Single lines down to a full width half maximum (FWHM) of ˜50 nm and as close as 125 nm are obtained with aspect ratios above 0.5 (line profiles in Figs. and). It should be noted that the extent of the deposition profile is in the order of the expected tip radius, which is typically regarded as the lower limit on resolution in most SPM-based nanofabrication . The clear Z-contrast of the deposited structure with the gold substrate as obtained from backscattered electrons in a scanning electron microscope (SEM) highlights the chemical nature of the deposited copper wire, as is also found by energy dispersive X-ray analysis from a larger structure (supplementary information SI-1). To enable the local writing however, we find two counter-intuitive results that disagree with previously reported micro-, and nano-SPM based electrochemical deposition mechanisms. First, we find that the local deposition only occurs when reducing the salt concentration down to a few µM. The panels in Fig. show the topography after the tip is translated over a 12 μm long path with the PF feedback enabled and ramping the voltage ( 1 < V < 2.7 V ) along the path. The figure clearly shows a topography emerging with increasing potential in the case with lowest ion concentration, which gradually disappears by increasing concentration. In fact, at concentrations above 10 µM, we are unable to observe any growth within our typical AFM scan range (˜15 μm) and parameters (supplementary information SI-2). While increasing the concentration of CuSO 4 should accelerate growth, mainly through facilitating mass transport, it is not in line with our observations. This is therefore a critical result, and rules out other known writing mechanisms, such as AFM-induced local depassivation of a metal substrate or electric-field enhanced mass transport .