Are Pentacene Monolayer and Thin-Film Polymorphs Really Substrate-Induced? A Molecular Dynamics Simulation Study

Molecular dynamics simulations were performed for mono- and multilayer pentacene films on a simple model surface to study polymorphs of the deposition-processed thin-films. The layer-by-layer deposition history was taken into account by successive layer-stacking simulations of monolayer, bilayer and up to five-layered systems. We found that the tilt-free monolayer polymorph and the molecular tilting upon stacking layers on this monolayer were realized in the simulations even without any substrate interactions. These results imply that neither the monolayer nor the small angle tilted thin-film polymorph is substrate-induced as commonly explained. Our results also show a crucial dependence on the layer stacking history for the thin-film polymorph, which indicate that the thin-film polymorph would be induced with the layering history starting from the tilt-free monolayer.