Accessing (Ba1-xSrx)Al2Si2O8:Eu Phosphors for Solid State White Lighting via Microwave-assisted Preparation: Tuning Emission Color by Coordination Environment

Microwave-Assisted Preparation of Luminescent Inorganic Materials: A Fast Route to Light Conversion and Storage Phosphors

Luminescent inorganic materials are used in several technological applications such as light-emitting displays, white LEDs for illumination, bioimaging, and photodynamic therapy. Usually, inorganic phosphors (e.g., complex oxides, silicates) need high temperatures and, in some cases, specific atmospheres to be formed or to obtain a homogeneous composition. Low ionic diffusion and high melting points of the precursors lead to long processing times in these solid-state syntheses with a cost in energy consumption when conventional heating methods are applied. Microwave-assisted synthesis relies on selective, volumetric heating attributed to the electromagnetic radiation interaction with the matter. The microwave heating allows for rapid heating rates and small temperature gradients yielding homogeneous, well-formed materials swiftly. Luminescent inorganic materials can benefit significantly from the microwave-assisted synthesis for high homogeneity, diverse morphology, and rapid screening of different compositions. The rapid screening allows for fast material investigation, whereas the benefits of enhanced homogeneity include improvement in the optical properties such as quantum yields and storage capacity.

Microwave-assisted solid-state synthesis of NaRE(MO4)2 phosphors (RE = La, Pr, Eu, Dy; M = Mo, W)

A synthetic method was developed to enable the microwave-assisted solid-state preparation of double molybdate and double tungstate scheelite-type phosphors of formula NaRE(MO4)2 (RE = La, Pr, Eu, Dy; M = Mo, W). Starting from subgram-scale stoichiometric mixtures of metal carbonates and oxides and with the aid of granular activated charcoal as a microwave susceptor, ternary (NaEu(MO4)2), quaternary (NaLa0.95Eu0.05(MO4)2), and quinary phosphors (NaLa0.95Pr0.025Dy0.025(MO4)2) were obtained upon heating in a countertop microwave oven. The synthesis of crystalline and phase-pure materials required heating times ranging from 18 to 27 min, significantly shorter than those typically encountered in solid-state reactions assisted by conventional heating. Depending on chemical composition, the speed-up factor ranged from 30 to 40. More importantly, photoluminescence studies performed on the compositionally complex quinary molybdate NaLa0.95Pr0.025Dy0.025(MoO4)2 showed that phosphors synthesized using microwave and conventional heating have nearly identical luminescence responses. The synthetic method described in this contribution is robust, fast, simple, and ideally suited for exploratory synthesis and rapid screening of group VI metalate phosphors, as well as for the preparation of binary precursors to these materials (e.g., Na2MoO4 and Na2WO4).

High-Throughput Synthesis and Characterization of Eu Doped Ba xSr2- xSiO4 Thin Film Phosphors

High-throughput techniques have been employed for the synthesis and characterization of thin film phosphors of Eu-doped Ba _xSr_{2- x}SiO₄. Direct synthesis from evaporation of the constituent elements under a flux of atomic oxygen on a sapphire substrate at 850 °C was used to directly produce thin film libraries (415 nm thickness) of the crystalline orthosilicate phase with the desired compositional variation (0.24 > x > 1.86). The orthosilicate phase could be synthesized as a pure, or predominantly pure, phase. Annealing the as synthesized library in a reducing atmosphere resulted in the reduction of the Eu while retaining the orthosilicate phase, and resulted in a materials thin film library where fluorescence excited by blue light (450 nm) was observable by the naked eye. Parallel screening of the fluorescence from the combinatorial libraries of Eu doped Ba _xSr_{2- x}SiO₄ has been implemented by imaging the fluorescent radiation over the library using a monochrome digital camera using a series of color filters. Informatics tools have been developed to allow the 1931 CIE color coordinates and the relative quantum efficiencies of the materials library to be rapidly assessed and mapped against composition, crystal structure and phase purity. The range of compositions gave values of CIE _x between 0.17 and 0.52 and CIE _y between 0.48 and 0.69 with relative efficiencies in the range 2.0 × 10^-4-7.6 × 10^-4. Good agreement was obtained between the thin film phosphors and the fluorescence characteristics of a number of corresponding bulk phosphor powders. The thermal quenching of fluorescence in the thin film libraries was also measured in the temperature range 25-130 °C: The phase purity of the thin film was found to significantly influence both the relative quantum efficiency and the thermal quenching of the fluorescence.

Recent progress in luminescence tuning of Ce3+and Eu2+-activated phosphors for pc-WLEDs

This review is devoted to several approaches to realize spectral tuning for improving the luminescence performance of Ce³⁺and Eu²⁺-activated pc-WLED phosphors.