An electron-energy-loss (EELS) study has been carried out on polycrystalline Sn before and after room-temperature exposures of 100, 500, 1500, and 3500 L [1 Langmuir (L) =10 Torr s] to Oz at low pressure (10 Torr) and to 02 at high pressures (160 Torr for 5 min and air at 1 atm for 5 min). Depthsensitive information was obtained from these surfaces by varying the primary-electron-beam energy from 100 to 600 eV and using inelastic mean-free-path calculations. The spectra have been interpreted based on features in the EELS spectra obtained from standard reference materials; Sn metal, SnO, and Sn02. During the 100-L exposure, 02 adsorbs dissociatively and forms SnO in the near-surface region.Subsurface SnO forms more deeply beneath the surface during the 500-L exposure, and a subsurface transitional oxide structure with a composition between that of SnO and Sn02 also forms. Higher exposures up to the EELS saturation exposure of 3500 L converts some of this transitional phase into subsurface Sn02. Angle-resolved EELS shows that the very near-surface region (outermost two or three atomic layers) consists almost entirely of SnO with Sn metal, transitional oxide, and Sn02 lying beneath the surface after a low-pressure, saturation exposure to 0&. After a high-pressure exposure, the near-surface region is fully oxidized to a mixture of SnO and Sn02 with no metallic Sn. The Sn02 concentration is maximum at about 1.4 nm beneath the surface, and both SnO and transitional oxide are present throughout the 3.0-nm oxidized layer in varying quantities. The presence of moisture appears to accelerate the oxidation process in some undetermined manner.
Energy-resolved ESD data obtained from Ag(ll0) after exposure to oxygen at room temperature are presented. The oxygen is found to desorb predominantly as a hydroxyl species. The surface hydrogen apparently originates from the bulk silver and is only removed slowly over many sample anneals. OH+ desorbs with an energy of 4.7 eV, and H + desorbs with an energy of 3.0 eV. The results suggest that oxygen adsorbs at two or more different bonding sites on this surface. OH+ desorbs by ESD from the most prevalent of these sites, and 0' desorbs from another site which seems to fill first upon oxygen exposure. The primary beam energy is an important ESD variable since exciting the Ag 3d core levels results in a substantial reduction in the OH+ desorption yield.
A detailed interpretation of reflection electron-energy-loss spectra taken from clean and oxygenexposed polycrystalline zirconium is described. Using a density-of-states model, the angular momentum character and binding energies of the unoccupied levels are discussed. Depth-sensitive oxidation state and plasmon information obtained by varying the primary electron beam energy from 100 to 600 eV is presented. The oxidation state of the zirconium appears to increase with depth through the oxgyen-containing layer suggesting that oxides such as ZrO, Zr2O3, and possibly nonstoichiometric oxides are present.
This study examines ( 1) the production of a clean poiycrystalline surface, (2) oxygen adsorption as a function of room-temperature exposure using ion scattering spectroscopy (ISS), Auger electron spectroscopy (A ES ), and electron spectroscopy for chemical analysis (ESCA), and (3) CO adsorption. Sand Cl are typical contaminants which modify the chemisorption properties of a Zr surface. However, they are difficult to detect using AES or ESCA but are readily observed using ISS. Sputtering a hot Zr surface provides an excellent method for producing a contaminantfree surface. The adsorption of oxygen proceeds in a stepwise fashion by rapidly populating sites in the outermost surface layer, filling subsurface sites and then forming a multilayer oxide film even at room temperature and low O 2 pressure (10-6 Torr). CO adsorbs dissociatively on Zr at room temperature. An oxide overlayer is formed which contains no C. The C lies beneath the oxide layer in a chemisorbed or interstitial form. Heating converts this C into a carbidic form and causes segregation of surface oxygen into the bulk Zr.
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