Dynamic Atomic-scale Metal Oxidation to Correlate with Multi-scale Simulations

材料暴露在空气或水中最重要的特性之一是其环境稳定性。随着材料系统的尺寸接近纳米尺度，在基本层面上理解它们如何在这些长度尺度上与环境相互作用是至关重要的。令人惊讶的是，最初的阶段是最不为人所知的氧化过程。经典的氧化模型假定薄膜生长均匀。这是因为经典的氧化分析主要依赖于热重技术，这种技术测量材料在氧化过程中的重量变化，因此不能提供材料结构的信息。然而，众所周知，在金属氧化过程中会发生结构变化。拟议的研究项目的潜在影响是对纳米级氧化过程的基本理解的发展。该研究小组利用原位和非原位环境透射电子显微镜扩展了对纳米级氧化的实验理解，直接与目前铜氧化原子模拟的理论成果进行了比较。原位和非原位实验的结果加速了计算工具的发展，这些工具需要增强用于关键反应氧化的预测材料设计的新兴领域。氧化在世界范围内具有重要意义，不仅对于腐蚀，而且作为一种自下而上的纳米氧化物处理方法。此外，该项目的一个关键方面是学生和博士后的教育和培训。互补的实验和理论工具之间的结合伙伴关系，特别是在现场，丰富了参与该项目的所有参与者的教育，并培养了未来的领导者，他们更有能力在预测科学和工程的新兴领域取得成功。这项研究的结果也被纳入研究生课程和高中社区外展项目，例如自2007年以来每年在匹兹堡举行的宾夕法尼亚初级科学院研讨会。技术总结：关于氧与金属表面的相互作用以及热力学稳定的氧化物的宏观生长，我们已经知道很多。然而，目前，纳米级氧化阶段——从金属氧化物成核到形成热力学稳定的氧化物——代表了一个科学上具有挑战性和技术上重要的未知领域。随着工程材料接近纳米级，在这种尺度上控制其环境稳定性变得至关重要。由于环境稳定性是大多数工程材料的基本特性，存在许多氧化理论来解释其机制。然而，大多数经典氧化理论都假设薄膜是均匀生长的，由于缺乏传统的实验程序来可视化这种在高度控制表面和杂质的条件下的非均匀生长，因此不考虑结构变化。然而，该研究小组最近的研究表明，铜氧化物岛是在铜氧化的早期阶段形成的，从而挑战了普遍认为的均匀氧化物形成的假设。该研究小组将实验结果与理论预测相关联，其中影响可能是对氧化的基本理解的范式转变，其中表面和缺陷控制氧化的早期阶段。具体而言，该研究团队将实验原位和非原位透射电子显微镜和x射线光电子能谱与理论模拟相结合，以获得对成核行为，氧化和聚并过程中氧化岛的形态演变以及扩散屏障等定量基本物理参数的关键见解。虽然重点是氧-金属反应，但所开发的方法适用于任何外延系统和气体表面反应。从结合独特的实验结果和直接相关的理论模型中获得的理解导致了纳米和中尺度材料、器件和利用表面气-金属反应的工艺的更智能的设计范例。这对于许多技术领域都是必不可少的，例如高温腐蚀、电化学、栅极氧化物和薄膜形成、用于环境保护的催化剂、能源产生和储存以及燃料电池反应。

NON-TECHNICAL SUMMARYOne of the most important properties for materials exposed to air or water is their environmental stability. As the dimensions of materials systems approach the nanoscale, it is critical to understand on a fundamental level how they interact with their environment at these length scales. Surprisingly, the initial stages are the least well-understood regime of oxidation. Classic models of oxidation assume uniform film growth. This is because classic oxidation analysis relied mostly on thermogravimetric techniques, which measure the weight change of the material during oxidation, and, hence, do not provide information on the materials' structure. Yet, structural changes are well-known to occur during metal oxidation. The potential impact of the proposed research project is the development of a fundamental understanding of nanoscale oxidation processes. This research team expands the experimental understanding of nanoscale oxidation using in situ and ex situ environmental transmission electron microscope to directly compare with a current theoretical effort on the atomistic simulation of oxidation of copper. The results from the in situ and ex situ experiments accelerates the development of computational tools needed to enhance the emergent field of predictive materials design for a critical reaction, oxidation. Oxidation is of world-wide importance, not only for corrosion but also as a bottom up approach to nano-oxide processing. Furthermore, a critical aspect of this project is the education and training of students and post-doc. The combined partnership between complementary experimental and theoretical tools, especially in situ, enriches the education of all participants involved in this project and the development of future leaders who are better equipped to bring to success the emergent field of predictive science and engineering. Results from this research are also incorporated into graduate courses and high school community outreach projects, such as Pennsylvania Junior Academy of Science workshop that has been held annually in Pittsburgh since 2007.TECHNICAL SUMMARYMuch is known about oxygen interaction with metal surfaces and about the macroscopic growth of thermodynamically stable oxides. At present, however, the nanoscale stages of oxidation - from nucleation of the metal oxide to formation of the thermodynamically stable oxide - represent a scientifically challenging and technologically important terra incognito. As engineered materials approach the nanometer regime, control of their environmental stability at this scale becomes crucial. As environmental stability is an essential property of most engineered materials, many oxidation theories exist to explain its mechanisms. However, most classical oxidation theories assume a uniform growing film, where structural changes are not considered due to the lack of traditional experimental procedure to visualize this non-uniform growth under conditions that allow highly controlled surfaces and impurities. Yet, recent studies by this research team reveal that the Cu oxide islands form during the early stages of Cu oxidation, and thereby challenge the common assumption of a uniform oxide formation. This research team correlates experimental results with theoretical predictions where the impact could be a paradigm shift in the fundamental understanding of oxidation where surfaces and defects control the early stages of oxidation. Specifically, this research team integrates experimental in situ and ex situ transmission electron microscopy and X-ray photoelectron spectroscopy with theoretical simulations in order to gain critical insights into the nucleation behavior, morphological evolution of oxide islands during oxidation and coalescence, and quantitative fundamental physical parameters such as diffusion barriers. Although the focus is on oxygen-metal reactions, the methodologies developed are applicable to any epitaxial system and gas-surface reaction. The understanding obtained from combining the unique experimental results and directly correlated theoretical models leads to smarter design paradigms for nano- and mesoscale materials, devices, and processes that utilize surface gas-metal reaction. This is essential to many technical areas, such as high temperature corrosion, electrochemistry, gate oxides and thin film formation, catalysis used for environmental protection, energy generation and storage, and fuel cell reactions.