Scanning Tunneling Microscopy of Metal Phthalocyanines: d7 and d9 Cases

Abstract

Scanning tunneling microscopy (STM) images of cobalt(II) phthalocyanine (CoPc), copper(II) phthalocyanine (CuPc), and mixtures of the two adsorbed on the Au(111) face are reported. Based upon the stability and ease of obtaining molecular images, CoPc appears to adsorb more strongly than CuPc on Au(111), but both species provide images showing submolecular structure. The mixed CoPc and CuPc films also provide high-quality images showing details of the internal structure of the metal phthalocyanine. A particularly exciting aspect of this work is the strong influence of the metal ion valence configuration on the observed tunneling images. Unlike CuPc, wherein the central metal appears as a hole in the molecular image, the cobalt atom in CoPc is the highest point (about 0.3 nm) in the molecular image. These data are interpreted as indicating that the Co(II) d7 system has significant d-orbital character near the Fermi energy while the Cu(II) d9 system does not. This interpretation is consistent with theoretical calculations that predict a large contribution of cobalt d-orbitals near the Fermi energy, and with inelastic electron tunneling spectra that show d-orbital-related bands within 1 eV of the Fermi energy. An intriguing aspect of this work is that it may be possible to chemically identify the different metal phthalocyanines simply by their appearance. This can be used to advantage in the study of surface diffusion, 2-d sublimation, and the surface thermodynamics and kinetics of adlayer formation.