The low-temperature initial oxidation stages of Cu(100) investigated by in situ ultra-high-vacuum transmission electron
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The nucleation and growth of Cu2O islands due to Cu(100) oxidation at temperatures from 200 to 350 °C have been observed by in situ ultra-high-vacuum transmission electron microscopy. For this temperature range, epitaxial Cu2O islands form a triangular shape with rounded edges when Cu(100) is exposed to dry oxygen at 5 × 10−4 Torr in situ. Our initial analysis on the nucleation and growth of these three-dimensional Cu2O islands agrees well with the heteroepitaxial model of surface diffusion of oxygen. I. INTRODUCTION
A fundamental understanding of oxidation mechanisms is essential, because these processes play crucial roles in numerous engineering applications, including environmental stability, gate oxides, catalytic reactions, and fuel cell reactions. The interaction of oxygen with metal surfaces ranges from single oxygen atom surface adsorption to bulk oxidation growth. Classical theories of oxidation of metals, such as Cabrera-Mott,1 are mainly concerned with the growth of an oxidation layer the information of which is mostly provided by bulk oxidation studies (an oxidation layer with a thickness of a few micrometers or more). Many elegant experiments using techniques such as ultra-high-vacuum (UHV) scanning tunneling microscopy (STM) have been performed to watch the interaction of oxygen on bare metal surfaces, which have provided insights into the atomic mechanisms of oxygen adsorption.2–4 However, these UHV STM studies are limited to a few monolayers, and do not provide the information about the nucleation, initial growth, and coalescence of the metal oxidation. A complete understanding of oxidation processes should include the investigation of the initial stage of oxidation (i.e., the nucleation and the initial growth of the oxides that occur in a nanometer-sized regime). The initial oxidation stage plays an influential role in the later oxidation process in various materials.5,6 Hence, we have chosen to
a)
Address all correspondence to this author. 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.2005.0236 1910
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J. Mater. Res., Vol. 20, No. 7, Jul 2005 Downloaded: 27 Mar 2015
study the nucleation and the growth of the oxides by in situ UHV transmission electron microscopy (TEM), which can bridge the gap between the monolayer and the bulk growth of oxidation. The UHV environment of UHV TEM provides excellent control of surface conditions. We can study quantitatively the dynamic aspect of the initial stage of oxidation by means of UHV TEM. The oxygen interaction with Cu surfaces is considered to be a model system for the oxidation of metal surfaces and has consequently been studied extensively.7–10 Our previous investigations11–14 of the kinetics of the initial stages of Cu(100) oxidation by in situ UHV TEM at a temperature of 350 °C have demonstrated t
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