It was observed that in alternating current scanning electrochemical microscopy (AC-SECM) the lateral contrast strongly depends on the frequency of the applied AC voltage [1]. Conditions of optimum imaging can be found by generating images for a set of frequencies and choosing the best image. It would be however more elegant to anticipate optimum measurement conditions prior to scanning. The analytical signal in AC-SECM can either be the current magnitude and the phase shift or the real and imaginary part of AC-current. They can be readily transformed into the corresponding impedance values. Each of these parameters reflects the local surface properties and can therefore be used for imaging. As in any kind of scanning probe microscopy the signal is dependent on the tip-to-sample distance. This relationship is represented by an approach curve, which is characteristic for the electrochemical activity of the surface. For this reason a comparison of approach curves to electrochemically different surfaces has to contain information on the optimum imaging contrast. This contrast is a function of the applied perturbation frequency. In this contribution a method is proposed to determine the frequency of interest based on the analysis of approach curves obtained for different frequencies. The optimum frequency may be different for each kind of analytical signal. Calculations allow to obtain a measure of lateral contrast for each analytical signal. Therefore optimum imaging conditions can be predicted prior to the measurement.

[1] K. Eckhard, C. Kranz, H. Shin, B. Mizaikoff, W. Schuhmann, Anal. Chem. 79 (2007) 5435-5438. Frequency dependence of the electrochemical activity contrast in AC-SECM and AFM-AC-SECM imaging

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