Studies of terbium bridge: Saturation phenomenon, significance of sensitizer and mechanisms of energy transfer, and luminescence quenching

Terbium chain in the form of S → (Tb³⁺)_n → A (S = Ce³⁺ or Eu²⁺, A = Eu³⁺), as a promising energy transfer (ET) approach, has been proposed to enhance Eu³⁺ emission for solid-state lighting. However, the viewpoint of ET from S to A via the terbium chain (Tb³⁺-Tb³⁺-Tb³⁺-⋯) is very doubtful. Here, hosts of Ba₃Ln(PO₄)₃, LnPO₄, LnBO₃, and Na₂Ln₂B ₂O₇ doped with Ce³⁺ → (Tb ³⁺)_n → Eu³⁺ or (Tb³⁺) _n → Eu³⁺ are synthesized to prove the universality of S → (Tb³⁺)_n → A in inorganic hosts and to study the unsolved issues. Saturation distance of Tb³⁺-Eu³⁺, estimated with the empirical data of different hosts, is proposed to be a criterion for determining whether a spectral chromaticity coordinate keeps constant. A branch model is put forward to replace the chain model to explain the role of (Tb³⁺)_n in ET from Ce³⁺ to Eu ³⁺ and the necessity of high content of Tb³⁺; the term "terbium bridge" is used to replace "terbium chain", and the value of n is determined to be two or three. The intensity quenching of Eu ³⁺ emission is attributed to the surface defects ascribed to the smaller particles and larger specific surface area rather than the concentration quenching of Tb³⁺. Based on the saturation distance and the mechanism of luminescence quenching, the necessary concentration of Tb ³⁺ for (Tb³⁺)_n can be estimated as long as the cell parameters are already known and the luminescent efficiency of Eu ³⁺ can be further improved by optimizing the synthesis method to decrease the quantity of surface defects.