Extended Data Fig. 2. SECCM measurements.
a, Their basic principle. A nanopipette with two reservoirs filled with 0.1 M HCl and fitted with identical quasi-reference electrodes (QRCEs) is used to inject protons through 2D crystals. A voltage applied between the QRCEs produces an ion current Idc that is used as a feedback signal. Another potential (Eapp) applied to one of the QRCEs determines the overall potential used to inject protons. Dashed arrows illustrate the scanning protocol: the probe approaches the crystal (red) and makes contact over the area marked by blue circle, it is then retracted (black arrows) and moved to another position (green). b, Steady state SECCM currents collected from various settings: green, without any crystal covering Nafion; blue, monolayer hBN on top of Nafion; black, monolayer graphene on Nafion; and red, the same graphene monolayer on SiNx away from apertures. This panel demonstrates that our experimental setup can probe proton conductivity of 2D materials within a 4-decade current range. c, Example of distance changes between the nanopipette and the sample during a typical scan obtained above proton-conducting (blue) and proton-blocking areas (red). d, Idc - t characteristics measured simultaneously show that Idc accurately detects the moments when meniscus wetting and dewetting occur. e, Corresponding Icollector-t characteristics (same colour coding). Dotted lines in panels c–e mark the moments when the probe wets and retracts from the surface.