Direct writing of copper nanostructures We control the local growth of copper on gold by applying a potential difference directly between a Au substrate and an AFM tip in a dilute aqueous electrolyte (1 µM CuSO 4 ). The tip apex, which is made of Pt, is the only part of the cantilever that is exposed to the liquid. During writing, the AFM is operated in peakforce (PF) tapping mode , where the cantilever is driven by a sinusoidal signal far below its resonance frequency (2 kHz in this work) (Fig. , and methods). In this mode, the force exerted by the tip is used as the feedback signal (3 to 30 nN in this work), while the intermittent contact results in minimal lateral forces on the substrate. By moving the tip along a programmed xy trajectory, arbitrary shapes can be written ( Figs. and).