Change in oxide epitaxy on Ni(111): Effects of oxidation temperature

Studies of the oxidation of Ni(111) in the temperature range 30–275°C by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and ellipsometry have shown that the epitaxy of the oxide is dependent on the growth temperature. Below 150°C the commonly reported hexagonal p(2×2)-O and NiO(111) patterns are seen. For oxidation temperature above 200°C, LEED observations at low oxygen exposures reveal a superlattice pattern consistent with a lightly expanded NiO(100) lattice on Ni(111) with NiO[011] rotated 45° with respect to Ni[011]. This is followed by the development of an additional square pattern with NiO(100)‖Ni(111) and NiO[011]‖NiO[011]. NiO(111) covers the Ni substrate (as indicated by the disappearance of the Ni(111) LEED beams) after about 60 L of oxygen exposure. NiO(100), however, does not coalesce on Ni(111) after this exposure but rather thickens as large islands; the substrate beams remain visible after oxygen exposures of at least 500 L. The growth of the NiO(100) face above 200°C may be due to the instability of the (111) face of a crystal such as NiO while the (100) face is one of low surface energy.