Combination of an electrochemical tunneling microscope (ECSTM) and a scanning electrochemical microscope (SECM): application for tip-induced modification of self-assembled monolayers

Abstract

A combined instrument for electrochemical scanning tunneling microscopy (ECSTM) and scanning electrochemical microscopy (SECM) was constructed. It brings a combined ECSTM–SECM probe into tunneling contact with a sample and retracts it by defined distances to perform electrochemical experiments outside the tunneling distance but within a separation of 5–30 nm from the sample. The capabilities have been illustrated by the investigation of self-assembled monolayers (SAM) of dodecanethiolate on flame-annealed gold. By establishing a tunneling contact between the tip and the SAM-covered gold surface the structure of the SAM is disturbed. This is evident from current–distance curves in the SECM feedback mode. Depending on the time allowed for relaxation, the signal changes from positive feedback to negative feedback as it would be expected above undisturbed SAM. Spatially correlated images of topography and reactivity could be recorded by acquiring an ECSTM line scan first and than using the topographic information to guide the probe parallel to the surface in a much larger distance after changing the probe potential to cause the diffusion-controlled oxidation of a mediator. This mode was used to image rough gold electrodes where an almost complete suppression of the topographic information in the SECM data could be achieved. A sample with an inhomogeneous distribution of electrochemical reactivity was created by tip-induced removal of two 100×100 nm 2 large regions of a dodecanethiolate SAM on a rough gold surface. The two areas with a midpoint–midpoint distance of 850 nm could clearly be resolved by SECM imaging in the feedback mode.