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Organic–inorganic halide perovskites have achieved remarkable success in various optoelectronic devices.

Large-sized single-crystal two-dimensional (2D) perovskites are highly desirable owing to their fundamental properties and intriguing ability to boost devices. Herein, 2-phenylethylammonium lead bromide [(PEA) 2 PbBr 4 ] single crystals, which are a violet-light-emitting 2D perovskite material, with typical lateral sizes of about one centimeter were successfully grown using a seeded solution method. The singlecrystal plates showed a well-defined shape (rectangle or hexagon), a natural thickness (300-500 mm) similar to that of conventional silicon and InP wafers, a large aspect ratio of $20, and a smooth surface (root mean square, $0.7 nm). We integrated these single crystal plates into an ultraviolet photodetector, achieving a low dark current of $10 À13 A and an efficient photoresponse (on/off ratio, $10 3 ). This experiment could easily be extended to grow freestanding 2D perovskite single crystals on a wafer scale for practical integrated optoelectronics.