A New Pentacene Polymorph Induced by Interaction with a Bi(0001) Substrate

Pentacene, one of the most studied organic semiconductors, crystallizes in different structures depending on the growth conditions. The structure is usually classified into one of the three polymorphs, the so-called single crystal phase, bulk phase, and thin film phase. In this report, we show that epitaxial crystalline pentacene thin films on Bi(0001), which have been believed to be the bulk phase, have a distinct molecular orientation in the same crystal lattice as the bulk phase. The calculated band structures are clearly different from those of the bulk phase and show an excellent agreement with the experimental data for the bandwidth. A tight-binding model indicates that one of the two main intermolecular interactions is almost identical to that of the single crystal phase, while the other one is almost identical to that of the bulk phase. The results implies that polymorphs in the same crystal lattice can be formed by a molecular-substrate interaction.