Spectral characterization of LiYbF upconverting nanoparticles.

In light of the recent developments on Yb-based upconverting rare-earth nanoparticles (RENPs), we have systematically explored the spectral features of LiYbF:RE/LiYF core/shell RENPs doped with various amounts of Tm, Er, or Ho. Tm-RENPs displayed photoluminescence from the UV to near-infrared (NIR), and the dominant high-photon-order upconversion emission of these RENPs was tunable by Tm doping. Similarly, Er- and Ho-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 RE doping concentration. Particularly in the case of Tm-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 Yb/Tm-doped systems, yet due to the high number of sensitizer ions in the LiYbF host these RENPs are expected to be brighter and thus better suited for applications such as controlled drug delivery or optogenetics. Overall, LiYbF:RE/LiYF RENPs are promising systems to effectively generate high-order upconversion emissions, owing to excitation energy confinement within the Yb network and its efficient funneling to the activator dopants.