Surface Charge Induced Order-Disorder Transition in an Organic Monolayer

Abstract

Monolayer $2-2^{\prime }$ 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 $\sim 3.3$ Å. The chains are randomly oriented at low charges, but they begin to align in parallel as the charge density reaches $\sim 13\mu \mathrm{C}/{\mathrm{cm}}^2$. 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.