Comprehensive study on the size effects of the optical properties of NaYF₄:Yb,Er nanocrystals

Monodisperse β-NaYF₄:Yb,Er nanocrystals with mean sizes of 11, 40, and 110 nm were synthesized by a thermal decomposition solvothermal process to better understand the relationship between particle size and optical properties. A systematic study of luminescence intensity versus size revealed that both visible upconversion and infrared downconversion emission intensities decrease with decreasing nanocrystal size. The intrinsic quantum efficiency of the infrared ⁴I_13/2 → ⁴I_15/2 downconversion transition was studied in great detail since this specific transition allows us to quantify the contribution of nonradiative losses more easily than the observed upconversion transitions. The intrinsic quantum efficiency of the ⁴I_13/2→⁴I_15/2 transition decreased from 50% (110 nm) to 15% (11 nm). Multiphonon relaxation and -OH quenching was studied in these materials by measuring the vibrational characteristics of β-NaYF₄:Yb,Er nanospheres. While multiphonon relaxation exhibited increased contribution to nonradiative decay, -OH quenching rates were calculated to be ∼4 orders of magnitude higher than that of the multiphonon relaxation. Therefore, surface -OH quenching effects were concluded to be primarily responsible for the observed dependence of emission intensity versus particle size.