Molecular rectifying behaviors of a planar binuclear phthalocyanine studied by scanning tunneling microscopy

A casting film of a kind of binuclear cobalt phthalocyanine-sulphonate (CoPc-CoPc) deposited on highly oriented pyrolytic graphite (HOPG) substrate was studied by scanning tunneling microscopy (STM) using a Pt/Ir nanotip in air at 22 °C. Molecules CoPc-CoPc crystallized on HOPG surface along one direction in a large scale. The asymmetrical I–V curves for the monolayer and the double layer reveal a rectifying behavior for a negative bias. The rectification mechanism can be explained by a modification of the Aviram–Ratner model. The electron transfer in the molecule can only along one direction and along another direction is disfavored. The electric resistance for the monolayer is smaller than that for the double layer, and thus the rectification of the monolayer is better than that of the double layer. The STM experiments have indicated that the asymmetry for I–V curves arises from the material asymmetry.