TECHNICAL: Surface oxidation processes play critical roles in environmental stability, high temperature corrosion, electrochemistry, catalytic reactions, gate oxides and thin film growth as well as fuel reactions. At present, however, the nanoscale stages of oxidation - from the nucleation of the metal oxide to the formation of the thermodynamically stable oxide - represent a scientifically challenging and technologically important terra incognito. The objective is to fundamentally understand nanoscale oxidation processes by coordinated experimental (in situ UHV-TEM) and theoretical efforts, where the impact is potentially a new paradigm for oxidation. To meet this objective, directly correlated experimental and theoretical investigations of the initial stages of Cu and Cu alloy (Cu-Au and Cu-Ni) oxidation will be performed. The in-situ experiments of the initial stages of Cu have been performed by PI in the past. The extension to Cu alloy oxidation will be conducted in an in situ UHV-TEM at various temperatures, pressures and oxidizing atmospheres in Yang's laboratory where a unique in--situ UHV TEM exists. To bridge the temporal gap between simulations and experiments, a newly developed dynamic TEM (DTEM) with nanoseconds time resolution, will be used as well at Lawrence Livermore National Lab (LLNL). Determination of the Cu/Cu2O interface structure formed by in-situ oxidation by cross-sectional TEM and scanning TEM (STEM) methods, including high-resolution electron microscopy (HREM), Z-contrast imaging, tomography and electron energy loss spectroscopy (EELS) and the comparison with theoretical simulations will provide critical insights into the oxidation transformation mechanisms. The sample preparation and TEM/STEM studies will be conducted in the new Peterson Institute of NanoScience and Engineering (PINSE) at Univ. of Pittsburgh (UPitt). The in-situ and ex-situ experimental TEM results will be directly correlated to theoretical models, where a first-principles kinetic Monte Carlo, called Thin Film Oxidation (TFOx) is being developed. TFOx is a C++ code that presently simulates 2D nucleation and growth, and will be developed to simulate 3D island formation where computer clusters at CMU and UF and the supercomputer facility at UPitt will be utilized for significantly enhanced simulation speed. The direct comparison between these simulations and in-situ experiments will lead to new knowledge. NON-TECHNICAL: The combined experimental with theoretical partnership between UPitt, LLNL,UF and CMU will significantly enrich and broaden the education of all of the graduate and undergraduate students involved in this program. Yang has a strong track record in advising women students. Dissemination of results will include a web-site: www.tfox.org. Yang will integrate her research program in thin films, gas surface reactions and electron microscopy into several of the Mechanical Engineering and Materials Science department (MEMS) undergraduate and graduate-level courses, such as undergraduate crystallography/diffraction laboratory and graduate nano-courses, such as thin films, nanocharacterization and electron microscopy as well as nanomaterials, at UPitt.
Oxidation reactions at the nanoscale are of tremendous importance to a wide variety of energy, sustainability and nano- applications, including corrosion, environmental stability, catalysis, fuel cells, gate oxides, sensors, thin film growth, such as chemical vapor deposition, and nano-oxide synthesis for a variety of applications in electronic, magnetic, and optical devices. Addressing the issues of corrosion costs the US a few percent of its GNP per year. As dimensions of materials systems approach nanoscale, it is critical to fundamentally understand their interactions with the environment at this length scale for their long-term durability. Hence, limiting oxidation is critical for environmental stability and controlled oxidation is used for nano-oxide processing (Fig. 1). Yet, the fundamental understanding of the atomic mechanisms of surface oxidation processes that occur at the nanoscale and below is currently very limited; classical theories of oxidation assume uniform oxide film growth due to traditional experimental methods’ inability to perform in situ characterization at the nanoscale. This project aims to significantly advance fundamental understanding of the nano-oxidation stage (Fig. 2), especially the initial oxidation stage using recent advances in in situ transmission electron microscopy (TEM) complemented with theoretical simulations. This project was a collaborative effort with theorists at Carnegie Mellon University, University of Pittsburgh, University of Florida, Gainesville, and other experimentalists where cutting edge in situ synchrotron X-ray diffraction and aberration-corrected environmental TEM with 1Šresolution resides at Argonne National Laboratory and Brookhaven National Laboratory, respectively. Our body of work on copper and copper alloy oxidation via in situ TEM demonstrated that oxidation involves nucleation and growth, surface diffusion and solid-state reactions, and bears a striking resemblance to heteroepitaxy (Fig. 3). Heteroepitaxy refers to thin film growth where the thin film material differs from the substrate material, such as Ge formation on Si. Heteroepitaxial growth has been extensively modeled and used successfully to describe metal on metal heteroepitaxy. We have shown that heteroepitaxial concepts describe surprisingly well the nucleation, and growth to coalescence of Cu2O islands on copper and copper-gold alloy thin films, thereby demonstrating a greater universality of the heteroepitaxial ideas. This project contributed to the training and education of 8 graduate students, including to 2 visiting graduate students and 3 female graduate students, ~ ten undergraduate research experience and senior projects, including 5 females and 1 African American. This project has produced ~25 journal publications including a book chapter and two review articles as well as a web-site on kinetic Monte Carlo simulations of adatom deposition and motion to simulate thin film oxidation.
