Initial oxidation kinetics of Cu(100), (110), and (111) thin films investigated by in situ ultra-high-vacuum transmissio
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The initial oxidation stages of Cu(100), (110), and (111) surfaces have been investigated by using in situ ultra-high-vacuum transmission electron microscopy (TEM) techniques to visualize the nucleation and growth of oxide islands. The kinetic data on the nucleation and growth of oxide islands shows a highly enhanced initial oxidation rate on the Cu(110) surface as compared with Cu(100), and it is found that the dominant mechanism for the nucleation and growth is oxygen surface diffusion in the oxidation of Cu(100) and (110). The oxidation of Cu(111) shows a dramatically different behavior from that of the other two orientations, and the in situ TEM observation reveals that the initial stages of Cu(111) oxidation are dominated by the nucleation of oxide islands at temperatures lower than 550 °C, and are dominated by two-dimensional oxide growth at temperatures higher than 550 °C. This dependence of the oxidation behavior on the crystal orientation and temperature is attributed to the structures of the oxygen-chemisorbed layer, oxygen surface diffusion, surface energy, and the interfacial strain energy.
I. INTRODUCTION
The classic theories of oxidation, such as the theory of Wagner1 and the Cabrera-Mott model,2 have proved to be highly successful in predicting the oxidation behavior of metals, but these models are focused on the growth of an oxide layer and assume uniform film growth controlled by the transport of ionic species through the continuous oxide film. Due to the development of wellcontrolled experiments and the increased experimental capabilities in terms of resolution and cleanliness, it is known now that the early stages of metal oxidation typically involve the nucleation and initial growth of metal oxide islands, rather than the formation and thickening of a continuous film,3–7 which represents a critical departure from previously held assumptions in the classic theories regarding metal oxidation.1,2 An ideal technique for probing the initial oxide formation is in situ transmission electron microscopy a)
Address all correspondence to this author. Present address: Materials Science Division, Argonne National Laboratory, Argonne, IL 60439. e-mail: [email protected] b) This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www. mrs.org/publications/jmr/policy.html. DOI: 10.1557/JMR.2005.0239 1684
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J. Mater. Res., Vol. 20, No. 7, Jul 2005 Downloaded: 17 Mar 2015
(TEM). Most TEM studies of oxide scales have relied on the examination of oxide films after they have been stripped from the metal. Such a technique has the disadvantage of changing the relative position of the oxide film on the substrate and prohibits the accurate determination of the orientation relationships between the oxide and the base metal. Furthermore, one cannot study the initial stages of the oxide scale since one cannot strip off discontinuous layers. Previous investigators h
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