Rapid dry exfoliation method for tuneable production of molybdenum disulphide quantum dots and large micron-dimension sheets

Abstract

Despite advances in two-dimensional (2D) transition metal dichalcogenide research owing to their outstanding physical properties, the synthesis of large micron-dimension single-layer sheets of these materials remains a challenge. Here, we present a novel and unique method to rapidly and flexibly exfoliate bulk molybdenum disulphide (MoS₂) into either small nanometre-dimension quantum dots (QDs) or large micron-dimension sheets comprising predominantly single- or few-layers. The exfoliation process is conducted in dry phase, i.e., without liquid, by exploiting nanometer-amplitude MHz-order surface vibrations in the form of surface acoustic waves (SAWs). To produce the small QDs, we take advantage of the unprecedentedly large surface acceleration—on the order of 10⁸ m s⁻²—to induce an iterative impaction mechanism involving successive ejection and collision of the bulk MoS₂ aggregates within a miniature enclosure in order to progressively thin and break their lateral dimensions into single- and few-layer QDs. In contrast, we suppress the impaction in the zero-height enclosure limit by confining the bulk MoS₂ under adhesive tape such that the shear that arises due to the travelling SAW on the substrate progressively thins the material whilst preserving their lateral dimension such that large, predominantly-monolayer, micron-sized sheets are produced with high substrate coverage up to around 80%. This fast, additive-free and dry exfoliation platform potentially presents a simple yet scalable micromechanical exfoliation method towards viable commercial production of 2D transition metal dichalcogenides.