Large Piezoelectric Response in Ferroelectric/Multiferroelectric Metal Oxyhalides MOX2 (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 piezoresponse is required for any practical applications. Based on first-principles calculations, we report that ferroelectric TiOX2 as well as multiferroelectric VOX2 (X = F, Cl, Br) monolayers exhibit significantly large in-plane stress (e11) and strain (d11) piezoelectric coefficients. For example, in-plane piezo-response of TiOBr2 (both e11 and d11) is about an order of magnitude larger than that of widely studied 1H-MoS2 monolayer, and also quite comparable to giant piezoelectricity of group-IV monochalcogenide monolayers e.g., SnS. Moreover, d11 of MOX2 monolayers - ranging from 29.028 pm/V to 37.758 pm/V - are significantly higher than the d11 or d33 of commonly used 3D piezoelectrics such as w-AlN (d33 = 5.6 pm/V) and α-quartz (d11 = 2.27 pm/V). Such large d11 of MOX2 monolayers originates from low in-plane elastic constants with large e11due to large Born effective charges (Zij ) and atomic sensitivity ( du/dη ) to an applied strain. We also show the possibility of opening a new way of controlling piezoelectricity by magnetic field.