The tunneling devices were prepared by mechanical exfoliation of graphite (NGS Naturgraphit) and hBN (hq graphene), dry stamp-transfer, and e-beam lithography (Zeiss EVO), as described in Supporting Fig. . Briefly, graphite was exfoliated to ca. 10-30 nm thickness onto a 300 nm SiO 2 /Si substrate. hBN was exfoliated onto poly(methyl methacrylate) PMMA substrate and then placed on the graphite using the stamp-transfer method. The resulting hBN/graphite heterostructure was spin-coated with an insulating PMMA layer (130 nm thick), in which two identical circular working electrode openings (typically 5 µm in diameter) over the basal planes of bare graphite and hBN were created using e-beam lithography (Fig. ). Electrical contact was made by scratch-exposing the graphite away from the openings and bonding it to a Cu wire using Ag paint. A Nikon Eclipse LV100ND optical microscope and a DS-Fi2 U3 CCD camera (Nikon Metrology, UK Ltd) were employed to acquire the optical images (Fig. ). The electrochemical cell was formed by dispensing a microdroplet of a liquid solution (typically 60-100 µm in diameter) around the opening (Fig. ). The number of hBN layers was determined by a Bruker Dimension 3100V atomic force microscope (AFM) in tapping mode (Fig. ). Raman spectra were collected using an inVia spectrometer (Renishaw) with a 532 nm laser excitation and X-ray photoelectron spectroscopy (XPS) was measured using a Thermo Scientific Al K-Alpha Theta Probe.