Surface radical chain reaction revisited: Comparative investigation of styrene and 2,4-dimethyl-styrene on hydrogenated Si(001) surface from density functional theory calculations

We report on periodic Density Functional Theory calculations of the adsorption and desorption of styrene and dimethyl-styrene on the H-terminated Si(001)-2 ×1 surface. Experimentally, the adsorption of these molecules on H-Si(001)-2 ×1 results in the formation of one-dimensional lines parallel to the Si dimer rows. At higher temperature, desorption of the molecules at the end of the lines is observed, leading to a shortening of the lines. The desorption temperature is ∼300 K for dimethyl-styrene and ∼400 K for styrene. To obtain insights into the atomic scale mechanisms of these processes, we have studied the initial steps of the radical chain and inverse chain reactions for the two molecules. Due to the existence of several metastable states, different competing reaction paths exist in both cases. We have found differences between styrene and dimethyl-styrene desorption barriers that are in agreement with experiments.