Concentration-driven structural evolution and enhanced photoluminescence dynamics in Ho3+-activated SrMoO4 nanophosphors.

Rare-earth-activated molybdate phosphors have attracted considerable interest because of their tunable structural and luminescence properties for photonic applications. In this work, Ho3+-doped SrMoO4 nanoparticles were synthesized through a controlled hydrothermal method employing Chikku (Manilkara zapota) leaf extract as a plant-derived processing additive. Structural characterization revealed the formation of a phase-pure tetragonal scheelite SrMoO4 structure for all investigated Ho3+ concentrations, with no detectable secondary crystalline phases. The observed structural changes are consistent with Ho3+ incorporation into the SrMoO4 host lattice, although direct confirmation of the substitutional site requires complementary characterization. The average crystallite size decreased from approximately 34 to 25 nm with increasing Ho3+ concentration from 1 to 7 mol%. UV-vis diffuse reflectance measurements showed a gradual modification of the absorption-edge profile, with the estimated optical band gap increasing from 4.10 to 4.17 eV. Under 451 nm excitation, the samples exhibited characteristic visible Ho3+ emissions, with prominent green and red emission bands near 545 and 658 nm, respectively. The relative emission intensity increased with Ho3+ concentration, reaching the highest value for the 7 mol% sample. Time-resolved photoluminescence measurements revealed a progressive increase in the emission lifetime from 148.62 µs for pristine SrMoO4 to 326.95 µs for the 7 mol% Ho3+-doped sample. No reduction in lifetime was observed within the investigated concentration range, indicating that significant concentration quenching is not evident up to 7 mol% Ho3+. Overall, the results demonstrate a concentration-dependent relationship between the structural characteristics, morphology, visible photoluminescence, and emission decay dynamics of Ho3+ activated SrMoO4 nanoparticles prepared through a plant-assisted hydrothermal route.