Adsorption structure of pyrazine on Si(100): Density-functional calculations

Abstract

We have studied the adsorption structure of pyrazine (C(4)H(4)N(2)) on the Si(100) surface by using density-functional theory calculations within a slab representation. We found that the adsorption energetics strongly depends on the C(4)H(4)N(2) coverage. At 0.25 monolayer (one monolayer is defined as one pyrazine molecule per Si dimer), two different molecular configurations are equally favored in energy: the end-on model where C(4)H(4)N(2) adsorbs on the down atom of a Si dimer and the cross-row model where C(4)H(4)N(2) connects two dimer rows. At 0.5 monolayer, the interactions between adsorbed molecules results in a far stabilized cross-row configuration in which molecules arrange in series along the row-perpendicular direction. This cross-row chain configuration is in good accordance with the images of a recent scanning tunneling microscopy experiment. When including the van der Waals interactions in our calculations, the cross-row structures were found to have larger energy gains, thus being more favored in energy than the end-on structures. The adsorption picture of the present slab-model calculations differs from previous cluster-model calculations, which questions the accuracy of the cluster representation of this adsorbed system.