Growth of NiO ultrathin films on Pd(100) by post-oxidation of Ni films: the effect of pre-adsorbed oxygen

This paper reports on the optimisation of the growth parameters of NiO ultrathin films on Pd(1 0 0). Growth is performed by means of UHV metal deposition and post-oxidation cycles. Chemical and structural characterisation of the deposits is achieved by means of electron spectroscopy (X-ray photoelectron spectroscopy, XPS; angle resolved XPS) and electron diffraction techniques (low energy electron diffraction, LEED; X-ray photoelectron diffraction, XPD). Three growth procedures have been investigated, which differ for the particular growth parameters adopted in each case. We demonstrate that post-oxidation is effective in order to obtain epitaxial NiO only if the initial dose of Ni evaporated on the clean Pd(1 0 0) substrate exceeds a critical value, corresponding approximately to two equivalent monolayers. However, the overlayer thus obtained is strongly understoichiometric in oxygen close to the metal/oxide interface and poorly ordered on the long range. When a Ni dose below this limiting first value is used, the layer evolves toward polycrystalline NiO, due to substantial oxidation of the Pd substrate promoted by the presence of Ni, very likely through a work function decrease upon direct metal/metal interface formation. On the contrary, epitaxial NiO(1 0 0) layers of good structural quality, with limited oxygen deficiency at the interface, with negligible substrate oxidation and with a good degree of long-range order are obtained if deposition and post-oxidation cycles are initiated on an oxygen pre-saturated Pd surface, characterised by the (√5×√5)-R27° O/Pd(1 0 0) LEED pattern. We therefore demonstrate that oxygen can act either as an inhibitor or as a promoter of NiO epitaxial growth on Pd(1 0 0), depending on the way it is used.