Topography Multiplicity of Titanyl Phthalocyanine on Ultrathin Insulating Films Observed by STM

Individual titanyl phthalocyanine (TiOPc) molecules on ultrathin sodium chloride striped films grown on Cu(110) exhibit two different topographies with 8-lobes and 6-lobes when imaged by scanning tunneling microscopy (STM). Direct images of the molecular orbitals of the molecules with 8-lobes are obtained, indicating that the electronic structure of the TiOPc molecule are decoupled from the metallic substrate. For the TiOPc molecule with 6-lobes, the STM images at negative and positive bias polarities show the same structures as 2-fold symmetry except for the 90° rotation with respect to each other. This phenomenon may be attributed to the splitting of the two former degenerate lowest unoccupied molecular orbitals due to the negative charging of the molecule. The identification of the molecular orbital splitting on the ultrathin insulating layer could deepen the understanding of the intrinsic properties of semi-conducting molecules.