Supplyment to the article"Layered group-IV-V-VI semiconductors with superior piezoelectricity and ferroelectricity"

Building upon the experimentally synthesized SnP2 Se6 crystal, we computationally propose a novel series of inherently asymmetric group-IV-V-VI semiconductors through elemental substitution. This series encompasses monolayers of SiP2 Se6 , SiAs2 S6 , SiAs2 Se6 , SiAs2 Te6 , GeP2 S6 , GeP2 Se6 , GeAs2 S6 , GeAs2 Se6 , GeSb2 S6 , SnP2 S6 , SnP2 Se6 , SnAs2 S6 , SnAs2 Se6 , SnAs2 Te6 and SnSb2 S6 .

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 SiAs2 Se6 , SiAs2 Te6 , GeP2 S6 , GeAs2 Se6 and SnAs2 Te6 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, SnP2 Se6 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.