Upconversion luminescence modulated by alkali metal (Li, Na, 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 up-conversion luminescence (UCL) of β-PbF2:Er3+/Yb3+ nanocrystals (NCs). The crystallization behavior of β-PbF2:Er3+/Yb3+ NCs in GCs are characterized by Xray diﬀraction (XRD), high-resolution transmission electron microscopy (HRTEM), EDX analysis, Raman spectra and photoluminescence (PL) spectra of Eu3+ ion as a structural probe. Li+, Na+ and K+ ions are inserted into glass matrix to destroy glass network structure which results in fluoride crystallization even without thermal treatment. Compared with free from alkali metal ions 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 for Pb2+ ions due to their similar ionic radius. Na+ ion as charge compensation ion doping into β-PbF2:Er3+/Yb3+ NCs results in lattice shrinkage sharply due to its smaller ionic radius than that of Pb2+ ion, β-PbF2:Er3+/Yb3+ NCs doped with Na+ ions grow more crowded because of tight limited by 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 nearly pure red emission (maximum intensity ratio of red to green (RRG)) UCL was obtained. Meanwhile, K+ ion as charge compensation ion doping into β-PbF2:Er3+/Yb3+ NCs results in slightly lattice expansion of β-PbF2:Er3+/Yb3+ NCs due to larger ionic radius of K+ ion than that of Pb2+ ions. K+ ion 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.