Structural changes of the W(211) surface induced by ultrathin films of Ph, Pt, and Pd

A W(211) surface covered with a thin film (between 0.5 and 1 physical monolayer) of Ph, Pt, or Pd is found to exhibit an n x 1 superstructure when annealed above a threshold temperature of similar to 900 K (500 K for Pd). The superstructure is observed using low energy electron diffraction; phase diagrams are presented to indicate the coverage range and the temperature threshold where the new structure appears. Scanning tunneling microscopy results indicate that in the case of Pd/W(211) the superstructure phase is due to missing overlayer rows. In the case of Rh/W(211), there are two possible interpretations to the observed periodic structure: either the adlayer forms an ordered superstructure or the W surface itself forms a microfaceted structure with {110} faces. On the basis of our STM observations of gas adsorption on the surface, we found the explanation involving microfacets more plausible. A careful calibration of overlayer coverage is performed using a quartz crystal monitor combined with temperature programmed desorption measurements performed at high temperatures (up to 2400 K). Auger electron spectroscopy used in combination with thermal annealing series demonstrates that overlayer coverages above one physical monolayer are thermally unstable, which is explained by the formation of an ultrathin alloy film.