Enhanced Upconversion Emission from K ^<b>+</b> -Modified NaGdF ₄ :Er ^{3 <b>+</b>} /Yb ^{3 <b>+</b>} Particles in Flexible Free-Standing Films for Thermal Sensing Application

Rare earth-based upconversion (UC) luminescent materials have potential use in optoelectronics, bioimaging, and thermal sensing. In this study, K⁺-modified NaGdF₄:Er³⁺/Yb³⁺ nanoparticles, embedded in transparent and flexible carboxymethyl cellulose-based free-standing films, exhibited enhanced UC luminescence and highly sensitive thermal sensing. Incorporating K⁺ ions into the NaGdF₄:Er³⁺/Yb³⁺ particles modified their rod-like morphology without altering their hexagonal crystalline phase, intensified their UC emission, and prolonged their excited-state lifetimes. Under 980 nm excitation, rising K⁺ content in the NaGdF₄:Er³⁺/Yb³⁺ particles prolonged the lifetimes of the Er³⁺ ⁴S_3/2 (538 nm) and ⁴F_9/2 (653 nm) → ⁴I_15/2 transitions from 54.6 to 308.6 μs and from 208.7 to 691.0 μs, respectively. In addition to enhanced UC emission intensity, the incorporation of K⁺ also influences the branching ratios of emission pathways, thereby altering the relative intensities of green and red emissions and increasing the ⁴S_3/2 → ⁴I_15/2 transition intensity. Systematic evaluation of the temperature-dependent luminescence response of the K⁺-modified NaGdF₄:Er³⁺/Yb³⁺ nanoparticles embedded in the polymeric films demonstrated that the resulting films can potentially function as efficient optical thermometers as the films presented high thermal sensitivity, excellent repeatability over multiple temperature cycles, and stable performance before and after they were embedded with the nanoparticles. These findings underscore that incorporating an alkali metal into a rare earth-doped material optimizes the luminescent and thermometric performance of the material, allows the material to be well dispersed in a polymeric film, and enables the preparation of free-standing films with potential use in advanced optical thermometry and multifunctional sensing.