Surface defects on MgO thin films: Their detection using metastable impact electron spectroscopy and interaction with pr
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Surface defects on MgO thin films: Their detection using metastable impact electron spectroscopy and interaction with probe molecules Jeffrey A. Stultz, Andrei Kolmakov, Xiaofeng Lai, Young Dok Kim, and D. Wayne Goodman Texas A&M University College Station, Texas 77842, U.S.A. ABSTRACT MgO thin films having different defect densities are explored in this study using metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), temperature programmed desorption (TPD), and scanning tunneling microscopy (STM). Surface point defects on MgO exhibit themselves in both the MIES and UPS spectra as a feature approximately 2 eV above the valance band, whereas extended defects are only observed spectroscopically as a broadening of the O 2p band. The interaction of NO and N2O with the MgO surface as a function of surface defect density is explored. Upon adsorption on MgO thin films at 100K, both NO and N2O show the development of three features which coincide with a standard gas phase N2O spectrum. The saturation coverage of N2O from NO adsorption increases with increasing defect density, indicating that defect sites are mainly responsible for N2O formation. STM images confirm the increase of thin film defect density upon thermal quenching. INTRODUCTION Surface defects on oxide surfaces have significant impact in a range of applications including catalysis, chemical sensors and electronic micro-devices [1]. In spite of their significance, there has been a shortage in experimental activity of the study of surface defects on metal oxides in comparison with their bulk counterparts due to a number of reasons: (i) Characterization of surface defects with surface science techniques is experimentally challenging because of their low concentration on the surface, and (ii) the fact that metal oxide samples accumulate charge when using electron spectroscopies. Thus, to investigate oxide surfaces most effectively, ultra thin films of metal oxides on metal substrates are employed [2]. This approach overcomes the main problem of charge accumulation on the surfaces of bulk samples while using electron spectroscopic techniques. Because the oxide films are a few monolayers thin, induced charge immediately dissipates via tunneling to/from the metal substrate. In addition, using thin films prepared in ultra high vacuum (UHV) conditions via vapor deposition allows the flexibility of controlling purity, composition, doping and other properties during film growth. Thin films of oxides prepared in this manner have been shown to have similar chemical and physical properties to those of the bulk crystals[3]. Once the regular thin films are prepared and characterized, the surface defects on the films were created to compare the properties of the as prepared and defective surfaces. MgO has been chosen because it has a considerable foundation P9.5.1
of experimental and theoretical study and its importance in catalysis and micro-electronic devices Point defects have been observed experimentally in previous electron energy
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