Nanoscale films covalently attached to conducting substrates: structure and dynamic behaviour of the layers
Nanoscale organic films, typically 1-5 nm thick, can be grafted to conducting substrates by the reduction of aryldiazonium salts. The grafting procedure results in covalent attachment of the film to the substrate, through a direct bond between the aryl group and a surface atom. The strength of this bond depends on the substrate: for carbon substrates, a high bond energy gives extremely stable layers. This paper describes our detailed electrochemical and atomic force microscopy studies of these films, which demonstrate that multilayer films do not incorporate close-packed layers of aryl groups. A loosely packed structure is consistent with the mechanism of the surface attachment process, which proceeds via the formation of aryl radicals. It is also consistent with the proposed route for multilayer formation which yields branching film growth. Our studies of the films have revealed that they can respond in an, at least partially, reversible manner to their environment, undergoing changes in thickness, barrier properties and wettability as they are exposed to different solvents and/or applied potentials. These switchable properties are proposed to originate in film swelling and shrinking which are possible because of the loosely packed multilayer structure.