Data and materials availability: All data are available in the manuscript or supplementary information. All materials are available upon request to L.D. Methods Solution-phase synthesis of pure 2D halide perovskite sheets In this study, ten types of pure 2D halide perovskite sheets were synthesized via a quaternary solvent method.

Over the last several years, there has been tremendous progress in the development of nanoscale halide perovskite materials and devices that possess a wide range of band gaps and tunable optical and electronic properties. Particularly, the emerging two-dimensional (2D) forms of halide perovskites are attracting more interest due to the long charge carrier lifetime, high photoluminescence quantum efficiency, and great defect tolerance. Interfacing 2D halide perovskites with other 2D materials including graphene and transition metal dichalcogenides (TMDs) significantly broadens the application range of the 2D materials and enhances the performance of the functional devices. The synthesis and characterization of 2D halide perovskite nanostructures, the interface of the 2D halide perovskites with other 2D materials, and the integration of them into high-performance optoelectronic devices including solar cells, photodetectors, transistors, and memory devices are currently under investigation. In this article, we review the progress of the above-mentioned topics in a timely manner and discuss the current challenges and future promising directions in this field.

Edges of two-dimensional (2D) halide perovskites are found to exhibit unusual properties such as enhanced photoluminescence lifetime and reduced photoluminescence emission energy. Here, we report the formation mechanism and the dynamic nature of edge states on exfoliated 2D halide perovskite thin crystals. In contrast to other 2D materials, the edge states in 2D perovskites are extrinsic and can be triggered by moisture with a concentration as low as ∼0.5 ppm. High-resolution atomic force microscopy and transmission electron microscopy characterizations reveal the width of the low-energy states is ∼40 nm wide. A temperature-dependent photoluminescence study suggests the edge states are a combination of several lower-energy states. Importantly, we demonstrate that the charge carriers on the dynamically formed edge states are not only long-lived but also highly mobile and can be conducted along the edges effectively with high mobilities of 5.4−7.0 cm 2 V −1 s −1 . This work provides significant insights on the origin of the edge states in 2D perovskites and provides routes to manipulate their optical and electrical properties through controlling their edges.

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Two‐dimensional (2D) halide perovskites can be regarded as natural organic‐inorganic hybrid quantum wells, which exhibit very promising light‐emitting applications due to their high photoluminescence quantum yield, narrow emission bandwidth, and large exciton binding energy. However, it remains a grand challenge to achieve reliable devices for both light‐emitting diodes (LEDs) and lasers utilizing phase‐pure 2D perovskites. Recently, exciting progresses have been made with respect to molecular design, optoelectronic property, and device fabrication for novel 2D perovskite hybrid quantum‐wells. In this article, we critically review the key challenges of exciton losses, charge injections, and triplet issues associated with the light‐emitting applications of such phase‐pure 2D perovskites after examining their recent breakthroughs in LEDs and lasers. Lastly, we provide a new perspective on molecular engineering strategies to address the above‐mentioned fundamental issues, which may open up a new avenue to the development of highly efficient quantum‐well emitters for solid‐state lighting and display.