Self-organization and Patterning of Multilayers of Molecules
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Self-organization and Patterning of Multilayers of Molecules David Salac and Wei Lu Department of Mechanical Engineering, University of Michigan Ann Arbor, MI 48109, USA ABSTRACT A phase field model is developed to simulate the molecular motion and patterning under the combined actions of dipole moments, intermolecular forces, entropy, and external electric field. The study reveals self-alignment, pattern conformation and the possibility to reduce the domain sizes via a layer by layer approach. INTRODUCTION Adsorbate molecules usually carry electric dipole moments, and the magnitude can be engineered by adding polar groups [1]. These molecules are mobile on a surface [2, 3], and may spontaneously form domain patterns [4, 5]. Two competing actions determine the feature size: coarsening to reduce the domain boundary energy and refining to reduce the electric dipole interaction energy. A dipole type of interaction is characterized by a 1/distance3 variation in the energy, which can be induced by electric, magnetic or elastic fields. Similar patterns and analogous mechanisms have been observed in diverse material systems, such as Langmuir films at the air-water interface [6], ferrofluids in magnetic fields [7], organic molecules on metal surfaces [8, 9], and surface stress induced self-organization on elastic substrates [10]. The mechanism of monolayer pattern formation gives insight into the study of analogous phenomena in multilayers, where pertinent experiments are lacking. Electrostatic interactions have been utilized to construct functional multilayer systems by the approach of electrostatic self-assembly (ESA) [11, 12]. ESA processing involves dipping a chosen substrate into alternate aqueous solutions containing anionic and cationic molecules or nanoparticles, such as complexes of polymers, metal and oxide nanoclusters or proteins. This leads to alternating layers of polyanion and polycation monolayers. Design of the precursor molecules and control of the sequence of the multiple molecular layers allow control over macroscopic electrical, optical, mechanical and other properties. While applications such as nano-filtration and photovoltaic devices have been demonstrated, the ESA process is generally limited to simple, laminar multilayer systems, with little or no lateral variation in the layer. We will show that for molecules carrying electric dipoles, dipole interaction can induce self-assembled patterns within each layer in a multilayer system. The capability is desirable for making complex structures, especially the formation of nanointerfaces and three dimensional nanocomposites. MODEL Figure 1 shows a multilayer of molecules adsorbed onto a substrate. The first layer is in contact with the substrate, and the nth layer is the top layer. Each layer has a thickness of hm , with m varying from 1 to n. The total thickness of the multilayer system is h f . The space above the top layer can be either air or a dielectric fluid. The substrate surface coincides with the plane of x3 = 0 . An array of ele
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