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The initial growth stages of nickel deposited on Ag(0 0 1) have been investigated by LEIS, XPS, STM and XPD. Embedded atom method (EAM) calculations were performed to obtain information about the dynamics of Ni adatoms on the Ag(0 0 1) surface. Amounts of Ni ranging from submonolayers up to 4 monolayers were deposited on the Ag(0 0 1) surface under UHV conditions at room temperature. The LEIS spectra indicate that the Ni islands are almost completely covered by silver. In agreement with previous studies, STM images show that Ni grows on Ag(0 0 1) forming three dimensional islands. A significant fraction of islands with a thickness of 2–3 atomic layers is present even at coverages as low as 0.1 ML. The observation of islands protruding from the surface with heights below that expected for one atomic layer of Ni covered by one atomic layer of Ag suggests that Ni atoms are incorporated in the outermost layer of the substrate. From the EAM calculations the energy barrier for incorporation of a nickel atom into the outermost layer was found to be 0.19 eV, substantially lower than that for bridge site hopping over the substrate (0.61 eV). On the basis of the EAM calculations, a diffusion mechanism for Ni atoms incorporated within the first layer of the surface is proposed to explain the growth of Ni islands. As far as the geometric structure of the Ni film is concerned, the XPD curves indicate that at a coverage of ≈0.3 ML, the structure of the film is distorted with respect to the fcc lattice. The displacement of the forward scattering peaks towards higher polar angles is consistent with the formation of pseudomorphic Ni islands (i.e. with the same in-plane lattice parameter of the substrate). The XPD results lead to the conclusion that pseudomorphic Ni clusters coexist with undistorted fcc Ni islands. Even at the highest coverage studied here, a fraction of the islands maintains a strained fcc structure. |
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