Negative Longitudinal Piezoelectricity Coexisting with both Negative and Positive Transverse Piezoelectricity in a Hybrid Formate Perovskite

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]2MX4 (M = Ge, Sn, Pb; X = Cl, Br, I) were investigated using first-principles calculations. The excellent piezoelectricity of 2D-HOIP-[C6H11NH3]2MX4 has been analyzed in detail. More importantly, 2D-[C6H11NH3]2MX4 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]2SnBr4 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]2MX4 with potential applications in ultrathin piezoelectric cantilever and diaphragm devices.

Persistent and Quasipersistent Spin Textures in Halide Perovskites Induced by Uniaxial Stress

Persistent spin textures are highly desirable for applications in spintronics, as they allow for long carrier spin lifetimes. However, they are also rare, as they require a delicate balance between spin-momentum coupling parameters. We used density functional theory simulations to predict the possibility of achieving these desirable spin textures through the application of uniaxial stress. Hybrid organic-inorganic perovskite MPSnBr3 (MP = CH3PH3) is a ferroelectric semiconductor which exhibits persistent spin textures near its conduction band minimum and mostly Rashba type spin textures in the vicinity of its valence band maximum. Application of uniaxial stress leads to the gradual evolution of the valence band spin textures from mostly Rashba type to quasipersistent ones under a tensile load and to pure Rashba or quasipersistent ones under a compressive load. The material exhibits flexibility, a rubber-like response, and both positive and negative piezoelectric constants. A combination of such properties may create opportunities for flexible and rubbery spintronic devices.

Novel Piezoelectricity in Two-Dimensional Metallic/Semimetallic Materials with Out-of-Plane Polarization

Normally, the good conductivity of metals and semimetals is incompatible with the piezoelectricity since the internal electric current will dismiss any polarization. However, here, we reveal that the out-of-plane piezoelectric effect can exist in two-dimensional (2D) metallic/semimetallic materials due to their giant anisotropy. A method is developed to calculate the out-of-plane polarization in 2D systems, where the modern theory of polarization based on a Berry-phase approach is not applicable. Detailed calculation and analysis on a Dirac material, the FeB2 monolayer, show that it has an out-of-plane polarization of 8.3 pC/m and the piezoelectric coefficient of e31 = -59.3 pC/m and d31 = -0.25 pm/V. This work provides a formalism to discover more piezoelectric materials within the vast 2D metallic/semimetallic materials.