Shu Fen Hu

Thermally stable luminescence of KSrPO4:Eu2+ phosphor for white light UV light-emitting diodes

A novel blue phosphor based on phosphate host matrix, KSrPO4 doped with Eu2+, was prepared by solid state reaction. The phosphor invariably emits blue luminescence with a peak wavelength at 424nm under ultraviolet excitation at 360nm. Eu2+-doped KSrPO4 phosphors show higher thermally stable luminescence which was found to be better than commercially available Y3Al5O12:Ce3+ phosphor at temperature higher than 225°C.

Quantum dot monolayer sensitized ZnO nanowire-array photoelectrodes: True efficiency for water splitting

Increasing demand for clean energy has motivated considerable effort to exploit the properties of various materials in photovoltaics and related solar-harvesting devices. Splitting of water by sunlight to generate hydrogen is one of the forms of energy production with the most potential. Metal oxides such as TiO2, ZnO, and WO3 with various morphologies have been investigated for use in splitting water. However, most of these metal oxides have large band gaps, which limit light absorption in the visible region and overall efficiency. To reduce the band gaps of nanostructured metal oxides, doping and utilization of transition metals, carbon, or nitrogen have been investigated. One possibility is the use of semiconductor nanocrystals, known as quantum dots (QDs), as an alternative to photosensitive dyes. Quantum dots generally offer various significant advantages over dyes. It was recently established that QDs generate multiple electron– hole pairs per photon, improving device efficiency. Quantum dot sensitized nanostructures are widely studied for use in solar cells. However, little work has been done on metal oxide and semiconductor QD-based composite structures for use in water-splitting nanodevices. To elucidate this fundamental issue, we examined a combination of CdTe QDs and ZnO nanowires for splitting water photoelectrochemically (Scheme 1). One-dimensional nanostructures offer the additional potential advantage of improved charge transport over zero-dimensional nanostructures such as nanocrystals. Additionally, the typical electron mobility in ZnO is 10–100 times higher than that in TiO2, so the electrical resistance is lower and the electron-transfer efficiency higher. However, since the overall water-splitting reaction is tough, sacrificial reagents are commonly adopted to evaluate the photocatalytic activity for water splitting. When the photocatalytic reaction is carried out in an aqueous solution that contains a reductant, electron donors, or hole scavengers such as sulfide ions or selenium ions, photogenerated holes irreversibly oxidize the reductant rather than the water. Employment of CdTe QDs in water splitting system has major advantages. CdTe with a more favorable conduction band energy (ECB= 1.0 V vs. NHE) can inject electrons into ZnO faster than CdSe (ECB= 0.6 V vs. NHE). In addition, monolayer deposition of CdTe QDs on the surface of ZnO nanowires would further improve the stability in electrochemical reaction, by avoiding anodic decomposition/corrosion of CdTe and thus enhancing the overall watersplitting performance. During the photoirradiation of CdTe, two reactions can be expected to dominate after initial charge separation [Eqs. (1) and (2)].

Origin of thermal degradation of Sr 2-x Si 5 N 8 :Eu x phosphors in air for light-emitting diodes

The orange-red emitting phosphors based on M(2)Si(5)N(8):Eu (M = Sr, Ba) are widely utilized in white light-emitting diodes (WLEDs) because of their improvement of the color rendering index (CRI), which is brilliant for warm white light emission. Nitride-based phosphors are adopted in high-performance applications because of their excellent thermal and chemical stabilities. A series of nitridosilicate phosphor compounds, M(2-x)Si(5)N(8):Eu(x) (M = Sr, Ba), were prepared by solid-state reaction. The thermal degradation in air was only observed in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.10, but it did not appear in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.02 and Ba analogue with x = 0.10. This is an unprecedented investigation to study this phenomenon in the stable nitrides. The crystal structural variation upon heating treatment of these compounds was carried out using the in situ XRD measurements. The valence of Eu ions in these compounds was determined by electron spectroscopy for chemical analysis (ESCA) and X-ray absorption near-edge structure (XANES) spectroscopy. The morphology of these materials was examined by transmission electron microscopy (TEM). Combining all results, it is concluded that the origin of the thermal degradation in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.10 is due to the formation of an amorphous layer on the surface of the nitride phosphor grain during oxidative heating treatment, which results in the oxidation of Eu ions from divalent to trivalent. This study provides a new perspective for the impact of the degradation problem as a consequence of heating processes in luminescent materials.

Combinatorial approach to the development of a single mass YVO4: Bi3+,Eu3+ Phosphor with red and green dual colors for high color rendering white light-emitting diodes

Instead of developing a novel red phosphor individually, this work proposes the production of white light by combining a near-ultraviolet/ultraviolet diode chip with blue and special yellow phosphors: the yellow phosphor includes the red and green components with high color saturation. The availability of this scheme is demonstrated by preparing a white light-emitting diode (WLED) with color rendering index (Ra) up to 90.3. The desired single-mass yellow phosphor is successfully screened out from the YVO(4):Bi(3+),Eu(3+) system by using a combinatorial chemistry approach. When the emission color and luminous efficiency are both considered, the best composition for producing white light is (Y(1-s-t)Bi(s)Eu(t))VO(4) with 0.040 < or = s < or = 0.050 and 0 < t < or = 0.015. The red component that is required for a high-Ra WLED is obtained through sensitizing luminescence of Eu(3+) by Bi(3+) in a YVO(4) host; meanwhile, both Bi(3+) and Eu(3+) emission are improved by keeping the Bi(3+) and Eu(3+) contents close to the critical concentration.

High-performance lithium-ion battery and symmetric supercapacitors based on FeCo2O4 nanoflakes electrodes

A successive preparation of FeCo2O4 nanoflakes arrays on nickel foam substrates is achieved by a simple hydrothermal synthesis method. After 170 cycles, a high capacity of 905 mAh g(-1) at 200 mA g(-1) current density and very good rate capabilities are obtained for lithium-ion battery because of the 2D porous structures of the nanoflakes arrays. The distinctive structural features provide the battery with excellent electrochemical performance. The symmetric supercapacitor on nonaqueous electrolyte demonstrates high specific capacitance of 433 F g(-1) at 0.1 A g(-1) and 16.7 F g(-1) at high scan rate of 5 V s(-1) and excellent cyclic performance of 2500 cycles of charge-discharge cycling at 2 A g(-1) current density, revealing excellent long-term cyclability of the electrode even under rapid charge-discharge conditions.

Highly stable three-band white light from an InGaN-based blue light-emitting diode chip precoated with (oxy)nitride green/red phosphors

A three-band white light-emitting diode (LED) was fabricated using an InGaN-based blue LED chip that emits 455nm blue light, and green phosphor SrSi2O2N2:Eu and red phosphor CaSiN2:Ce that emit 538nm green and 642nm red emissions, respectively, when excited by the 455nm blue light. The luminous efficacy of this white LED is about 30lm∕W at a dc of 20mA. With increasing dc from 5.0to60mA, both the coordinates x and y of the white LED tend to be the same, and consequently the Tc is the same and the Ra increases to 92.2.

Near-ultraviolet excitable orange-yellow Sr3(Al2O5)Cl2:Eu2+ phosphor for potential application in light-emitting diodes

Sr3(Al2O5)Cl2 phosphor doped with Eu2+ was prepared by a soli-state reaction. This phosphor emits a broad orange-yellow luminescence with a peak wavelength of 620nm and a full width at half maximum of about 175nm under near-ultraviolet (NUV) excitation at ∼400nm. Yellow light-emitting diodes (LEDs) for general lighting were fabricated by combining Sr3(Al2O5)Cl2:Eu2+ phosphor with an NUV chip. The phosphor-converted LEDs had a color temperature of about 2300K and their color rendering index was 74.

A low-temperature co-precipitation approach to synthesize fluoride phosphors K2MF6:Mn4+ (M = Ge, Si) for white LED applications

A new class of Mn4+ activated alkali-metal hexafluoride red phosphors are emerging for white light-emitting diodes because of their sharp red line 2Eg → 4A2g emissions (600–650 nm) excited by irradiation of 4A2g → 4T1g (320–380 nm) and 4A2g → 4T2g (380–500 nm) transitions. However, these phosphors have the drawbacks of difficult control of the Mn valence state during synthesis and lack of underlying mechanisms for structure–photoluminescence relationships. In this study, we explore a novel, highly productive route to the quantifiable synthesis of K2GeF6:Mn4+ by the chemical co-precipitation method at room temperature. The prepared yellowish K2GeF6:Mn4+ powders exhibit a hexagonal shape and high crystallinity without significant defects. The photoluminescence thermal stability and white light-emitting diodes applicability of K2GeF6:Mn4+ suggest that it is a promising commercial red phosphor because of its efficient emission intensity, high color purity and excellent thermal stability. Structural analyses and theoretical calculations reveal that the red shift of the K2GeF6:Mn4+ red phosphor compared with K2SiF6:Mn4+ is due to the longer Ge–F distance and lower effective Mulliken charge of F ions in coordination environments of the MnF62− octahedron. The split feature in K2GeF6:Mn4+ is due to the hexagonal distortion in the host. The structure–photoluminescence mechanism is predicted to be general in hexafluoride red phosphors to tune the optical properties through cationic substitutions and crystal structure adjustments.

Full-Color and Thermally Stable KSrPO4 : Ln ( Ln = Eu , Tb , Sm ) Phosphors for White-Light-Emitting Diodes

The novel phosphors of KSrPO 4 doped with Eu 2+ , Tb 3+ , and Sm 3+ were synthesized by solid-state reaction, and their luminescence properties were investigated. The phosphors show pure three-basal-color (red-green-blue) luminescence in the CIE1931 chromaticity diagram, and it was observed that the concentration quenching was dependent on the different dopant contents. This series of phosphate-based phosphors shows higher thermally stable luminescence, which was found to be better than commercially available Y 3 Al 5 O 12 :Ce 3+ phosphor at temperatures higher than 200°C. Considering the situation of high color-rendering index and chemical stability, we have demonstrated that KSrPO 4 :Ln (Ln = Eu, Tb, Sm) are potentially useful new scintillation materials for white-light-emitting diodes.

Enhanced luminescence of SrSi2O2N2:Eu2+ phosphors by codoping with Ce3+, Mn2+, and Dy3+ ions

The authors report here the enhanced luminescence properties of SrSi2O2N2 doped with Eu and M (M=Ce, Dy, and Mn). The Eu and Eu, Mn-codoped powders were prepared by a solid state reaction at temperatures between 1400 and 1600°C under H2 (25%)–N2 (75%) atmosphere. The Eu, M-codoped Sr1−x−ySi2N2O2 phosphors have the monoclinic structure with lattice parameters a∼15.6A, b∼16.2A, c∼9.4A, and β∼91°. The phosphors can be efficiently excited in the UV to visible region, making them attractive as conversion phosphors for a light emitting diode application. A green-yellow emission was observed for Eu,M-codoped Sr1−x−ySi2N2O2. The addition of M in the Eu site in SrSi2O2N2 remarkably enhances the luminescent intensity by the factor of 144%, 148%, and 168% for Ce, Dy, and Mn, respectively.