3-Dimensional Structure of a Prototypical Ionic Liquid-Solid Interface: Ionic Crystal-Like Behavior Induced by Molecule-Substrate Interactions

The molecular structure of the ionic liquid-electrode interface has been recently investigated by atomic force microscopy (AFM) methods focusing either on the vertical structure of the ion layers or on the lateral structure of the innermost layer. Here, live combine high-resolution AFM imaging with atomic force spectroscopy measurements to elucidate the structure of the interface between the ionic liquid propylammonium nitrate (PAN) and highly ordered pyrolytic graphite (HOPG). The lateral structure of the innermost layer of adsorbed molecules (i.e., the Stem layer) is resolved on the molecular scale by means of amplitude Modulation atomic force microscopy (AM-AFM). A quasi (4 x 4)R0 degrees overlayer is formed by the ionic liquid molecules on the HOPG surface. Additional dynamic mode force spectroscopy measurements reveal the existence of a layered structure of the ionic liquid normal to the surface plane and allow for a precise determination of the layer spacing. We are able to infer the three-dimensional structure of the PAN HOPG interface from a combination of both information, i.e., the experimentally observed lateral and normal structures. The obtained 3D lattice structure is in accordance with a zincblende-type (ZnS) crystal structure.