Fig. 1: Characterization of mechanically exfoliated monolayer hexagonal boron nitride (h-BN).

a, b and d, e Conductive atomic force microscopy (CAFM) topography maps and current maps collected without applying bias on a monolayer mechanically exfoliated h-BN flake transferred on a 5 nm Ru/30 nm Ta/300 nm SiO₂/Si substrate. The Ru film has been connected to the metallic sample holder using Ag ink to allow electrical measurements. d and e Are the zoom-in images marked as the white squares in (a and b). c Thickness analysis of the exfoliated h-BN flake in (d). The solid blue and red curves in c represent the Gaussian fittings for the original curves (in the colour of green), while the dashed blue and red lines mark the peak locations of each Gaussian distribution curve. f I–V curves collected at 24 randomly selected locations of the surface of the mechanically exfoliated monolayer h-BN flake on Ru substrate, in (a). The value of onset potential (V_ON) for each I–V curve and the resistance (read right before current saturation) are also shown, where the V_ON is defined as the minimum voltage detected when the current just exceeds the noise level (~3 pA). g and h are the high-resolution current maps inside the mechanically exfoliated monolayer h-BN flake collected with biases of 0 and 0.8 V, respectively. i Evolution of the resistance of 4 current spots in 4 consecutive current maps (see Supplementary Fig. 7). For each spot, the left, middle-left, middle-right and right columns indicate the resistance detected in the 1^st, 2^nd, 3^rd and 4^th scan, respectively.