Figure 4.

Fabrication and characterization of WS₂ nanopore devices. (a) Schematic of the nanopore device setup. The WS₂ flake (green) is suspended on a 200–500 nm FIB hole in a 50 nm thick and 50 µm × 50 µm dimensions SiN_x (deep blue) window, fabricated on a SiO₂/Si (orange/yellow) wafer. A 1 or 3 M KCl solution (light blue) is separated by the nanopore in the WS₂ flake. A bias voltage (V_B) is applied across the nanopore using Ag/AgCl electrodes (black), and the ionic current through the nanopore (I_B) is measured. (b) Optical image of a triangular WS₂ flake on a SiN_x window with a FIB hole. (Inset) Scanning electron microscopy image of a 300 nm FIB hole. (c) AC-HRSTEM image of (i) an undamaged suspended WS₂ membrane, (ii) a 0.3 nm nanopore, and (iii) a 1.3 nm nanopore drilled with accelerating voltage of 80 kV. (d) Plot of open pore conductance of WS₂ nanopores with the corresponding nanopore diameter. Inset shows the plot over a larger d_TEM range. The solution conductivity was calculated by fitting the equation for 2D nanopores (G₀ = σd_TEM) and found to be 13.5 ± 0.3 S/m. The nanopore diameter was calculated from the corresponding STEM image by defining a threshold for pixel intensities and selecting the pixels within that range in the vicinity of the pore in ImageJ software. This was repeated multiple times for a single pore, and the standard deviation was taken as the measurement error. Standard deviations in the slope of I_B vs V_B plots for each nanopore were used as the error in open pore conductance, which are very small and lie within each data point area.