DESIGNED FORMATION OF NANOCOMPOSITES VIA DIPOLE INTERACTION David Salac and Wei Lu Department of Mechanical Engineering University of Michigan, Ann Arbor, MI 48109 Email: weilu@umich.edu ABSTRACT The formation of designed nanocomposites by multiple layers of adsorbate molecules is studied. We consider the presence of two types of molecules in each layer, which are characterized by different dipole moments. The nanostructures are characterized by the non-uniform distribution of the two molecules. A phase field model is developed to simulate the molecular diffusion and patterning under the combined actions of dipole moments, intermolecular forces, entropy, and external electric field. The study reveals self-alignment, structure conformation and the possibility to reduce the domain sizes via a layer by layer approach. It is also shown that the structure in a layer may define the roadway for molecules to travel on top it. This combined with electrodes embedded in the substrate gives much flexibility to guide the molecular organization and fabrication of designed nanocomposites. Keywords: self-assembly, phase field model, dipole interaction. INTRODUCTION Self-organization is an efficient way to grow nanostructures with regular sizes and spacings on a substrate surface. As a representative system, the spontaneous domain pattern formation of adsorbate monolayers has attracted wide spread interest [1-3]. Adsorbate molecules carry electric dipole moments, and the magnitude can be engineered by adding polar groups [4]. These molecules are mobile on a surface [5, 6]. Domains coarsen to reduce the domain boundary energy and refine to reduce the electric dipole interaction energy. The competition determines the feature sizes and leads to stable patterns. A dipole type of interaction is characterized by a 1/distance 3 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 [7, 8], ferrofluids in magnetic fields [9, 10], organic molecules on metal surfaces [11-13], and surface stress induced self- organization on elastic substrates [14, 15]. An external field may be applied to direct the self- organization process. Molecular monolayers composed of electric dipoles can be manipulated with an electric field induced by an AFM tip, a ceiling above the layer, or an electrode array in the substrate [16-19]. The electric field also takes effect through dielectric inhomogeneity, which has been shown to cause domain patterns in dielectric films [20-22]. 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) [23-25]. 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 , with m varying from 1 to n. The total thickness of the multilayer m h 1 Copyright © 2006 by ASME Proceedings of IMECE2006 2006 ASME International Mechanical Engineering Congress and Exposition November 5-10, 2006, Chicago, Illinois, USA IMECE2006-14991 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 10/13/2014 Terms of Use: http://asme.org/terms