Photoluminescence tuning via energy transfer in Eu-doped Ba₂(Gd,Tb)₂Si₄O₁₃ solid-solution phosphors

Abstract

We report on the phase formation of a Ba₂(Gd,Tb)₂Si₄O₁₃ solid-solution, and the coexistence of Eu²⁺/Eu³⁺ was identified after Eu ion doping although the samples were prepared in a reducing atmosphere. Under 377 nm near-ultraviolet (UV) light excitation, Ba₂Tb₂Si₄O₁₃ exhibits the characteristic emission originating from Tb³⁺ corresponding to ⁵D₄–⁷F_6,5,4,3 transitions; whereas Ba₂Tb₂Si₄O₁₃:Eu emits bright red emission from Eu³⁺ with peaks around 594, 613 and 623 nm. Accordingly, photoluminescence tuning of Eu-doped Ba₂(Gd,Tb)₂Si₄O₁₃ phosphors has been realized from green, yellow, orange, to red emission light. Decay time and time-resolved luminescence results revealed that the tunable luminescence behavior should be ascribed to the existence of energy migration in the terbium subset, and successive energy transfer processes Eu²⁺–Eu³⁺(Tb³⁺) and Tb³⁺–Eu³⁺ appear to occur in the Ba₂Tb_2−ySi₄O₁₃:yEu (y = 0–0.12) solid-solution phosphors under investigation.

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1. Introduction

Lanthanide-doped inorganic phosphor materials have attracted much attention in recent years owing to their potential applications in display technology, solid state lighting, and other fields.^1,2 Amongst them, Eu ions have been widely used as activators because both Eu³⁺ and Eu²⁺ can function as emission centers in the host lattices.³ Compared with the 4f–5d transition of Eu²⁺, the emission of the 4f–4f transition of Eu³⁺ is relatively insensitive to the host and temperature because the 4f shell is shielded by the outer filled 5s and 5p shells. However, the 4f–5d transition of Eu²⁺ is parity allowed and therefore intense emission bands can be achieved. In addition, for Eu²⁺ the absorption and emission wavelengths of can be tuned by the host lattice because of the strong interaction between the 5d electron and the ligand anions through covalency and crystal field. Eu³⁺ ions are employed in luminescent devices such as fluorescent lamps and cathode ray tubes, and phosphors containing Eu³⁺ ions as activators can emit red light; their excitation bands usually consist of host lattice excitation bands, charge-transfer bands, and direct excitation bands of Eu³⁺ ions.⁴ However, the low oscillator strength and narrow line width of Eu³⁺ 4f–4f absorption transitions leads to a weak absorption in the near-UV and blue region.⁵ As a comparison, Eu²⁺ emits in the UV- to visible region and the emission wavelength strongly depends on the nature of the host lattice due to the participation of d orbitals. Therefore, as the characteristic broad-band transition excitation and emission of Eu²⁺ have been well recognized, and it might be very exciting and interesting if an efficient Eu²⁺–Eu³⁺ energy transfer can occur in a single-phase valence-varied Eu-doped phosphor, which can provide an opportunity to tailor the absorption of red-emitting phosphors containing Eu³⁺ ions. However, the existence of metal–metal charge transfer (MMCT) effect will quench the luminescence of the sensitizer for the designed Eu²⁺–Eu³⁺ energy transfer. Therefore, Setlur firstly proposed that energy migration in the terbium subset can be used as an intermediate to alleviate the MMCT effect.⁶ After that, such a strategy has been realized in Ba₃Ln(PO₄)₃:Ce³⁺,Tb³⁺,Eu³⁺, LnPO₄:Ce³⁺,Tb³⁺,Eu³⁺, and Ba₂(Ln_1−zTb_z)(BO₃)₂Cl:Eu phosphors systems, and some interesting luminescence properties have been observed.^7,8 In the present work, Eu-doped Ba₂(Gd,Tb)₂Si₄O₁₃ solid-solution phosphors have been designed, the coexistence of Eu²⁺/Eu³⁺ has been evidenced and energy transfer of the type Eu²⁺–Eu³⁺(Tb³⁺) and Tb³⁺–Eu³⁺ has been observed.

The crystal structures of the Ba₂Gd_2−xTb_xSi₄O₁₃ phase under investigation were derived from the Ba₂Gd₂Si₄O₁₃ phase via Gd/Tb substitution. Ba₂Gd₂Si₄O₁₃ possesses a monoclinic structure and a space group C2/c with lattice parameters a = 12.896(3) Å, b = 5.212(1) Å, c = 17.549(4) Å, β = 104.08(3) Å.⁹ Up to now, several studies regarding the luminescence properties of rare earth doped Ba₂Gd₂Si₄O₁₃ have been reported. Guo et al. investigated the tunable photoluminescence properties of Ba₂Gd₂Si₄O₁₃:Ce³⁺,Tb³⁺,¹⁰ and the spectroscopic properties of Ba₂Gd₂Si₄O₁₃:Eu³⁺.¹¹ Zhang et al. studied the vacuum UV spectroscopic properties of Ba₂Gd₂Si₄O₁₃:Ln³⁺ (Ln³⁺ = Ce³⁺, Tb³⁺, Dy³⁺, Eu³⁺, Sm³⁺).¹² In 2015, Zhou et al. studied the green emitting phosphor BaGd₂Si₄O₁₃:Eu²⁺.¹³ Herein, we have investigated the phase formation of Ba₂(Gd,Tb)₂Si₄O₁₃, and the coexistence of Eu²⁺/Eu³⁺; the energy transfer processes Eu²⁺–Eu³⁺(Tb³⁺) and Tb³⁺–Eu³⁺ in Ba₂Tb₂Si₄O₁₃ were investigated in detail.

2. Experimental section

2.1 Sample preparation

Ba₂Gd_2−xSi₄O₁₃:xTb and Ba₂Tb_2−ySi₄O₁₃:yEu phosphors were synthesized by high temperature solid–state reaction, starting from a mixture containing commercially-available BaCO₃ (99.9%), Gd₂O₃ (99.995%), SiO₂ (99.95%), Tb₄O₇ (99.995%) and Eu₂O₃ (99.995%) in the given stoichiometric proportions. After mixing and grinding, the mixtures were placed into an alumina crucible and then fired in air at 1360 °C for 5 h under a 5% H₂/N₂ gas mixture. After this, the samples were furnace-cooled to room temperature, and ground again into powder for the following measurements.

2.2 Characterization methods

The powder X-ray diffraction (XRD) measurements were conducted on a D8 Advance diffractometer (Bruker Corporation, Germany) operating at 40 kV and 40 mA with Cu Kα radiation (λ = 0.15406 Å), and the scanning rate was fixed at 4° min⁻¹. Diffuse reflectance spectra were measured on a UV-vis-NIR spectrophotometer (SHIMADZU UV-3600) attached to an integrating sphere. BaSO₄ was used as a reference standard. Room temperature photoluminescence (PL) excitation and emission spectra were measured on a fluorescence spectrophotometer (F-4600, HITACHI, Japan) with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp used as the excitation lamp. The time-resolved PL spectra were measured by an Edinburgh FLS920 spectrofluorometer with the monitoring wavelength of 377 nm, and the room-temperature decay curves were recorded on the same spectrofluorometer and the corresponding wavelengths (365 nm for Eu²⁺ emission, 377 for Tb³⁺ emission and 393 nm for Eu³⁺ emission) were used to monitor the decay. The temperature-dependent luminescence properties were measured on the same spectrophotometer, which was combined with a self-made heating attachment and a computer-controlled electric furnace. Quantum efficiency was measured using the integrating sphere on the FLS920 fluorescence spectrophotometer (Edinburgh Instruments Ltd., UK).

3. Results and discussion

Fig. 1(a) and (b) show the typical XRD patterns of the as-prepared Ba₂Gd_2−xSi₄O₁₃:xTb (x = 0, 0.5, 1.0, 1.5, 2.0) and Ba₂Tb_2−ySi₄O₁₃:yEu (y = 0, 0.01, 0.03, 0.06, 0.15, 2.0) samples, respectively. It can be found that all the diffraction peaks of these two series of samples can be exactly assigned to the corresponding standard data for the hexagonal phase of Ba₂Gd₂Si₄O₁₃ (ICSD card no. 260737), indicating that this series of Ba₂Gd_2−xSi₄O₁₃:xTb (x = 0, 0.5, 1.0, 1.5, 2.0) solid solution samples with different Tb/Gd ratios can form a single phase, and a small amount of Eu also will not change the crystal structure. We find that the characteristic diffraction peak (1 1 4) shifts to higher diffraction angles from 29.28° to 29.40° for Ba₂Gd_2−xSi₄O₁₃:xTb, while it shifts to lower diffraction angles from 29.40° to 29.22° for Ba₂Tb_2−ySi₄O₁₃:yEu. This variation could be due to the different ionic radii for Gd³⁺, Tb³⁺ and Eu³⁺, further suggesting that a continuous solid solution of Ba₂Gd_2−xSi₄O₁₃:xTb and Ba₂Tb_2−ySi₄O₁₃:yEu has formed in the crystal structure.

Fig. 1 XRD patterns of as-prepared (a) Ba₂Gd_2−xSi₄O₁₃:xTb (x = 0, 0.5, 1.0, 1.5, 2.0) and (b) Ba₂Tb_2−ySi₄O₁₃:yEu (y = 0, 0.01, 0.03, 0.06, 0.15, 2.0) samples. The standard data for Ba₂Gd₂Si₄O₁₃ (ICSD card no. 260737) is shown as a reference.

The evolution of the unit cell parameters a and V of Ba₂Gd_2−xSi₄O₁₃:xTb and Ba₂Tb_2−ySi₄O₁₃:yEu are given in Fig. 2(a) and (b), as a function of x and y, respectively. The linear behavior of the cell parameters illustrates that these two series of compounds both belong to a continuous iso-structural solid solution. Fig. 2(c) presents the crystal structure of Ba₂(Gd,Tb)₂Si₄O₁₃. The structure is based on finite zigzag-shaped C₂-symmetric Si₄O₁₃ chains and Gd₂O₁₂ dimers built of edge-sharing GdO₇ polyhedra of C₁ symmetry. The [9 + 1]-coordinated Ba atoms are located in voids of the atomic arrangement, as highlighted in Fig. 2(d).

Fig. 2 Unit cell parameters a, V of (a) Ba₂Gd_2−xSi₄O₁₃:xTb (x = 0, 0.5, 1.0, 1.5, 2.0) and (b) Ba₂Tb_2−ySi₄O₁₃:yEu (y = 0, 0.01, 0.03, 0.06, 0.15)are plotted. (c) Crystal structure of Ba₂Gd₂Si₄O₁₃ viewed along the b axis. The SiO₄ tetrahedra and GdO₇ polyhedra are highlighted in the unit cell, and Ba₂Tb₂Si₄O₁₃ is isostructural with Ba₂Gd₂Si₄O₁₃. (d) The bonding of O²⁻ anions with Ba²⁺, Si⁴⁺ and Gd³⁺ cations in the independent part of the Ba₂(Gd,Tb)₂Si₄O₁₃ unit cell.

Room temperature photoluminescence excitation (PLE) and emission (PL) spectra of the Ba₂Gd_1.99Si₄O₁₃:0.01Eu phosphors are shown in Fig. 3(a). Upon excitation at 365 nm, the PL spectra of Ba₂Gd_1.99Si₄O₁₃:0.01Eu present a broad band in the green region (350–600 nm) and several small peaks on the shoulders near 600 nm. It is well known that Eu²⁺ ions provide a broad emission band corresponding to the 4f⁶5d–4f⁷ transition, while Eu³⁺ ions emit discrete narrow lines belonging to the ⁵D₀–⁷F_J transitions (J = 0–6). Moreover, the two excitation spectra for Ba₂Gd_1.99Si₄O₁₃:0.01Eu give the corresponding typical spectral profile for the two ions. Although the europium precursor was Eu₂O₃ containing the trivalent ion and the samples were prepared in a reducing atmosphere, it seems that the trivalent europium coexists with divalent europium in this compound. In order to prove it, a Eu³⁺ singly doped Ba₂Gd₂Si₄O₁₃ phosphor was prepared in air and its PLE and PL spectra were comparatively investigated. As also shown in Fig. 3(a), there are three narrow peaks centered at 594, 613 and 624 nm in the PL spectrum of Ba₂Gd_1.99Si₄O₁₃:0.01Eu³⁺ upon 393 nm excitation, which agrees with the position of Eu³⁺ emission in Ba₂Gd_1.99Si₄O₁₃:0.01Eu²⁺/Eu³⁺. Furthermore, the PLE spectral profile of Ba₂Gd_1.99Si₄O₁₃:0.01Eu³⁺ is similar to that of Ba₂Gd_1.99Si₄O₁₃:0.01Eu²⁺/Eu³⁺ monitored at 613 nm; this confirms the consistence of Eu²⁺ and Eu³⁺ in the Ba₂Gd₂Si₄O₁₃ host when prepared in reducing atmosphere. Fig. 3(b) shows the PLE (λ_em = 543 nm) and PL (λ_em = 377 nm) spectra of Ba₂Gd_1.99Si₄O₁₃:0.01Tb³⁺. The PLE spectrum observed in the range of 200–300 nm was attributed to the 4f⁸–4f⁷5d¹ (⁷F₆–⁷D) transition of Tb³⁺ and the one in the range of 300–400 nm come from the 4f–4f transition of Tb³⁺. The Tb³⁺ emission lines are located at 487 nm, 542 nm, 585 nm, and 624 nm, and are assigned to the ⁵D₄–⁷F_6,5,4,3 transitions, respectively.¹⁴

Fig. 3 PLE and PL spectra of (a) Ba₂Gd_1.99Si₄O₁₃:0.01Eu²⁺/Eu³⁺ prepared in a reducing atmosphere, Ba₂Gd_1.99Si₄O₁₃:0.01Eu³⁺ prepared in air, and (b) Ba₂Gd_1.99Si₄O₁₃:0.01Tb³⁺ prepared in air.

The decay curves and lifetime values of Eu²⁺/Eu³⁺ (excitation wavelength 365 nm for Eu²⁺ and 393 nm for Eu³⁺) and Tb³⁺ (excitation wavelength 377 nm) in Ba₂Gd_1.99Si₄O₁₃:0.01Eu/Tb phosphor are shown in Fig. 4. Since there are several different cations with variable environments for the doped rare earth ions in this system, we have tried different exponential functions. It is found that, in order to get the best fitting (with the confidence coefficient of 99%) and evaluate the decay time, the experimental curves were fitted by the sum of two exponential decays using the formula:¹⁵

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (1)

where I is the luminescence intensity, A₁ and A₂ are constants, τ is the time, τ₁ and τ₂ are the rapid and slow decay exponential components, respectively. On this basis, the effective decay constant (τ*) can be calculated as:¹⁶

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (2)

Fig. 4 (a) Decay time curves and the corresponding lifetime values of Eu²⁺ in Ba₂Gd_1.99Si₄O₁₃:0.01Eu phosphor. (b) Eu³⁺ and Tb³⁺ decay curves of Ba₂Gd_1.99Si₄O₁₃:0.01Eu/Tb phosphor.

The effective decay time (τ*) were calculated to be 2.01 μs, 1.96 ms, 1.89 ms and 2.21 ms for the 4f⁶5d–4f⁷ (Eu²⁺ emission at 507 nm) in Ba₂Gd_1.99Si₄O₁₃:0.01Eu, ⁵D₀–⁷F₂ (Eu³⁺ emission at 613 nm) in Ba₂Gd_1.99Si₄O₁₃:0.01Eu, ⁵D₀–⁷F₂ (Eu³⁺ emission at 613 nm) in Ba₂Gd_1.99Si₄O₁₃:0.01Eu³⁺, and ⁵D₄–⁷F₅ (Tb³⁺ emission at 543 nm) in Ba₂Gd_1.99Si₄O₁₃:0.01Tb³⁺, respectively. The decay profile of the ⁵D₄ emission for the latter sample shows a clear rise at short times, due to the slow relaxation from ⁵D₃ to ⁵D₄, attributed to multiphonon relaxation in the silicate host containing high frequency vibrations. The non-exponential character of the decay profiles is presumably due to the presence of several slightly different sites for the dopants and, in the case of Ba₂Gd_1.99Si₄O₁₃:0.01Eu, to possible Eu²⁺–Eu³⁺ energy transfer processes.

The possibility of the Eu²⁺/Eu³⁺ coexistence can be verified by the luminescence behavior. Fig. 5(a) shows the PLE and PL spectra of Ba₂Tb₂Si₄O₁₃. It can be seen that the PLE spectrum of Tb³⁺ exhibits two obvious broad bands from 200 to 300 nm with two peaks at 252 and 279 nm and some weak transitions from 300 to 400 nm. Fig. 5(b) represents the PLE and PL spectra of Ba₂Tb_1.97Si₄O₁₃:0.03Eu obtained at different experimental conditions. When Eu ions with a low concentration (y = 0.03) were introduced into this matrix, the PLE spectrum of Ba₂Tb_1.97Si₄O₁₃:0.03Eu phosphor prepared in reducing atmosphere has been obviously broadened and the intensity of the sharp lines corresponding to the emission of Tb³⁺ has obviously increased. Moreover, the excitation band edge has shifted to 450 nm, showing a broad-band character. By comparing the two excitation bands of Ba₂Tb_1.97Si₄O₁₃:0.03Eu phosphor obtained at different experimental conditions, there is an obvious difference in the excitation band owing to the appearance of a broad excitation feature, which supports the existence of Eu²⁺. Upon excitation at 377 nm, the Tb³⁺ emission intensity decreases accompanied by a remarkable increase of the emission intensities at 594, 613, and 624 nm for Ba₂Tb_1.97Si₄O₁₃:0.03Eu phosphor, which are the characteristic red emission peaks of Eu³⁺ ions corresponding to the transitions from ⁵D₀ to ⁷F_J (J = 1 and 2).¹⁷ Generally, we can propose that the enhancement of the red-emitting luminescence can be ascribed to the energy transfer from Tb³⁺ to Eu³⁺.^18,19 Furthermore, it is notable that the emission intensity of the Ba₂Tb_1.97Si₄O₁₃:0.03Eu phosphor prepared in reducing atmosphere is relatively higher than the one of the Ba₂Tb_1.97Si₄O₁₃:0.03Eu fired in air, indicating an efficient energy transfer between Eu²⁺–Eu³⁺ (Tb³⁺) and the Eu²⁺ absorption.

Fig. 5 PLE and PL spectra of (a) Ba₂Tb₂Si₄O₁₃ and (b) Ba₂Tb_1.97Si₄O₁₃:0.03Eu phosphor.

The diffuse reflectance spectra of Ba₂Gd_2−xSi₄O₁₃:xTb (x = 0, 0.5, 1.0, 1.5, 2.0) phosphors are shown in Fig. 6(a). The spectrum of the Ba₂Gd₂Si₄O₁₃ host shows a high reflectance in the visible range, and the band gap estimation in the Ba₂Gd₂Si₄O₁₃ host is shown by the inset in Fig. 6(a). The band gap of the Ba₂Gd₂Si₄O₁₃ host can be estimated according to eqn (3)²⁰

[F(R_∞)hν]ⁿ = A(hν − E_g) (3)

where hν is the photon energy; A is a proportional constant; E_g is the value of the band gap; n = 2 for a direct transition or 1/2 for an indirect transition; and F(R_∞) is the Kubelka–Munk function defined as²¹

F(R_∞) = (1 − R)²/2R = K/S (4)

where R, K, and S are the reflection, absorption, and scattering coefficient, respectively. From the linear extrapolation of [F(R_∞)hν]² = 0 in the inset of Fig. 6(a), the E_g value was estimated to be about 4.20 eV. As Tb³⁺ ions were doped into the Ba₂Gd₂Si₄O₁₃ host, a strong absorption in the range of 200–350 nm assigned to the f–d transition of Tb³⁺ was observed, as found in the excitation spectrum. The E_g value gradually increases, and the absorption intensity is enhanced at higher Tb³⁺ ion concentrations. As a comparison, Ba₂Tb_2−ySi₄O₁₃:yEu phosphors show an obvious broad absorption band near 400 nm (Fig. 6(b)), which should be caused by the 4f⁷–4f⁶5d¹ transition of Eu²⁺.²² The reflectance spectrum and PLE spectrum indicate that the absorption of Ba₂Tb₂Si₄O₁₃:Eu phosphor matches well with near-UV chips for applications in near-UV LEDs.

Fig. 6 The diffuse reflectance spectra of (a) Ba₂Gd_2−xSi₄O₁₃:xTb (x = 0, 0.5, 1.0, 1.5, 2.0) and (b) Ba₂Tb_2−ySi₄O₁₃:yEu (y = 0, 0.01, 0.04, 0.06) phosphors. The extrapolation of the band gap energy for (inset of a) Ba₂Gd₂Si₄O₁₃, (inset of b) Ba₂Tb₂Si₄O₁₃, (c) Ba₂Gd_1.5Tb_0.5Si₄O₁₃, (d) Ba₂GdTbSi₄O₁₃, and (e) Ba₂Gd_0.5Tb_1.5Si₄O₁₃.

Fig. 7(a) illustrates the emission spectra of Ba₂Tb_2−ySi₄O₁₃:yEu (y = 0–0.12) samples upon excitation at 377 nm. With increasing Eu concentration, one can see that the relative emission intensities of Tb³⁺ becomes weak, whereas the emission intensities of Eu³⁺ firstly increase until they reach saturation at y = 0.03, reflecting the result of energy transfer from Tb³⁺ to Eu³⁺. In addition, it is surprising to find the appearance of a broad band centered around 507 nm with increasing y, which can be assigned to the 4f⁶5d¹–4f⁷ transition of Eu²⁺, further supporting the coexistence of Eu²⁺ and Eu³⁺.²³ Because of the Eu²⁺–(Tb³⁺)_n–Eu³⁺ energy transfer process, the CIE coordinates upon 377 nm excitation of Ba₂Tb_2−ySi₄O₁₃:yEu (y = 0, 0.01, 0.03, 0.09) progressively shift from green (0.2966, 0.5798) to red (0.5048, 0.3955) with increasing y as displayed in Fig. 7(b). As also given in the photographs of the emitting phosphors, the as-observed emitting color is obvious, which means that tunable luminescence can be realized in the novel Ba₂Tb_2−ySi₄O₁₃:yEu phosphors via Eu²⁺–(Tb³⁺)_n−Eu³⁺ energy transfer processes.

Fig. 7 (a) PL spectra for the phosphors Ba₂Tb_2−ySi₄O₁₃:yEu (y = 0, 0.005, 0.01, 0.03, 0.04, 0.06, 0.09, 0.12) phosphors, and (b) the corresponding CIE coordinates and photographs.

In order to demonstrate the energy transfer from Tb³⁺ to Eu³⁺, the decay curves of Tb³⁺ emission at 542 nm and Eu³⁺ emission at 613 nm of Ba₂Tb₂Si₄O₁₃ and Ba₂Tb_1.995Si₄O₁₃:0.005Eu phosphors are shown in Fig. 8(a) (excitation at 377 nm). Using eqn (1) and (2), the values of the effective decay time were found to be 2.59, 0.96 and 1.83 ms for the ⁵D₄–⁷F₅ (Tb³⁺ emission at 543 nm) in Ba₂Tb₂Si₄O₁₃, ⁵D₄–⁷F₅ (Tb³⁺ emission at 543 nm) in Ba₂Tb_1.995Si₄O₁₃:0.005Eu, and ⁵D₀–⁷F₂ (Eu³⁺ emission at 613 nm) in Ba₂Tb_1.995Si₄O₁₃:0.005Eu, respectively. Accordingly, the energy transfer efficiency (η_T) can be calculated using the following expression:²⁴

here τ₀ and τ_x are the corresponding lifetime values of donor Tb³⁺ in the absence and the presence of the acceptor Eu³⁺, and η_T is the energy transfer efficiency. As shown in the inset of Fig. 8(a), the total η_T is found to be 62.9% from Tb³⁺ to Eu³⁺ in the Ba₂Tb_1.995Si₄O₁₃:0.005Eu phosphor. We also measured the decay curves of the Ba₂Tb_2−ySi₄O₁₃:yEu phosphors excited at 377 nm, as given in Fig. 8(b). It was found that the lifetimes for Tb³⁺ decreased with increasing Eu concentration, which offers clear evidence for the presence of energy transfer from Tb³⁺ to Eu³⁺, and the energy transfer efficiencies are 62.9%, 68.7%, 73.7%, 81.5% and 89.2% depending on different Eu concentration, 0.005, 0.01, 0.02, 0.03 and 0.06, respectively. Fig. 8(c) shows the decay curves of the ⁵D₄–⁷F₅ transition of Tb³⁺ upon excitation at 273, 365, and 377 nm. Based on the decay curves in Fig. 8(c), eqn (1) and (2), the value of decay time were found to be practically independent of the excitation wavelength, with values 0.49, 0.45 and 0.48 ms for excitation of 273, 365, and 377 nm, respectively.

Fig. 8 (a) Decay time curves and lifetimes of Eu³⁺ in Ba₂Tb_1.995Si₄O₁₃:0.005Eu phosphor and Tb³⁺ in Ba₂Tb₂Si₄O₁₃ and Ba₂Tb_1.995Si₄O₁₃:0.005Eu phosphor. (b) Tb³⁺ decay curves of Ba₂Tb_2−ySi₄O₁₃:yEu phosphors monitoring 542 nm emission. (c) Tb³⁺ decay curves of Ba₂Tb_1.97Si₄O₁₃:0.03Eu phosphors monitoring 542 nm emission under 273, 365 and 377 nm excitation.

To study the energy transfer process from Tb³⁺ to Eu³⁺ in Ba₂TbSi₄O₁₃:Eu phosphor in more detail, time-resolved emission spectra of Ba₂Tb_1.995Si₄O₁₃:0.005Eu were measured upon excitation into the Tb³⁺ band with a delay time ranging from 0 to 3.12 ms (Fig. 9). At t = 0 ms, only the characteristic emission of Tb³⁺ is observed; no Eu³⁺ emission is present because the excitation energy Tb³⁺ has not been transferred yet to Eu³⁺. As also shown in Fig. 9, when t = 0.78 ms, the emission of Tb³⁺ starts to decrease accompanied by the presence of Eu³⁺ emissions due to the efficient energy transfer of Tb³⁺ to Eu³⁺. The emission of Tb³⁺ decreases gradually with further increase of the decay time, whereas that of Eu³⁺ begins to increase due to transfer of more excitation energy from Tb³⁺ to Eu³⁺. Therefore, the Eu³⁺ ion acts as the terminal of the thermal of the energy transfer processes in the Ba₂TbSi₄O₁₃:Eu phosphor. Thus, we can obtain bright red luminescence in the Ba₂TbSi₄O₁₃:Eu phosphor upon near UV excitation.

Fig. 9 The time-resolved spectra of Ba₂Tb_1.995Si₄O₁₃:0.005Eu phosphor under 377 nm excitation.

Fig. 10 illustrates the energy level model for the energy transfer processes of the type Eu²⁺–Tb³⁺–Eu³⁺ in the Ba₂Tb₂Si₄O₁₃ host. As can be seen from Fig. 10, the excitation features of Ba₂Tb₂Si₄O₁₃:Eu phosphors in the n-UV region come from the combined absorption of Tb³⁺, Eu³⁺, and Eu²⁺, so that a broad and strong absorption band can be observed in the PLE spectrum (Fig. 3(b)). The energy absorbed by Eu²⁺ can also be efficiently transferred from the 4f⁶5d¹ configuration to the excited levels of Eu³⁺ and Tb³⁺; accordingly, the energy absorbed by Tb³⁺ can be transferred from the ⁵D₄ level of Tb³⁺ to the ⁵D₀ (⁵D₁) level of Eu³⁺.^25,26

Fig. 10 Energy level diagram for the ET processes among Eu²⁺, Tb³⁺, and Eu³⁺ in Ba₂Tb₂Si₄O₁₃ phosphor.

The thermal stability of phosphor is one of the important parameters for possible applications. For this reason, the emission spectra of the selected Ba₂Tb_1.97Si₄O₁₃:0.03Eu phosphor at various temperatures are shown in Fig. 11(a). The variations of the relative emission intensities as a function of temperature are plotted in Fig. 11(b). When the temperature is increased to 150 °C, the PL intensities of Tb³⁺ and Eu³⁺ ions decrease to 94.9% and 88.2% of the corresponding initial value (25 °C). In Fig. 11(b), we have also given the temperature dependent PL intensities variation of the commercial BaMgAl₁₀O₁₉:Eu²⁺ (BAM) blue phosphor, and the comparable thermal stability can be found. The small intensity decrease indicates that Ba₂Tb₂Si₄O₁₃:Eu could be used for high-power LED application. In general, the thermal quenching of emission intensity can be explained by several mechanisms. A widely accepted mechanism is the electronic transition through the intersection between the ground and excited states. In other words this mechanism is described as a large displacement between the potential energy curves of the ground and excited state in the configuration coordinate diagram.²⁷ In the present case, this mechanism would be valid in the case of excitation in the absorption profile of Eu²⁺. To better understand the thermal quenching phenomena, the activation energy for the thermal quenching was fitted using the equation describing the crossover between Franck Condon displaced potential energy curves:^28,29

where I₀ is the initial PL intensity of the phosphor at room temperature, I_T is the PL intensity at different temperatures, c is a constant, ΔE is the activation energy for thermal quenching, and k is Boltzmann constant (8.62 × 10⁻⁵ eV). According to the equation, the activation energy ΔE can be calculated from a plotting of ln[(I₀/I) − 1] against 1/kT, where a straight slope equals −ΔE. As shown in Fig. 11(b), ΔE were found to be 0.351 and 0.296 eV for Tb³⁺ and Eu³⁺, respectively. The relatively high activation energy results in a good thermal stability for this phosphor. The quantum efficiency of the phosphor is another important parameter to be considered for practical application, and we have also measured the internal quantum efficiency (QE) of Ba₂Tb_2−xSi₄O₁₃:xEu (x = 0, 0.005, 0.03 and 0.09) phosphors. The measured internal QE values are 30.06%, 31.66%, 39.35% and 23.39%, respectively, for x = 0, 0.005, 0.03, 0.09 under 377 nm excitation.

Fig. 11 (a) Temperature-dependent PL spectra of Ba₂Tb_1.97Si₄O₁₃:0.03Eu. (b) The variations of the relative emission intensities as a function of temperature of the Ba₂Tb_1.97Si₄O₁₃:0.03Eu phosphor and commercial BAM phosphor. The inset shows relative PL intensities of Tb³⁺, and Eu³⁺ in Ba₂Tb₂Si₄O₁₃ host with raised temperatures. The inset shows the activation energy (ΔE) of Tb³⁺ and Eu³⁺ in Ba₂Tb_1.97Si₄O₁₃:0.03Eu phosphor.

4. Conclusion

In summary, color-tunable Ba₂Gd_2−xTb_xSi₄O₁₃:yEu solid-solution phosphors were prepared by the high-temperature solid–state reaction in reducing atmosphere. The terbium ions in the host can not only form an iso-structural phase originated from the Ba₂Gd₂Si₄O₁₃, but they can be also used to realize the Eu²⁺–(Tb³⁺)_n–Eu³⁺ energy transfer channel between broad-band absorption of Eu²⁺ and the narrow-line emission of Eu³⁺. On the basis of this process, the emission color of the obtained phosphors can be varied from green to yellow and finally to red by adjusting the concentration of Eu. Moreover, the temperature dependence of the luminescence intensity shows that this phosphor has an excellent thermal stability against temperature quenching.

Acknowledgements

The present work was supported by the National Natural Science Foundations of China (Grant No. No.51572023, No.51272242), Natural Science Foundations of Beijing (2132050), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0950), Beijing Nova Program (Z131103000413047), Beijing Youth Excellent Talent Program (YETP0635), the Funds of the State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University (KF201306), and Fundamental Research Funds for the Central Universities (FRF-TP-15-005A2).

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