Quantum yield and energy transfer in up-conversion SrGd2O4:Yb, Er nanoparticles obtained via sol-gel assisted combustion

Nanopowders of up-conversion SrGd2O4 orthorhombic (Pnma) phase co-doped with different Yb3+ (1, 2.5 and 5 at%) and constant Er3+ (0.5 at%) ions were successfully prepared via sol-gel assisted combustion. Rietveld refinement indicated unit cell lattice parameters increase with Yb3+ and Er3+ ions doping. Scanning transmission electron microscopy with corresponding energy-dispersive X-ray spectroscopy revealed that obtained powders are composed of agglomerated nanoparticles that have a uniform distribution of all constituting elements. Photoluminescence measurements implied intensification of the up-conversion (UC) emission in the visible part of spectrum with the increase of Yb3+ content, which is followed by a significant change in the green to red ratio. Two-photon UC processes are established as a result of Er3+ f-f electronic transitions: green emission at 523 and 551 nm (2H11/2, 4S3/2 → 4I15/2) as well as a red emission at 661 nm (4F9/2 → 4I15/2). The highest value of absolute quantum yield of 0.055% is determined for SrGd2O4 nanoparticles doped with 0.5 at% of Er3+ and co-doped with 5 at% of Yb3+ (λexc = 976 nm, power density 200 W/cm2).