The physicochemical origins of coincident epitaxy in molecular overlayers:Lattice modeling vs potential energy calculations

The physicochemical basis for epitaxial stabilization of coincident molecul ar overlayers is illustrated by comparison of optimum overlayer-substrate c onfigurations calculated with potential energy (PE) methods and a simple ge ometric lattice misfit modeling algorithm (EpiCalc) that neglects molecular orientations and intermolecular potentials. Using beta-bis(ethylenedithio) tetrathiafulvalene triiodide (beta-ET2I3), perylenetetracarboxylic diimide (PTCDI), and perylenetetracarboxylic dianhydride (PTCDA) overlayers on a gr aphite substrate as examples, both methods predict identical optimum azimut hal overlayer orientations for each overlayer that also agree with experime ntal observations. PE calculations for three hypothetical PTCDA overlayers, with identical lattice parameters but different molecular orientations, pr edict coincidence at the same azimuthal orientation for all overlayers. Ide ntical results are achieved for PE calculations performed with this lattice when it is occupied by naphthalenetetracarboxylic dianhydride (NTCDA) mole cules or argon atoms. These results demonstrate that the epitaxial orientat ion of coincident overlayers is governed more by geometric lattice matching than specific molecule-substrate interactions and that unambiguous determi nation of the optimum azimuthal orientation relies on establishing the phas e relationship between several overlayer supercells and the substrate. In t he case of PE methods, calculations with large overlayer sizes are computat ionally prohibitive and the energy differences between alternative orientat ions typically are smaller than the confidence limits of the method. In con trast, the calculation time required by EpiCalc is independent of overlayer size, providing a significant advantage over PE methods with respect to co mputational speed while enabling unambiguous assignment of the optimum coin cident configuration. The reliability of EpiCalc in predicting observed epi taxial overlayer orientations indicates that geometric lattice misfit model ing can be used to screen efficiently for the most favorable epitaxial conf iguration (overlayer lattice parameters and azimuthal angle), which can the n be used in subsequent PE calculations that allow for other degrees of fre edom such as molecular orientation.