Laser-induced changes on the upconversion luminescence properties of BiF3:Yb,Er nanoparticles.

BiF 3 nanoparticles doped with Yb3+ and Er3+ ions in varying ratios were prepared by co-precipitation method and its upconversion luminescence (UCL) properties were investigated in detail before and after high-intensity laser treatment. Green-to-red ratio (GRR) of luminescence from cubic-BiF 3 :Yb,Er nanoparticles is found to increase with an increase in laser power for as prepared sample. Unlike this, for conventional Yb,Er doped upconversion nanoparticles (UCNPs) such as cubic NaYF 4 :Yb,Er and hexagonal-NaYF 4 :Yb,Er, a decrease in GRR values is observed with an increase in laser power. This has been explained based on the reduced extent of Er3+-Er3+ interaction in BiF 3 :Yb,Er nanoparticles compared to that existing in cubic and hexagonal forms of NaYF 4 :Yb,Er, nanoparticles. Reduced interaction is brought about by higher lattice parameter of BiF 3 host compared to the NaYF 4 host which leads to a lesser extent of cross-relaxation in the former host thereby favoring the population of 4S 3/2 and 2H 11/2 levels of Er3+. Effect of high-intensity laser (180W/cm2) irradiation on BiF 3 :Yb,Er nanoparticles resulted in an irreversible five-fold increase in upconversion luminescence intensity and this is attributed to combined effect of partial conversion of BiF 3 to cubic Bi 2 O 3 and removal of adsorbed water molecules. These inferences are confirmed by XRD and TEM measurements carried out on laser-treated sample. Improved luminescence intensity from the laser-treated sample is also confirmed by the high-contrast clear images obtained from impressions made using high-intensity laser irradiation on BiF 3 :Yb,Er nanoparticles. BiF 3 :Yb,Er nanoparticles showed an excellent thermal sensitivity of 0.3% K−1 at room temperature and the value is comparable with that of conventional β-NaYF 4 :Yb,Er nanoparticles. • Green to red ratio of luminescence increases with increase in laser power. • Luminescence intensity increases for BiF 3 :Yb,Er nanoparticles upon laser treatment. • Laser treatment leads to partial conversion of BiF 3 to Bi 2 O 3. [ABSTRACT FROM AUTHOR]