Large piezoelectric response in ferroelectric/multiferroelectric metal oxyhalide MOX (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 and multiferroelectric VOX (X = F, Cl, and Br) monolayers exhibit large in-plane stress () and strain () piezoelectric coefficients. For example, the in-plane piezo-response of TiOBr (both = 28.793 × 10 C m and = 37.758 pm V) is about an order of magnitude larger than that of the widely studied 1H-MoS monolayer, and also quite comparable to the giant piezoelectricity of group-IV monochalcogenide monolayers, , SnS. Moreover, the of MOX monolayers - ranging from 29.028 pm V to 37.758 pm V - are significantly higher than the or of commonly used 3D piezoelectrics such as w-AlN ( = 5.1 pm V) and α-quartz ( = 2.3 pm V). Such a large of MOX monolayers originates from low in-plane elastic constants with large due to large Born effective charges () 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.