Comparative study of the carbide-modified surfaces C/Mo(110) and C/Mo(100) using high-resolution x-ray photoelectron spectroscopy

The preparations of single (monolayer) and bulk carbides on the Mo(110) and Mo(100) single crystals are followed in situ at 1200 K using synchrotron-based high-resolution x-ray photoelectron spectroscopy of the C 1s and core Mo 3d(5/2) levels. By comparing the experimental results to first principles calculations using density functional theory, we suggest real-space surface structures for the carbide-modified surfaces. For a monolayer carbide on Mo(110), carbon dimers adsorb in the long-bridge site, most likely at a coverage of 3/8 ML carbon atoms per Mo surface atom. For the bulk carbide, we find a coverage of similar to 0.5ML on the surface, and the calculations show that single carbon atoms are more stable than dimers on the surface. The monolayer carbide on Mo(100) exhibits a coverage of similar to 1ML and agrees with previous studies, while the bulk carbide preparation probably leads to a faceting of the surface.