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

Abstract

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, similar to 1.19 mu m), and 5I7 -> 5I8 (MIR, similar to 2.0 mu m). Based on the mutual intensities of recorded individual emission bands, their percentage contributions (beta) were evaluated. It was found that the calculated values of beta coefficients are strongly dependent on the applied annealing temperature (700 or 900 degrees 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.e., (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 degrees C -> 900 degrees 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 similar to 1.19 mu m (up to tau(5I6) = 10.19 ms) and similar to 2.0 mu m (up to tau(5I7) = 8.44 ms), which could predispose them for use in optoelectronic devices.