Surface composition and structure of nickel ultra-thin films deposited on Pd(111)

Ultra-thin nickel films deposited on the Pd(111) surface were characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). Up to 3 ML coverage, a LEED (1 x 1) pattern with a diffuse background due to a random distribution of Ni atoms on the surface is observed. Annealing at 600 deg. C reduced the background drastically and sharp (1 x 1) spots appeared on the screen, but XPS showed no presence of nickel on the surface, indicating diffusion into the bulk. Annealing at 300 deg. C for 30 min yielded also a sharp (1 x 1) LEED pattern, and the XPS Ni/Pd intensity ratio decreased with annealing time. The comparison between experimental and theoretical XPD results indicated that the surface was covered partially by Ni islands and partially by a random Ni-Pd surface alloy