Temperature-dependent structures of heteroepitaxial metallo-phthalocyanine (Mpc, M = Cu, Fe) thin films on silver.
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Metallo-phthalocyanines (MPcs, M = Cu, Fe) thin films were formed on Ag(111) surface by vapor deposition in ultra high vacuum (UHV). The morphology and structure of the MPcs were studied by scanning tunneling microscope (STM). Submonolayer (sub-ML) MPcs deposited at room temperature (RT) without subsequent annealing did not form ordered film regions. After annealing the sub-ML films to 500 K for 30 minutes, ordered regions as well as individual molecules therein were observed. By successive annealing MPc, the symmetry of the overlayer increased from oblique to the square lattice. This is understood by a delicate balance between intermolecular van der Waals (vdW) interaction and molecule-substrate diffusion potential. Furthermore, the number of molecular vacancies within the ordered regions was reduced. Annealing the thin films well beyond the sublimation temperature, much of the CuPc molecules were desorbed. However, the remaining molecules reacted and formed dendrite-like chains, in which each CuPc molecule was linked through its isoindole rings. A possible model for the formation of dendritic chains through dehydrogenation and subsequent polymerization is discussed. Another system of FePc adsorbed on Ag(111) was also studied. The ordered region shows an oblique lattice of FePc. Within the unit cell, the molecules at the opposite corners align their isoindole rings such that the nearest and the next nearest neighbor distances are increased substantially compared to those in the close-packed, square lattice observed in CuPc/Ag(111). In addition to the unit cell, which is about 38 % larger than that of CuPc, the oblique lattice of FePc is commensurate to the substrate, and the molecular axes align along the high-symmetry azimuths of the substrate. The overlayer lattice structure and the distinct molecular orientation are attributed to the dxz,yz orbital interaction between the central Fe2+ and Ag(111).