Orientation transition of pentacene molecule on Bi(111)

Abstract

Abstract 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.