Graphene has been broadcasted as a promising choice of electrode and substrate for flexible electronics. To be truly useful in this regime, graphene has to prove its capability in ordering the growth of overlayers at an atomic scale, commonly known as epitaxy. Meanwhile, graphene as a diffusion barrier against atoms and ions has been shown in some metal-graphene-dielectric configurations for integrated circuits. Guided by these two points, this work explores a new direction of using graphene as a bifunctional material in an electrochemical metallization memory, where graphene is shown to (i) order the growth of a low-ionicity semiconductor ZnS single-crystalline film and (ii) regulate the ion migration in the resistive switching device made of Cu/ZnS/graphene/Cu structures. The ZnS film is confirmed to be van der Waals epitaxially grown on single-crystal graphene with X-ray structural analysis and Raman spectroscopy. Charge transport studies with controlled kinetic parameters reveal superior ion regulating characteristic of graphene in this ZnS-based resistive switching device. The demonstration of the first graphene-directed epitaxial wide band gap semiconductor resistive switching suggests a possible and promising route toward flexible memristors.
Keywords: ZnS; electrochemical metallization memories; graphene; ion migration; resistive switching; van der Waals epitaxy.