Steric effects on the adsorption of alkylthiolate self-assembled monolayers on Au (111)

Steric effects on the adsorption of self-assembled monolayers (SAMs) formed by alkylthiolates on the Au(111) surface were investigated using density functional theory. Based on the (root3 x root3) R30degrees structure, the current results on methylthiolate (CH3S) show that the adsorption prefers the face-centered cubic-bridge and hexagonal close-packed-bridge sites. Furthermore, the adsorption energy decreases slightly compared to the CH3S adsorption on the p(2 x 2) structure due to lateral interactions. Comparison between the results on CH3S and 1-propylthiolate (C3H7S) illustrates that the adsorption energy increases with chain length. Strong steric effects were found due to the chain length of the alkylthiolates and the hydrogen atoms in the CH2 unit adjacent to the S atom (alpha hydrogens). The energetically favored tilt angle is 20degrees for C3H7S. The preferred geometry for both CH3S and C3H7S adsorptions has the two alpha hydrogens pointing toward the bridge Au atoms of the surface. The results suggest a flat potential energy surface, which correlates well with the dynamic nature of alkylthiolate SAMs observed experimentally.