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Strain-Patterned Self-Assembly In a Binary Alloy Thin Film

Alex M. Nieves1, Vaclav Vitek2, and Talid Sinno1. (1) Chemical and Biomolecular Engineering, University of Pennsylvania, Towne Building, Room 311A, 220 South 33rd Street, Philadelphia, PA 19104, (2) Material Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104

The use of external fields (e.g. magnetic, mechanical, and electrical) to direct nanostructural evolution in hard materials is a promising avenue for fabricating devices at length scales beyond current limits [1]. In order to take advantage of such avenues, the interactions between atomic transport, equilibrium properties, and external fields must be mechanistically understood. Here, we focus on the coupling between externally applied stress fields and phase segregation in a binary thin film alloy. Previous experimental [2] and simulation [2,3] studies have shown that applied strain fields in a crystalline system can strongly influence atomic diffusion and lead to patterned microstructural evolution. On the other hand, it is still not clear how to devise optimal operating conditions for maximizing the quality of the resulting microstructure.

We perform Metropolis Monte Carlo simulations in a binary thin film consisting of about 50,000 Lennard-Jones atoms. The top and bottom surfaces of the film are subjected to fixed vertical displacement fields obtained as solutions to a continuum mechanics model of a film subjected to an array of nano-indenters. The binary Lennard-Jones alloy is generated by making the length parameter for the A-A interaction smaller than the B-B interaction, creating a lattice mismatch between A and B atomic species. This mismatch couples the atomic system to the externally imposed strain field, and drives the smaller species towards the indenter regions. Moreover, a surface energy penalty between the A and B phases is created by adjusting the A-B LJ interaction, which drives the system to phase separate.

Our simulations demonstrate that it is possible to control the quality of the resulting patterning by applying variable temperature anneals. In particular, delaying the onset of phase segregation until some atomic diffusion has occurred in the film, greatly increases the overall kinetics of patterning. We develop guidelines for optimal annealing schedules as a function of film/indenter geometry, mismatch magnitude and surface energy. The results of these simulations should prove valuable in guiding experiments in strain-directed assembly.

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[1] C.-Y. Hung, A. F. Marshall, D.-K. Kim, W. D. Nix, J. S. Harris, Jr. and R. A. Kiehl, J. Nanoparticle Research 1, 329-347 (1999).

[2] W. Lu and Z. Suo, J. Mech. Phys. Solids 49, 1937 (2001).

[3] T. R. Mattsson and H. Metiu, Appl. Phys. Lett. 75, 926 (1999).