Investigation of the Scanning Tunneling Microscopy Image, the Stacking Pattern and the Bias-voltage Dependent Structural Instability of 2,2'-Bipyridine Molecules Adsorbed on Au(111) in Terms of Electronic Structure Calculations

A self-assembled monolayer of 2,2'-bipyridine (22BPY) molecules on Au(111) underwent a structural phase transition when the polarity of a bias voltage was switched in scanning tunneling microscopy (STM) experiments. The nature of two bright spots representing each 22BPY molecule on Au(111) in the high-resolution STM images was identified by calculating the partial density plots for a monolayer of 22BPY molecules adsorbed on Au(111) using tight-binding electronic structure calculations. The stacking pattern of the chains of 22BPY molecules on Au(111) was explained by examining the intermolecular interactions between the 22BPY molecules based on first principles electronic structure calculations for a 22BPY dimer, (22BPY)2. The structural instability of the 22BPY molecule arrangement caused by a change in the bias voltage switch was investigated by estimating the adsorbate-surface interaction energy using a point-charge approximation for Au(111).