Cooperative and non-cooperative sensitization upconversion in lanthanide-doped LiYbF₄ nanoparticles

Lanthanide (Ln³⁺)-doped upconversion nanoparticles (UCNPs) have attracted tremendous interest owing to their potential bioapplications. However, the intrinsic photophysics responsible for upconversion (UC) especially the cooperative sensitization UC (CSU) in colloidal Ln³⁺-doped UCNPs has remained untouched so far. Herein, we report a unique strategy for the synthesis of high-quality LiYbF₄:Ln³⁺ core-only and core/shell UCNPs with tunable particle sizes and shell thicknesses. Energy transfer UC from Er³⁺, Ho³⁺ and Tm³⁺ and CSU from Tb³⁺ were comprehensively surveyed under 980 nm excitation. Through surface passivation, we achieved efficient non-cooperative sensitization UC with absolute UC quantum yields (QYs) 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 0.0085% under excitation at a power density of 70 W cm^-2. 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:Tb³⁺ UCNPs.