Haochen Liu

Optically Active CdSe-Dot/CdS-Rod Nanocrystals with Induced Chirality and Circularly Polarized Luminescence

Ligand-induced chirality in semiconductor nanocrystals (NCs) has attracted attention because of the tunable optical properties of the NCs. Induced circular dichroism (CD) has been observed in CdX (X = S, Se, Te) NCs and their hybrids, but circularly polarized luminescence (CPL) in these fluorescent nanomaterials has been seldom reported. Herein, we describe the successful preparation of l- and d-cysteine-capped CdSe-dot/CdS-rods (DRs) with tunable CD and CPL behaviors and a maximum anisotropic factor ( glum) of 4.66 × 10-4. The observed CD and CPL activities are sensitive to the relative absorption ratio of the CdS shell to the CdSe core, suggesting that the anisotropic g-factors in both CD and CPL increase to some extent for a smaller shell-to-core absorption ratio. In addition, the molar ratio of chiral cysteine to the DRs is investigated. Instead of enhancing the chiral interactions between the chiral molecules and DRs, an excess of cysteine molecules in aqueous solution inhibits both the CD and CPL activities. Such chiral and emissive NCs provide an ideal platform for the rational design of semiconductor nanomaterials with chiroptical properties.

Electric Bias Induced Degradation in Organic-Inorganic Hybrid Perovskite Light-Emitting Diodes

For organic-inorganic perovskite to be considered as the most promising materials for light emitting diodes and solar cell applications, the active materials must be proven to be stable under various conditions, such as ambient environment, heat and electrical bias. Understanding the degradation process in organic-inorganic perovskite light emitting diodes (PeLEDs) is important to improve the stability and the performance of the device. We revealed that electrical bias can greatly influence the luminance and external quantum efficiency of PeLEDs. It was found that device performance could be improved under low voltage bias with short operation time, and decreased with continuous operation. The degradation of perovskite film under high electrical bias leads to the decrease of device performance. Variations in the absorption, morphology and element distribution of perovskite films under different electrical bias revealed that organic-inorganic perovskites are unstable at high electrical bias. We bring new insights in the PeLEDs which are crucial for improving the stability.

Alloyed multi-shell quantum dots with tunable dual emission

In this work, we synthesized ZnxCd1−xSe/ZnS/ZnyCd1−ySe/ZnS dual emission-alloyed QDs for the first time. Through introducing a ZnS barrier layer, we avoid exciton diffusion to realize dual emission. By adjusting the composition of the core and the quantum well (QW) layer, the emission wavelength can be effectively tuned from 550 to 710 nm and 500 to 550 nm, respectively. The photoluminescence (PL) intensity of the QW layer can also be adjusted by changing the annealing time and the thickness of the outmost ZnS layer, which was the result of reduction of the defects of the QW layer. The highest quantum yield (QY) was over 50% for the dual emission-alloyed QDs. Moreover, this material also exhibited high photostability, and even being exposed to 365 nm Hg lamp irradiation for 12 h, the QY was still maintained at about 85% of the unirradiated level. The energy transfer efficiency between the core and the QW is calculated to be 48% using time-resolved PL spectra.

Less-Lead Control toward Highly Efficient Formamidinium-Based Perovskite Light-Emitting Diodes

A formamidinium (FA)-based perovskite is an ideal option for the potential efficient light-emitting diode (LED) in view of its high tolerance factor closer to 1. In this work, FA cation-based perovskite nanocrystals FA0.8Cs0.2Pb xBr3 ( x = 1.0, 0.8, 0.7, and 0.6) are fabricated with stoichiometric modification. The adoption of less-lead precursor is confirmed to be a feasible and effective approach in inhibiting nonradiative recombination by diminishing the presence of uncoordinated metallic Pb atoms. Note that the subsequent devices require the optimized lead ratio for an optimum behavior, a clear influence of Pb ratio on a perovskite LED has been established. No surprisingly, the less-lead perovskites exert positive roles on the perovskite LED performance, not only in terms of efficiency but also in stability. With an optimized composition FA0.8Cs0.2Pb0.7Br3, the perovskite LED displays the prominent performance with a current efficiency of 28.61 cd A-1, about 11-fold improvement than the previous best record of pure FA-based perovskite. Additionally, the perovskite device degradation can be mitigated under operating conditions by properly altering precursor stoichiometry, which can be attributed to the hydrogen reaction under moisture-induced ambient. The stoichiometric optimization of the metal Pb in the perovskite is an important strategy on the road to the further development of perovskite LEDs.