Multi-color Upconversion Luminescence Tuning by Thermal Process with Silica Nanoparticles

Department of Chemical EngineeringMulticolor luminescence tuning of upconversion nanocrystals (UCNs) is important to realize efficient photon upconversion for versatile applications ranging from bioimaging to encoding systems. Commonly used luminescence emission tuning methods, however, is limited by the complex synthetic process with precise stoichiometric control of lanthanide dopants. Here, we describe a novel macro-synthesis method of in which SiO2 nanoparticles involved sintering process influences the inter-atomic distance to simultaneously manipulate the crystal phase (hexagonal to apatite), and multi-color luminescent efficiency of the UCNs. In the presence of SiO2 nanoparticles, the hexagonal-to-apatite phase transition accompanies the color transition from yellow to green with high luminescence property, however, the hexagonal-to-cubic phase transition without SiO2 nanoparticles raised to color transition from yellow to red which is weak and invisible under near-infrared light. Through this SiO2 nanoparticles assistance, we also observed luminescence enhancement after a sintering process. These effects are explained by DFT calculation. We show that emission wavelength of UCNs can be manipulated through crystal phase transition with the change of SiO2 NPs (nanoparticles) concentration and sintering temperature. Our results demonstrate potential application for covert multi-level security (MLS) system, including multi-colored emissive upconversion microcrystals (UCM) fabrication.ope