Figure 1: IDM imaging of Rb⁺ ions adsorbed at the surface of mica in an aqueous solution.

The solution contains 3 mM RbCl. Individual ions can be distinguished both in topography and in phase (a, arrow), often forming geometrical domains or rows. Occasionally, a bright ‘tail’ is visible adjacent to the dark spots attributed to adsorbed ions. This tail is induced by a momentary increase of the tip vibration amplitude as the feedback loop completes its correction, and is due to the particularly gentle imaging conditions. The atomic structure of mica evidences the position of the ion binding sites, which are occupied by K⁺ ions in the bulk crystal (b). Repeatedly scanning over a same binding site (c) yields substantial height variations in time as single Rb⁺ ions adsorb and desorb from the site (see also Supplementary Fig. 3). Scale bar, 3 nm (a), and the colour bars represent 1 nm (topography) and 15° (phase).