Hongping Ma

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 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.

The crystal structure and luminescence properties of novel Ce3+ and Ce3+, Sm3+-activated Y4SiAlO8N phosphors for near-UV white LEDs

A series of novel Ce3+ and/or Sm3+ activated Y4SiAlO8N (YSAON) phosphors are successfully synthesized by a conventional solid-state reaction method. The formation of monophasic YSAON is confirmed by X-ray powder diffraction (XRD) analysis. The crystal structure of YSAON is solved by XRD refinement data. DFT calculations indicate that Y4SiAlO8N has an indirect band gap of 3.819 eV. Based on the analysis of crystal structure and lifetime curves, four emission sites of YSAON:0.02Ce3+ are identified, which are Y1 (413 nm, 448 nm), Y2 (428 nm, 469 nm), Y3 (619 nm, 698 nm) and Y4 (512 nm, 569 nm), respectively. The concentration quenching of Ce3+ in YSAON is investigated in detail and it is confirmed to be due to dipole–dipole interactions. Photoluminescence excitation (PLE) and photoluminescence (PL) spectral measurements showed that the Ce3+ and Sm3+ co-doped phosphor could be efficiently excited by near-UV light, and exhibited four emission bands peaked at 485 nm, 565 nm, 605 nm and 650 nm, respectively. The CIE chromaticity coordinates of the obtained white light can be tuned by adjusting the content of Ce3+ and Sm3+ ions. Therefore, we anticipate that these materials can be used in near-UV chip pumped white LEDs.

Synthesis, luminescence properties and electronic structure of Tb3+-doped Y4−xSiAlO8N:xTb3+ – a novel green phosphor with high thermal stability for white LEDs

A series of novel green Y4−xSiAlO8N:xTb3+ phosphors have been prepared by a high temperature solid state reaction. The phase formation and structural properties were analyzed by X-ray powder diffraction. The XRD results and SEM images show that the Y3+ can be substituted by Tb3+ for the max content of x = 1.5 and the most suitable sintering temperature is about 1500 °C. The PLE spectra of Y4−xSiAlO8N:xTb3+ phosphors exhibit a wide excitation band ranging from 200 to 500 nm, which matches well with the characteristic emission of n-UV chips. Under excitation of 380 nm, the phosphor shows four intense emission bands with emission peaks at 488 nm, 543 nm, 585 nm and 624 nm, respectively. These emissions were attributed to the characteristic 5D4 → 7FJ (J = 6, 5, 4, 3) transitions of Tb3+ ions. The optimum doping concentration of Tb3+ was found to be x = 2.0 which indicated that the concentration quenching effect in the Y4−xSiAlO8N:xTb3+ phosphor is very weak. The critical distance for the Tb3+ ions calculated by the concentration quenching is 3.64 A. The detailed nonradiative energy transfer mechanism between Tb3+ ions is confirmed to be via a dipole–dipole interaction by the fluorescence decay analysis. Furthermore, with the introduction of Tb3+ ions, the reflectance spectra shows an obvious increment of reflectance in the region of 200–250 nm, which is due to the variation of electronic structure in the conductance band derived from Y3+ ions. Finally, the excellent thermal stability of the phosphors was demonstrated by the temperature dependence of the PL spectra. Compared to the initial intensity at room temperature (293.0 K), the relative PL intensity maintains high value of 96.5% at 475.2 K. The detailed thermal quenching behavior has also been interpreted.