First-principles study of adsorption of methanethiol onCo(0001)

Abstract

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 $\mathrm{Co}(0001)$ surface using density functional theory calculations. We find that the dissociative adsorption of ${\mathrm{CH}}_{3}\mathrm{SH}$ forming an adsorbed methylthiolate $({\mathrm{CH}}_{3}\mathrm{S})$ 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 $\ensuremath{\sim}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ 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 $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$, $(2\ifmmode\times\else\texttimes\fi{}2)$ and $(2\ifmmode\times\else\texttimes\fi{}3)$ adsorptions, we find that the $\mathrm{S}\penalty1000-\hskip0pt\mathrm{C}$ bond tends to be normal to the surface, whereas for the $(2\ifmmode\times\else\texttimes\fi{}1)$ adsorption it tilts away from the surface normal direction by $\ensuremath{\sim}40\ifmmode^\circ\else\textdegree\fi{}$. The $(2\ifmmode\times\else\texttimes\fi{}1)$ 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 $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$ structure may form in the process of methylthiolate adsorption on $\mathrm{Co}(0001)$ due to its adsorption energy being only $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ lower than that for the $(2\ifmmode\times\else\texttimes\fi{}2)$ and $(2\ifmmode\times\else\texttimes\fi{}3)$ structures. We find that there is a charge transfer from the substrate surface atoms to the $\mathrm{S}$ atoms, and that the $\mathrm{S}\penalty1000-\hskip0pt\mathrm{Co}$ bond is strongly polar. The surface $\mathrm{Co}$ atoms bound to $\mathrm{S}$ have a magnetic moment of $\ensuremath{\sim}1.66{\ensuremath{\mu}}_{B}$, while the surface $\mathrm{Co}$ atoms unbound to $\mathrm{S}$ have a larger magnetic moment of $\ensuremath{\sim}1.85{\ensuremath{\mu}}_{B}$. The $\mathrm{S}$ atom in the adsorbed ${\mathrm{CH}}_{3}\mathrm{S}$ acquires a magnetic moment of $\ensuremath{\sim}0.08{\ensuremath{\mu}}_{B}$.