EPITAXIALLY DRIVEN ASSEMBLY OF CRYSTALLINE MOLECULAR FILMS ON ORDEREDSUBSTRATES

Crystalline, molecularly thick organic films mimicking layer motifs ob served in bulk crystals of conducting (ET)(2)X charge-transfer salts ( ET = bis(ethylenedithiolo)tetrathiafulvalene, X = I-3, ReO4) form on h ighly oriented pyrolytic graphite (HOPG) electrodes upon electrochemic al oxidation of ET in electrolytes containing I-3(-) 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 de duced from lattice images obtained by AFM under growth conditions. AFM data reveal two different (ET)(2)I-3 overlayers that are distinguishe d by the orientation of the ET molecules. One of these overlayers (typ e I) exhibits lattice structure and thickness corresponding to the (00 1) layer of bulk beta-(ET)(2)I-3, while the other (type II) exhibits s tructural characteristics consistent with a slightly reconstructed ver sion of the ((1) over bar 10) layer in crystalline beta-(ET)(2)I-3. In contrast, (ET)(2)ReO4 overlayers exhibit only the type II orientation , which resembles the (011) layer of bulk(ET)(2)ReO4. Comparison of th e overlayer azimuthal orientation with respect to the underlying HOPG substrate, determined directly by AFM, reveals that each overlayer for ms 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 value s predicted by either potential energy calculations or an analytical m odel of the overlayer-substrate interface. Strong two-dimensional intr alayer interactions in the type I (001) beta-(ET)(2)I-3 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 b onding vector remains, facilitating slight reconstructions from the bu lk layer structures so that coincidence can be achieved. Calculations of overlayer-substrate and overlayer energies and elastic constants in dicate that although coincident epitaxy between the (001) (ET)(2)ReO4 overlayer and HOPG is possible, the accumulation of interfacial stress es from noncommensurate overlayer sites within its large supercell pre vents its formation. These observations, when combined with analysis o f the intralayer and overlayer-substrate elastic constants, indicate t hat the overlayer structure and its orientation with respect to the su bstrate are governed by the epitaxial relationship between the substra te and large ordered arrays of molecules, reflecting a delicate balanc e of intralayer and overlayer-substrate energetics. The design strateg y based an bulk crystallographic layers and the overlayer-substrate ep itaxy represents a ''crystal engineering'' approach to the fabrication of molecular thin films.