title: Metal Adatoms and Clusters on Ultrathin Zirconia Films.
abstract: Nucleation and growth of transition metals on zirconia has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Since STM requires electrical conductivity, ultrathin ZrO2films grown by oxidation of Pt3Zr(0001) and Pd3Zr(0001) were used as model systems. DFT studies were performed for single metal adatoms on supported ZrO2films as well as the (1̅11) surface of monoclinic ZrO2. STM shows decreasing cluster size, indicative of increasing metal-oxide interaction, in the sequence Ag < Pd ≈ Au < Ni ≈ Fe. Ag and Pd nucleate mostly at steps and domain boundaries of ZrO2/Pt3Zr(0001) and form three-dimensional clusters. Deposition of low coverages of Ni and Fe at room temperature leads to a high density of few-atom clusters on the oxide terraces. Weak bonding of Ag to the oxide is demonstrated by removing Ag clusters with the STM tip. DFT calculations for single adatoms show that the metal-oxide interaction strength increases in the sequence Ag < Au < Pd < Ni on monoclinic ZrO2, and Ag ≈ Au < Pd < Ni on the supported ultrathin ZrO2film. With the exception of Au, metal nucleation and growth on ultrathin zirconia films follow the usual rules: More reactive (more electropositive) metals result in a higher cluster density and wet the surface more strongly than more noble metals. These bind mainly to the oxygen anions of the oxide. Au is an exception because it can bind strongly to the Zr cations. Au diffusion may be impeded by changing its charge state between -1 and +1. We discuss differences between the supported ultrathin zirconia films and the surfaces of bulk ZrO2, such as the possibility of charge transfer to the substrate of the films. Due to their large in-plane lattice constant and the variety of adsorption sites, ZrO2{111} surfaces are more reactive than many other oxygen-terminated oxide surfaces.
authors.affiliation: Institute of Applied Physics and Center for Computational Materials Science, TU Wien , 1040 Vienna, Austria.
authors.forename: Joong Il Jake
authors.initials: JI
authors.lastname: Choi
authors.affiliation: Institute of Applied Physics and Center for Computational Materials Science, TU Wien, 1040 Vienna, Austria; Institute of Applied Physics and Center for Computational Materials Science, TU Wien, 1040 Vienna, Austria.
authors.forename: Wernfried
authors.initials: W
authors.lastname: Mayr-Schmölzer
authors.affiliation: Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università di Modena e Reggio Emilia, 41125 Modena, Italy; Istituto Nanoscienze, Consiglio Nazionale delle Ricerche, 41125 Modena, Italy.
authors.forename: Ilaria
authors.initials: I
authors.lastname: Valenti
authors.affiliation: Istituto Nanoscienze, Consiglio Nazionale delle Ricerche , 41125 Modena, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Luches
authors.affiliation: Institute of Applied Physics and Center for Computational Materials Science, TU Wien, 1040 Vienna, Austria; Institute of Applied Physics and Center for Computational Materials Science, TU Wien, 1040 Vienna, Austria.
authors.forename: Florian
authors.initials: F
authors.lastname: Mittendorfer
authors.affiliation: Institute of Applied Physics and Center for Computational Materials Science, TU Wien, 1040 Vienna, Austria; Institute of Applied Physics and Center for Computational Materials Science, TU Wien, 1040 Vienna, Austria.
authors.forename: Josef
authors.initials: J
authors.lastname: Redinger
authors.affiliation: Institute of Applied Physics and Center for Computational Materials Science, TU Wien , 1040 Vienna, Austria.
authors.forename: Ulrike
authors.initials: U
authors.lastname: Diebold
authors.affiliation: Institute of Applied Physics and Center for Computational Materials Science, TU Wien , 1040 Vienna, Austria.
authors.forename: Michael
authors.initials: M
authors.lastname: Schmid