Up-conversion in Er3+/Yb3+ co-doped LaPO4 submicron-sized spheres

• The Er3+/Yb3+ co-doped LaPO4 sub-micron spheres were synthesized.
• The up- and down-luminescence conversion was characterized.
• We investigated the influence of thermal treatment on luminescence efficiency.
• The efficiency after thermal treatment was significantly improved.

Er3+/Yb3+ co-doped materials have been extensively used for imaging in biomedical applications using either visible up-converted (UC) or near-infrared (NIR) emissions. The UC spectrum is composed mainly by two Erbium emissions in the green (2H11/2:4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) spectral range, while the NIR spectrum includes Er3+ (λ ∼ 1.5 μm, 4I13/2 → 4I15/2) and Er3+/Yb3+ (λ ∼ 980 nm, 2F5/2 → 2F7/2 (Yb3+):4I11/2 → 4I15/2 (Er3+)) transitions; which relative intensities are dependent on several physical parameters. In the present work, we present the preparation and optical characterization of Er3+/Yb3+ co-doped LaPO4 submicron-sized spheres. The luminescence (CW and pulsed) characteristics, after different post-annealing treatments, are studied. It is found that such treatments strongly increment the emission efficiency, possibly due to the suppression of residual impurities. After calcination at 1100 °C the material behaves as an excellent UC and NIR–NIR wavelength converter.