Impedance feedback control for scanning electrochemical microscopy.
A new constant-distance imaging method based on the relationship between tip impedance and tip-substrate separation has been developed for the scanning electrochemical microscope. The tip impedance is monitored by application of a high-frequency ac voltage bias between the tip and auxiliary electrode. The high-frequency ac current is easily separated from the dc-level faradaic electrochemistry with a simple RC filter, which allows impedance measurements during feedback or generation/collection experiments. By employing a piezo-based feedback controller, we are able to maintain the impedance at a constant value and, thus, maintain a constant tip-substrate separation. Application of the method to feedback and generation/collection experiments with tip electrodes as small as 2 microm is presented.