Instrumentation Development for Interfacial Studies and Analysis of Substrate-Supported Molecular Thin Films by Reflection High-Energy Electron Diffraction
Rajagopal, Karjini 1987-
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Chemical and physical properties at interfaces play key roles in understanding a number of diverse phenomena. A better molecular-level understanding of the interfacial interactions is needed to utilize them in the potential applications. Different surface-analysis techniques have their own distinct level of surface sensitivity and probe different surface characteristics of the system of interest. Our interest is to directly visualize the structure of atoms and molecules at interfaces of the substrate-supported molecular thin films using reflective high-energy electron diffraction (RHEED). The shorter wavelength of an electron compared to typical bond lengths and its larger scattering cross section make it ideal for surface and interfacial probing. Interfacial behaviors in structure, interactions, phase transition, and thermal evolution of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]) vapor deposited ionic liquid (IL) thin films on substrate surfaces such as highly oriented pyrolytic graphite (HOPG), Hydrogen terminated Si(111), mica, Cu(111) and Ni(111) were studied. By comparing the ordered structural behavior of 3 nm thick [EMIM][Tf2N] film on HOPG with other surfaces, it is found that the terrace surface morphology of HOPG plays a role as a template for vertical stacking of IL ion pairs. The phase transition of [EMIM][Tf2N] on HOPG happened at 256 K, which matches with the melting point of bulk [EMIM][Tf2N] IL. The desorption of 3 nm [EMIM][Tf2N] IL from substrate happened starting from 340 - 360 K to ~ 375 K. As a second system, the interfacial structural ordering of water molecules on cadmium telluride (CdTe(111)A) surface and oxidized CdTe(111)A surface was investigated. A molecular beam doser system was developed to deposit the water molecules in a controlled and quantitative way. On CdTe(111)A surface, the deposited water molecules became ordered ice crystallites by adopting the geometry of the underlying substrate as evidenced by the Bragg diffraction spots. On oxidized CdTe(111)A surface upon deposition, initially formed amorphous water layer turned into Debye–Scherrer diffraction rings indicating the randomly oriented ice crystallites of cubic ice form. From the studies of both systems, the main role of the supported substrate in the ordering of molecules at the interface is found.