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

0932814周该奖项由2009年美国复苏和再投资法案资助（公法111-5）.尽管通过金属的低温氧化形成非晶氧化物薄膜对于包括多相催化、电子学、防腐蚀和表面涂层的许多技术应用具有重要意义，低温氧化领域和对无定形氧化物形成机理的基本理解在实验上由于缺乏简单系统的大量数据而受到阻碍。几个原因导致这种数据的缺乏：由于在低温下缓慢的氧化动力学测量的困难，杂质的控制很少，原始表面的不完整表征，以及非晶氧化物的结构表征的长期挑战。对低温氧化的有限理解也受到传统实验技术的阻碍，传统实验技术无法在氧化过程中在纳米尺度上对结构和反应动力学进行原位测量。本计画的主要目的是利用臭氧（O3）的强氧化能力来提高金属表面氧化物的形成速率，并利用原位超高真空（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，以研究非晶氧化物形成的微观过程。在原位可视化实验的同时，还将辅以波动电子显微镜，以建立氧化机制与非晶氧化物薄膜纳米级原子结构之间的相关性。这项工作将为非晶氧化物形成的行为提供新的机理见解，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，而且对于非晶氧化物膜的受控生长是关键的实际应用的进展也是如此。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Al表面O_3氧化过程的原位观察和氧化膜纳米尺度原子结构的非原位表征，本论文将研究以下问题：（1）O原子在Al表面的迁移和化学吸附，（2）非晶Al_2 O_3岛的成核和生长动力学，（3）非晶Al_2 O_3膜中孔形成的微观起源，（4）非晶Al_2 O_3膜中O原子在Al表面的迁移和化学吸附，（5）非晶Al_2 O_3岛的成核和生长动力学，（6）非晶Al_2 O_3膜中孔形成的微观起源，（7）非晶Al_2 O_3膜中O原子的迁移和化学吸附，（8）非晶Al_2 O_3岛的成核和生长动力学。（4）动力学粗糙化对连续非晶氧化物膜生长的影响;（5）氧化参数对纳米级原子结构的影响（即，中程有序）的非晶氧化物膜。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，因此这些发现将揭示其他材料系统（例如Cr、Ta、Zr、Si、Ge）和反应（例如阳极氧化），其中控制非晶氧化物膜形成的基本过程的原型与铝的低温氧化表现出极大的相似性。这项研究的重点是通过纳米级成像技术原位可视化纳米级氧化物的形成，从而使其有资格作为开展计划的纳米级科学教育活动的工具。作为该研究计划的一部分，研究生和本科生将学习新的显微镜技术和材料问题，这些技术和材料问题处于当前材料研究的最前沿。除了开发以纳米科学和纳米技术为重点的材料科学新课程外，PI还将开发一个新网站，专门介绍原位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，这将对作为通信工具和促进纳米科学公众教育的媒介产生广泛影响。为了弥合纳米技术研究和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，毕业生和高中学生聚集在一起，进行多层次的学习体验。该HSSD计划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和现实生活中的研究经验来激发高中生的自然好奇心。科学教师参与该计划将导致新的教育模块的开发，随后在课堂上教授给学生，因此具有显着的放大效应。

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.