Opposite size dependences of the red/green upconversion intensity ratio in sub-20 nm Yb3+,Er3+-doped β-NaGdF4 nanophosphors

Upconversion nanophosphors (UCNPs) of lanthanide-doped efficient ternary fluoride (β-NaGdF4:Yb3+,Er3+) with sizes below 20 nm are synthesized in a facile solvothermal route. Upconversion luminescence (UCL) of the UCNPs is found to increase with increase of the nanoparticle size prepared at a higher solvothermal-treatment temperature, due to reduction of the surface defects. Interestingly, opposite size dependences of the red/green UCL intensity ratio are observed for the same series of UCNPs in different surface conditions, i.e., with increase of the nanoparticle size, the ratio decreases for as-prepared dry powders, but increases for moisturized counterparts after being exposed in air for some days, and interesting the ratio decreases again after annealing of the moisturized samples. The phenomenon is explained due to the effect of surface defects and adsorbed water molecules, respectively, dominating in nonradiative relaxations, which usually facilitate red UCL. The disclosed phenomenon clarifies some contradictory observations on UCL in the literature.