On the topography multiplicity of non-planar titanyl (IV) phthalocyanine molecules and the STM imaging mechanism

Titanyl phthalocyanine (TiOPc) molecules exhibit topography multiplicity in their two-dimensional self-assembled adlayers on highly oriented pyrolytic graphite (HOPG) surfaces and Au(1 1 1) substrates when imaged by scanning tunneling microscopy (STM). TiOPc molecules show various shapes while the packing arrangements preserve hexagonal symmetries in the STM images, which is unique in comparison with other metallophthalocyanines that mostly appear as fourfold shapes. Furthermore, the individual TiOPc molecules isolated by 1,3,5-tris(10-carboxydecyloxy)benzene (TCDB) cavities appear characteristic fourfold shaped features. The origin of the topography multiplicity in homogenous TiOPc adlayers with hexagonal symmetries is suggested to be associated with three possible reasons: the immediate environment of the packing structure, the distortion of the adsorption configuration or deformation of the adsorbed macrocyclic molecules, and the inverse imaging effect due to titanyl groups acting as the effective tip structures.