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.