Spontaneous polarity formation in thin crystalline films of host-guest materials

Statistical mechanics calculations show that in thin crystalline films of c hannel-forming host-guest lattices asymmetric but achiral molecules undergo spontaneous polar order under conditions of thermal equilibrium. Depending on the state of the substrate, unipolar or bipolar layers are obtained. Fo r less than about L=25 layers, a nearly homogeneous polarisation [S-x] (x = 0, 1,..., L; x is the layer index) close to its maximum values normalised to 1 or -1 is possible. A comparison of the polarisation state under condit ions of either thermal equilibrium or metastable states resulting from a Ma rkov process of growth reveals similar functions of [S-x] up to a sufficien tly low L. for both cases when assuming particular, but realistic. interact ion energies; Using crystalline host-guest thin films opens up the possibil ity of obtaining polar order of guest molecules, which, when crystallised a s pure compounds into layers or the bulk, would to a high probability form centrosymmetric structures. (C) 2000 Elsevier Science B.V. All rights reser ved.