Scanning Tunneling Microscopy Fractal Characterization of Poly (o-toluidine) Films Produced Electrochemically on Polyfaceted Gold Single Crystal Spheres

Abstract

The topography evolution of a poly(o-toluidine) layer (the object) electrodeposited on a polyfaceted Au single crystal from 0.5 M H₂SO₄ + 0.65 M o-toluidine aqueous solution at 25 °C has been studied combining nanometer scale ex-situ scanning tunneling microscopy and electrochemical techniques. A thin polymer layer structure consists of nodular elements following the substrate topography, whereas a thick polymer structure corresponds to a rather disordered structure made up of agglomerates formed by nodular elements with average diameter size d_s ≅ 15 nm. In contrast to the former one, this structure is independent of the substrate topography. For thick layers ξ, the root-mean-square roughness of the polymer coating, and L_s, the scan length, fulfill a ξ ∝ L^α relationship with a = 0.33 ± 0.05 for L_s > d_s and α = 0.85 ± 0.05 for L_s < d_s- Accordingly, the polymer layer surface behaves as a self-affine fractal. For L_s < d_s the value of a agrees with that predicted for moving interfaces generated by the Eden model, whereas for L_s < d_s the value of a approaches that expected from an Euclidean surface. The topographic analysis of these polymer layers can be related to the kinetics of electrochemical reactions at polymer-coated Au electrodes.