Making Superior Alumina Thin Films via Ozone Oxidation of Aluminum: Correlation between Oxidation Mechanism and Nanoscale Atomic Structure in Amorphous Oxide Films

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。尽管金属低温氧化形成非晶态氧化薄膜对许多技术应用具有重要意义，包括非均相催化、电子、防腐和表面涂层，但由于缺乏简单系统的大量数据，低温氧化领域和对非晶态氧化形成机制的基本理解在实验上受到阻碍。造成这种数据缺乏的原因有以下几个：由于低温氧化动力学缓慢导致测量困难，杂质控制不足，原始表面表征不完整，以及长期以来在非晶氧化物结构表征方面的挑战。传统的实验技术无法在纳米尺度上对氧化过程的结构和反应动力学进行原位测量，这也阻碍了人们对低温氧化的有限理解。该项目的主要目的是研究非晶态氧化物的微观形成过程，利用臭氧（O3）的强氧化能力来提高金属表面氧化物的形成速度，并采用原位超高真空（UHV）扫描探针显微镜来监测从氧表面化学吸附到氧化物成核和生长的反应顺序。为了建立氧化机理与非晶氧化膜纳米级原子结构之间的关系，将在原位可视化实验的基础上辅以波动电子显微镜。智力优势：这项工作将为非晶氧化物形成行为提供新的机制见解，这不仅需要构建自然连接不同氧化阶段的分层多尺度氧化模型，而且还需要在非晶氧化物膜的受控生长的实际应用中取得进展。实验将在单晶铝（Al）的简单模型系统上进行，这将导致清晰的解释和基本概念的建立。基于O3氧化Al的原位可视化和氧化膜纳米级原子结构的非原位表征，将解决以下问题：(1)从捕获的O3分子中解离的O原子在Al表面的迁移和化学吸附；(2)非晶态Al2O3岛的成核和生长动力学；(3)非晶Al2O3薄膜孔隙形成的微观成因；(4)动力学粗化对连续非晶氧化膜生长的影响；(5)氧化参数对非晶氧化膜纳米级原子结构（即中阶有序）的影响。由于该项目解决了有助于理解表面结构、反应性、质量传输、生长形态和纳米级原子结构之间的相关性的基本问题，这些发现将为其他材料系统（例如Cr、Ta、Zr、Si、Ge）和反应（例如阳极氧化）提供启发，其中控制非晶氧化膜形成的基本过程原型与铝的低温氧化非常相似。更广泛的影响：本研究的重点是通过纳米级成像技术对纳米级氧化物形成的现场可视化，从而使其成为开展计划中的纳米级科学教育活动的工具。作为这项研究计划的一部分，研究生和本科生将学习新的显微镜技术和材料问题，这些问题处于当前材料研究的前沿。除了开发以纳米科学和纳米技术为重点的材料科学课程新课程外，PI还将开发一个新的网站，专门用于原位SPM技术的仪器开发以及这些技术在各个研究领域的新应用，这将作为促进纳米科学公共教育的交流工具和媒介产生广泛的影响。为了弥合目前纳米技术研究进展与中学科学内容之间的差距，PI将建立一个“高中科学日”（HSSD）计划，将科学教师、毕业生和高中生聚集在一起，进行多层次的学习体验。这个HSSD项目旨在通过展示最先进的仪器，介绍纳米技术研究的最新进展，以及实际的研究经验，激发高中生的自然好奇心。科学教师的参与将导致新的教育模块的发展，并随后在课堂上教授给学生，因此具有显著的放大效应。

0932814ZhouThis award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Although the formation of amorphous oxide thin films by low-temperature oxidation of metals is of significant importance for many technological applications including heterogeneous catalysis, electronics, corrosion protection, and surface coatings, the field of low-temperature oxidation and the fundamental understanding of the mechanism of amorphous oxide formation have been experimentally handicapped by a lack of extensive data on simple systems. Several reasons contribute to this paucity of data: the difficulty of measurements due to slow oxidation kinetics at low temperatures, little control of impurities, incomplete characterization of the original surface, and the longstanding challenge in structural characterization of amorphous oxides. The limited understanding of low temperature oxidation has also been hindered by the inability of traditional experimental techniques to perform in situ measurements of the structure and reaction kinetics at the nanoscale as the oxidation progresses. The main thrust of this project is to investigate the microscopic processes of amorphous oxide formation by utilizing the strong oxidation power of ozone (O3) to enhance the rate of oxide formation on metal surfaces and employing in situ ultrahigh vacuum (UHV) scanning probe microscopy to monitor the reaction sequence from oxygen surface chemisorption to oxide nucleation and growth. The in situ visualization experiments will be complemented by fluctuation electron microscopy for establishing the correlation between oxidation mechanism and nanoscale atomic structure of the amorphous oxide films.Intellectual Merit: This work will provide new mechanistic insights into the behavior of amorphous oxide formation, which are needed not only for construction of hierarchical multi-scale oxidation models that naturally link different oxidation stages, but also for advances toward practical applications where the controlled growth of amorphous oxide films is critical. Experiments will be performed on the simple model system of single-crystal aluminum (Al), which will lead to clear interpretations and the establishment of fundamental concepts. Based on in situ visualization of O3 oxidation of Al and ex situ characterization of nanoscale atomic structure of the oxide films, the following issues will be addressed: (1) the migration and chemisorption of O atoms dissociated from trapped O3 molecules on Al surfaces; (2) the nucleation and growth kinetics of amorphous Al2O3 islands; (3) the microscopic origin of pore formation in amorphous Al2O3 films; (4) the effect of kinetic roughening on the growth of continuous amorphous oxide film; (5) the effect of oxidation parameters on the nanoscale atomic structure (i.e., medium-range order) of amorphous oxide films. Because the project addresses fundamental issues that help to understand the correlations among surface structure, reactivity, mass transport, growth morphology, and nanoscale atomic structure, the findings will shed light on other material systems (e.g. Cr, Ta, Zr, Si, Ge) and reactions (e.g. anodic oxidation), where the prototypes of basic processes governing the formation of amorphous oxide films exhibit great similarity to the low-temperature oxidation of aluminum.Broader Impact: This research focuses on in situ visualization of nanoscale oxide formation via nanoscale imaging techniques, thus qualifying it as a vehicle to carry out planned nanoscale science education activities. As part of this research program, students at the graduate and undergraduate levels will learn about new microscopy techniques and materials issues that are at the forefront of current materials research. In addition to the development of new courses in materials science curricula focusing on nanoscience and nanotechnology, the PI will develop a new website dedicated to the instrumentation development in in situ SPM techniques and the novel applications of these techniques to various research fields, which will have broad impact for serving as a communication tool and a medium for facilitating public education on nanoscience. To bridge the gap between current progress in nanotechnology research and secondary science content, the PI will establish a "High-School-Science-Day" (HSSD) program by bringing together science teachers, graduates, and high school students in a multi-level learning experience. This HSSD program will be designed to excite the natural curiosity of high-school students through demonstration of state-of-the-art instruments, presentations on the latest progress in nanotechnology research, and real-life research experience. The involvement of science teachers in this program will lead to the development of new educational modules to be subsequently taught to students in their classrooms and therefore have significant amplification effect.