Electron tunneling across an interfacial water layer inside an STM junction: tunneling distance, barrier height and water polarization effect

We have studied electron tunneling characteristics across an aqueous STM capacitor junction by measuring the tunneling current and local barrier height as functions of junction distance and applied bias voltage for HOPG, Au and TaTe2 surfaces in air. The electron tunneling distance ranges from 7-20 Angstrom when the junction resistance is of the order of 10(8) Ohm. We suggest that the unusually low value of the barrier height (< 1.5 eV) originates from the three-dimensional nature of the electron tunneling through the interfacial water layer present inside the junction. An asymmetric variation in barrier height is observed with respect to the applied bias voltage at a junction distance of water monolayer thickness, which reflects the polarization of the water layer on the surface.