Strategies For Improving Luminescence Properties of Glycothermally Synthesized Ce:YAG Nanophosphors

Abstract

Ce:YAG phosphors used in white LED backlit LCD displays suffer from two major issues: the phosphor powder is too large for next generation microLED display technology, and there is a lack of red in its emission spectrum. To address this, Ce:YAG based nanophosphors were synthesized using the glycothermal method. They were found to contain ~10x higher concentrations of octahedrally coordinated Al atoms compared to bulk Ce:YAG, confirmed by NMR spectroscopy. Photoluminescence and photostability were found to be enhanced with the growth of an intrinsic shell layer, made using a two-step growth process. Shell thickness was controlled by the addition of intrinsic precursor material as a ratio of the initial precursor concentration, denoted as the shell-to-seed ratio (SSR). The absorption spectra of Ce:YAG could be tuned up to 40 nm with the addition of Mg-Si pairs. An enhancement of Ce:YAG nanophosphor luminescence was seen when the reaction temperature was slightly elevated, attributed to increased precursor reactivity due to the higher kinetics of precursor ligand cleavage. The introduction of Mn2+ as a co-dopant was found to provide a red component to the Ce:YAG emission spectrum. XRD structural analysis indicated that Mn2+ should preferentially occupy the Y site, though the 594 nm emission characteristic of 4T1(4G)  6A1(6S) transitions in octahedral co-ordination was observed. Mn2+ atoms which occupy Al octahedral sites are well supported for luminescence due to the high density of octahedral sites present after gycothermal synthesis. Mn2+ emission was enhanced using Si4+ as a charge compensator, and further enhanced with the addition of an intrinsic shell layer. Non-radiative energy transfer of Ce to Mn2+ was observed, indicating that the parity allowed 5d  4f transition of Ce acts as an effective sensitizer for Mn2 luminescence. Glycothermal synthesis was found to preserve the luminescent trivalent and divalent oxidation states of Ce and Mn, respectively.