Negative thermal quenching effect of NaYbF4:Tm nanoparticles: Towards high performance LIR temperature sensing

Abstract

Lanthanide-doped upconversion nanoparticles excel in many fields of applications due to their unique anti-Stokes spectroscopic properties, narrow emission bands, and high photochemical stability. Yb-based self-sensitized fluorides nanoparticles have higher absorption efficiency at 980 nm near-infrared laser than Y-based nanoparticles. This enhanced absorption makes NaYbF₄ an outstanding upconversion host material for pumping more photons to high-energy excited states of Tm³⁺ that are rich in energy levels. For these purposes, NaYbF₄:Tm nanoparticles seem to be more favorable for photoluminescence applications. In this study, size-adjustable NaYbF₄:Tm nanoparticles were prepared using a high-temperature co-precipitation method. Under 980 nm excitation, the upconversion emission spectra appeared four emission bands in visible region. By increasing the size of nanoparticles, we well enhanced upconversion luminescence, indicating that surface quenching sites play a major role in inhibiting upconversion luminescence. In addition, the negative thermal quenching effect of NaYbF₄:Tm nanoparticles was observed with temperature dependent luminescence spectra, and we found that smaller sized nanoparticles got stronger negative thermal enhancement and better temperature sensing performance. We fitted the luminescence intensity ratio (LIR) between the ¹D₂ → ³F₄ (451 nm) and ¹G₄ → ³F₄ (647 nm) transitions of Tm³⁺, where these nanoparticles exhibited both good absolute sensitivity S_A (1.38 % K⁻¹, at 453 K) and relative sensitivity S_R (2.56 % K⁻¹, at 293 K). These results suggested that NaYbF₄:Tm nanoparticles could be a good candidate for thermometers even at high temperatures (up to 500 K).