Up- and Down-Convertible LaF₃:Yb,Er Nanocrystals with a Broad Emission Window from 350 nm to 2.8 μm: Implications for Lighting Applications

In this study, emissions between 350 nm and 2.8 μm are produced from molten-salt-synthesized LaF3:Yb3+,Er3+ (LFYE) nanocrystals (NCs) under 980 nm excitation. In fact, such wide spectral emissions from a single phosphor would be highly favorable in lighting applications ranging from light-emitting diodes, optical fibers, and telecommunications. Upconversion luminescence (UCL) spectra show intense red emission compared to the green band below 6 mW, wherein a strong green emission is achieved compared to a red emission when the laser power is increased beyond 85 mW. To further enhance their UCL and downconversion luminescence (DCL), the LFYE NCs are annealed at 400 °C for 30, 60, 90, and 120 min. Their UCL and DCL intensities are improved with up to 90 min of annealing but drastically reduced after annealing for 120 min. The latter is caused by both phase transition and shape changes, that is, from the cubic LFYE NCs to tetragonal LaOF:Yb3+,Er3+ (LOFYE) NCs with a square pyramidal shape. Density functional theory calculations show that the LOFYE NCs are more thermodynamically favorable to form due to their lower defect formation energy than the LFYE NCs. Moreover, LaF3 is dynamically more unstable in comparison to LaOF, as indicated by a greater number of imaginary modes for LaF3 in the phonon dispersion plot. Interestingly, the highest phonon energy for LaF3 is found to be 439 cm–1 while that of LaOF is 527 cm–1, which is expected to play a role in the different emission behaviors between LaF3 and LaOF. Our work shows exemplary potential of the LFYE NCs with near-ultraviolet to mid-infrared emissions as UCL and DCL for laser power-induced color tunability for a wide range of applications.