LOCAL DECOMPOSITION OF NIO ULTRA-THIN FILMS FORMED ON NI(111)

AES, LEED and STM have been combined to study the decomposition of NiO (111) and NiO(100) thin films formed on Ni(111) after annealing betwee n 500 and 700 K in UHV. The annealing of the two to four layer thick f ilms leads to their partial decomposition and produces stable NiO(111) crystallites and the Ni(111)-(2 x 2) phase of adsorbed oxygen. The de composition of the NiO(111) thin films produces an intermediate (root 3 x 2) rect. phase assigned to 2D arrangements of residual NiO rows on reduced Ni(111) planes, before the formation of the (2 x 2) adsorbed phase. The loss of oxygen of the (2 x 2) adsorbed phase leads to the u ncovering of the substrate terraces preferentially on the upper side o f the steps, indicating a preferential removal of the oxygen atoms by adsorption in the metal or reactive desorption at the ledges of the su perstructure. The formation of a surface monolayer of nickel hydroxide on the NiO(111) thin film shifts the observation of the early stages of decomposition from 550 to 600 K with respect to the NiO(111) surfac e covered by a monolayer of hydroxyl groups. This is attributed to the increase of the film thickness resulting from the surface hydroxidati on. The NiO(111) orientation has a higher resistance to decomposition than the Ni0(100) one, which is already totally decomposed into the ad sorbed phase at 550 K. This is assigned to the higher stability of the NiO(111)/Ni(111) interface. (C) 1998 Published by Elsevier Science B. V. All rights reserved.