Combined scanning electrochemical-atomic force microscopy (SECM-AFM): Simulation and experiment for flux-generation at un-insulated metal-coated probes

The use of metal-coated AFM probes as electrodes to quantitatively generate chemical species above a target interface is described. This SECM-AFM approach is of interest as it enhances conventional AFM, by providing a simple method of generating a reactant or perturbing the solution conditions adjacent to a surface electrochemically, while allowing the effect on the topography of the surface to be imaged. The case of reactant generation by simple electron transfer at the SECM-AFM probe is considered. This process is treated through finite element simulations and generation-collection experiments. The SECM-AFM electrode reaction is operated under diffusion-limited conditions and simulations focus on identifying the spatial resolution of the technique, by calculating the diffusion field of the electrogenerated species as a function of time following the potential step. Three types of surface are considered: (i) an infinite conductive surface; (ii) a surface containing a 10 mu m diameter ultramicroelectrode; and (iii) an inert surface. Significantly, it is shown that although the SECM-AFM probe is electroactive over a large distance (>= 100 mu m length dimension) of the cantilever, the effect of the perturbation on the underlying surface can be confined to the micron scale or smaller by employing short duration potential pulses. Thus, high spatial resolution can be achieved for either the reagent-generation or equilibrium perturbation modes, without the need for a small area electrode integrated into the AFM probe. The results of the simulations are examined experimentally through generation-collection studies under the conditions of case (ii). (c) 2005 Elsevier B.V. All rights reserved.