APT Being Applied in Er:Yb:KY(WO4)2 Green Phosphor with High Color Purity

Trivalent lanthanide ions (Ln3+) have caught a lot attention due to its applications as upconverting luminescent materials in biological analysis, counterfeit in printed documents, and back-lighting in liquid crystal displays.

In upconversion (UC) or anti-Stokes emission, Yb3+ is used to act as sensitizer due to its high absorption cross section, allowing the efficient excitation around 980 nm with cheap diode lasers and followed by energy transfer (ET) to other trivalent Ln3+ ions, such as Er3+, Tm3+ and Ho3+. The Ln3+ ions are suitable for emission process due to their energy level structure and narrow emission bands.

Recently it is reported that Er:Yb::KYW nanocrystals for green phosphor application had been prepared applying ammonium paratungstate (APT) as tungsten source. The produced green phosphor had seen a high color purity. Lanthanide concentration was fixed to 40 at% Yb3+ to synthesize a first group of sample varying Er3+ concentration, for the second group lanthanide concentration was fixed to 4 at% Er3+ varying Yb3+.

The production process of Er:Yb:KY(WO4)2 green phosphors is as below:

Citric acid was utilized as chelation agent and ethylene glycol as esterification agent. Optimal molar ratios were determined previously to 1:1 metal-to-citric acid and 1:2 metal-to-ethylenglycol. Lanthanide oxides Er2O3 (99.99%), Yb2O3 (99.9%) and Y2O3 (99.99%) were added separately to a hot nitric acid solution until nitrate salts were obtained producing a clear aqueous solution. Ammonium paratungstate hydrate (APT, 99.99%) and citric acid were added to the solution. After some hours, ethylene glycol was added and slow water evaporation allowed the formation of a polymer foam. After annealing processes at 573 K and 1073 K, an organic-free white powder was obtained.

In conclusion, Er:Yb::KYW green phosphor had been prepared applying ammonium paratungstate (APT) as tungsten source by the modified Pechini method. Synthesized samples possess single phase showing irregular shape morphology. High UC emission was achieved for a concentration of 4 at% Er3+ 20 at% Yb3+ KY(WO4)2. Doped KY(WO4)2 double tungstate nanocrystals possess high purity color and short dominant wavelength displacement can be considered as good candidate for green upconversion phosphor in displays field and fluorescence technologies.

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