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Synthesis and luminescence properties of double perovskite Ba2MgGe2O7:Mn4+ deep red phosphor
Journal of Luminescence 203 (2018) 420–426
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Synthesis and luminescence properties of double perovskite Ba2MgGe2O7:Mn4+ deep red phosphor Zuizhi Lu, Anjie Fu, Fangfang Gao, Xiaoshan Zhang, Liya Zhou
T
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School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Deep red-emitting Mn4+ Phosphors Fluorescence lifetime
A deep red-emitting Ba2MgGe2O7:Mn4+ (BMG:Mn4+) phosphor was synthesized by conventional solid-state reaction. 4A2g→4T1g, 4A1g→4T2g, and 4A2g→2T2g transitions of Mn4+ ions resulted in three absorption bands centered at 305, 330, and 420 nm, respectively. The phosphor showed a broad emission band from 600 nm to 700 nm due to 2E→4A2 transition of Mn4+ ions. The highest luminous peak located at 660 nm, and the highest luminous intensity was achieved at high Mn4+ concentration (0.013). Multipolar interaction was the major mechanism for fluorescence quenching, and dipole–dipole interaction was the type of interaction mechanism between Mn4+ ions. Fluorescence lifetimes decreased from 0.898 ms to 0.746 ms, which accorded with the requirement of white light-emitting diode phosphor. Color coordinates of BMG:Mn4+ phosphors (0.721, 0.279) were close to the standard values of the National TV Standards Committee.
1. Introduction
exposed and is susceptible to the influence of crystal field and coordination environment. The d–d electron transition shows broadband absorption in the UV and near-UV regions and features a strong peak emission in the blue and red regions [14] Mn4+-doped red phosphor shows characteristics of good stability, high color purity, and low decline. Mixing with YAG: Ce3+ yellow phosphor can be hopefully applied to production of W-LEDs. Double perovskite Ba2MgGe2O7 (BMG) features the advantages of good stability and easy synthesis. The mass ratio of Ge in BMG is far less than those of phosphors such as SrGe4O9 [15] and A2Ge4O9 (A = K, Rb) [16]. In the past, a variety of phosphors based on BMG have been developed (e.g., Ba2MgGe2O7:Nb3+ [17] and Ba2MgGe2O7:Cr4+ [18]. However, Nb and Cr are expensive; thus, Ba2MgGe2O7:Nb3+ and Ba2MgGe2O7:Cr4+ phosphors have not been widely used in W-LEDs. For Mn4+-doped BMG:Mn4+ phosphor, no previous studies have been reported. Therefore, studying BMG phosphor is necessary. In this paper, BMG:xMn4+ (x = 0.002, 0.004, 0.006, 0.008, 0.01, 0.013, 0.016, 0.02, 0.025) phosphors were synthesized by conventional solid-state reaction (Mixing the raw materials by mole ratio, and the mixture is calcinated directly without adding any substance, then the samples are obtained). Crystal structure, morphology, and luminescent properties of samples were tested and analyzed.
In recent years, white light-emitting diodes (LEDs) have received considerable attention because of their high efficiency, environmental friendliness, and long service lives [1–3]. At present, white LEDs mainly comprise blue light chip and YAG: Ce3+ yellow phosphor [4,5]. However, this kind of white light due to lack of red components caused by color and high-color-temperature low shortcomings develop gradually, thus hindering the development of warm white LEDs (color temperature of 4000–8000 K) and restricting their application in indoor lighting. Therefore, developing red phosphors with near- ultraviolet (UV) excitation is urgent [6,7]. The red phosphor used in white light (W-LEDs) plays an important role in modulating color temperature of white light and improving color rendering [8]. Luminescent properties of phosphors directly influence their practical value. Therefore, the search for appropriate matrix composition and activators is a key factor in the synthesis of LEDs using red fluorescent materials [9,10]. At present, rare-earth iondoped red phosphors (e.g., Eu3+ and Sm3+ ions) are limited in the visible light region due to the narrow band of emission spectrum, thus limiting their application in W-LEDs [11]. In recent years, Mn4+-doped fluoride red phosphor has become a new research focus because of its characteristic red light narrow band emission, UV and blue light excitation, and cheapness as raw material. In addition, rare-earth europium resources are scarce and expensive, which cause some difficulties in industrial production [12,13]. The outermost orbital of Mn4+ 3d is
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Corresponding author. E-mail address: [email protected] (L. Zhou).
https://doi.org/10.1016/j.jlumin.2018.06.061 Received 4 November 2017; Received in revised form 17 June 2018; Accepted 19 June 2018 Available online 20 June 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
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2. Experiments
in host BMG lattice [19]. The patterns of the 28–30° range is amplified and displayed in Fig. 1(b). With the increase of the concentration of doped Mn4+, the position of the diffraction peak remains unchanged. According to Bragg’s law, 2dsinθ = nλ, Mn4+ replaced an ion close to its ionic radius, that is, Ge4+. Mn4+ (r = 0.53 Å) replaced an ion with a radius equal to it, that is, Ge4+ (r = 0.53 Å). Fig. 2(a) shows the scanning electron microscopy image of the obtained BMG:xMn4+ (x = 0.013) powder. The phosphor exhibited a flake structure morphology, and particle size of BMG:Mn4+ powder reached 3–5 µm. The phosphor also presented a regular appearance and no clear reunion phenomenon. A bright red light was emitted under the excitation of UV light at 365 nm [Fig. 2(b)]. The samples were homogeneous white powders under natural light [Fig. 2(c)]. The X-ray photoelectron spectroscopy (XPS) survey spectra of BMG:0.008Mn4+ phosphor is shown in Fig. 3, which can obtain the chemical compositions of manganese and the valence state in the samples. As shown in the figure, the signals of barium (Ba), magnesium (Mg), germanium (Ge), and oxygen (O) species were clearly detected. The signals of carbon (C) species are mainly due to the adsorption of CO2. The signal from manganese (Mn) is weakly recorded because of its lower concentration relative to that of the other elements. Fig. 4 displays the UV–vis absorption spectra of BMG:xMn4+ (x = 0, 0.006, 0.01, 0.02). The phosphors manifested a broad absorption band in the range of 250–500 nm. Absorption intensity decreased rapidly in the range of 250–290 nm due to charge transfer (CT) band transitions of Mn4+→O2+. Given the 4A2g→4T1g, 4A1g→4T2g, and 4A2g→2T2g transitions of Mn4+ ions, three absorption bands centered at 305, 330, and 420 nm, respectively. These absorption bands all conformed to absorption characteristics of Mn4+ [20]. Fig. 5 shows the excitation spectrum of BMG:0.013Mn4+ phosphor at 660 nm monitoring wavelength and emission spectra under an excitation of 289 nm excitation wavelength. Four Gaussian-deconvoluted peaks centered at ~ 260, ~ 302, ~ 324, and ~ 420 nm in the excitation spectrum. The 260 nm peak was due to CT band transitions of Mn4+→ O2+. The other three Gaussian-deconvoluted peaks centered at ~ 260, ~ 302, ~ 324, and ~ 420 nm were due to the 4A2g→4T1g, 4A1g→4T2g, and 4A2g→2T2g transitions of Mn4+ ions, respectively. This result agrees with the findings of UV absorption spectra. The emission spectrum consisted of two distinct emission peaks. The highest emission peak located at 660 nm was attributed to 2Eg→A2g transition of Mn4+ [21]. These results showed that BMG:Mn4+ phosphors can be excited by near-UV or UV light and emit red light. Therefore, in terms of luminescence properties, BMG:Mn4+ phosphor is an excellent potential red phosphor used in W-LEDs. The W-LEDs can be made by combination of red phosphors, green phosphors, and blue phosphors [22]. The influence of crystal field on 3dn configuration energy level can
2.1. Materials and synthesis A series of BMG:xMn4+ (x = 0.002, 0.004, 0.006, 0.008, 0.01, 0.013, 0.016, 0.02) phosphors were prepared by high-temperature solid-state reaction. The raw materials included BaCO3 (A.R. 99.5%), MgO (A.R. 98.5%), GeO2 (99.99%), and MnCO3 (A.R. 99.5%). Samples were weighed at a corresponding molar ratio and fully ground in a mortar. Then, the samples were preheated for 4 h at 873 K. After preheating, the samples were cooled down to room temperature and ground again. Subsequently, the samples were heated at 1273 K for 6 h under air atmosphere. Finally, the prepared phosphors were cooled to room temperature, and subsequent measurements were performed. 2.2. Sample characterization Ground samples were characterized by powder X-ray diffraction (XRD) analysis with a RIGAKU D/max 2200 vpc X-ray diffractometer. Step-scan was conducted over an angle range of 10–80° with a step size of 0.02. The X-ray photoelectron spectroscopy (XPS) survey spectra was recorded by a Synchro thermal analyzer (Netzsh, STA 449 F5). Morphologies of the as-synthesized products were investigated by electron microscopy (Hitachi, S-3400 N). UV–vis absorption spectra of the powders were recorded using a Cary 5000 UV–vis spectrophotometer. Photoluminescence (PL) excitation and spectra were recorded by Hitachi-2500 fluorescence spectrometer at 150 W excitation using a xenon lamp as excitation source. Excitation slit and emission slit were 5.0 nm, and the scan speed was 300 nm/min. An FLS920 was used to record luminescence decay curve. The quantum efficiency (QE) measurements was carried out by a barium sulfate coated integrating sphere. Excitation and emission wavelengths measured 289 and 660 nm, respectively. An FLS-980 Edinburgh fluorescence spectrometer was used to measure temperature-dependent spectra. All measurements were carried out at room temperature. 3. Results and discussions Fig. 1(a) shows the XRD spectra of phosphors with different Mn4+ doping concentrations. Diffraction peaks of all samples matched those reported for the PDF (JCPDS 37-1478). No obvious diffraction peaks for detectable impurity phases were observed in the samples, demonstrating that samples pure, and Mn4+ ions were well-doped into the host with no significant influence on the structure of BMG. According to similar ionic radius principle (Ba2+:1.35 Å, Mg2+:0.72 Å, Ge4+:0.53 Å, and Mn4+:0.53 Å), Mn4+ ions may replace mainly the Ge4+ ions sites
Fig. 1. XRD spectra of BMG:xMn4+ (x = 0.002, 0.008, 0.013, 0.02) phosphor and the PDF (JCPDS 37-1478) standard pattern. 421
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Fig. 2. (a) SEM image of BMG:0.013Mn4+ (b) BMG:Mn4+ phosphors under ultraviolet lamp (c) BMG:Mn4+ phosphors under daylight.
Fig. 3. The XPS) survey spectra of BMG:0.008Mn4+ phosphor.
Fig. 4. UV–vis absorption spectra of BMG:xMn4+ (x = 0, 0.006, 0.01, 0.02).
be explained by the diagram of Tanabe-Sugano. Fig. 6 illustrates the Tanabe-Sugano diagram of the 3dn configuration of Mn4+ ions. The 3d–3d electronic transition of Mn4+ ions was significantly affected by the external environment. 2Eg and 2T1, 2T2, 2A2 levels belonged to the t23 electron orbit, whereas 4T1 and 4T2 levels were located in the t22e electron orbit. Therefore, the 4A2→4T2 electron transition produced large transverse shifts. However, the electron transition between 4A2 and 2Eg produced relatively small lateral displacement due to the same electron orbit. The large transverse displacement implied a strong interaction between electrons and phonons, which resulted in broadening of the optical band. According to the spin selection law, spin allows ground state 4A2 energy level transition to the high energy state 4T1 or 4 T2 energy level. Therefore, strong excitation peaks of 4A2→4T1 and 4 A2→4T2 electron transitions of Mn4+ are broadbands. When electrons are excited to high-energy states 4T1 and 4T2, no radiative relaxation to the lower energy states 2Eg usually occurs, causing further spinning of the forbidden 2Eg–4A2 electron transitions. Therefore, the emission spectrum of Mn4+ in the matrix is generally a broadband [23].
Fig. 7(a) shows the emission spectrum of BMG:xMn4+ phosphors (x = 0.002, 0.004, 0.006, 0.008, 0.01, 0.013, 0.016, 0.02, 0.025) phosphor under 289 nm near-UV excitation. Two shrill emission peaks were observed in the range of 560–750 nm and were specifically located at 632 and 660 nm. As we know, the red light located at about 650 nm is suitable for human eye sensitivity. The emission peaks located at 632 and 660 nm are close to 650 nm, therefore, in terms of the wavelength of the emission peak, BMG:xMn4+ phosphor is suitable for W-LEDs [24]. Between x = 0 and x = 0.013, luminescence intensity of BMG:xMn4+ phosphors increased with increasing concentration of doped Mn4+ ions. When x = 0.013, luminous intensity reached the maximum value, and intensity decreased with increasing Mn4+ concentration due to concentration quenching. Occurrence of resonancetype energy transfer was attributed to the following aspects: multipolar interaction and exchange interaction [25]. To determine whether the primary mechanism for concentration quenching of BMG:xMn4+ phosphor is multipolar interaction or exchange interaction, critical distance (Rc) should be calculated. Rc can be calculated by the following 422
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Fig. 5. Excitation (λem = 660 nm) and emission (λex = 289 nm) spectra of BMG:0.013Mn4+ phosphors.
Fig. 7. (a) Emission spectra of BMG:xMn4+ (x = 0.002, 0.004, 0.006, 0.008,0.01, 0.013, 0.016, 0.02) phosphors. (b) Fitting curves between lg(x) and lg(I/x).
Fig. 6. Tanabe–Sugano diagram of Mn4+ ions.
equation [26]:
R c = 2[3V /(4πCN )]1/3 ,
(1)
where V is the volume of host lattice, C corresponds to the concentration of Mn4+ at which emission intensity reaches the maximum value, and N represents the number of cations in the unit cell. According to JCPDS 37-1478, V and N are equal to 389.4 Å3 and 2, respectively. C can be determined by emission spectrum, which is 0.013. Thus, Rc is 30.58 Å. Rc between the activator and sensitizer totals < 5 Å when exchange interaction is the primary mechanism. Therefore, multipolar interaction was identified as the major mechanism for fluorescence quenching. According to Dexter’s energy transfer formula of multipolar interaction, the following formula can be used to confirm the type of interaction mechanism among Mn4+ ions [27]:
I / χ = K [1 + β (χ )θ/3]−1 ,
(2) 4+
phoswhere I refers to the luminescence intensity of BMG:0.013Mn phors, and x is doping Mn4+ ion concentration. θ = 6 corresponds to dipole–dipole interaction, and θ = 8 and θ = 10 correspond to dipole–quadrupole interaction and quadrupole–quadrupole interaction, respectively [28]. Fig. 7(b) shows the linear fitting curve of the relationship between lg(I/x) and lg(x). The slope of the fitted line was
Fig. 8. Decay curve of BMG:xMn4+ (x = 0.002, 0.006, 0.01, 0.016, 0.02).
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–1.89, that is, –θ/3 = –1.89, and θ = 5.67. The value of θ was close to 6. Therefore, the type of interaction mechanism between Mn4+ ions can be confirmed as a dipole–dipole interaction. Light emission by Mn4+ is due to the 3d–3d electron transition, and its lifetime usually lasts for millisecond. To study luminescence dynamics of Mn4+ in the BMG matrix, fluorescence decay curves of Mn4+ phosphors with different concentrations were measured, and results are shown in Fig. 8. Fitting the measured attenuation curves, all curves are well fitted with the following formula [29]:
I = A1 exp(−t / τ1) + A2 exp(−t / τ2).
and temperature. Luminous intensity decreased with increasing temperature due to thermal quenching. To explain the phenomenon of thermal quenching, activation energy Ea must be calculated. Ea can be calculated using the Arrhenius equation [34]:
I=
I E ln ⎛ 0 − 1⎞ = ln A − a kT ⎝I ⎠
(3)
(4)
(5)
where WOR is the probability of radiative transition, and WNR is the probability of nonradiative transition. In general, WOR is not affected by external factors, but WNR is. Excessive amounts of Mn4+ led to adsorption of Mn4+ on the surface, causing defects on the phosphor surface and thereby increasing nonradiative transition channels. Therefore, when high concentration of Mn4+ was doped on the matrix, a part of Mn4+ can be found in the nominative case and the light, whereas other parts of Mn4+ may be adsorbed on the surface of phosphor powders. These phenomena cause fluorescence lifetime to decay in a double exponential manner [30,31]. The quantum efficiencies of BMG:Mn4+ phosphor was calculated by using the method described by de Palsson [32] and Mello [33]. The following equation should be used:
ηQE =
∫ LS ∫ ER − ∫ ES
(8)
where I0 is the luminous intensity initial temperature; I stands for the luminous intensity at varying temperatures; A and k are constants; T is varying temperatures. The corresponding values were calculated and fitted, as shown in Fig. 10(b). The slope of the fitted line was 0.337, that is, Ea = 0.337 eV. The value of Ea was higher than that of commercial red phosphor CaAlSiN3:Eu2+ (~ 0.2 eV). To explain the influence of temperature on fluorescence intensity, a schematic diagram of the energy of the coordinate of the Mn4+ cluster ions was designed [Fig. 8(c)]. When the energy at 289 nm was absorbed, electrons migrated from the O point of the ground state to 4T2g and 4T1g. For instance, when electrons reached the excited state 4T1g for electron transition to A point, they transitioned to the bottom of the excited state 2 Eg along the green arrow. When electrons reached C, they continually jumped backed to the bottom of the ground state 2Eg; however, the transition process involves two different paths [35]. In normal circumstances, electrons move back along path 1 to O point. However, thermal excitation can cause electrons to shift in equilibrium state C in accordance with path 2. When temperature increases, more electrons at the C point arrive at the D point in accordance with path 2 and then return from the D point to the ground state 4A2g, in which path 2 accommodates nonradiative transition that leads to the decrease in luminous intensity [36]. This phenomenon explains thermal quenching. Using the same method can also explain the emission spectra of samples, which constantly exhibited the same shape at different excitation wavelengths. The intersections (e.g., D points) in the configuration graph are independent of energy excitation. Fig. 11 shows color coordinates of BMG:Mn4+ phosphors. Color coordinates of the prepared BMG:Mn4+ phosphor lie in the deep red region (0.721, 0.279). Compared with some common red phosphors, K2TiF6:Mn4+ (0.73, 0.26) is closer to the standard values of the National TV Standards Committee. Therefore, BGM:Mn4+ phosphors are suitable for warm W-LED in color coordinates.
The calculated results shown in Fig. 8 meet the requirements of WLED phosphors for fluorescence lifetime. Fluorescence lifetime can also be calculated using the following formula:
τ = 1/(WOR + WNR (T ))
(7)
Eq. (6) can be rearranged as follows:
Fluorescence decays in a double exponential manner, and the following formula can be used to calculate the fluorescence lifetime:
τs = (A1 τ12 + A2 τ22)/(A1 τ1 + A2 τ2).
I0 1 + Ae(−Ea/ kT )
(6)
where LS means the emission spectra of samples, ER is the excitation light with integrating sphere, and ES is the excitation light without integrating sphere. As shown in Fig. 9, the QE of BMG:0.013Mn4+ phosphor was measured and calculated to be 14.28%. Fig. 10(a) shows emission spectra (λex = 289 nm) of BMG:0.013Mn4+ phosphors at different temperatures (273–498 K). The illustration shows the relationship between the value of emission peak
4. Conclusion A novel double perovskite BMG:Mn4+ phosphor was successfully synthesized by conventional solid-state reaction. 4A2g→4T1g, 4 A1g→4T2g, and 4A2g→2T2g transitions of Mn4+ ions resulted in three absorption bands centered at 305, 330, and 420 nm, respectively. This result indicates good absorption property of the phosphor near-UV light. The phosphor showed two emission peaks at 632 and 660 nm. The emission peaks were caused by the 2E→4A2 transition of Mn4+ ions. Luminescence intensity was the strongest when Mn4+ doping concentration was 0.013. Rc between the activator and sensitizer totaled > 5 Å, and multipolar interaction was the major mechanism for fluorescence quenching. Dipole–dipole interaction was the type of interaction mechanism among Mn4+ ions. Fluorescence lifetime decayed in a double exponential manner. Luminous intensity of phosphors decreased with increasing temperature. Ea = 0.337 eV, which was higher than that of commercial red phosphor CaAlSiN3:Eu2+ (~ 0.2 eV). The color coordinates of BMG:Mn4+ phosphors (0.721, 0.279) were close to the standard values of the National TV Standards Committee. All results indicate that BMG:Mn4+ phosphor is a potential red phosphor for WLED applications.
Fig. 9. Excitation line of the background and emission spectra of BMG:0.013Mn4+ phosphor collected using an integrating sphere. Inset shows a magnification of the emission spectra from 600 nm to 700 nm. 424
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Fig. 10. (a) Temperature-dependent PL spectra of BMG:0.013Mn4+ phosphor (b) Fitting graph of ln(I0/I-1) and 1/KBT (c) The configuration coordinate of Mn4+.
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Fig. 11. CIE chromaticity coordinates of BMG:Mn4+ phosphor.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and luminescence properties of double perovskite Ba2MgGe2O7:Mn4+ deep red phosphor Zuizhi Lu, Anjie Fu, Fangfang Gao, Xiaoshan Zhang, Liya Zhou
T
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School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Deep red-emitting Mn4+ Phosphors Fluorescence lifetime
A deep red-emitting Ba2MgGe2O7:Mn4+ (BMG:Mn4+) phosphor was synthesized by conventional solid-state reaction. 4A2g→4T1g, 4A1g→4T2g, and 4A2g→2T2g transitions of Mn4+ ions resulted in three absorption bands centered at 305, 330, and 420 nm, respectively. The phosphor showed a broad emission band from 600 nm to 700 nm due to 2E→4A2 transition of Mn4+ ions. The highest luminous peak located at 660 nm, and the highest luminous intensity was achieved at high Mn4+ concentration (0.013). Multipolar interaction was the major mechanism for fluorescence quenching, and dipole–dipole interaction was the type of interaction mechanism between Mn4+ ions. Fluorescence lifetimes decreased from 0.898 ms to 0.746 ms, which accorded with the requirement of white light-emitting diode phosphor. Color coordinates of BMG:Mn4+ phosphors (0.721, 0.279) were close to the standard values of the National TV Standards Committee.
1. Introduction
exposed and is susceptible to the influence of crystal field and coordination environment. The d–d electron transition shows broadband absorption in the UV and near-UV regions and features a strong peak emission in the blue and red regions [14] Mn4+-doped red phosphor shows characteristics of good stability, high color purity, and low decline. Mixing with YAG: Ce3+ yellow phosphor can be hopefully applied to production of W-LEDs. Double perovskite Ba2MgGe2O7 (BMG) features the advantages of good stability and easy synthesis. The mass ratio of Ge in BMG is far less than those of phosphors such as SrGe4O9 [15] and A2Ge4O9 (A = K, Rb) [16]. In the past, a variety of phosphors based on BMG have been developed (e.g., Ba2MgGe2O7:Nb3+ [17] and Ba2MgGe2O7:Cr4+ [18]. However, Nb and Cr are expensive; thus, Ba2MgGe2O7:Nb3+ and Ba2MgGe2O7:Cr4+ phosphors have not been widely used in W-LEDs. For Mn4+-doped BMG:Mn4+ phosphor, no previous studies have been reported. Therefore, studying BMG phosphor is necessary. In this paper, BMG:xMn4+ (x = 0.002, 0.004, 0.006, 0.008, 0.01, 0.013, 0.016, 0.02, 0.025) phosphors were synthesized by conventional solid-state reaction (Mixing the raw materials by mole ratio, and the mixture is calcinated directly without adding any substance, then the samples are obtained). Crystal structure, morphology, and luminescent properties of samples were tested and analyzed.
In recent years, white light-emitting diodes (LEDs) have received considerable attention because of their high efficiency, environmental friendliness, and long service lives [1–3]. At present, white LEDs mainly comprise blue light chip and YAG: Ce3+ yellow phosphor [4,5]. However, this kind of white light due to lack of red components caused by color and high-color-temperature low shortcomings develop gradually, thus hindering the development of warm white LEDs (color temperature of 4000–8000 K) and restricting their application in indoor lighting. Therefore, developing red phosphors with near- ultraviolet (UV) excitation is urgent [6,7]. The red phosphor used in white light (W-LEDs) plays an important role in modulating color temperature of white light and improving color rendering [8]. Luminescent properties of phosphors directly influence their practical value. Therefore, the search for appropriate matrix composition and activators is a key factor in the synthesis of LEDs using red fluorescent materials [9,10]. At present, rare-earth iondoped red phosphors (e.g., Eu3+ and Sm3+ ions) are limited in the visible light region due to the narrow band of emission spectrum, thus limiting their application in W-LEDs [11]. In recent years, Mn4+-doped fluoride red phosphor has become a new research focus because of its characteristic red light narrow band emission, UV and blue light excitation, and cheapness as raw material. In addition, rare-earth europium resources are scarce and expensive, which cause some difficulties in industrial production [12,13]. The outermost orbital of Mn4+ 3d is
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Corresponding author. E-mail address: [email protected] (L. Zhou).
https://doi.org/10.1016/j.jlumin.2018.06.061 Received 4 November 2017; Received in revised form 17 June 2018; Accepted 19 June 2018 Available online 20 June 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
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2. Experiments
in host BMG lattice [19]. The patterns of the 28–30° range is amplified and displayed in Fig. 1(b). With the increase of the concentration of doped Mn4+, the position of the diffraction peak remains unchanged. According to Bragg’s law, 2dsinθ = nλ, Mn4+ replaced an ion close to its ionic radius, that is, Ge4+. Mn4+ (r = 0.53 Å) replaced an ion with a radius equal to it, that is, Ge4+ (r = 0.53 Å). Fig. 2(a) shows the scanning electron microscopy image of the obtained BMG:xMn4+ (x = 0.013) powder. The phosphor exhibited a flake structure morphology, and particle size of BMG:Mn4+ powder reached 3–5 µm. The phosphor also presented a regular appearance and no clear reunion phenomenon. A bright red light was emitted under the excitation of UV light at 365 nm [Fig. 2(b)]. The samples were homogeneous white powders under natural light [Fig. 2(c)]. The X-ray photoelectron spectroscopy (XPS) survey spectra of BMG:0.008Mn4+ phosphor is shown in Fig. 3, which can obtain the chemical compositions of manganese and the valence state in the samples. As shown in the figure, the signals of barium (Ba), magnesium (Mg), germanium (Ge), and oxygen (O) species were clearly detected. The signals of carbon (C) species are mainly due to the adsorption of CO2. The signal from manganese (Mn) is weakly recorded because of its lower concentration relative to that of the other elements. Fig. 4 displays the UV–vis absorption spectra of BMG:xMn4+ (x = 0, 0.006, 0.01, 0.02). The phosphors manifested a broad absorption band in the range of 250–500 nm. Absorption intensity decreased rapidly in the range of 250–290 nm due to charge transfer (CT) band transitions of Mn4+→O2+. Given the 4A2g→4T1g, 4A1g→4T2g, and 4A2g→2T2g transitions of Mn4+ ions, three absorption bands centered at 305, 330, and 420 nm, respectively. These absorption bands all conformed to absorption characteristics of Mn4+ [20]. Fig. 5 shows the excitation spectrum of BMG:0.013Mn4+ phosphor at 660 nm monitoring wavelength and emission spectra under an excitation of 289 nm excitation wavelength. Four Gaussian-deconvoluted peaks centered at ~ 260, ~ 302, ~ 324, and ~ 420 nm in the excitation spectrum. The 260 nm peak was due to CT band transitions of Mn4+→ O2+. The other three Gaussian-deconvoluted peaks centered at ~ 260, ~ 302, ~ 324, and ~ 420 nm were due to the 4A2g→4T1g, 4A1g→4T2g, and 4A2g→2T2g transitions of Mn4+ ions, respectively. This result agrees with the findings of UV absorption spectra. The emission spectrum consisted of two distinct emission peaks. The highest emission peak located at 660 nm was attributed to 2Eg→A2g transition of Mn4+ [21]. These results showed that BMG:Mn4+ phosphors can be excited by near-UV or UV light and emit red light. Therefore, in terms of luminescence properties, BMG:Mn4+ phosphor is an excellent potential red phosphor used in W-LEDs. The W-LEDs can be made by combination of red phosphors, green phosphors, and blue phosphors [22]. The influence of crystal field on 3dn configuration energy level can
2.1. Materials and synthesis A series of BMG:xMn4+ (x = 0.002, 0.004, 0.006, 0.008, 0.01, 0.013, 0.016, 0.02) phosphors were prepared by high-temperature solid-state reaction. The raw materials included BaCO3 (A.R. 99.5%), MgO (A.R. 98.5%), GeO2 (99.99%), and MnCO3 (A.R. 99.5%). Samples were weighed at a corresponding molar ratio and fully ground in a mortar. Then, the samples were preheated for 4 h at 873 K. After preheating, the samples were cooled down to room temperature and ground again. Subsequently, the samples were heated at 1273 K for 6 h under air atmosphere. Finally, the prepared phosphors were cooled to room temperature, and subsequent measurements were performed. 2.2. Sample characterization Ground samples were characterized by powder X-ray diffraction (XRD) analysis with a RIGAKU D/max 2200 vpc X-ray diffractometer. Step-scan was conducted over an angle range of 10–80° with a step size of 0.02. The X-ray photoelectron spectroscopy (XPS) survey spectra was recorded by a Synchro thermal analyzer (Netzsh, STA 449 F5). Morphologies of the as-synthesized products were investigated by electron microscopy (Hitachi, S-3400 N). UV–vis absorption spectra of the powders were recorded using a Cary 5000 UV–vis spectrophotometer. Photoluminescence (PL) excitation and spectra were recorded by Hitachi-2500 fluorescence spectrometer at 150 W excitation using a xenon lamp as excitation source. Excitation slit and emission slit were 5.0 nm, and the scan speed was 300 nm/min. An FLS920 was used to record luminescence decay curve. The quantum efficiency (QE) measurements was carried out by a barium sulfate coated integrating sphere. Excitation and emission wavelengths measured 289 and 660 nm, respectively. An FLS-980 Edinburgh fluorescence spectrometer was used to measure temperature-dependent spectra. All measurements were carried out at room temperature. 3. Results and discussions Fig. 1(a) shows the XRD spectra of phosphors with different Mn4+ doping concentrations. Diffraction peaks of all samples matched those reported for the PDF (JCPDS 37-1478). No obvious diffraction peaks for detectable impurity phases were observed in the samples, demonstrating that samples pure, and Mn4+ ions were well-doped into the host with no significant influence on the structure of BMG. According to similar ionic radius principle (Ba2+:1.35 Å, Mg2+:0.72 Å, Ge4+:0.53 Å, and Mn4+:0.53 Å), Mn4+ ions may replace mainly the Ge4+ ions sites
Fig. 1. XRD spectra of BMG:xMn4+ (x = 0.002, 0.008, 0.013, 0.02) phosphor and the PDF (JCPDS 37-1478) standard pattern. 421
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Fig. 2. (a) SEM image of BMG:0.013Mn4+ (b) BMG:Mn4+ phosphors under ultraviolet lamp (c) BMG:Mn4+ phosphors under daylight.
Fig. 3. The XPS) survey spectra of BMG:0.008Mn4+ phosphor.
Fig. 4. UV–vis absorption spectra of BMG:xMn4+ (x = 0, 0.006, 0.01, 0.02).
be explained by the diagram of Tanabe-Sugano. Fig. 6 illustrates the Tanabe-Sugano diagram of the 3dn configuration of Mn4+ ions. The 3d–3d electronic transition of Mn4+ ions was significantly affected by the external environment. 2Eg and 2T1, 2T2, 2A2 levels belonged to the t23 electron orbit, whereas 4T1 and 4T2 levels were located in the t22e electron orbit. Therefore, the 4A2→4T2 electron transition produced large transverse shifts. However, the electron transition between 4A2 and 2Eg produced relatively small lateral displacement due to the same electron orbit. The large transverse displacement implied a strong interaction between electrons and phonons, which resulted in broadening of the optical band. According to the spin selection law, spin allows ground state 4A2 energy level transition to the high energy state 4T1 or 4 T2 energy level. Therefore, strong excitation peaks of 4A2→4T1 and 4 A2→4T2 electron transitions of Mn4+ are broadbands. When electrons are excited to high-energy states 4T1 and 4T2, no radiative relaxation to the lower energy states 2Eg usually occurs, causing further spinning of the forbidden 2Eg–4A2 electron transitions. Therefore, the emission spectrum of Mn4+ in the matrix is generally a broadband [23].
Fig. 7(a) shows the emission spectrum of BMG:xMn4+ phosphors (x = 0.002, 0.004, 0.006, 0.008, 0.01, 0.013, 0.016, 0.02, 0.025) phosphor under 289 nm near-UV excitation. Two shrill emission peaks were observed in the range of 560–750 nm and were specifically located at 632 and 660 nm. As we know, the red light located at about 650 nm is suitable for human eye sensitivity. The emission peaks located at 632 and 660 nm are close to 650 nm, therefore, in terms of the wavelength of the emission peak, BMG:xMn4+ phosphor is suitable for W-LEDs [24]. Between x = 0 and x = 0.013, luminescence intensity of BMG:xMn4+ phosphors increased with increasing concentration of doped Mn4+ ions. When x = 0.013, luminous intensity reached the maximum value, and intensity decreased with increasing Mn4+ concentration due to concentration quenching. Occurrence of resonancetype energy transfer was attributed to the following aspects: multipolar interaction and exchange interaction [25]. To determine whether the primary mechanism for concentration quenching of BMG:xMn4+ phosphor is multipolar interaction or exchange interaction, critical distance (Rc) should be calculated. Rc can be calculated by the following 422
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Fig. 5. Excitation (λem = 660 nm) and emission (λex = 289 nm) spectra of BMG:0.013Mn4+ phosphors.
Fig. 7. (a) Emission spectra of BMG:xMn4+ (x = 0.002, 0.004, 0.006, 0.008,0.01, 0.013, 0.016, 0.02) phosphors. (b) Fitting curves between lg(x) and lg(I/x).
Fig. 6. Tanabe–Sugano diagram of Mn4+ ions.
equation [26]:
R c = 2[3V /(4πCN )]1/3 ,
(1)
where V is the volume of host lattice, C corresponds to the concentration of Mn4+ at which emission intensity reaches the maximum value, and N represents the number of cations in the unit cell. According to JCPDS 37-1478, V and N are equal to 389.4 Å3 and 2, respectively. C can be determined by emission spectrum, which is 0.013. Thus, Rc is 30.58 Å. Rc between the activator and sensitizer totals < 5 Å when exchange interaction is the primary mechanism. Therefore, multipolar interaction was identified as the major mechanism for fluorescence quenching. According to Dexter’s energy transfer formula of multipolar interaction, the following formula can be used to confirm the type of interaction mechanism among Mn4+ ions [27]:
I / χ = K [1 + β (χ )θ/3]−1 ,
(2) 4+
phoswhere I refers to the luminescence intensity of BMG:0.013Mn phors, and x is doping Mn4+ ion concentration. θ = 6 corresponds to dipole–dipole interaction, and θ = 8 and θ = 10 correspond to dipole–quadrupole interaction and quadrupole–quadrupole interaction, respectively [28]. Fig. 7(b) shows the linear fitting curve of the relationship between lg(I/x) and lg(x). The slope of the fitted line was
Fig. 8. Decay curve of BMG:xMn4+ (x = 0.002, 0.006, 0.01, 0.016, 0.02).
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–1.89, that is, –θ/3 = –1.89, and θ = 5.67. The value of θ was close to 6. Therefore, the type of interaction mechanism between Mn4+ ions can be confirmed as a dipole–dipole interaction. Light emission by Mn4+ is due to the 3d–3d electron transition, and its lifetime usually lasts for millisecond. To study luminescence dynamics of Mn4+ in the BMG matrix, fluorescence decay curves of Mn4+ phosphors with different concentrations were measured, and results are shown in Fig. 8. Fitting the measured attenuation curves, all curves are well fitted with the following formula [29]:
I = A1 exp(−t / τ1) + A2 exp(−t / τ2).
and temperature. Luminous intensity decreased with increasing temperature due to thermal quenching. To explain the phenomenon of thermal quenching, activation energy Ea must be calculated. Ea can be calculated using the Arrhenius equation [34]:
I=
I E ln ⎛ 0 − 1⎞ = ln A − a kT ⎝I ⎠
(3)
(4)
(5)
where WOR is the probability of radiative transition, and WNR is the probability of nonradiative transition. In general, WOR is not affected by external factors, but WNR is. Excessive amounts of Mn4+ led to adsorption of Mn4+ on the surface, causing defects on the phosphor surface and thereby increasing nonradiative transition channels. Therefore, when high concentration of Mn4+ was doped on the matrix, a part of Mn4+ can be found in the nominative case and the light, whereas other parts of Mn4+ may be adsorbed on the surface of phosphor powders. These phenomena cause fluorescence lifetime to decay in a double exponential manner [30,31]. The quantum efficiencies of BMG:Mn4+ phosphor was calculated by using the method described by de Palsson [32] and Mello [33]. The following equation should be used:
ηQE =
∫ LS ∫ ER − ∫ ES
(8)
where I0 is the luminous intensity initial temperature; I stands for the luminous intensity at varying temperatures; A and k are constants; T is varying temperatures. The corresponding values were calculated and fitted, as shown in Fig. 10(b). The slope of the fitted line was 0.337, that is, Ea = 0.337 eV. The value of Ea was higher than that of commercial red phosphor CaAlSiN3:Eu2+ (~ 0.2 eV). To explain the influence of temperature on fluorescence intensity, a schematic diagram of the energy of the coordinate of the Mn4+ cluster ions was designed [Fig. 8(c)]. When the energy at 289 nm was absorbed, electrons migrated from the O point of the ground state to 4T2g and 4T1g. For instance, when electrons reached the excited state 4T1g for electron transition to A point, they transitioned to the bottom of the excited state 2 Eg along the green arrow. When electrons reached C, they continually jumped backed to the bottom of the ground state 2Eg; however, the transition process involves two different paths [35]. In normal circumstances, electrons move back along path 1 to O point. However, thermal excitation can cause electrons to shift in equilibrium state C in accordance with path 2. When temperature increases, more electrons at the C point arrive at the D point in accordance with path 2 and then return from the D point to the ground state 4A2g, in which path 2 accommodates nonradiative transition that leads to the decrease in luminous intensity [36]. This phenomenon explains thermal quenching. Using the same method can also explain the emission spectra of samples, which constantly exhibited the same shape at different excitation wavelengths. The intersections (e.g., D points) in the configuration graph are independent of energy excitation. Fig. 11 shows color coordinates of BMG:Mn4+ phosphors. Color coordinates of the prepared BMG:Mn4+ phosphor lie in the deep red region (0.721, 0.279). Compared with some common red phosphors, K2TiF6:Mn4+ (0.73, 0.26) is closer to the standard values of the National TV Standards Committee. Therefore, BGM:Mn4+ phosphors are suitable for warm W-LED in color coordinates.
The calculated results shown in Fig. 8 meet the requirements of WLED phosphors for fluorescence lifetime. Fluorescence lifetime can also be calculated using the following formula:
τ = 1/(WOR + WNR (T ))
(7)
Eq. (6) can be rearranged as follows:
Fluorescence decays in a double exponential manner, and the following formula can be used to calculate the fluorescence lifetime:
τs = (A1 τ12 + A2 τ22)/(A1 τ1 + A2 τ2).
I0 1 + Ae(−Ea/ kT )
(6)
where LS means the emission spectra of samples, ER is the excitation light with integrating sphere, and ES is the excitation light without integrating sphere. As shown in Fig. 9, the QE of BMG:0.013Mn4+ phosphor was measured and calculated to be 14.28%. Fig. 10(a) shows emission spectra (λex = 289 nm) of BMG:0.013Mn4+ phosphors at different temperatures (273–498 K). The illustration shows the relationship between the value of emission peak
4. Conclusion A novel double perovskite BMG:Mn4+ phosphor was successfully synthesized by conventional solid-state reaction. 4A2g→4T1g, 4 A1g→4T2g, and 4A2g→2T2g transitions of Mn4+ ions resulted in three absorption bands centered at 305, 330, and 420 nm, respectively. This result indicates good absorption property of the phosphor near-UV light. The phosphor showed two emission peaks at 632 and 660 nm. The emission peaks were caused by the 2E→4A2 transition of Mn4+ ions. Luminescence intensity was the strongest when Mn4+ doping concentration was 0.013. Rc between the activator and sensitizer totaled > 5 Å, and multipolar interaction was the major mechanism for fluorescence quenching. Dipole–dipole interaction was the type of interaction mechanism among Mn4+ ions. Fluorescence lifetime decayed in a double exponential manner. Luminous intensity of phosphors decreased with increasing temperature. Ea = 0.337 eV, which was higher than that of commercial red phosphor CaAlSiN3:Eu2+ (~ 0.2 eV). The color coordinates of BMG:Mn4+ phosphors (0.721, 0.279) were close to the standard values of the National TV Standards Committee. All results indicate that BMG:Mn4+ phosphor is a potential red phosphor for WLED applications.
Fig. 9. Excitation line of the background and emission spectra of BMG:0.013Mn4+ phosphor collected using an integrating sphere. Inset shows a magnification of the emission spectra from 600 nm to 700 nm. 424
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Fig. 10. (a) Temperature-dependent PL spectra of BMG:0.013Mn4+ phosphor (b) Fitting graph of ln(I0/I-1) and 1/KBT (c) The configuration coordinate of Mn4+.
Acknowledgment This work was financially supported by grants from the Science Foundation of Guangxi Province (No. 2016GXNSFDA380036); the National Natural Science Foundation of China (No. 61664002). References [1] Z.W. Zhang, D.Q. Ma, Y. Yue, M.Z. Ma, R.P. Liu, Wide-band excited LaBMoO6:Eu3+, red phosphor for white-light-emitting diode, J. Alloy. Compd. 636 (2015) 113–116. [2] D. Qin, W.J. Tang, Crystal structure, tunable luminescence and energy transfer properties of Na3La(PO4)2:Tb3+,Eu3+ phosphors, RSC Adv. 7 (2017) 2494–2502. [3] C.F. Guo, W. Zhang, L. Luan, T. Chen, H. Cheng, D.X. Huang, A promising redemitting phosphor for white light emitting diodes prepared by sol–gel method, Sens. Actuators B-Chem. 133 (2008) 33–39. [4] I. Ahemen, D.K. De, A.N. Amah, A review of solid state white light emitting diode and its potentials for replacing conventional lighting technologies in developing countries, Appl. Phys. Res. 6 (2014) 1188–1194. [5] C.C. Lin, R.S. Liu, Advances in phosphors for light-emitting diodes, J. Phys. Chem. Lett. 2 (2011) 1268–1277. [6] C.J. Humphreys, Solid-state lighting, MRS Bull. 33 (2008) 459–470. [7] X.G. Zhang, C.Y. Zhou, J.H. Song, L.Y. Zhou, M.L. Gong, High-brightness and thermal stable Sr3La(PO4)3:Eu3+, red phosphor for NUV light-emitting diodes, J. Alloy. Compd. 592 (2014) 283–287. [8] J.H. Li, J.Y. Yan, D.W. Wen, U.K. Wasim, J.X. Shi, M.M. Wu, Q. Su, A.T. Peter, Advanced red phosphors for white light-emitting diodes, J. Mater. Chem. C 4 (2006) 8611–8623. [9] S. Ye, Z.S. Liu, J.G. Wang, X.P. Jing, Luminescent properties of Sr2P2O7: Eu,Mn phosphor under near UV excitation, Mater. Res. Bull. 43 (2008) 1057–1065.
Fig. 11. CIE chromaticity coordinates of BMG:Mn4+ phosphor.
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