Caiman Yan

Efficient synthesis of highly fluorescent carbon dots by microreactor method and their application in Fe3 + ion detection

Rapidly obtaining strong photoluminescence (PL) of carbon dots with high stability is crucial in all practical applications of carbon dots, such as cell imaging and biological detection. In this study, we proposed a rapid, continuous carbon dots synthesis technique by using a microreactor method. By taking advantage of the microreactor, we were able to rapidly synthesized CDs at a large scale in less than 5min, and a high quantum yield of 60.1% was achieved. This method is faster and more efficient than most of the previously reported methods. To explore the relationship between the microreactor structure and CDs PL properties, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were carried out. The results show the surface functional groups and element contents influence the PL emission. Subsequent ion detection experiments indicated that CDs are very suitable for use as nanoprobes for Fe3+ ion detection, and the lowest detection limit for Fe3+ is 0.239μM, which is superior to many other research studies. This rapid and simple synthesis method will not only aid the development of the quantum dots industrialization but also provide a powerful and portable tool for the rapid and continuous online synthesis of quantum dots supporting their application in cell imaging and safety detection.

Full spectral optical modeling of quantum-dot-converted elements for light-emitting diodes considering reabsorption and reemission effect

Quantum dots (QDs) have attracted significant attention in light-emitting diode (LED) illumination and display applications, owing to their high quantum yield and unique spectral properties. However, an effective optical model of quantum-dot-converted elements (QDCEs) for (LEDs) that entirely considers the reabsorption and reemission effect is lacking. This suppresses the design of QDCE structures and further investigation of light-extraction/conversion mechanisms in QDCEs. In this paper, we proposed a full spectral optical modeling method for QDCEs packaged in LEDs, entirely considering the reabsorption and reemission effect, and its results are compared with traditional models without reabsorption or reemission. The comparisons indicate that the QDCE absorption loss of QD emission light is a major factor decreasing the radiant efficacy of LEDs, which should be considered when designing QDCE structures. According to the measurements of fabricated LEDs, only calculation results that entirely consider reabsorption and reemission show good agreement with experimental radiant efficacy, spectra, and peak wavelength at the same down-conversion efficiency. Consequently, it is highly expected that QDCE will be modeled considering the reabsorption and reemission events. This study provides a simple and effective modeling method for QDCEs, which shows great potential for their structure designs and fundamental investigations.

Tuning the emission spectrum of highly stable cesium lead halide perovskite nanocrystals through poly(lactic acid)-assisted anion-exchange reactions

We demonstrate an organic macromolecule-assisted anion-exchange reaction method for tuning the emission spectrum of cesium lead halide perovskite (CsPbX3) nanocrystals (NCs) from green to near-ultraviolet using a microreactor. Using poly(lactic acid) (PLA), the emission peak of CsPbX3 NCs can be tuned from 514 nm to 420 nm while maintaining high photoluminescence (PL) quantum yields (QYs) of 33–90%. By taking advantage of the microreactor, we synthesize parent CsPbBr3 NCs and complete anion-exchange reactions at the same time, which is more efficient than most previously reported methods. The stability of CsPbX3 NCs is improved by PLA coating, especially for CsPbCl3, which shows long-term stability under ambient conditions for at least two weeks. The CsPbBr3 NCs are utilized with a red phosphor on a blue light emitting-diode (LED) chip, achieving white light emission with a luminous efficacy of 62.93 lm W−1 under a 20 mA driving current. Highly efficient white LEDs (wLEDs) demonstrate the potential of halide perovskite NCs for optoelectronic applications, including low-cost displays, lighting, and optical communication.

Effect of ZnO nanostructures on the optical properties of white light-emitting diodes

White light produced by blue LEDs with yellow phosphor is the most widely used methods, but it results in poor quality in angular CCT uniformity. In this work, a novel technique was introduced to solve this problem by integrating different ZnO nanostructures into white light-emitting diodes. The experiment of ZnO doped films and the simulation of Finite-Difference Time-Domain (FDTD) were carried out. The result indicated scattering effect of ZnO nanoparticles could improve uniformity of scattering energy effectively. Moreover, the effect of ZnO nanostructures on white light-emitting diodes (wLEDs) devices was also investigated. The CCT deviation of wLEDs devices would decrease from 3455.49 K to 96.30 K, 40.03 K and 60.09 K when the node-like (N-ZnO), sheet-like (S-ZnO) and rod-like ZnO (R-ZnO) respectively applied. The higher CCT uniformity and little luminous flux dropping were achieved when the optimal concentrations of N-ZnO, S-ZnO, and R-ZnO nanostructures were 0.25%, 0.75%, and 0.25%. This low-cost and green manufacturing method has a great impact on development of white light-emitting diodes.

Improvement in Luminous Efficacy and Thermal Performance Using Quantum Dots Spherical Shell for White Light Emitting Diodes

White light-emitting diodes (WLEDs) based on quantum dots (QDs) are gaining increasing attention due to their excellent color quality. QDs films with planar structure are universally applied in WLEDs for color conversion, while they still face great challenges in high light extraction and thermal stability. In this study, a QDs film with a spherical shell structure was proposed to improve the optical and thermal performance for WLEDs. Compared with the conventional planar structure, the luminous efficacy of the QDs spherical shell structure is improved by 12.9% due to the reduced total reflection effect, and the angular-dependent correlated color temperature deviation is decreased from 2642 to 283 K. Moreover, the highest temperature of the WLED using a QDs spherical shell is 4.8 °C lower than that of the conventional WLED with a planar structure, which is mainly attributed to larger heat dissipation area and separated heat source. Consequently, this QDs spherical shell structure demonstrates superior performance of QDs films for WLEDs applications.

Regulating the Emission Spectrum of CsPbBr3 from Green to Blue via Controlling the Temperature and Velocity of Microchannel Reactor

The ability to precisely obtain tunable spectrum of lead halide perovskite quantum dots (QDs) is very important for applications, such as in lighting and display. Herein, we report a microchannel reactor method for synthesis of CsPbBr3 QDs with tunable spectrum. By adjusting the temperature and velocity of the microchannel reactor, the emission peaks of CsPbBr3 QDs ranging from 520 nm to 430 nm were obtained, which is wider than that of QDs obtained in a traditional flask without changing halide component. The mechanism of photoluminescence (PL) spectral shift of CsPbBr3 QDs was investigated, the result shows that the supersaturation control enabled by the superior mass and heat transfer performance in the microchannel is the key to achieve the wide range of PL spectrum, with only a change in the setting of the temperature controller required. The wide spectrum of CsPbBr3 QDs can be applied to light-emitting diodes (LEDs), photoelectric sensors, lasers, etc.

ACU improvement of CCT-tunable LED device by electrospun nanofiber film

We fabricated PMMA nanofiber composite diffusion films to improve the angular color uniformity (ACU) of correlated-color-temperature-tunable light emitting diode (CCT-tunable LED) device. Based on electrospinning technology, we prepared diffusion films with different scattering properties by controlling the fiber deposition time. Detailed optical measurement shows that the scattering intensity of PMMA nanofiber films increases with the increase of thickness. The diffuse transmittance and haze are 88%, 96.6% in the visible light, respectively. Furthermore, when applied to the CCT-tunable LED device, the device witnesses a reduced CCT deviation from 630 K to 159 K in a typical CCT range of 5000– 6000 K. Therefore, the proposed nanofiber films can be applied to lighting systems as diffusion films to improve their ACU.

Effect of chips surface morphology on optical performance of remote and conformal phosphor-converted light-emitting diodes

In this paper, we use optical simulations to study the influence of chips surface morphology on remote phosphor-converted light-emitting diodes (rpcLEDs) and conformal phosphor-converted light-emitting diodes (cpcLEDs), respectively. Their radiant efficacy, luminous efficacy, spectra and correlated color temperature (CCT) are given and discussed. Results show that the rough surface more contributes to the light extraction of rpcLEDs compared with that of cpcLEDs; moreover, rpcLEDs have a more excellent CCT consistency when using vertical LED chips with different surface morphology. It can provide a new perspective to understand the differences between the remote and conformal phosphor structures.

Color Uniformity Enhancement for WLEDs Using Inverted Dispensing Method

The inverted dispensing (ID) method is proposed to fabricate remote phosphor layers (RPLs) with cone-like shapes for white light-emitting diodes (WLEDs) to improve their color uniformity. We introduce surface evolver packages and optical simulations to study the effect of the surface tension on the optical performance of these WLEDs during the ID process. The results show that a decreasing surface tension contributes to a sharpened geometry of the RPL. This sharpened surface geometry is responsible for the optimization of the color uniformity by deflecting blue light towards central angles. The proposed ID method is simple and efficient. The color uniformity performance can be tuned by selecting silicone with a proper surface tension without affecting the total correlated color temperature and sacrificing the WLED’s optical power.