Abstract
We directly observe, using the scanning tunneling microscope, a two-dimensional crystal of the organic compound 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA). The surface unit cell dimension is found to be 21.6±2.2 Å by 15.2±1.6 Å, or approximately 20% larger than the bulk unit cell. Furthermore, the organic lattice is oriented with respect to the graphite substrate even though the two lattices are incommensurate. These observations are consistent with reflection high energy electron diffraction measurements, and energy minimization calculations, assuming that the van der Waals bond is the predominant intermolecular force which determines the equilibrium crystal structure. The combination of measurement and theory provides the first step in developing tools for predicting the conditions which lead to quasiepitaxial growth of these technologically important van der Waals solids.
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