Energy Loss Mechanism of Upconversion Core/Shell Nanocrystals

Small-sized upconversion nanocrystals (<10 nm) show a quite low luminescence efficiency. Even if these nanocrystals are coated by a 2 nm thick inert shell, the core/shell nanocrystals still exhibit weak upconversion luminescence. The involved energy loss mechanism is under debate. Here, we have demonstrated that the major contribution to the low upconversion efficiency is ascribed to an overtone vibrational energy transfer from electronic transition of the Yb³⁺ excited state to overtone transitions of deactivating group vibrations by dipole–dipole coupling. The maximum coupling distance reaches ∼11 nm. Moreover, we first find that an ultrathick inert shell (>11 nm) is not beneficial for upconversion luminescence due to a strong scattering effect. A novel lifetime model is proposed to precisely describe the decay times of the Yb^{3+ 2}F_5/2 state and Er^{3+ 4}S_3/2 state as a function of inert-shell thickness. Based on an insight into the luminescence loss, we design β-Na91%YbF₄:9%Er@NaGdF₄ nanocrystals with a 11.2 nm inert shell to completely block surface quenching and achieve more efficient and stronger upconversion emission.