Orientation transition of pentacene molecule on Bi(111)

The pentacene (Pn) thin films of different coverages were prepared on semimetal Bi(111) substrate at 100 K or room temperature(RT). With the coverage increasing gradually, isolated Pn molecules, 1D bands, 2D self-assembled islands and crystalline monolayer were observed on the Bi(111) substrate, respectively. Flat-lying, side-lying and standing-up orientations of Pn molecules have been identified with a low temperature scanning tunneling microscopy (LT-STM). The molecular orientation exhibits flat-lying orientation for isolated molecules and 2D islands while side-lying orientation for 1D bands, in which the lattice constant is according with the b-c plane in crystal. When the coverage reaches to 1 ML, crystalline monolayer is formed by standing-up Pn molecules, and its lattice constant is very similar to the a-b plane in crystal. More importantly, a molecular orientation transition from flat-lying to side-lying in sub-monolayer and finally to standing-up in crystalline monolayer was observed by high resolved STM images. The transition can be attributed to the competition between molecule-molecule interaction and molecule-substrate interaction as well as shallow energy minima for structural rearrangements as coverage increasing gradually.