A clear knowledge of structures is essential to the understanding and potential control of complex interfacial phenomena that involve multiple intermolecular and surface interactions of different strengths. Molecules with the ability to form hydrogen bonds are often of particular interest. Here, we report the observation of 2- and 3-dimensional ordered assemblies of methanol molecules on hydrophobic silicon surfaces, using reflection high-energy electron diffraction. Direct structure probing reveals that the crystallization temperatures and the structural transformations of the hydrogen-bonded networks are far beyond a single-stage description and strongly depend on the thermal annealing procedures used. Such results elucidate the unique self-assembling behavior of interfacial methanol even without much guidance from the smooth substrate.
In
this report, reflection high-energy electron diffraction as
a direct structure-probing method is used to reveal an unanticipated
vertical ordering and long-range crystallinity in interfacial acetonitrile
physisorbed on highly oriented pyrolytic graphite (HOPG), whose assembly
structures are relevant to further development of batteries. Even
without significant guiding forces in this solid–molecule system,
the surface morphology and terraces of HOPG play a critical role in
molecular ordering as a result of the lattice-matching template effect.
More surprisingly, a phase transition between two acetonitrile polymorphs
solely due to surface topography of HOPG is observed for the first
time. By careful examination of the crystal structures, the interfacial
ordering and the structural transition can be understood as the results
of a competition between the kinetically controlled Ostwald process
and thermodynamic energetics. Finally, the present observations add
to the mounting evidence for the need to explicitly take surface morphology
of HOPG into consideration and the unique strength of reflection electron
diffraction for interfacial studies.
Reflection high-energy electron diffraction is presented as a contactless, surface-specific method to probe the ion organization and layering at the ionic liquid-solid interfaces. Three regimes can be identified for the structure of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]) on highly oriented pyrolytic graphite, which is strongly dependent on the distances of ions from the surface. Direct observations showed that the ultrathin ionic liquid (IL) assembly can exhibit bulk-like phase-transition behaviours as a result of the structural matching between the IL and graphite layers and the confinement template effect due to the surface topography of graphite. The present study illustrates the opportunities for conducting further studies of the structures and ultrafast dynamics of IL-solid interfaces.
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