Effects of chain–chain interaction on the configuration of short-chain alkanethiol self-assembled monolayers on a metal surface

Surface-specific sum frequency generation vibrational spectroscopy is applied to study the molecular configuration of short-chain n-alkanethiol self-assembled monolayers (SAMs with n = 2–6) on the Au surface. For monolayers with n ≥ 3, the alkanethiols are upright-oriented, with the CH3 tilt angle varying between ∼33° and ∼46° in clear even–odd dependency. The ethanethiol monolayer (n = 2) is, however, found to exhibit a distinct lying-down configuration with a larger methyl tilt angle (67°–79°) and a smaller CH2 tilt angle (56°–68°). Such a unique configurational transition from n = 2 to n ≥ 3 discloses the steric effect owing to chain–chain interaction among neighboring molecules. Through density functional theory calculations, the transition is further confirmed to be energetically favorable for thiols on a defective reconstructed Au(111) surface but not on the pristine one. Our study highlights the roles of the chain–chain interaction and the substrate surface atomic structure when organizing SAMs, offering a strategic pathway for exploiting their applications.