Oxidation kinetics forNi(111)and the structure of the oxide layers

Abstract

The oxidation kinetics for $\mathrm{Ni}\left(111\right)$ surface and the structure of the oxide layers grown at room temperature were analyzed by high-resolution medium energy ion scattering using isotopically labeled ${}^{18}\mathrm{O}_2$. Initially, the surface showed a reflection high energy electron diffraction (RHEED) pattern of the $\mathrm{Ni}\left(111\right)\text{−}(2\times 2)\text{−}\mathrm{O}$ chemisorption structure at an oxygen exposure of a few langmuir $(1\mathrm{L}:{10}^{-6}\mathrm{Torr}\mathrm{s})$ and started to form $\mathrm{NiO}\left(111\right)$ layers above $10\mathrm{L}$. The oxide thickness was saturated with six atomic layers at oxygen exposure of $160\mathrm{L}$. The saturated $\mathrm{NiO}\left(111\right)$ surface shows a $(1\times 1)$ RHEED pattern not $p(2\times 2)$ and consists of two domains: $\mathrm{NiO}\left(111\right)\text{−}\left[11\overline{2}\right]∕∕\mathrm{Ni}\left(111\right)\text{−}\left[11\overline{2}\right]$ (primary) and $\mathrm{NiO}\left(111\right)\text{−}\left[1\overline{1}0\right]∕∕\mathrm{Ni}\left(111\right)\text{−}\left[11\overline{2}\right]$(a small fraction). The elemental depth profile corresponds to the octopolar reconstruction terminated with Ni $\left(0.25\mathrm{ML}\right)$ [$1\mathrm{ML}=1.33\times {10}^{15}\mathrm{atoms}∕{\mathrm{cm}}^2$: areal density of $\mathrm{NiO}\left(111\right)$]. The oxide surface grown epitaxially probably takes a precursor state of the octopolar structure with significant distortion because of a large lattice mismatch of 19% between $\mathrm{NiO}\left(111\right)$ and $\mathrm{Ni}\left(111\right)$. Tow step oxidation using ${}^{18}\mathrm{O}_2$ and ${}^{16}\mathrm{O}_2$ clearly indicates that oxidation takes place at the top surface obeying the Cabrera-Mott mechanism. The potential barrier for the jump from Ni into the $\mathrm{NiO}$ layer is derived to be $2.36\mathrm{eV}$.