Materials and Methods Supplementary Text Fig. S1 Reference (35) Database S1 Atomically Thin Two-dimensional Organic-inorganic Hybrid Perovskites

Flat perovskite crystals Bulk crystals and thick films of inorganic-organic perovskite materials such as CH3NH3PbI3 have shown promise as active material for solar cells. Dou et al. show that thin films—a single unit cell or a few unit cells thick—of a related composition, (C4H9NH3)2PbBr4, form squares with edges several micrometers long. These materials exhibit strong and tunable blue photoluminescence. Science, this issue p. 1518 Several inorganic-organic perovskite materials grown as atomically thin crystals exhibit strong photoluminescence. 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.