Structural Characterization of Thin Films of Molecules on Solid Surfaces: Development of a Portable UHV Chamber to Bridge RHEED and SFG Spectroscopy Techniques



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In surface science, it is often essential to visualize and obtain the structures of molecules at solid-liquid and solid-vacuum interfaces, etc. Each surface characterization method has its advantages and disadvantages, and there is no single technique that can provide all possible information about one sample. Therefore, the combined use of complementary surface science methods may offer a more detailed picture of the system. One must then consider multiple factors depending on the nature of the methods or the samples under study. This thesis describes studies of selected interfacial structures, taking advantage of the strengths of reflection high-energy electron diffraction (RHEED) and sum-frequency generation (SFG) spectroscopy techniques, which include thin films of ionic liquids (ILs) deposited by physical vapor deposition and self-assembled monolayers on gold. A thickness-dependent structural transition of ILs on highly-oriented pyrolytic graphite (HOPG) was observed by RHEED, where an ordered structure for 2–10-nm films was found at low temperatures while no apparent 3D crystallization was found for films thinner than 2 nm nominally and thicker than ~10 nm. An open question about the structural details is discussed, which motivates the design of a portable setup to maintain the sample under vacuum and bridge the RHEED and SFG techniques. Furthermore, a combined study using RHEED and SFG on rubrene single crystals and a temperature-dependent SFG study in a high-vacuum cell are two other case studies in this thesis. In summary, this work enables a vast possibility of future experiments to investigate structures of thin films, their interactions with other systems, studies on both sides of a thin film (film-solid, film-vacuum) interfaces, etc. The support of an R. A. Welch Foundation grant (E-1860) and a National Science Foundation grant (CHE-1653903) is acknowledged.



RHEED, SFG, Thin Films, Characterization, UHV, Rubrene, Ionic Liquids, SAMs, PVD.