Youjie Hua

Modification of the crystal structure of Sr2−xBaxSi(O,N)4: Eu2+phosphors to improve their luminescence properties

A series of Sr1.98−xBaxSi(O,N)4: 0.02Eu2+ (0 ≤ x ≤ 0.5) phosphors were synthesized by a conventional solid state reaction method. The Sr–N and Si–N bonds could be observed in FT-IR spectra. The XRD refinement results indicated that N3− would substitute for O22− and form Si–NO3 tetrahedrons during the process of forming the Sr1.98Si(O,N)4: Eu2+ (SSON: Eu2+) substitutional solid solution. The lattice constants of Sr1.98−xBaxSi(O,N)4: 0.02Eu2+ (SBSON) were increased due to the longer bond length of Ba–O. Compared with Sr2SiO4: Eu2+, the SSON: Eu2+ showed a remarkable red-shift, which is due to the nephelauxetic effect and the strong crystal field splitting originating from the stronger covalent bonding effect of the Eu–N bond. The SSON: Eu2+ presented a β phase structure before the introduction of Ba2+, whereas the α′-SBSON phase was obtained by the substitution of Ba2+ for Sr2+. Ba2+ doping led to an obvious blue-shift under 375 nm and 460 nm excitation. The 5d orbital of Ba2+ is coupled with the 5d orbital of Eu2+ on the higher energy level position in the host crystal. Under 375 nm excitation, the PL intensity gradually increased with the increase of the Ba2+ content. Under 460 nm excitation, the PL intensity gradually declined with the increase of Ba2+ content. The thermostability of α′-SBSON: Eu2+ was significantly improved compared with β-SSO: Eu2+ and β-SSON: Eu2+. On the basis of their adjustable emission wavelength, enhanced PL intensity and excellent thermostability in SBSON: Eu2+, we anticipate that these materials can be used as green or red phosphors in white light emitting diodes.

Luminescence properties of novel emission-tunable NaSr(4−x−y)Bax(BO3)3:yEu2+ phosphors for white light emitting diodes

A series of emission-tunable NaSr(4−x−y)Bax(BO3)3:yEu2+ phosphors have been prepared by a conventional solid-state reaction method. The structures of NaSr(4−x−y)Bax(BO3)3:yEu2+ have been investigated by Rietveld refinement of the X-ray diffraction (XRD) patterns. The results indicated that the as-prepared samples showed the same crystal structure of NaSr4(BO3)3 with a cubic unit cell and space group of Iad. With the increase of Ba2+ concentration, the Sr2+ sites were replaced by Ba2+ completely and the lattice parameter of the unit cell increased from a = b = c = 15.0710 A to 15.7266 A. Both emission spectra and decay curves of NaSr3.98(BO3)3:0.02Eu2+ and NaBa3.98(BO3)3:0.02Eu2+ showed the existence of two different Eu2+ emission centers named Eu1 and Eu2. Eu2 was six-coordinated and Eu1 was eight-coordinated of oxygen. With the increase of Eu2+ concentration in the NaSr3−yBa(BO3)3:yEu2+ sample, the emission intensity increased and reached a maximum at y = 0.02. Then the concentration quenching phenomenon emerged due to the electric dipole–dipole interaction. Upon the cation substitutions (Sr2+ for Ba2+) in the NaSr(4−x−y)Bax(BO3)3:yEu2+ host, the emission peaks of Eu2+ blue-shifted from 609 nm to 544 nm and the thermal stability decreased, which was ascribed to the change of the covalency and the crystal field strength that the 5d orbital of the Eu2+ ion experiences. The CIE chromaticity coordinates of the obtained phosphors can be continuously tuned from orange-red (0.4795, 0.4070) to yellow-green (0.3432, 0.4665) by adjusting the Ba2+ concentration. The results demonstrate that the emission-tunable NaSr(4−x−y)Bax(BO3)3:yEu2+ phosphors have a potential application for white light emitting diodes (w-LEDs).

Luminescence properties and crystal structure of α′-Sr2Si3x/4O2Nx:Eu2+ phosphors with different concentrations of N3− ions

A series of disordered α′-Sr2Si3x/4O2Nx:Eu2+ (1.333 ≤ x ≤ 2.4) phosphors were synthesized by the conventional solid state reaction method. The disordered α′-Sr2Si3x/4O2Nx:Eu2+ (α′-SSON:Eu2+) phosphors have two distinct activation centers: Eu(I) and Eu(II). With the increase of N concentration, both the luminescence intensity and the dominant peak wavelength (DPWs, which is about 490 nm) of the Eu(I) site were extraordinarily unchanged. In comparison with the yellow emissions (∼580 nm) of the Eu(II) site of the disordered α′-Sr2SiO4:Eu2+, the DPWs of Eu(II) emissions were at red spectral regions (609–618 nm), which depends on the amount of N3−. The PL intensity of the Eu(II) emission band increased first and then decreased, and reached a maximum at x = 2. The disordered α′-SSON is a substitutional solid solution. Compared with the disordered α′-Sr2SiO4, all the lattice constants of disordered α′-SSON became smaller which led to the decrease of the cell volume. The peaks of the Si–N and Sr–N bond could be observed in FT-IR spectra. The Si–(N/O)4 tetrahedrons transformed from Si–O4, Si–NO3, and Si–N2O2 into Si–N3O with the increase of N content. The bond lengths of Si–N and Sr–(N/O) were within the normal ranges compared with other silicon-based oxynitrides. The Si–O bond lengths became shorter due to the extrusion effects of longer Si–N bonds. Both of the average bond lengths of Sr1–(N/O) and Sr2–(N/O) in disordered α′-SSON became longer than that of disordered α′-Sr2SiO4. Due to the red emission and high photoluminescence intensity of the disordered α′-Sr2Si3x/4O2Nx:Eu2+ (1.333 ≤ x ≤ 2.4), we anticipate that these materials can be used as red phosphors in white light emitting diodes.

Spectroscopic investigations on Er3+/Yb3+-doped oxyfluoride glass ceramics containing YOF nanocrystals

Spectroscopic properties of Er3+/Yb3+-doped transparent oxyfluoride borosilicate glass ceramics containing YOF nanocrystals were systematically investigated. X-ray diffraction (XRD) confirmed the formation of YOF nanocrystals in the glassy matrix. Based on the Judd-Ofelt theory, the intensity parameters Ωi (i=2, 4, 6), spontaneous emission probability, radiative lifetime, radiative quantum efficiency and the effective emission bandwidth were investigated. The upconversion luminescence intensity of Er3+ ions in the glass ceramics increased significantly with the increasing crystallization temperature. The transition mechanisms of the green and red upconversion luminescence were ascribed to a two-photon process, and the blue upconversion luminescence was a three-photon absorption process.

Tunable luminescence of Ce3+/Li+, Eu2+ co-doped Ca4(PO4)2O phosphor for white light emitting diodes

Abstract Phosphors Ca 4 (PO 4 ) 2 O:Ce 3+ /Li + , Eu 2+ were prepared by solid state reaction successfully, and characterized by X-ray diffraction, UV-Vis spectrometer and photoluminescence spectrometry. The as-prepared phosphors exhibited strong absorption in the UV-visible region and dual-emission bands centered at approximately 460 and 630 nm. Energy transfer from Ce 3+ to Eu 2+ was also observed. By varying the excitation wavelength, these new phosphors exhibited tunable emissions from blue to white and then to yellow, making them potential candidates as UV-convertible phosphors for white light emitting diodes.

Luminescence properties of Mn2+ ions doped Lu2CaMg2Si3O12 garnet phosphors

Yttrium aluminum garnet structure phosphors Lu2CaMg2Si3O12:Mn2+ were synthesized by conventional high temperature solid-state reaction in reductive atmosphere. The structure and optical properties of samples were characterized by application of powder X-ray diffraction (XRD) and photoluminescence spectroscopy. Results of X-ray diffraction (XRD) analysis showed that the phosphors mainly presented garnet structure with a few weak peaks of impurity phases. Lu2–xCaMg2Si3O12:xMn2+ (x=0.01–0.8) phosphors showed a broad emission band peaking at around 590 nm under ultraviolet (UV) light of 408 nm when Mn2+ concentration was less than 0.08 mol. With an increase in the Mn2+ concentration (above 0.08), another broad emission band peaking at 720 nm besides 590 nm was observed, which may be due to manganese ion having different valence and occupying different host lattice. The critical quenching concentrations of manganese ion in the wavelength of 590 and 720 nm were about 0.06 and 0.2 mol, respectively. With 408 nm excitation wavelength, emission color of the samples had a red shift trend as the Mn2+ concentration increased. All the results indicated that the Lu2CaMg2Si3O12:Mn2+ phosphors could be applicable to n-UV based white LEDs.

Luminescence properties of single-phase color-tunable Li4SrCa(Si2O4N8/3):Eu2+ phosphor for white light-emitting diodes

A novel single-phase color-tunable phosphor, Eu2+-activated Li4SrCa(Si2O4N8/3):Eu2+ (LSCSN:Eu2+), was synthesized via the conventional solid-state reaction method. The crystal structure of LSCSN has been determined by Rietveld refinement on X-ray powder diffraction data, and it was found that LSCSN crystallizes in an orthorhombic unit cell with the space group Pbcm(57). Under the excitation of UV light, the phosphor shows two main emission bands peaking at 430 nm and 578 nm, which have been confirmed to correspond to Eu2+ occupying the ten-coordination Sr sites and six-coordination Ca sites, respectively. Subsequently, the energy transfer between different Eu2+ emission centers was verified by the overlap of the emission and excitation spectra and the variation of fluorescence decay lifetimes. Meanwhile, when the excitation was changed to 375 nm light, it was found that the color hue can be tuned from blue to yellow, and even white if the Eu2+ ion concentration was increased to some extent. The thermal stability of the phosphor has been also investigated, which performs well with a high value of stability along with its integrated emission intensity remaining as 90.9% at 150 °C of that measured at room temperature. Furthermore, a warm white LED has been fabricated with a LSCSN:0.03Eu2+ phosphor and a 375 nm LED chip, and when the applied current is set to be 80 mA, the white LED was found to have a color rendering index (Ra) of 82.5 at the CIE coordinates of (0.3568, 0.3111) and correlated color temperature of 3318 K. The outstanding luminescence properties suggested that our prepared samples have potential application in w-LEDs.