The Impact of Silver Codoping on Tb³⁺ Luminescence in Lithium Tetraborate Glasses

Spectroscopic properties of Tb-doped and Tb-Ag codoped lithium tetraborate (LTB) glasses with Li₂B₄O₇ (or Li₂O-2B₂O₃) composition are investigated and analysed using electron paramagnetic resonance (EPR), optical absorption, photoluminescence (PL) and photoluminescence excitation (PLE) spectra, PL decay kinetics and absolute quantum yield (QY) measurements. PL spectra of the investigated glasses show numerous narrow emission bands corresponding to the ⁵D₄ → ⁷F_J (J = 6-0) and ⁵D₃ → ⁷F_J (J = 5-3) transitions of Tb³⁺ (4f⁸) ions. The most intense PL band of Tb³⁺ ions at 541 nm (⁵D₄ → ⁷F₅ transition) is characterised by a lifetime slightly exceeding 2.6 ms. PL spectra of the Ag-containing glass show two broad weakly resolved emission bands in the violet-green spectral range attributed to Ag⁺ ions and nonplasmonic Ag nanoclusters. Decay kinetics of these bands are nonmonoexponential, characterised by an average lifetime in the microsecond range. A significant enhancement of the PL intensity and PL QY of Tb³⁺ ions was observed in Tb-Ag codoped LTB glass in comparison with Tb-doped LTB glass. The excitation energy transfer (EET) mechanisms from Ag⁺ ions and Ag nanoclusters to Tb³⁺ ions were explored.