Energy transfer: origin of concentration quenching

1.3 Optical properties

1.3.4 Energy transfer: origin of concentration quenching

When the excitation energy is absorbed by the system (e.g. Eu₂O₃ and Tb₂O₃), all the Eu³⁺/Tb³⁺ ions within the system do not absorb the energy. The ions that absorb the excitation energy are in the excited state, but the energy is not confined to those ions (excited state). The excitation energy can be transferred from one Eu³⁺/Tb³⁺ ion

Figure 1.9: Schematic energy level diagram of Gd₂O₃with emission transitions. The lowest- energy emission transition (⁶P_7/2 →⁸S_7/2) corresponds to an emission peak in UV (∼ 313 nm) region.

(excited state) to the other Eu³⁺/Tb³⁺ ion (ground state). Energy transfer between the two ions is possible only if (a) the energy differences between the ground state and excited state of the two ions are equal, which is known as energy resonance and (b) a suitable interaction between the two ions is present. The interaction between the two ions may be of exchange type (overlap of wave functions) or an electric or magnetic multipolar type.

The electric multipolar interaction is strong if the electric-multipole transitions are allowed.

Since the electric-dipole transitions within the 4f energy levels of Eu³⁺/Tb³⁺ ions are forced, but not allowed, the multipolar interactions are weak. Energy transfer between the Eu³⁺/Tb³⁺ ions is possible only if the exchange interaction is strong. The strength of the exchange interaction is influenced by the distance between the ions. Typically, the exchange interaction is effective when the distance between the ions is shorter than 5-8 ˚A. At high concentration of the ions (e.g. Eu₂O₃ and Tb₂O₃), the distance between them is short. Since the two identical Eu³⁺/Tb³⁺ions fulfill the energy resonance criterion and the exchange interaction is effective, the energy transfer occurs. Considering the long radiative lifetimes (∼1 ms) of Eu³⁺and Tb³⁺ions, the energy transfer can occur multiple times. The excitation energy can relocate far away from the original location where it was absorbed, which is called energy migration. If the excitation energy migrates to a defect or impurity site where it is lost non-radiatively, the radiative emission (luminescence) is quenched. This process leads to reduction in the luminescence efficiency. This phenomenon is defined as concentration-dependent luminescence quenching or concentration quenching. A schematic of the energy migration process that leads to concentration quenching is shown in figure 1.10.

The Eu³⁺/Tb³⁺-doped RE₂O₃ is preferred over the pure Eu₂O₃/Tb₂O₃ for their higher luminescence efficiency. Common host lattices are Y₂O₃ (lattice constant = 10.60

˚A, space group: Ia3) and Gd₂O₃ (lattice constant: 10.81 ˚A, space group: Ia3). The crystal structure of these host lattices is identical to that of Eu₂O₃ and Tb₂O₃. Typically, Y₂O₃ is employed because yttrium (Y) is abundant compared to gadolinium (Gd). Reducing

Figure 1.10: Schematic of excitation migration mechanism.

the concentration of luminescent ions minimizes the exchange interaction between them.

For example, all lattice sites occupying RE³⁺ ions are filled by Eu³⁺ ions in pure Eu₂O₃. In contrast, 10%Eu³⁺:Gd₂O₃ has only 10% of the RE³⁺ ion occupying lattice sites filled by Eu³⁺ ions. The Eu³⁺ ions occupy these lattice sites in a statistical way. Hence, the Eu³⁺-Eu³⁺distance increases as the Eu³⁺ion concentration decreases, which minimizes the exchange interaction between them at low concentration. The excitation energy migration is reduced and it is confined to the ion where it was absorbed. Thus, the luminescence efficiency is increased.

Low concentration of luminescent ions is required to enhance the luminescence effi- ciency, but a certain concentration should be employed to quench higher energy-level emis- sion transitions by cross-relaxation. In cross-relaxation, a part of the excitation energy of one luminescent ion is transferred to another identical luminescent ion via phonon-assisted energy transfer. Cross-relaxation is the preferred energy transfer mechanism because cer- tain higher energy level emission transitions are quenched at the expense of lower energy level excitation transitions, which can be easily explained for Eu³⁺ and Tb³⁺ ions. Fig- ure 1.11 shows cross-relaxation process using the energy level diagrams of Eu³⁺ and Tb³⁺

ions. For example, the ⁵D₁ → ⁵D₀ emission transition is quenched in one Eu³⁺ ion at the

Figure 1.11: Schematic of cross-relaxation mechanism in Eu³⁺ and Tb³⁺ ions. The ⁵D₁

→ ⁵D₀ emission transition is quenched in one Eu³⁺ ion at the expense of the ⁷F₀ → ⁷F₃ excitation transition in another Eu³⁺ ion. Similarly, the⁵D₃ → ⁵D₄ emission transition is quenched in one Tb³⁺ ion at the expense of the⁷F₆ →⁷F₀ excitation transition in another Tb³⁺ ion.

expense of the⁷F₀ → ⁷F₃ excitation transition in another Eu³⁺ ion. Such transitions are preferred because electrons are populated at the⁵D₀ energy level and emission transitions from the higher energy level [⁵D₁ → ⁷F_J (J = 0-6)] are avoided. Similarly, the⁵D₃ →⁵D₄ emission transition is quenched in one Tb³⁺ ion at the expense of the⁷F₆ →⁷F₀ excitation transition in another Tb³⁺ ion. Thus, electrons are populated at the ⁵D₄ energy level and emission transitions from the higher energy level [⁵D₃ → ⁷FJ (J = 6-0)] are avoided. The cross-relaxation is observed above a critical concentration of luminescent ions as it depends on the interaction between the luminescent ions. The critical concentration depends on the host lattice (3% Eu³⁺-doped Y₂O₃).⁵ Doping of the host lattice with the luminescent ions to the critical concentration is desired. Doping of the luminescent ions in the host lattice above the critical concentration results in lower luminescence efficiency as described earlier.