White light and near-infrared emissions of Pr³⁺ ions in SiO₂-LaF₃ sol-gel nano-glass-ceramics

In this work, we report the results of our investigations on the structural and luminescence properties of SiO2-LaF3:Pr3+ nano-glass-ceramics synthesized using the sol-gel method. Based on XRD, microscopic (TEM), and ATR-IR measurements, the crystallization of LaF3 nanocrystals favorably occupied by Pr3+ ions and overall transformations within the silicate sol-gel hosts dependent on heat-treatment conditions of the as-prepared amorphous xerogels were characterized. The fabricated oxyfluoride nano-glass-ceramics revealed the emissions within the greenish-blue (P-3(0,1) -> H-3(4), P-3(0,1) -> H-3(5)), reddish-orange (P-3(0,1) -> H-3(6), D-1(2) -> H-3(4), P-3(0) -> F-3(2,3)), and NIR spectral scopes (D-1(2) -> F-3(4),(1)G(4), (1)G(4) -> H-3(5), F-3(3,4) -> H-3(4)). Based on the luminescence spectra in the VIS range, the CIE chromaticity coordinates, correlated color temperatures (CCT), and color purities (CP) were calculated. The obtained results clearly indicate that the prepared Pr3+-doped sol-gel nano-glass-ceramics exhibit warm or neutral white light emissions with CCT values in the range from 2567 K to 3962 K. The lowest CP value was estimated at 12.8%, indicating that the fabricated samples are able to emit bright white light. Additionally, the NIR emissions cover E, S, C, and L bands, which are important for devices applicable in telecommunication technologies. For further characterization, the tau(P-3(0)) and tau(D-1(2)) decay times were estimated. It was established that the emissions from the P-3(0) and the D-1(2) excited states of Pr3+ ions, as well as the participation of cross-relaxation (CR) processes, are dependent on the size of crystallized LaF3 phase, distribution of optically active Pr3+ ions between amorphous and crystalline phase (determining the Pr3+-Pr3+ inter-ionic distances), and relative content of OH groups in the prepared sol-gel hosts.