Tunneling Conductivity Switching by Reversible Electric-Field-Induced Proton Transfer for a Hydrogen-Bonding Heterobilayer Film

Heteromolecular interactions are responsible for a variety of functions. We report the first observation of reversible tunneling conductivity switching induced by external electric field (EEF) for a proton (H⁺) donor/acceptor bilayer film on Au substrates under ambient conditions at room temperature. The heterobilayer film was self-assembled through two-step immersion: the H⁺-donor, the H⁺-electron-correlated molecule (catechol-fused bis(propylthio)tetrathiafulvalene), was hydrogen-bonded to the H⁺-accepting imidazole-terminated undecanethiolate self-assembled monolayer on Au(111). The bilayer film topographies, molecular adsorption states, and physical properties were characterized by using spectroscopic and microscopic methods. In particular, scanning tunneling microscopy and spectroscopy measurements revealed reversible changes in the tunneling conductivity of the bilayer film depending on EEF stimulation. This is attributed to reversible EEF-induced H⁺-transfer in the bilayer, based on deuteration of H⁺, vibrational spectrum changes, and theoretical model calculations. Notably, the reversible response exhibited hysteresis, indicating that the bilayer film could function as a molecular memory driven by the H⁺-switch.