Evaluation of relaxation dynamics from excited states of Ho³⁺ ions in sol–gel nano-glass-ceramic materials

In this paper, a series of oxyfluoride SiO₂-LaF₃ nano-glass-ceramics (GCs) activated by Ho³⁺ ions with variable concentrations were synthesized by the sol-gel method. The crystallization of the LaF₃ fluoride phase for all prepared series was verified by XRD measurements and TEM microscopy. For the fabricated GCs, a series of specific luminescence bands within VIS, NIR, and MIR regions were recorded, and the most prominent emissions were assigned to the following 4f¹⁰-4f¹⁰ transitions of Ho³⁺ ions: (⁵S₂,⁵F₄) → ⁵I₈ (green), ⁵F₅ → ⁵I₈ (red), ⁵I₆ → ⁵I₈ (NIR, ∼1.19 μm), and ⁵I₇ → ⁵I₈ (MIR, ∼2.0 μm). Based on the mutual intensities of recorded individual emission bands, their percentage contributions (<i>β</i>) were evaluated. It was found that the calculated values of <i>β</i> coefficients are strongly dependent on the applied annealing temperature (700 or 900 °C), the average size of precipitated LaF₃ crystals, and the concentration of Ho³⁺ ions. Thus, the results indicate that the connotation in the mutual population of the individual excited states (<i>i.e.</i>, (⁵S₂,⁵F₄), ⁵F₅, ⁵I₆, and ⁵I₇) - correlated with the relaxation dynamics from the above-mentioned levels - relies on the phonon energy (modified during an increase in annealing temperature (700 °C → 900 °C)) in the nearest proximity around Ho³⁺ ions by activating (or suspending) the non-radiative multiphonon channels (MPR). The performed luminescence studies tentatively showed that increasing the heat-treatment temperature favors Ho³⁺ migration from the silicate network into the low-oscillation energy LaF₃ phase, unfavoring the non-radiative MPR processes between the (⁵S₂,⁵F₄) and the ⁵F₅ states, and also between the excited ⁵I₆/⁵I₇ states and the ⁵I₈ ground level. The prepared Ho³⁺-doped GCs are able to emit green light with high color purity (CP) reaching even 98%, and generate long-lived NIR/MIR emissions at ∼1.19 μm (up to <i>τ</i>(⁵I₆) = 10.19 ms) and ∼2.0 μm (up to <i>τ</i>(⁵I₇) = 8.44 ms), which could predispose them for use in optoelectronic devices.