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

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The preparations of single (monolayer) and bulk carbides on the $\mathrm{Mo}(110)$ and $\mathrm{Mo}(100)$ single crystals are followed in situ at 1200 K using synchrotron-based high-resolution x-ray photoelectron spectroscopy of the $\mathrm{C}\phantom{\rule{0.28em}{0ex}}1s$ and core Mo $3{d}_{5/2}$ 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 $\mathrm{Mo}(110)$, carbon dimers adsorb in the long-bridge site, most likely at a coverage of $3/8\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$ carbon atoms per Mo surface atom. For the bulk carbide, we find a coverage of $\ensuremath{\sim}0.5\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$ on the surface, and the calculations show that single carbon atoms are more stable than dimers on the surface. The monolayer carbide on $\mathrm{Mo}(100)$ exhibits a coverage of $\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$ and agrees with previous studies, while the bulk carbide preparation probably leads to a faceting of the surface.