This 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.
