Zhengmao Yin

Enhancement of light extraction in GaN-based light-emitting diodes using rough beveled ZnO nanocone arrays

A remarkable enhancement of light extraction efficiency in GaN-based blue light-emitting diodes (LEDs) with rough beveled ZnO nanocone arrays grown on the planar indium tin oxide (ITO) layer is reported. The light output power of LEDs with rough beveled ZnO nanocone arrays was increased by about 110% at 20 mA compared with conventional LEDs with planar ITO. The light extraction efficiency of GaN-based LEDs with rough-beveled ZnO nanocones is measured much greater than with smooth-surface hexagonal ZnO nanorods. The light-ray tracing analysis showed that ZnO nanocones with rough surfaces enlarge the light escape cone of GaN-based LEDs and have a greater advantage for extracting light compared with ZnO nanorods.

Improving the Quality of GaN Crystals by Using Graphene or Hexagonal Boron Nitride Nanosheets Substrate

The progress in nitrides technology is widely believed to be limited and hampered by the lack of high-quality gallium nitride wafers. Though various epitaxial techniques like epitaxial lateral overgrowth and its derivatives have been used to reduce defect density, there is still plenty of room for the improvement of gallium nitride crystal. Here, we report graphene or hexagonal boron nitride nanosheets can be used to improve the quality of GaN crystal using hydride vapor phase epitaxy methods. These nanosheets were directly deposited on the substrate that is used for the epitaxial growth of GaN crystal. Systematic characterizations of the as-obtained crystal show that quality of GaN crystal is greatly improved. The fabricated light-emitting diodes using the as-obtained GaN crystals emit strong electroluminescence under room illumination. This simple yet effective technique is believed to be applicable in metal-organic chemical vapor deposition systems and will find wide applications on other crystal growth.

Growth of single-crystalline rutileTiO2 nanorod arrays on GaN light-emitting diodes with enhanced light extraction

TiO2 is a wide band gap semiconductor with important applications in photovoltaic cells and photocatalysis. In this paper, single-crystalline rutileTiO2 nanorod arrays were fabricated on the GaN based light emitting diodes (LEDs) with a TiO2 seed layer by an acid hydrothermal process. The morphologies and the crystallinity of the rutileTiO2 nanorod arrays on GaN wafer were characterized by scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. The optical properties of surface-textured TiO2/GaN were measured and analyzed by photoluminescence. The influence and dependence of the TiO2 nanorod array with the different densities and diameters on the light output of the fabricated GaN wafers were investigated. The light extraction efficiency of GaN LEDs was enhanced by the single-crystalline rutileTiO2 nanorod array, which showed an eight-fold increase in photoluminescence intensity compared to the normal planar surface. The mechanisms for the enhanced light output of GaN LEDs by the single-crystalline rutileTiO2 nanorod arrays were discussed.

Synthesis and characterization of a novel cyanostilbene derivative and its initiated polymers: aggregation-induced emission enhancement behaviors and light-emitting diode applications

A novel cyanostilbene derivative (I) is synthesized and sufficiently characterized, which is also utilized as the initiator to prepare poly(e-caprolactone) (I-PCL) and poly(D,L-lactide) (I-PLA) via ring-opening polymerization (ROP). Notably, molecule I exhibits superior aggregation-induced enhanced emission (AIEE) behavior in the mixed solvent of THF and water. The bulky cyano groups are linked to vinylene moieties in molecule (I) generating intramolecular coplanarity and preventing π–π stacking, which leads to enhanced emission in the aggregated state. Crystallographic data of I confirm that the existence of multiple intermolecular interactions among adjacent molecules restricts intramolecular vibration and rotation. In addition, molecule I can self-organize into ordered microtubes in appropriate THF/water mixtures exhibiting significant optical waveguide properties. Furthermore, the polymers, I-PCL and I-PLA, also show AIEE behaviors since the single chromophore I is anchored on the polymer chains. As compared with the corresponding small molecule I, the target polymers possess not only preferable thermostabilities but also the property of easy fabrication. The luminogen aggregation of the initiator is well-realized and even enlarged after polymerization, due to the linkage of polymer chains. In the end, the polymer I-PLA has a great prospect in white light-emitting diode (LED) application, which emits daylight white light.

Light Extraction Enhancement of GaN LEDs by Hybrid ZnO Micro-Cylinders and Nanorods Array

A hybrid patterned ZnO micro-cylinders and nanorods array (MCNR) was fabricated to improve the light extraction efficiency for high brightness GaN-based light-emitting diodes (LEDs). The light extraction efficiency of the LEDs with the ZnO MCNR is increased by 86.4% compared with conventional planar LEDs. LEDs with ZnO MCNR have greater light extraction efficiency than those only with ZnO micro-cylinders or ZnO nanorods array. The optical-electrical characteristics and far-field radiation patterns are measured and analyzed. The local optical field distribution patterns of the LEDs with ZnO micro-cylinders, ZnO nanorods, and ZnO MCNR are investigated using confocal scanning electroluminescence microscopy. The higher light extraction enhancement effect of ZnO MCNR is attributed to that it combines the light outputting effect from side walls of ZnO micro-cylinders and the light scattering effect of ZnO nanorods array.

Light transmission enhancement from hybrid ZnO micro-mesh and nanorod arrays with application to GaN-based light-emitting diodes

A hybrid ZnO micro-mesh and nanorod arrays (MMNR) was fabricated as a light output window for GaN-based light-emitting diodes (LEDs) to enhance the light extraction efficiency. The light output power of GaN-based LEDs with the ZnO MMNR is improved by 95% compared to the original planar LEDs. The ZnO MMNR is manufactured by photolithography techniques and a two-step wet chemical growth process. The incident angle-resolved light transmission of the ZnO MMNR beyond the critical angle of total internal reflection is greatly enhanced. The light diffraction pattern of the ZnO MMNR shows that it possesses both the two-dimensional diffraction grating effect of a ZnO micro-mesh and the light scattering effect of a ZnO nanorod array. LEDs with the ZnO MMNR have greater light extraction efficiency than those with only a ZnO micro-mesh or a ZnO nanorod array. The local optical field patterns of the ZnO micro-mesh and the ZnO MMNR are investigated using confocal scanning electroluminescence microscopy. The microscopic light extraction mechanism of the ZnO MMNR is analyzed in-depth.

White Light-Emitting Diodes Based on Individual Polymerized Carbon Nanodots

A search for new phosphor materials that exhibit high light-emission, spectral purity, long-time stability and processability capture particular attention to modern solid-state lighting. Here, polymerizable silane pre-functionalized carbon dot (SiCD) fluids were dripped and co-polymerized or completely bulk polymerized to build color conversion and encapsulation coatings of commercially available GaN blue LEDs. Most parameters of SiCD-based white LEDs were similar to or even better than those of phosphor-based white LEDs, particularly the insensitivity to excitation wavelength and working current. Thus, SiCDs were superior to those phosphors in terms of broadband properties, high transparency (no light blocking and leaking), as well as arbitrary doping of its content as color conversion and encapsulation layers simultaneously, unique solubility, flexible chemical, optical and mechanical processability. Thus, designing new CD-based white LEDs, instead of inorganic rare earth phosphor-based LEDs, is possible for better performance solid state lighting devices.