Initial Growth of the Pentacene Monolayer on a Si(001)-2 × 1 Substrate: A Lattice Model View

In this work, we investigate the initial growth of the pentacene monolayer on the reconstructed Si(001)-2 × 1 surface with a detailed lattice gas model. We employ a combination of electron density functional theory (DFT) and tensor renormalization group (TRG) methods to reveal the equilibrium structures of the monolayer and driving forces of its self-assembly. The planar adsorption complexes are found to be predominant in the pentacene monolayer on the reconstructed Si(001)-2 × 1 in a wide range of pressure and surface coverages. We found two equilibrium structures of the monolayer differing in the surface coverage. Both structures of the monolayer consist of a mixture of adsorption complexes in which pentacene molecules are oriented either along or perpendicular to the rows of Si═Si dimers. The low coverage structure is the network with two-dimensional rectangular pores. As the pressure increases, this structure transforms into a dense phase by filling the pores of the network with extra pentacene molecules. Our TRG calculations demonstrate that self-assembly of the pentacene adsorption monolayer on the Si(001)-2 × 1 surface is driven by the subtle interplay between relative stability of different adsorption complexes and intermolecular interactions.