Materials Science & Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2792
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Item LOW TEMPERATURE PLASMA-METAL INTERACTIONS: PLASMA-CATALYSIS AND ELECTRON BEAM-INDUCED METAL ETCHING(2024) Li, Yudong; Oehrlein, Gottlieb G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Low-temperature plasma can generate different types of chemically reactive species at gas temperatures far below what is required to form such species from thermal excitation. Interactions between these reactive plasma-generated species and material surfaces have great potential for various applications, such as semiconductor etching or gas conversion. Synergistic effects, where the production rate with two inputs is greater than the sum of the consequences of each individually, have been demonstrated by combining the plasma with other energy inputs such as heat or kinetic energy from ions or electrons. Understanding the mechanisms by which these species interact with relevant surfaces is vital for the future development of plasma processing, chemistry and physics. In this work, we focus on the interaction of long-lived plasma species, particularly neutrals, with metal. A remote plasma-surface configuration was applied, where the plasma itself does not directly contact the surface. Two examples of plasma-metal interactions will be discussed, one taking place at atmospheric and the other at low pressure. The first case is plasma-assisted catalytic oxidation of methane (CH4) using a nickel (Ni) catalyst at atmospheric pressure, implemented by combining a remote plasma jet. The interrelation of real-time measurements of reaction products and surface adsorbates and plasma diagnostics allowed the identification of atomic oxygen as the key plasma-generated species that drives the synergistic plasma-catalytic reaction. The in-situ characterizations of the surface and gas phase reactions reveal the possible key reaction pathways for the plasma-catalysis reactions. We also observed the activation of the catalyst resulting from long-lasting catalyst surface modification induced by plasma species interaction. The second case is the damage-free etching of refractory metals, ruthenium (Ru) and tantalum (Ta), at low pressure. This was implemented by combining a remote plasma source (RPS) with an electron beam (EB) source. We investigated the effects of CF4 and Cl2 additions to Ar/O2 RPS effluents and we find that Ar/O2 with Cl2 addition induces the highest Ru etch rate (ER) and best removal selectivity over Ta. The surface chemistry characterization by spatially-resolved XPS reveals the possible mechanism of the electrons and neutrals induced materials etching. We also proposed a model that considers the fundamental aspects of the etching reaction and successfully predicts the major features of the electron and neutral induced etching reactions.Item Low and Atmospheric Pressure Plasma Interactions with Biomolecules and Polymers(2015) Bartis, Elliot Andrew James; Oehrlein, Gottlieb S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cold atmospheric plasma (CAP) sources have emerged as economical and environmentally friendly sources of reactive species with promising industrial and biomedical applications. Many different sources are studied in the literature for advanced applications including surface disinfection, wound healing, and cancer treatment, but the underlying mechanisms for these applications are not well-understood. The overall goals of this dissertation are to 1) identify how plasma treatments induce surface modifications and which plasma species are responsible for those modifications; 2) identify how changes in surface and plasma chemistry contribute to changes in biological activity of biomolecules; and 3) investigate how fluxes of reactive species produced by atmospheric pressure plasma devices can be controlled. As a first step, a well-studied low pressure plasma system was used to isolate the effects of ions, high energy photons, and radicals using Ar and H2 plasma. The finding that plasma-generated radicals can biodeactivate and modify films with negligible etching motivated further study at atmospheric pressure. Two very different CAP sources were used under mild, remote conditions to study the biological deactivation of two immune-stimulating biomolecules: lipopolysaccharide (LPS), found in bacteria such as Escherichia coli, and peptidoglycan, found in bacteria such as Staphylococcus aureus. The surface chemistry was measured to understand which plasma- generated species and surface modifications are important for biological deactivation. To simplify the complex molecular structure of the biomolecules and study specific moieties, model polymer films were studied including polystyrene, poly(methyl methacrylate), polyvinyl alcohol, and polypropylene. The interaction of the plasma plume with the environment was studied as a parameter to tune surface modifications. It was found that increasing ambient N2 concentrations in an N2/Ar ambient decreased surface modifications of LPS, similarly to how adding N2 to the O2/Ar feed gas decreased the plasma-generated O3 density and O atom optical emission. In this work, we first observed the formation of surface-bound NO3 after plasma treatment, which had not been reported in the literature. The plasma-ambient interaction was further studied using polystyrene as a model system. This detailed study demonstrated a competition between surface oxidation and nitridation, the latter of which occurs under very specific conditions. It was found that NO3 formed on all the materials studied in this dissertation after plasma treatment. This NO3 formed after treatment by both sources, but in different concentrations. The surface-bound NO3 correlated better with changes in biological activity than general oxidation, demonstrating its importance. Studying model polymers revealed that this surface moiety preferentially forms on – OH containing surfaces. Since the atmospheric pressure plasma jet (APPJ) operates with low N2/O2 admixtures to Ar and the surface microdischarge (SMD) operates with N2/O2 mixtures, the mechanisms that cause biological deactivation must be different, and are discussed.