Extended Data Fig. 2: Filling nanochannels with water.
From: In-plane dielectric constant and conductivity of confined water
a, AFM topography image of a representative device with h ≈ 5 nm before filling it with water—same image as in Extended Data Fig. 1f but using a flattening filter to better visualize the sagging of the top hBN layer into nanochannels. The image was taken in the intermittent-contact attractive mode with no voltage applied. b,c, Same as a but with applying a low-f electric field (vAC = 8 V, 1 kHz) before (b) and after (c) filling the nanochannels with water. The contrast above the channels changes from light cyan in b to red in c, indicating successful filling of water. This contrast in c, which shows an apparent roughness change of several nanometres, arises from the large in-plane conductivity of water and not from actual topographical features. The top surface of the device becomes essentially atomically flat after water filling, with residual roughness <3 Å for all of our devices, as demonstrated in f and g (see also the ‘Device fabrication’ section in Methods). d, Zoom-in of the central region in a. The top hBN layer is partially sagged into the empty channels. e, Further zoom-in showing the region indicated by the dashed rectangle in d. f, Same as e but after filling nanochannels with water. It shows that the top hBN layer became flat, no longer sagging inside the nanochannels. g, Topography profiles over three channels in e and f, as indicated by colour-coded lines. h, Dielectric image taken from the same region as in f at a constant height and at 1 GHz. The plotted values of |dC/dz| are variations after subtracting the |dC/dz| values found at the centre of the hBN spacer region. Scale bars: 3 μm (a–c); 2 μm (d), 1 μm (e,f,h).
