Large piezoelectric response in ferroelectric/multiferroelectric metal oxyhalide MOX 2 (M = Ti, V and X = F, Cl and Br) monolayers.

Flexible two-dimensional (2D) piezoelectric materials are promising for applications in wearable electromechanical nano-devices such as sensors, energy harvesters, and actuators. A large piezo-response is required for any practical applications. Based on first-principles calculations, we report that ferroelectric TiOX 2 and multiferroelectric VOX 2 (X = F, Cl, and Br) monolayers exhibit large in-plane stress ( e 11 ) and strain ( d 11 ) piezoelectric coefficients. For example, the in-plane piezo-response of TiOBr 2 (both e 11 = 28.793 × 10 -10 C m -1 and d 11 = 37.758 pm V -1 ) is about an order of magnitude larger than that of the widely studied 1H-MoS 2 monolayer, and also quite comparable to the giant piezoelectricity of group-IV monochalcogenide monolayers, e.g. , SnS. Moreover, the d 11 of MOX 2 monolayers - ranging from 29.028 pm V -1 to 37.758 pm V -1 - are significantly higher than the d 11 or d 33 of commonly used 3D piezoelectrics such as w-AlN ( d 33 = 5.1 pm V -1 ) and α-quartz ( d 11 = 2.3 pm V -1 ). Such a large d 11 of MOX 2 monolayers originates from low in-plane elastic constants with large e 11 due to large Born effective charges ( Z ij ) and atomic sensitivity to an applied strain. Moreover, we show the possibility of opening a new way of controlling piezoelectricity by applying a magnetic field.