, F27), which showed no sign of exciplex formation and had the opposite emission behavior with respect to increasing the dye layer film thickness when compared to the charged dyes.
The overall findings of these studies show that there are complicated dynamics that can occur in solid phase, layered thin film systems. Interfacial interactions, whether they originate in physical or chemical nature, can have a vast effect on the performance of these devices. This dissertation demonstrates how having a fundamental understanding of such systems can reveal information necessary for the complete optimization of sensors and other nano-sized devices.
Mullen, Matthew J., "An Investigation into the Influence of Interfacial Effects in a Three Layer Solid Phase Sensing Platform" (2017). Open Access Dissertations. Paper 577.