Comparative study of the carbide-modified surfaces $\mathrm{C}/\mathrm{Mo}\left(110\right)$ and $\mathrm{C}/\mathrm{Mo}\left(100\right)$ using high-resolution x-ray photoelectron spectroscopy

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

Figure 1

Selected XP spectra of the monolayer carbide preparation on $\mathrm{Mo}\left(110\right)$ at different doses of ethylene both in the (a) Mo $3d_{5/2}$ and the (b) $\mathrm{C}1s$ core levels. (c), (d) Respective spectra of the subsequent oxidation are presented $(T_S=1200\mathrm{K},h\nu =380\mathrm{eV})$.

Figure 2

Quantitative analysis of the monolayer carbide preparation on $\mathrm{Mo}\left(110\right)$ both in the (a) Mo $3d_{5/2}$ and (b) the $\mathrm{C}1s$ core levels. (c), (d) Analysis of the subsequent oxidation is presented $(T_S=1200\mathrm{K},h\nu =380\mathrm{eV})$.

Figure 3

Comparison between the monolayer (a) and (b) and bulk (c) and (d) carbide overlayers of $\mathrm{C}/\mathrm{Mo}\left(110\right)$ at low temperatures for the (a) and (c) Mo $3d_{5/2}$ core level and (b) and (d) C $1s$ core level $(T=140\mathrm{K},h\nu =380\mathrm{eV})$.

Figure 4

Relative peak ratios in the Mo $3d_{5/2}$ core level of the monolayer carbide on $\mathrm{Mo}\left(110\right)$ for different kinetic energies of the photoelectrons.

Figure 5

DFT optimized structures of monolayer $\mathrm{C}/\mathrm{Mo}\left(110\right)$ with $3/8\mathrm{ML}$ coverage based on an orthorhombic unit cell (C atoms are depicted as gray, Mo atoms as gold), which gave the lowest energies in case of adsorption of three isolated carbon atoms and of a carbon dimer together with one isolated carbon atom, respectively.

Figure 6

Calculated adsorption energies per carbon atom for different coverages on $\mathrm{Mo}\left(110\right)$.

Figure 7

Calculated constant current STM images of the energetically most favorable structures for monolayer $\mathrm{C}/\mathrm{Mo}\left(110\right)$ with different coverages (in ML). The $p(4\times 4)$ unit cell is indicated by black lines. (Left panel) STM image overlayered with the surface structure; only the topmost Mo layer is indicated, (right panel) only STM. Isodensity surfaces of $3.3\times {10}^{-7}$ atomic units are shown to simulate the STM images within the Tersoff-Hamann approach. Coloring as in Fig. 5.

Figure 8

Selected XP spectra of the bulk carbide preparation on $\mathrm{Mo}\left(110\right)$ at different doses of ethylene both in the (a) Mo $3d_{5/2}$ and the (b) C $1s$ core levels. (c), (d) Respective spectra of the subsequent oxidation are presented $(T_S=1200\mathrm{K},h\nu =380\mathrm{eV})$.

Figure 9

Quantitative analysis of the bulk carbide preparation of $\mathrm{Mo}\left(110\right)$ for both the (a) Mo $3d_{5/2}$ and (b) C $1s$ core levels. (e), (f) Respective data of the oxidation experiment is presented. Between the two experiments (c) and (d), the surface was monitored without dosing for 300 s $(T_S=1200\mathrm{K},h\nu =380\mathrm{eV})$.

Figure 10

Relative peak ratios of the bulk carbide on $\mathrm{Mo}\left(110\right)$ for different kinetic energies of the photoelectrons in the (a) Mo $3d_{5/2}$ and (b) $\mathrm{C}1s$ core levels, respectively.

Figure 11

Calculated adsorption energies per carbon atom on $\mathrm{Mo}\left(110\right)$ with increasing number of subsurface carbon layers for $3/8$ surface coverage. See Fig. 5 for arrangements of the surface carbon atoms.

Figure 12

Selected XP spectra of the monolayer carbide preparation on $\mathrm{Mo}\left(100\right)$ at different doses of ethylene both in the (a) Mo $3d_{5/2}$ and the (b) C $1s$ core levels. (c), (d) Respective spectra of the subsequent oxidation are presented $(T_S=1200\mathrm{K},h\nu =380\mathrm{eV})$.

Figure 13

Quantitative analysis of the monolayer carbide preparation on $\mathrm{Mo}\left(100\right)$ both in the (a) Mo $3d_{5/2}$ and (b) the $\mathrm{C}1s$ core levels. (c), (d) Analysis of the subsequent oxidation is presented $(T_S=1200\mathrm{K},h\nu =380\mathrm{eV})$.

Figure 14

Comparison between the monolayer (a) and (b) and bulk (c) and (d) carbide overlayers of $\mathrm{C}/\mathrm{Mo}\left(100\right)$ at low temperatures for the (a) and (c) Mo $3d_{5/2}$ core level and (b) and (d) $\mathrm{C}1s$ core level $(T<140\mathrm{K},h\nu =380\mathrm{eV})$.

Figure 15

Relative ratios of Mo $3d_{5/2}$ core level peaks of the monolayer carbide $p(1\times 1)-\mathrm{C}/\mathrm{Mo}\left(100\right)$ for different kinetic energies of the photoelectrons.

Figure 16

(a) Selected XP spectra of the thick carbide preparation on $\mathrm{Mo}\left(100\right)$ at different doses of ethylene in the $\mathrm{C}1s$ core level. Note that the experiment started with only a low amount of carbon preadsorbed; (b) quantitative analysis of the respective experiment $(T_S=900\mathrm{K},h\nu =380\mathrm{eV})$.

Figure 17

XP spectra in the $\mathrm{C}1s$ core level showing the difference of the carburization process with and without flashes to $1300\mathrm{K}(T=900\mathrm{K},h\nu =380\mathrm{eV})$. See the text for details.