Highly luminescent Ca3Sc2Si3O12:Ce3+ silicate garnet nano- and microparticles with 50-70% photoluminescence quantum yields as efficient phosphor converters for white LEDs

I. Levchuk, F. Schröppel, L. Römling, A. Osvet, N. Khaidukov, Y. Zorenko, R. Van Deun, M. Batentschuk, C.J. Brabec
University of Erlangen-Nuremberg,

Keywords: silicate garnets, nanoparticles, photoluminescence, quantum yield, white LED


Solid state lighting based upon a combination of violet/blue light emitting diodes (LEDs) and a phosphor converter created enormous commercial interest because of the potentially higher efficacies and long lifetimes of these light sources. Most of the phosphors used in white light emitting diodes (WLED) absorb and emit light as a result of inter-configurational 5d-4f transitions in rare-earth (RE) ions, such as Ce3+ or Eu2+. In particular, the standard WLED structure is based on the combination of a blue LED chip and a yellow phosphor converter (PC). Notably, the most commonly used PC in WLEDs is the Ce3+ doped Y3Al5O12 garnet (YAG:Ce). However, the combination of a blue chip and YAG:Ce PC shows a low color rendering index (CRI) of about 80 due to the weak emission in the red spectral range. Moreover, this combination yields cool WLEDs with high correlated color temperatures (CCT) above 5000 K, which often does not meet the requirements for ambient lighting. Several attempts were undertaken to improve the emission spectrum of phosphors with the garnet structure. A common approach has been the substitution of Y3+ ions at the dodecahedra sites of the crystal lattice by Tb3+, Gd3+ or Lu3+. However, it is difficult to optimize all the important parameters like CRI, excitation wavelength and the thermal stability of the emission at temperatures over 100 °C. A relatively new approach is to develop new luminescent materials with garnet structure, namely silicates of calcium and yttrium garnets {Ca2,Y}[Mg,Sc,Al,Ga]2(Ga,Al,Si)3O12 doped with Eu2+ and Ce3+ ions. As a first step, research aiming at high quantum efficiency of Ca3Sc2Si3O12:Ce3+ in µm-powder and nanocrystalline-form is considered. Here, we present a novel synthesis method of Ca3Sc2Si3O12:Ce3+ nano- and microparticles with outstandingly bright photoluminescence. Nanoparticles were synthesized by the co-precipitation method, where a long-chain fatty acid salt was used as the precipitation agent for Ca, Sc and Ce water solutions in combination with SiO2 nanoparticles or TEOS as a silica source. Further calcination in reducing atmosphere results in brightly luminescent Ca3Sc2Si3O12:Ce3+ nanopowders with up to 50% photoluminescence quantum yield (PLQY). The modified conventional solid-state reaction was utilized for synthesizing microparticles. Pre-synthesis of metal and silicon oxide solid blends with subsequent high-temperature post-treatment enables to obtain Ca3Sc2Si3O12: Ce3+ microparticles with the record-high PLQY of about 70%. Both samples display typical double emission peaks centered at 506 and 547 nm which correspond to the 5d-4f transition for Ce3+ in the garnet of this composition. Additionally, Ca3Sc2Si3O12:Ce3+ displays significantly higher thermal stability of photoluminescence in comparison to a commercial YAG:Ce phosphor. Variable particle sizes, high PLQY and photoluminescent thermal stability make this optimized Ca3Sc2Si3O12:Ce3+ phosphor a very competitive candidate for white LED application.