Ultrathin films of Pd on Au(111): Evidence for surface alloy formation

Pd ultrathin films on Au (111) have been prepared by vapor deposition and studied over the substrate temperature range 150--650 K using Auger electron spectroscopy (AES), low-energy ion-scattering spectroscopy (LEISS), x-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). With regard to assessing the growth mode of Pd adlayers on Au (111), AES uptake plots (AES signal intensity versus deposition time) are of limited utility. LEISS data are critical for establishing the surface composition of the deposited film and the existence of alloying. The growth mode of vapor-deposited Pd films on Au (111) at 150 K is described well by an epitaxial layer-by-layer mechanism for the first few layers. However, for Pd deposition at substrate temperatures of 300 K and above, LEISS data taken using 1 keV Ne^+ ions indicate surface alloy formation, with the extent of Au interdiffusion increasing with temperature. Other experiments involving annealing monolayer Pd films show interdiffusion (alloying) at temperatures as low as 240 K. XPS data indicate very little (0. 2 eV) or no shifts in the Pd (3d), Au (4f), or Pd x-ray-excited Auger (M₅VV) regions as a function of Pd coverage or substrate temperature. We discuss the surface stoichiometry in relation to prior published work on this system, the relevance of this system for studies of Pd monolayer and thin film properties, and our results pertaining to the surface chemistry of small molecule adsorbates on Pd films deposited on Au (111).