Dy³⁺ → Eu³⁺ Energy Transfer in SrLaGa₃O₇:Dy³⁺/Eu³⁺ Phosphors

SrLaGa₃O₇:0.04Dy³⁺, SrLaGa₃O₇:0.02Eu³⁺ and SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ (x = 0.01, 0.02, 0.03, 0.04 and 0.05) phosphors were prepared by the calcinations at 1400 °C for 3 h. The reagents of SrCO₃, La₂O₃, Ga₂O₃, Eu₂O₃ and Dy₂O₃ with the purity of 99.99% were used for the preparation. In the preparation, the reagents were weighted firstly according to the stoichiometric proportion and grinded in a mortar. Then, the mixed reagents with different stoichiometric proportions were put into the corundum crucibles and calcined at 1400 °C for 3 h. Thirdly, the products were collected and re-grinded for the following measurements. The X-ray diffraction (XRD) data were measured by a Germany Bruker D8 Advance X-ray diffractometer. The excitation, emission and decay data were measured by an Edinburgh Instrument FLS980 spectrophotometer.

We display the normal XRD patterns of pure SrLaGa₃O₇ (PDF # 86–1839) and the XRD patterns of the prepared SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ (x = 0.01, 0.02, 0.03, 0.04 and 0.05) phosphors in Fig. 1. As revealed by the XRD patterns, the diffraction lines match well with the XRD data of tetragonal SrLaGa₃O₇, indicating the successful preparation of Dy³⁺/Eu³⁺ doped SrLaGa₃O₇ phosphors with the tetragonal phase. No diffraction lines corresponding to other materials can be detected, indicating the single phase of them. In Dy³⁺/Eu³⁺ doped SrLaGa₃O₇ phosphors, Dy³⁺/Eu³⁺ ions occupy lattice sites and substitute La³⁺ ions. ^6,19

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Figure 2 displays the luminescence spectra of SrLaGa₃O₇:0.02Eu³⁺ (A), SrLaGa₃O₇:0.04Dy³⁺ (B) and SrLaGa₃O₇:0.04Dy³⁺/0.02Eu³⁺ (C) phosphors. As we can see in Fig. 2A, SrLaGa₃O₇:0.02Eu³⁺ has an excitation spectrum within 225–575 nm. Among them, the broad band within 225–310 nm is a typical a charge transfer band (CTB) induced by the O²⁻ → Eu³⁺ charge transfer absorption, but other sharp bands come from the f-f transitions of Eu³⁺. ¹⁹ The transitions of Eu³⁺ are assigned and marked in detail in the left spectrum of Fig. 2A. The emission spectrum (recorded at 472 nm excitation), as shown in the right spectrum in Fig. 2A, includes the emission bands coming from the ⁵D₀ → ⁷F_{1, 2, 3, 4} transitions of Eu³⁺. Among theses emission bands, the band coming from the ⁵D₀ → ⁷F₂ transition is strongest, indicating the sites of Eu³⁺ without inversion in SrLaGa₃O₇. ²⁰ As we can see in Fig. 2B, SrLaGa₃O₇:0.04Dy³⁺ has an excitation spectrum containing a broad band within 225–275 nm and several narrow bands within 275–475 nm. The broad band in the shorter wavelength region is a CTB induced by the O²⁻ → Dy³⁺ charge transfer absorption, but other bands come from the f → f transitions of Dy³⁺. ^6,11 All of the transitions are distinguished and signaled detailedly in the left spectrum of Fig. 2B. The wide distribution of excitation spectrum indicates that the SrLaGa₃O₇:Dy³⁺ phosphors are suitable for the excitation of ultraviolet, near ultraviolet and blue light. The emission spectrum (recorded at 365 nm excitation), as shown in the right spectrum in Fig. 2B, includes the emission bands coming from the ⁴F_9/2 → ⁶H_{15/2, 13/2, 11/2} transitions of Dy³⁺. Among the emission bands, the band coming from the ⁴F_9/2 → ⁶H_13/2 transition appears to be highest, indicating the sites without inversion for Dy³⁺ ions in SrLaGa₃O₇. ²¹ As we can see in Figs. 2A and 2B, there are spectral overlaps between the excitation spectrum of SrLaGa₃O₇:0.02Eu³⁺ with the emission spectrum of SrLaGa₃O₇:0.04Dy³⁺. Generally, the spectral overlap indicates the possibility of Dy³⁺ → Eu³⁺ energy transfer in phosphors. Thus, the luminescence of SrLaGa₃O₇:0.04Dy³⁺/0.02Eu³⁺ phosphor was investigated. Figure 2C provides the excitation and emission spectra of SrLaGa₃O₇:0.04Dy³⁺/0.02Eu³⁺ phosphor. As shown in the left spectrum of Fig. 2C, the excitation spectrum of SrLaGa₃O₇:0.04Dy³⁺/0.02Eu³⁺ contains the excitation bands corresponding to transitions of Dy³⁺ and Eu³⁺ while monitoring the emission of Eu³⁺ (619 nm). The excitation bands corresponding to transitions of Dy³⁺ in the excitation spectrum indicate the existence of Dy³⁺ → Eu³⁺ energy transfer. Upon the excitation at 365 nm, the emission spectrum of SrLaGa₃O₇:0.04Dy³⁺/0.02Eu³⁺ shows the emission bands of Dy³⁺ and Eu³⁺, as shown in the right spectrum of Fig. 2C.

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To show the luminescence characteristics of SrLaGa₃O₇:Dy³⁺/Eu³⁺ phosphors in detail, the emission of SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ (x = 0.01, 0.02, 0.03, 0.04 and 0.05) phosphors was measured subsequently, as shown in Fig. 3. Upon the excitation at 365 nm, SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ show emission bands coming from the ⁴F_9/2 → ⁶H_{15/2, 13/2, 11/2} transitions of Dy³⁺ and the ⁵D₀ → ⁷F_{1, 2, 3, 4} transitions of Eu³⁺. The increasing Eu³⁺ concentrations lead to the decreasing emission intensity of Dy³⁺. These luminescence characteristics are similar to the characteristics of other Dy³⁺/Eu³⁺ codoped phosphors. ^10–12 Meanwhile, we find that the Eu³⁺ emission intensity in SrLaGa₃O₇:0.04Dy³⁺/0.02Eu³⁺ is ∼1.18 times of that of SrLaGa₃O₇:0.02Eu³⁺. The continuous reduction of Dy³⁺ emission intensity in SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ and the higher Eu³⁺ emission intensity in SrLaGa₃O₇:0.04Dy³⁺/0.02Eu³⁺ than that of SrLaGa₃O₇:0.02Eu³⁺ are induced by Dy³⁺ → Eu³⁺ energy transfer. The Dy³⁺ → Eu³⁺ energy transfer leads to the tunable luminescence of SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ phosphors. On the basis of emission spectra, the Commission International de L'Eclairage (CIE) chromaticity coordinates were calculated. The CIE coordinates of SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ phosphors are displayed by Table I and marked in the CIE chromaticity diagram (Fig. 4). As we can see in Fig. 4, the light moves from the yellow region to the yellow-red region following the enchantments of Eu³⁺ concentration.

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The exchange and electric multipolar interactions are generally taken into consideration while we investigate the energy transfer process. As a rule, the exchange interaction plays the dominant role while the critical distance (R_c) is shorter than 5 Å or else the electric multipolar interaction is the key. The R_c can be calculated via the formula written as ${R}_{c}=2{\left(3V/4\pi {X}_{c}N\right)}^{1/3}.$ ²² In phosphors with sensitizes/activator pairs, the X_c refers to the entire concentration of sensitizer and activator while the emission intensity of the sensitizer in the phosphor with the activator is half that of without the activator. ¹² Meanwhile, V and N mean the volume and the number of cations in the unit cell. Figure 5A provides the relative Dy³⁺ emission intensities of SrLaGa₃O₇:0.04Dy³⁺ and SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ phosphors. In SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ phosphors, the X_c is found to be 0.0843. The numbers of V and N are 346.1 ų and 5, respectively. Form the formula, the R_c is calculated to be 11.6 Å. Therefore, we can conclude that the electric multipolar interaction plays the dominant role for Dy³⁺ → Eu³⁺ energy transfer in SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ phosphors. The electric multipolar interaction includes the dipole-dipole (d–d), dipole-quadrupole (d–q) and quadrupole-quadrupole (q–q) interactions and the actual interaction is generally confirmed by the formula written as $\left({{\rm{I}}}_{{\rm{s}}0}/{{\rm{I}}}_{{\rm{s}}}\right)\propto {{\rm{C}}}^{{\rm{n}}/3},$ where I_s0 and I_s mean the emission intensities of Dy³⁺ without and with Eu³⁺, C means the total concentration of Dy³⁺ and Eu³⁺, the values of 6, 8, 10 for n correspond to d–d, d–q, q–q interactions. ¹² The plots of ${{\rm{I}}}_{{\rm{S}}0}/{{\rm{I}}}_{{\rm{S}}}{\text{vs}C}^{{\rm{n}}/3},$ as shown in Figs. 5B–5D, indicate that the consistency between the experimental data with the fitting line is best for n = 10. Thus, the actual interaction of Dy³⁺ → Eu³⁺ energy transfer is q–q interaction.

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In general, the lifetime of sensitizer decreases if energy transfer occurs in sensitizer/activator codoped phosphors. Thus, the decay characteristics can also be used to verify the occurrence of energy transfer. Figure 6 displays the decay curves of Dy³⁺ emission in SrLaGa₃O₇:0.04Dy³⁺ and SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ phosphors. Meanwhile, the experimental data fit well with the single-order exponential formula written as ${{\rm{I}}}_{{\rm{t}}}={{\rm{I}}}_{0}\exp \left(-{\rm{t}}/\tau \right),$ where I_t and I₀ are the emission intensities of phosphors at time of t and 0, and τ is the lifetime. ²³ The characteristics of decay curves indicate the sole crystallographic sites for Dy³⁺ in SrLaGa₃O₇. The τ values for SrLaGa₃O₇:0.04Dy³⁺ and SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ phosphors are displayed in Table I. With the increasing Eu³⁺ concentrations, the τ values decrease gradually. This gives further evidence to Dy³⁺ → Eu³⁺ energy transfer in SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ phosphors. The Dy³⁺ → Eu³⁺ energy transfer efficiency (${\eta }_{{\rm{t}}}$) is generally computed by the formula written as ${\eta }_{{\rm{t}}}=1-{\tau }_{{\rm{s}}}/{\tau }_{{\rm{s}}0},$ where ${\tau }_{{\rm{s}}}$ and ${\tau }_{{\rm{s}}0}$ are lifetimes of Dy³⁺ emission when the Eu³⁺ is presence and absence. ⁹ From the changing lifetimes of Dy³⁺ emission, the Dy³⁺ → Eu³⁺ energy transfer probability (P_t) can also be computed by the formula written as ${{\rm{P}}}_{{\rm{t}}}=\left(1/{\tau }_{{\rm{s}}}\right)-\left(1/{\tau }_{{\rm{s}}0}\right).$ The τ and P_t values are displayed in Table I. The calculated values manifest that the energy transfer efficiency and energy transfer probability become higher as the Eu³⁺ concentration becomes larger. These changes are induced by the shorter distance between neighboring Eu³⁺ ions.

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From the luminescence performance of SrLaGa₃O₇:0.04Dy³⁺ and SrLaGa₃O₇:0.04Dy³⁺/xEu³⁺ phosphors, we ascertain the Dy³⁺ → Eu³⁺ energy transfer. Whereafter, we conclude the possible processes for luminescence and energy transfer, which is revealed by a simplified schematic energy level diagram shown in Fig. 7. Firstly, electrons of Dy³⁺ were excited to the ⁶P_7/2 energy level following the absorption of energy from the exciting light. Then, the excited electrons at ⁶P_7/2 energy level relaxed to the ⁴F_9/2 energy level via a nonradiative process. At this moment, part of the electrons relaxed to the ⁶H_{15/2, 13/2, 11/2} levels following the multicolor emission and part of energy transferred to the ⁵D₁ energy level of Eu³⁺. Herein, a nonradiative process from the ⁵D₁ level to ⁵D₀ level occurred immediately. Meanwhile, electrons of Eu³⁺ were also excited to the ⁵D₄ level from the ground ⁷F₀ level upon the excitation. Subsequently, electrons at the ⁵D₄ level relaxed to the ⁵D₀ level via a nonradiative process. Finally, electrons of Eu³⁺ at the ⁵D₀ energy level relaxed to the ⁷F_4,3,2,1,0 levels following the multicolor emission.

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Dy³⁺/Eu³⁺ doped SrLaGa₃O₇ phosphors were prepared successfully by the calcinations at 1400 °C for 3 h. The XRD patterns of the prepared phosphors indicate their pure phase. This means that the Dy³⁺/Eu³⁺ doping barely influences the phase and the ions have doped into the host lattices. SrLaGa₃O₇:Dy³⁺ phosphor shows the emission bands coming from the ⁴F_9/2 → ⁶H_{15/2, 13/2, 11/2} transitions of Dy³⁺ while they are excited at 365 nm and SrLaGa₃O₇:Eu³⁺ phosphor shows the emission bands coming from the ⁵D₀ → ⁷F_{1, 2, 3, 4} transitions of Eu³⁺ while they are excited at 472 nm. For Dy³⁺/Eu³⁺ codoped SrLaGa₃O₇ phosphors, the emission bands of Dy³⁺ and Eu³⁺ are both observed while they are excited at 365 nm. The Dy³⁺ → Eu³⁺ energy transfer occurs via the quadrupole-quadrupole interaction and tunable luminescence is generated by reason of the energy transfer process.