Bilayer-by-bilayer growth of vanadyl phthalocyanine molecules on Bi(111) surface

Abstract

The self-assembly of vanadyl phthalocyanine (VOPc) molecules grown on Bi(111) surfaces has been studied using low-temperature scanning tunneling microscopy (STM). It is found that the isolated VOPc molecules adsorbed on Bi(111) rotate around the center stemming from the weak interaction between the molecules and Bi(111) surface. When VOPc molecules assemble into 2D domains, they adopt the bilayer-by-bilayer mode from the beginning of growth. In VOPc ultrathin films, there are almost no domains with one molecular layer (ML) or 3 ML, only domains with 2 ML and 4 ML. Furthermore, the VOPc bilayer possesses a nested flat-laying structure, where the molecules have the orientation O-up in the lower layer and O-down in the upper layer. This results in a strong interlayer coupling within the nested bilayer and a relatively weak coupling between the bilayers, leading to bilayer-by-bilayer growth.