Ionic conduction in a ferroelectric leads to anomalous polarization switching kinetics but prevents retention failure.

Ferroelastic-switching-driven large shear strain and piezoelectricity in a hybrid ferroelectric

A key question is whether the long-range order can survive against the formidable destabilizing field and thermal fluctuation when the ferroic material approaches 2D limit. For example, dimension/size effect is notorious for ferroelectric ultrathin films due to the suppression of ferroelectricity by depolarization effect. [5] In conventional 3D ferroelectric heterostructures, the interfacial bonding and charge mismatch can play a deterministic role in the stability and switchability of the polarization. [6,7] In contrast, van der Waals (vdW) ferroelectrics exhibit much weaker interaction between layers and with the supporting substrate. By employing sufficiently conductive electrodes, it was previously shown that ferroelectricity can be sustained down to few-layer thickness in a vdW ferroelectric material with out-of-plane polarization. [8] Another way to circumvent the depolarization effect is to explore ferroelectrics with in-plane polarization. [9] One of the Collective ferroic orders in van der Waals (vdW) crystals are receiving increasing attention in 2D materials research. The interplay between spatial quantum confinement and long-range cooperative phenomena not only broadens the horizon of fundamental physics, but also enables new device paradigms and functionalities built upon vdW heterostructures. Here, the in-plane ferroelectric properties in thin flakes of vdW hybrid perovskite bis(benzylammonium) lead tetrachloride are studied. The ordering of electric dipoles along the layer plane circumvents the depolarization field and preserves the ferroelectricity down to one unit-cell thickness or two vdW layers at room temperature. The superior performance of the electromechanical energy conversion is demonstrated by exploiting its in-plane piezoelectricity. The successful isolation of ferroelectric order in atomically thin vdW hybrid perovskite paves the way for nonvolatile flexible electronic devices with the cross-coupling between strain, charge polarization, and valley degrees of freedom. Flexible ElectronicsDespite their fundamental and technological significance, ferroic orders in 2D materials, such as ferroelectricity, ferromagnetism, and ferroelasticity, remain largely unexplored. [1][2][3][4] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Two-dimensional (2D) layered lead halide perovskites with large exciton binding energies, efficient radiative recombination, and outstanding environmental stability are regarded as supreme candidates for realizing highly compact and ultralow threshold lasers. However, continuous-wave (CW) pumped lasing of 2D lead halide perovskites, as the precondition for the electrically pumped lasing, is still challenging. Here, we tackled this challenge by demonstrating lasing emission in phenylethylammonium lead iodide [(PEA)2PbI4] embedded in a vertical microcavity under continuous pumping at room temperature. The millimeter-sized (PEA)2PbI4 single crystal was obtained from a two-step seed-growth method, showing high crystallization, excellent thermal stability, and outstanding optical properties. We used the exfoliated (PEA)2PbI4 thin flake as the gain medium to construct a vertical-cavity surface-emitting laser (VCSEL), showing robust single-mode CW lasing operation with an ultra-low threshold of 5.7 W cm−2 at room temperature, attributed to strong optical confinement in the high-Q cavity. Our findings provide a strategy to design and fabricate solution-based 2D perovskite VCSELs and mark a significant step toward the next-generation of coherent light sources.