Tb3+: A Green-Emitting Phosphor with High

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Sr2LiScB4O10:Ce3+/Tb3+: A Green-Emitting Phosphor with High Energy Transfer Efficiency and Stability for LEDs and FEDs Hang Chen†,‡,§ and Yuhua Wang*,†,‡,§

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†

National & Local Joint Engineering Laboratory for Optical Conversion Materials and Technology, Lanzhou University, Lanzhou, 730000, People’s Republic of China ‡ Key Laboratory for Special Function Materials and Structural Design of the Ministry of Education, Lanzhou University, Lanzhou, 730000, People’s Republic of China § Gansu Province Engineering Laboratory for Optical Functional Materials, Lanzhou University, Lanzhou, 730000, People’s Republic of China S Supporting Information *

ABSTRACT: Exploring new, efficient, and stable phosphors is important and meaningful work for both LEDs and FEDs. Here, the new Ce3+/Tb3+-codoped green-emitting phosphor Sr2LiScB4O10 with high energy transfer efficiency and outstanding stability was designed and prepared by a solid-state reaction. The phosphors show a broad excitation spectrum in the range of 235−375 nm and a narrow green emission spectrum around 544 nm. The strong Tb3+ emission with weak Ce3+ emission and the decreasing lifetime of Ce3+ indicate that the samples have high energy transfer efficiency up to more than 90%. The green emission intensity at 250 °C can keep 75% of its intensity at room temperature with changeless color coordinates, which reveals that the phosphors have excellent thermal stability and color stability. The CL performance indicates that the phosphor possesses high saturation current and saturation voltage with good color stability, and a related luminescence mechanism was proposed to explain the remarkable CL properties. On the basis of the outstanding luminescence performance and thermal and color stability in PL and CL, the phosphor shows promise for applications in LEDs and FEDs.

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Mn2+-doped phosphors which have been reported, such as βSiAlON,10 (BaSr)2SiO4,11 LuAG,12 γ-AlON,13 and so on. Although the oxynitride-based phosphors have good luminescent performances and thermal stability, the high sintering temperature and harsh preparation conditions result in high production costs which limit their broad application. In addition, the oxide-based phosphor suffers from poor thermal stability.11,12 Therefore, discovering a novel efficient green phosphor with low cost and mild preparation process for WLEDs is necessary. Furthermore, in the display area, FEDs have been the hopeful technologies for next-generation flat panel displays due to their outstanding performances, such as self-emission, quick response, wide viewing angle, thin panel thickness, low power consumption, and distortion-free images.4,14,15 As an important part of FEDs, the phosphor must have high efficiency and perfect stability under high current densities (10−100 μA/ cm2) and low excitation voltage (≤5 kV) in practical applications.16,17 On the basis of these requirements, many sulfur compounds have been developed for FEDs such as ZnS:Ag,Cl,18 Gd2O2S:Tb3+,19 Y2O2S:Tb3+/Eu3+,19,20 Zn(Cd)S:Cu,Al,21 and SrGa2S4:Tb3+.22 However, their good performance in luminescence cannot cover their disadvantages. For

INTRODUCTION Following the rapid development of the economy and society, environmental issues and energy issues have become more and more prominent due to the sharply increasing consumption of resources in a great number of fields. As an essential part in the display and lighting fields, particularly in field emission displays (FEDs) and phosphor-converted white-light-emitting diodes (pc-WLEDs), rare-earth-doped phosphors have stolen much limelight on account of their outstanding performance, such as energy savings, high efficiency, nontoxicity, environment friendliness, and so forth.1−5 In the illumination field, pc-WLEDs have replaced traditional light sources, becoming a new generation of light source in the past few years. Generally, blue chips with yellow phosphors YAG:Ce3+ are employed to fabricate the pc-WLED. Obviously, because of the absence of a red light element, the obtained white light is very cold with high correlated color temperature and low color index.6,7 In addition, the poor thermal stability of YAG:Ce3+ also limits its widespread application.8 Alternatively, the combination of blue-, green-, and red-emitting phosphors with UV/near-UV LED chips is a promising way to overcome the aforementioned issues, and the method could dominate the market of WLEDs in the future.9 Therefore, researchers have spent much time and effort to explore novel phosphors for these requirements. For green phosphors, there are some commercialized Eu2+-, Ce3+-, and © XXXX American Chemical Society

Received: March 5, 2019

A

DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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X-ray) spectroscopy on the TEM. The PL (photoluminescence) and PLE (PL excitation) spectra were obtained by a FLS-920T fluorescence spectrophotometer, and the thermal stability of samples was tested by an aluminum plaque with cartridge heaters. The cathodoluminescence (CL) properties were determined with a modified Mp-Micro-S instrument (Horiba Jobin Yvon). The DRS (diffusion reflection spectra) were measured with a UV−vis spectrophotometer (PE Lambda 950).

example, the poor stability of sulfide results in decomposition under high-energy electron bombardment, which can decrease the luminescence efficiency,16,23 and the decomposition including the S element is harmful for both people and the environment, as it is not environmentally friendly or harmless. Thus, it is meaningful to develop a novel phosphor with excellent luminescence efficiency and outstanding chemical stability. As an alternative, the rare earth Tb3+ usually serves as a green-emitting activator and the main emission peak is located around 544 nm due to the 5D4−7F5 transition of Tb3+.24 The sharp peak around the green area means that the Tb3+-doped phosphors have high color purity. Unfortunately, the Tb3+doped materials have narrow and weak absorption in the nearUV region on account of the spin-forbidden 4f−4f transitions. For the purpose of increasing the absorption in the near-UV area, researchers have spent much time and effort. Until now, codoping Ce3+ as a sensitizer is the most common and efficient way to intensify Tb3+ absorption due to the broad and strong absorption in the near-UV region of Ce3+.25 However, most Ce3+/Tb3+-codoped phosphors suffer from nonnegligible Ce3+ emission in the UV and blue region, which can result in a decrease in the the color purity of green light.26−28 In many oxide compounds, borates have attracted much attention on account of their particular advantages, such as high thermal stability, low synthesis temperature, nontoxicity, and especially their interesting optical performance.29−33 Therefore, activator-doped borates have been investigated, such as Ba2(Lu/Y)5B5O17,25,29,34 M3Ln2(BO3)4 (M = Ba, Sr, Ln = Y, Gd, La),33,35 ZnB2O4,17 and so on. The results indicate that the borates can be regarded as good host lattices for phosphors. As a representative, the pyroborate Sr2LiScB4O10 (SLSBO) not only has above the advantages but also possesses a particular structure for ionic replacement.36 On the basis of all the above, we prepared the borate host material SLSBO and the Ce3+/Tb3+-codoped SLSBO green phosphor was prepared through a solid-state method in this work. The crystal structure, luminescence, and cathode-luminescence performances of phosphors were studied minutely. In addition, the influence of temperature on luminescence and the effects of probe current and accelerating voltage on cathodoluminescence were also researched. The energy transmission mechanism from Ce3+ to Tb3+ was also studied in detail.

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RESULTS AND DISCUSSION Phase and Structure. Figure 1 shows the Rietveld structural refinement of the powder diffraction of the SLSBO

Figure 1. Rietveld refinement of the powder XRD profile of SLSBO.

Table 1. Structural Data of Sr2LiScB4O10 from the Rietveld Refinement formula space group cryst syst cell params

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cell ratio cell volume Z reliability factors

EXPERIMENTAL SECTION Raw Materials and Preparation Process. The Ce3+/ Tb3+-codoped Sr2LiScB4O10 samples were synthesized through a traditional high-temperature solid-state method. The raw materials SrCO3, Li2CO3, Sc2O3, H3BO3, CeO2, and Tb4O7 were weighed in a stoichiometric ratio. Then the materials were put into an agate mortar and ground evenly. The mixed material was then transferred into a tube furnace and calcinated at 800 °C under a reducing atmosphere of N2/H2 (50/1). After 6 h, the as-prepared samples were cooled to room temperature naturally. Measurement Instruments. The phase formation was identified by an XRD instrument (D2 PHASER X-ray Diffractometer, Germany), operating at 15 mA and 30 kV with Cu Kα radiation (λ = 1.54056 Å). The morphologies were characterized by HRTEM (high-resolution transmission electron microscopy, FEI Tecnai F30) and SEM (scanning electron microscopy, S-340, Hitachi, Japan), and the elemental analysis of phosphors was detected by EDX (energy-dispersive

Sr2LiScB4O10 P121/n1 (No. 14) monoclinic a = 12.6318(2) Å, b = 5.2543(1) Å, c = 13.7382(2) Å, β = 116.69(0)° c/a = 1.0876, b/c = 0.3825, a/b = 2.4041 814.67(37) Å3 4 Rp = 9.23%, Rwp = 13.61%, χ2 = 1.894

compound using the GSAS program to obtain the crystal data of Sr2LiScB4O10 (PDF#80-0626). The green stars and red lines depict the measured and calculated patterns, respectively. The locations of the Bragg reflections of the calculated pattern (purple lines) and the difference between measured and calculated patterns (blue line) are shown under the diffraction peaks. The observed peaks satisfy the reflection conditions, and the refinement eventually converges to χ2 = 1.894, Rwp = 13.61%, and Rp = 9.23%, which indicate that there is no other impurity phase. SLSBO belongs to a monoclinic unit cell, and the space group is P121/n1 (No. 14). Table 1 shows the related structural data of the SLSBO compound. The crystal structure of a SLSBO cross-sectional view in the [010] direction and the environment around Sr and Sc ions are B

DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) Crystal structure of SLSBO, (b) coordination environments of Sc ions, and (c, d) coordination environments of Sr ions in SLSBO.

difference. The peak positions shifted to the right first following the increase in Ce3+/Tb3+ concentration, and then remained steady gradually with a further increase in Tb3+ contents. From the structure analysis, we know that the activator ions were inclined to occupy the larger Sr sites due to the small radius difference, instead of the smaller Sc sites. As a result, the crystal lattice will shrink, and the related diffraction peaks will shift toward higher angles at the same time. When Tb3+ ions were doped, they also tended to occupy the Sr sites, but due to the larger radius difference (0.23 Å) between Tb3+ and Sr2+, there is greater lattice contraction and the adjacent Sc sites will be stretched. On further increase in the Tb3+ concentration, there will be more stretched Sc sites, which means that the Sc sites are larger than the initial Sc sites (0.745 Å) and the radius difference between Tb3+ and Sc3+ will be lower than 0.178 Å. In order to balance the tensile stress, the Tb3+ will also occupy the Sc sites. Therefore, there will be a balance, and some of the Tb3+ ions occupy the Sr sites while others occupy the Sc sites. Accordingly, the crystal lattice also remains almost the same. In order to further verify the lattice parameter change, the lattice parameters of samples with different Tb3+ concentrations were obtained by Rietveld refinement, as shown in Figure S1. The data proved that the activator ions occupy not only the Sr sites but also the Sc sites. The electric structure and band gap of Sr2LiScB4O10 were investigated by the density functional theory (DFT) method, as shown in Figure 4. From the electronic structure, the compound haswns a direct band gap of 3.659 eV and the valence band top and the conduction band bottom are both located at the Brillouin zone G point. Figure 4b shows the calculated partial and total density of states of SLSBO. The conduction band was mainly attributed to Li 2s, B 2s2p, O 2p, Sc 3s3p, and Sr 4p4d states, while the valence band was composed of Li 2s, B 2s2p, O 2s2p, Sc 3s3p3d, and Sr 4s4p4d states. A typical SEM image is given in Figure 5a; it is clear that the particles with good dispersibility exhibit an irregular schistose shape, and the size is about several micrometers. The dispersive micro size of particles is applicable for encapsulation in a solid-state lighting device.38 The smooth surface of the

Figure 3. XRD pattern of the series samples with various Tb3+ contents.

given in Figure 2. The structure was composed of boron− oxygen trigonal-planar units, scandium−oxygen octahedra, and strontium−oxygen polyhedra. It is clear that there are three cationic sites (Sc1, Sr1, Sr2) which could be occupied by activator ions. Sc sites are six-coordinated with ionic radii of about 0.745 Å, while Sr sites are seven-coordinated with anionic radii of about 1.21 Å.37 Therefore, when the activator ions Tb3+ (0.98 Å for CN = 7 and 0.923 Å for CN = 6) and Ce3+ (1.07 Å for CN = 7 and 1.01 Å for CN = 6) were doped,37 they tended to occupy the Sr sites. The XRD patterns of Ce3+/Tb3+-codoped SLSBO samples with various Tb3+ concentrations are shown in Figure 3. Obviously, all of the diffraction peaks are in correspondence with the calculated data, which indicates that all of the obtained samples have the same phase, and the incorporation of Ce3+/Tb3+ did not change the main crystal structure. In addition, on further increase in the dopant concentration, the intensity of diffraction peaks decreased gradually; this can be attributed to the crystal lattice distortion because of the radius C

DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Electronic structure and (b) partial and total density of states of the SLSBO host.

Figure 5. (a) SEM image, (b) TEM image, (c) HTEM image, and (d) EDX spectrum of SLSBO:Ce3+/Tb3+.

sample can be clearly observed from the low-magnification TEM image in Figure 5b, which illustrates that there is the existence of a preferential orientation in the process of preparation. From the HRTEM image of the sample in Figure 5c, distinct lattice fringes were observed. There are also two obvious planes with interplanar spacings of about 3.71 and 2.75 Å, which matched well with the (−303) and (−314) interplanar distances, respectively. The clear crystal face of the sample not only means that it has good crystallinity and stability but also reveals the existence of a preferential orientation, which has been verified by the relatively strong

peaks in the XRD patterns (as illustrated in Figure 3). The related EDX spectrum is also given in Figure 5d; the spectrum indicates that the chemical components of sample include B, O, Sr, Sc and Tb, while the Li is too small to be detected. There is no other impurity element except for the Cu and C signals, which are attributed to the instrument. Photoluminescence. Figure 6 depicts the diffuse DRS of the SLSBO host, Ce3+-doped SLSBO, and Ce3+/Tb3+-codoped SLSBO samples. Because of the host absorption, both the doped and undoped samples have a strong absorption between 200 and 300 nm. The Ce3+-doped SLSBO sample has a D

DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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absorption in the near-UV region indicates that the SLSBO:Ce3+/Tb3+ phosphor can be matched well with a near-UV LED. The emission and excitation spectra of Ce3+-doped SLSBO phosphor are given in Figure 7a. Under monitoring at 385 nm, the excitation spectrum shows two bands around 300 and 350 nm, and they can be ascribed to the 4f−5d transitions of Ce3+. The emission spectrum shows an asymmetric broad band in the range of 350−500 nm under 347 nm excitation, which can be well fitted to two Gaussian peaks located at 380 nm (26315.8 cm−1) and 412 nm (24271.8 cm−1). In theory, there should be four peaks in the emission bands because there are two Sr sites for Ce ions, and every kind of luminescence center will have two peaks due to the spin−orbit splitting of the ground state 2F5/2 and 2F7/2 of Ce3+.39 The Van Uitert empirical equation can be employed to clarify the attribution of the two bands:40 ÄÅ ÉÑ 1/ V ÅÅ Ñ ij V yz ÅÅ ÑÑ −nEa × r /80Ñ E = Q ÅÅ1 − jj zz 10 ÑÑ ÅÅ ÑÑ 4 k { (1) ÅÇ ÑÖ

Figure 6. DRS of SLSBO host, SLSBO: Ce3+, and SLSBO: Ce3+/ Tb3+.

where E refers to the emission position of activator ions (Ce3+), Q represents the energy of the lowest d-band edge for the free activator ions (for Ce3+, Q = 50000 cm−1), and V, n, Ea, and r are the valence state of the activator (for Ce3+, V = 3), the coordination number of activator ions, the anion’s electron affinity, and the radius of the replaced ions by activator ions,

broader absorption in the range of 200−400 nm, and it can be ascribed to the 4f−5d transition of Ce3+. In comparison to the spectrum of Ce3+-doped SLSBO, the absorption of Ce3+/Tb3+codoped SLSBO is much stronger. This is because of the existence of the 4f−5d transition of Ce3+, as well as the 4f8− 4f75d1 transition of Tb3+ at around 250 nm. The increase in

Figure 7. PL and PLE spectra of (a) Ce3+-doped, (b) Tb3+-doped, (c) Ce3+- and Tb3+-doped, and (d) Ce3+/Tb3+-codoped SLSBO samples. E

DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. Energy-level scheme of SLSBO: Ce3+/Tb3+ phosphor.

Figure 9. Series of PL of SLSBO: 0.03Ce3+, xTb3+ (0 ≤ x ≤ 0.30).

respectively. In this system, the two Sr sites have the same coordination number and radius, and this means that the emission positions (E) of the two luminescence center are identical. Thus, the two peaks ought to belong to the spin− orbit splitting of the ground state 2F5/2 and 2F7/2 of Ce3+, and the energy difference between the two emissions was calculated to be 2044 cm−1; the value is in accordance with the theoretical difference value of about 2000 cm−1.41 The PL and PLE spectra of Tb3+-doped SLSBO phosphor are illustrated in Figure 7b. Under 276 nm excitation, the Tb3+doped SLSBO shows a proverbial green luminescence with the strongest peak located at 544 nm, deriving from the 5D4 → 7F5 transition, as well as three weak peaks located at 488, 585, and

Figure 10. CIE chromaticity coordinates of SLSBO: Ce3+, xTb3+ (0 ≤ x ≤ 0.30) phosphors.

Table 2. CCT, AE, and IQE of Partial Samples sample SLSBO:0.03Ce SLSBO:0.03Ce,0.05Tb SLSBO:0.03Ce,0.10Tb SLSBO:0.03Ce,0.20Tb

F

CIE (0.175, (0.292, (0.330, (0.340,

0.062) 0.458) 0.540) 0.566)

IQE/%

AE/%

37.61 39.67 43.49 45.87

68.31 72.67 75.19 82.15

DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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verified in Figure 7d. Under the monitoring at 544 nm, the PLE of Ce3+/Tb3+-codoped SLSBO appears as a broad band from 235 to 375 nm, and it is same as for Ce3+-doped samples instead of Tb3+-doped samples. At the same time, in comparison with the broad excitation band in near-UV, the characteristic excitation of the 4f8 → 4f75d1 transition of Tb3+ around 250 nm becomes weaker. The results reveal that there is an energy transition from Ce3+ to Tb3+.43 With 325 nm excitation of Ce3+, it is clear that the emissions of Tb3+ and Ce3+ both exist, and the emission intensity of Tb3+ is much greater than that of Ce3+, which signifies that the Ce3+/Tb3+codoped SLSBO can act as an efficient green-emitting phosphor for near-UV LEDs. For the purpose of investigating the mechanism of luminescence and energy transfer from Ce3+ to Tb3+, the energy-level scheme of Ce3+ /Tb 3+ -codoped SLSBO is displayed to explain the process, as shown in Figure 8. For Ce3+ single-doped SLSBO, when Ce3+ absorbs near-UV radiation, the electrons can be excited from the ground state to excited states. In general, most of the electrons in excited states return to ground state of both 2F5/2 and 2F7/2 with the radiation of energy hν. However, for Ce3+/Tb3+-codoped SLSBO, the electrons in excited states also jump to the excited state of the Tb3+ 5d level (energy transfer process), which is attributed to the large overlap between the 4f → 4f transition of Tb3+ and the 5d−4f transition of Ce3+. The electrons in the 5d level of Tb3+ relax to the 5D4 and 5D3 levels, and then the electrons return to 7FJ (J = 0−6) levels with multiple peaks. However, in this host there are only four peaks which are ascribed to 5D4 → 7FJ (J = 3, 4, 5, 6), and the other peaks do not appear. This is not only because of the cross relaxation with nonradiative transition from 5D3 to 5D4 but also on account of the high Tb3+ concentration, which results in the peaks of 5D3 → 7FJ (J = 0−6) being too weak to be observed.44−46 In order to obtain an efficient green phosphor, the emission spectra of a series of Ce3+/Tb3+-codoped SLSBO samples with different Tb3+ doping contents were studied, as shown in Figure 9. (The Ce3+ concentration is constant, and the optimum doping concentration of Ce3+ is 0.03 by a series of experiments, as shown in Figure S2.) It can be seen that the peak intensity of Tb3+ continuously increases with an increase in Tb3+ contents, until the Tb3+ content is above 0.20. In contrast, the peak intensity of Ce3+ decreased monotonically, which indicates that when the Ce3+ doping concentration is unaltered, the energy transition efficiency from Ce3+ to Tb3+ is more and more efficient. The high quenching concentration of Tb3+ denotes the weak interaction between the host lattice and Tb3+ ions.33 The Commission Internationale de L’Eclairage (CIE) was employed to calculate the chromaticity coordinates of samples through their emission spectra. From Figure 10, following an increase in Tb3+ concentration, the chromaticity coordinates varied from (0.175, 0.062) to (0.338, 0.582); the related chromaticity coordinates are listed an the inset of Figure 10. Correspondingly, the digital photographs of Ce3+-doped and partial Ce3+/Tb3+-codoped SLSBO phosphors under 365 nm light excitation are illustrated in an inset of Figure 10. In addition, the AE (absorption efficiency) and IQE (internal quantum efficiency) of Ce3+-doped and Ce3+/Tb3+-codoped SLSBO samples were also evaluated, as given in Table 2. The results reveal that the Ce3+/Tb3+-codoped SLSBO can emit bright and pure green light under near-UV excitation.

Figure 11. Decay curves of the SLSBO series samples monitored at 385 nm.

Figure 12. (a) Related emission intensity and lifetime of Ce3+. (b) Energy transfer efficiency calculated by eqs 4 and 5.

625 nm from the 5D4 → 7FJ (J = 6, 4, 3) transitions of Tb3+. The PLE spectrum includes two parts; the strong absorption from 235 to 300 nm is ascribed to the 4f8 → 4f75d1 transition of Tb3+, and the other weak absorption from 350 to 400 nm belongs to the 4f8 → 4f8 transition. From Figure 7c, it is conspicuous that the spectral overlap between the excitation band of Tb3+ and emission band of Ce3+ appears from 350 to 400 nm, which is most important for an energy transition in terms of Dexter’s theory.42 In general, the greater the overlap, the higher the probability of an energy transition. The great overlap signifies a high possibility of energy transition from Ce3+ to Tb3+. The energy transition from Ce3+ to Tb3+ ions is G

DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 13. Relationships between I0/Is of Ce3+ and concentration of Ce3+ and Tb3+ with different n values.

where τ0 and τs refer to the lifetime of Ce3+ in the absence and presence of Tb3+ ions, respectively. Another alternative equation is usually employed to calculate ηT:51

The efficient energy transfer can be verified by the decay curves of Ce3+ in Ce3+/Tb3+-codoped SLSBO, and the related lifetimes were also calculated as shown in Figure 11. The decay curves of samples could be fitted well by the doubleexponential formula47,48 ji −t zy ji −t zy I(t ) = A1 expjjj zzz + A 2 expjjj zzz jτ z jτ z k 1{ k 2{

ηT = 1 −

A1τ12 + A 2 τ22 A1τ1 + A 2 τ2

where I0 and IS refer to the emission intensity of Ce in the absence and presence of Tb3+ ions. On the basis of the data in Figure 12a, the calculated results according to eqs 4 and 5 are illustrated in Figure 12b. It is obvious that the ηT values calculated by the two equations are approximate and with an increase in Tb3+ content, the energy transfer efficiency continuously increases. When the Tb3+ concentration is 0.30, the efficiency can reach above 90%, which reveals that the energy transfer from Ce3+ to Tb3+ is very efficient. In general, the energy transfer from Ce3+ to Tb3+ in codoped systems occurs through an electronic multipolar interaction or exchange interaction. If the energy transfer is led by exchange interaction, the critical distance between dopants ought to be shorter than 5 Å.52 In contrast, if the critical distance is more than 5 Å, the energy transfer belongs to an electronic multipolar interaction. The critical distance (Rc) can be obtained by the equation17,52

(2)

(3)

With increasing Tb3+ content, the average lifetime of Ce3+ gradually decreased. The existence of energy transfer from Ce3+ to Tb3+ was strongly verified by the decreasing lifetime of Ce3+.27 In a general way, the energy transfer efficiency can be obtained by two methods. According to the decay lifetimes, the energy transfer efficiency (ηT) can be simply defined by the equation49,50 ηT = 1 −

τs τ0

(5) 3+

where t and I are time (ns) and luminescence intensity, respectively, and A1, A2 and τ1, τ2 are constants and the exponential components of lifetimes. On the basis of the above formula, the related average lifetimes (τ) can be obtained easily by the equation τ=

Is I0

i 3V yz zz R c = 2jjj k 4πxN {

1/3

(6)

in which χ, V, and N represent the ultimate quenching concentration of Ce3+ and Tb3+, the unit cell volume, and the number of molecules in one unit cell, respectively. In this

(4) H

DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 14. (a) Ce3+ and Tb3+ emission of SLSBO:0.03Ce3+, 0.20Tb3+ at different temperatures. (b) Thermal stability properties of SLSBO:0.03Ce3+,0.20Tb3+ and (BaSr)2SiO4:Eu2+ phosphors. (c) Color correlation at different temperatures. (d) Relationship between temperature and emission intensity. (e) Relationship between 1/kT and ln[(I0/IT) − 1] of SLSBO:0.03Ce3+,0.20Tb3+.

system, χ = 0.23, N = 4, and V = 797.64 Å3, and the calculated Rc is 11.83 Å, which is much larger than 5 Å. This indicates that the energy transfer is relevant to an electronic multipolar interaction rather than exchange interaction. To understand the electric multipolar interaction deeply, the relationship between Dexter’s energy transfer formula of multipolar interaction and Reisfeld’s approximation was determined out by the expression33,53 η0 ηs

n /3 ∝ CCe 3+ + Tb3 +

illustrated in Figure 14. As exhibited in the inset of Figure 14a, both Tb3+ emission intensity and Ce3+ emission intensity decrease with increasing temperature. When the temperature reaches 250 °C, the emission intensity of Ce3+ remains about 41% of that at room temperature, but the Tb3+ intensity still retains above 75% of its value at room temperature, and the result reveals that the phosphor has better thermal stability than the commercial green phosphor (BaSr)2SiO4:Eu2+, as illustrated in Figure 14b. Furthermore, the CIE color coordinates of samples remain almost invariable during the unremitting heating process from room temperature to 250 °C, as shown in Figure 14c, which means the phosphor has outstanding color stability. The temperature dependence of the emission intensity projection figure of SLSBO:0.03Ce3+/ 0.20Tb3+ phosphor is also given in Figure 14d to show the relationship between temperature and emission intensity. To determine the reason for the outstanding thermal stability of the phosphor, the thermal quenching mechanism of Ce3+/Tb3+ is discussed briefly. Generally, there is a competition among the energy transfer, thermal activation rate, and intrinsic decay rate in the 5d level of Ce3+, and when the temperature increases, the large transfer rate can slow the decrease in transfer efficiency.25,54 The reduction of lifetime of Ce3+ verifies that there is a fast transfer rate; thus, when the transfer rate is fast, the Tb3+ has outstanding thermal stability. For Ce3+, most of the electrons in the 5d level not only transfer to the 5d level of Tb3+ but also reach ground state by nonradiative transition due to a high thermal activation rate at high temperature, which results in the PL intensity of Ce3+ dropping continuously. In order to study the PL performance on thermal

(7)

where η0 and ηs are the luminescence efficiencies of Ce3+ ions without and with Tb3+, respectively, the value of η0/ηs can be replaced approximately by I0/Is (I0 and Is are the relative luminescence intensity of Ce3+ without and with Tb3+, respectively), and C represents the contents of both Tb3+ and Ce3+. Different n values represent different interaction mechanisms of energy transfer; n = 10, 8, 6, 3 corresponds to quadrupole−quadrupole, dipole−quadrupole, dipole−dipole, and exchange interactions, respectively. On the basis of the above relation, the relationships between I0/Is of Ce3+ and concentrations of Ce3+ and Tb3+ are plotted in Figure 13. As shown in Figure 13, it can be seen that the best linear fitting relationship appears when the value of n is 6, and the result manifests that the dipole−dipole interaction is relevant to an energy transfer mechanism in this system. The thermal stability of phosphors as an important part of performance is always a concern for applications. As a representive, the PL intensity of 0.03Ce3+/0.20Tb3+-codoped SLSBO at different temperatures was studied minutely, as I

DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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Tb3+-codoped samples were measured with increasing temperature, as shown in Figure S3. The Ce3+-doped TL curves show one defect level in the temperature range 30−125 °C, associated with Ce3+-related defects. The Ce3+/Tb3+-codoped sample displayed two TL curves, in the ranges 30−125 and 150−250 °C, revealing another trap appearance, which can be attributed to the Tb3+-related defects. The TL peaks reveal that the defects have some influence on the PL thermal stability. However, it is notable that TL peaks were not strong, which means that the effect is not obvious. Cathodoluminescence. In view of the excellent properties in photoluminescence, the low-voltage cathodoluminescence of SLSBO:0.03Ce3+,0.20Tb3+ was investigated to explore its potential as an efficient green phosphor for FEDs. With excitation of a low-voltage electron beam, the phosphor shows the typical emissions of Tb3+ at 625, 587, 550, and 490 nm, which are attributed to 5D4 → 7FJ (J = 3−6) transitions of Tb3+, respectively. The strongest peak is located around 550 nm. In order to further investigate the influence of probe current on CL, the CL spectra were depicted with different probe currents from 30 to 100 mA under a 5 kV electron beam excitation, as illustrated in Figure 15a. Obviously, the emission intensity of CL exponentially increases with increasing probe current, and no saturation current occurs. Similarly, as shown in Figure 15b, it is evident that, when the probe current is persistent at 60 mA, the CL intensity also is enhanced linearly following the incremental accelerating voltage from 3 to 9 eV with no appearance of saturation voltage. Following both the increasing probe current and accelerating voltage, the electrons can penetrate into the phosphor body deeply due to the larger electron-beam current density, which is the reason for enhancement of emission intensity.56 The related electron penetration depth (L) can be calculated through the empirical equation57,58 i A yi E yzn zzz L (Å) = 250jjjj zzzzjjjj k ρ {k Z {

Figure 15. CL spectra of SLSBO:0.03Ce3+/0.20Tb3+ with different (a) probe currents and (b) accelerating voltages.

)

where IT and I0 are the emission intensities at different temperatures and room temperature, respectively, and c, ΔE, and k are a constant, the activation energy, and Boltzmann’s constant, respectively. The relationship between 1/kT and ln[(I0/IT) − 1] can be plotted through the above equation, as shown in Figure 14e, and the activation energy can be obtained from the slope of the line. The high activation energy of 0.15347 eV of the sample verifies the high thermal quenching temperature. In addition, we also investigated the effect of defects on the PL thermal stability. In this system, the activator ions A (Ce3+/ Tb3+) ions occupying the Sr2+ sites will result in some defects by the equation 3SrSr + 2A3 + → 2A Sr * + 3Sr 2 + + VSr′′

1.2 1 − 0.29 log Z

(10)

where Z, E, ρ, and A represent the number of electrons in one molecule, the accelerating voltage (kV), the density of the phosphor, and the relative molecular mass, respectively. For this system, Z = 230, ρ = 3.51 g/cm3, and A = 430.37, and when the accelerating voltages are 3, 5, 7, and 9 kV, the electron penetration depths are 6.4, 89.7, and 420.7 nm, respectively. It is well-known that the CL spectrum is caused by plasma produced by the incident electrons.59 As a result, a deeper electron penetration depth can produce more plasma, and an increase in plasma results consequently in stronger emission intensity. In comparison with the PL spectrum, from the CL spectrum the Ce3+ emission can hardly be seen because of the very strong Tb3+ emission, and the difference in spectra is attributed to the different excitation mechanisms. As illustrated in Figure 16, the CL process of the sample is given by the schematic diagram. When the primary electrons interact with the lattice host, many secondary electrons will be produced. If the secondary electrons excite the host lattice, they can create many electron−hole pairs, which lead to bound exciton formation.4,59 The bound excitons can transfer their energy to the excited states of activators through a quasi-resonant or resonant process, which gives their characteristic emission. At the same time, the secondary electrons with different energy can also get to the 5d level of activators directly, which causes

stability deeply, the activation energy (ΔE) of the phosphor was calculated by a modified Arrhenius equation:55 I0 IT = ΔE 1 + c exp − kT (8)

(

n=

(9)

In order to realize the effect of defects on PL thermal stability, the thermoluminescent (TL) spectra of Ce3+-doped and Ce3+/ J

DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 16. CL process of SLSBO:Ce3+/Tb3+ phosphor with schematic diagram of the mechanism.

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the strong luminescence. In addition, an energy transfer of Ce3+ to Tb3+ also exists. As a result, the intensity of Tb3+ emission in CL is much stronger than that in PL.

*E-mail for Y.W.: [email protected].

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ORCID

CONCLUSION In summary, the novel green-emitting phosphor Sr2LiScB4O10:Ce3+/Tb3+ was successfully obtained through traditional solid-state action. Because of the energy transfer, the codoped samples have a broad excitation spectrum from 235 to 375 nm, which can match well with near-UV LED chips. The emission spectrum is mainly dominated by the emission of 5D4 → 7F5 of Tb3+ instead of Ce3+ emission because of the high energy transfer efficiency up to more than 90% following an increase in Tb3+ content. The related energy transition from Ce3+ to Tb3+ was analyzed through an energylevel scheme, and the energy transfer mechanism in this system is relevant to a dipole−dipole interaction. Following the increase in temperature, the phosphor has perfect color stability and thermal stability. Furthermore, the phosphor also has high saturation current and saturation voltage with high color stability. From a consideration of its excellent luminescence performance in CL and PL, Sr2LiScB4O10:Ce3+/ Tb3+ can possibly act as efficient green phosphor for application in LEDs and FEDs.

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AUTHOR INFORMATION

Corresponding Author

Yuhua Wang: 0000-0002-5982-8799 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly appreciate the financial support from the National Natural Science Foundation of China (grant no. 51672115) and Gansu Development and Reform Commission (no. 51502122).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b00639 Inorg. Chem. XXXX, XXX, XXX−XXX