ULTRAHIGH-VACUUM QUASIEPITAXIAL GROWTH OF MODEL VAN-DER-WAALS THIN-FILMS .1. THEORY

We introduce a model to calculate the equilibrium crystal configuration of a monolayer lattice of large, planar organic molecules bonded to a substrate by van der Waals (vdW) forces. The model significantly simplifies analysis by replacing the conventional atom-atom vdW potential summation with a single ellipsoidal potential centered in the molecular plane. Our results indicate that recent observations of crystalline quasiepitaxial vacuum growth of incommensurate lattices of these planar molecular films result from the relatively large intralayer stiffness as compared to the interlayer shear stress. Good agreement between calculated and observed structures is achieved using no adjustable parameters. The model is used to predict molecular structures which are likely to form quasiepitaxial layers. Comparison with previous models describing physisorption of incommensurate layers of atomic vdW systems on graphite substrates is also made. In the subsequent paper (paper II) we present experimental data comparing the grown structures of several model compounds with the theoretical predictions made here.