Influence of the synthesis method on preferential clustering of Yb3+ in CaF2:Yb3+/Er3+ upconverting nanoparticles

Abstract Yb3+/Er3+ co-doped CaF2 upconverting nanoparticles have been synthesized by three preparation methods. Their 12 K absorption measurements exhibit signatures of preferentially formed Yb3+ clusters as the predominant centres along with a single ion cubic (Oh) centre. By changing the synthesis method from decomposition to co-precipitation and finally to hydrothermal, the ratio between negatively charged clusters and non-locally charge-compensated cubic centres decreases becoming similar to that in bulk crystals. For nanoparticles prepared by co-precipitation and decomposition the Er3+ 4S3/2,4F9/2 → 4I15/2 luminescence measured monitoring the heterogeneous Yb–Er cluster centres is insensitive to the excitation wavelength, giving a similar intensity when excited at around 10,204 cm−1 (980 nm) and 10,250 cm−1 (975 nm). However, for nanoparticles prepared by a hydrothermal growth method the intensity when excited at 10,250 cm−1 (975 nm), coinciding with the absorption maximum, is twice the yield when excited at 10,204 cm−1 (980 nm). By varying the optimum excitation wavelength and growth method, a factor of 102 times enhancement of the upconversion luminescence is achieved.