Epitaxially Driven Assembly of Crystalline Molecular Films on Ordered Substrates

Crystalline, molecularly thick organic films mimicking layer motifs observed in bulk crystals of conducting (ET)₂X charge-transfer salts (ET = bis(ethylenedithiolo)tetrathiafulvalene, X = I₃, ReO₄) form on highly oriented pyrolytic graphite (HOPG) electrodes upon electrochemical oxidation of ET in electrolytes containing I₃^- or ReO₄^- anions. The assembly of these molecular overlayers can be observed directly by in situ atomic force microscopy (AFM), and their structures can be deduced from lattice images obtained by AFM under growth conditions. AFM data reveal two different (ET)₂I₃ overlayers that are distinguished by the orientation of the ET molecules. One of these overlayers (type I) exhibits lattice structure and thickness corresponding to the (001) layer of bulk β-(ET)₂I₃, while the other (type II) exhibits structural characteristics consistent with a slightly reconstructed version of the (1̄10) layer in crystalline β-(ET)₂I₃. In contrast, (ET)₂ReO₄ overlayers exhibit only the type II orientation, which resembles the (011) layer of bulk (ET)₂ReO₄. Comparison of the overlayer azimuthal orientation with respect to the underlying HOPG substrate, determined directly by AFM, reveals that each overlayer forms by coincident epitaxy in which strict commensurism is achieved only at the vertexes of a supercell comprising an array of primitive unit cells. The observed azimuthal orientations are in agreement with values predicted by either potential energy calculations or an analytical model of the overlayer-substrate interface. Strong two-dimensional intralayer interactions in the type I (001) β-(ET)₂I₃ overlayer and a coincident lattice match favor the formation of a crystalline layer in which the structure mimicks the bulk layer structure. However, the type II overlayers are oriented such that only one strong intralayer bonding vector remains, facilitating slight reconstructions from the bulk layer structures so that coincidence can be achieved. Calculations of overlayer-substrate and overlayer energies and elastic constants indicate that although coincident epitaxy between the (001) (ET)₂ReO₄ overlayer and HOPG is possible, the accumulation of interfacial stresses from noncommensurate overlayer sites within its large supercell prevents its formation. These observations, when combined with analysis of the intralayer and overlayer-substrate elastic constants, indicate that the overlayer structure and its orientation with respect to the substrate are governed by the epitaxial relationship between the substrate and large ordered arrays of molecules, reflecting a delicate balance of intralayer and overlayer-substrate energetics. The design strategy based on bulk crystallographic layers and the overlayer-substrate epitaxy represents a "crystal engineering" approach to the fabrication of molecular thin films.