Large piezoelectric response in ferroelectric/multiferroelectric metal oxyhalide MOX₂ (M = Ti, V and X = F, Cl and...

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 TiOX₂ and multiferroelectric VOX₂ (X = F, Cl, and Br) monolayers exhibit large in-plane stress (e₁₁) and strain (d₁₁) piezoelectric coefficients. For example, the in-plane piezo-response of TiOBr₂ (both e₁₁ = 28.793 × 10^-10 C m^-1 and d₁₁ = 37.758 pm V^-1) is about an order of magnitude larger than that of the widely studied 1H-MoS₂ monolayer, and also quite comparable to the giant piezoelectricity of group-IV monochalcogenide monolayers, e.g., SnS. Moreover, the d₁₁ of MOX₂ monolayers - ranging from 29.028 pm V^-1 to 37.758 pm V^-1 - are significantly higher than the d₁₁ or d₃₃ of commonly used 3D piezoelectrics such as w-AlN (d₃₃ = 5.1 pm V^-1) and α-quartz (d₁₁ = 2.3 pm V^-1). Such a large d₁₁ of MOX₂ monolayers originates from low in-plane elastic constants with large e₁₁ due to large Born effective charges (Z) 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.