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

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.