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

非技术总结暴露在空气或水中的材料最重要的特性之一是它们的环境稳定性。随着材料系统的尺寸接近纳米级，从根本上了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，最初的阶段是人们最不了解的氧化过程。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，该技术测量材料在氧化过程中的重量变化，因此不能提供关于材料结构的信息。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发展对纳米级氧化过程的基本理解。该研究小组使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，以直接与当前在原子模拟铜氧化方面的理论工作进行比较。现场和非现场实验的结果加速了计算工具的发展，这些计算工具需要加强关键反应-氧化的预测材料设计的紧急领域。氧化不仅对腐蚀，而且作为一种自下而上的纳米氧化物加工方法，在世界范围内都具有重要意义。此外，该项目的一个关键方面是对学生和博士后的教育和培训。相互补充的实验和理论工具之间的结合伙伴关系，特别是现场合作，丰富了对参与这一项目的所有参与者的教育，并促进了未来领导者的发展，他们更有能力使新兴的预测科学和工程领域取得成功。这项研究的结果也被纳入研究生课程和高中社区推广项目，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。TECHNICAL SUMMARY许多人都知道氧气与金属表面的相互作用以及热力学稳定氧化物的宏观生长。然而，目前，氧化的纳米级阶段--从金属氧化物的成核到热力学稳定氧化物的形成--代表着一个在科学上具有挑战性和在技术上具有重要意义的领域。随着工程材料接近纳米体系，在这种规模上控制它们的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性质，因此存在许多氧化理论来解释其机理。然而，大多数经典的氧化理论假定薄膜是均匀生长的，由于缺乏传统的实验方法来可视化这种在允许高度控制表面和杂质的条件下的非均匀生长，因此没有考虑结构变化。然而，该研究小组最近的研究表明，铜氧化岛是在铜氧化的早期阶段形成的，因此挑战了普遍认为的均匀氧化物形成的假设。这个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。具体地说，该研究小组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、氧化和合并过程中氧化岛的形态演变以及扩散势垒等定量基本物理参数的关键见解。虽然重点放在氧-金属反应上，但所开发的方法适用于任何外延系统和气体-表面反应。通过将独特的实验结果和直接相关的理论模型相结合而获得的理解，导致了利用表面气体-金属反应的纳米和中尺度材料、器件和工艺的更智能的设计范例。这对许多技术领域是必不可少的，如高温腐蚀、电化学、栅氧化物和薄膜的形成、用于环境保护的催化、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.