Topography feedback mechanism for the scanning electrochemical microscope based on hydrodynamic forces between tip and sample

Abstract

The distance between microelectrode and sample is a crucial parameter in scanning electrochemical microscopy. Here we report on a technique where the hydrodynamic coupling between tip and sample is employed to measure and control this distance. We built an apparatus, where a microelectrode is vibrated laterally and the damping of the amplitude upon approach to the surface is measured optically. We show that amplitudes of up to 5 μm are easily obtained in an aqueous environment and that amplitudes below 1 nm can be detected with phase sensitive amplifiers. We measured the different vibration modes of the electrodes and identified the ones best suited for distance measurements. On hard surfaces we found the characteristic decay length of the coupling to be less than 1 μm and, within our measured range, independent of frequency and amplitude of the vibration. Measurements on an elastic surface of known spring constant revealed that the vibrating electrode exerts a repulsive normal force on the sample which may displace the surface and result in an increased apparent decay length of several μm. Under typical conditions this frequency- and amplitude-dependent force is in the order of several 100 μN. In parallel electrochemistry experiments we found that the reduction current at the microelectrode was only slightly increased by the vibration of the electrode and was a linear function of the amplitude.