Crystallization and activator reduction simultaneously for calcium sulfide nanophosphors and evaluation of red-light compensation - Environmental reliability

Abstract

Calcium sulfide nanophosphors (CaS:Eu2+) with red emissions were synthesized by hydrothermal-derived 250 nm carbon-sphere templates to induce a reduction atmosphere to activate an activator valence transition and crystallize (Ca,Eu)S simultaneously at 800 °C without extra sulfur atmosphere. The cationic concentration z = [Ca2++Eu2+] and ionic ratio y = [S2−]/[Ca2++Eu2+] were controlled in an innovative chemical solution reflux process to synthesize a sulfide nanophosphor precursor on carbon-sphere template. The particle size and crystallinity of the prepared nanophosphors increased with the “y” value increase. The photoluminescence (PL) intensity of the 652 nm emission excited by 450 nm increased to y = 1 and kept nearly constant to y = 20. High PL intensity was achieved by 0.025CaS-1 and 0.025CaS-10 samples with individually sphere-like nanoparticles, enough crystallinity and homogeneous Eu doping as well as fully reduced Eu2+ in the nanophosphors. The CaS:Eu2+ nanoparticles prepared by carbon-sphere templates showed thermal quenching with temperature. The Eu2+-doped CaS emitted 652 nm red light by 450 nm but not by 380 nm excitation that was only for CaS host at 555 nm emission. The prepared 0.025CaS-10 paste of only 7 wt% was individually coated on a blue-light LED and commercially available YAG-LED. The red emission intensity was enhanced and broadened by the prepared sulfide nanophosphor compared to a commercially available red LED. The CRI value for R9 increased to higher than 90, and the measured Ra was higher than 95 for the prepared sulfide nanophosphor-coated commercial WLED. The prepared sulfide nanophosphors could keep the red-light brightness over six months' exposure at rainy marina to show the environmental reliability.