Tetragonal GeO2.9Nb2O5 phase stabilization-induced modulation of visible and NIR emission and excited-state dynamics in Ho3 +/Yb3+ co-doped GeO2–Nb2O5 nanocomposites under NIR excitation

Ho3+/Yb3+ co-doped GeO2–Nb2O2 nanocomposites were synthesized via the sol–gel method, aiming to investigate the role of tetragonal GeO2.9Nb2O5 phase stabilization on their luminescent properties in the visible and near infrared (NIR). Structural, morphological, and compositional analyses were performed using XRD, Raman spectroscopy, electron microscopy, and energy-dispersive X-ray spectroscopy (EDS). XRD and Rietveld refinement confirmed that increasing the tetragonal phase content (up to 30 wt%) was achieved with higher annealing temperatures. Under 980 nm excitation, upconversion emissions at 541, 650, 755 nm, and a NIR emission at 1220 nm were observed, corresponding to the 5S2,5F4 → 5I8, 5S2, 5F4 → 5I7, and 5I6 → 5I8 transitions, respectively, confirming an efficient energy transfer mechanism from Yb3+ ions. Green and NIR emissions were significantly enhanced in tetragonal-rich samples, while red emission (5F5 → 5I8 transition) was suppressed. Comparison with isolated Ho3+/Yb3+ co-doped GeO2 and GeO2.9Nb2O5 phases confirmed the pivotal role of the tetragonal mixed oxide modulating the emission profile. The longer lifetimes of the Ho3+ excited states observed in samples with a higher fraction of the tetragonal phase suggest that this structure not only reduces non-radiative losses but also provides a more favorable crystal field environment for the Ho3+ ions, contributing to enhanced luminescent efficiency. CIE 1931 chromaticity diagrams illustrate a shift from orange to green, achieving over 87.4 % color purity, which showcases the potential for tunable luminescence in photonic applications.