The probe in SECPM is a sharp metal needle. Detector signal is electric potential difference between tip and sample (or between tip and a reference electrode) in an ionic or polar liquid, where an electric double-layer exists at the liquid/sample interface. SECPM maps the electric potential profile across the depth of the double-layer (versus Z, the tip-sample distance) at tip XY location. In feedback mode, the output signal usually adjusts the Z position of the scanner.