Collaborative Research: Experimental and Computational Studies of Solid-State Diffusion and New Phase Formation in Bimetallic Nanostructures

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Non-Technical Summary The diffusive motions of atoms can lead to changes in the physical properties of solids over time that range from simple composition variations to the formation and growth of new structures. The effects of these changes (which can be advantageous or disadvantageous) become significant at lower temperatures, shorter time periods, and smaller length scales as the sample size is reduced. As a result, a fundamental understanding of the origins and consequences of these processes is particularly important in nanostructured materials. The research component of this project consists of an experimental and computational study of diffusive atomic motion in bimetallic samples at the nano-scale. The analysis of the experimental and computational results will provide a detailed microscopic picture of diffusive motion in these nanostructures over a wide range of length and time scales, and ultimately will contribute to the design, synthesis, and processing of new materials with useful properties. The educational and outreach component of the project consists of the development of a new undergraduate/graduate level course in the practical application of x-ray diffraction methods, providing meaningful research opportunities for undergraduate and graduate students, and an outreach program targeted at junior and senior level high school students by participating in the Delaware Science Olympiad. Technical Summary While solid-state diffusion (SSD) and new phase formation (NPF) in multicomponent bulk and two-dimensional (2D) thin film systems has been studied for many years, much less is known about SSD and NPF in zero-dimensional (0D) and one-dimensional (1D) diffusion couples. The research component of this project has been designed to address this issue through a systematic experimental and computational study of the structural evolution in 0D core/shell nanoparticles and 1D multilayered nanowires. In particular, the structural evolution of chemically prepared 0D and electrochemically prepared 1D diffusion couples will be experimentally studied using both optical pump/x-ray probe ultrafast time-resolved x-ray diffraction (TRXRD) and conventional high temperature x-ray powder diffraction (cXRD) measurement techniques. These experimental studies will be complemented by computational studies of SSD and NPF in model 0D and 1D diffusion couples. The experimental component of the research project will consist of pump/probe TRXRD measurements carried out using in-house facilities and facilities available at the Advanced Photon Source at Argonne National Laboratory. In both cases a Ti:sapphire laser will be used to pump the sample with an approximately 50 femtosecond optical pulse followed by an x-ray probe pulse. The optical pump leads to a very rapid increase in the same temperature, and by delaying the arrival of the x-ray pulse relative to the pump pulse temperature depend x-ray diffraction patterns can be acquired with a time resolution of 10's - 100's of femtoseconds over time periods from 10's to 100's of nanoseconds. Conventional high temperature powder x-ray diffraction measurements (cXRD) will allow complementary structural measurements to be carried out over much longer time periods (although with limited time resolution). The computational component of the research project will consist of modeling SSD and NPF in 0D and 1D diffusion couples by combining density functional theory calculations with cluster expansion methods and kinetic Monte Carlo simulations. Computational studies of this kind are of particular importance because they are applicable at the short length scales over which continuum models break down. These experimental and computational studies will provide a detailed microscopic picture of SSD and NPF in 0D and 1D nanostructures over both very short and very long time scales, and ultimately will contribute to the design, synthesis, and processing of new materials with useful properties.