Understanding Energy Transfer Mechanisms for Tunable Emission of Yb³⁺-Er³⁺ Codoped GdF₃ Nanoparticles: Concentration-Dependent Luminescence by Near-Infrared and Violet Excitation

Abstract

Energy transfer (ET) is an important route to manage the population density of excited states, giving rise to spectrally tunable emission that is valuable for multicolor imaging and biological tracking. In this paper, a case study of GdF3 nanoparticles (NPs) codoped with Yb3+ and Er3+ was used to experimentally and theoretically investigate the ET mechanisms under near-infrared and violet excitation. Red-to-green ratio (RGR) is used as a primary evaluating protocol, and the power-dependent luminescence and Er3+ 4I13/2 luminescence behavior are used to identify the corresponding conjectures about ET mechanisms. Compared with the four common upconversion (UC) models, a joint effect of energy-back-transfer, multiphonon relaxation, and linear decay depletion mechanisms for the Er3+ 4I13/2 manifold was proposed for the UC process based on UC spectra for samples with different dopant concentrations. Meanwhile, the varying RGR could also be observed from downshifting (DS) emission spectra. The ET mechanism for the DS process, where three cross-relaxation processes coexisted including the Yb3+ 2F5/2 manifold as energy in-transit state, was proposed for the first time. The findings are expected to provide an approach for understanding ET mechanisms in many Yb3+/Er3+ codoped UC and DS systems and enable spectrally tunable emission properties for applications that require precisely defined optical transitions.

Up-conversion luminescence, temperature sensing properties and laser-induced heating effect of Er3+/Yb3+ co-doped YNbO4 phosphors under 1550 nm excitation

“…Finally, the UC luminescence intensity enhances, but the intensity ratio I R /I G of Er 3+ decreases. In the case of higher concentration of Yb 3+ (greater than 6 mol%) doping, the interionic distance between Er 3+ and Yb 3+ decreases and those aggregated Yb 3+ may become quenching center, resulting in the quenching of UC luminescence of Er 3+  35 . In addition, at higher doping concentration of Yb 3+ , the population of 4 I 11/2 level may be reduced via BET process much easier than that of 4 I 13/2 (as can be seen in Fig.…”

Lanthanide-Doped KLu2F7 Nanoparticles with High Upconversion Luminescence Performance: A Comparative Study by Judd-Ofelt Analysis and Energy Transfer Mechanistic Investigation

“…A new UC mechanism involving the population of 4 F 9/2 manifold through an EBT process from high-lying level 4 G 11/2 was proposed3839. From our previous study40, the population of Er 3+ red-emitting manifold should not be tailored mainly by cross-relaxation or multiphonon processes in Yb 3+ -Er 3+ system. Therefore, we compare the two disputed EBT processes (see Supplementary Fig.…”

Morphology evolution and pure red upconversion mechanism of β-NaLuF4 crystals

“…As is known, CR is dependent on the distance among activators46. The average distance between Er 3+ reduces with the increase of Er 3+ dopant dose, which would result in the enhancement of CR effect.…”