Particle-size-dependent upconversion luminescence of NaYF₄: Yb, Er nanoparticles in organic solvents and water at different excitation power densities

A systematic study of the luminescence properties of monodisperse β-NaYF₄: 20% Yb³⁺, 2% Er³⁺ upconversion nanoparticles (UCNPs) with sizes ranging from 12-43 nm is presented utilizing steady-state and time-resolved fluorometry. Special emphasis was dedicated to the absolute quantification of size- and environment-induced quenching of upconversion luminescence (UCL) by high-energy O-H and C-H vibrations from solvent and ligand molecules at different excitation power densities (P). In this context, the still-debated population pathways of the ⁴F_9/2 energy level of Er³⁺ were examined. Our results highlight the potential of particle size and P value for color tuning based on the pronounced near-infrared emission of 12 nm UCNPs, which outweighs the red Er³⁺ emission under "strongly quenched" conditions and accounts for over 50% of total UCL in water. Because current rate equation models do not include such emissions, the suitability of these models for accurately simulating all (de)population pathways of small UCNPs must be critically assessed. Furthermore, we postulate population pathways for the ⁴F_9/2 energy level of Er³⁺, which correlate with the size-, environment-, and P-dependent quenching states of the higher Er³⁺ energy levels.