Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Metal halide perovskite nanocrystals offer a range of interesting properties and are being studied extensively for applications in solar cells, photodetectors and light-emitting devices. This perspective provides a number of best practices for the synthesis, purification, and characterization of metal halide perovskite nanocrystals, with detailed discussion of CsPbI3, CsPbBr3, CH3NH3PbI3 (MAPI), and Cs2AgBiBr6 as examples. The choice of reactants and ligands for hot-injection reactions are discussed, as well as how various reaction conditions, including temperature and time, affect yield, uniformity, and crystal phase. We extensively discuss the use of antisolvent precipitation methods for purification, since ligand coordination to most perovskite nanocrystals is weak and the nanocrystals are sensitive to degradation. Finally, we discuss some of the strategies for imaging these nanocrystals using transmission electron microscopy (TEM).

Capping ligand bonding and the thermal and colloidal stability of Cs 2 AgBiBr 6 nanocrystals were studied. Oleylamine and oleic acid bonding to Cs 2 AgBiBr 6 nanocrystals was studied with 1 H nuclear magnetic resonance and nuclear Overhauser effect spectroscopy. Both molecules are present in the ionic metathesis synthesis reaction, but only oleylamine remains bound to the nanocrystals after purification. Oleic acid is not a capping ligand; however, the synthesis requires it, and its concentration determines the yield of the reaction while oleylamine primarily affects the uniformity of the sample. Substitution of oleic acid in the reaction with diisooctylphosphinic acid still yielded nanocrystals with similar size, cuboidal shape, uniformity, cubic double-perovskite crystal structure, and optical properties. Nanocrystals were assembled into superlattices and heated in air. Grazing incidence smallangle and wide-angle X-ray scattering showed that the nanocrystals sinter at 250 °C, but the crystal structure and preferred crystal orientation on the substrate does not change. When nanocrystals were dispersed in hexane and exposed to light, they precipitated within 24 h. Scanning electron microscopy showed that the aggregated nanocrystals still retained their initial size and shape and had not coalesced.

The equilibrium phase of cesium lead iodide (CsPbI3) at room temperature is yellow and optically inactive due to its indirect band gap. The metastable black phase of CsPbI3 on the other hand exhibits optical properties that are suitable for photovoltaic and light-emitting devices. Here, we examine the stability of the black phase of ligand-stabilized CsPbI3 nanocrystals heated in humid air. Water vapor is known to catalyze the transition of CsPbI3 from the black phase to the yellow phase. Uniform nanocrystals with cube shape were synthesized with capping ligand mixtures of oleylamine and oleic acid or diisooctylphosphinic acid, assembled into superlattices with preferred crystal orientation, and studied using grazing incidence small- and wide-angle X-ray scattering with in situ heating. The black-phase nanocrystals are found to inhabit the γ-orthorhombic phase and do not revert to the equilibrium yellow δ-orthorhombic phase until reaching a relatively high temperature, between 170 and 200 °C, coinciding with superlattice degradation.

circuits. [4][5][6][7] PbS CQDs are well-suited to SWIR photodetection in light of their sizetuned bandgap and strong absorption in the IR region. The latest CQD photodetectors have adopted the photodiode architecture consisting of a CQD-based active layer, a CQD-based hole-transport layer (HTL), and a metal oxide-based electrontransport layer (ETL). [7][8][9][10] High-efficiency photodetectors require accurate control of the optoelectronic properties of each layer. To reach a peak sensitivity in the wavelength range of 1400-1500 nm, small bandgap PbS CQDs are used as active layers, and their relatively deep conduction band position limits the selection of ETs. Known metal oxides such as ZnO or TiO 2 do not offer the combination of properties needed for SWIR CQD detectors: the requisite union of band alignment at the interface, and stable performance at the operating wavelengths. [8,10] CQD-based transport layers benefit from tunable optoelectronic properties through surface ligand engineering and quantum-size effect tuning. While CQD HTLs have been extensively studied and improved, CQD ETLs are rarely explored. [11][12][13] Here we develop n-type CQDs as ETLs and tailor their band-edge positions for efficient charge extraction at the active layer/ETL interface. We fabricate all-CQD photo detectors in which CQDs of the same composition but with different sizes and ligand passivation were used to produce functionalities optimized separately for the active layer and charge-transport layers. The CQD photodetectors operating at 1450 nm exhibit a high external quantum efficiency (EQE) of 66% and a low dark current of ≈1 × 10 −3 mA cm −2 at 1 V, which has not been realized simultaneously in the prior works that used metal oxides as ETLs.We find that the surface passivation of CQDs is crucial to suppressing ion migration and preventing performance degradation in CQD photodetectors. Therefore, we develop a strategy to improve the passivation of ETL CQDs using strongly bound organic ligands. Specifically, we demonstrate that ETLs employing a strong organic ligand trans-4-(trifluoromethyl) cinnamic acid (TFCA) improve the dark current stability of CQD photodetectors by 50× compared to ETLs employing a weakly bound inorganic ligand tetrabutylammonium iodide (TBAI). Solution-processed photodetectors based on colloidal quantum dots (CQDs)are promising candidates for short-wavelength infrared light sensing applications. Present-day CQD photodetectors employ a CQD active layer sandwiched between carrier-transport layers in which the electron-transport layer (ETL) is composed of metal oxides. Herein, a new class of ETLs is developed using n-type CQDs, finding that these benefit from quantum-size effect tuning of the band energies, as well as from surface ligand engineering. Photodetectors operating at 1450 nm are demonstrated using CQDs with tailored functionalities for each of the transport layers and the active layer. By optimizing the band alignment between the ETL and the active layer, CQD photodetectors that combine a low d...