Growth of Ultrathin Cobalt oxide films on Pd(100): Refined Structural Model

We present an atomistic-level study on the growth of cobalt oxide ultra-thin films prepared on Pd(100) substrate performed by a combination of scanning tunneling microscopy (STM) and Low Energy Electron Diffraction (LEED). In this contribution, we report a new surface structure c(12×2)-CoO(111), which co-exist with the previously reported extended c(4×2)-CoO(100) domains at low Co deposition coverages. We demonstrate that the lattice parameters derived for the newly observed hexagonal c(12×2)-CoO(111) phase fit the three- or sixfold lattice parameter of the c(4×2)-CoO(100) layer. The c(12×2)-CoO(111) layer was shown to grow directly on top of the c(4×2)-CoO(100) structure, offering a transition mechanism from rectangularly to hexagonally terminated CoO surface. At growing Co coverages, HEX-CoO(111) domains exhibiting a Moiré pattern followed by the formation of well-defined Co3O4(111) islands, were prepared and structurally characterized. Both latter phases have been previously reported in the literature. Finally, we provide a refined model for the epitaxial growth of cobalt oxide thin films on Pd(100), including the formation of new c(12×2)-CoO(111) phase.