Structural phase transition in self-assembled 1,10′ phenanthroline monolayer on Au(111)

The self-assembly of 1,10' phenanthroline (phen) on Au(111) from aqueous solutions has been studied as a function of the substrate potential with in situ scanning tunneling microscopy (STM). The phen molecules adsorb spontaneously onto the substrate with a preference to decorate the reconstruction stripes of Au(111). The adsorbed molecules stand vertically with their nitrogen atoms facing the Au(111) and stack, like rolls of coins, into polymer-like chains. At high potentials, the chains pack closely in parallel and form an ordered monolayer. Decreasing the potential to a critical value, the chains become randomly oriented via a reversible order-disorder phase transition that resembles the nematic-isotropic transition in liquid crystal materials. High resolution images reveal each phen molecule as two blobs located at the two nitrogen atoms, indicating that the coupling between the nitrogen atoms and Au(111) is responsible for the tunneling current probed by STM. The phen monolayer contains pits with a depth of about one Au layer, which may be attributed to surface stress induced by the strong adsorption of the phen molecules on the surface.