Direct investigation of the electrochemical double layer using the STM

A first investigation of the electrochemical double layer in the tunneling gap between a WSe2 semiconductor electrode and a Pt:Ir STM tip was performed by current-distance spectroscopy. The magnitude of the current between tip and semiconductor depends exponentially on the barrier height limiting this current. The local barrier height can be derived from the slope of a semi-logarithmic plot of the current versus the distance. For a fully developed Helmholtz double layer the Schottky barrier height within the semiconductor surface dominates the tunneling barrier. However, the charge of the semiconductor surface and thereby the Schottky barrier height also depends on the counter charges located on the tip and within the Helmholtz double layers in the electrolyte inside the gap. Changes in the distribution of the charges lead to a change of the Schottky barrier height. Thereby, the influence of the first and the second water layers can be detected as well as the penetration of positive and negative ions into the gap when the tip is removed from the surface. Thereby the space needed by specifically adsorbed as well as by solvated ions to fit into the gap and to built up the inner Helmholtz layer can directly be measured.