Influence of H₂O molecules on sub-nanometre scale gaps between Au leads

Electric-field induced surface modification of Au is used to create stable sub-nanometre-scale gaps between Au leads. Electron tunnelling current is measured as a function of the molecular environment. We explore the influence of water molecules on tunnelling conductance. At small voltage, water molecules do not influence the conductance. At large voltage, a strong static electric field between the leads induces trapping of H₂O molecules from the water vapour. At room temperature, trapped water molecules increase the tunnelling barrier height, in agreement with the workfunction of water. We estimate that the resistance of a single trapped water molecule at 5 V is of order 100 GΩ.