Structural transition and interconversion between the 2D self-assembled structures of pentacene.

The 2D self-assemblies and structural transitions of pentacene on a Cd(0001) surface have been investigated with low temperature scanning tunneling microscopy (STM). With increasing coverage, pentacene molecules show a structural evolution from the initial disordered gas-like phase through the porous network phase to the herringbone phase, and finally to the brickwall phase at the full monolayer. In particular, orientational frustration and cooperative rotation of pentacene molecules take place in the herringbone phase. Furthermore, successive STM scanning leads to structural interconversions between the porous network phase, herringbone phase, and brickwall phase, indicating the metastability of the 2D assembled structures of pentacene on Cd(0001). These structural transitions and interconversion can be attributed to the interplay between the repulsive electrostatic forces resulting from the charge transfer from the substrate to pentacene and the attractive effects originating from dipole-dipole interactions and intermolecular van der Waals forces.