Incommensurate epitaxy of tetrathiophene on potassium hydrogen phthalate: Insights from molecular simulation

The (010) surface of potassium hydrogen phthalate (KAP) has a strong orientational effect on the growth of crystalline tetrathiophene (4T) films from the vapor phase, even if there is complete incommensurism between their crystal lattices. Here we report on molecular dynamics simulations of the deposition of 4T on KAP, with several degrees of coverage and different initial conditions. We analyze both the behavior of preassembled crystalline monolayers of one tetrathiophene polymorph and the dynamics of molecules randomly distributed on the organic substrate. The results clearly show the presence of two preferred orientations of the long axes of the 4T molecules, in agreement with experiment. The aggregation of molecules into clusters is also observed at higher degrees of coverage. Again, the molecules are parallel to each other along the preferred orientations, and even a second layer adopts the same alignment.