Xianqing Piao

Characterization and luminescence properties of Sr2Si5N8:Eu2+ phosphor for white light-emitting-diode illumination

Eu2+-doped ternary nitride phosphor, Sr2Si5N8:Eu2+, was prepared by the carbothermal reduction and nitridation method. The Rietveld refinement analysis showed that the single phase products were obtained. Two main absorption bands were observed on the diffuse reflection spectra peaking at about 330 and 420nm, so that the resultant phosphor can be effectively excited by InGaN light-emitting diodes. The emission peak position of (Sr1−xEux)2Si5N8:Eu2+ series varied from 618to690nm with increasing Eu2+ ion concentration. The redshift behavior of the emission band was discussed on the basis of the configuration coordination model.

Preparation of ( Sr1 − x Ca x ) 2Si5N8 ∕ Eu2 + Solid Solutions and Their Luminescence Properties

A series of ternary nitride solid solutions with a general formula of (Sr 1-x Ca x ) 2 Si 5 N 8 /Eu 2+ (2 atom %) were synthesized by the carbothermal reduction and nitridation (CRN) method. The structure and luminescence properties were characterized for practical applications as a potential red phosphor for the phosphor-converted white light-emitting diodes (LEDs). The solid solutions were formed as the uniformly single phases with orthorhombic (Pmn2 1 ) and monoclinic (Cc) symmetry at each end of (Sr 1-x Ca x ) 2 Si 5 N 8 , while in the range of 0.5 < x < 0.75, such solid solution phases coexisted. All of the obtained phosphors were well crystallized and the grains grew from 5 to 20 μm with increasing Ca 2+ content. These phosphors showed broadened excitation spectra originated from the 4f 7 → 4f 6 5d transition of Eu 2+ ions, so that the intense orange-red emission bands were observed under the excitation of 380-470 nm corresponding to the output lights of near-UV or blue LEDs.

Self-propagating high temperature synthesis of yellow-emitting Ba2Si5N8:Eu2+ phosphors for white light-emitting diodes

High-performance nitride phosphors, Ba2Si5N8:Eu2+, were prepared by the self-propagating high temperature synthesis method. The broadband absorption in the range of 380–500nm indicated that the resultant phosphors were effectively excited by an InGaN blue light-emitting diode (400∕470nm). The emission peak position of Ba2−xEuxSi5N8 (x=0–0.3) varied from 572to650nm with increasing Eu2+ concentration, and the intensity was maximized at x=0.04. The optimized sample gave a strong yellow emission peak at 580nm. The redshifting behavior of the emission band was attributed to three factors: energy migration, crystal field strengthening, and reabsorption.

Synthesis of Nitridosilicate CaSr1-xEuxSi5N8 (x = 0-1) Phosphor by Calcium Cyanamide Reduction for White Light-Emitting Diode Applications

A convenient calcium cyanamide (CaCN 2 ) reduction route was developed to synthesize the Eu 2+ -doped nitridosilicate phosphors, CaSr 1-x Eu x Si 5 N 8 (x = 0-1), and the photoluminescence properties of the obtained phosphors were characterized for solid-state lighting applications [white light-emitting diodes (LEDs)]. The Rietveld refinement analysis for undoped CaSrSi 5 N 8 was carried out in space group Pmn2 1 (no. 31) with a = 569.0(4) pm, b = 673.0(1) pm, and c = 932.0(6) pm. The Eu 2+ ions formed a complete solid solution by occupying the Sr 2+ sites in CaSrSi 5 N 8 host lattice. A typical sample doped with an optimized amount of Eu 2+ (2 atom % vs Ca/Sr) showed a broadband absorption covering the range from UV to visible region (380-500 nm) and strong red emission peaking at 629 nm, revealing that this phosphor was a potential candidate for the phosphor-converted white LEDs. Under excitation of 460 nm, this phosphor gave the intensity comparable (about 120%) to that of yttrium aluminum garnet YAG:Ce 3+ (P46-Y3) standard. The emission peaks of CaSr 1-x Eu x Si 5 N 8 :Eu 2+ varied from 625 to 680 nm with increasing Eu 2+ ion concentration. The redshifting behavior of the emission band dominantly contributed to the energy migration among Eu 2+ ions.

Novel carbon sphere@Bi2MoO6 core–shell structure for efficient visible light photocatalysis

Carbon sphere (CS)@Bi2MoO6 core–shell structure (CS@BMO) composites were successfully synthesized via a solvothermal reaction of CSs and Bi2MoO6 precursors in the mixed solution of ethylene glycol and ethanol. The morphology, structure and photocatalytic performance of the composites in the degradation of Rhodamine B (RhB) were characterized by scanning electron microscopy, X-ray diffraction, UV-vis absorption spectroscopy, electrochemical impedance spectra and nitrogen adsorption–desorption, respectively. The results show that the CS@BMO composites exhibit enhanced photocatalytic performance for degradation of RhB with a maximum degradation rate of 95% under visible light irradiation compared with the pure Bi2MoO6. The improved photocatalytic performance is ascribed to the enhanced specific surface area and light absorption as well as the reduced electron–hole pair recombination with the presence of CSs in the composites.

Long afterglow SrAl2O4:Eu,Dy phosphors for CdS quantum dot-sensitized solar cells with enhanced photovoltaic performance

An efficient bifunctional structured layer composed of long afterglow SrAl2O4:Eu,Dy phosphors on top of a transparent layer of nanocrystalline TiO2 was fabricated for use in CdS quantum dot-sensitized solar cells (QDSSCs). The introduction of SrAl2O4:Eu,Dy increases the photocurrent of the QDSSCs mainly due to enhanced light harvesting abilities via improved light absorption and scattering. Under one sun illumination (AM 1.5G, 100 mW cm−2), cells containing SrAl2O4:Eu,Dy show a marked improvement in their conversion efficiency (1.24%) compared with cells without SrAl2O4:Eu,Dy (0.98%). After one sun illumination for 1 min, after which the light source was turned off, cells containing SrAl2O4:Eu,Dy show an efficiency of 0.07% under dark conditions due to the irradiation by the long persistent light from SrAl2O4:Eu,Dy. The present strategy should provide a possibility to fulfil the operation of solar cells even in the dark.

Synthesis and luminescent properties of low oxygen contained Eu2+-doped Ca-α-SiAlON phosphor from calcium cyanamide reduction

Abstract A new convenient calcium cyanamide (CaCN2) reduction route was developed to synthesize the Eu2+ activated Ca-α-SiAlON phosphors containing low oxygen content. The luminescence properties of the obtained products were investigated for white LEDs application. The critical Eu2+ concentration in various hosts and its effect on the photoluminescence properties were studied. The optimized sample (10at.% Eu2+ vs. Ca2+) could be efficiently excited by the current GaN/InGaN blue LED chips and provided emission intensity competitive with that of YAG:Ce3+ (P46-Y3) standard, revealing that this phosphor was a potential candidate for phosphor-converted white LEDs.

Enhanced performance of cadmium selenide quantum dot-sensitized solar cells by incorporating long afterglow europium, dysprosium co-doped strontium aluminate phosphors.

CdSe quantum dot-sensitized solar cells based on an efficient bifunctional structured layer composed of long afterglow SrAl2O4:Eu,Dy phosphors on top of a transparent layer of nanocrystalline TiO2 were fabricated and their photovoltaic performances were investigated. The results show that a high power conversion efficiency of 1.22% is achieved for the cell with SrAl2O4:Eu,Dy at one sun illumination (AM 1.5 G, 100 mW cm(-2)), which is an increase of 48% compared to the cell without SrAl2O4:Eu,Dy (0.82%). After one sun illumination for 1 min and subsequent turn off of the light source, the cell with SrAl2O4:Eu,Dy still shows an efficiency of 0.04% under dark condition due to the irradiation by the long persistent light from SrAl2O4:Eu,Dy. The present strategy should provide a possibility to fulfill the operation of solar cells even in the dark.

Preparation of Sr2Si5N8:Eu2+ phosphors using various novel reducing agents and their luminescent properties

Divalent europium-doped metal nitride phosphors, Sr2Si5N8:Eu2+, were synthesized using three novel reducing agents, C3H6N6, C2H4N4, and Sr(CH3COO)2. The samples obtained showed higher emission intensity and lower residual oxygen/carbon content as compared with the sample prepared by the conventional carbothermal reduction and nitridation (CRN) method. A maximum emission intensity of 155% relative to that of the commercially available YAG:Ce3+ (P46-Y3) standard phosphor was achieved using the Sr(CH3COO)2 route. White LEDs fabricated using synthesized phosphors emitted white light with a correlated color temperature in the range 2900–6300 K. Moreover, a general color rendering index (Ra) of 80 was obtained. The reducing methods in this study were shown to be more effective than the conventional CRN route; these phosphors can be used to compensate for a red color deficiency of the current white LED.

All carbon nanotube based flexible field emission devices prepared through a film transfer method

Utilizing carbon nanotubes (CNTs) as substitutes for the conventional rigid indium–tin oxide (ITO) electrodes, all CNT based flexible field emission devices (FEDs) were possible to prepare with a novel film transfer method. In the method, both CNT-based flexible cathode and anode were fabricated by first attaching the vacuum filtered CNT film/mixed cellulose eater (MCE) filter membrane to an adhesive semi-cured poly(dimethyl siloxane) substrate, and then peeling off the MCE filter membrane. Effects of bending direction and bending radius on field emission properties were studied. The field emission performance of the convex bending device was worse than that of the flat device and deteriorated further with the decrease of bending radius. In contrast, the concave bending device exhibited better field emission properties than the flat device; nevertheless, its field emission properties decreased as the concave bending radius reduced below a certain value. The method achieved two successes. One is that the interfacial interaction between CNT films and the flexible substrate was greatly enhanced. The other is that the transfer process improved the field emission properties due to the increase of vertically aligned CNT tips. Compared with traditional methods applied in FED preparation, the novel modified transfer method is organic free, easy to control, cheap, and environment friendly.