Organic-inorganic hybrid perovskites, which have proved to be promising semiconductor materials for photovoltaic applications, have been made into atomically thin two-dimensional (2D) sheets. We report the solution-phase growth of single- and few-unit-cell-thick single-crystalline 2D hybrid perovskites of (C4H9NH3)2PbBr4 with well-defined square shape and large size. In contrast to other 2D materials, the hybrid perovskite sheets exhibit an unusual structural relaxation, and this structural change leads to a band gap shift as compared to the bulk crystal. The high-quality 2D crystals exhibit efficient photoluminescence, and color tuning could be achieved by changing sheet thickness as well as composition via the synthesis of related materials.

Nonequilibrium processes occurring in functional materials can significantly impact device efficiencies and are often difficult to characterize due to the broad range of length and time scales involved. In particular, mixed halide hybrid perovskites are promising for optoelectronics, yet the halides reversibly phase separate when photo-excited, significantly altering device performance. By combining nanoscale imaging and multiscale modeling, we elucidate the mechanism underlying this phenomenon, demonstrating that local strain induced by photo-generated polarons promotes halide phase separation and leads to nucleation of light-stabilized iodide-rich clusters. This effect relies on the unique electromechanical properties of hybrid materials, characteristic of neither their organic nor inorganic constituents alone. Exploiting photo-induced phase separation and other nonequilibrium phenomena in hybrid materials, generally, could enable new opportunities for expanding the functional applications in sensing, photoswitching, optical memory, and energy storage.Photovoltaic and light-emitting devices typically operate under conditions far from equilibrium. As such, elucidating the response of functional materials to nonequilibrium driving forces is vital to understanding their fundamental properties and to determining their suitability for device applications. In particular, photo-induced dynamic processes are of major importance to the performance of hybrid perovskite-based devices.1-3 Hybrid perovskites are low-cost, solution processable materials that are promising for many device applications, including photovoltaics 4-9 and light-emitting diodes (LEDs). 10The high device efficiencies have been attributed to their high brightness, long charge carrier migration lengths, + 2 , FA), and X is either iodide, bromide, chloride, or iodide/bromide or bromide/chloride mixtures. By varying the halide ratios in hybrid perovskites, the bandgap can be tuned across the visible spectrum.1,14,15 Precise control of the bandgap presents promising opportunities for color tuning perovskite-based LEDs and lasers, and for incorporating hybrid perovskites in tandem solar cells. 9,16Light-induced effects, however, restrict the practical use of mixed halide hybrid perovskites.1,17,18 Photoluminescence (PL) and X-ray diffraction (XRD) measurements suggest that MAPb(I x Br 1−x ) 3 (0.1 < x < 0.8) undergoes reversible phase separation into iodide-rich and bromiderich regions when photo-excited.1 Such demixing is detrimental to photovoltaic performance, since it leads to charge carrier trapping in the iodide-rich regions. Determining the microscopic mechanism behind phase separation is essential for furthering approaches to mitigate adverse photo-induced effects in devices and should expand the range of their functional applications into areas such as optical memory storage and sensing.19,20 Unfortunately, the microscopic mechanism behind this effect has been elusive because of the multiple length and time scales involved in characterizing the ...