Ordered Structures of Molecular Assemblies at Interfaces

dc.contributor.advisorYang, Ding-Shyue
dc.contributor.committeeMemberBaldelli, Steven
dc.contributor.committeeMemberXu, Shoujun
dc.contributor.committeeMemberGuloy, Arnold M.
dc.contributor.committeeMemberCheung, Margaret S.
dc.creatorWu, Chengyi 1990-
dc.date.createdMay 2019
dc.date.submittedMay 2019
dc.description.abstractHeterogeneous interfaces play a significant role in various fields of science and technology, including catalysis, corrosion, electrochemistry, and even glass and phase transitions. Understanding and visualization of solid-adsorbate interfaces serve as one of the major aspects in surface science to interpret fundamental processes and behaviors. Compared to conventional optical spectroscopic methods which have poor spatial resolution due to the diffraction limit, reflection high-energy electron diffraction (RHEED) provides atomic structural resolution via reciprocal-space imaging and surface specificity even at the sub-nm level. These advantages make RHEED applicable, not only to material science for traditional thin-film fabrication, but also to studies of molecular assemblies at interfaces. In this project, RHEED is applied to investigate three nm-thick interfacial assemblies with different intermolecular interactions. In Chapter 3, diffraction results indicate that thin films of ionic liquids (ILs) exhibit bulk-like ordered structures on highly oriented pyrolytic graphite (HOPG). Such ordered structures are attributed to the lattice-matching template effects between ILs and graphite layers. Although the Coulombic forces in ILs alone may not be enough to form ordered structures with large domains on smooth surfaces, methanol with a hydrogen-bonding network, in contrast, shows 2- and 3-dimensional ordered structures on hydrophobic surfaces. In Chapter 4, the crystallization temperatures and structural transformations of methanol strongly depend on the thermal annealing procedures used. These observations reveal the unique self-assembled property of interfacial methanol even without the topographic guidance from smooth surfaces. In Chapter 5, unexpected 3-dimensional single crystals of acetonitrile are found to form on HOPG, where the main intermolecular forces are dipole-dipole interactions. Unlike the thermal-driven phase transition from β to α in bulk, a morphology-induced α-to-β phase transition is observed by using HOPG samples with different step-edge densities and increasing the film thickness. In summary, this work demonstrates the capability of RHEED in structural studies of molecular assemblies at interfaces and elucidates the influences of intermolecular forces and template effects in different solid-adsorbate systems. It also presents opportunities for further studies of structural dynamics, using time-resolved electron diffraction. An example is given by the slowed energy transfer rate across the 2-dimensional methanol layers in Chapter 4.
dc.description.departmentChemistry, Department of
dc.format.digitalOriginborn digital
dc.identifier.citationPortions of this document appear in: He, Xing, Chengyi Wu, and Ding-Shyue Yang. "Communication: No guidance needed: Ordered structures and transformations of thin methanol ice on hydrophobic surfaces." J. Chem. Phys. 145 (2016), 171102. And in: He, X., Wu, C., Rajagopal, K., Punpongjareorn, N., Yang, D.-S. Ordered ionic liquid structure observed at terraced graphite interfaces. Phys. Chem. Chem. Phys.2016, 18, 3392–3396.
dc.rightsThe author of this work is the copyright owner. UH Libraries and the Texas Digital Library have their permission to store and provide access to this work. UH Libraries has secured permission to reproduce any and all previously published materials contained in the work. Further transmission, reproduction, or presentation of this work is prohibited except with permission of the author(s).
dc.subjectReflection high-energy electron diffraction (RHEED)
dc.subjectTemplate effect
dc.titleOrdered Structures of Molecular Assemblies at Interfaces
thesis.degree.collegeCollege of Natural Sciences and Mathematics
thesis.degree.departmentChemistry, Department of
thesis.degree.grantorUniversity of Houston
thesis.degree.nameDoctor of Philosophy


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