Rectifying behaviors of Langmuir–Blodgett films of an asymmetrically substituted phthalocyanine

A multilayer Langmuir–Blodgett (LB) film of an asymmetrically substituted phthalocyanine derivative (NtBuPc), deposited onto a HOPG substrate, was studied by scanning tunneling microscopy using a Pt/Ir nanotip in air at 23°C. The asymmetrical I–V curves for 3- and 7-layer LB films of NtBuPc reveal a rectifying behavior for a negative and a positive bias. This suggests that NtBuPc molecules cause the rectification and not the Schottky barrier formed at the electrode–LB interface. The rectification mechanism can be explained by a modification of the Aviram–Ratner model. The electron transfer between bilayers might be interpreted as a quantum well effect through interlayer tunneling. Such quantum tunneling between layers would accelerate charge transport between bilayers, and thus rectification of 7-layer films is better than that of three layers.