First-principles study of adsorption of methanethiol on Co(0001)

Authors

Ligen Wang, University of Nebraska - LincolnFollow
Evgeny Y. Tsymbal, University of Nebraska-LincolnFollow
Sitaram Jaswal, University of NebraskaFollow

Date of this Version

August 2004

Comments

Published in Physical Review B, 70, 075410 (26 August 2004)
Copyright © 2004 The American Physical Society. Used by permission
URL: http://link.aps.org/abstract/PRB/v70/e075410
doi:10.1103/PhysRevB.70.075410

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, CH₃SH, on the ferromagnetic Co (0001) surface using density functional theory calculations.We find that the dissociative adsorption of CH₃SH forming an adsorbed methylthiolate (CH₃S) and an adsorbed H atom is energetically favorable, and that the CH₃S molecule adsorbed at the threefold fcc and hcp hollow sites is most stable. The adsorption energy at the bridge site is only ~ 0.2 eV smaller than that at the threefold hollow site, and the adsorption of CH₃S at the atop site is unstable. For the (√3 × √3)R30°, (2 × 2) and (2 × 3) adsorptions, we find that the S-C bond tends to be normal to the surface, whereas for the s231d adsorption it tilts away from the surface normal direction by ~ 40°. The (2 × 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 (√3 × √3)R30° structure may form in the process of methylthiolate adsorption on Co (0001) due to its adsorption energy being only 0.1 eV lower than that for the (2 × 2) and (2 × 3) structures. We find that there is a charge transfer from the substrate surface atoms to the S atoms, and that the S-Co bond is strongly polar. The surface Co atoms bound to S have a magnetic moment of ~ 1.66μ_B, while the surface Co atoms unbound to S have a larger magnetic moment of ~ 1.85μ_B. The S atom in the adsorbed CH3S acquires a magnetic moment of ~0.08 μ_B.