A scanning tunneling microscopy and spectroscopy study of vanadyl phthalocyanine on Au(111): The effect of oxygen binding and orbital mediated tunneling on the apparent corrugation

Submolecular resolution STM images of vanadyl phthalocyanine (VOPc) are reported. Unlike previously studied metal phthalocyanines, the image has neither a simple dark or bright spot in the center. Rather, it appears to have nodal planes running through the center of the molecule. The dark central region is attributed to the oxygen atom reducing the number of available states near the Fermi energy. The apparent height of the Pc ring is very bias dependent having a value of similar to 0.1 nm between -1.5 and 0.2 V (sample) bias. The apparent height increases rapidly to a plateau of about 0.55 nm at +1.0 V sample bias. I - V and dI/dV data taken over VOPc islands are very different from the same measurements made on exposed Au(lll) substrate adjacent to the islands. Strong peaks in dI/dV are seen at +1.38 and -1.8 V sample bias. The +1.38 V feature is associated with orbital mediated tunneling via the pi* LUMO of the Pc ring. Occasionally, the tip-surface interaction generates anomalous dark areas in the VOPc islands.