Reactive deposition of NiO ultrathin films on Pd(100)

NiO ultrathin films have been grown on Pd(100) following a reactive deposition procedure. Ni has been dosed at room temperature on the substrate surface in an oxygen partial pressure of 4 · 10-6 mbar. The electronic and structural evolution of the resulting NiO(100) ultrathin films has been followed by means of X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and scanning tunnelling microscopy (STM). XPS, XPD and STM data indicate a 2D growth of the first NiO monolayer, while further growth leads to the nucleation of 3D islands, in a Stranski–Krastanov growth scheme. Combined XPD and LEED data indicate an initially pseudomorphic growth, characterised by in-plane compressive tetragonal strain of the NiO film, with a consequent out-of-plane interlayer expansion. Partial strain relaxation occurs abruptly, very likely between the second and the third atomic layer of the 3D islands, while a completely bulk-like cubic environment is reached only gradually as a function of thickness. NiO(100) films even ca 50 equivalent monolayers thick can be grown with good long-range order, as shown by (1 · 1) LEED images