Luminescent Rare-Earth-Doped LiYbF₄ Nanoparticles And Their Upconversion Properties

Lanthanide(Ln3+)-doped upconversion nanoparticles(UCNPs)have recently attracted tremendous interest in the field of biomedicine owing to their superior physicochemical feature and unique optical properties.In this dissertation,we report a unique strategy for the synthesis of high-quality LiYbF4:Ln3+ core-only and core/shell UCNPs with tunable particle size and shell thickness through a modified high-temperature co-precipitation method.Based on upconversion(UC)spectroscopy and UC quantum yield(QY)measurement under 980-nm excitation,we investigated systematically the optical properties and excited-state dynamics of the energy transfer UC from Er3+,Ho3+ and Tm3+ and the cooperative sensitization upconversion(CSU)from Tb3+ in LiYbF4:Ln3+UCNPs.It was found that the ultraviolet(UV)UC emission of Tm3+ in LiYbF4 was unusually stronger than in other Tm3+-activated UCNPs ever reported under excitation at a low power density of 5 W·cm-2,which can be attributed to the unique crystal structure of LiYbF4 and energy migration among Yb3+ sub-lattice.The intense UC luminescence(UCL)in UV region is highly desired for triggering the photochemical reactions in several promising biomedical applications,such as the remote control of drug delivery and cell adhesion.Through surface passivation,we achieved efficient non-CSU with absolute UCQYs of 3.36%,0.69%and 0.81%for Er3+,Ho3+ and Tm3+,respectively.Particularly,we for the first time quantitatively determined the CSU efficiency for Tb3+with an absolute QY of 0.009%under excitation at a power density of 70 W·cm-2.Furthermore,by means of temperature-dependent steady-state and transient UC spectroscopy,we unraveled the dominant mechanisms of phonon-assisted cooperative energy transfer(T>100 K)and sequential dimer ground-state absorption/excited-state absorption(T<100 K)for the CSU process in LiYbF4:Tb UCNPs,thus lying a foundation for their optical performance optimization towards versatile bioapplications.Finally,based on multi-layer core/shell nanostructured design,we realized UCL multicolor tuning and filtration-shell-mediated orthogonal excitations-emissions UCL in a single LiYbF4:Ln3+ core/shell UCNPs.