A Bipolar Electrochemical Approach to Constructive Lithography: Metal/Monolayer Patterns via Consecutive Site-Defined Oxidation and Reduction

Abstract

Experimental evidence is presented, demonstrating the feasibility of a surface-patterning strategy that allows stepwise electrochemical generation and subsequent in situ metallization of patterns of carboxylic acid functions on the outer surfaces of highly ordered OTS monolayers assembled on silicon or on a flexible polymeric substrate. The patterning process can be implemented serially with scanning probes, which is shown to allow nanoscale patterning, or in a parallel stamping configuration here demonstrated on micrometric length scales with granular metal film stamps sandwiched between two monolayer-coated substrates. The metal film, consisting of silver deposited by evaporation through a patterned contact mask on the surface of one of the organic monolayers, functions as both a cathode in the printing of the monolayer patterns and an anodic source of metal in their subsequent metallization. An ultrathin water layer adsorbed on the metal grains by capillary condensation from a humid atmosphere plays the double role of electrolyte and a source of oxidizing species in the pattern printing process. It is shown that control over both the direction of pattern printing and metal transfer to one of the two monolayer surfaces can be accomplished by simple switching of the polarity of the applied voltage bias. Thus, the patterned metal film functions as a consumable "floating" stamp capable of two-way (forward-backward) electrochemical transfer of both information and matter between the contacting monolayer surfaces involved in the process. This rather unusual electrochemical behavior, resembling the electrochemical switching in nanoionic devices based on the transport of ions in solid ionic-electronic conductors, is derived from the nanoscale thickness of the water layer acting as an electrolyte and the bipolar (cathodic-anodic) nature of the water-coated metal grains in the metal film. The floating stamp concept introduced in this report paves the way to a series of unprecedented capabilities in surface patterning, which are particularly relevant to nanofabrication by chemical means and the engineering of a new class of molecular nanoionic systems.