| With the continous comsuption of pertrochemical energy all over the world, using the energy effectively has become the hot issue that attracts people’s attention. In the field of lighting, white light-emitting diodes (WLED) have attracted substantial attention owing to the extraordinary low power cusumption, environmental friendliness and longlife. The LED will replace the existing light source as the next generation light source. The phosphors, as the key role of the WLED, have attracted wide attention.At present, one involving hiving the emission from red, green and blue(RGB) phosphors with UV-LED or nUV-LED chip has attracted more and more attention. However, the most widely use red phosphors for UV InGaN-based LED is Y2O2S:Eu3+, but the lifetime is insufficient under UV or nUV irradiation due to its chemical instability and low efficiency. Therefore, there is an urgent need to seek an effiencient red phosphors with strong absorption in UV or nUV region with excellent chemical stability. In this study, optical properties of as-prepared phosphors were characterized via X-ray powder diffraction(XRD), Scanning elelctron microscope(SEM) and Photoluminescence spectrum(PL), respectively. A part of as-prepared red phosphors were used as phosphor for red-emitting LED with 395 nm-emitting InGaN chip and the performance of fabricated red LED were evaluated. The main contibutions were as follows:(1) ZnNb2O6:Eu3+ and MgNb2O6:Eu3+ phosphors were prepared by sol-gel method. The XRD characterization showed the pure phase of ZnNb2O6 and MgNb2O6 were obtained after calcined at 1100 ℃ for 5 h. The emission spectra showed red emission at 614 nm, corresponding to the 5D0â†'7F2 transition of Zn0.89Nb2O6:Eu3+0.05,Bi3+0.005, M+(M= Li, Na, K) and Mg0.89Nb2O6:Eu3+0.05, Bi3+0.005, M+(M= Li, Na, K) phosphors near-UV excitation(395 nm). With co-activator Bi3+, a broad band from 300-350 nm appeared due to the Bi3+â†'Nb5+ and Bi3+â†'O2- charge transfer transition in ZnNb2O6:Eu3+, Bi3+ and MgNb2O6:Eu3+, Bi3+ phosphors. The PL intensities of the Li+ and Na+ doped in Zn0.945Nb2O6:Eu3+0.05, Bi3+0.005 and Mg0.945Nb206:Eu3+0.05,Bi3+0.005 phosphors were higher than those of doped without Li+ and Na+, the increasing ratio is 1.9 and 1.2 times in ZnNb2O6 system,9.0 and 2.6 times in MgNb2O6 system, respectively. The CIE chromaticity of Zn0.89Nb206:Eu3+0.05, Bi3+0.005, Li+0.055 and Mg0.89Nb2O6:Eu3+0.05, Bi3+0.005, Li+0.055 phosphors are (x= 0.67, y= 0.34) and (x = 0.61, y= 0.39), respectively, which approximated NTSC standard values, demonstrating that the above-described phosphors hold considerable potential in white LED application.(2) Mg1-xZnxMoO4:Eu3+ and Li3Ba2Gd3(MoO4)8:Eu3+(LBGM:Eu3+) red phosphors were synthesized using a solid-state reaction. The XRD pattern showed that when calcined at 700 ℃ for 5 h and 900 ℃ for 10 h, the pure phase of ZnMoO4 and LBGM were obtained, respectively. The crystal structure of ZnMoO4 did not changed when doping with Mg2+ or Bi3+, and Bi3+ or Sm3+ codoping did not changed either in the LBGM host. The emission spectra exhibited a strong red emission at 616 nm, corresponding to the 5D0â†'7F2 transition of MgxZn0.95-xMoO4:Eu3+ 0.05 and LBGM:Eu3+ phosphors under near-UV excitation(395 nm). Compared with the single-doped ZnMoO4:Eu3+ phosphors, the samples codoped with Mg2+ had 1.7 times higher emission intensity than the single-doped ZnMoO4:Eu3+ phosphor, which has attributed to the tiny adjustment of non-inversion symmetry of the Eu3+-coordinated environment. The co-doped Bi3+ ions absorbed and transferred energy to the Eu3+ ions efficiently, the color purity of the Mg0.10Zn0.84MoO4:Eu3+0.05, Bi3+0.01 was caculated to be 91.80%, and the maximum energy transfer efficiency of Bi3+ in the Mg1-xZnxMoO4 host was caculated to be 85.60%. The codoped Bi3+ or Sm3+ ions demonstrated efficient energy transfer to Eu3+ ions and strengthened the emission intensity at 616 nm. The color purity of LBGM:Eu3+0.05, Bi3+0.03 and LBGM:Eu3+0.05, Sm3+0.005 phosphors are 94.81% and 95.04%, respectively. The maximum energy transfer efficiency of Bi3+ and Sm3+ in the LBGM host were caculated to be 35.5% and 36.5%, respectively.(3) The red phosphors NaGd1-x-y-z(WO4)2-a(MoO4)a:Eu3+x, Bi3+y, Sm3+z(a= 0-2, x= 0-0.60, y= 0-0.05, z= 0-0.05) were synthesized by high-temperature solid-state reaction. The XRD patterns showed that the pure phase of NaGd(WO4)2 was obtained when calcined at 900 ℃ for 5 h, and the pure phase of NaGd(WO4)2 didn’t changed when codoped with a certain amount of Eu3+, Bi3+ or Sm3+. The emission spectra showed that all the samples could be excited by UV-light 395 nm and emit red light at 616 nm, corresponding to 5D0â†'7F2 electric dipole transition of Eu3+. The substitution W6+ with Mo6+ changed the sublattice environment of Eu3+, making more amount of Eu3+ to locate in the site of non-inversion center, resulting in the enhancement of luminescent intensity. Only the characteristic transition emission of Eu3+ were detected, indicating that the co-doped Bi3+ and Sm3+ ions absorbed and transferred energy to Eu3+ ions efficiently and effectively extended the absorption area ranging from 300 nm to 400 nm. The color purity of all samples are more than 95%, the results indicated that the samples can be pontentially applied to WLED.(4) A series of Eu3+-active boron tungstate phosphors Gd3BWO9:Eu3+ have been prepared by a solid-state reaction route. XRD results reveal a pure phase was obtained when calcined at 900 ℃ for 12 h, the doping of Mo6+, Bi3+ or Sm3+ have not influence in the crystalline structure of Gd3BWO9. Evenmore, the incorporation of Mo6+ into the lattice of Gd2.95B(W1-yMoy)09:Eu3+0.05, which in turn leads to the shrinkage of unit cell volume, resulting in the enhancement of non-inversion symmetry properties. The Gd2.95B(W1-yMoy)09:Eu3+0.05 phosphors show broad charge transfer(CT) band from 300-400 nm due to CT from oxygen to tungsten/molybdenum. The doped Bi3+ or Sm3+ proved efficient in sensitizing the emission of Eu3+ at 620 nm when excited under 275 nm light, and the doped of Bi3+ extending the adsorption of near-UV light with wavelength at approximately from 300 nm to 350 nm. In addition, the chromaticity coordinates of all as-prepared samples, which approximated the color standard values mandated by the NTSC.(5) The red phosphors LiCa9Gd2/3(PO4)7:Eu3+, Tb3+(LCGP:Eu3+, Tb34) have been systhesized through solid-state reaction. XRD results showed that the pure phase of LCGP with rhombohedrol crystal structure was obtained after calcined at 1000 ℃ for 10 h. Under 395 nm excitation, the phosphors show characteristic red emission at 616 nm. The characteristic emission of Tb3+ were not observable, indicating that the energy can be transferred from Tb3+ to Eu3+ with multi-photon relaxation. The chromaticity coordination of LCGP:Eu3+0.20, Tb3+0.20 is (x= 0.65, y= 0.35), which approximated the standard values mandated by the NTSC.(6) In order to study the luminescent properties of Eu3+ which occupied the different center of inversion in lattice, Red phosphors of Eu3+-dopped BaM2Zn05 (M= La, Gd) were prepared by solid-state reaction. Structure and photoluminescence properties of the phosphors were characterized by X-ray diffraction(XRD), scan electron microscopy(SEM), photoluminescence spectrophotometer(PL). XRD results showed that the pure phase of BaM2Zn05(M= La, Gd) could be obtained available after being fired at 1000℃. The particles have a slight agglomerate phenomenon with the diameter of about 3-5 μm. BaLa2ZnO5:Eu phosphors could be effectively excited by the near UV irridiation (395 nm) and emit orange light at 595 nm due to the magnetic dipole 5D0â†'7F1 transitions on Eu3+, which indicated that Eu3+ occupied the symmetry sites in the BaLa2ZnO5 host lattice. BaGd2ZnO5:Eu phosphors could be effectively excited by the near UV irridiation (395 nm)and emit orange light at 627 nm due to the forced electric dipole 5D0â†'7F2 transitions on Eu3+, which indicated that Eu3+ occupied the site without center of symmetry in the BaGd2Zn05 host lattice.(7) An intense red light-emitting diode (LED) was fabricated by combining the as-prepared ZnNb2O6:Eu3+, MgZnMoO4:Eu3+, LBGM:Eu3+, NaGd(WO4)(MoO4):Eu3+, Gd3BWO9:Eu3+ phosphor with a 395 nm-emitting InGaN chip, and the performance of the LED were tested. The LEDs emit red light under 395 nm excitation, the results are in good agreement with the photoluminscent properties of the phosphors. The codoping of Li+ and Na+ ions as charge compensation agent make obvious increasement of luminous efficiency; The emission intensity increased effectively by the doping Mg2+to ZnMo04 and Mo6+ to NaGd(WO4)2, Gd3BWO9 host, and the as-prepared phosphors which doped with Mg2+ and Mo6+ were more efficient under the excitation of near-UV light with wavelength in 395 nm compared to those doped without Mg2+ and Mo6+. It indicated that by doping small amount of Bi3+ or Sm3+ into Eu3+ sites, the CT band of Bi-O, Sm-O and Eu-O were significantly enhanced. As a result, the emission intensity and absorptance on the near-UV light could be improved efficiently. Red LED was fabricated by combining LBGM:Eu3+0.05, Sm3+0.005, LBGM:Eu3+0.05, Bi3+0.01 NaGd0.93(WO4)(MoO4):Eu3+0.05, Bi3+0.01, Sm3+0.01 and Gd2.95B(W0.70Mo0.30):Eu3+0.05 phosphors with 395 nm-emitting GaInN chip, the CIE of the red LED are located beside the CIE of standard red light. |
PDF Full Text Request