Dramatic Effect of Temperature on Metal-oxide Nanostructures: Oxidation of Cu Films by In situ UHV-TEM
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Dramatic Effect of Temperature on Metal-oxide Nanostructures: Oxidation of Cu Films by In situ UHV-TEM Guangwen Zhou and Judith C. Yang Materials Science and Engineering Dept, University of Pittsburgh, Pittsburgh, PA 15261 ABSTRACT We investigated the temperature effect on the Cu2O morphology by oxidizing Cu(100) thin films at the temperature ranging from 350°C to 1000°C. We demonstrated that dramatically different morphologies of oxide nanostructures can be achieved by modifying the oxidation temperature. Quasi-one-dimensional Cu2O structures with aspect ratios as large as 40:1 were formed at the oxidation temperature of 600°C. The in situ observation data on the elongation of Cu2O islands agree with the energetic calculations based on the balance between surface and interface energies and the elastic stress relaxation in the three dimensional islands. INTRODUCTION From the standpoint of thermodynamics all of the metals exhibit a tendency to oxidize. The oxidation driving force depends on the free-energy change for oxide formation. But the morphological changes of oxides during oxidation depend on kinetics, and microstructural considerations. Visualizing the oxidation process will provide essential insights into the complex kinetics and energetics of nano-oxide formation [1,2]. Furthermore, oxidation can be viewed as a processing tool for creating self-ordered nanostructures and the understanding of exact formation process of oxide would provide the guidance to control the oxide nanostructures [3]. In situ ultra high vacuum transmission electron microscope (UHV-TEM) allows us to study the nucleation and growth processes of oxide at nanometer scale, provides a unique view of dynamic reactions, and enables us to understand and therefore manipulate surface reactions. Since Cu has been chosen by many investigators as a model system to understand oxidation kinetics [4-7], we chose Cu films as a model system to study the formation of the oxide nanostructures by in situ UHV-TEM. We have examined the dependence of island density, size distribution, morphology on the oxidation parameters, such as substrate temperature, oxygen pressure, and orientation of the substrate. The focus of this paper is the dramatic effect of temperature on the oxide morphology formed on Cu(100) thin films. Copper forms two thermodynamically stable oxides, Cu2O and CuO. Cu2O is simple cubic lattice (space group pn3m) with 4Cu and 2O atoms in its basis, and a lattice parameter of 4.22Å. The Cu atoms form a FCC lattice and the O atoms form a BCC lattice, where each O atom is surrounded by a tetrahedron of Cu atoms. CuO has a monoclinic structure. Cu is a FCC metal with a lattice parameter of 3.6Å. For the temperatures and very low oxygen partial pressures used in our experiments, only Cu2O is expected to form [8]. EXPERIMENTAL The microscope used in this work was a modified JEOL 200CX [9]. A UHV chamber was attached to the middle of the column, where the base pressure was less than 10-8 torr without the
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use of the cryoshroud. The
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