Synthesis and Luminescence Properties of Bi3+-Activated K2MgGeO4

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Synthesis and Luminescence Properties of Bi3+-Activated K2MgGeO4: A Promising High-Brightness Orange-Emitting Phosphor for WLEDs Conversion Huimin Li,†,‡ Ran Pang,*,† Guanyu Liu,† Wenzhi Sun,§ Da Li,† Lihong Jiang,† Su Zhang,† Chengyu Li,*,† Jing Feng,† and Hongjie Zhang†

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/10/18. For personal use only.

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State key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, P. R. China S Supporting Information *

ABSTRACT: In this article we synthesized a series of phosphors K2MgGeO4:Bi3+ with high brightness for white light-emitting diodes (WLEDs) conversion and investigated their crystal structures and luminescence properties using powder X-ray diffraction, diffuse reflectance spectra, X-ray photoelectron spectroscopy, photoluminescence spectra, and absolute quantum efficiency. K2MgGeO4:Bi3+ phosphor exhibits intense absorption in near-UV area and presents a broad asymmetric emission band with the main peak located at 614 nm, which was ascribed to the 3P1 → 1S0 transition of Bi3+. The absolute quantum efficiency of the K2MgGeO4:0.01Bi3+ phosphor was measured to be 66.6%. Also, this orange emission with color chromaticity coordinates of (0.4989, 0.4400) has an excellent resistance to thermal quenching: its integrated intensity at 393 K still maintained ∼85% of the one at room temperature. The WLEDs devices with Ra = 93.8 were fabricated by employing K2MgGeO4:0.01Bi3+ as an orange phosphor, which contains abundant red light component in its emission spectrum. The excellent luminescent performance of K2MgGeO4:0.01Bi3+ suggests that it is a promising orange-emitting phosphor for near-ultraviolet WLEDs.

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ions like Mn4+; however, some innate drawbacks still appear simultaneously. Therein, for Eu3+-doped phosphors, most of their effective absorption region is remarkably narrow, which can result in relatively lower utilization efficiency of the WLEDs chip emitting light. Also their sharp line emissions converge on the few characteristic peak positions, which easily leads to the incomplete color display and deviates from the spectrum of sunlight. For Mn4+-, Ce3+-, and Eu2+-doped phosphors, most of them have intense absorption in visible light region. When applied in the WLEDs model, these phosphors may absorb the short wavelength light components from other phosphors, causing serious reabsorption of the justgenerated white net spectrum and resulting in serious color distortion.5 Further, Ce3+ and Eu2+ usually give red emission in nitrides such as (Ca, Sr, Ba)2Si5N8:Ce3+ and CaAlN3:Eu2+. Besides the incidental reabsorption phenomenon, the defect of critical synthesis conditions cannot be well overcome.7,8 In addition, the previously reported red phosphors usually suffer from either their relatively low luminescence efficiency under UV excitation or serious thermal quenching at high temper-

INTRODUCTION White light-emitting diodes (WLEDs) have been rapidly developed and become the most promising candidates as light sources for future indoor domestic lighting and display applications. Incontestably, WLEDs will be more widely used and lead to a new phosphor research program, owing to their good environmentally friendly traits, low energy consumption, as well as great stability and durability compared with traditional incandescent or fluorescent lamps.1−4 Current commercial WLEDs are mostly driven by blue-emitting (450−470 nm) GaN chips combined with commercial Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor. Some intrinsic deficiencies exist in these WLEDs, for instance, the high correlated color temperatures (CCT) and poor color rendering index (CRI) caused by the insufficiency of red light component. To overcome these shortcomings, the composite of an ultraviolet (UV) chip with tricolor (that is blue, green, and red) phosphors named as UV-pc-WLED was employed to optimize the WLEDs light.5 Although the green and blue phosphors tend to be mature, the shortage of commercially available red phosphors is still obvious.6 Plentiful investigations on long wavelength phosphors have been implemented, especially on rare-earth ions such as Eu2+/3+ and Ce3+ and transition-metal © XXXX American Chemical Society

Received: July 19, 2018

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DOI: 10.1021/acs.inorgchem.8b02025 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Edinburgh Instruments FLS 920 spectrofluorometer with a temperature controller and a 450 W xenon lamp as the excitation source. Photoluminescence absolute quantum efficiency (QE) was acquired in the integrating sphere in a Photonic Multichannel Analyzer (C10027, Hamamatsu Photonics K. K.). WLEDs devices were fabricated using a combination of the phosphors of the K2MgGeO4:Bi3+, BaMgA110O17:Eu2+, and Y3(Al,Ga)5O12:Ce3+ phosphors and UV-LED chips. The parameters of the fabricated WLEDs were obtained by an HAAS-2000 system in an integrating sphere. The electroluminescence (EL) spectra that covered the UV range were detected by QE65000 spectrometer (Ocean Optics). Sample morphologies and element mappings were measured with scanning electron microscope (SEM, S-4800, Hitachi) with an energydispersive X-ray spectroscopy (EDS). Except the temperaturedependent spectra, all the measurements were performed at room temperature.

atures, which further restrict their commercial applications. Nowadays it is imperative to develop some novel red phosphors to improve the optical properties of UV WLEDs with blending multicolor phosphors method.9 Therefore, our efforts have been devoted to the investigation of novel UVexcited long-wavelength phosphors with high luminescence efficiency, broad emission band with abundant red light component, and excellent thermal stability. In this work we shift our attention to trivalent bismuth, a favored luminescent material activator, which has been widely investigated in luminescence material field.10,11 Bi3+ has the electronic configuration of [Xe]4f145d106s2, and its naked 6s and 6p electrons are sensitive to the surrounding field.12−14 So the emission is even susceptible to the microenvironment, and that makes it feasible to control Bi3+ emission through structural design of the matrix.5,15 However, to the best of our knowledge, few Bi3+-doped phosphors with long-wavelength emission were utilized to fabricate WLEDs.9,15,16 That may be because most Bi3+-doped phosphors cannot meet the requirements of applicable light output efficiency or emission wavelength distribution. Here, a Bi3+-doped potassium magnesium germinate phosphor K2MgGeO4:Bi3+ (KMGO:Bi3+) is reported. Upon the excitation of UV light, the phosphor yields bright orange emission peaking at 614 nm with a broad band whose full width at half-maximum (fwhm) is 148 nm (3820 cm−1). Also, the KMGO:xBi3+ exhibits excellent resistance to thermal quenching and relatively high quantum efficiency. By coupling the phosphors of KMGO:Bi3+ (orange), BaMgA110O17:Eu2+ (BAM:Eu, blue), and Y3(Al,Ga)5O12:Ce3+ (YAGG:Ce, green) with a commercial UV-LED chip, a WLEDs device is constructed, and this prototype device exhibits good WLEDs emission properties.

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RESULTS AND DISCUSSION Phase Identification and Crystal Structure. To verify the phase homogeneity of the as-prepared samples, the powder X-ray diffraction was performed. Figure 1 depicts XRD profiles

EXPERIMENTAL AND COMPUTATIONAL SECTION

Materials and Synthesis. K2MgGeO4:xBi3+ (KMGO:xBi3+) (x = 0−0.05) phosphors were prepared by the traditional high-temperature solid-state reaction method. The raw materials K2CO3 (99.99%, Aladdin), MgO (99.99%, Aladdin), GeO2 (99.99%, Aladdin), and Bi2O3 (99.99%, Aladdin) were weighed in the stoichiometric ratio. These raw materials were mixed and ground in an agate mortar uniformly and then sintered in an alumina crucible at 1050 °C for 5 h in air. After they naturally cooled to room temperature the assynthesized samples were grounded into fine powder for the next measurements. Characterizations. The powder X-ray diffraction (XRD) data were obtained by a D8 Focus diffractometer (Bruker) operating at 40 kV and 40 mA with graphite-monochromated Cu Kα radiation (λ = 1.5406 Å). The Rietveld refinements were achieved by general structure analysis system (GSAS) software. The fluorescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) coupled with a 928 PMT using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source. The composition of the phosphor was determined by inductively coupled plasma optical emission spectrometer (ICP-OES, X Series 2, Thermo Scientific). The diffuse reflectance spectra were recorded using a 3600 UV−vis−NIR (NIR = near-infrared) spectrophotometer (Shimadzu) with reference of barium sulfate. The valence state and partitioning of the bismuth ion was received by X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with a focused monochromatic Al Kα X-ray beam (with 500 μm spot size and Lens Mode of LargeAreaXL); the binding energies were corrected by referring to the C 1s orbital of 284.6 eV. The room-temperature photoluminescence excitation (PLE) as well as the photoluminescence (PL) spectra of the phosphors was measured by a Hitachi F-7000 spectrophotometer with the excitation source of a 150 W xenon lamp. The temperaturedependent spectra of PL were measured by a high-resolution

Figure 1. Representative XRD patterns of KMGO:xBi3+ (x = 0, 0.001, 0.005, 0.01, 0.03, and 0.05) and the standard data of K2MgGeO4 (JCPDS 49-0614) as well as the magnified XRD patterns nearby 32°.

of standard data K2MgGeO4 (JCPDS card No. 39-0720) and KMGO:xBi3+ (x = 0, 0.001, 0.005, 0.01, 0.03, and 0.05) samples. Evidently, all of the diffraction peaks of the samples can be well-indexed to the standard data of KMGO, which indicates that the obtained KMGO:xBi3+ samples in our work were single phase, and no other impurity phases emerged after the doping of Bi3+ ions. The Rietveld refinements of KMGO:0.01Bi3+ and KMGO matrix were performed to receive the detailed crystal structure information. Figure 2 shows the refinement patterns of KMGO matrix (a) and KMGO:0.01Bi3+ (b). Rietveld refinement and their respective refinement results including the lattice parameters are listed in Table 1. The final obtained residual R-factors of Rwp and Rp are determined to be 9.56 and 7.29% for KMGO:0.01Bi3+, 9.89% and 7.43% for KMGO sample. The results further verify the phase purity of the as-prepared samples, which also confirm the fact that Bi3+ was successfully doped into KMGO crystal. The EDS spectrum and elemental maps of the representative particle of KMGO:0.01Bi3+ were performed to check whether the dopants would induce the inhomogeneity such as element aggregation in the samples (see Figure S1). The results show that the sample comprises the elements K, Mg, Ge, O, and Bi (Figure S1a). All the elements K, Mg, Ge, O, and Bi are distributed homogeneously throughout the particle (Figure B

DOI: 10.1021/acs.inorgchem.8b02025 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

are inserted in the cavities created by the tetrahedral network, which is formed by MgO4 and GeO4 tetrahedrons. The crystal structure model of KMGO in view direction of b-axis as well as four types of K+ crystallographic sites is depicted in Figure 3. In KMGO crystal structure, two types of

Figure 3. Structural projection of KMGO and the coordinated environment of K+ sites.

Mg2+ and Ge4+ exist in the form of tetrahedron. MgO4 and GeO4 tetrahedrons are arranged alternately, and four types of K+ cations are inserted in the cavities created by the tetrahedral networks. Therein, K1 and K4 are coordinated with five oxygen atoms, respectively, while K2 and K3 are coordinated with four surrounded oxygen atoms and formed tetrahedron. The selected K−O distance in KMGO sample is presented in Table S1. Considering the doped Bi3+ ions have the tendency to replace the cations inserted in the cavities rather than the cations that constructed network, we suggest that K+ was preferentially occupied by Bi3+.10,19 Moreover, the similarity of the radii between K+ (r = 1.38 Å, CN = 6) and Bi3+ (r = 1.03 Å, CN = 6) also supports this viewpoint. According to the report of Davolos, an allowed radii difference between substituted and doped ions must be within the limits of 30%.20 The radius difference percentage can be calculated according to the formula below:21

Figure 2. Observed (×) and calculated (red solid line) powder XRD patterns of (a) KMGO:0.01Bi3+ and (b) KMGO sample, Bragg reflection (green sticks), and the profile difference between the observed and calculated data (blue solid line).

Table 1. Rietveld Refinement and Lattice Parameters of KMGO:0.01Bi3+ and KMGO formula space group a (Å) b (Å) c (Å) α = β = γ (deg) units, Z V (Å3) Rp (%) Rwp (%) χ2

KMGO:0.01Bi3+ Pca21 (29) 11.196 5.581 15.852 90 8 990.510 7.29 9.56 3.464

KMGO Pca21 (29) 11.206 5.592 15.865 90 8 994.164 7.43 9.89 4.638

Dr = 100 ×

R m(CN) − R d(CN) R m(CN)

where CN represents the coordination number; Dr is the radius difference percentage; Rd(CN) and Rm(CN) represent the radii of the doped ion and host cation, respectively. Because of the lack of the data for K+ (CN = 5) and Bi3+ (CN = 4), to calculate the Dr between Bi 3+ and tetracoordinated Mg2+/Ge4+, we took the data of CN = 6 (for Bi3+, r = 1.03 Å; for Mg2+, r = 0.72 Å; for Ge4+, r = 0.53 Å) as a reasonable approximation.22 The values of Dr between Bi3+ and host cations are 25.3%, −43.0%, and −94.3% for K+, Mg2+, and Ge4+, respectively, as listed in Table 2. In conclusion, K+ is the only cation that can be substituted, which is consistent with the formerly discussed results. The magnified XRD patterns nearby 32° are also presented in Figure 1. The peaks shift to a larger 2θ angle when doping Bi3+,

S1b), without any phase separation and traceable element aggregation in KMGO:0.01Bi3+ sample. K2MgGeO4 was first described in the cubic system by Torres-Martinez et al. in 1988.17 Afterward, Colbeau-Justin et al.18 further analyzed the crystal structure and determined that the K2MgGeO4 crystal structure shows distortions linked to rotations of tetrahedron and K+ repositioning, which is very close to ideal cubic structure type. The compounds crystallize in the β-SiO2 cristobalite structure type, in which GeO4 tetrahedron built network and connected by their corners. In addition, the Mg−O−Ge angle is 130° rather than 180° in the structure due to the distortions linked to rotations of tetrahedron and monovalent cations repositioning, and K+ C

DOI: 10.1021/acs.inorgchem.8b02025 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

where R, K, and S represent the reflection, absorption, and scattering parameter, respectively. The inset of Figure 4 depicts the absorption spectrum of KMGO calculated using the Kubelka−Munk equation. The Eg value of KMGO was calculated ∼5.85 eV from the extrapolation of the line for [F(R∞)hν]2 = 0, which is relatively low compared with that of YAG (6.7−6.9 eV).34 Room-Temperature PL and Decay Curve Analysis. The PLE spectrum of KMGO:0.01Bi3+ phosphor monitored at 614 nm is presented in Figure 5. The spectrum exhibits two

Table 2. Ionic Radii Difference Percentage (Dr) between Matrix Cations and Bismuth Ion ions in matrix +

K (CN = 6) Mg2+(CN = 6) Ge4+(CN = 6)

bismuth ion 3+

Bi (CN = 6) Bi3+(CN = 6) Bi3+(CN = 6)

Rm

Rd

Dr (%)

1.38 Å 0.72 Å 0.53 Å

1.03 Å 1.03 Å 1.03 Å

25.3 −43.0 −94.3

which indicates the shrinkage of the cell and further demonstrates Bi3+ substituting for the K+ ions. As magnesium vacancies can be easily produced in the phosphors that contain magnesium,23−28 when Bi3+ ions are doped into the host, more magnesium vacancies may be generated simultaneously to compensate the net charges in Bi3+ sites resulting from the nonequivalence substitution. To confirm the existence of magnesium vacancies, ICP-OES analysis, which is widely used for verifying the vacancies, was performed,29−31 and the result showing in Table S2 proves that the magnesium vacancies are the dominating cation vacancies, and also, there are a relatively small amount of germanium vacancies in the phosphor. Reflectance of Bi3+-Doped K2MgGeO4. The diffuse reflectance spectra of KMGO:xBi3+ are shown in Figure 4.

Figure 5. PLE and PL spectra of KMGO:0.01Bi3+ phosphor, λex = 335 nm and λem = 614 nm. (inset) The schematic pattern for the PL mechanism at room temperature.

bands covering the region from 200 to 400 nm with maximum value at 335 nm. The range coincides with the absorption spectrum of KMGO:0.01Bi3+ presented in Figure 4. As the bismuth valence was identified by the XPS analyzing in Figure S2, it is confirmed that the two excitation bands originate from the absorption of Bi3+ corresponding to the spin-allowed 1S0 → 1 P1 (225 nm) and 1S0 → 3P1 transition (335 nm).10,19 The PL spectrum of KMGO:0.01Bi3+ phosphor under 335 nm excitation is also presented in Figure 5. The PL spectrum extends from 440 to 800 nm centered at 614 nm with fwhm being 148 nm (3820 cm−1). Obviously, this broad emission stems from Bi3+ ions in the KMGO:xBi3+ crystal, since the matrix without Bi3+ did not show any photoluminescence phenomenon. For trivalent bismuth, the ground state is 1S0 with the 6s2 electronic configuration, and the excited states are split into four levels of 3P0, 3P1, 3P2, and 1P1 in order of increasing energy. The transitions of 1S0 → 3P0 and 1S0 → 3P2 are completely spin-forbidden. Nevertheless, the 3P1 level undergoes mixing with 1P1 by spin−orbit coupling, and that renders the transitions of 1S0 → 3P1 and 1S0 → 1P1 have fairly strong absorption strength. Thereby, for KMGO:xBi3+, the electrons can be excited to 3P1 and 1P1 energy levels from 1S0 ground state under 200 to 400 nm UV excitation. The electrons situated in 1P1 level fall into the 3P1 level in a nonradioactive way, and the transition of 3P1 → 1S0 will proceed along with the orange emission afterward, as illustrated in the schematic diagram in the inset of Figure 5.10,19 As Bi3+ ions are sensitive to their surrounding environment and the PL spectrum exhibits an asymmetric character, we proposed that different emission centers exist in the phosphor. We classified the K sites into two categories based on coordination environment. Category I including K1 (penta-

Figure 4. UV−Vis diffuse reflectance spectra of KMGO:xBi3+ (x = 0, 0.01, 0.02, and 0.03). (inset) The KMGO matrix absorption spectrum calculated using the Kubelka−Munk theoretic equation.

The KMGO host exhibits high reflection in the range from 320 to 600 nm and an intense absorption band ranging from 200 to 320 nm corresponding to the matrix absorption. When 1 mol % Bi3+ is incorporated into KMGO, the absorption intensity increased in the range from 200 to 400 nm, which mainly attributes to the 1S0 → 3P1 electronic transition of Bi3+.32 As the Bi3+concentration increases, the absorption band edge continuously expands to long wavelength side and reaches ∼525 nm when the Bi3+ concentration is 0.03. The band gap of the matrix can be estimated according to the equation:33 [F(R ∞)hν ]n = C(hν − Eg )

(1)

In the above equation, F(R∞) represents the Kubelka− Munk function, R∞ = Rsample/Rstandard, hν is the energy per photon, C represents a proportionality constant, n represents a transition coefficient (n = 2 for direct allowed transition) and Eg is the value of the band gap. The Kubelka−Munk function of F(R∞) could define as33 F(R ∞) = (1 − R )2 /2R = K /S

(2) D

DOI: 10.1021/acs.inorgchem.8b02025 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry coordinated, average bounds 2.7105 Å) and K4 (pentacoordinated, average bounds 2.6735 Å) is distinct from category II (K2 and K3), in which the coordination numbers of K2 and K3 are four, and the average bonds of them are 2.7839 and 2.7739 Å, respectively (see Table S1). Hence, the emission bands located at 22 026 cm−1 (614 nm) and 16 286 cm−1 (454 nm) originate from Bi3+ ions substituting for the two categories of K+.10 The emission spectrum curve can be well-fitted and decomposed into two symmetrical Gaussian components curves corresponding to two emission centers (Bi1 and Bi2 corresponding to 454 and 614 nm, respectively) (see Figure S3a). To further clarify and distinguish the two emission centers, the excitation spectra monitoring at 454 and 614 nm were detected and presented in Figure S3b. The obtained two curves are obviously different. Although both of the two peaks are located at ∼335 nm, the spectrum monitored at 454 nm is much broader than the one at 614 nm. Similarly, the shape of normalized emission spectra with λex = 300 and 335 nm are also different. The emission curve generated by 300 nm light excitation shows slightly higher intensity in 400−500 nm range than the one generated by 335 nm (see Figure S3c). Considering the site occupancy analyzed above, we suggest that the two emission centers stem from the Bi3+ ions substituting for two different K+ sites located in different crystalline fields. The difference between the fluorescence decay curves monitored at 454 nm (1.562 μs) and 614 nm (1.701 μs) also support this viewpoint (Figure S4). The PL spectra of KMGO:xBi3+ (x = 0.001−0.05) presented in Figure 6a depict the effect of Bi3+ concentration on the PL spectra of KMGO:xBi3+. When x < 0.01, the intensity elevates with the augment of x value and reaches the maximum at x = 0.01. Afterward, the intensity weakens sharply along with the increment of Bi3+, which can be attributed to concentration quenching effect. To investigate the concentration quenching mechanism of KMGO:xBi3+ phosphor, the critical distance could be determined by the formula: ij 3V yz zz R c ≈ 2jjj j 4πxcZ zz k {

Figure 6. (a) PL spectra of KMGO:xBi3+ (x = 0.001−0.05), λex = 335 nm. (inset) Illustration of the variation of Bi3+ emission intensity as a function of x, (b) dependence of log(I/xBi3+) on log(xBi3+) for KMGO:xBi3+ phosphors.

slope is −2.149, as shown in Figure 6b, θ is determined to be 6.447, which is approximately equal to 6, which indicates that the dipole−dipole interaction dominates the energy transfer mechanism between Bi3+ ions.35 The decay curves as well as the corresponding exponential fitting curves of KMGO:xBi3+ monitored at 614 nm (λex = 335 nm) are portrayed in Figure 7. The decay curves obey the second-order exponential equation and could be perfectly fitted:35

1/3

(3)

where xc represents the critical concentration of dopant ions; Z is the number of cations per unit cell; and V is the volume of the unit cell. For KMGO:0.01, Z = 8, V = 990.510 Å3, and xc is 0.01 for Bi3+. Consequently, Rc was calculated to be ∼28.70 Å. As we know, exchange and multipolar interations are two types of resonant energy transfer modes. In the earlier works, Blass et al. reported that the exchange interation could occur when the critical distance is ∼5 Å or less. As the Rc value of Bi3+ is far beyond this scale, we suggest that the mutipolar interaction is the dominant mechanism in the concentration quenching in KMGO:xBi3+ phosphors. According to the report of Dexter and Van Uitert, the PL intensity (I) per activator keeps to the following equation:5,35 I /x = [(1 + β(x)θ /3 )]−1

(4)

where x is the concentration of Bi3+ ions; β represents a constant for a given host lattice and excitation conditions. The values of θ = 6, 8, and 10 corresponding to the dipole−dipole (d−d), dipole−quadrupole (d−q), and quadrupole−quadrupole (q−q) interactions, respectively. As the dependence of log(I/xBi3+) on log(xBi3+) can be well-fitted as a line with the

Figure 7. Fluorescence decay curves of KMGO:xBi3+ monitored at 614 nm (λex = 335 nm). E

DOI: 10.1021/acs.inorgchem.8b02025 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry I(t) = I0 + A1 exp( −t /τ1) + A 2 exp( −t /τ2)

increased electron−phonon coupling at elevated temperature. Remarkably, the magnitude of the reduction is really small (less than 15%) in the temperature-rise process from 303 to 393 K. As illustrated in Figure 9a, although the magnitude of

(5)

where t denotes the time, I(t) represents the corresponding luminescence intensity, A1 and A2 are the constants of the fitted function, and τ1 and τ2 correspond to the rapid and slow decay times for the exponential components. The fitting parameters of different concentration are listed in Table S2. With the known parameters, the average luminescence decay time τ* can be obtained with the formula:36 τ * = (A1τ12 + A 2 τ2 2)/(A1τ1 + A 2 τ2)

(6)

The calculated average decay time τ* of Bi3+ is 2.106, 1.949, 1.701, and 1.564 μs, corresponding to the concentration x of 0.001, 0.004, 0.01, 0.02, respectively. Generally, fluorescence decay curves could probe the energy transfer process in phosphors. With the increasing of activator concentration, the distance between activator ions reduces, and that gives rise to energy transfer, which is embodied in a form of extra luminescence decay time. Namely, the decay time would decline. In the current work, the average lifetimes monitored at 614 nm decline gradually as the x increases, which could further confirm that energy transfer proceeds among Bi3+ ions. Moreover, as the transition of 3P0 → 1S0 is usually forbidden and its decay time is approximately several hundred microseconds, the result verifies that the emission at room temperature originates from the transition of 3P1 → 1S0, which is Laporte-allowed and usually has decay time ranging from 1 × 10−6 to 1 × 10−8 s (orders of magnitude).11,37 The results are consistent with those of the literature.11,37 Thermal Stability Analysis. As the phosphor mounted surface temperature of driven WLEDs chips is ∼373 K, thermal stability is an important technological parameter for WLEDs.38 Generally, for a phosphor, with the increasing of the temperature, the emission intensity would gradually decrease due to the magnified population of higher vibration levels, density of phonons, and probability of nonradiative transfer. Therefore, it is imperative to study how KMGO:0.01Bi3+ performs at high temperature. The high-temperature-dependent PL spectra of KMGO:0.01Bi3+ phosphor under 335 nm excitation are measured, as presented in Figure 8. The spectra show a slight blue shift of λem,max, which is common in other phosphors,39,40 owing to the changes in the Bi3+ crystal field splitting as a function of temperature. Furthermore, the spectra peaks broaden at relatively high temperature because of

Figure 9. High-temperature-dependent (a) PL intensity and (b) Arrhenius fitting of the PL intensity in KMGO:0.01Bi3+.

the reduction enlarges with the temperature rising from 393 to 453 K, the integrated intensity at 423 K remains ∼72% of the initial one, which is roughly similar to famous commercial yellow YAG:Ce3+ phosphor, whose integrated intensity at 423 K could remain up to 80% of the room-temperature one.41 When the phosphor is heated to 453 K, the integrated intensity still remains 65% of the initial one, which is really valuable for an orange phosphor based on Bi3+ emission with such a large Stokes shift value (∼1.68 eV, 13 564 cm−1). The activation energy (ΔE) for thermal quenching can be determined by the Arrhenius equation:35 I0 IT = ΔE 1 + c exp − kT (7)

(

)

where IT and I0 denote the integrated PL intensity at testing temperature and room temperature (300 K), respectively, c denotes a constant for a certain host, and k represents the Boltzmann constant (8.629 × 10−5 eV). Figure 9b shows the relationship of ln(I0/IT − 1) versus 1/kT, from which ΔE is calculated to be ∼0.397 eV. The thermal stability analysis signifies the phosphor has good resistance to thermal quenching. Beyond that, the absolute QE of KMGO:0.01Bi3+ sample is acquired by the measurement system at room temperature, and the QE value is

Figure 8. High-temperature-dependent PL spectra (λex = 335 nm) of KMGO:0.01Bi3+ sample measured between 303 and 453 K. F

DOI: 10.1021/acs.inorgchem.8b02025 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

chromaticity coordinates and the correlate color temperature (CCT) of the fabricated WLEDs were measured as (x, y) = (0.3588, 0.3429) and 4437 K. The color rendering index (CRI, Ra) value and the R9 value of the white WLEDs are 93.8 and 96.5, respectively. Beyond that, the color parameter, photometric parameter, and electrical parameter are listed in Table S4. The results indicate that the orange-emitting KMGO:0.01Bi3+ phosphor has great potential in WLEDs based on the UVLED chip.

up to 66.6% under 335 nm excitation, which is higher than widely studied ScVO4:Bi3+ phosphor (∼56%).5,9,37,42,43 In addition, in consideration of nonoverlapping between the PLE spectra of KMGO:0.01Bi3+ and visible light, when the phosphor is used as orange component in UV-WLEDs, selfabsorption that in general brings color distortion in WLEDs applications may be avoided. The efficient orange emission output also makes the phosphor promising for UV-WLEDs applications. To further evaluate the performance of the KMGO:0.01Bi3+ phosphor, a series of LED devices were fabricated using phosphors and a commercial 310 nm-emitting UV-LED chip. Performances of WLEDs. First, the LEDs were fabricated using a 310 nm UV-LED chip driven by 50 mA current and KMGO:0.01Bi3+ phosphor. As the PLE spectrum of the phosphor matches well with the emission of the chip, the phosphor could be efficiently excited by the working UV-LED chip and give broad-band orange emission. The electroluminescence (EL) spectrum of the KMGO:0.01Bi3+ phosphor is shown in Figure S5. The inset of Figure S5 shows the photograph of the working LEDs. Then, the KMGO:0.01Bi3+ phosphor was incorporated into a WLEDs device with a 310 nm UV-LED chip and commercially available phosphors of BAM:Eu (blue) and YAGG:Ce (green). Its luminous spectrum together with the photograph is obtained and demonstrated in Figure 10a. The Commission International de l’Eclairage (CIE) coordinates of KMGO:0.01 Bi3+, BAM:Eu, and YAGG:Ce phosphors are presented in Figure 10b. The CIE

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CONCLUSION In summary, we have successfully synthesized a novel orange emission phosphor KMGO:0.01Bi3+ for near-UV WLEDs conversion. After the characterization, we found that the phosphor exhibits intense absorption in the near-UV range and presents a broad asymmetric emission band with the main peak located at 614 nm. The QE value of the phosphor was measured to be 66.6%. The excitation spectrum contains two main bands at 225 and 335 nm, which are ascribed to 1S0−1P1 and 1S0−3P1 transitions of Bi3+, respectively. The broad emission band covering from 440 to 800 nm attributes to 3 P1 → 1S0 transition of Bi3+ in two kinds of K+ sites based on the analysis of crystal structure, bismuth valence, and photoluminescence spectra. The phosphor has an excellent resistance to thermal quenching, of which the integrated intensity at 423 K still remains without considerable deterioration. The WLEDs device with Ra = 93.8 was fabricated by employing K2MgGeO4:0.01Bi3+ as a red light component. In view of the experimental results, we conclude that KMGO:Bi3+ is a promising high-brightness orangeemission phosphor for near-UV WLEDs.

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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.8b02025. EDS spectrum and elemental maps, ICP-AES, fitted emission spectrum along with its decomposed Gaussian components curves, excitation spectra (λem = 480 and 614 nm) and normalized emission spectra (λex = 300 and 335 nm) of KMGO:0.01Bi3+; XPS, fluorescence decay curves (monitored at 614 nm, λex = 337 nm) together with their fitting parameters of KMGO:xBi3+; selected K−O distance in KMGO; EL spectrum of an LED fabricated using a UV-LED chip and KMGO:0.01Bi3+; color parameter, photometric parameter, and electrical parameter of the WLEDs device (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone:+86-0431-85262208. E-mail: [email protected]. (C.Y.L.) *E-mail: [email protected]. (R.P.) ORCID

Su Zhang: 0000-0002-9261-1620 Chengyu Li: 0000-0003-1844-0429 Hongjie Zhang: 0000-0001-5433-8611

Figure 10. (a) EL spectrum of a WLED using a UV-LED chip (310 nm) and KMGO:0.01Bi, BAM:Eu, and YAGG:Ce phosphors. (b) The CIE coordinates of KMGO:0.01Bi3+, BAM:Eu, and YAGG:Ce phosphors and fabricated WLED.

Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.inorgchem.8b02025 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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ACKNOWLEDGMENTS The research is financially supported by the National Basic Research Program of China (973 Program, Grant No. 2014CB643801), the Key Program of the Frontier Science of the Chinese Academy of Sciences (Grant No.YZDY-SSWJSC018), the Fund for Creative Research Groups (Grant No. 21221061), and National Key R&D Plan (No. 2016YFB0701003).

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DOI: 10.1021/acs.inorgchem.8b02025 Inorg. Chem. XXXX, XXX, XXX−XXX