Guided Formation of Nanostructures in Thin Films
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U11.10.1
Guided Formation of Nanostructures in Thin Films Wei Lu and Dongchoul Kim Mechanical Engineering, University of Michigan Ann Arbor, MI 48109-2125, U.S.A. ABSTRACT A thin two-component epilayer grown on a solid surface may separate into distinct phases. Sometimes the phases select sizes about 10 nm, and order into an array of stripes or disks. However, the pattern types are limited and the location of the features is not controlled. This paper develops a dynamic model to simulate guided self-assembly. In particular, we look at the effect of surface chemistry on the pattern formation process. The simulations suggest that diverse patterns may be produced by tuning the surface chemistry of a substrate. In addition, the self-assembled features may be anchored at specific locations. INTRODUCTION Experiments have shown that a two-phase epilayer on an elastic substrate may self-assemble into nanoscale patterns [1-4]. For instance, a submonolayer of oxygen on a Cu (110) surface can form stable periodic stripes of alternating oxygen overlayer and bare copper [1]. The stripes had a width of about 10 nm and run in the direction. Plass et al. found that a monolayer of Cu and Pb on a Cu (111) surface could form ordered patterns of dots or stripes, depending on the percentage of Pb atoms in the epilayer [2]. These nanoscale self-assembly behaviors are intriguing since they are lack in a bulk system. If a bulk two-phase alloy is annealed, phases will coarsen to reduce the total area of phase boundary. Time permitting, coarsening will continue until only one large particle is left in a matrix. For a two-phase epilayer, surface stress provides a refining action [5,6]. Surface stress can be roughly viewed as the residual stress in an epilayer multiplied by the layer thickness. It has a unit of force per length and can be measured experimentally [7-9]. For a non-uniform epilayer, the surface stress is also nonuniform, inducing a fringe elastic field in the substrate. When the phase size is reduced, the fringe field depth is reduced, and so is the elastic energy. It is this reduction in the elastic energy that drives phase refining. The two competing actions, coarsening due to phase boundaries and refining due to surface stress, select an equilibrium phase size. Furthermore, a superlattice of dots or stripes may minimize the total free energy, so that the competing actions also drive the self-assembly into the superlattices [10].
U11.10.2
Surface chemistry may be utilized to guide the self-assembly process. To illustrate the idea, consider a two-phase epilayer composed of atomic species A and B. During annealing, the atoms diffuse on the substrate to reduce the free energy. When the substrate surface is homogeneous, the competition of phase boundary and surface stress determines the patterns. However, the locations of self-assembled features cannot be predetermined due to the translational symmetry. Now imagine an inhomogenous substrate surface and two regions with different affinity to Aand B-atoms. The two regions may bot
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