Ordered Structures of Molecular Assemblies at Interfaces
Heterogeneous 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.