Plasticity-induced oxidation reactivity on Ni(100) studied by scanning tunneling spectroscopy
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Plasticity-induced oxidation reactivity on Ni(100) studied by scanning tunneling spectroscopy F.W. Herbert and K.J. Van Vliet, Department of Materials Science & Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 B. Yildiz, Department of Nuclear Science & Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Address all correspondence to K.J. Van Vliet at [email protected] and B. Yildiz at [email protected] (Received 25 August 2011; accepted 3 October 2011)
Abstract Using in situ indentation, we show that highly localized and well-defined mechanical deformation can be coupled with structural and electronic characterization in the scanning tunneling microscope. Dislocations induced in Ni(100) were topographically imaged and probed by scanning tunneling spectroscopy to assess their effect on local surface electronic structure. Compared with undamaged terraces, dislocation regions exhibited a significant increase in local density of states near the Fermi level, and enhanced reactivity toward oxidation. In the context of the d-band electronic structure model, we suggest that the undercoordination of atoms and residual strain resulting from plastic deformation serve to locally accelerate adsorption-driven chemical reactions with species such as molecular oxygen.
The adsorption and reactivity of oxygen on nickel surfaces has attracted considerable research effort over the last 40 years. An atomic-scale understanding of this complex interaction offers fundamental insights crucial to many important technological processes such as corrosion, bulk oxidation in electronic systems, and heterogeneous catalysis. The chemisorption of oxygen on the (100) surface of nickel at room temperature has been well documented.[1–3] Molecular oxygen dissociates at the surface and an oxygen monolayer of p(2 × 2) or c(2 × 2) arrangement forms prior to the nucleation of epitaxial nickel monoxide (NiO) nuclei. The adsorption of oxygen and subsequent nucleation of the oxide phase can be highly dependent on the presence of topographical defects such as surface vacancies.[4] Moreover, scanning tunneling microscopy (STM) studies have demonstrated that monatomic steps on a vicinal surface provide the most active sites for NiO nucleation on Ni(100)[5] and Ni(111).[6] Dislocations also play a major role in surface reactions; for example, the catalytic activity of nickel has long been known to increase upon cold rolling.[7] Superficial steps associated with near-surface dislocations are likely to influence the oxygen adsorption and oxidation activity in several ways, including altered local electronic structure, lower atomic coordination, and facilitated migration of charged adsorbate species. In this study, using controlled in situ indentation in the STM we induce deformation structures with well-defined topographies, interpreted as dislocation loops intersecting the surface. We record the tunneling current–voltage characteristics w
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