First-principles study of adsorption of methanethiol on $\mathrm{Co}\left(0001\right)$

Investigation of the resident site and the adsorption phase structure of thiolates is of fundamental importance for understanding the formation of self-assembled organic monolayers on metal substrate surfaces. In the present study, we have investigated adsorption of methanethiol, ${\mathrm{CH}}_3\mathrm{SH}$, on the ferromagnetic $\mathrm{Co}\left(0001\right)$ surface using density functional theory calculations. We find that the dissociative adsorption of ${\mathrm{CH}}_3\mathrm{SH}$ forming an adsorbed methylthiolate $\left({\mathrm{CH}}_3\mathrm{S}\right)$ and an adsorbed $\mathrm{H}$ atom is energetically favorable, and that the ${\mathrm{CH}}_3\mathrm{S}$ molecule adsorbed at the threefold fcc and hcp hollow sites is most stable. The adsorption energy at the bridge site is only $\sim 0.2\mathrm{eV}$ smaller than that at the threefold hollow site, and the adsorption of ${\mathrm{CH}}_3\mathrm{S}$ at the atop site is unstable. For the $\left(\sqrt{3}\times \sqrt{3}\right)\mathrm{R}30°$, $\left(2\times 2\right)$ and $\left(2\times 3\right)$ adsorptions, we find that the $\mathrm{S}‐\mathrm{C}$ bond tends to be normal to the surface, whereas for the $\left(2\times 1\right)$ adsorption it tilts away from the surface normal direction by $\sim 40°$. The $\left(2\times 1\right)$ adsorption phase is much less stable. The reduction of the adsorption energy with the increasing coverage is attributed to the repulsive interaction between the adsorbates. Our calculations show that the $\left(\sqrt{3}\times \sqrt{3}\right)\mathrm{R}30°$ structure may form in the process of methylthiolate adsorption on $\mathrm{Co}\left(0001\right)$ due to its adsorption energy being only $0.1\mathrm{eV}$ lower than that for the $\left(2\times 2\right)$ and $\left(2\times 3\right)$ structures. We find that there is a charge transfer from the substrate surface atoms to the $\mathrm{S}$ atoms, and that the $\mathrm{S}‐\mathrm{Co}$ bond is strongly polar. The surface $\mathrm{Co}$ atoms bound to $\mathrm{S}$ have a magnetic moment of $\sim 1.66{\mu }_B$, while the surface $\mathrm{Co}$ atoms unbound to $\mathrm{S}$ have a larger magnetic moment of $\sim 1.85{\mu }_B$. The $\mathrm{S}$ atom in the adsorbed ${\mathrm{CH}}_3\mathrm{S}$ acquires a magnetic moment of $\sim 0.08{\mu }_B$.

Figure 1

(Color online) Surface unit cells and adsorption sites (left panel): (a) $(1\times 1)$, (b) $(2\times 1)$, (c) $\left(\sqrt{3}\times \sqrt{3}\right)\mathrm{R}30°$, (d) $(2\times 2)$, and (e) $(2\times 3)$. The right panel shows an example of the $(2\times 2)$ adsorption supercell with methylthiolate adsorbed at the hcp hollow site. The $\mathrm{Co}$, $\mathrm{S}$, $\mathrm{C}$, and $\mathrm{H}$ atoms are represented by the largest, second largest, third largest, and smallest spheres, respectively. $\mathrm{Co}\left(\mathrm{I}\right)$ denotes the surface $\mathrm{Co}$ atoms bound to $\mathrm{S}$, whereas $\mathrm{Co}\left(\mathrm{II}\right)$ denotes the surface $\mathrm{Co}$ atoms unbound to $\mathrm{S}$ in the surface unit cell.

Figure 2

Adsorption energy for various adsorption phase structures with methylthiolate adsorbed at the fcc hollow site. For the ‘unrelaxed’ case (see explanation in the caption to Table ) the adsorption energy is represented by the star symbol.

Figure 3

(Color online) (a) Optimized atomic structure (topview), (b) total valence electron density, and (c) difference electron density for the $\left(\sqrt{3}\times \sqrt{3}\right)\mathrm{R}30°$ adsorption with ${\mathrm{CH}}_3\mathrm{S}$ at the fcc hollow site. The difference electron density is defined by $\Delta \rho =\rho ({\mathrm{CH}}_3\mathrm{S}∕\text{substrate})-\rho \left(\text{substrate}\right)-\rho \left({\mathrm{CH}}_3\mathrm{S}\right)$.