XPS, LEED and STM study of thin oxide films formed on Cr(110)

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The growth, thickness, composition and structure of chromium oxide thin films formed by exposing Cr(110) single-crystal surfaces to gaseous oxygen at 300 and 625 K have been investigated by XPS, LEED and STM measurements. The oxide films formed at the two temperatures are significantly different. At 300 K, a granular and non-crystalline oxide is formed, which grows with a constant ∼Cr 2O 3 stoichiometry up to a limiting thickness of 0.9 nm. The film is hydrated with a water content of 10–20%, which decreases upon annealing. Nuclei of oxide with a lateral dimension of ∼0.7 nm and a height of ∼0.2 nm have been observed in the nucleation stage. These nuclei grow predominantly laterally and coalesce to fully cover the substrate surface prior to the thickening stage. At 625 K, a first stage of oxygen adsorption is observed in which stripes 1.5–2.3 nm wide and parallel to the Cr[001] direction are observed after annealing in UHV. They correspond to narrow segments of mixed and close-packed planes of O atoms and ions having a geometry and orientation similar to those of the anions planes in the oxide crystals. Rows of adatoms, possibly Cr 3+ ions of oxide nuclei, are observed above the stripes. Thickening at 625 K leads to the formation of a non-crystalline oxide, which grows up to a limiting thickness of 4.6 nm. The presence of Cr 3+ vacancies related to a significant cation transport through the oxide film in this temperature regime is detected. After UHV annealing at 825 or 925 K, the film is anhydrous. The Cr 3+ vacancies are accumulated at the metal/oxide film interface. The film crystallizes in epitaxy with the substrate in the following orientation: α-Cr 2O 3(0001)‖Cr(110) and α-Cr 2O 3[213̄0]‖Cr[001]. The STM measurements of the unit cell of the α-Cr 2O 3(0001) surface are consistent with a termination by a cation plane and show three tunneling sites assigned to the various possible locations of the Cr 3+ ions at room temperature due to surface diffusion.