Hydrostatic strain-induced reversible phase transition in bilayer MoS₂

In current topics of van der Waals (vdW) homo- and hetero-bilayers, strain has been recognized as a new tuning tool to control the electronic structures of vdW bilayers. Both in-plane and out-of-plane strains can impact the electronic coupling between the vdW layers and can be used to tailor the electronic structure. We have used a diamond cell to apply hydrostatic pressure to tune the coupling of vdW bilayers and observe the dynamic response of the electronic and atomic structures using in-situ optical and Raman spectroscopies. Most interestingly, we find there exists a critical pressure above which the bilayer stacking configuration changes from a conventional H_MX-stacking into an H_MM stacking, with a sudden shift in in-plane alignment. Moreover, we find this is a reversal phase transition accompanied by a hysteresis loop analogous to a ferroelectric phase transition. First-principle calculations reproduce quantitatively the phase transition pressure. The accompanied hysteresis can be understood in terms of the kinetic barrier between the two configurations.