Composition, structure and stability of surfaces formed by Ni deposition on Pd(111)

Abstract

Surface composition and structure of deposited Ni ultrathin films grown on a Pd(111) surface and their thermal stability have been studied using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS) and scanning tunneling microscopy (STM). In experiments where up to 2 monolayers (ML) of Ni was deposited onto Pd(111) at 300K, the initial film growth followed a non-ideal layer-by-layer growth mode, in which the majority of the surface was covered by a single atomic layer of Ni, but the second Ni layer started to appear before the first layer was completed. Annealing the Ni/Pd(111) surface to 600K caused Ni interdiffusion into subsurface layers and the outermost surface was mainly Pd. This structure, designated as Pd–Ni–Pd(111), was not stable in the presence of surface oxygen. Ni segregated to the topmost surface layer to form a (2×2) superstructure after exposing the Pd–Ni–Pd(111) surface at 590K to 350L O2. The oxygen-induced segregation of Ni is consistent with predictions from density functional theory (DFT) calculations.