We report a novel CsX-stripping mechanism that enables the efficient chemical transformation of nonluminescent CsPbX (X = Cl, Br, I) nanocrystals (NCs) to highly luminescent CsPbX NCs. During the transformation, CsPbX NCs dispersed in a nonpolar solvent are converted into CsPbX NCs by stripping CsX through an interfacial reaction with water in a different phase. This process takes advantage of the high solubility of CsX in water as well as the ionic nature and high ion diffusion property of CsPbX NCs, and produces monodisperse and air-stable CsPbX NCs with controllable halide composition, tunable emission wavelength covering the full visible range, narrow emission width, and high photoluminescent quantum yield (up to 75%). An additional advantage is that this is a clean synthesis as CsPbX NCs are converted into CsPbX NCs in the nonpolar phase while the byproduct of CsX is formed in water that could be easily separated from the organic phase. The as-prepared CsPbX NCs show enhanced stability against moisture because of the passivated surface. Our finding not only provides a new pathway for the preparation of highly luminescent CsPbX NCs but also adds insights into the chemical transformation behavior and stabilization mechanism of these emerging perovskite nanocrystals.

The practical applications of CsPbX nanocrystals (NCs) have been limited by their poor stability. Although much effort has been devoted to making core-shell nanostructures to enhance the stability of CsPbX NCs, it is still very difficult to coat CsPbX NCs with another material on a single-particle level. In this work, we report a facile one-pot approach to synthesize CsPbBr@SiO core-shell nanoparticles (NPs), in which each core-shell NP has only one CsPbBr NC. The formation process has been carefully monitored. It has been found that the formation rates, determined by reaction temperature, precursor species, pH value, etc., of both CsPbBr and SiO are critical for the successful preparation of core-shell NPs. Thanks to the protection of SiO shell, the product shows much higher long-term stability in humid air and enhanced stability against ultrasonication treatment in water than that of naked CsPbBr NCs. This work not only provides a robust method for the preparation of core-shell nanostructures but also sheds some light on the stabilization and applications of CsPbX NCs.

The poor stability of CsPbX (X = Cl, Br, I) nanocrystals (NCs) has severely impeded their practical applications. Although there are some successful examples on encapsulating multiple CsPbX NCs into an oxide or polymer matrix, it has remained a serious challenge for the surface modification/encapsulation using oxides or polymers at a single particle level. In this work, monodisperse CsPbX/SiO and CsPbBr/TaO Janus nanoparticles were successfully prepared by combining a water-triggered transformation process and a sol-gel method. The CsPbBr/SiO NCs exhibited a photoluminescence quantum yield of 80% and a lifetime of 19.8 ns. The product showed dramatically improved stability against destruction by air, water, and light irradiation. Upon continuous irradiation by intense UV light for 10 h, a film of the CsPbBr/SiO Janus NCs showed only a slight drop (2%) in the PL intensity, while a control sample of unmodified CsPbBr NCs displayed a 35% drop. We further highlighted the advantageous features of the CsPbBr/SiO NCs in practical applications by using them as the green light source for the fabrication of a prototype white light emitting diode, and demonstrated a wide color gamut covering up to 138% of the National Television System Committee standard. This work not only provides a novel approach for the surface modification of individual CsPbX NCs but also helps to address the challenging stability issue; therefore, it has an important implication toward their practical applications.

Emerging all-inorganic perovskite nanocrystals can retain a desired crystal structure under ambient conditions and offer easy solution processability. In this work, we have demonstrated CsPbI 3 perovskite quantum dot (QD) solar cells with a remarkable efficiency approaching 13% and an extremely low energy loss of 0.45 eV by employing a series of dopant-free polymeric hole-transporting materials (HTMs). The CsPbI 3 QD solar cells use polymer HTMs to achieve efficient charge extraction at QD/polymer interfaces and avoid device instability caused by complex doping and oxidation processes required by conventional Spiro-OMeTAD. Meanwhile, the CsPbI 3 QD photovoltaic devices can be fabricated at room temperature and exhibit more reproducible film quality, showing potential advantages over current all-inorganic thin-film perovskite solar cells. We believe that our findings will catalyze the development of new device structures, specifically for perovskite QDs, and help realize the promising potential of all-inorganic perovskite solar cells.

All inorganic perovskite nanocrystals (NCs) of CsPbX (X = Cl, Br, I, or their mixture) are regarded as promising candidates for high-performance light-emitting diode (LED) owing to their high photoluminescence (PL) quantum yield (QY) and easy synthetic process. However, CsPbX NCs synthesized by the existing methods, where oleic acid (OA) and oleylamine (OLA) are generally used as surface-chelating ligands, suffer from poor stability due to the ligand loss, which drastically deteriorates their PL QY, as well as dispersibility in solvents. Herein, the OA/OLA ligands are replaced with octylphosphonic acid (OPA), which dramatically enhances the CsPbX stability. Owing to a strong interaction between OPA and lead atoms, the OPA-capped CsPbX (OPA-CsPbX) NCs not only preserve their high PL QY (>90%) but also achieve a high-quality dispersion in solvents after multiple purification processes. Moreover, the organic residue in purified OPA-CsPbBr is only ∼4.6%, which is much lower than ∼29.7% in OA/OLA-CsPbBr. Thereby, a uniform and compact OPA-CsPbBr film is obtained for LED application. A green LED with a current efficiency of 18.13 cd A, corresponding to an external quantum efficiency of 6.5%, is obtained. Our research provides a path to prepare high-quality perovskite NCs for high-performance optoelectronic devices.

Recently, all-inorganic cesium lead halide (CsPbX 3 , X = Cl, Br, I) perovskite nanocrystals have drawn much attention because of their outstanding photophysical properties and potential applications. In this work, a simple and efficient solvothermal approach to prepare CsPbX 3 nanocrystals with tunable and bright photoluminescent (PL) properties, controllable composition, and morphology is presented. CsPbX 3 nanocubes are successfully prepared with bright emission high PL quantum yield up to 80% covering the full visible range and narrow emission line widths (from 12 to 36 nm). More importantly, ultrathin CsPbX 3 (X = Cl/Br, Br, and Br/I) nanowires (with diameter as small as ≈2.6 nm) can be prepared in a very high morphological yield (almost 100%). A strong quantum confinement effect is observed in the ultrathin nanowires, in which both the absorption and emission peaks shift to shorter wavelength range compared to their bulk bandgap. The reaction parameters, such as temperature and precursors, are varied to investigate the growth process. A white light-emitting device prototype device with wide color gamut covering up to 120% of the National Television System Committee standard has been demonstrated by using CsPbBr 3 nanocrystals as the green light source. The method in this study provides a simple and efficient way to prepare high-quality CsPbX 3 nanocrystals.The ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/adfm.201701121.light-emitting devices (LEDs), [3] fieldeffect transistors, [4] solar cells, [5] and so on. Although much progress has been made in the preparation of traditional colloidal semiconductor nanocrystals, the current synthetic strategies are still suffering from several challenges, including low yield, complex procedures, and toxic chemicals. It is thus highly desired to develop new types of semiconductor nanocrystals that possess excellent photoluminescent properties and can be prepared in a simple, reproducible, and cost-effective way. For this purpose, recently, a new family of perovskite semiconductor nanocrystals has proven to be a great candidate for optoelectronic applications due to their low cost, easy preparation, high reproducibility, and great photoluminescent properties. [6] For example, the organic-inorganic hybrid perovskite, i.e., CH 3 NH 3 PbX 3 (X = Cl, Br, I), have been used in solar cell and showed great performance. [7] However, the hybrid perovskite nanocrystals are highly sensitive to moisture and oxygen, resulting in low stability. Since the pioneered work by Kovalenko and co-workers in 2015, intensive research efforts have been devoted to studying the all-inorganic cesium lead halide (CsPbX 3 , X = Cl, Br, I) perovskite nanocrystals, which show outstanding photophysical properties, including composition-and size-controlled photoluminescent property, high PL quantum yield (PLQY), narrow emission widths, and short radiative lifetime. [8] The all-inorganic perovskite nanocrystals have been prepared through...

X-type ligands, for example, the pair of oleylamine (OAm) and oleic acid (OA), have been widely used to prepare CsPbX3 nanocrystals (NCs). However, the proton exchange between coordinated OAm and OA may induce the detachment of ligands, resulting in poor performance after cleaning or long-time storage. Herein, density functional theory calculations predict that primary amines (L-type ligands) can stabilize a PbBr x -rich surface and yield a trap-free material with fully delocalized valence band maximum and conduction band minimum states, which can significantly improve the photophysical properties and stability of CsPbBr3 NCs. Along this prediction, a room-temperature reprecipitation method using L-type ligands (OAm, n-octylamine, or undecylamine) as the sole capping ligand has been developed to synthesize high-quality CsPbBr3 NCs with near-unity photoluminescence quantum yield and dramatically improved stability against purification and water treatment. The enhancement can be attributed to the strong binding of unprotonated amines to lead atoms and the effective surface passivation provided by the resulted PbBr x -rich surface, which are highly consistent with the theoretical predictions. This work not only offers an approach to synthesize high-quality perovskite NCs but also provides an in-depth understanding of the surface modification of CsPbX3 NCs for practical applications.

Hydrochromic materials that can reversibly change color upon water treatment have attracted much attention owing to their potential applications in diverse fields. Herein, for the first time, we report that space‐confined CsPbBr3 nanocrystals (NCs) are hydrochromic. When CsPbBr3 NCs are loaded into a porous matrix, reversible transition between luminescent CsPbBr3 and non‐luminescent CsPb2Br5 can be achieved upon the exposure/removal of water. The potential applications of hydrochromic CsPbBr3 NCs in anti‐counterfeiting are demonstrated by using CsPbBr3 NCs@mesoporous silica nanospheres (around 100 nm) as the starting material. Owing to the small particle size and negatively charged surface, the as‐prepared particles can be laser‐jet printed with high precision and high speed. We demonstrate the excellent stability over repeated transformation cycles without color fade. This new discovery may not only deepen the understanding of CsPbX3, but also open a new way to design CsPbX3 materials for new applications.