Giant intrinsic piezoelectricity in 2D hybrid organic-inorganic perovskites [C₆H₁₁NH₃]₂MX₄ (M = Ge, Sn, Pb; X = Cl, Br, I)

2D-piezoelectric materials are attractive for micro-electromechanical systems (MEMS), medical implants and wearable devices because of their numerous exceptional properties. 2D-hybrid organic-inorganic perovskites (HOIPs) have attracted extensive research interest due to their merits of structural diversity, good mechanical flexibility, and ease of fabrication. The electronic energy band, charge density and the elastic properties of 2D-HOIP-[C6H11NH3]2MX(4) (M = Ge, Sn, Pb; X = Cl, Br, I) were investigated using first-principles calculations. The excellent piezoelectricity of 2D-HOIP-C6H11NH3]2MX(4 )has been analyzed in detail. More importantly, 2D-[C6H11NH3]2MX(4) have giant intrinsic positive and negative out-of-plane piezoelectric coefficients under the effect of van der Waals interaction. The d31 and d32 of [C6H11NH3](2)SnBr(4 )are 82.720 pm V-1 and -36.139 pm V-1, respectively, which are among the largest piezoelectric coefficients among all kinds of atomic-thick 2D materials reported. The high flexibility together with the giant out-of-plane piezoelectricity would endow these 2D-HOIP-[C6H11NH3]2MX(4) with potential applications in ultrathin piezoelectric cantilever and diaphragm devices.