A surface charge induced order-disorder phase transition in organic monolayers

Adenine and 2-2\u27 bipyridine monolayer films adsorbed on Au (111) have been studied by Scanning Tunneling Microscopy (STM) in solution as a function of surface charge and bulk concentration. Both molecules form polymer chains. The length of the polymer chains increases as the concentration increases. At low charges they are randomly oriented, but they begin to align in parallel as the charge density reaches a critical value. The transition, which resembles the isotropic-nematic phase transition encountered in liquid-crystals, is fully reversible by lowering the charge below the critical value