Surface Charge Induced Order-Disorder Transition in an Organic Monolayer

Abstract

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