Growth and thermal behaviour of NiO nanolayers on Pd(100)

The growth of ultrathin nickel oxide overlayers (nanolayers) on Pd(100) from submonolayer coverages up to 20 monolayer thick films has been investigated by scanning tunneling microscopy (STM), in conjunction with LEED and high-resolution electron energy loss spectroscopy (HREELS). The first nickel oxide monolayer on Pd(100) forms a wetting layer with a c(4 x 2) structure, which has been interpreted in terms of an interface stabilised Ni3O4-type structure. Stoichiometric NiO grows on top of the c(4x 2) monolayer in a strained lattice with the (100) surface orientation parallel to the substrate. The lattice relaxation of the NiO phase occurs gradually and is completed after 10 monolayers, where a bulk-type NiO phase is obtained. The phonon spectra of the c(4 x 2) monolayer and the subsequent NiO layers, as measured by HREELS, are characteristic and of diagnostic value to identify the different oxide phases. The morphology of the nickel oxide nanolayers has been analysed by quantitative evaluation of the STM images in terms of their roughness parameters. The roughness of nickel oxide nanolayers prepared by different kinetic routes has been compared and the optimal preparation conditions for obtaining smooth morphologies are proposed.