Study On The Growth Mechanism Of Copper Phthalocyanine On Semimetallic Bi(111) Surface

In the past decades,the investigation of the growth mechanism of organic thin films deposited on various substrate surfaces has already caused wide attention.The organic molecular self-assembly film materials,such as phthalocyanine compounds,in gas sensors,organic light-emitting devices,organic field-effect transistors and solar cells has broad application prospect.Due to their high thermal stability of the self-structure and the electronic properties of the phthalocyanine molecules,it is conducive to the further development of the new technology.Low temperature scanning tunneling microscopy(LT-STM)is considered to be a very beneficial characterization technique in the research field of nanometer science and the technology.It can be invariably used to characterize the topographic image of nanostructures at the atomic scale,at the same time,it can deeply explore the information of electronic structure combined with scanning tunneling spectrum(STS)function.Therefore,accompanied by the function of the STM increasingly gradual improvement,it has become widely applied in the detection of solid surface information.This experiment using the LT-STM to measure the self-assembled structures and the chiral features of the copper phthalocyanine(CuPc)molecules deposited on the surface of Bi(111).Firstly,under the very low coverage,we found that the individual CuPc molecules adsorbed on Bi(111)surface keep rotating under liquid nitrogen temperature(78K)until they interacted with other CuPc molecules to form clueters,owing to the relatively weak interfacial interactions between the CuPc molecules and the Bi(111).As the molecular coverage continues to increase,the molecular chain,clusters,can be observed in succession,eventually formed the self-assembled two-dimensional(2-D)molecular film.We observed that the CuPc molecules adopt a flat-lying adsorption orientation without any rotation on the substrate,and each CuPc molecules exhibits a cross shape with four perpendicular lobes consistented with the molecular model.The self-assembled CuPc thin films deposited on Bi(111)surface having identical in-plane orientations,which can be attributed to the mutual coupling of the symmetry of Bi(111)and CuPc.In particular,when CuPc coverage reached a certain degree,high resolution STM topography shows that the molecules are self-assembled into 2-D domains with each molecule exhibiting chiral features.This phenomenon is thought to be the combined effect of asymmetric charge transfer between the CuPc molecules and the Bi(111)surface and the formation of asymmetric intermolecular van der Waals interactions.Beyond one monolayer(1ML),the CuPc molecules adopt an orientational transition from a flat-lying to a standing-up adsorption gesture.A study of the orientational transformation in films of CuPc molecules on Bi(111)demonstrates that the CuPc-Bi(111)interaction can be ignored owing to the decoupling of the first CuPc layer with the Bi(111)surface.In summary,as coverage increases,CuPc molecules show a structural evolution and molecular adsorption orientational transformation in films on Bi(111)substrate.