Unusual Diffusion Behavior of Copper-Hexadecafluoro-Phthalocyanine Molecules on Au(111)

Abstract

The unusual bias-voltage dependence of copper-hexadecafluoro-phthalocyanine (F16CuPc) molecules on Au(111) was investigated by using scanning tunneling microscopy/spectroscopy (STM/STS) at low temperatures. Depending on the bias polarity, the molecules seem to cover the whole surface of Au(111) at positive bias voltages while they are apparently invisible in STM images at negative bias voltages. Here, we experimentally demonstrate that the molecular diffusivity on Au(111) varies according to the applied electric field induced between the STM tip and the Au(111) surface rather than the intrinsic molecule-substrate interaction. Such results are corroborated by STS measurements indicating that attraction/repulsion of a molecule indeed occurs under an STM tip at positive/negative bias voltages during sweeps of the bias voltage. In addition, we found that the diffusive molecules are immobilized at a temperature of 8 K at both positive and negative bias voltages due to a lack of thermal energy for diffusion.