SURFACE-CHARGE INDUCED ORDER-DISORDER TRANSITION IN AN ORGANIC MONOLAYER

Monolayer 2-2' bipyridine on Au(111) has been studied by scanning tunneling microscopy as a function of surface charge density. At positive charges, the planar molecules stand vertically on the surface and form polymeric chains in which they stack with a repeat distance of similar to 3.3 Angstrom. The chains are randomly oriented at low charges, but they begin to align in parallel as the charge density reaches similar to 13 mu C/cm(2). The transition is fully reversible by lowering the charge below the critical density. The real-space images reveal the molecular-scale processes during the transition.