Structural transition of pentacene monolayer on Cd(0001)

Pentacene is a typical organic semiconductor molecule, which has important application value in organic electronics. In this paper, the two-dimensional self-assembled structures of pentacene on the Cd(0001) surface were investigated by means of organic molecular beam deposition (OMBD) and low-temperature scanning tunneling microscopy (LT-STM). Three structural transitions induced by temperature were found. (1) Pentacene molecules form a disordered molecular monolayer under the room-temperature deposition conditions. (2) For the disorder monolayer, annealing the sample at 320 K leads to the transition from disordered phase to a well ordered monolayer, in which two kinds of molecular chains with different molecular orientations are alternately arranged, constituting the herringbone structure. (3) When the annealing temperature is increased to 350 K, the herringbone packing transforms into a unique chiral clover structure that has never been reported: three same pentacene dimers arrange into a chiral pinwheel structure. (4) Further increasing the annealing temperature to 400 K, the chiral clover structure transforms into a common and ordered brick-wall structure. With the combination of density functional theory (DFT) calculation, it is found that the change of charge density at both ends of individual pentacene molecule is opposite, which indicates that the each pentacene molecule has the net dipole moment. It is particularly interesting that the organizational chirality of chiral clover structure can be switched from one enantiomer to another driven by the electric filed from a STM tip.