Di Yang

From Nonluminescent Cs4PbX6 (X = Cl, Br, I) Nanocrystals to Highly Luminescent CsPbX3 Nanocrystals: Water-Triggered Transformation through a CsX-Stripping Mechanism

We report a novel CsX-stripping mechanism that enables the efficient chemical transformation of nonluminescent Cs4PbX6 (X = Cl, Br, I) nanocrystals (NCs) to highly luminescent CsPbX3 NCs. During the transformation, Cs4PbX6 NCs dispersed in a nonpolar solvent are converted into CsPbX3 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 Cs4PbX6 NCs, and produces monodisperse and air-stable CsPbX3 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 Cs4PbX6 NCs are converted into CsPbX3 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 CsPbX3 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 CsPbX3 NCs but also adds insights into the chemical transformation behavior and stabilization mechanism of these emerging perovskite nanocrystals.

Microwave-assisted synthesis of high-quality “all-inorganic” CsPbX3 (X = Cl, Br, I) perovskite nanocrystals and their application in light emitting diodes

As an emerging material for optoelectronic applications, cesium lead halide (CsPbX3) nanocrystals (NCs) have drawn considerable attention because of their excellent photophysical properties. For the first time, we report a fast and efficient microwave-assisted strategy for the synthesis of high-quality CsPbX3 NCs with controllable morphologies (nanocube, nanoplate and nanorod). The prepared CsPbX3 nanocubes show high PLQY, approximately 75%, and narrow full width at half maximum emission line (from 9 to 34 nm) covering the entire spectrum of the visible range. Plate-like and rod-like CsPbBr3 NCs can also be obtained by attenuating the reaction parameters. A feasible kinetics-controlled growth mechanism of such NCs has been proposed and verified. The as-synthesized CsPbBr3 NCs show excellent optical performance as a green light source in a white light emitting diode prototype device. This work not only provides a new synthetic path for the preparation of high-quality CsPbX3 NCs, but also sheds some light on the fundamental study and practical applications of such NCs.

Interfacial Synthesis of Highly Stable CsPbX3/Oxide Janus Nanoparticles

The poor stability of CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) has severely impeded their practical applications. Although there are some successful examples on encapsulating multiple CsPbX3 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 CsPbX3/SiO2 and CsPbBr3/Ta2O5 Janus nanoparticles were successfully prepared by combining a water-triggered transformation process and a sol-gel method. The CsPbBr3/SiO2 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 CsPbBr3/SiO2 Janus NCs showed only a slight drop (2%) in the PL intensity, while a control sample of unmodified CsPbBr3 NCs displayed a 35% drop. We further highlighted the advantageous features of the CsPbBr3/SiO2 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 CsPbX3 NCs but also helps to address the challenging stability issue; therefore, it has an important implication toward their practical applications.

Improving the Stability and Size Tunability of Cesium Lead Halide Perovskite Nanocrystals Using Trioctylphosphine Oxide as the Capping Ligand

Recently, all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, and I) nanocrystals (NCs) have drawn wide attention because of their excellent optoelectronic properties and potential applications. However, one of the most significant challenges of such NCs is their low stability against protonic solvents. In this work, we demonstrate that by incorporating a highly branched capping ligand, trioctylphosphine oxide (TOPO), into the traditional oleic acid/oleylamine system, monodisperse CsPbX3 NCs with excellent optoelectronic properties can be achieved at elevated temperatures (up to 260 °C). The size of such NCs can be varied in a relatively wide range. The capping of TOPO on NCs has been verified through Fourier transform infrared spectroscopy measurement. More importantly, the presence of TOPO can dramatically improve the stability of CsPbX3 NCs against ethanol treatment. After ethanol treatment for 100 min, the emission intensity of the TOPO-capped sample dropped only 5%, whereas that of non-TOPO-capped NCs dropped up to 86%. This work may shed some light on the preparation and application of CsPbX3 NCs with higher stability.

Highly Luminescent and Stable Perovskite Nanocrystals with Octylphosphonic Acid as a Ligand for Efficient Light-Emitting Diodes

All inorganic perovskite nanocrystals (NCs) of CsPbX3 (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, CsPbX3 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 CsPbX3 stability. Owing to a strong interaction between OPA and lead atoms, the OPA-capped CsPbX3 (OPA-CsPbX3) 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-CsPbBr3 is only ∼4.6%, which is much lower than ∼29.7% in OA/OLA-CsPbBr3. Thereby, a uniform and compact OPA-CsPbBr3 film is obtained for LED application. A green LED with a current efficiency of 18.13 cd A-1, 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.

One-Pot Synthesis of Highly Stable CsPbBr3@SiO2 Core–Shell Nanoparticles

The practical applications of CsPbX3 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 CsPbX3 NCs, it is still very difficult to coat CsPbX3 NCs with another material on a single-particle level. In this work, we report a facile one-pot approach to synthesize CsPbBr3@SiO2 core-shell nanoparticles (NPs), in which each core-shell NP has only one CsPbBr3 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 CsPbBr3 and SiO2 are critical for the successful preparation of core-shell NPs. Thanks to the protection of SiO2 shell, the product shows much higher long-term stability in humid air and enhanced stability against ultrasonication treatment in water than that of naked CsPbBr3 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 CsPbX3 NCs.

A general and facile approach to disperse hydrophobic nanocrystals in water with enhanced long-term stability

We report a general and fast approach that can transfer any hydrophobic nanocrystals into water within several seconds without vigorous stirring or ultrasonication treatment at room temperature. The key factor is to find a suitable solvent that can be soluble in both organic solvent and water using the classical “like dissolves like” rule. Hydrophobic nanocrystals covered with oleic acid, oleylamine, and TOPO have been successfully transferred into water. The second ligand could be any amphiphilic ligands, such as sodium oleate, decanoate, dopamine, CTAB, and so on. The surface charge could be tuned to negatively or positively charged by using different ligands. After phase transfer, the original properties, including the plasmonic, magnetic, catalytic, and fluorescence properties, of the nanocrystals are well retained.

Colloidal Synthesis of Au@Pd Core–Shell Nanorods with Tunable Dimensions and Enhanced Electrocatalytic Activities

Pd-based bimetallic nanoparticles have attracted much attention due to their promising applications in diverse fields. For example, in electro-oxidation of alcohol, the synergistic effects between different metals can significantly improve the resistance of Pd to sintering, coke deposition and CO poisoning. Although great progress has been achieved in the synthesis of Pd-based nanostructures, it is still lacking a robust protocol to synthesize Pd bimetallic nanostructures with desired shapes and dimensions. In this work, Au@Pd core–shell nanorods with tunable size of both core and shell were synthesized using a one-pot method. Both the dimensions of Pd shell and Au core can be tuned separately by altering reaction conditions. Compared with Pd cube and octahedron, Au@Pd core–shell nanorods exhibit superior activity in the electrochemical oxidation of ethanol due to the synergistic effects between Au core and Pd shell. This work not only develops a facile one-pot protocol for synthesizing Au@Pd core–shell nanostructures, but also sheds some light on the design and preparation of catalysts with enhanced electrocatalytic performance.

Cs4PbX6 (X = Cl, Br, I) Nanocrystals: Preparation, Water-Triggered Transformation Behavior, and Anti-Counterfeiting Application

As a promising material, Cs4PbX6 (X = Cl, Br, I) nanocrystals (NCs) have attracted much attention. However, their luminescent property is still under debate. In this work, we first systematically studied the colloidal preparation of Cs4PbX6 NCs. It is found that the critical parameter for the formation of Cs4PbX6 NCs is the ratio between Cs and Pb. Pure Cs4PbX6 NCs are nonluminescent. The luminescence property of previous reported Cs4PbX6 NCs may come from the impurity of luminescent CsPbX3 NCs. No coexistence of both Cs4PbX6 and CsPbX3 phase has been found in one single nanoparticle. The water-triggered transformation from nonluminescent Cs4PbX6 NCs to luminescent CsPbX3 NCs has been quantitatively studied. The potential application of Cs4PbX6 NCs in humidity sensor and anticounterfeiting have been demonstrated. This work is important because it not only confirmed the nonluminescent nature of Cs4PbX6 NCs but also demonstrated the potential application of such NCs.