Spectral characterization of LiYbF₄ upconverting nanoparticles

In light of the recent developments on Yb³⁺-based upconverting rare-earth nanoparticles (RENPs), we have systematically explored the spectral features of LiYbF₄:RE³⁺/LiYF₄ core/shell RENPs doped with various amounts of Tm³⁺, Er³⁺, or Ho³⁺. Tm³⁺-RENPs displayed photoluminescence from the UV to near-infrared (NIR), and the dominant high-photon-order upconversion emission of these RENPs was tunable by Tm³⁺ doping. Similarly, Er³⁺- and Ho³⁺-RENPs with green and red upconversion showed wide color tuning, depending on the doping amount and excitation power density. From steady-state power plot and photoluminescence decay studies we have observed respective changes in upconversion photon order and average lifetime that attest to a number of cross-relaxation processes occurring at higher RE³⁺ doping concentration. Particularly in the case of Tm³⁺-RENPs, cross-relaxation promotes four- and five-photon order upconversion emission in the UV and blue spectral regions. The quantum yield of high-order upconversion emission was on par with classic Yb³⁺/Tm³⁺-doped systems, yet due to the high number of sensitizer ions in the LiYbF₄ host these RENPs are expected to be brighter and thus better suited for applications such as controlled drug delivery or optogenetics. Overall, LiYbF₄:RE³⁺/LiYF₄ RENPs are promising systems to effectively generate high-order upconversion emissions, owing to excitation energy confinement within the Yb³⁺ network and its efficient funneling to the activator dopants.