An incremental strategy for tunable anisotropic piezoelectricity in 2D materials

Anisotropic flexible piezoelectric materials hold potential for designing flexible sensor capable of screening, distinguishing, and storing different forms of energy. However, suitable two-dimensional (2D) candidate materials exhibiting such properties within a single system are rare. Here, we propose an incremental crystal design strategy to construct 2D MnAXm (n = 1, 2; m = 2, 4) with tetragonal lattice, including experimentally synthesized Ag2WS4, Cu2MoS4, and Cu2WS4. The presence of a fourfold roto-inversion operation (S4) results in bipolar out-of-plane piezoelectric coefficients with high anisotropy, characterized by e31×e32 < 0. High-throughput calculations revel the descriptors linking piezoelectric response and structural parameters, such as lattice constants and coordination number density. Furthermore, incremental tunable strategies, including interlayer stacking, Janus configuration, and element substitution, facilitate customized piezoelectricity and supporting dual piezoelectric modes with large piezoelectric anisotropy |e33 / e31| ≈51. Our findings not only provide an effective design strategy for realizing highly anisotropic piezoelectric materials, but also hold potential for application in directional detection, pressure localization as well as direction-dependent flexible nano-devices.