Phase transitions in a methyl-terminated monolayer self-assembled on Au{111}

Molecular dynamics simulations have been performed to study the behavior of CH₃(CH₂)₁₂S chain molecules self-assembled on a Au{111} surface. The temperature dependence of the packing of the chains to form the monolayer has been investigated. The self-assembled monolayer is found to undergo at least two phase transitions. The first transition is a change in tilt direction from nearest neighbor to next-nearest neighbor below room temperature. At temperatures higher than room temperature, there is a transition from the next-nearest neighbor tilt to an orientationally disordered state. The Au-S interface region exhibits characteristics similar to those of two-dimensional melting prior to completion of the second phase transition.