Synthesis and photoluminescence properties of phosphate/silicate-based phosphors to realize white light/tunable emissions for LEDs application

Abstract

Abstract The phosphor materials have attained great achievement and progress in various fields including solid-state lighting, optical temperature sensors, flat panel displays, solar cells, and optical biomarkers. As next-generation lighting devices, phosphor-converted white light-emitting diodes (w-LEDs) have received much more attention since w-LEDs provide extraordinary superiorities, such as low electric consumption, high electro-optical conversion efficiency, high brightness, good stability, fast response, and environmental friendliness. The color tunability is important for phosphors because it facilitates w-LED color tuning. There are two strategies to achieve the color tunability. One is based on the mechanism of the energy transfer from sensitizer to an activator. The other is based on the design of elemental substitution in the inorganic functional materials, since the overall nature and the lattice parameters of the host lattice are well modified and thereby induce white light/tunable emissions. The purpose of this dissertation is to develop the new single-phased phosphate/silicate-based phosphors for various LEDs application. The phosphate and silicate groups as the end building blocks have high crystal flexibility and stability in the phosphor-based substrates. For the phosphate-based apatite like-compound Sr10(PO4)6F2, we tried two kinds of elemental substitutions. One is a silicate-substituted apatite following the pattern: (PO4)3- + F- = (SiO4)4-; another is two divalent alkaline earth cations replaced by one trivalent rare earth ion and one monovalent alkaline earth cation following the pattern: 2Sr2+ = La3+ + Na+. In the Eu2+ doped new phosphor-silicate apatite Sr3LaNa(PO4)2SiO4 phosphors, all the samples exhibited broad absorption bands from 200 to 450 nm, revealing the phosphor-silicate phosphor interesting for application in the UV/near-UV used phosphor-converted LED chips. All of the phosphors showed the strong asymmetric broad emission bands, and the broadband consists of three Gaussian fitting symmetry bands located at 440 nm, 484 nm, and 535 nm. In addition, by partial substitution of silicate groups for phosphors, an obvious red-shift variation was found from the reflection and emission spectra. Meanwhile, both CIE color coordinates were consistent with the red-shift of reflection and emission spectra. Our results show that the studied phosphors have the potential for UV/near-UV pumped w-LEDs. To further study the elemental substitution in a known Sr3LaNa(PO4)2SiO4 phosphors, we synthesized the new Sr3CeNa(PO4)2SiO4 phosphors with the substitution of Ce3+ for La3+ ions. It is noteworthy that the Eu2+-activated Sr3CeNa(PO4)2SiO4 phosphor matches well with commercial near-UV chips, suggesting its promising application in phosphor-converted white LEDs. Interestingly, by incorporating La3+ ions in Sr2.99CeNa(PO4)2SiO4:0.01Eu2+, the regulation of Eu2+ luminescence intensities in two centers were achieved by two methods: one is to increase the La3+ concentration from 0 to 0.9 mol and the other is to adjust the excitation wavelength from 300 to 390 nm, and in the meantime, a significant blue-shift appeared since the band gap showed a widening trend from 5.548 to 5.621 eV, and a continuous red-shift was also observed due to the reabsorption. The orderly changes in Eu2+ emission intensities over the two Eu2+ luminescent centers suggested that the incorporation of La3+ ions in Sr2.99CeNa(PO4)2SiO4:0.01Eu2+ can affect the performance and characteristics of Eu2+ ions. More specifically, the luminescence dynamic process and internal quantum efficiency were also changed by La3+ doping in Sr2.99CeNa(PO4)2SiO4: 0.01Eu2+. Therefore, these phenomena illustrate that incorporating La3+ ions in Sr2.99CeNa(PO4)2SiO4:0.01Eu2+ can affect the performance and characteristics of Eu2+ ions. Another important phosphate/silicate-based inorganic functional material is eulytite-type M3IIMIII (PO4)3 (where MII = Ca2+, Sr2+, Ba2+, Pb2+; MIII = La3+, Y3+, Sc3+, Ce3+, Tb3+, Lu3+) compound, which has been intensively studied as host materials for phosphors application. Eu-activated Sr3La(PO4)3 phosphor has been synthesized and investigated in detail. Detailed crystal structure analysis confirmed that Eu2+ and Eu3+ activators were able to replace Sr2+ and La3+ sites, respectively. The Eu2+ ions occupied two types of Sr/La luminescent centers exhibited blue and green emissions, while the Eu3+ ions resided one type of Sr/La luminescent centers exhibited typical red emission. It is also noteworthy that the Eu-activated Sr3La(PO4)3 phosphor had a wide absorption band at 250-450 nm, matching well with the commercial UV/near-UV chips. Interestingly, an abnormal blue shift behavior of Eu2+ emission was observed in Sr3La(PO4)3:Eu2+,Eu3+ system because of the strong covalent character of Eu3+-O2- bond and the release of adjacent Eu3+-induced stress. Therefore, the result suggests that the Eu3+ can affect the performance of the Eu2+ ions in this system. In addition, our work can also encourage the study of the complicated luminous behavior among multi-oxidation states in single Wyckoff site with huge potential applications. At last, Bi3+ and Eu3+ ion co-doped Ba9Lu2Si6O24 single-phased phosphor was synthesized successfully via a conventional high-temperature solid-state reaction. X-ray diffraction, crystal structure analysis, diffuse reflectance and luminescent spectra, quantum efficiency measurements, and thermal stability analysis were applied to investigate the phase, structure, luminescent and thermal stability properties. From the analyses of the crystal structure and luminescent spectra, we observed four discernible Bi3+ luminescent centers with peaks at ~363.3, ~403.1, ~437.7, and ~494.5 nm. Moreover, due to the complex energy transfer processes among these Bi3+ centers, their relative emission intensity tightly depended on the incident excitation wavelength. Interestingly, the as-prepared phosphor could generate warm white light/tunable emission by changing the concentration of Eu3+ ions or adjusting the excitation wavelength. The energy transfer mechanism from Bi3+ to Eu3+ was confirmed via an electric dipole-dipole interaction. The energy transfer efficiencies (η_T) from Bi3+ to Eu3+ were 50.84% and 40.17% monitoring at 410 and 485 nm, respectively. The internal quantum efficiency of the optimized Ba9Lu2Si6O24:Bi3+,Eu3+ phosphor was calculated to be 42.6%. In addition, the configurational coordinate model was carried out to explain the energy decrease of the phonon-electron coupling effect. All the results show that the Ba9Lu2Si6O24: Bi3+, Eu3+ phosphor has potential application in phosphor-converted w-LEDs.