Amplified Spontaneous Emission in Single-Layerd 2D Tin Perovskites

Metal halide perovskites are attracting increasing interest for applications in photonics, light emitting devices and lasers. However, the need to overcome trap-mediated non-radiative losses, increase the radiative recombination efficiency, improve the material stability, and find non-toxic alternatives to the standard lead-based materials urgently calls for the study of different perovskite structures.<br/>Currently, perovskite lasers remain restricted to 3D architectures with limited chemical tunability, or multidimensional perovskites of the Ruddlesden-Popper series where the radiative recombination still relies on the phases with highest dimensionalities due to energy funneling processes. 2D perovskites have long been proposed as promising alternatives to improve the luminescence efficiency, thanks to their extremely high exciton binding energy which allows the formation of stable excitons and favours fast radiative decay. At the same time, their layered architecture can grant increased environmental stability, block detrimental ionic diffusion, and relax geometrical constraints giving access to a wider range of lead-free compositions. Nevertheless, compared to the parental 3D structures, lasing in single-layered 2D perovskites has been very scarcely reported, [1-3] and their ability to sustain coherent emission remains unclear and highly debated. [4-6] Notably, to our knowledge, amplified spontaneous emission (ASE) in single-layered 2D perovskites has never been reported.<br/>In this work, we investigate the gain properties of 2D tin-based perovskites. By combining temperature-dependent structural characterization and optical spectroscopy, we show that the choice of the templating cation critically affects the structural rigidity, defectivity, photostability and morphology of the perovskite film determining its ASE properties. Through proper synthetic design, we observe intense ASE with very low threshold (down to 30 μJ/cm²), optical gain coefficient comparable to that of the archetypical methylammonium lead iodide [7] (up to 3500 cm^-1 at 77 K) and ASE operational stability up to room temperature in tin layered perovskites. We further discuss the integration of such 2D perovskites with optical resonators to achieve optically-pumped lasing.<br/>Our findings represent the first characterization of the gain properties of single-layered perovskites, providing fundamental synthetic guidelines to improve their coherent emission properties and opening the way to the design and exploitation of a new class of lead-free perovskite lasers.<br/>[1] T. Kondo <i>et al</i>, <i>Solid State Commun.</i> (1998), 105, 253-255.<br/>[2] E. P. Booker <i>et al</i>, <i>Adv. Optical</i><i> </i><i>Mater. </i>(2018), 6, 1800616.<br/>[3] C. M. Raghavan <i>et al</i>, <i>Nano Lett. </i>(2018) , 18, 3221-3228.<br/>[4] W. K. Chong <i>et al</i>, <i>Phys</i>. <i>Chem. Chem. Phys.</i> (2016), 18, 14701.<br/>[5] M. R. Layden <i>et al</i>, <i>Phys</i>. <i>Chem. Chem. Phys.</i> (2018), 20, 15030.<br/>[6] Y. Liang <i>et al</i>, <i>Adv</i>. <i>Mater. </i>(2019), 31, 1903030.<br/>[7] A. L. Alvarado-Leaños <i>et al, Adv. Opt. </i><i>Mater. </i>(2021), 9, 31, 1903030.