Wonkeun Chung

Extremely high color rendering white light from surface passivated carbon dots and Zn-doped AgInS2 nanocrystals

In this study, highly luminescent carbon dots with diameters of 3–5 nm were synthesized via the carbonization of citric acid, and the effects of surface passivation and carbonization temperature on the optical properties were investigated. A red-shift in the emission wavelength was observed with increasing excitation wavelength, and oleylamine-capped carbon dots showed the highest quantum yield of approximately 43% when excited at 380 nm, whereas the reaction temperature had no influence on the emission wavelength and morphology of the dots. Additionally, I–III–VI AgInS2 nanocrystals (NCs) were prepared by thermal decomposition at a low temperature, and the emission wavelength was tuned by adjusting the growth temperature or introducing Zn ions, which enhanced the quantum yield up to 50%. For carbon dot application, a white LED was fabricated by combining a 380 nm UV LED with the carbon dots and Zn-doped AgInS2 NCs. White light from this LED exhibited an excellent color rendering index of greater than 95 with a warm color temperature, and demonstrated its potential for use in solid state lighting.

Fabrication of high color rendering index white LED using Cd-free wavelength tunable Zn doped CuInS2 nanocrystals.

Highly luminescent Cd-free Zn doped CuInS(2) nanocrystals (ZCIS NCs) were synthesized, and their properties were evaluated using X-ray diffraction, Raman, UV, and photoluminescence. The crystal structure of the ZCIS NCs was similar to the zinc blende, and the lattice constant decreased with increasing Zn concentration. By incorporation of Zn, the emission wavelength was tuned from 536 to 637 nm with concomitant enhancement of the quantum yield up to 45%. A white light emitting diodes, integrating dual ZCIS NCs (λ(em) = 567, and 617 nm) and a 460 nm InGaN LED, exhibited a high color rendering index of 84.1 with a warm color temperature of 4256.2K. The CIE-1931 chromaticity coordinates were slightly shifted from (0.3626, 0.3378) at 20 mA to (0.3480, 0.3206) at 50 mA.

Spray pyrolysis prepared yellow to red color tunable Sr 1-x Ca x Se:Eu 2+ phosphors for white LED

The spherical and submicron size of Sr₁-xCaxSe:Eu²⁺ phosphors were successfully prepared by ultrasonic spray pyrolysis. The phosphors adopted a cubic structure, and the replacement of Sr²⁺ with Ca²⁺ decreased the lattice parameter. The Sr₁-xCaxSe:Eu²⁺ showed broad and strong excitation under 420-460 nm blue light, and the emission band could be tuned from 565 to 607 nm by increasing the Ca²⁺ ratio in the host lattice. In addition, the doping of Zn²⁺ into Sr²⁺ or Ca²⁺ enhanced the emission intensity with a small red shift due to the change in crystal field strength and nephelauxetic effects. The warm and high CRI of white LED was achieved using blue LED pumped with blending phosphors of 612 nm emitting Ca₀.₉₈Zn₀.₀₂Se:Eu²⁺ and 565 nm emitting YAG. The correlated color temperatures and CRI were 4719.2K, and 86.3, respectively, and an acceptable color variation was also observed at operating currents ranging from 20 to 70 mA.

Characterization of surface modified ZnCuInS2 nanocrystals and its application to white light-emitting diodes.

Red-emitting ZnCuInS2 semiconductor nanocrystals (NCs) were synthesized and surface modification was performed on the NCs with oleylamine, trioctylphosphine, and 3-mercaptopropionic acid by the ligand exchange strategy. UV-visible spectroscopy, photoluminescence (PL) spectroscopy, time-resolved PL analysis, and attenuated total reflection Fourier transform infrared spectroscopy were used to verify the surface modification of NCs. Additionally, white light-emitting diodes (LEDs) were fabricated using surface modified red-emitting ZnCuInS2 NCs as red phosphor to compensate for the deficiency of red emission in white LEDs, consisting of blue LEDs as excitation sources and YAG:Ce as yellow phosphor.

White emission from CdSe/ZnSe nanoparticles combining with 400, 430 and 460nm InGaN LED

White light emitting diodes (LEDs) were fabricated combining InGaN LED chip as the excitation source and CdSe/ZnSe as the phosphor. CdSe/ZnSe nanoparticles were synthesized via wet chemical method using CdO, zinc stearate, TOP Se as precursors. 4.5 and 5.3nm diameter CdSe/ZnSe nanoparticles with emission peak at 540 and 620nm, respectively, were obtained. CdSe nanoparticle phosphors were coated onto the 460, 430 and 400nm LED chips to fabricate white LEDs. As the shorter wavelength of LED chip was applied to excitation source, the luminous efficiency of white LED was enhanced. The luminous efficiencies of 400, 430, and 460nm excitation white LEDs were 2.15, 3.96 and 6.57 lm/W, respectively at 20 mA. The corresponding CIE-1931 coordinates were (0.34, 0.30), (0.30, 0.32), and (0.31, 0.29). The color rendering index (CRI) of white LED showed similar values ; 65.1, 65.5, and 66.2.

Fabrication of highly stable silica coated ZnCuInS nanocrystals monolayer via layer by layer deposition for LED application

In this study, non-toxic and highly stable silica coated ZnCuInS NCs were synthesized by a reverse microemulsion method. The single NCs were uniformly encapsulated in a silica shell with a diameter of ~30 nm. Although hydrolyzed TEOS caused a QY reduction, and a 12.5 nm red shift occurred after silica coating, the photo and thermal stabilities were extremely improved. For LED application, the silica coated ZnCuInS NCs phosphor layer was arrayed on the InGaN LED surface by layer-by-layer deposition utilizing electrostatic attraction. When the ZnCuInS/SiO2 NCs single monolayer was fabricated, 6.73% high color conversion efficiency was achieved.

White light emission from blue InGaN LED with hybrid phosphor

In this work we fabricated white LEDs using a blue InGaN LED precoated conjugated copolymer / QDs composite (green-emitting Poly [(9,9-dioctyl-2,7-divinylenefluorenylene)-alto-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene] / red-emitting CdSe quantum dots) as hybrid phosphor. The emission peak wavelengths of blue, green and red emissions in spectra are 462 and 618 nm respectively. Green-emitting polymer has two emission peaks (515 and 543nm). The subpeak of polymer (515 nm) is covered by absorption of QDs. Therefore the emission of first peak is decreasing as increasing the ratio of QDs while the emission of QD (618 nm) is increasing. Therefore CIE-1931 coordinate, the color temperature (7c) and the color rendering index (Ra) were changed by the different ratio of QDs to copolymer. The white LED of the hybrid phosphor containing 20 wt% QDs has a luminous efficiency of 44.2 lm/W at 20 mA with a CIE-1931 coordinate of (0.3297, 0.3332), Tc of 5620 K and Ra of 75.3.