Eu3+-Doped LiCa3MgV3O12

Jul 11, 2018 - A series of LCMV:xBi3+,yEu3+ (x = 0–0.10, y = 0–0.15) compounds were .... could be identified via the expression(38) (1)where Z is ...
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Full Color Luminescence Tuning in Bi3+/Eu3+-Doped LiCa3MgV3O12 Garnet Phosphors Based on Local Lattice Distortion and Multiple Energy Transfers Peipei Dang,†,‡ Sisi Liang,†,‡ Guogang Li,*,§ Yi Wei,§ Ziyong Cheng,† Hongzhou Lian,† Mengmeng Shang,∥ Abdulaziz A. Al Kheraif,⊥ and Jun Lin*,†,#

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†

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China § Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences, 388 Lumo Road, Wuhan 430074, People’s Republic of China ‡ University of Science and Technology of China, Hefei, 230026, People’s Republic of China ∥ School of Chemistry and Chemical Engineering, Qingdao University, 308 Ningxia Road, Qingdao 266071, People’s Republic of China ⊥ Dental Health Department, College of Applied Medical Sciences, King Saud University, Riyadh, Saudi Arabia # School of Applied Physics and Materials, Wuyi University, Jiangmen, Guangdong 529020, People’s Republic of China S Supporting Information *

ABSTRACT: In the pursuit of high-quality W-LED lighting, the precise control of emission color of phosphor materials is indispensable. Herein we report a series of single-composition Bi3+-doped LiCa3MgV3O12 garnet-structure phosphors, whose emission colors under n-UV excitation could be tuned from bluish green (480 nm) to yellow (562 nm) on the basis of local lattice distortion and VO43− → Bi3+ energy transfer. Furthermore, full-color luminescence tuning from bluish green to orangish red across the warm white light region was successfully achieved by designing VO43− → Bi3+ → Eu3+ energy transfers. More interestingly, the thermal stabilities of as-prepared samples were gradually enhanced through designing VO43−/Bi3+ → Eu3+ energy transfers. This work provides a new perspective for color tuning originating from simultaneous local lattice distortion and multiple energy transfers.

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INTRODUCTION Phosphors with superb chemical and optical advantages are highly desired for white light emitting diodes (W-LEDs).1−4 To realize a high-quality indoor lighting, it is necessary to match phosphors with an appropriate spectral profile on the LED chips: that is, the development of phosphors with tunable optical properties is indispensible.5−8 The study of non rare earth ion doped phosphors is a hot topic.9 As some of the typical non rare earth activators, Bi3+-doped phosphors can present abundant emission colors from visible to near-infrared light due to the 3P1−1S0 and 1P1−1S0 transitions, which strongly depend on the coordination environment of the host lattice, including the bond distance, the coordination number, the lattice symmetry, and so on.10−15 Currently, many works have also been committed to improving the luminescence performance and design of novel controllable color-tuning phosphor materials via the chemical modification of the host lattice such as adjusting the crystal field splitting, changing the excitation wavelength, designing energy transfers, thermal expansion or contraction, and so on.14−25 Despite these © XXXX American Chemical Society

achievements, the basic role of the host lattice in affecting the emission transitions of activators has yet to be fully understood.26 Accordingly, the precise tuning over emission color based on modification of the host lattice remains challenging and needs to be further discussed. Due to the intraconfigurational 4f−4f transitions, Eu3+ activated materials commonly give a typical red emission.27−32 However, their application is hampered by the very low absorption cross section in the near-UV region and especially within the blue spectral area. Except for being a luminescence center, a Bi3+ ion is an efficient sensitizer to transfer excitation energy to an Eu3+ ion in many matrices.22,33−35 Nevertheless, the luminescence tuning properties of Bi3+/Eu3+-doped LiCa3MgV3O12 garnet vanadate phosphors have not previously been reported. Moreover, a full color luminescence tuning via simultaneous lattice distortion and multiple energy transfers has also not been involved in the previously reported phosphor systems. Received: May 9, 2018

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DOI: 10.1021/acs.inorgchem.8b01271 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. (a) Representative powder XRD patterns of LCMV:xBi3+,yEu3+ (x = 0−0.1; y = 0−0.07) samples together with the standard data of LiCa3MgV3O12 (JCPDS No. 24-1212). (b) Data (black dots) and fitted (red line) XRD patterns as well as a difference profile (blue line) for Rietveld refinement of the LCMV:0.04 Bi3+,0.04Eu3+ sample carried out by using the GSAS program. The short vertical lines show the positions of Bragg reflections of the fitted pattern. (c) Schematic crystal structure of LCMV and the connection of the [CaO8] dodecahedrons, [MgO6] octahedrons, and [VO4] tetrahedrons. room temperature and crushed into powders for 10 min, generating the final phosphor powders. LED Fabrication. LEDs were fabricated by combining the representative LCMV:xBi3+,yEu3+ (x = 0, y = 0; x = 0, y = 0.02; x = 0.10, y = 0; x = 0.10, y = 0.07; x = 0.15, y = 0) phosphors and 370 nm InGaN chips. The proper amounts of phosphors were added into the epoxy resins and mixed thoroughly for 20 min. The acquired mixture was coated on the surface of the 370 nm InGaN chips and dried at 70 °C to produce LEDs. All measurements were carried out at 100 mA drive current. Characterization. The crystalline phases of as-prepared samples were identified by means of X-ray diffraction (XRD) using a Bruker D8 diffractometer. The samples were measured at a scanning rate of 1° min−1 with Cu Kα radiation (λ = 1.54 Å) that was operated at 40 kV and 40 mA. Crystal structure refinements were conducted with a GSAS program, and the crystal structure diagrams were acquired via the CrystalMaker Demo program. Digital photographs of luminescent samples were obtained with a CCD camera. The PL and PLE spectra were collected on an Edinburgh Instruments FLSP-920 fluorescence spectrometer equipped with a 450 W xenon lamp as the excitation source at room temperature. In addition, the lifetimes of samples were also measured on a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width 4 ns, gate 50 ns) as the excitation (Continuum Sunlite OPO). The internal quantum yields (IQYs) of samples were acquired with a C9920-02 system. Performances of the fabricated LEDs were measured via a Starspec SSP6612 apparatus. The temperature-dependent (25−200 °C) PL spectra were also measured by fluorescence spectrometers equipped

In this work, we synthesized a series of single-phase garnetstructured phosphors (LiCa3−x−yMgV3O12:xBi3+,yEu3+ abbreviated as LCMV:xBi3+,yEu3+) through a high-temperature solid-state process, and their crystallographic structures were identified via the Rietveld refinement method. On the basis of local lattice distortion and multiple energy transfers, full-color luminescence tuning from bluish green to orangish red was achieved. The VO43− → Bi3+ → Eu3+ multiple energy transfers in LCMV:xBi3+,yEu3+ phosphors were investigated in detail. Moreover, the thermal quenching behavior of LCMV:xBi3+,yEu3+ samples was also revealed systematically, which could be decreased by designing efficient energy transfers. In view of the experimental results, the as-prepared single-phase LCMV:xBi3+,yEu3+ phosphors with full color tuning suggest promising applications in W-LED devices.

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EXPERIMENTAL SECTION

Sample Preparation. A series of LCMV:xBi3+,yEu3+ (x = 0−0.10, y = 0−0.15) compounds were synthesized via a high-temperature solid-state reaction process. Stoichiometric amounts of Li2CO3 (A.R.), CaCO3 (A.R.), MgO (A.R.), NH4VO3 (A.R.), Bi2O3 (A.R.), and Eu2O3 (99.99%) were thoroughly mixed using an agate mortar and pestle with an appropriate alcohol as the solvent. After they were dried at 70 °C for 30 min, the powder mixtures were placed in aluminum oxide crucibles and sintered in a box furnace at 800 °C for 6 h in air. After the calcination, the samples were slowly cooled to B

DOI: 10.1021/acs.inorgchem.8b01271 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (a) Normalized photoluminescence excitation (PLE, λem 480−562 nm) and (b) photoluminescence (PL, λex 320−340 nm) spectra of LCMV:xBi3+ (x = 0−0.10) samples. The insert in (b) shows the relative emission intensities of LCMV:xBi3+ (x = 0−0.10) samples with various Bi3+ contents. (c) Digital photographs of LCMV:xBi3+ (x = 0−0.10) samples under 320 nm UV light. (d) Increase in the maximum excitation and emission wavelengths for LCMV:xBi3+ (x = 0−0.10) by the increased distortion of the Bi3+ coordination geometry, indexed by the bond length ratio d88/d81. (e) Interconnectivity of the garnet structure as illustrated with Bi3+ ions (cyan) and tetrahedron (orange) and octahedron (red), highlighting d88 and d81.

CN = 6). The V5+ ions in the (C) sites with a 24d position form [VO4] tetrahedral units. Due to their similar ionic radii, Bi3+ (r = 1.17 Å, CN = 8; r = 0.96 Å, CN = 6) and Eu3+ (r = 1.07 Å, CN = 8; r = 0.95 Å, CN = 6) could be expected to occupy Ca2+ (r = 1.06 Å, CN = 8) sites to form {Bi/Eu}O8 dodecahedrons. Mg2+ (r = 0.72 Å, CN = 6) and Li+ (r = 0.76 Å, CN = 6) sites are too small for them to enter in. The average lengths between cation and oxygen ligand for the series LCMV:xBi3+,yEu3+ samples are summarized in Table S3. The Ca1/Bi1−O1 bond lengths are observed to exhibit a monotonous increase with gradual substitution of the larger Bi3+/Eu3+ ions for the smaller Ca2+ ions. To alleviate the lattice strain, the Mg1−O1 bond lengths at neighboring MgO6 polyhedrons would then be compressed. This phenomenon further indicates that Bi3+/Eu3+ ions successfully occupied Ca2+ lattice sites. Photoluminescence Properties. Figure 2a,b presents the normalized PLE and PL spectra of LCMV:xBi3+ (x = 0−0.10) samples monitored at the correspondingly optimal emission and excitation wavelengths. The PLE spectrum of LCMV presents a broad absorption band from 250 to 380 nm centered at 330 nm, which is attributed to the 1A1 → 1T1 transitions of the VO43− group. Under 330 nm UV light excitation, LCMV mainly exhibits an asymmetric bluish green light emission band ranging from 360 to 700 nm peaking at 480 nm due to the self-activated 3T1 → 1A1 transitions of the VO43− group. There is also a weak emission peak at 430 nm attributed to the 3T2 → 1A1 transitions of the VO43− group. When Bi3+ ions were introduced into the host lattice, the excitation peaks were obviously broadened; in particular the absorption in the longer wavelength region (350−380 nm) was strengthened due to the superimposition of a Bi3+ excitation transition (1S0−3P1). Interestingly, a large emission red shift from 480 to 562 nm is

with a xenon lamp as the excitation source (Edinburgh Instruments FLSP-920) and a temperature controller.

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RESULTS AND DISCUSSION Phase Recognition. Figure 1a shows XRD profiles of the representative LCMV:xBi3+,yEu3+ (x = 0, y = 0; x = 0.04, y = 0; x = 0, y = 0.04; x = 0.04, y = 0.04) samples, which can be well indexed with the standard data of LiCa3MgV3O12 (JCPDS No. 24-1212). This result proves the formation of a pure LCMV phase and the successful doping of Bi3+ and Eu3+ ions into the host lattice. To further verify the phase purity of the designed samples, Figure 1b plots the Rietveld fitting of the XRD pattern of the representative LCMV:0.04 Bi3+,0.04Eu3+ sample. The starting model was built with crystallographic data taken from the previously reported LiCa3MgV3O12. According to the refinement results, the LCMV:0.04Bi3+,0.04Eu3+ sample crystallized in a cubic crystal system with space group Ia3̅d (230), a = b = c = 12.4497(28) Å, α = β = γ = 90°, V = 1929.65(8) Å3, and Z = 8 and yielded Rwp = 6.56%, Rp = 4.62%, and χ2 = 2.413. Tables S1 and S2 give the refined lattice parameters, atom positions, and thermal vibration parameters of the LCMV host and the representative LCMV:0.04 Bi3+,0.04Eu3+ as well as the other LCMV:xBi3+,yEu3+ (x = 0−0.10, y = 0−0.07) samples. These results indicate that the series of phosphor samples still maintains single-phase compositions even though Bi3+ and Eu3+ ions are introduced. It is well-known that the garnet structure {A}3[B]2(C)3O12 (cubic, Ia3̅d) contains a three-dimensional dodecahedral {A}O8, octahedral [B]O6, and tetrahedral (C)O4 framework by sharing borders and corners.36 In the LiCa3MgV3O12 structure, the Ca2+ ions occupy the {A} sites with 24c symmetry (Figure 1c). The Mg2+ and Li+ ions are both located in the [B] sites perch at 16a positions on the basis of the close ionic radii of Mg2+ (r = 0.72 Å, CN = 6) and Li+ (r = 0.76 Å, C

DOI: 10.1021/acs.inorgchem.8b01271 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

430 nm corresponding to the 3T2 → 1A1 transition of the VO43− group is weaker with an increase in the Bi3+ doping concentration, which slightly proves that the VO43− group transfers energy to Bi3+ ions. The normalized Gaussian fitting PL spectra of LCMV:xBi3+ samples at two luminescence centers are shown in Figure 3a, and the correlative emission wavelengths, fwhms, and internal quantum yields (IQYs) are summarized in Table S4. As the Bi3+ doping concentration continues to increase, the VO43− emission intensity decreases gradually, while the intensity of the Bi3+ emission initially increases until the Bi3+ concentration reaches 0.06 (IQY = 43.9%), as shown in Figure 3b. The subsequent decrease in the Bi3+ emission intensity and IQY is the result of a concentration quenching effect. Thus, a wide range of tunable emission from bluish white to yellow was observed, which could be corroborated by their luminescence photographs in Figure 2c. To further confirm the VO43− → Bi3+ energy transfer, the lifetimes decay curves of as-prepared LCMV:xBi3+ samples were measured at different excitations and emissions, as summarized in Figure 3c and Table S5, respectively. All decay curves could be fitted via the secondaryexponential decay equation

observed with an enhancement in the Bi3+ doping concentration and the corresponding full widths at half-maximum (fwhm) increase from 100 to 138 nm. On the basis of crystal field theory, the type of polyhedron is an important factor in determining the crystal field strength of the Ca2+ sites occupied by Bi3+ ions.37 In general, the crystal field strength (Dq) could be identified via the expression38 Dq =

1 2 r4 Ze 5 6 R

(1)

where Z is the anion charge, e is the electron charge, r is the radius of the d wave function, and R is the bond length between the central cation and its ligands. The Dq value is proportional to 1/R5. Theoretically, the Bi3+ doping should generate an emission blue shift due to the increased R value. This seems to be in contradiction with the observed emission red shift. Actually, except for the bond length R value, the lattice distortions also exert an important effect on Dq. When Bi3+ ions gradually replace the smaller Ca2+ ions, it could produce a local lattice distortion, which means there is stronger crystal field splitting. In detail, the coordination environment of the Bi3+ sites in the LCMV matrix can be described as a distorted cube with four longer and four shorter Bi−O bonds. The deviation from cubic symmetry at the Bi3+ site is mainly due to compression of the cube, which leads to a local distortion.26,39 As explained in Figure 2e, the d88/d81 ratio could be used to characterize the degree of distortion, where d88 is defined as the O−O distance shared between two adjacent dodecahedrons and d81 represents the edge of a Bi3+ dodecahedron that connects a tetrahedral vertex to an octahedral vertex. The d88 and d81 values were obtained through the bond lengths obtained from crystal structure refinement of as-prepared samples with a GSAS program. A plot of the maximum excitation and emission wavelength with increased distortion parameter d88/d81 is presented in Figure 2d. It can be seen that there is an approximately linear increase in the excitation and emission maxima with an increase in the local distortion of the Bi3+ site (also see Table 1). Therefore,

ij −t yz ij −t yz I(t ) = I0 + A1 expjjj zzz + A 2 expjjj zzz jτ z jτ z (2) k 1{ k 2{ The value of the average lifetime τ* could be obtained utilizing the expression τ* =

x

λex (nm)

λem (nm)

d88 (Å)

d81 (Å)

d88/d81

330 322 324 324 335 336 337 339 340

480 488 503 513 539 548 556 557 562

2.742 2.882 2.770 2.887 2.920 2.977 2.943 2.859 2.955

3.517 3.579 3.524 3.592 3.591 3.597 3.596 3.576 3.603

0.7796 0.8053 0.7860 0.8037 0.8131 0.8276 0.8184 0.7995 0.8202

(3)

where t is time, τ1 and τ2 are the short- and long-decay components, respectively, and A1 and A2 are constants. According to the above equations, the lifetimes of VO43− emission were determined to be 16.13 → 11.23 μs for x = 0− 0.10 samples. It is obvious that the lifetimes of the VO43− group decrease linearly with the increasing x values, which forcefully proves the existence of VO43− → Bi3+ energy transfer in LCMV:xBi3+ samples. Generally, the energy transfer efficiency (ηT) from the VO43− group to Bi3+ ions in LCMV:xBi3+ can be approximately calculated utilizing the lifetime variations of VO43− group by the equation τ ηT = 1 − S τS0 (4)

Table 1. d88/d81 Ratio of LCMV:xBi3+ (x = 0−0.10) Samples 0 0.01 0.02 0.03 0.04 0.05 0.06 0.08 0.10

A1τ12 + A 2 τ22 A1τ1 + A 2 τ2

where τS0 and τS represent the corresponding luminescence lifetimes of sensitizer ions in the absence and presence of activator ions, respectively. According to the inset of Figure 3c, the energy transfer efficiency (ηT) is low (only reaches 30.4% at x = 0.10), which is possibly caused by the reabsorption effect. In order to achieve the final full color emission tuning from bluish green/yellow light to red light through the ideal warm white light area, the VO43− → Eu3+ energy transfer and simultaneous Bi3+ → Eu3+ energy transfer were jointly designed in the LCMV host. Figure 4a depicts the PL spectra of LCMV:yEu3+ (y = 0−0.15) samples upon exciting into the host absorption at 330 nm. As the y value changes from 0.01 to 0.15, the emission intensity of VO43− decreases monotonically while the Eu3+ emission intensity increases. This is an obvious sign of energy transfer from VO43− groups to Eu3+ ions. The corresponding emission color can be tuned from bluish green to red with increasing y value; especially warm light was

the increasing local lattice distortion commonly results in an emission red shift. In the current LCMV:xBi3+ (x = 0−0.10) system, the local lattice distortions obviously have a greater effect on the Dq value of Bi3+ ions between the above two competitive effects, finally leading to the emission red shift. Furthermore, the possible VO43− → Bi3+ energy transfer is another factor that generates the emission red shift. As shown in Figure 2b, the asymmetric emissions could be observed in the PL spectra of LCMV:xBi3+ samples, which implies possible spectral overlaps resulting from the VO 4 3− and Bi 3+ luminescence centers. The intensity of the emission peak at D

DOI: 10.1021/acs.inorgchem.8b01271 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) Normalized Gaussian fitting PL spectra of LCMV:xBi3+ (x = 0−0.10) samples at two luminescence centers. (b) Relative emission intensity of VO43− and Bi3+ via various Bi3+ concentrations. (c) Lifetime decay curves of LCMV:xBi3+ (x = 0−0.10) samples monitored at different excitations and emissions. (d) Lifetime decay curves of LCMV:xEu3+ (x = 0−0.15) samples excited at 330 nm and monitored at 480 nm. The inserts in (c) and (d) show the relation of ηT versus Bi3+ and Eu3+ content, respectively.

realized at y = 0.02, which could be further confirmed by the color coordinates and luminescence photographs in the insets of Figure 4a. The decreasing lifetimes of VO43− with increasing Eu3+ doping content clearly proved the VO43− → Eu3+ energy transfer process in LCMV:yEu3+ samples, as shown by the decay curves of VO43− emission in Figure 3d. In addition, the PL properties and energy transfer of Bi3+/Eu3+-codoped LCMV phosphors were also revealed systematically. As presented in Figure 4b,c, upon 330 nm wavelength excitation, LCMV:xBi3+,0.04Eu3+ (x = 0−0.10) and LCMV:xBi3+,0.07Eu3+ (x = 0−0.10) samples could efficiently emit bluish white to yellow and yellow to orangish red light, respectively. The corresponding color coordinates and luminescence photographs of the above samples have verified this situation. Interestingly, the emission intensities of both Bi3+ and Eu3+ decrease monotonically, and Bi3+ emission undergoes a gradual red shift as the Bi3+ content increases. These results demonstrate that both VO43− → Bi3+ and Bi3+ → Eu3+ energy transfers exist in the LCMV:xBi3+,0.04Eu3+ and LCMV:xBi3+,0.07Eu3+ samples. As the Bi3+ content increases, the obvious emission red shift of Bi3+ will enlarge the Stokes shift, leading to a greater energy loss. Thus, the ηT value of Bi3+ → Eu3+ decreases at a higher Bi3+ doping level. The gradually decreasing lifetimes of Bi3+ emission for LCMV:xBi3+,0.04Eu3+ and LCMV:xBi3+,0.07Eu3+ with x value further prove the occurrence of Bi3+ → Eu3+ energy transfer. As a result, the simultaneous tuning of emission wavelengths of Bi3+ and Eu3+ ions could be successfully achieved, and the systematic full color emission tuning of emission color could be realized. A schematic variation of emission color in the CIE diagram by a

change in the activator concentration and the design of energy transfer in CaO8 sites is shown in Figure 4d. The detailed CIE color coordinate diagrams of all LCMV:xBi3+,yEu3+ (x = 0− 0.10, y = 0−0.15) samples are summarized in Figure S3. Generally, the emission color and photoluminescence properties can be systematically and efficiently tuned by local lattice distortion and design of multiple energy transfers. Photoluminescence Thermal Quenching. The excellent thermal stability of phosphors is an extremely important factor to ensure a high efficiency of phosphor-converted lightemitting devices.40−42 As shown in Figure 5a, the thermal stabilities of LCMV:xBi3+,yEu3+ samples progressively worsen with increasing Bi3+ ion concentration, in comparison with the LCMV sample. A possible reason is that the Bi3+−Bi3+ distance statistically becomes smaller at higher Bi3+ ion concentration, and the interaction will strengthen and eventually increase of the nonradioactive transition probability. The increasing substitution of Ca2+ ions by Bi3+ ions would attenuate the rigidity of the LCMV lattice, leading to a degradation of Bi3+ luminescence intensity with increasing temperatures. In addition, an emission red shift usually results in an increase in the Stokes shift, which means a reduction in thermal activation energy, thus finally increasing thermal quenching. It is obviously found that LCMV:Eu3+ samples have much less thermal quenching in comparison to those of LCMV:Bi3+ due to the closeness of their ionic radii to those of Ca2+ ions. Interestingly, the thermal stabilities of LCMV:xBi3+,yEu3+ gradually become better with an increase in Eu3+ ion concentration, as presented in Figure 5b. One reason is that a distortion of the local lattice resulting from the slight E

DOI: 10.1021/acs.inorgchem.8b01271 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. PL spectra (λex 330 nm) of (a) LCMV:yEu3+ (x = 0−0.15), (b) LCMV:xBi3+,0.04Eu3+ (x = 0−0.10), and (c) LCMV:xBi3+,0.07Eu3+ (x = 0−0.10) samples. The inserts in (a)−(c) are the corresponding CIE chromaticity coordinate diagrams and the luminescence photographs. (d) Dependence of the CIE chromaticity coordinates on x and y values in LCMV:xBi3+,yEu3+. Decay curves of (e) LCMV:xBi3+,0.04Eu3+ and (f) LCMV:xBi3+,0.07Eu3+. The inserts in (e) and (f) are the relation of energy efficiency (ηT) to Bi3+ content.

difference in the ionic radii of Bi3+ ions and Ca2+ ions is relieved via the incorporation of Eu3+ ions; another is that energy transfer becomes more and more efficient the ambient temperature is increased. Therefore, the thermal stability could be efficiently improved by the design of appropriate energy transfers. LED Applications. Finally, to evaluate the luminescence performance of LCMV:xBi3+,yEu3+ phosphors in practical applications, series LED lamps were fabricated via pumping the 370 nm chips on these phosphors with various x and y values. The emission of Bi3+ ions could be excited by 370 nm UV light on the basis of previous discussions. The electroluminescence (EL) spectra which were recorded under a 100 mA current and

the corresponding luminescence photographs of LED devices are illustrated in Figure 6. Obviously, the emission colors have changed from blue-green to red across yellow and white light. As depicted in Figure 6, the corresponding color temperature (CCT) continuously decreases from 4684 to 2096 K and the color rendering indexes (Ra) were calculated to be 69.9, 89.3, 75.7, 82.0, and 76.2, respectively. The above results demonstrate that excellent warm white and various colortunable emissions are successfully created, which could be further improved via process optimization, implying a potential application in practical WLED devices. F

DOI: 10.1021/acs.inorgchem.8b01271 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01271. XRD refinement of representative LCMV:xBi3+,yEu3+ (x = 0, y = 0; x = 0.04, y = 0; x = 0, y = 0.02; x = 0.04, y = 0.07), refined structure parameters, and selected interatomic distances, XRD patterns, emission wavelengths, fwhms, IQYs, and lifetimes of LCMV:xBi3+ (x = 0−0.10), and CIE color coordinate diagram of LCMV:xBi3+,yEu3+ (x = 0−0.10, y = 0−0.15) (PDF)

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Figure 5. Thermal quenching behavior of (a) LCMV:xBi3+ (x = 0.04, 0.10), LCMV, and LCMV:yEu3+ (y = 0.01, 0.04, 0.07, 0.15) samples and (b) LCMV:0.04 Bi3+, LCMV:yEu3+ (y = 0.04, 0.07), LCMV:0.04 Bi3+,yEu3+ (y = 0.04, 0.07) samples monitored at 330 nm UV on changes in temperature from 25 to 200 °C.

AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.L.: [email protected]. *E-mail for G.L.: [email protected]. ORCID

Jun Lin: 0000-0001-9572-2134 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC Nos. 51672265, 51672266, 21521092, 51750110511, 51672257, 51672259), the Key Research Program of Frontier Sciences, CAS (Grant No. YZDY-SSW-JSC018), the National Basic Research Program of China (2014CB643803), the Scientific and Technological Department of Jilin Province (Grant No. 20150520029JH, 20170414003GH), Jiangmen Innovative Research Team Program (No.[2017]385), the Major program of basic research and applied research of Guangdong Province (2017KZDXM083), and the Distinguished Scientist Fellowship Program of King Saud University as well as the Deanship of Scientific Research at King Saud University for funding this work through research group No. RG-1939-038.

Figure 6. Electroluminescence (EL) spectra of (a) LCMV, (b) LCMV:0.02Eu3+, (c) LCMV:0.10 Bi3+, (d) LCMV:0.04 Bi3+,0.07Eu3+, and (e) LCMV:0.15Eu3+ samples incorporated into 370 nm InGaN LED chips at 100 mA forward bias current.

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REFERENCES

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CONCLUSIONS

Herein, a series of single-phase LCMV:xBi3+,yEu3+ phosphors with typical garnet structures were successfully prepared. The introduction of Bi3+/Eu3+ into the host can lead to local lattice distortion and multiple energy transfers of VO43− → Bi3+ and VO43− → Bi3+ → Eu3+. This enables the systematic color tuning from bluish green (480 nm) through yellow (564 nm) to red (609 nm) in LCMV:xBi3+,yEu3+ on excitation into the intrinsic absorption of the VO43− group. Moreover, the thermal stability of LCMV:xBi3+,yEu3+ samples were gradually improved with increasing Eu 3+ concentration. Finally, representative LED devices with a combination of 370 nm UV chips and these phosphors could present excellent EL performance. In view of the above results, full-color luminescence adjustments based on simultaneous local lattice distortion and multiple energy transfers may be helpful for the design of single-phase warm white lights and novel colortunable phosphors in the future. G

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DOI: 10.1021/acs.inorgchem.8b01271 Inorg. Chem. XXXX, XXX, XXX−XXX