3:Tb3+, Sm3+ for Warm White LEDs - ACS Publications - American

Jun 16, 2017 - MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of ... the large-scale emission color tunability is realized in Ba3L...
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Concentration-Driven Selectivity of Energy Transfer Channels and Color Tunability in Ba3La(PO4)3:Tb3+, Sm3+ for Warm White LEDs Weijie Zhou,† Meng Gu,† Yiyi Ou,† Caihua Zhang,† Xuejie Zhang,† Lei Zhou,† and Hongbin Liang*,† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Here, we report the large-scale emission color tunability in Ba3La(PO4)3:Tb3+, Sm3+ (BLPO:TS) system based on the detailed discussion on the concentration-driven selectivity of energy transfer (ET) channels from Tb3+ to Sm3+. It is induced by the concentration-dependent 5D3 and 5D4 emissions of Tb3+ and the different interaction mechanisms of ET from Tb3+ to Sm3+ via 5D3 and 5D4 channels. In the diluted Tb3+ scenario, the red emission of Sm3+ is efficiently sensitized via the 5D3 channel, while in the concentrated Tb3+ case, the contribution of 5D4 channel is dominant. Therefore, by simply adjusting the doping concentrations of Tb3+ and Sm3+, the emission color of the phosphors can be tuned from green to red. In view of the phosphors with red emissions are critical to the warm white light-emitting diodes (WLEDs), an orange-red Tb3+, Sm3+ coactivated phosphor Ba3La0.90Tb0.05Sm0.05(PO4)3 (BLPO:5T5S) with good thermal and chromaticity stability and internal quantum efficiency ∼67% is developed in the system. Then, a near-UV WLED (CCT ≈ 4500 K, Ra ≈ 81) is fabricated using this phosphor. These findings not only indicate that the orange-red phosphor BLPO:5T5S is available for near-UV warm white LEDs but also deliver new insights into the ET processes in Tb3+ and Sm3+ activated phosphors. To examine the sensitization of Sm3+ emission through 5D3 level of Tb3+ and gain clear insights into the different energy transfer channels from Tb3+ to Sm3+, Ba3La(PO4)3 is selected as the host compound in this work due to its appropriate distance between two nearest adjacent La3+ ions, effective phonon frequency, excellent chemical stability, and easy doping of lanthanide ions.14,15 The concentration and temperaturedependent luminescence properties of Tb3+/Sm3+ singly and Tb3+, Sm3+ codoped samples are systematically investigated, respectively, to elucidate the concentration-driven selectivity of energy transfer (ET) channels from Tb3+ to Sm3+. As a result, the large-scale emission color tunability is realized in Ba3La(PO4)3:Tb3+, Sm3+ (BLPO:TS) system, and an orange-red Tb3+, Sm3+ coactivated phosphor Ba3La0.90Tb0.05Sm0.05(PO4)3 (BLPO:5T5S) with good thermal and chromaticity stability and internal quantum efficiency of ∼67% is developed, which proves available for near-UV WLEDs.

1. INTRODUCTION Nowadays, the phosphor-converted white light-emitting diodes (pc-WLEDs) have become commercially available solid-state lighting sources.1−5 The red light is a critical component to obtain warm white light-emitting diodes (WLEDs) with high color rendering index (CRI) and low correlated color temperature (CCT).6,7 Sm3+ ions usually exhibit red emissions due to 4G5/2−6HJ (J = 5/2−11/2) transitions in most compounds. Its simple energy level structure, abundant reserves, and cheap price make this lanthanide ion a possible red emitter for WLEDs. Nevertheless, the red emission of Sm3+ is often self-quenched by cross-relaxation (CR) processes between Sm3+ ions.8,9 Consequently, enhancing its luminescence intensity and absorption in the near-ultraviolet (n-UV) or blue range are significant challenging tasks for the utilization of this lanthanide resource in luminescent materials. The energy transfer between Sm3+ ion and another codoping ion may be a promising way to sensitize the luminescence of Sm3+. Depending on the structure of host compounds, Tb3+ ions exhibit blue and green emissions originated from the 5D3 and 5 D4 excited states, respectively.10,11 Both the 5D3 blue and 5D4 green emissions of Tb3+ have clear spectral overlapping with the excitation of Sm3+. Although the green emission from Tb3+ 5 D4 multiplet is usually used to sensitize the luminescence of other doping ions,12 the sensitizing feature of 5D3 blue emission is rarely reported.13 © 2017 American Chemical Society

2. EXPERIMENTAL SECTION Series of Sm3+/Tb3+ singly doped and Sm3+, Tb3+ codoped Ba3La(PO4)3 powder samples were synthesized by a high-temperature solid-state reaction method. Stoichiometric amounts of analytical reagents BaCO3, NH4H2PO4, and 99.99% pure rare-earth oxides (La2O3, Sm2O3, Tb4O7) were thoroughly ground in an agate mortar. Received: March 21, 2017 Published: June 16, 2017 7433

DOI: 10.1021/acs.inorgchem.7b00737 Inorg. Chem. 2017, 56, 7433−7442

Article

Inorganic Chemistry The obtained mixtures were heated at 873 K in air for 3 h, then annealed at 1573 K in CO atmosphere for 8 h. The final products were obtained after cooling to room temperature (RT). The structure of synthesized samples was characterized by powder X-ray diffraction (PXRD) employing a Bruker D8 Advanced powder diffractometer with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. Highquality XRD data for Rietveld refinement was recorded over a 2θ range of 10° to 110° at an interval of 0.02°, and the refinement was performed using TOPAS Academics software.16 The excitation and emission spectra and luminescence decay curves were collected on an Edinburgh Instrument FSP920 fluorescence lifetime and steady-state spectrometer. A 450 W xenon lamp was used as the excitation source for steady-state UV−vis spectra, while a 60 W μs flash lamp with a pulse width of 1.5−3.0 μs was employed as the excitation source for luminescence decay curves. The measurement temperature above RT was regulated by a temperature controller (Oxford, OptistatDN2). The internal quantum efficiency of sample was measured by employing a barium sulfate-coated integrating sphere (150 mm in diameter) attached to the FSP920 fluorescence spectrometer.

with 35% and 65% occupancies, respectively). The XRD patterns of Sm3+/Tb3+ singly and Sm3+, Tb3+ doubly doped Ba3La(PO4)3 samples were also recorded at RT, and the representative diffractograms are displayed in Figure S1. All the patterns are similar to one another and agree well with the refined pattern of Ba3La(PO4)3, which indicates that all are of a single Ba3La(PO4)3 phase. Because of the comparable effective ionic radii [r(Sm3+) = 113.2 pm, r(Tb3+) = 109.5 pm, r(La3+) = 121.6 pm, r(Ba2+) = 147.0 pm (CN 9)]17 (CN = coordination number) and the same valence state (3+), the doped Sm3+ and Tb3+ ions are thought to enter La3+ sites in the host. 3.2. Luminescence Properties of Sm3+ Doped Ba3La(PO4)3. Figure 2a shows the excitation (λem = 597 nm)

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Phase Purity. The Rietveld refinement of laboratory XRD data of synthesized compound Ba3La(PO4)3 was performed by using the I4̅3d (cubic) structure as an initial model (Figure 1).14 The obtained

Figure 2. (a) Excitation (λem = 597 nm) and (b) emission (λex = 402 nm) spectra of sample Ba3La0.995Sm0.005(PO4)3 at RT.

spectrum of sample Ba3La0.995Sm0.005(PO4)3 at RT. There are several excitation peaks located in the wavelength range of 300−500 nm, which are originated from 4f intra-configurational transitions of Sm3+. Among them, the peak at ∼402 nm with the strongest intensity is attributed to the transition from the ground state 6H5/2 to the excited state 4F7/2. The emission spectrum upon 402 nm excitation at RT is presented in Figure 2b. Four emission peaks are observed at ∼561, ∼597, ∼644, and ∼707 nm, corresponding to the transitions from 4G5/2 to 6 HJ/2 (J = 5−11), respectively. Considering the crystal structure of host Ba3La(PO4)3, the multifold electrostatic environments induced by random occupancies of cations (La3+ and Ba2+) and oxygen ions will make the 4f intra-configurational transitions of Sm3+ have wider energy ranges than those in other hosts.18,19 Consequently, the transition peaks in Figure 2, especially the emission peaks, of Sm3+ ions turn broadening to some extent, which is similar to those of Sm3+ in noncrystalline glass hosts.20 Figure S2 shows the luminescence decay curve (λex = 402 nm, λem = 597 nm) of sample Ba3La0.995Sm0.005(PO4)3 at RT. It appears single exponential property and the lifetime of Sm3+ emission is fitted to be ∼2.83 ms.

Figure 1. Rietveld refinement of laboratory XRD data of the synthesized compound Ba3La(PO4)3 at RT. (inset) The crystal structure viewed in a axis.

reliability factors Rwp, Rp, and RB all imply a good fitting quality, and no impurity phase was found. The refined structural parameters are listed in Table 1. The inset of Figure 1 shows the crystal structure viewed in a axis. The compound Ba3La(PO4)3 has a cubic structure with the space group I4̅3d (220) and the lattice parameters a = 10.53(1) Å, V = 1167 Å3, and Z = 4. Both the Ba2+ and La3+ enter the Wyckoff 16c sites with the occupancies of 0.75 and 0.25, respectively. They are coordinated by nine oxygen ions with C3 point symmetry, and the average bond length is ∼2.815(1) Å. The distance between two nearest La3+ ions is ∼4.191(1) Å. The oxygen ions are distributed over two kinds of Wyckoff 48e sites (O1 and O2

Table 1. Refined Structural Parameters of Compound Ba3La(PO4)3a at RT

a

atom

site

x

y

z

occ

biso (Å2)

Ba La P O1 O2

16c 16c 12a 48e 48e

0.06126(7) 0.06126(7) 3/8 0.4663(17) 0.5733(12)

0.06126(7) 0.06126(7) 0 0.3614(11) 0.3432(5)

0.061 26(7) 0.061 26(7) 1/4 0.7075(16) 0.7021(8)

0.75 0.25 1 0.3542 0.6458

1.77(2) 1.77(2) 1.89(1) 1.89(1) 1.89(1)

Symmetry, cubic; space group, I4̅3d; a = 10.53(1) Å, V = 1167 Å3; Z = 4. 7434

DOI: 10.1021/acs.inorgchem.7b00737 Inorg. Chem. 2017, 56, 7433−7442

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Figure 3. (a) The emission (λex = 402 nm) spectra of samples Ba3La1−xSmx(PO4)3 (x = 0.005−0.20) at RT. (inset) The concentration dependence of emission intensities of Sm3+ ions at 597 nm wavelength. (b) Concentration-dependent luminescence decay curves (λex = 402 nm, λem = 597 nm) of samples Ba3La1−xSmx(PO4)3 (x = 0.005−0.20) at RT and the corresponding fitting curves via Inokuti−Hirayama model. (inset) The possible cross relaxation channels of Sm3+. (c) The fitting results of decay curves via Yokota−Tanimoto model. (d) The fitting results of decay curves via Burshtein̆ model.

can be estimated to be ∼22.3 Å, which is well-consistent with the results of other works.19,24 This long critical distance of ET between Sm3+ ions implies that an effective multipolar interaction is dominant in the ET process.25 To further study the ET between Sm3+ ions, the luminescence decay curves (λex = 402 nm, λem = 597 nm) of samples Ba3La1−xSmx(PO4)3 (x = 0.005−0.20) at RT were collected and presented in Figure 3b. For low doping sample (x = 0.005), due to the long separation distance, the ET between the isolated Sm3+ is inefficient to influence the luminescence decay. Consequently, the luminescence intensity I(t) of Sm3+ in low doping sample (x = 0.005) can be simply described using a single exponential function as follows:

The influence of doping concentration on the Sm 3+ luminescence is discussed in detail below. Figure 3a shows the emission (λ ex = 402 nm) spectra of samples Ba3La1−xSmx(PO4)3 (x = 0.005−0.20) at RT. With the increase of doping content, the emission intensity of Sm3+ first goes rising and comes to the maximum at x = 0.05, as clearly illustrated in the inset; subsequently, the concentration quenching effect dominates, giving rise to the decrease of Sm3+ emission. Basically, the energy transfer (ET) between Sm3+ ions via cross relaxation (CR) processes plays a main role in the concentration quenching of Sm3+. Numerous CR channels have been previously reported as depicted in the inset of Figure 3b, such as [4G5/2, 6H5/2]→[6F11/2, 6F5/2], [4 G5/2 , 6 H 5/2 ]→[ 6F 5/2, 6F 11/2 ], 21 [ 4G 5/2 , 6 H5/2]→[6 F 9/2 , 6 F9/2],22 and so on. As suggested by Blasse,23 the critical distance (Rc) of energy transfer between Sm3+ ions is approximately equal to twice the radius of a sphere that one ion averagely possesses: ⎛ 3V ⎞1/3 R c ≈ 2⎜ ⎟ ⎝ 4πxcN ⎠

⎡ ⎛ t ⎞⎤ I (t ) = exp⎢ −⎜ ⎟⎥ ⎢⎣ ⎝ τ0 ⎠⎥⎦ I(0)

(2)

where I(0) is the initial emission intensity, and τ0 is the intrinsic lifetime of Sm3+. The fitting result has been shown in Figure S2. When further increasing the Sm3+ concentration, the luminescence decay curves gradually deviate from the single exponential property, which indicates that the nonradiative processes depopulating the Sm3+ emitting levels, such as cross relaxation energy transfer, is more active. Herein, the luminescence intensity I(t) of Sm3+ is expressed by

(1)

where xc is the critical concentration, N is the number of possible sites that Sm3+ ions can enter in the unit cell of host, and V is the unit cell volume. In our case, xc is equal to 0.05, N is 4, and V is 1167 Å3. Consequently, the critical distance of ET 7435

DOI: 10.1021/acs.inorgchem.7b00737 Inorg. Chem. 2017, 56, 7433−7442

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Inorganic Chemistry ⎡ ⎛ t ⎞⎤ N I (t ) = exp⎢ −⎜ ⎟⎥ ∏ exp[−tn(R k)] I(0) ⎣⎢ ⎝ τ0 ⎠⎥⎦ k = 1

models, although the better fitting qualities have been realized compared to those via I−H model. Consequently, the energy migration process between Sm3+ is only active in the Sm3+ concentrated sample. The temperature-dependent luminescence decay curves (λex = 402 nm, λem = 597 nm) of sample Ba3La0.995Sm0.005(PO4)3 in a temperature range of 300−475 K are presented in Figure S3. The well overlapping of these curves with single exponential properties indicates that no evident thermal quenching of Sm3+ emission occurs. In our case, both the absence of high-lying quenching states and the big energy gap (∼32 197 cm−1) from 4 G5/2 excited state to 6F11/2 state make the possible quenching channels for Sm3+ emission, such as the thermal activated quenching or multiphonon relaxation, appear inert. As a consequence, the red emission from Sm3+ exhibits a good thermal stability. 3.3. Luminescence Properties of Tb3+ Doped Ba3La(PO4)3. Figure 4a shows the excitation (λem = 540 nm)

(3)

3+

where N is the total number of Sm ions around a hypothetical Sm3+ donor in a spherical volume, and n(Rk) denotes the rate constant of energy transfer from a donor Sm3+ to an acceptor Sm3+ at a distance of Rk. According to the former works,26−29 the relationship between rate constant n(Rk) and distance Rk is determined by an inverse-power rate model in consideration of multipolar interaction for energy transfer. Consequently, we first obtained the Inokuti−Hirayama (I−H) formula [eq 4] to study the luminescence decay of Sm3+ ions.30 ⎡ ⎛ t ⎞ 4π ⎤ ⎛ I (t ) 3⎞ CA Γ⎜1 − ⎟(C DA )3/ S t 3/ S ⎥ = exp⎢ −⎜ ⎟ − ⎝ ⎢⎣ ⎝ τ0 ⎠ ⎥⎦ 3 I(0) S⎠ (4)

where CA denotes the concentration of Sm , Γ(x) is the gamma function, S value implies the type of multipolar interaction [6 for electric dipole−dipole (EDD), 8 for electric dipole−quadrupole (EDQ), and 10 for electric quadrupole− quadrupole (EQQ)], and CDA is the energy transfer microparameter. For S = 6, the best fitting results for the concentration-dependent luminescence decay curves of Sm3+ were achieved as shown in Figure 3b. It indicates that the main mechanism responsible for the ET between Sm3+ is the electric dipole−dipole (EDD) interaction. The fitting results are listed in Table S1. The average CDA is ∼5.337 × 10−54 m6/s. Further, when considering the possible energy migration along with the energy transfer between Sm3+ governed by EDD interaction, two other theoretical models for different migration regimes, that is, the diffusion model of Yokota−Tanimoto (Y−T)31,32 ̆ 33,34 were applied to study and the hopping model of Burshtein, 3+ the decay properties of Sm . The diffusion model for dipole− dipole interaction gives the following equation for the luminescence decay of Sm3+: 3+

Figure 4. (a) Excitation (λem = 540 nm) and (b) emission (λex = 377 nm) spectra of sample Ba3La0.97Tb0.03(PO4)3 at RT.

spectrum of sample Ba3La0.97Tb0.03(PO4)3 at RT. The excitation band beyond 300 nm is attributed to the f−d transitions of Tb3+. The sharp absorption lines in the wavelength range of 300−500 nm is originated in the f−f transition of Tb3+. Among them, the peak at ∼377 nm corresponds to the transition from the ground state 7F6 to excited state 5G6. Upon the 377 nm excitation, the emission spectrum (curve b) consists of two sets of emission lines from the 5D3 and 5D4 excited states to the 7FJ ground states, respectively, as separated by the black dash line in Figure 4. They are (i) 5D3→7F5 (∼415 nm) and 5D3→7F4 (∼434 nm) and (ii) 5D4→7F6 (∼485 nm), 5D4→7F5 (∼540 nm), 5D4→7F4 (∼581 nm), and 5D4→7F3 (∼620 nm). The blue emission from 5D3 state is weaker than green emission from 5D4 state under the excitation to 5G6 level. Generally, the intensity ratio of 5D3/5D4 emissions of Tb3+-activated phosphor with specific doping concentration can be first adjusted by the different excitation energies. When the phosphor is excited by the host absorption energy or to the 5d states, the visible quantum cutting of Tb3+ will occur and then give the different intensity ratio of 5D3/5D4 emissions compared to the cases that the systems are only excited to the 4f excited states by the lowenergy photons.36,37 Recently, Lin et al. has also reported a UV/VUV switch-driven green-blue two-color-reversal effect of Tb3+ in an oxonitridosilicate host and proposed different feeding channels for Tb3+ when the different energy photons were used to excite the samples.10 In our case, the system is only excited to the 5G6 state of Tb3+ upon the 377 nm excitation, so the quantum cutting process is ruled out. Actually, the excited electrons will quickly relax to the lower 4f states,

⎡ ⎛ ⎞ I (t ) 4 t = exp⎢ −⎜ ⎟ − π 3/2CA(C DA )1/2 t 1/2 ⎢⎣ ⎝ τ0 ⎠ 3 I(0) ⎛ 1 + 10.87x + 15.50x 2 ⎞3/4 ⎤ ⎟ ⎥ ⎜ 1 + 8.743x ⎠ ⎥⎦ ⎝

(5)

DCDA−1/3t2/3,

where x = and D is the diffusion parameter derived from the energy migration microparameter CDD via D = 0.5(4πCA/3)4/3CDD.35 This model is valid in the limit CDD ≪ CDA. The hopping model for dipole−dipole interaction is expressed by ⎤ ⎡ ⎛t ⎞ I (t ) 4 = exp⎢ −⎜ ⎟ − π 3/2CA(C DA )1/2 t 1/2 − Wt ⎥ ⎢⎣ ⎝ τ0 ⎠ 3 I(0) ⎦⎥

(6)

where W = π(2π/3) The hopping model is valid in the limit CDA ≪ CDD. The fitting results for the concentration-dependent luminescence decay curves of Sm3+ via these two models were shown in Figure 3c,d and Table S2. Only the fitting procedure for the decay curve of sample Ba3La0.80Sm0.20(PO4)3 via Y−T model is valid. The obtained energy migration microparameter CDD (∼0.2410 × 10−54 m6/s) is much less than the energy transfer microparameter CDA (∼7.105 × 10−54 m6/s; Table S2). Other fitting results are meaningless or contradictory to the conditions required for the 5/2

CA2CDA1/2CDD1/2.

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DOI: 10.1021/acs.inorgchem.7b00737 Inorg. Chem. 2017, 56, 7433−7442

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Inorganic Chemistry

Figure 5. (a) The schematic configurational coordinate diagram of Tb3+ in Ba3La(PO4)3 host; (b) the temperature-dependent lifetime values of Tb3+ D3 level in the temperature range of 78−475 K; (c) the area-normalized concentration-dependent emission (λex = 377 nm) spectra of samples Ba3La1−yTby(PO4)3 (y = 0.03−0.30) at RT. (inset) The emission intensities at 434 nm from 5D3 state and 540 nm from 5D4 state with the increase of Tb3+ content.

5

different temperatures estimated by the equation τa = [∫ ∞ 0 t· 43 I(t)dt]/[∫ ∞ are also given in Figure 5b. The decay 0 I(t)dt] time of 5D3 emission shows slight decrease by ∼9% up to 475 K compared to that at 78 K, which is in line with the prediction of eq 7. Of course, except for the MPR process, the phononassisted CRET between Tb3+ will also take effect when taking the fact of the nonresonant CR channels into account.44 Second, the contribution of CRET between Tb3+ is considered. Figure 5c shows the area-normalized concentration-dependent emission (λex = 377 nm) spectra of samples Ba3La1−yTby(PO4)3 (y = 0.03−0.30) at RT. With the increase of Tb3+ content, the blue emission (400−475 nm) from 5D3 state gradually decreases, while the green emission (475−650 nm) from 5D4 progressively increases, as evidently illustrated in the inset. This is related to the CRET between Tb3+ ions, and the main CR channels such as [5D3, 7F6] (5800 cm−1)→[5D4, 7 F0] (6000 cm−1) and [5D3, 7F6] (20 300 cm−1)→[7F0, 5D4] (20 500 cm−1) are illustrated in Figure 5a. The 5D3 state serves as the donor character during this CRET process, so the blue emission from 5D3 is then seriously quenched. In fact, the contribution of CRET process for 5D3 state depopulation is larger than that of MPR process as discussed above. To study the main interaction mechanism responsible for CRET between Tb3+ ions, the concentration-dependent luminescence decay curves (λex = 377 nm, λem = 434 nm) of donor 5D3 state of samples Ba3La1−yTby(PO4)3 (y = 0.03−0.30) at RT were collected and presented in Figure 6a. It shows that, even for the diluted sample (y = 0.03), the decay curve exhibits the nonexponential rather than single exponential property. Further, with the increase of Tb3+ concentration, the curves drop more rapidly. These observations imply that the energy transfer between Tb3+ is efficient and the corresponding critical distance is long enough, so a multipolar interaction is more

such as 5D3 and 5D4, and then return to the ground states with different emissions, respectively, as depicted in Figure 5a. As a consequence, the weaker blue emission from 5D3 state in Figure 4 is mostly related to the preferential nonradiative depopulation of 5D3 state compared to 5D4 state. Referring to the schematic energy levels of Tb3+ in Figure 5a, there are two possible mechanisms responsible for the nonradiative depletion of 5D3: (1) the multiphonon relaxation (MPR) from 5D3 to 5D4 state;38 (2) the cross-relaxation energy transfer (CRET) between Tb3+.39 The following parts will discuss the contributions of these two processes. First, the nonradiative de-excitation of 5D3 state to the next lower 5D4 state could be achieved by the MPR process. The possibility of this process, according to the stimulated phonon emission model,40 depends on temperature, the energy difference between the 5D3 and 5D4 state, and the effective host compound phonon energy involved in the relaxation process, as given in the following equation: −p ⎡ ⎛ hν ⎞⎤ W MPR (T ) = W0MPR ⎢1 − exp⎜ − ⎟⎥ ⎝ kT ⎠⎦ ⎣

(7)

MPR

where W (T) is the multiphonon relaxation rate at temperature T (K), while WMPR is the rate at 0 K. In our 0 case, the parameter hν of host Ba3La(PO4)3 is estimated to be the same as its highest phonon energy,40 which is ∼1100 cm−1 as most phosphate compounds possessing.40,41 Consequently, the phonon number p (p = ΔE/hν) that is required to bridge the energy gap (ΔE ≈ 5800 cm−1) between the 5D3 and 5D4 states of Tb3+ is calculated to be ∼5, which implies that the multiphonon relaxation channel plays a role in the depopulation of 5D3 state.42 Figure S4 shows the temperaturedependent luminescence decay curves (λex = 377 nm, λem = 434 nm) of 5D3 emission. The average lifetimes of 5D3 level at 7437

DOI: 10.1021/acs.inorgchem.7b00737 Inorg. Chem. 2017, 56, 7433−7442

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Inorganic Chemistry

3.4. Concentration-Driven Selectivity of Energy Transfer Channels from Tb3+ to Sm3+ in Ba3La(PO4)3. Figure 7a

Figure 6. (a) The concentration-dependent luminescence decay curve (λex = 377 nm, λem = 434 nm) of samples Ba3La1−yTby(PO4)3 (y = 0.03−0.10) at RT and the fitting results via Inokuti−Hirayama model; (b) the concentration-dependent luminescence decay curve (λex = 377 nm, λem = 540 nm) of samples Ba3La1−yTby(PO4)3 (y = 0.03−0.10) at RT. (inset) The enlargement of curves in the time range of 0−3 ms.

likely to take effect. Similar to the case of Sm3+ above, the Inokuti−Hirayama model is employed to analyze. When S = 6, the best fitting results for the concentration-dependent luminescence decay curves of 5D3 state were achieved as shown in Figure 6a, which indicate that the main mechanism responsible for the CRET between Tb3+ is the EDD interaction. The energy transfer microparameter CDA is ∼2.726 × 10−53 m6/s (Table S3). Meanwhile, the intrinsic lifetime of 5D3 state is fitted to be ∼2.03 ms in Ba3La(PO4)3 host, and the simulated decay curve of 5D3 state with single exponential property is also presented [Figure 6a]. The analysis on the possible energy migration between Tb3+ ions is similar to the above-mentioned Sm3+ case. The fitting results via Y−T and Burshtein̆ models (Table S4) imply that the migration processes between Tb3+ ions could be negligible in the investigated concentration range. The CRET process depopulates the 5D3 state but populates the 5D4 state. Figure 6b shows the luminescence decay curves (λex = 377 nm, λem = 540 nm) of sample Ba3La1−yTby(PO4)3 (y = 0.03−0.30) at RT. For the diluted sample (y = 0.03), the decay curve comprises two main processes: (1) the build-up process in the initial time range of 0−0.5 ms due to the CRET from 5D3 to 5D4 state; (2) the decay process of 5D4 state. With the increase of y the build-up process turns more faster, so that the initial rising phenomenon becomes more unobvious as displayed in the inset of Figure 6b. This observation confirms the more efficient CRET process to populate the 5D4 state.

Figure 7. (a) The excitation and emission spectra of Tb3+ and Sm3+ in Ba3La(PO4)3 at RT; (b) the schematic configurational coordinate diagram of Tb3+ and Sm3+ in Ba3La(PO4)3; (c) the emission (λex = 377 nm) spectra of samples Ba3La0.95−yTbySm0.05(PO4)3 (y = 0.01−0.20) at RT. (inset) The concentration dependencies of emission intensities of Sm3+ at 597 nm and of Tb3+ at 540 nm.

compiles the excitation and emission spectra of Ba3La0.97Tb0.03(PO4)3 and Ba3La0.995Sm0.005(PO4)3 at RT. The 5 D3 and 5D4 emissions of Tb3+ have ample spectral overlapping with the excitation spectrum of Sm3+, which presage that the Tb3+ ions might sensitize the Sm3+ emissions via two possible CRET channels, that is, (1) 5D3 channel: [5D3(Tb3+), 6 H5/2(Sm3+)]→[7F5(Tb3+), 4F7/2(Sm3+)] and (2) 5D4 channel: [5D4(Tb3+), 6H5/2(Sm3+)]→[7F6(Tb3+), 4I11/2(Sm3+)], as shown in Figure 7b. Actually, the sensitizing feature of Tb3+ via 5D4 channel for other luminescent ions has been widely studied,12,45 while the report on the contributions of 5D3 channel is scarce.13 To efficiently sensitize other luminescent ions, such as Eu2+, Sm3+, etc., via the 5D3 blue emission of Tb3+, the MPR and CRET processes that quench the 5D3 emission should be restricted. The host compounds with suitable cationic distance and effective phonon energy would be carefully selected to incorporate the Tb3+ ions to give the 7438

DOI: 10.1021/acs.inorgchem.7b00737 Inorg. Chem. 2017, 56, 7433−7442

Article

Inorganic Chemistry

Figure 8. (a) The luminescence decay curves (λex = 377 nm, λem = 434 nm) of Ba3La0.95−xTb0.05Smx(PO4)3 (x = 0−0.05) at RT; (b) the luminescence decay curves (λex = 377 nm, λem = 540 nm) of Ba3La0.95−xTb0.05Smx(PO4)3 (x = 0.005−0.05) at RT; (c) the luminescence decay curves (λex = 377 nm, λem = 540 nm) of Ba3La0.80−xTb0.20Smx(PO4)3 (x = 0.005−0.05) at RT; (d) the schematic diagram for the concentration-driven selectivity of ET channels from Tb3+ to Sm3+.

intense 5D3 blue emission. Herein, the moderate separation distance (∼4.19 Å) between La3+ sites and phonon energy (∼1100 cm−1) make compound Ba3La(PO4)3 a good example to reveal the energy transfer properties from Tb3+ to Sm3+ via different channels. To experimentally check and investigate the sensitizing feature of Tb3+ for Sm3+ emissions, a series of codoping samples was designed and synthesized. First, the emission (λex = 377 nm) spectra of samples Ba3La0.95−yTbySm0.05(PO4)3 (x = 0.01− 0.20) are presented in Figure 7c. Upon the 377 nm excitation of Tb3+ transition (7F6→5G6), the increasing Tb3+ content gives rise to not only the more intense emissions from Tb3+ but also the increasing emissions from Sm3+, whose concentration is fixed to be 0.05. The concentration dependencies of emission intensities of Sm3+ at 597 nm and Tb3+ at 540 nm in the inset show a clear presentation. Figure S5 also displays the excitation (λem = 597 nm) spectra of samples Ba3La0.80−xTb0.20Smx(PO4)3 (x = 0.005−0.05) at RT. When monitored the 597 nm emission mainly from Sm3+ ion, the excitation intensities of Tb3+ at constant concentration (0.20) turn stronger with the increasing Sm3+ concentrations, especially the f−d absorptions beyond the 300 nm. Surely, the Sm3+ emissions can be sensitized by Tb3+ ions. To further study the energy transfer dynamics from Tb3+ to Sm3+ via two possible ET channels, specifically, from 5D3 and 5 D4 states of Tb3+ to Sm3+, two series of samples that contain diluted and concentrated Tb3+ ions with increasing Sm3+ content, respectively, are analyzed with a view to the concentration-dependent 5D3 and 5D4 emissions of Tb3+. For the diluted Tb3+ case with more dominant 5D3 blue emission, Figure 8a presents the luminescence decay curves (λex = 377 nm, λem = 434 nm) of Ba3La0.95−xTb0.05Smx(PO4)3 (x = 0− 0.05) at RT. As discussed above, the curve of Tb3+ singly doped sample exhibits a nonexponential property, which is related to

the CRET between Tb3+ accelerating the decay of 5D3 emissions. With the increase of Sm3+ content, the lifetime values of 5D3 emissions are further largely decreased. This proves the CRET process from Tb3+ to Sm3+ via the 5D3 channels is efficient. However, because of the coexistence of CRET from Tb3+ to Tb3+ and from Tb3+ to Sm3+, we failed to fit the decay curves of codoped samples as the analysis conducted above on Sm3+ or Tb3+ singly doped samples. Herein, an approximate method is available that the S value featuring the cooperative interaction mechanism of Tb3+→Tb3+ and Tb3+→Sm3+ CRET processes can be simply obtained by fitting the straight line acquired from the data in Figure 8a with the following formula derived from the eq 4:46,47 ⎛ ⎛ I (t ) ⎞ t ⎞ 3 ⎛t ⎞ ln⎜ −ln⎜ ⎟ − ⎟ = B + ln⎜ ⎟ S ⎝ τ0 ⎠ τ0 ⎠ ⎝ ⎝ I(0) ⎠

(8)

where τ0 is set to be ∼2.03 ms for the 5D3 intrinsic lifetime as obtained above, and B is a control factor. The fitting results in Table 2 and Figure S6 show the average S value is approximately equal to 6, which implies that an EDD interaction is responsible for the cooperative contributions of Tb3+→Tb3+ and Tb3+→Sm3+ CRET processes. Considering the Table 2. Obtained S Values from the Fitting Procedures for Samples Ba3La0.95−xTb0.05Smx(PO4)3 (x = 0.005−0.05) at RT

7439

Sm3+ content x

fitting S values

average S value

0.005 0.01 0.02 0.03 0.05

6.107 6.312 5.922 5.701 6.088

6.026

DOI: 10.1021/acs.inorgchem.7b00737 Inorg. Chem. 2017, 56, 7433−7442

Article

Inorganic Chemistry EDD mechanism for Tb3+→Tb3+ CRET process and the comparative shortening of 5D3 lifetimes in the Tb3+ singly doped and Tb3+, Sm3+ codoped samples with similar total doping content in Figure S7, we propose that the CRET from Tb3+ to Sm3+ through 5D3 channel is also mainly governed by EDD interaction mechanism. Consequently, the efficient CRET from the 5D3 level of Tb3+ to Sm3+ can serve as a new strategy to sensitize the Sm3+ red emission, which is rarely reported to our knowledge. Meanwhile, the contribution of 5D4 channel is also investigated in terms of the luminescence decay curves (λex = 377 nm, λem = 540 nm) of Ba3La0.95−xTb0.05Smx(PO4)3 (x = 0.005−0.05) at RT in Figure 8b. Unlike the former 5D3 case, the lifetime of 5D4 state just slightly decreases with the increase of Sm3+ content, which means that the CRET between Tb3+ and Sm3+ via 5D4 channel is inefficient and subordinate to the one via 5D3 channel in the diluted Tb3+ scenario. For the concentrated Tb3+ case with increasing Sm3+ content, the CRET between ions becomes more efficient to quench the 5 D3 blue emission due to the increasing total doping concentrations and shorter separation distances between ions. Thus, the decay signals of 5D3 emissions (434 nm) of Tb3+ in samples Ba3La0.80−xTb0.20Smx(PO4)3 (x = 0.005−0.05) are too weak to collect. Despite of the same short separation distances and efficient energy transfer (EDD) in Tb3+→Tb3+ and Tb3+→ Sm3+ pairs, the dominant amount of Tb3+ ions determine that the 5D3 blue emission is to a large extent quenched by Tb3+→ Tb3+ CRET process [Figure 5a] rather than Tb3+→Sm3+ via 5 D3 channel [Figure 7b]. That is to say, the sensitizing contribution of 5D3 channel to Sm3+ emissions in the concentrated Tb3+ case is quite limited. While the 5D4 green emission of Tb3+ becomes stronger compared to those in the diluted Tb3+ case, so the sensitizing feature of 5D4 channel turns obvious and dominating. Figure 8c presents the luminescence decay curves (λex = 377 nm, λem = 540 nm) of Ba3La0.80−xTb0.20Smx(PO4)3 (x = 0.005−0.05) at RT. The lifetime of 5D4 state decreases with the increase of Sm3+ content. So far, on the basis of the concentration-dependent 5 D3 and 5D4 emissions of Tb3+ and the different interaction mechanisms of CRET from Tb3+→Sm3+ via 5D3 and 5D4 channels, a concentration-driven selectivity of CRET channels in Ba3La(PO4)3:Tb3+, Sm3+ phosphors is proposed as illustrated in Figure 8d, which is essential for the development of phosphors based on Tb3+ and Sm3+ ions toward advanced applications.8,48,49 3.5. Color Tunability of Ba 3 La(PO 4 ) 3 :Tb 3+ , Sm 3+ Phosphors under Near-UV Excitation. The emission color tunability is one of the important properties for phosphors to fulfill the different technical demands for applications.50 Because of the different emissions from Tb3+ and Sm3+ along with the CRET from Tb3+ to Sm3+ via concentrationdependent channels, a large-scale color tuning can be achieved by controlling their doping concentrations. Figure 9 shows the CIE chromaticity coordinates of emission spectra (λex = 377 nm) and luminescence photos under 377 nm excitation light of two sets of codoped samples, that is, Ba3La0.80−xTb0.20Smx(PO4)3 (x = 0−0.05) and Ba3La0.95−yTbySm0.05(PO4)3 (y = 0−0.20). An evident color evolution from green (0.311, 0.571) to red (0.590, 0.395) is presented. More information is given in Figure S8 and Table S5. To evaluate the potential of these phosphors for near-UV LED applications, sample Ba 3 La 0.90 Tb 0.05 Sm 0.05 (PO 4 ) 3 (BLPO:5T5S) with orange-red emission is selected to

Figure 9. CIE chromaticity coordinates of the emission (λex = 377 nm) spectra and luminescence photos under 377 nm excitation light of Ba3La(PO4)3:Tb3+, Sm3+ phosphors with different doping concentrations.

investigate. The internal quantum efficiency at RT is measured to be ∼67%. Figure 10a shows the emission (λex = 377 nm) spectra in a temperature range of 300−500 K. The integrated emission intensity gradually decreases with the increase of

Figure 10. (a) The emission (λex = 377 nm) spectra of sample Ba3La0.90Tb0.05Sm0.05(PO4)3 (BLPO:5T5S) in the temperature range of 300−500 K. (inset) The temperature-dependent integrated emission intensities and chromaticity coordinates; (b) the electroluminescence (EL) spectrum of packed LED device under a 100 mA forward bias current and the luminescence photo. 7440

DOI: 10.1021/acs.inorgchem.7b00737 Inorg. Chem. 2017, 56, 7433−7442

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Inorganic Chemistry temperature, and the intensity at 500 K drops to ∼72% of that at 300 K. The chromaticity coordinates of emission spectra at increasing temperature keep quite stable. These results show that the orange-red emission of phosphor BLPO:5T5S possesses good thermal stability and chromaticity stability, which are critical characteristics for general lighting applications.51,52 To further obtain a warm white LED device, the phosphor BLPO:5T5S along with the commercial blue phosphor BAM (BaMgAl10O17:Eu2+) and yellow phosphor Ca6BaP4O17:Eu2+53 were packaged with a near-UV chip (370− 380 nm). Figure 10b shows the corresponding electroluminescence (EL) spectrum of packaged LED device under a 100 mA forward bias current and the luminescent photo. It presents a low CCT (∼4500 K) and high CRI (∼81), which is superior to those of the commercial white LED composed of blue chip and YAG:Ce3+ phosphor (CCT ≈ 7756 K; Ra ≈ 75). The CIE chromaticity coordinates are calculated to (0.368, 0.404). These properties of fabricated LED indicate that the orange-red phosphor BLPO:5T5S is available for near-UV warm white LEDs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Weijie Zhou: 0000-0001-5199-9836 Hongbin Liang: 0000-0002-3972-2049 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (21171176, U1232108, U1432249, 21671201 and U1632101) and the Natural Science Foundation of Guangdong Province (S2013030012842).

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4. CONCLUSIONS In summary, we have discussed the different ET channels from Tb3+ to Sm3+ and emission color tunability from green to red in Ba3La(PO4)3 host. Simultaneously, an orange-red Tb3+, Sm3+ coactivated phosphor Ba 3 La 0 . 9 0 T b 0 . 0 5 Sm 0 . 0 5 (PO 4 ) 3 (BLPO:5T5S) with good thermal and chromaticity stability and internal quantum efficiency ∼67% is developed, which is available for near-UV warm white LEDs. The Rietveld refinement gives the cubic structure of synthesized host with the space group I4̅3d, and the doping Tb3+ and Sm3+ ions are thought to enter the La3+ sites. The concentration- and temperature-dependent luminescence properties of Tb3+/Sm3+ singly doped samples are investigated in detail, respectively. The red emission from Sm3+ is thermally stable but will be quenched due to the efficient CRET governed by the EDD mechanism with the increase of concentrations. The blue emission from 5D3 level of Tb3+ is weaker than green emission from 5D4 level, especially for concentrated samples, which is due to the MPR and CRET featuring EDD mechanism. Then the sensitization of Sm3+ red emission by Tb3+ is discussed. In the diluted Tb3+ case, the efficient ET from Tb3+ to Sm3+ is through 5D3 channel, which proves governed by the EDD mechanism, whereas in concentrated Tb3+ scenario, sensitization of Sm3+ emission is mainly via 5D4 channel. The serial phosphors present tunable emission from green to red by simply adjusting the doping concentrations of Tb3+ and Sm3+. Finally, a near-UV warm white LED (CCT ≈ 4500 K, Ra ≈ 81) is fabricated by using the efficient orange-red phosphor Ba3La0.90Tb0.05Sm0.05(PO4)3 (BLPO:5T5S).

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samples BLPO:Tb3+, Sm3+; fitting results of luminescence decay curves of Sm3+ and Tb3+ via InokutiHirayama, Yokota-Tanimoto, and Burshtein̆ models; CIE chromaticity coordinates of emission spectra of samples BLPO:Tb3+, Sm3+ (PDF)

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00737. Representative XRD patterns of samples BLPO:Tb3+, Sm3+; temperature-dependent luminescence decay curves of different samples; excitation spectra of samples BLPO:0.20Tb3+, xSm3+ at RT; fitting curves for determining the S values of ET; luminescence decay curves of different samples at RT; emission spectra of 7441

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