Red-emitting NaSrY(BO 3 ) 2 :Er 3+ /Yb 3+ upconversion phosphors: synthesis, characterisation, and optical thermometry

Red-emitting upconversion (UC) phosphors NaSrY0.97-xYb0.03Erx(BO3)2 (x = 0–0.15) and NaSrY0.93-yYbyEr0.07(BO3)2 (y = 0–0.07), with a single-phase structure and wide band gaps that decreased with increasing Yb3+ content, were synthesized by a combustion method. SEM images revealed cauliflower-like agglomerates with an average size of ~250 nm, while elemental analysis confirmed the presence of all constituent elements. Under 932 nm excitation, the phosphors exhibited dominant red UC emission with weak green components arising from Er3+ transitions. The optimum dopant concentrations were 7 mol% Er3+ and 3 mol% Yb3+ at higher power. Power-dependent studies indicated a two-photon UC process. Temperature-dependent luminescence showed thermal quenching, whereas NaSrY0.90Yb0.03Er0.07(BO3)2 demonstrated promising optical thermometry performance with maximum absolute and relative sensitivities of 0.02708 K−1 and 0.00708 K−1 at 313.15 K. Stable red emission highlights potential applications in bioimaging, displays, and photovoltaics.