First-principles study of the adsorption and reaction of 2,3-butanediol on Si(001)

The adsorption and reaction of 2,3-butanediol on the Si(001) surface are investigated by first-principles density-functional calculations within the generalized-gradient approximation. We consider the two different reaction pathways forming adsorptions on top of a single dimer (termed the "on-top" structure) and across the ends of two adjacent dimers in the same dimer row (termed the "end-bridge" structure). For both reaction pathways, the O atom of the hydroxyl group initially bonds to the down Si atom of the buckled dimer and, subsequently, O-H dissociation takes place. However, the reaction of another hydroxyl group shows a drastic difference between the two reaction pathways. Along the former reaction pathway, it is difficult to form the second Si - O bond because of a repulsive interaction between an O lone pair and a single Si dangling bond, thereby prohibiting formation of the on-top structure. On the other hand, along the latter reaction pathway, an O lone pair of the hydroxyl group is attracted to the down Si atom of a neighboring dimer, proceeding to the formation of the end-bridge structure. These results provide an explanation for a recent scanning tunneling microscope observation that 2,3-butanediol occupies predominantly the end-bridge structure.