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.

The rapidly growing field of nanoscale lasers can be advanced through the discovery of new, tunable light sources. The emission wavelength tunability demonstrated in perovskite materials is an attractive property for nanoscale lasers. Whereas organic-inorganic lead halide perovskite materials are known for their instability, cesium lead halides offer a robust alternative without sacrificing emission tunability or ease of synthesis. Here, we report the low-temperature, solution-phase growth of cesium lead halide nanowires exhibiting low-threshold lasing and high stability. The as-grown nanowires are single crystalline with well-formed facets, and act as high-quality laser cavities. The nanowires display excellent stability while stored and handled under ambient conditions over the course of weeks. Upon optical excitation, Fabry-Pérot lasing occurs in CsPbBr 3 nanowires with an onset of 5 μJ cm −2 with the nanowire cavity displaying a maximum quality factor of 1,009 ± 5. Lasing under constant, pulsed excitation can be maintained for over 1 h, the equivalent of 10 9 excitation cycles, and lasing persists upon exposure to ambient atmosphere. Wavelength tunability in the green and blue regions of the spectrum in conjunction with excellent stability makes these nanowire lasers attractive for device fabrication.nanowire | perovskite | laser | inorganic | stability M iniaturized light sources hold great promise for advancing the field of optoelectronics. The development of highly stable, wavelength-tunable light sources on the nanoscale can unlock the potential for commercial applications in optical communications (1, 2), sensing (3), imaging (4), and data storage (5), among many others. Nanowire lasers represent one promising approach toward miniaturized light sources. Acting both as the laser cavity and gain medium (6), nanowires may be easily incorporated into optoelectronic circuits based on their size as well as recent advances in electrical pumping (7-10). A wide range of nanowire lasers has been reported consisting of a multitude of compositions including many II-VI and III-V semiconductors (11). Unfortunately, fabrication of many of these nanowires requires expensive hightemperature or low-pressure conditions. Additionally, whereas most are stable under ambient conditions, only a few of these materials have demonstrated broad wavelength tunability. The recent discovery of the favorable properties of methyl ammonium lead halide perovskite materials has triggered a paradigm shift in what is possible in optoelectronics. Stoichiometric wavelength tunability, low trap state density, solution-phase processability, as well as excellent light absorption and emission, make these materials well suited to applications in solar cells (12-15), light-emitting diodes (16, 17), photodetectors (18, 19), and lasers (20)(21)(22).Recently, Zhu et al. reported low lasing thresholds and recordbreaking quality factors for methyl ammonium lead halide (CH 3 NH 3 PbX 3 , X = I, Br, Cl) nanowire lasers as well as excellent wavelength tuna...

Smart photovoltaic windows represent a promising green technology featuring tunable transparency and electrical power generation under external stimuli to control the light transmission and manage the solar energy. Here, we demonstrate a thermochromic solar cell for smart photovoltaic window applications utilizing the structural phase transitions in inorganic halide perovskite caesium lead iodide/bromide. The solar cells undergo thermally-driven, moisture-mediated reversible transitions between a transparent non-perovskite phase (81.7% visible transparency) with low power output and a deeply coloured perovskite phase (35.4% visible transparency) with high power output. The inorganic perovskites exhibit tunable colours and transparencies, a peak device efficiency above 7%, and a phase transition temperature as low as 105 °C. We demonstrate excellent device stability over repeated phase transition cycles without colour fade or performance degradation. The photovoltaic windows showing both photoactivity and thermochromic features represent key stepping-stones for integration with buildings, automobiles, information displays, and potentially many other technologies.

Here, we demonstrate the successful synthesis of brightly emitting colloidal cesium lead halide (CsPbX3, X = Cl, Br, I) nanowires (NWs) with uniform diameters and tunable compositions. By using highly monodisperse CsPbBr3 NWs as templates, the NW composition can be independently controlled through anion-exchange reactions. CsPbX3 alloy NWs with a wide range of alloy compositions can be achieved with well-preserved morphology and crystal structure. The NWs are highly luminescent with photoluminescence quantum yields (PLQY) ranging from 20% to 80%. The bright photoluminescence can be tuned over nearly the entire visible spectrum. The high PLQYs together with charge transport measurements exemplify the efficient alloying of the anionic sublattice in a one-dimensional CsPbX3 system. The wires increased functionality in the form of fast photoresponse rates and the low defect density suggest CsPbX3 NWs as prospective materials for optoelectronic applications.

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.