We developed a microreactor with porous copper fibers for synthesizing nitrogen-doped carbon dots (N-CDs) with a high stability and photoluminescence (PL) quantum yield (QY). By optimizing synthesis conditions, including the reaction temperature, flow rate, ethylenediamine dosage, and porosity of copper fibers, the N-CDs with a high PL QY of 73% were achieved. The PL QY of N-CDs was two times higher with copper fibers than without. The interrelations between the copper fibers with different porosities and the N-CDs were investigated using X-ray photoelectron spectroscopy (XPS) and Fourier Transform infrared spectroscopy (FTIR). The results demonstrate that the elemental contents and surface functional groups of N-CDs are significantly influenced by the porosity of copper fibers. The N-CDs can be used to effectively and selectively detect Hg²⁺ ions with a good linear response in the 0~50 μM Hg²⁺ ions concentration range, and the lowest limit of detection (LOD) is 2.54 nM, suggesting that the N-CDs have great potential for applications in the fields of environmental and hazard detection. Further studies reveal that the different d orbital energy levels of Hg²⁺ compared to those of other metal ions can affect the efficiency of electron transfer and thereby result in their different response in fluorescence quenching towards N-CDs.

Quantum dots (QDs) show a great potential for light-emitting diodes (LEDs) packaging, which still face great challenges compared with the matured phosphor downconversion materials. These are probably caused by the unique scattering and absorption properties of QDs, which are extremely different to the traditional phosphor due to their several nanometers size, while their effect on QDs-converted LEDs (QCLEDs) is barely studied. In this paper, we have experimentally and theoretically investigated the effect of scattering and absorption of CdSe/ZnS QDs on the optical performance for QCLEDs by comparing with the traditional yttrium aluminum garnet phosphor. Results indicate that the strong absorption (reabsorption) of QDs causes low radiant efficacy and stability for QCLEDs; their weak scattering also leads to a low color uniformity. It demonstrates that their unique scattering and absorption properties are key factors leading to low optical performance of QCLEDs compared with the traditional phosphor-converted LEDs. For purpose of gaining the white LED with high efficiency and stability, it is highly suggested to use a low QD concentration to reduce the reabsorption loss and the total internal reflection loss. We believe that this paper can provide a better understanding of improving the optical performance for QCLEDs from the prospective of scattering and absorption. In the future, it is important to use low QD concentration to gain high-performance white LEDs with high downconversion efficiency by optimizing packaging structures.

We demonstrate an ultrasonication-assisted synthesis without polar solvent of CsPbBr₃ and Cs₄PbBr₆ perovskite nanocrystals (PNCs) and their reversible transformation. The as-prepared CsPbBr₃ PNCs and Cs₄PbBr₆ PNCs exhibit different optical properties that depend on their morphology, size, and structure. The photoluminescence (PL) emission and quantum yield (QY) of the CsPbBr₃ PNCs can be tuned by changing the ultrasound power, radiation time, and the height of the vibrating spear. The optimized CsPbBr₃ PNCs show a good stability and high PL QY of up to 85%. In addition, the phase transformation between CsPbBr₃ PNCs and Cs₄PbBr₆ PNCs can be obtained through varying the amount of oleylamine (OAm) and water. The mechanism of this transformation between the CsPbBr₃ PNCs and Cs₄PbBr₆ PNCs and their morphology change are studied, involving ions equilibrium, anisotropic growth kinetics, and CsBr-stripping process.