Adsorption and ordered phase formation of 2,2′-bypyridine on Au(111): a combined surface-enhanced infrared and STM study

The structure and bonding of 2,2′-bipyridine (bpy) adsorbed on Au(111) electrodes have been investigated as a function of applied potential by means of in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and scanning tunneling microscopy (STM). We observed orientational and two-dimensional phase transitions in the bpy adlayer, which facilitate the packing of the molecules more densely at more positive potentials. Bpy is adsorbed flat below about −0.4 V (SCE) and vertically at more positive potentials. The vertically oriented bpy has a cis conformation and coordinates to the surface via two nitrogen atoms. Associated with the flat to vertical orientational transition, ordered domains in which molecules are stacked into rows like ‘rolls of coins’ are formed. The ordered domains grow in size as the potential increases and cover the whole surface at about 0 V. At potentials between −0.4 and 0.2 V, the molecules in the stacking rows are rotated, by either about 55° or about −75° from the row direction to form a zigzag structure with a (3×4√3) symmetry. When the electrode potential is scanned towards more positive values, the (3×4√3) phase is transformed to the commensurate (4×2√3) phase reported previously in the literature through a disordered phase.