Upconversion luminescence modulated by alkali metal (Li, Na, and K) induced crystallization in Er3+/Yb3+ co-doped β-PbF2 oxyfluoride glass ceramics
Alkali metal ions (Li+, Na+ and K+) are introduced into oxyfluoride glass ceramics (GCs) by a traditional melt-quenching and thermal treatment method to regulate crystallization behavior and upconversion luminescence (UCL) of β-PbF2:Er3+/Yb3+ nanocrystals (NCs). The crystallization behavior of β-PbF2:Er3+/Yb3+ NCs in GCs is characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), EDX analysis, Raman spectroscopy and photoluminescence (PL) spectroscopy with Eu3+ ions as structural probes. Li+, Na+ and K+ ions are inserted into a glass matrix to destroy its glass network structure, which results in fluoride crystallization even without thermal treatment. Compared with alkali metal ion-free doped GCs, crystallization behavior and UCL properties of β-PbF2:Er3+/Yb3+ NCs in Li+ doped GCs are almost unchanged because Li+ ions are too small to incorporate into the β-PbF2:Er3+/Yb3+ NCs. Na+ and K+ ions are believed to enter the β-PbF2 lattice by substituting Pb2+ ions due to their similar ionic radii. Na+ ions as charge compensation ions doped into the β-PbF2:Er3+/Yb3+ NCs result in sharp lattice shrinkage due to their smaller ionic radius than Pb2+ ions. β-PbF2:Er3+/Yb3+ NCs doped with Na+ ions become more crowded because of a tight network structure limited by a Si–O layer. The crowded growth of β-PbF2:Er3+/Yb3+ NCs results in a very small distance between Er3+ and Yb3+ ions, which leads to a strong EBT process between Er3+ and Yb3+ ions, then a nearly pure red UCL emission (the maximum intensity ratio of red to green (RRG)) was obtained. Meanwhile, K+ ions as charge compensation ions doped into the β-PbF2:Er3+/Yb3+ NCs result in slight lattice expansion of the β-PbF2:Er3+/Yb3+ NCs due to the larger ionic radius of K+ ions than that of Pb2+ ions. K+ ions could act as a self-promoting agent for the enhanced crystallization of the β-PbF2:Er3+/Yb3+ NCs. UCL intensity of β-PbF2:Er3+/Yb3+ NCs doped with K+ ions increases by about 30 times due to high crystallinity and lattice distortion around Er3+ ions. The results show that simultaneous co-doping of alkali metal ions (Li+, Na+ and K+) in oxyfluoride GCs is a very effective way to integrate this material for advanced photonic applications.