Effect of Yb³⁺ concentration on the upconversion emission properties of sub 10 nm RbY₂F₇:Yb, Er nanoparticles

Upconversion nanoparticles (UCNPs) are a fascinating group of luminescent materials known for their ability to convert low-energy photons to high-energy photons. Many inorganic UCNPs have been studied to understand the underlying mechanism of upconversion phenomena, out of which alkali rare-earth fluorides (AREFs) such as NaREF₄, LiREF₄ and KREF₄ were found to exhibit high upconversion efficiencies. This work investigates a similar AREF upconversion nanoparticle viz RbY₂F₇:Yb, Er. This UCNP system was synthesised by a modified thermal decomposition method by varying the Yb concentration from 20% to 98%. Structural analysis using XRD revealed that all the synthesised samples were found to be formed in the orthorhombic phase irrespective of the increasing Yb concentration. The thermal decomposition method greatly aided in the reduction of particle size. HRTEM analysis revealed that the as-synthesised UCNPs have a spherical morphology with an average particle size of 7.9 ± 0.2 nm. The upconversion emission studies taken by exciting the samples with a 975 nm laser show three distinct peaks at 527, 542 and 656 nm. The experimental results indicate that the increased Yb³⁺ concentration improves the red to green intensity ratio by supporting the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ through the energy back transfer process between Er³⁺ and Yb³⁺. Notably, there is a reduction in the overall emission intensity with increasing Yb³⁺ concentration. Furthermore, the decay lifetime studies show a decreasing trend at 542 nm and 656 nm emission lines with an increase in the Yb concentration, which is because of the concentration quenching effect of the increasing Yb concentration.