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) elec trodes have been investigated as a function of applied potential by means o f in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and sc anning tunneling microscopy (STM). We observed orientational and two-dimens ional phase transitions in the bpy adlayer, which facilitate the packing of the molecules more densely at more positive potentials. Bpy is adsorbed fl at below about - 0.4 V (SCE) and vertically at more positive potentials. Th e vertically oriented bpy has a cis conformation and coordinates to the sur face via two nitrogen atoms. Associated with the flat to vertical orientati onal transition, ordered domains in which molecules are stacked into rows l ike 'rolls of coins' are formed. The ordered domains grow in size as the po tential increases and cover the whole surface at about 0 V. At potentials b etween - 0.4 and 0.2 V, the molecules in the stacking rows are rotated, by either about 55 degrees or about - 75 degrees from the row direction to for m a zigzag structure with a (3 x 4 root 3) symmetry. When the electrode pot ential is scanned towards more positive values, the (3 x 4 root 3) phase is transformed to the commensurate (4 x 2 root 3) phase reported previously i n the literature through a disordered phase. (C) 2000 Elsevier Science S.A. All rights reserved.