Layered group-IV-V-VI semiconductors with superior piezoelectricity and ferroelectricity

Two-dimensional (2D) layered materials lacking inversion symmetry are promising candidates for nano-electromechanical and electronic technologies. Building upon the experimentally synthesized SnP2Se6 crystal, we computationally propose a novel series of inherently asymmetric group-IV-V-VI semiconductors through elemental substitution. This series encompasses monolayers of SiP2Se6, SiAs2S6, SiAs2Se6, SiAs2Te6, GeP2S6, GeP2Se6, GeAs2S6, GeAs2Se6, GeSb2S6, SnP2S6, SnP2Se6, SnAs2S6, SnAs2Se6, SnAs2Te6 and SnSb2S6. All these structures exhibit robust energetic and dynamic stability, along with excellent resistance to oxidation when exposed to air. These monolayers have moderate bandgaps ranging from 1.04 to 2.36 eV; notably, the monolayers of SiAs2Se6, SiAs2Te6, GeP2S6, GeAs2Se6 and SnAs2Te6 are direct-bandgap semiconductors. More importantly, these group-IV-V-VI monolayers exhibit significant in-plane piezoelectricity, with coefficients (d11) as high as 22.94 pm/V. Additionally, SnP2Se6 bilayer exhibit significant out-of-plane piezoelectricity (d33) with coefficients exceeding 5 pm/V. Moreover, the coexistence of room-temperature ferroelectricity in these monolayers is unambiguously confirmed by their intrinsic spontaneous polarization and remarkably low ferroelectric switching barriers. These findings establish a new paradigm for integrating piezoelectricity and ferroelectricity with diverse 2D materials, thereby opening exciting opportunities for designing multifunctional materials and developing novel electronic devices.