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

Abstract

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 one-pot solvothermal method. Under 980 nm excitation, NaBiF₄:Yb³⁺/Er³⁺(Tm³⁺) UCNPs show excellent UCL. The UCL intensities of NaBiF₄:Yb³⁺/Er³⁺ and NaBiF₄:Yb³⁺/Tm³ 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 (r² 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.