Benefits of surfactant effects on quantum efficiency enhancement and temperature sensing behavior of NaBiF4 upconversion nanoparticles

Upconversion luminescent materials have received much attention due to their unique luminescence properties and potential applications in various fields. However, the weak upconversion luminescence (UCL) intensity and low quantum yield often limit their development. Herein, Yb3+/Er3+(Tm3+) codoped NaBiF4 upconversion nanoparticles (UCNPs) have been successfully synthesized through a facile onepot solvothermal method. Under 980 nm excitation, NaBiF4: Yb3+/Er3+(Tm3+) UCNPs show excellent UCL. The UCL intensities of NaBiF4: Yb3+/Er3+ and NaBiF4: Yb3+/Tm-3 UCNPs could be further enhanced through polyacrylic acid (PAA) modification on the surface to be 4.2 and 3.1 times stronger than those of their unmodified counterparts, respectively. The UCL quantum yields of the PAA-modified NaBiF4 UCNPs are 0.32% (Er3+) and 3.70% (Tm3+), which are higher than those of the unmodified NaBiF4: Yb3+/Er3+(Tm3+) UCNPs and NaYF4: Yb3+/Er3+(Tm3+) UCNPs (the most efficient UC materials). Moreover, the temperature-dependent UCL properties of NaBiF4: Yb3+/Er3+ and PAA-modified NaBiF4: Yb3+/Er3+ were investigated systematically under 980 nm excitation. The modification of PAA on the surface of the NaBiF4: Yb3+/Er3+ UCNPs could effectively improve the fit degree (r(2) value) of the experimental data and the maximum thermal sensitivity of PAA-modified NaBiF4: Yb3+/Er3+ could reach 0.0040 K-1 at 498 K. Therefore, PAA-modified NaBiF4: Yb3+/Er3+ UCNPs with strong UCL and high quantum yield could be a potential candidate for sensitive temperature sensors.