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

BiF3 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-BiF3: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 NaYF4:Yb,Er and hexagonal-NaYF4: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 BiF3:Yb,Er nanoparticles compared to that existing in cubic and hexagonal forms of NaYF4:Yb,Er, nanoparticles. Reduced interaction is brought about by higher lattice parameter of BiF3 host compared to the NaYF4 host which leads to a lesser extent of cross-relaxation in the former host thereby favoring the population of 4S3/2 and 2H11/2 levels of Er3+. Effect of high-intensity laser (180W/cm2) irradiation on BiF3: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 BiF3 to cubic Bi2O3 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 BiF3:Yb,Er nanoparticles. BiF3: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-NaYF4:Yb,Er nanoparticles.