Shu-Ru Chung

Zn(x)Cd(1-x)S quantum dots-based white light-emitting diodes.

In this study, two kinds of colloidal ternary semiconductor white light-emitting quantum dots (WQDs), Zn(0.5)Cd(0.5)S and Zn(0.8)Cd(0.2)S, are prepared and used as nanophosphors in a UV light-emitting diode (UV-LED) pumping device. When the weight ratio of Zn(0.5)Cd(0.5)S WQDs is 9.1 wt. % in silicone and the drive current is set at 20 mA, the chromaticity coordinates (CIE), correlated color temperature (CCT), color rendering index (CRI), and luminous efficiency are (0.43,0.37), 2830 K, 90, and 0.94 lm/W, respectively. On the other hand, under the same weight ratio in silicone, the CIE, CCT, CRI, and luminous efficiency of Zn(0.8)Cd(0.2)S WQDs are (0.36,0.33), 4240 K, 86, and 4.12 lm/W, respectively. Based on the above results, we can conclude that WQDs-based LED can be obtained by controlling the compositions of Zn(x)Cd(1-x)S QDs due to the coexistence of band-edge and surface state emission.

The effect of surface structures and compositions on the quantum yields of highly effective Zn0.8Cd0.2S nanocrystals

We have synthesized white light emitting ternary Zn0.8Cd0.2S nanocrystals with high quantum yields of about 99%. Through detailed surface and structural characterization with the investigation of Cd and Zn coordination numbers, it can be found that besides surface state emission, the highly effective white light emission is strongly related to the surface chemical states of Zn and Zn–O/Cd–O bonding.

Correlation between surface state and band edge emission of white light ZnxCd1−xS nanocrystals

In this study, we have demonstrated a facile thermal pyrolyzed route to prepare ternary ZnxCd1−xS nanocrystals (NCs) with high quantum yield (QY) and white light emission. The detailed surface and structural characterizations based on the X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) measurements have illustrated that by controlling the Zn content, the surface chemical states as well as the surface state emission of NCs can be manipulated, and their optical properties can be tuned accordingly The Zn shell of NCs may enhance the surface state emission, especially for Zn0.8Cd0.2S, thereby promoting the white light emission and increasing the QY to 56%. Moreover, the QY of Zn0.8Cd0.2S decreases from 56 to 49% after aging in hexane for 2 months, implying that the stability of NCs is excellent. The surface oxide-rich structure could be the reason that influences the surface state emission and stability of the ZnxCd1−xS NCs.

Promotion of PdCu/C Catalysts for Ethanol Oxidation in Alkaline Solution by SnO2 Modifier

The development of high performance PdCu/C catalysts through manipulation of reduction temperatures and addition of SnO2 promoter has been elucidated systematically in order to enhance ethanol oxidation reaction (EOR) performance. The structure, morphology, surface species, and electroactivity of the PdCu/C and SnO2‐modified PdCu/C catalysts are characterized by use of X‐ray diffraction, high resolution transmission electron microscopy, X‐ray photoelectron spectroscopy, and cyclic voltammetry. For the PdCu/C catalysts, the ratio of the forward anodic peak current density to that of the reverse, that is, the If/Ib ratio, the degree of alloying (Dalloy), grain size, and EOR performance increases as the reduction temperature increases to 570 K. However, severe sintering at 670 K deteriorates EOR performance. Therefore, 570 K is a suitable reduction temperature for the preparation of PdCu/C catalysts with fine alloy structure, small crystal size, good Dalloy, good anti‐poisoning ability, and excellent EOR performance. For the SnO2 modified PdCu/C catalysts, addition of 20 wt % Sn may cause a large amount of SnO2 to form on the surface and block the active site of the catalysts, thus decreasing EOR performance. However, addition of 10 wt % Sn promotes stability in EOR performance, attributable to the significant amount of PdSn phase in the bulk and SnO2 species on the surface, which enhance the CO oxidation ability and electrochemical surface area. Consequently, based on the chronoamperometric measurements after 1 hr, the EOR activity of PdCu/C catalysts with 10 wt % SnO2 modification is enhanced by approximately 230 % compared with the as‐reduced sample.

Promotion of solid-state lighting for ZnCdSe quantum dot modified-YAG-based white light-emitting diodes

In this study, the effect of quantum dot (QD) addition and photoluminescence quantum yield (PL QY) of QDs on the luminous efficacy and color rendering index (CRI) of white light-emitting diodes (LEDs) has been investigated. Two kinds of red-emitting materials, ZnCdSe QDs and commercial CaSiAlN3:Eu2+ (nitride) phosphor, are mixed with commercial Y3Al5O12:Ce3+ (YAG) phosphor as white LED materials. The as-prepared ZnCdSe QDs have a PL QY of about 44%, which drops to 29% after purification due to the removal of the surfactants. When the as-prepared QDs are used, both the CRI and luminous efficacy of the YAG/QDs-based white LED can be improved to meet the requirements of solid-state lighting (SSL), suggesting that the addition of QDs with high QY can overcome the trade-off property between CRI and luminous efficacy. Besides, the correlated color temperature (CCT) of this white LED can be tuned toward cold light by incorporating YAG with QDs, while it tunes toward warm light for a YAG/CaSiAlN3-based white LED. This special phenomenon can be solely observed in QD-modified white LEDs. Moreover, the long term operation stability of QD-modified white LEDs is excellent.

Ultra high luminous efficacy of white Zn x Cd 1-x S quantum dots-based white light emitting diodes

In this study, the colloidal ternary ZnxCd1-xS (x = 0.5 and 0.8, named as Zn0.8 and Zn0.5) white quantum dots (WQDs) have been prepared and used to form WQDs-based white light emitting diodes (WLED) with three different encapsulation types (convert, remote and conformal type). Moreover, acyclic-based UV curing polymer was used to incorporate with WQDs. The optical properties of Znx WQDs-based WLEDs including Commission Internationale de l’Eclairage (CIE) chromaticity coordinates, correlated color temperature (CCT), color rendering index (CRI) and luminous efficacy can be tuned by controlling the compositions, blending content of Znx WQDs and encapsulated methods. The optimum result for Zn0.8 and Zn0.5-based WLEDs can be observed in the remote type with the CIE, CRI, CCT and luminous efficacy under 20 mA of (0.34, 0.32), 87, 5000 K and 11.93 lm/W and (0.40, 0.37), 86, 3400 K and 2.76 lm/W, respectively. The higher luminous efficacy and stability of WLEDs can be obtained due to use an acyclic-based UV curing polymer, high QY of WQDs and suitable encapsulated method. The results also have exhibited the potential applications of Znx WQDs as nanophosphors in WLEDs.

The structure-dependent quantum yield of ZnCdS nanocrystals

In this study, we demonstrate the effect of atomic arrangement and valence band structures on the optical properties of ZnxCd1−xS nanocrystals (NCs) by controlling the Zn ratios. Our results indicate that the increase in coordination number of Zn–O bonding within the NCs and the structural changes in the valence band (VB) would enhance their quantum yield (QY) due to the influence of the surface states and charge recombination rate in VB, respectively. Consequently, the ZnxCd1−xS NCs with an optimal x = 0.4 have the highest QY of about 56%. By combining structural and optical analysis, we systematically elucidate the effect of composition on the local structure and the charge distribution in the VB of the ZnxCd1−xS NCs. Thus, this study provides mechanistic insight into the development of ZnCdS NCs with high QY.

Green light emission of Zn x Cd 1-x Se nanocrystals synthesized by one-pot method

We present a facile one-pot synthesis to prepare ternary ZnxCd1-xSe (x = 0.2, 0.5, 0.8, and 1) nanocrystals (NCs) with high emission quantumyield (QY, 45∼89%). The effect of Zn content (x) of ZnxCd...We present a facile one-pot synthesis to prepare ternary (x = 0.2, 0.5, 0.8, and 1) nanocrystals (NCs) with high emission quantum yield (QY, 45~89%). The effect of Zn content (x) of NCs on their physical properties is investigated. The NCs have a particle size of 3.2 nm and face centered cubic structure. However, the actual compositions of the NCs are Zn0.03Cd0.97Se, Zn0.11Cd0.89Se, and Zn0.38Cd0.62Se when Zn content is 0.2, 0.5, and 0.8, respectively. In terms of the optical properties, the emission wavelength shifts from 512 to 545 nm with increasing Zn content from 0 to 0.8 while the QY changes from 89 to 45, respectively. Partial replacement of Cd by Zn is beneficial to improve the QY of Zn0.2 and Zn0.5 NCs. The optical properties of ternary NCs are affected by compositional effect rather than particle size effect.

Developed one-pot synthesis of dual-color CdSe quantum dots for white light-emitting diode application

Quantum dots (QDs) have attracted tremendous attention due to their outstanding optical properties, which show great potential for next generation lighting and displays. In particular, dual-color QDs (green- and red-emitting) offer significant advantages over the conventional phosphors used in white light-emitting diode (WLED) applications, which are replacing traditional fluorescent lamps due to their improved energy efficiency. However, a facile synthesis of well-controlled dual-color QDs is still one of the major constraints on a wider application of QD-WLEDs. Here, we report a developed one-pot synthesis method to produce dual-color QDs for WLED application. The size and fraction of red and green QDs can be controlled precisely during the synthetic process using HDA and HDA/TOPO as sophisticated capping reagents, respectively, thus manifesting decent optical properties and remarkably enhanced stability. One of our samples, GR0.75 QDs, is demonstrated to produce pure white color in a fabricated WLED device up to 18 hours. Based on advanced characterization, our findings reveal the critical role of the specific capping reagent in the synthesis and structure control of dual-color QDs, while the results regarding the enhanced stability may broaden the avenues for future QD-WLED applications.

Application of quantum dots in solid state lighting

In this study, binary CdSe and ternary Zn0.5Cd0.5Se quantum dots (QDs) with monochromatic light are blended with conventional phosphors and those monochromatic lights of Zn0.5Cd0.5Se QDs are also mixed together and then packaged as remote type. Besides, white light CdSe QD are used alone to fabricate white light-emitting diode (LED). We have demonstrated that the color rendering index (CRI) and luminous efficacy can be improved by blending conventional phosphors with QDs. The Zn0.5Cd0.5Se QDs incorporated with yellow phosphors have better device performance and optical properties. Zn0.5Cd0.5Se QDs with different emission wavelength are blended with each other. The result reveals that mixing ratios and photoluminescence efficiency (PLE) between different colors of QDs are important factors. When white CdSe QD is excited by ultraviolet LED (UV-LED) without mixing multi-colors of phosphor, high CRI can be obtained for CdSe-based white LED.