Upconversion luminescence modulated by alkali metal (Li, Na, and K) induced crystallization in Er3+/Yb3+ co-doped β-PbF2 oxyfluoride glass ceramics

Abstract

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.