Largely enhanced out-of-plane electromechanical coupling effects in two-dimensional molybdenum-disulfide/boron-nitride heterostructures

The electromechanical coupling effects in two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted great interest. However, for 2D TMDs, piezoelectricity is confined to the basal plane, and the flexoelectricity-derived out-of-plane electromechanical response is usually faint, limiting the applications of this material family using the out-of-plane electromechanical effects. Here, this work reports a facile strategy to greatly enhance the out-of-plane electromechanical response of hexagonal molybdenum disulfide (2H-MoS₂) nanoflakes by stacking monolayer hexagonal boron nitride (h-BN) on 2H-MoS₂ nanoflakes to form MoS₂/BN heterostructures. The $d_{\text{33}}^{\text{eff}}$ coefficient of MoS₂/BN can reach a value comparable to that of commonly used wurtzite bulk piezoelectric materials, such as AlN and GaN. The strong out-of-plane electromechanical response of MoS₂/BN is due to the breaking of the out-of-plane structural symmetry. Kelvin probe force microscopy (KPFM) results show an increased effective work function of MoS₂/BN, indicating polar-structure formation at the heterostructure interface, which also accounts for the enhanced out-of-plane piezoresponse. This study gives an insight into the role of heterostructure engineering in the electromechanical performances of 2D TMDs, and provides this material family an opportunity for applications using out-of-plane electromechanical effects.