Consequences of ET and MMCT on Luminescence of Ce3+-, Eu3+-

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Consequences of ET and MMCT on Luminescence of Ce3+-, Eu3+-, and Tb3+-doped LiYSiO4 Rui Shi,† Guokui Liu,‡ Hongbin Liang,*,† Yan Huang,§ Ye Tao,§ and Jing Zhang§ †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China ‡ Chemical Science and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, P. R. China S Supporting Information *

ABSTRACT: Ce3+, Eu3+, and Tb3+ singly doped, Ce3+-Tb3+, Tb3+Eu3+, and Ce3+-Eu3+ doubly doped, as well as Ce3+-Tb3+-Eu3+ triply doped LiYSiO4 phosphors were prepared by a high-temperature solid-state reaction technique. Rietveld refinement was performed to determine the structure of host compound. The cross-relaxation (CR) of Tb3+ is quantitatively analyzed with the Inokuti−Hirayama model of energy transfer (ET), and the site occupancy is confirmed by emission spectra of Eu3+. ET and metal−metal charge transfer (MMCT) are systematically investigated in Ce3+-Tb3+, Tb3+-Eu3+, and Ce3+-Eu3+ doubly doped systems. The combined effects of ET and MMCT on luminescence and emission color of Ce3+-Tb3+-Eu3+ triply doped samples are discussed in detail, showing that the photoluminescence emission is tunable in a large color gamut. critical distance of Ce3+-Eu3+ MMCT, this system is also convenient to elucidate the MMCT effect.12,13 On the basis of structure refinement of LiYSiO4, in this paper we report the luminescence of Ce3+, Tb3+, and Eu3+ singly doped, Ce3+-Tb3+, Tb3+-Eu3+, and Ce3+-Eu3+ doubly doped, and Ce3+-Tb3+-Eu3+ triply doped in this host compound. The ET processes are discussed in detail, and the impacts of MMCT on luminescence are systematically verified.

1. INTRODUCTION As visible emitting lanthanide ions, luminescence of Ce3+, Eu3+, and Tb3+ has gained actual applications in lighting and displays. Y2SiO5:Ce3+ is recommended to be a blue-emitting phosphor in field emission displays (FEDs); Y2O3:Eu3+ and CeMgAl11O19:Tb3+ are commercially available red- and green-emitting phosphors in tricolor light tubes, respectively. For solid-state lamps application, the combination of blue-emitting GaInN chip and yellow-emitting phosphor Y3Al5O12:Ce3+ is the most convenient approach to fulfill white-emitting from lightemitting diodes (LEDs). To decrease the correlated color temperature and to increase the color rendering index of whiteemitting LEDs, the phosphors doped with Ce3+ and other longwavelength emitting lanthanide ions such as Sm3+, Eu2+, Eu3+, Tb3+, and Dy3+ have been extensively studied in recent years.1−9 Although the tunable emission has been successfully achieved through Ce3+-Tb3+-Eu3+ triply doped in different systems, the combined influences of energy transfer (ET) and metal−metal charge transfer (MMCT) should be further clarified. LiYSiO4 is an important host compound for luminescence of lanthanide ions.10,11 In this compound, the emission of Ce3+ extends from ∼375 to ∼475 nm, which covers the excited states of 5D3 and 5L6 of Tb3+ and Eu3+, respectively. Therefore, it is in favor of investigation of potential ET from Ce3+ to Tb3+ or Eu3+. Meanwhile, two nearest adjacent Y3+ ions have a suitable distance 4.0516 Å, which is proper for study of the influence of doping concentration on possible ET of Eu3+ and Tb3+ through cross relaxation. In addition, because of a relatively larger © XXXX American Chemical Society

2. EXPERIMENTAL SECTION Several series of powder samples with Ce3+, Tb3+, and Eu3+ singly doped, Ce3+-Tb3+, Tb3+-Eu3+, and Ce3+-Eu3+ doubly doped, as well as Ce3+-Tb3+-Eu3+ triply doped in LiYSiO4 were prepared by a hightemperature solid-state reaction route. The details of preparation and measurements are described in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Structure Characterization. The samples LiY1−xLnxSiO4 (Ln = Ce, Tb, Eu) and LiTmSiO4 (ICSD 75538) are isomorphic, which belong to orthorhombic phase. Therefore, we adopt LiTmSiO4 to refine LiYSiO4 structure. To obtain the standard LiYSiO4 structure data, Rietveld structure refinement of the high-quality powder XRD data was performed using the TOPAS program. The fitting result for LiYSiO4 is plotted in Figure 1. Refined positions of all atoms and unit cell parameters are listed in Table S1. The values of Rp Received: May 23, 2016

A

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that the substitution of Y3+ by Ln3+ do not significantly influence the crystal structure. Moreover, the dependences of lattice parameters with Ce3+ doping content in the right-hand side of Figure S1 show that the lattice parameters (a, b, c) and unit cell volume (V) increase with Ce3+ content. Because of the radius difference (∼0.11 Å) between Ce3+ and Y3+, it is clear that the expansion of lattice parameters relate to the replacement of Y3+ by the dopant Ce3+. 3.2. X-ray Absorption Near Edge Structure of Representative LiY0.99−xCe0.01EuxSiO4 Samples. In consideration of the general characteristics of valence state transformation of Ce (+3 → +4) and Eu (+3 → +2) in inorganic materials, Ce/Eu-L3 edge X-ray absorption near edge structure (XANES) spectra were measured. Figure S2a shows the Ce-L3 edge XANES of singly/doubly doped samples along with that of standard CeO2 sample for comparison. Two intense bands (labeled A, B) at 5730 and 5736 eV are usually detected in XANES spectrum of CeO2, which arise from the L3-edge absorption of Ce4+.15 On the contrary, Ce3+ ion shows a single absorption band at 5726.3 eV due to an electron excitation from 2p3/2 shell to its 5d shell.16 A single intense absorption band (labeled C) is clearly observed in the spectra of singly/ doubly doped samples, indicating that Ce3+ ions exist predominantly in all samples and that the codoping of Eu3+ has little influence on the valence state of Ce3+. Figure S2b exhibits the Eu-L3 edge XANES of LiY0.99−xCe0.01EuxSiO4 (x = 0.01, 0.03, and 0.09) samples along with that of standard Eu2O3 sample for comparison. An intense absorption at ∼6981 eV is clearly observed in the spectrum of standard Eu2O3, which is attributed to the typical transition from Eu3+ 2p3/2 shell to its 5d shell. The energy of L3-edge of Eu2+ is usually lower by ∼7.5 eV than that of Eu3+.17−20 A dominant absorption edge is observed in each XANES spectrum of LiY0.99−xCe0.01EuxSiO4, and every absorption band shape or peak position is in good line with that of Eu2O3. So the dominated transition (labeled D, 6981 eV) in the spectra states that the valence state of Eu ions are stable at +3 in codoping samples.21 3.3. Luminescence of Ce3+, Tb3+, and Eu3+ Singly Doped LiYSiO4. Although the luminescence properties of Ce3+

and Rwp indicate that the refined crystal structure data are reliable.

Figure 1. Experimental (×) and calculated (red solid line) XRD patterns and the difference (blue solid line) for the Rietveld fit of LiYSiO4 by Topas program. Short vertical black lines represent Bragg reflection positions of the calculated pattern.

In LiYSiO4, the Y3+ site is coordinated by six oxygen atoms with Cs symmetry, and the refined bond lengths are listed in Table S2. The average Y3+−O2− bond length is 2.2772 Å. The distance of two nearest adjacent Y3+ ions is 4.0516 Å. Since the apparent difference between Li+ and Ln3+ (Ln = Ce, Eu, Tb), when Ln3+ ions are doped into LiYSiO4, the dopants should occupy the Y3+ sites because of the similarity of valence and the ionic radii of Ce3+ (1.01 Å), Tb3+ (0.923 Å), Eu3+ (0.947 Å), and Y3+ (0.9 Å) for sixfold coordination.14 The XRD patterns of representative samples LiY1−xLnxSiO4 (Ln = Ce, Tb, Eu) at room temperature (RT) are shown in the left-hand side of Figure S1, which are in good agreement with the refined LiYSiO4 diffraction pattern, and no second phase can be found, indicating that each sample is purity phase and

Figure 2. (a) Synchrotron radiation VUV-UV excitation spectrum of LiY0.999Ce0.001SiO4 of 400 nm emission at 15 K, (b) emission spectrum of Ce3+ under 348 nm excitation, and (c) decay curve of Ce3+ in LiY0.999Ce0.001SiO4 at RT. B

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Figure 3. Excitation (a, b, λem = 541 nm) and normalized emission [λex = 280 (c) and 377 (d) nm] spectra of LiY0.99Tb0.01SiO4 at RT.

The normalized emission spectra of LiY0.99Tb0.01SiO4 upon different excitation wavelengths (280 and 377 nm) are displayed in curves c and d of Figure 3. No significant difference can be detected when the excitation wavelength changes. The observed f-f emissions are assigned to the electron transitions from 5D3 and 5D4 excited multiplets to 7FJ (J = 1−6) ground states of Tb3+. Among them, the strongest 5D4-7F5 transition is at ∼550 nm. Figure S3 shows the normalized emission spectra of LiY1−xTbxSiO4 (x = 0.001, 0.01, 0.03, 0.05, 0.09, 0.13, 0.17) under 377 nm excitation at RT. When the doping concentrations increase, the emission intensities of Tb3+ 5D3 → 7FJ transitions proportionally decrease, but those of 5D4 → 7 FJ transitions increase. The cross-relaxation (CR) between Tb3+ ions is responsible for this variation as shown in the inset of Figure S3. Meanwhile, the multiphonon relaxation (MPR) processes occur in all samples when we consider the 5D3-5D4 energy gap and the effective phonon energy.26−28 The decay curves of Tb3+ 5D3 432 nm emission of samples LiY1−xTbxSiO4 (x = 0.001, 0.01, 0.03, 0.05, and 0.09) under 377 nm excitation at RT are plotted in Figure 4a. Because of extremely low doping content and exponential decay, Tb3+ in LiY0.999Tb0.001SiO4 is approximately regarded as an isolated ion, and the CR is assumed to be negligible. Accordingly, the 5D3 intrinsic decay time of Tb3+ is estimated to be ∼1.38 ms. The decay curves of Tb3+ 5D3 emission apparently deviate gradually from single exponential behavior with the increase of doping contents. So the Inokuti−Hirayama model29 is adopted to investigate the energy transfer between Tb3+ states due to multipolar interaction. The simulation model is constructed by eqs 1−4.

in LiYSiO4 have been discussed in detail in our previous work,10 to facilitate the discussions in following sections, herein we first discuss the spectra of Ce3+ through a typical sample LiY0.999Ce0.001SiO4 as shown in Figure 2. The bands with maxima at ∼350, ∼315, ∼305, ∼206, and ∼185 nm in Figure 2a are five Ce3+ 4f → 5dJ (J = 1−5) excitation bands, and the band at ∼154 nm is host-related absorption band. Under 348 nm excitation, the sample exhibits a broad band emission with a maximum at ∼400 nm (curve b) due to the transitions from the lowest 5d state of Ce3+ to its 2F5/2, 7/2 states. The curve c confirms that emission of Ce3+ decays exponentially in this sample with a decay time ∼38.1 ns. Figure 3 shows the excitation and emission spectra of LiY0.99Tb0.01SiO4 at RT. In the synchrotron radiation VUV-UV excitation spectrum of 541 nm emission (curve a), the peak of the host absorption band is found at ∼154 nm, which is in line with the result in Figure 2. The broad excitation bands at ∼220, ∼237, ∼258, and ∼286 nm are assigned to the spin-allowed (SA) and the spin-forbidden (SF) 4f-5d transitions of Tb3+ when the 5d orbit is in low-spin (LS) or high-spin (HS) state, respectively.22 Normally, because of the approximate effect of crystal field splitting and centroid shift for different trivalent lanthanides in a specific lattice site,23 the f-d excitation energies of Tb3+ can be estimated from those of Ce3+. The difference between Tb3+ SA f-d transition energy and Ce3+ f-d transition energy is usually in the range of 1.66 ± 0.12 eV. The energy of Tb3+ SF f-d transition can be readily predicted by consideration of a shift −1 eV to the energy of SA f-d transition.24 Accordingly, three lowest SA and SF f-d transitions of Tb3+ are predicted at ∼237, ∼220, ∼212 nm (SA) and ∼293, ∼267, ∼256 nm (SF), which are plotted as blue vertical bars in the left side of Figure 3, respectively. Obviously, low-intensity broad bands at ∼258 and ∼286 nm in the spectrum belong to the SF f-d transitions, and the high-intensity broad bands at ∼220 and ∼237 nm are mainly attributed to Tb3+ SA f-d transitions.25 The UV−vis excitation spectrum (curve b) consists of Tb3+ f-f transitions in the host. The comparison of the band intensities of the bands between 300 and 360 nm in curves a and b shows that the f-f transitions of Tb3+ are very weak relative to the f-d transitions.

⎛ ⎛ I (t ) ⎞ 3 t⎞ In⎜ −ln⎜ ⎟ − ⎟ = B + ·In(t ) s ⎝ ⎝ I(0) ⎠ t0 ⎠

(1)

⎞ ⎛ t I(t ) = I(0)exp⎜ − − Q ·t 3/ s⎟ ⎠ ⎝ t0

(2)

4π ⎛⎜ 3⎞ (s) 3/ s ) Γ 1 − ⎟ ·CA ·(C DA ⎝ 3 s⎠

(3)

Q= C

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0.09) with the increase of Tb3+ content, demonstrating that multipolar interaction becomes more remarkable. The kinetics parameter CDA and critical doping content CA(k) are estimated to be ∼7.33 × 10−50 m6·s−1 and ∼1.7%, respectively. With increasing contents from 0.1% to 9%, the values of PSA increase from 2.53 s−1 to 2.326 × 104 s−1, revealing that CR process becomes predominated gradually. The last simulated CR rate 2.326 × 104 s−1 for 9% doping sample is ∼32 times of 5D3 intrinsic decay rate 7.25 × 102 s−1 (estimated from the decay time ∼1.38 ms), so the 5D3 emission of this sample is almost completely quenched as displayed in Figure S3. Figure S4 shows the decay curves of Tb3+ 5D4 541 nm emission of samples LiY1−xTbxSiO4 (x = 0.01, 0.03, 0.09, and 0.17) under 377 nm excitation at RT. The initial rising process is clearly observed in diluted LiY0.99Tb0.01SiO4 sample before the exponential decay. Since the excitation wavelength corresponds to 7F6 → 5D3 transition of Tb3+, the observation of initial rising process demonstrates that the population of 5D4 through 5D3 state is with a relatively slow rate in this low Tb3+ doping sample. With the increase of doping content, the rising part becomes more inconspicuous because of more rapid CR process between Tb3+ ions. This result is highly coincident with the observation in Figure 4 and simulation from the Inokuti− Hirayama model. The decay time of 5D4 state is estimated to be ∼2.1 ms from simulation of the exponential decay part. Moreover, the decay property of 5D4 multiplet still follows exponential in quite highly Tb3+-doped content (17%). It demonstrates that the other energy transfer processes such as excitation state absorption (ESA) and cooperative upconversion process of Tb3+ 5D3,4 multiplets are negligible.30−32 Figure 5 shows the VUV-UV (λem = 611 nm), UV−vis excitation (λem = 611 nm), and emission (λex = 393 nm) spectra of LiY0.99Eu0.01SiO4 at RT. The VUV-UV−vis excitation spectra of 611 nm emission consist of a broad band below wavelength ∼275 nm and some sharp lines in the 280−550 nm range. The broad band mainly arises from Eu3+-O2− charge transfer band (CTB) with a maximum at ∼233 nm and also contains hostrelated absorption at short-wavelength hand.11 The sharp f-f excitation lines at ∼393, ∼466, and ∼531 nm are attributed to

Figure 4. (a) Decay curves of Tb3+ 5D3 432 nm emission of samples LiY1−xTbxSiO4 (x = 0.001, 0.01, 0.03, 0.05, and 0.09) under 377 nm excitation at RT, the red solid lines denote the fitting results using the Inokuti−Hirayama model; (b) the fitting results based on eq 1.

PSA =

(s) C DA s R SA

(4)

where B is a control factor, s is the multipolar effect parameter that the value s = 6, 8, 10 represents the dominant mechanism being dipole−dipole, dipole−quadrupole or quadrupole− quadrupole interaction, respectively; Q is the multipolar interaction parameter, CA is the acceptor content; CDA denotes the multipolar ion−ion interaction kinetic microparameter, PSA represents the energy transfer rate between donor and acceptor. The physical meanings of other parameters have been defined elsewhere.10 When PSA is equal to 1/t0, RSA(k) and CA(k) are called “critical distance” and “critical doping content”, respectively. The fitting results shown in Figure 4b demonstrate that the dominant mechanism of this CR process is dipole− dipole interaction with the mean s value of ∼6.10. Q values become larger (5.06 × 10−4 for x = 0.01 to 4.3 × 10−3 for x =

Figure 5. (a) VUV-UV (λem = 611 nm), (b) UV−vis excitation (λem = 611 nm), and (c) emission (λex = 393 nm) spectra of LiY0.99Eu0.01SiO4 at RT. D

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Figure 6. Emission spectrum of LiY0.99Eu0.01SiO4 in 560−640 nm range under 531 nm excitation at RT.

Figure 7. (a) Excitation spectra of LiY0.94Tb0.05Eu0.01SiO4 of Tb3+ 541 nm (a1) and Eu3+ 611 nm (a2) emissions; (b) emission spectra of LiY0.94Tb0.05Eu0.01SiO4 under Tb3+ 352 nm (b1) and Eu3+ 393 nm (b2) excitations at RT.

584.6, 591.1, and 596.0 nm, five 5D0 → 7F2 transition lines at 605.9, 608.7, 611.1, 612.8, and 621.6 nm, respectively. The results confirm that Eu3+ ions occupy the Cs symmetry Y3+ sites in LiYSiO4.33 Figure S5 displays the decay curves of Eu3+ 5D0-7F2 emission at 611 nm in samples LiY1−xEuxSiO4 (x = 0.001, 0.01, 0.03, 0.05, 0.1) under 393 nm excitation at RT. All curves almost overlap each other, following exponential characteristic and decay times are estimated to be ∼1.65 ± 0.02 ms. It indicates that concentration quenching effect is negligible. 3.4. Luminescence of Tb3+-Eu3+, Ce3+-Tb3+ and Ce3+Eu3+ Doubly Doped LiYSiO4. Figure 7a represents the excitation spectra of LiY0.94Tb0.05Eu0.01SiO4 by monitoring the emissions of Tb3+ at 541 nm (curve a1) and Eu3+ at 611 nm (curve a2), respectively. Curve a1 only contains the typical Tb3+ broad f-d transitions and f-f sharp lines corresponding to the

transitions of 7F0-5L6, 5D2, and 5D1 of Eu3+ within the 4f6 electron configuration, respectively. The comparison of two excitation curves in 260−360 nm displays that the broad bands are more intense than the f-f transitions. In the emission spectrum of 393 nm 7F0-5L6 transition excitation, a group of 5 D0-7FJ transitions are observed, and the strongest emission is the induced 5D0-7F2 electric dipole transition at ∼611 nm. Because Y3+ ions occupy Cs symmetry sites in LiYSiO4, it is expected that the degeneracy of each 7F0,1,2 energy level of Eu3+ would be completely lifted provided that Eu3+ ions enter Y3+ sites. To ascertain the site symmetry of Eu3+ in LiYSiO4, a more detailed emission spectrum of sample LiY0.99Eu0.01SiO4 in 560− 640 nm range was recorded with step size 0.1 nm under 531 nm 7F0 → 5D1 transition excitation at RT as shown in Figure 6. It can be observed that one 5D0 → 7F0 transition line with a maximum at 577.6 nm, three 5D0 → 7F1 transition lines at E

DOI: 10.1021/acs.inorgchem.6b01249 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry results in Figure 4a,b; no excitation signals of Eu3+ are detected, implying that the ET from Eu3+ to Tb3+ does not occur. When the monitoring emission wavelength shifts to 611 nm of Eu3+, the curve a2 consists of the excitation signals of Eu3+ and Tb3+, which shows the possible ET from Tb3+ to Eu3+.9,24,34 Because Tb3+ has a relatively strong absorption at 352 nm [Figure 3b], but the absorption of Eu3+ is almost undetectable at this wavelength [see blue dotted arrow in Figure 5b], we selected 352 nm as excitation wavelength in Figure 7b1. Besides the intrinsic Tb3+ 5D4-7FJ emissions, the Eu3+ 5D0-7FJ (J = 0, 1, 2) emissions appear unambiguously in the region of 575−625 nm, which confirm the ET from Tb3+ to Eu3+. At the same time, the emission spectrum of LiY0.94Tb0.05Eu0.01SiO4 under Eu3+ 393 nm excitation is plotted in Figure 7b2. No Tb3+ emission can be observed in this spectrum when Eu3+ is effectively excited, demonstrating that the energy transfer from Eu3+ to Tb3+ is absent when excited at the 393 nm transition in Eu3+. The Tb3+ → Eu3+ ET can be further confirmed by the Tb3+ 5 D4 emission decay dynamics. The decay curves of 5D4 541 nm emission in representative samples LiY0.95−xTb0.05EuxSiO4 (x = 0, 0.03, 0.05) under 483 nm (7F6-5D4 transition) excitation at RT are plotted in Figure 8. With the increase of Eu3+ content,

Figure 9. Decay curves of Tb3+ 5D3 432 nm emission in LiY0.999−xTb0.001EuxSiO4 (x = 0, 0.01, 0.03, 0.05) under 377 nm excitation at RT. (inset) Schematic ET processes.

seems plausible that this deviation and shortening is an evidence of Tb3+ → Eu3+ energy transfer via Tb3+ 5D3 state. In consideration of the energy levels match of Tb3+ and Eu3+, we infer that the possible ET channels may contain CR processes such as (5D3, 7F0-7F6, 5G2−6/5L6,7), (5D3, 7F0-7F5, 5D3), and (5D3, 7F0-7F1−3, 5D2) and so on as plotted in the inset of Figure 9. Obviously, the lifetimes of the excited states 5G2−6/5L6,7 and 5 D3 of Eu3+ are shorter than that of 5D3 of Tb3+; the electrons in these excited states of Eu3+ would swiftly relax to its subsequent states through multiphonon relaxation. Therefore, the CR processes such as 7F0 (Eu3+) + 5D3 (Tb3+) → 5 G2−6/5L6,7 (Eu3+) + 7F6 (Tb3+), 7F0 (Eu3+) + 5D3 (Tb3+) → 5 D3 (Eu3+) + 7F5 (Tb3+), and 7F0 (Eu3+) + 5D3 (Tb3+) → 5D2 (Eu3+) + 7F1−3 (Tb3+), which start from 5D3 excited states of Tb3+, would be dominant in comparison to the opposite processes, resulting in that the CR energy transfer is from Tb3+ to Eu3+. Using the Inokuti−Hirayama model, the ET kinetics parameter CDA is estimated to be ∼1.45 × 10−50 m6·s−1, and the ET rates PSA are increasing from ∼56.9 s−1 (1%) to ∼1385.39 s−1 (5%). It demonstrates the more efficient ET from Tb3+ to Eu3+ through 5D3 state with the increase of Eu3+ in these codoping samples. Figure S6a represents the excitation spectrum of LiY0.98Ce0.01Tb0.01SiO4 of 550 nm Tb3+ emission at RT. Besides the Tb3+ f-d excitation bands marked by black arrow in figure and weak f-f transitions (5D4-7F6) located at ∼483 nm, strong Ce3+ f-d excitations are clearly observed in the spectrum, denoting the ET occurring from Ce3+ to Tb3+. The emission spectra of LiY0.99−xCe0.01TbxSiO4 (x = 0.001−0.09) under 345 nm excitation are shown in Figure S6b. With the increase of Tb3+ content (x = 0.001−0.09) in LiY0.99−xCe0.01TbxSiO4 samples, Ce3+ emission intensities decrease gradually along with the enhancement of Tb3+ 5D4 emissions. The decay properties of Ce3+ in these samples at RT are plotted in Figure S6c. The Ce3+ decays become faster when Tb3+ content increases. All these observations demonstrate the occurrence of ET from Ce3+ to Tb3+, and the ET efficiencies increase with increasing doping content of Tb3+. Figure 10a shows the excitation spectra of LiY0.98Ce0.01Eu0.01SiO4 by monitoring Ce3+ 400 nm or Eu3+ 611 nm emissions at RT. It can be found that the Ce3+ excitation band intensity by monitoring Eu3+ 611 nm emission

Figure 8. Decay curves of Tb 3+ 5D4 541 nm emission in LiY0.95−xTb0.05EuxSiO4 (x = 0, 0.03, 0.05) under 483 nm excitation at RT. (inset) Schematic ET processes.

the decay curves show an anticipated deviation from exponential and shortened decay times, suggesting the efficient ET. The inset of Figure 8 schematically gives the possible ET channels, which are ascribed to phonon-assisted crossrelaxation processes (5D4, 7F0-7F6, 5D1) and (5D4, 7F0-7F4,5, 5 D0) because of the energy match between these multiplets.35,36 Using the Inokuti−Hirayama model, the ET kinetics parameter CDA is estimated to be ∼1.66 × 10−51 m6·s−1, and the ET rates PSA are increasing with the increase of Eu3+ content from ∼7.65 s−1 (at 1% doping) to ∼190.76 s−1 (at 5% doping). It is interesting to notice that the decay curves of Tb3+ 5D3 432 nm emission deviate from exponential characteristics and that the decay times are also proportionally shortened with increasing Eu3+ doping content in samples LiY0.999−xTb0.001EuxSiO4 (x = 0, 0.01, 0.03, 0.05) under 377 nm excitation at RT as shown in Figure 9. Because the Tb3+ doping concentration is fixed in these samples, the change certainly relates to the introduction of Eu3+. Accordingly, it F

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Figure 10. (a) Excitation spectra by monitoring Ce3+ 400 nm and Eu3+ 611 nm emissions of LiY0.98Ce0.01Eu0.01SiO4, and (b) emission spectra under 345 nm excitation of samples LiY0.99−xCe0.01EuxSiO4 (x = 0, 0.01, 0.03, 0.05) at RT.

Figure 11. (a) Decay curves of Ce3+ in LiY0.99−xCe0.01EuxSiO4 samples at RT; (b) emission spectra of samples LiY0.97Eu0.03SiO4 and LiY0.96Ce0.01Eu0.03SiO4 with 393 nm excitation at RT. (inset) Concentration-dependent Eu3+ emission intensities in single/doubly doped samples, respectively.

is conspicuously weaker than that by monitoring Ce3+ 400 nm emission, implying that the energy transfer from Ce3+ to Eu3+ is inefficient.13 Figure 10b shows the emission spectra of LiY0.99−xCe0.01EuxSiO4 under 345 nm (the first f-d transition band of Ce3+) excitation at RT. No obvious Eu3+ emissions can be observed in low Eu3+ doped sample LiY0.98Ce0.01Eu0.01SiO4, and Eu3+ emissions are not significantly enhanced with the increase of Eu3+ content. Referring to Figure S5 the concentration quenching does not occur in this doping range; the observation further confirms inefficient energy transfer from Ce3+ to Eu3+. In addition, Figure 10b clearly reveals that the emission intensities of Ce3+ decrease sharply after incorporation of Eu3+ into the samples. Meantime, the fluorescence decays of Ce3+ are faster and gradually deviate from exponential characteristic with the increase of Eu3+ content as shown in Figure 11a. The variations of Ce3+ emission intensities and decays imply the

existence of another crucial nonradiative channel since the inefficient energy transfer from Ce3+ to Eu3+. Figure 11b represents the emission spectra of samples LiY0.97Eu0.03SiO4 and LiY0.96Ce0.01Eu0.03SiO4 under Eu3+ 393 nm excitation at RT. Clearly, the emission intensity of Eu3+ in LiY0.96Ce0.01Eu0.03SiO4 is weaker than that in LiY0.97Eu0.03SiO4 although the two samples have same Eu3+ doping concentrations. The inset of Figure 11b exhibits the Eu3+ emission intensities decrease after incorporation of Ce3+ ions for all samples. These phenomena indicate that Eu3+ ions kill the Ce3+ emission and the Ce3+ ions kill the Eu3+ emission. Because of the stable valence states (+3) of Ce and Eu ions in the doubly doped samples (shown in Figure S2), the formation of Ce4+Eu2+ pairs during the synthesis process can be excluded. Generally, the metal−metal charge transfer (MMCT) between RE3+ ions with opposite reduction and oxidation properties, such as Ce3+-Eu3+ and Ce3+-Yb3+, could result in strong luminescence quenching for relevant ions after excitation. G

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Inorganic Chemistry Therefore, the MMCT of Ce3+ + Eu3+ → Ce4+ + Eu2+ is likely responsible for quenching the luminescence of Ce3+ and Eu3+.12,13,37 3.5. Luminescence of Ce3+-Tb3+-Eu3+ Triply Doped LiYSiO4. Figure 12 shows the excitation spectra of (a) Ce3+,

of Eu3+ by Ce3+ can be fulfilled through the incorporation of Tb3+. Figure 13a shows the emission spectra of tridoped samples LiY0.96−xCe0.01Tb0.03EuxSiO4 with different Eu3+ content (x = 0, 0.01, 0.03, 0.05, and 0.09) under 345 nm excitation. With the increase of x values, the emission intensities of Ce3+ and Tb3+ decrease regularly, and the weak emissions of Eu3+ at 611 nm also show a bit unexpectedly decrease. Because the concentrations of Ce3+ and Tb3+ are fixed in this series of samples, the variations of Ce3+/Tb3+ emission intensities must be the result of introduction of Eu3+. We infer that MMCT between Ce3+ and Eu3+ still occurs predominantly in these samples. It diminishes the intensities of Ce3+/Eu3+ emission and subsequently reduces the Ce3+ → Tb3+ energy transfer efficiency, so the simultaneous decreases of emission intensities of Ce3+/Eu3+/ Tb3+ in these samples are observed. When we fixed the doping concentrations of Ce3+/Eu3+ in samples LiY0.96−yCe0.01TbyEu0.05SiO4 (y = 0.03, 0.05), the emission spectra (λex = 345 nm, RT) are shown in Figure 13b. It can be found that the emission intensities of Ce3+ decrease, while those of Eu3+/Tb3+ increase. We believe that the observed results are due to combined effects of Ce3+ → Tb3+, Tb3+ → Eu3+, and Ce3+ → (Tb3+)n → Eu3+ ET processes as well as the Ce3+-Eu3+ MMCT. Because the concentrations of Ce3+ and Eu3+ are fixed, we first consider that the variations of Ce3+/Tb3+ emission intensities are due to the increase of Ce3+ → Tb3+ ET efficiencies with increasing Tb3+ doping content, when we presume that the influence of Ce3+-Eu3+ MMCT is fixed in these samples. Second, it is reasonable that ET efficiencies of Tb3+ → Eu3+ would also increase due to increasing Tb3+ content. Because the increase of Tb3+ → Eu3+ ET efficiency is lower than that of Ce3+ → Tb3+ ET efficiency, the emission intensities of Tb3+ and Eu3+ increase simultaneously. Third, Ce3+ → (Tb3+)n → Eu3+ ET also has its considerable contribution to the changes of Ce3+/Tb3+/Eu3+ emission intensities.34 Figure 14 shows the CIE chromaticity coordinates of LiY0.99Ce0.01SiO4 (A), LiY0.95Tb0.05SiO4 (B), LiY0.97Eu0.03SiO4 (C), and LiY0.96−xCe0.01Tb0.03EuxSiO4 [x = 0 (D1), 0.01 (D2), 0.03 (D3), 0.05 (D4), and 0.09 (D5)] and the luminescence

Figure 12. Excitation spectra (a) λem = 400 nm; (b) λem = 550 nm; (c) λem = 611 nm of LiY0.91Ce0.01Tb0.05Eu0.03SiO4 at RT.

(b) Tb3+, and (c) Eu3+ emissions in tridoped sample LiY0.91Ce0.01Tb0.05Eu0.03SiO4 at RT. In curve (a) only the f-d excitation bands of Ce3+ can be found, which is completely consistent with the excitation spectrum of Ce3+ in singly doped samples. The excitation spectrum of Tb3+ emission (curve b) clearly consists of the broad 4f-5d bands of Ce3+ and Tb3+ (