On the Mechanism of Scanning Electrochemical Potential Microscopy

Scanning electrochemical potential microscopy (SECPM) is a type of probe microscopy, in which a sharp tip similar to those employed in electrochemical tunnelling microscopy is connected to a high impedance amplifier, but the tip potential instead of tip current is used as the signal in the feedback loop. SECPM has been found to provide much higher spatial resolution than would be expected on the basis of a mechanism in which the tip responds to the local electrochemical potential of the solution; in fact, it can obtain atomic resolution similar to STM, but is a superior technique for imaging electronically insulating objects such as proteins on a metal surface. We suggest a mechanism for these high-resolution images based on electron exchange between tip and substrate coupled to faradaic processes at the tip/solution interface. This mechanism operates alongside the conventional mechanism in which the tip responds to the local potential in the diffuse layer of the substrate and allows a simple description of the sigmoidal tip potential−distance curves that have been reported.