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.
