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

随着时间的推移，原子的扩散运动可以导致固体物理性质的变化，从简单的组成变化到新结构的形成和生长。随着样本量的减少，这些变化的影响（可能是有利的，也可能是不利的）在较低的温度、较短的时间周期和较小的长度尺度下变得显著。因此，对这些过程的起源和后果的基本理解在纳米结构材料中尤为重要。该项目的研究部分包括在纳米尺度上对双金属样品中的扩散原子运动进行实验和计算研究。对实验和计算结果的分析将提供这些纳米结构在大范围长度和时间尺度上扩散运动的详细微观图像，并最终将有助于具有有用性能的新材料的设计、合成和加工。该项目的教育和推广部分包括开发一门新的本科/研究生水平的课程，介绍x射线衍射方法的实际应用，为本科生和研究生提供有意义的研究机会，并通过参加特拉华州科学奥林匹克竞赛，为初中和高中学生提供一个推广项目。技术概述虽然多组分块体和二维（2D）薄膜系统中的固态扩散（SSD）和新相形成（NPF）已经研究多年，但对零维（0D）和一维（1D）扩散偶中的固态扩散（SSD）和新相形成（NPF）知之甚少。该项目的研究部分旨在通过系统的实验和计算研究0D核/壳纳米粒子和1D多层纳米线的结构演变来解决这一问题。特别地，将利用光泵浦/x射线探针超快时间分辨x射线衍射（TRXRD）和传统的高温x射线粉末衍射（cXRD）测量技术，实验研究化学制备的0D和电化学制备的1D扩散偶的结构演变。这些实验研究将通过在模型0D和1D扩散偶中的SSD和NPF的计算研究来补充。研究项目的实验部分将包括使用内部设备和阿贡国家实验室先进光子源的设备进行泵/探针TRXRD测量。在这两种情况下，Ti：蓝宝石激光器将使用大约50飞秒的光脉冲和x射线探针脉冲来泵送样品。光泵浦导致相同温度的快速增加，并且通过延迟x射线脉冲相对于泵浦脉冲温度的到达，可以在10到100纳秒的时间周期内以10 - 100飞秒的时间分辨率获得x射线衍射图。传统的高温粉末x射线衍射测量（cXRD）将允许在更长的时间内进行互补结构测量（尽管时间分辨率有限）。该研究项目的计算部分将包括将密度泛函理论计算与簇展开方法和动力学蒙特卡罗模拟相结合，在0D和1D扩散对中对SSD和NPF进行建模。这类计算研究特别重要，因为它们适用于连续体模型失效的短长度尺度。这些实验和计算研究将在很短和很长的时间尺度上提供固态固态和NPF在零维和一维纳米结构中的详细微观图像，并最终将有助于具有有用性能的新材料的设计、合成和加工。

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 SummaryWhile 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.