Adsorption and Interfacial Chemistry of Pentacene on the Clean Si(100) Surface: A Density Functional Study

Abstract

Density functional theory calculations have been performed on the main adsorption configurations of pentacene on the Si(100) surface and on the possible pathways for the following C−H bond cleavage. We considered possible candidates for all the orientations of pentacene experimentally observed with STM, i.e., on the top of silicon dimer rows, perpendicular to the dimer rows, diagonal to the dimer rows and between two adjacent dimer rows (“in between”). Our calculations indicate that the most stable adsorption configuration of pentacene on the Si(100) surface is the symmetric perpendicular structure with an adsorption energy of −128.3 kcal mol-1, with the in between structure 10.5 kcal mol-1 and the symmetric parallel structure 13.0 kcal mol-1 higher in energy. Transition states for the dissociation of C−H and formation of Si−H bonds from the main adsorption configurations of pentacene have been characterized and the corresponding energy barriers estimated. We identified two kinds of adsorbed configurations of pentacene from which the breaking of two C−H bonds can be accessible: one on top of a silicon dimer row with one or both outer benzene rings di-σ−bonded through a [2 + 2] cycloaddition; one with one or more pentacene rings 1,4 di-σ-bonded across two dimer rows, such as the in between structure. The kinetically most favorable reactive channel is that from the in between configuration and involves the separate abstraction of two hydrogen atoms on the sp3 carbon atoms by the two silicon atoms of the two dimers bearing an unpaired electron, with an energy barrier of 29−30 kcal mol-1.