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 ∼3.3 Å. The chains are randomly oriented at low charges, but they begin to align in parallel as the charge density reaches ∼13 μC/cm2. 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.