Surface Reaction of Alkynes and Alkenes with H-Si(111) : A DFT study

There is currently a strong interest in the organic functionalization of semiconductor surfaces. One of the most promising approaches for preparing such functionalized surfaces is via a radical-initiated surface chain reaction of terminally unsaturated molecules with hydrogen-terminated surfaces. The adsorbing organic molecule reacts with the Si dangling bond, and forms an intermediate metastable state in which a carbon centered radical is present. Abstraction of a hydrogen atom from a neighboring H-Si surface unit results in a stable adsorbed species and a new hydrogen vacancy. We have studied the initial stages of this chain reaction on the H-Si(111) surface using the first principles string molecular dynamics approach [1] which couples the Car-Parrinello scheme with an efficient method to determine reaction pathways. We find that the relative values of the energy barrier for hydrogen abstraction and for the desorption of the adsorbed carbon centered radical at the metastable state is the crucial factor in determining the viability of the surface chain reaction. The roles of the molecule and surface electronic structures in the reaction are discussed. Results for the adsorption of C$_2$H$_2$, C$_2$H$_4$, Phenylacetylene, and Styrene on the H-Si(111) surface are presented. \newline \newline [1] Y. Kanai, A. Tilocca, A. Selloni, and R. Car, J. Chem. Phys. 121, 3359 (2004).