Out-of-plane electromechanical coupling in transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) are intrinsically piezoelectric within the plane of their atoms, but out-of-plane piezoelectric response should not occur due to the symmetry of the crystal structure. Recently, however, MoS2 was shown to exhibit out-of-plane electromechanical coupling consistent with the flexoelectric effect. In this study, MoSe2, WS2, and WSe2 are investigated to determine the existence and strength of out-of-plane electromechanical coupling in other monolayer TMD semiconductor materials. Piezoresponse force microscopy measurements show that monolayer MoS2, MoSe2, WS2, and WSe2 all exhibit out-of-plane electromechanical response. The relative magnitudes of their out-of-plane electromechanical couplings are calculated and compared with one another and to predictions made from a simple model of flexoelectricity. This simple model correctly predicts the magnitude of out-of-plane electromechanical response in these materials, and the measured values provide useful guidance for both more detailed understanding of flexoelectric response in monolayer TMDs, and assessment of their consequences in devices incorporating these materials.