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Red-green-blue-tunable emission from Eu3+ and Tb3+ codoped pyrophosphate phosphors
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Red-green-blue-tunable emission from Eu3+ and Tb3+ codoped pyrophosphate phosphors J. Zhanga, Z.Y. Maa, J.G. Guob, G.M. Caia,c,∗, L. Mac, X.J. Wangc,∗∗ a
School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, PR China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, PR China c Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA b
A B S T R A C T
Color-tunable phosphors based on a single host are superior to traditional mixed primary-color phosphors in fluorescent lamps, due to the better consistency of different color components in service. In this work, a series of Eu3+/Tb3+ doped MgIn2P4O14 phosphors were synthesized by a conventional solid state reaction method. The phosphors exhibit strong absorption around 254 nm with negligible absorption in the visible light region. Under the joint action of the co-activation, cross-relaxation, sensitization, and phonon-assistance, color-tunable reddish-orange-yellow-green-blue emission is achieved in MgIn2P4O14: Eu, Tb phosphors by adjusting the doping content, which demonstrates great potential for the application in fluorescent lamps.
1. Introduction Three band fluorescent lamps are widely applied in household lighting because the three primary color components (red, green, and blue) of luminescence provide a high color rendering index (CRI). The stable performance of a fluorescent lamp requires a good match of the phosphors in some of their properties such as temperature dependence of emission and phosphor lifespan [1–3]. Since those properties are usually reliant on the structures of host compounds, it is a desirable way to integrate the three primary color components into a same host compound [4,5]. Therefore, color-tunable phosphors based on a single host have gained much attention in the past years [6–9]. Eu3+ and Tb3+ are prominent rare earth activators often jointly used in luminescent materials, due to their strong characteristic f-f emissions and the efficient energy transfer (ET) from Tb3+ to Eu3+10−11. The 5D0 → 7Fj transitions of Eu3+ usually generate emissions within the wavelength range of 580–700 nm, especially at 590 nm and 610 nm. As a result, Eu3+ is a commonly used red luminescent center [10–12]. Tb3+ serves as a green center because its dominant 5D4 → 7Fj transitions are normally located around 540 nm [13]. Meanwhile, the 5D3 → 7Fj transitions of Tb3+ in the blue region (370–450 nm) are generally quenched through the cross-relaxation between neighboring Tb3+ ions [1,14]. However, the cross-relaxation process, especially at low doping concentration, was found to be weak in some hosts, where the 5D3 → 7Fj blue emissions of Tb3+ may be strong, such as in Zn (PO3)2: Tb3+ [15], La2O3: Tb3+ [16], Y2O2S: Tb3+ [17], Ba3LaK
∗
(PO4)3F: Tb3+ [18] and Sr2MgSi2O7: Tb3+ [19]. In those cases, a supplementary, even major blue component could be added into the Tb3+ doped phosphors. In our previous reports, the monoclinic magnesium indium phosphate MgIn2P4O14 (S G.: C2/c, No.15) with a novel layered structure was found, and the Tm3+, Dy3+ or Ce3+ ions doped MgIn2P4O14 phosphors showed outstanding luminescent characteristics [20,21]. Recently, we found that 5D3→ 7Fj transitions of Tb3+ could produce blue emissions in MgIn2P4O14 host at low Tb3+ concentrations under UV excitation. This inspires us to fabricate three-primary-color phosphors for fluorescent lamps with MgIn2P4O14 host. In the present work, we focus on the Tb3+ activated luminescence based on the host and also investigate the simultaneous double activation of Eu3+ and Tb3+ in this compound. We have obtained multicolor emissions over red-yellowgreen-blue region by adjusting the doping concentrations of Eu3+ and Tb3+ in the MgIn2P4O14: Eu3+, Tb3+ phosphors. Energy transfer, absorption competition, and cross relaxation play the roles for the tunable emissions. 2. Experimental 2.1. Sample preparations Conventional high-temperature solid-state reaction method was adopted for the syntheses of a series of Eu3+/Tb3+ single- and co-doped Mg(In1-x-yEuxTby)2P4O14 (MgIn2P4O14: Eux, Tby, x = 0–0.07,
Corresponding author. School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, PR China. Corresponding author. E-mail addresses: [email protected] (G.M. Cai), [email protected] (X.J. Wang).
∗∗
https://doi.org/10.1016/j.jlumin.2019.116732 Received 29 June 2019; Received in revised form 11 July 2019; Accepted 4 September 2019 0022-2313/ © 2019 Published by Elsevier B.V.
Please cite this article as: J. Zhang, et al., Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.116732
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In order to achieve higher service efficiency, luminescent devices usually require phosphors to have strong absorption in the ultraviolet region to ensure that they can be effectively stimulated; secondly, phosphors should have high transmittance in the visible light to prevent the energy loss caused by the re-absorption of light emitted by themselves. From the UV–Vis diffuse reflection spectra in Fig. 3, it can be seen that the reflectance curves of all the doped and undoped samples present approximate horizontally with an R value of ~90% in the visible-near infrared area. This deviation from complete transmission is caused by limited test conditions (incomplete transmittance of quartz slice). The small peaks at around 390 nm and 465 nm for the MgIn2P4O14: Eu curve belong to the characteristic f-f transition 7F0→5L6 and 7F0→5D2, respectively. By applying formula A = log(1/R), in which A refers to absorbance and R reflectance, absorbance of those samples can be obtained [22] and shown as the inset of Fig. 3. Clearly, after Eu3+/Tb3+ doping, the absorbance of MgIn2P4O14 before 400 nm is significantly enhanced, which is advantageous to the luminescence properties.
y = 0.005–0.11). Firstly, stoichiometric amount of starting materials MgO (Aladdin, 99.99%), In2O3 (Aladdin, 99.99%), NH4H2PO4 (Xilong Scientific Co., LTD., 99.5%), Eu2O3 (Aladdin, 99.99%) and Tb4O7 (Aladdin, 99.99%) were ground in an agate mortar for 30 min to form homogeneous mixtures. Then, the mixtures were heated in corundum crucibles at 600 °C for 12 h to get the precursors. Subsequently, the precursors underwent two 24-hour calcinations at 1000 °C after being ground for each time. Finally, the products were gradually cooled to room temperature in the furnace and were ground again into homogeneous powders for the following analyses. 2.2. Characterization X-ray Rigaku diffractometer D/MAX-2500 were used to record powder XRD patterns. The field-emission scanning electron microscope (FE-SEM, Nova Nano SEM 230, FEI, Czech Republic), equipped with an energy dispersive spectroscopy (EDS), was used to observe the morphology and conduct elemental analysis. UV–Vis diffuse reflectance (UV-DR) spectra were recorded on a Shimadzu UV-2600 spectrophotometer. The regular photoluminescence excitation and emission (PLE/PL) spectra were obtained on a Hitachi F-7000 fluorescence spectrophotometer. Time-resolved emission decay behavior were evaluated on an Edinburgh Instruments Ltd. FLS920 spectrometer with μF900 lamps as the excitation source and a R928-PA photo multiplier as the signal detector. The vacuum ultraviolet (VUV) spectra were obtained at Beam line 4B8 in Beijing Synchrotron Radiation Facilities.
3.2. Photoluminescence properties of single-doped MgIn2P4O14: Tb UV excited luminescence properties of MgIn2P4O14: Tb phosphors. Under the monitoring of the optimum emission wavelength of Tb3+ at 548 nm, a wide absorption band appears in the excitation spectrum (PLE) in the range of 200–275 nm, which is the characteristic d-f absorption of trivalent Tb ions, i.e., the inter-configuration 4f8→ 4f75d1 transition (see Fig. 4(a)). The absorption peaks around 223 and 264 nm correspond to two different transition types of 4f8→4f75d1, i.e., spin-allowed and spin-forbidden transitions, respectively. The inset in Fig. 4(a) presents the changes of peak intensity (~223 nm) versus Tb3+ doping concentration with each data point labeled with peak position value. It can be noticed that a slight red shift occurs with Tb3+ content increasing. This is because the crystal lattice expands and the band gap gets narrow, as more In3+ ions (0.80 Å) are substituted by Tb3+ ions (0.92 Å) in host matrix. In contrast, the inter-configuration 4f8→4f8 absorption band of Tb3+ is much weaker (within the wavelength range of 275–380 nm), and its intensity increases with the increase of Tb3+ concentration, while the peak position does not shift. Excited by optimal d-f absorption, the PL spectra of the MgIn2P4O14: Tb phosphors (Fig. 4(b)) exhibit a set of typical peaks within 370–650 nm, which are the characteristic f-f emission of Tb3+. Apparently, the relative peak intensity varies with Tb3+ concentration. Peaks at wavelength below 470 nm can be attributed to the transitions from 5D3 to 7Fj (j = 6, 5 and 4) level of Tb3+ ions and the intensity of the emission from the 5D3 level decreases gradually with the increase in Tb3+ concentration. For those peaks corresponding to 5D4→7Fj (j = 6, 5, 4 and 3) at wavelength longer than 470 nm, the peak intensity rises with increasing Tb3+ content. The energy difference between the 5D3 and 5D4 excited states matches approximately the energy difference between the 7F6 ground state and higher 7F0 state, which promotes the cross-relaxation process between neighboring Tb3+ ions: 5D3 (Ion I) + 7F6 (Ion II) → 5D4 (Ion I) + 7F0 (Ion II). At large Tb–Tb distances (low Tb content), the probability of cross-relaxation is low, and the emissions from both the 5D3 and 5D4 excited states are observed in the PL spectra. At high Tb concentration, cross-relaxation quenches the emission from the 5D3 level, and the emission peaks on the PL spectra are mainly from the 5D4 level.
3. Results and discussion 3.1. Phase analysis Previously, the crystal structure of MgIn2P4O14 compound was analyzed and refined based on the single-phase XRD diffraction data [20]. Fig. 1 compares the XRD patterns of several representative Eu3+/ Tb3+ doped MgIn2P4O14 phosphors and un-doped MgIn2P4O14 host compound. By contrast, the XRD spectra of the doped ones were consistent with those of the matrix without doping, and no redundant diffraction peaks were observed, indicating that the substitution of Eu and Tb for In was successful and no second phase was produced. Fig. 2 presents the SEM images of MgIn2P4O14: Eu0.005,Tb0.05 sample and the EDS elemental maps from the corresponding area. The EDS elemental mapping results corroborate that Eu and Tb disperse homogeneously in the MgIn2P4O14 matrix.
3.3. Photoluminescence properties of Eu/Tb co-doped MgIn2P4O14 phosphors Possible energy transfer (ET) of MgIn2P4O14: EuTb phosphors. Fig. 5 depicts the PLE and PL spectra of Eu3+ and Tb3+ single doped MgIn2P4O14 phosphors. It is possible that, in certain energy conditions, ET process from Tb3+ to Eu3+ appears due to the proximity between the 5D3→7F6 emission of Tb3+ and the 7F0→5L7 absorption of Eu3+ as
Fig. 1. XRD patterns of representative MgIn2P4O14: Eu, Tb phosphors. 2
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Fig. 2. SEM images (a) and SEM-EDS elemental mapping of Mg, In, P, O, Eu, and Tb (b–g) for MgIn2P4O14: Eu0.005,Tb0.05 sample.
ET mechanism of MgIn2P4O14: EuTb phosphors. Fig. 6(a) depicts the PL spectra of MgIn2P4O14: Eu0.03Tby phosphor excited at 254 nm. The intensity of Eu3+ characteristic emission peaks increases slightly when a small amount of Tb was doped into the MgIn2P4O14: Eu0.03 phosphor. This reveals the sensitizing effect of Tb3+ to Eu3+, namely, the ET process from Tb3+ to Eu3+. As the content of Tb further increases to 0.03 and above, the Eu3+ characteristic emission declines gradually because of the competition from Tb3+ absorption. At low Tb concentration, the influence of Tb3+ absorption on Eu3+ absorption is limited, so the contribution of ET to Eu3+ emission exceeds the competition from Tb3+ absorption. In contrast, at high Tb concentration, a considerable portion of incident light is absorbed by Tb3+, and the ET process cannot compensate the decrease in Eu3+ absorption. With Tb content increasing, the Tb3+ characteristic emission is intensified gradually. However, the intensity of Tb3+ emission still cannot match that of Eu3+ emission. This is because absorption of Eu3+ is much stronger than that of Tb3+ at 254 nm in MgIn2P4O14 host (according to Fig. 5) and the emission of Eu3+ is strengthened by ET from Tb3+ to Eu3+ (seen from inset of Fig. 6(a)). Fig. 6(b) shows the PL spectra of MgIn2P4O14: EuxTb0.05 phosphors excited at 254 nm. With the Eu content increasing from 0 to 0.07, the characteristic emission of Eu3+ (at above 580 nm) is intensified linearly (inset in Fig. 6(b)). However, the addition of Eu, even at low concentrations, causes drastic decline in the intensity of characteristic Tb3+ emission (at around 540 nm). This is also attributed to the energy transfer from Tb3+ to Eu3+ and the competition of absorption between the two species. In order to further verify the process of Tb3+→Eu3+ energy transfer, vacuum ultraviolet (VUV) fluorescence properties were
Fig. 3. UV–Vis diffuse reflection spectra of MgIn2P4O14, MgIn2P4O14: Eu0.05 and MgIn2P4O14: Tb0.03. The inset presents the absorbance of MgIn2P4O14, MgIn2P4O14: Eu0.05 and MgIn2P4O14: Tb0.03.
shown in the shaded area of Fig. 5. Besides, the significant overlapped region within 200–300 nm between the PLE spectra of Eu3+ and Tb3+ single doped MgIn2P4O14 phosphors indicate that simultaneous double activation is feasible. Thus, a set of Eu, Tb double-doped MgIn2P4O14 phosphors were prepared with luminescence investigated. 3
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Fig. 4. (a) PLE spectra of MgIn2P4O14: Tb phosphors monitored at 548 nm. Inset is the plot of peak intensity versus Tb content; (b) PL spectra of MgIn2P4O14: Tb. Inset is schematic energy level diagram illustrating characteristic excitation and emission of Tb3+ in MgIn2P4O14 (NRT and CR denotes non-radiative transition and crossrelaxation, respectively).
MgIn2P4O14: Eu, MgIn2P4O14: Tb and MgIn2P4O14: Eu,Tb. Monitored at the characteristic emission of Eu3+ (592 nm), it is obvious that the PLE spectra profile of MgIn2P4O14: Eu, Tb before 325 nm contains the characteristics of 4f-5d absorption of Tb3+ and CTB band of Eu3+ ions, while the absorption peak located after 325 nm is a combination of the 4f-4f transitions of Eu3+ and Tb3+. The appearance of Tb3+ characteristic absorption under Eu3+ monitoring confirms the occurrence of Tb3+→Eu3+ ET process. Meanwhile, the 5D4 → 7F4 emission of Tb3+ ions are also located at around 592 nm (see Fig. 4(b)), which makes it plausible to obtain the Tb3+ excitation peak by monitoring the emission of Eu3+. Fig. 7(b) and (c) show the PLE spectra of MgIn2P4O14: Eu0.01,Tb0.05 phosphor excited by 540 and 592 nm wavelengths measured at various low temperatures within 27–293 K ranges, respectively. Both spectra exhibit an absorption peak at ~224 nm, which is ascribed to the characteristic 4f8 → 4f75d1 transition of Tb3+. Fig. 7(d) presents the VUV PL spectra of MgIn2P4O14: Eu0.01,Tb0.05 excited by 224 nm at different temperatures within 25–298 K. This practically presents complete results of strong thermal quenching effect at room temperature. By the way, it can be seen from Fig. 7(d) that the ratio of Tb3+ emission intensity to that of Eu3+ increases with the decrease of temperature, indicating that the energy transferred from Tb3+ to Eu3+ is temperature-dependent. This is consistent with the theory that the ET process requires thermal energy to provide additional phonon assistance (lattice vibration) [23–25]. VUV-Excited Luminescence Properties of MgIn2P4O14: Eu0.01,Tb0.05 Phosphor. Under general PL measurement, only the luminescence performance after 200 nm can be obtained. Here, we studied the vacuum ultraviolet fluorescence properties of the co-doped
Fig. 5. Comparison of both excitation and emission spectra of Eu and Tb singly doped MgIn2P4O14.
investigated since the synchrotron radiation source has higher intensity and resolution level, which can provide information not only at the wavelength range before 200 nm but also at low temperatures within 27–293 K. Fig. 7(a) shows the comparison of the VUV PLE spectra of
Fig. 6. PL spectra of MgIn2P4O14: Eu0.03Tby (a) and MgIn2P4O14: EuxTb0.05 (b) phosphors with different Eu/Tb doping concentrations under the excitation of 254 nm. 4
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Fig. 7. (a) The VUV PLE spectra of Eu/Tb single doped and Eu, Tb co-doped MgIn2P4O14 phosphors; (b), (c) and (d) are the vacuum ultraviolet (VUV) PLE and PL spectra of MgIn2P4O14: Eu0.01,Tb0.05 measured at various low temperatures within 27–293 K ranges.
MgIn2P4O14: Eu0.01,Tb0.05 phosphor under synchrotron radiation. Fig. 7(a) and (b) show the PLE spectra of MgIn2P4O14: Eu0.01,Tb0.05 phosphor excited by 540 and 592 nm wavelengths measured at various low temperatures within 27–293 K ranges, respectively. Both spectra exhibit an absorption peak at ~224 nm. Fig. 7(c) presents the vacuum ultraviolet PL spectra of MgIn2P4O14: Eu0.01,Tb0.05 phosphor excited by 224 nm measured at different temperatures from 25 K to 298 K. This practically presents complete results of strong thermal quenching effect at room temperature. Similar to the general UV PLE spectra in Fig. 4(a), the VUV PLE spectra of MgIn2P4O14: Tb in Fig. 7(d) exhibit a peak at ~224 nm, which is also ascribed to the characteristic 4f8 → 4f75d1 transition of Tb3+. As is shown in Fig. 7(b), when the monitoring wavelength is changed to the characteristic emission of Eu3+ (592 nm), the absorption peaks of Tb3+ ions are also present in the PLE spectra. There are two reasons for this phenomenon. One is that the Tb3+→ Eu3+ ET process takes place in the Eu/Tb co-doped MgIn2P4O14 phosphor, which makes it plausible to obtain the Tb3+ excitation peak by monitoring the emission of Eu3+. The other is that the 5D4 → 7F4 emission peaks of Tb3+ ions are also located at around 592 nm (see Fig. 4(b)). Therefore, the 224 nm excitation peaks of Tb3+ can be obtained by using 592 nm monitoring wavelength. Here, further discussion of the mechanism of Tb3+→Eu3+ energy transfer in MgIn2P4O14 will be carried out. The resonant-type energy transfer is generally performed through exchanged interaction or multipolar interaction. The former type occurs only when the critical distance Rc between sensitizers and activators shorter enough (usually < 5 Å), which can be calculated by the equation proposed by G. Blasse [26–28]:
V=
4 R 3 π ⎛ c ⎞ ⋅x c ⋅N , 3 ⎝2⎠
(1)
where N stands for the number of lattice sites that can be occupied by activator ions in unit cell and V is the volume of unit cell (12 and 1032.56 Å3, respectively). For the case Rc < 5 Å, xc is calculated to be larger than 1, which is obviously unreasonable. Therefore, the ET mechanism in MgIn2P4O14: Eux,Tb0.05 phosphors is governed by the multipolar interaction. In the light of Dexter's energy transfer theory and Reisfeld's approximation, details of multipolar interaction can be revealed on the basis of relation [29,30]:
IS0 θ ∝ C3, IS
(2) 3+
where IS0 and IS represents the emission intensity of Tb ions in the absence and presence of Eu3+ ions, respectively, C is the sum of doping ions concentration and θ is an indicator of the ET mechanism type. θ = 6, 8, and 10 corresponds to dipole-dipole (d-d), dipole-quadrupole (d-q), and quadrupole-quadrupole (q-q) interaction, respectively. The θ
dependences of Is0/ Is on C 3 are all fitted with linear relations in Fig. 8 and the best fit appears at θ = 6 with an R-squared factor of 0.9993. This clearly indicates that the ET process from Tb3+ to Eu3+ follows the non-radiative dipole-dipole interaction mechanism. Luminescence decay properties of MgIn2P4O14: Eu, Tb phosphors. Luminescence decay time measurements were performed to further analyze the ET process. Fig. 9 shows the typical luminescence decay curve of 5D4 → 7F5 emission of the Tb3+ at 548 nm for the MgIn2P4O14: Eu0.01,Tb0.05 phosphors under the excitation of 254 nm 5
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Fig. 8. Energy transfer efficiencies from Tb3+ to Eu3+ in MgIn2P4O14: Eux,Tb0.05 phosphor.
Fig. 10. CIE chromaticity diagram and digital photographs of Tb single-doped MgIn2P4O14 and Eu/Tb co-doped MgIn2P4O14 phosphors under the excitation of 254 nm.
increase in the Eu3+ doping concentration results in a slight downturn in the decay time of Tb3+ emission, which is in consistence with the literature results [31]. In the case of energy transfer, the decay lifetime of a sensitizer will be shortened, because of the presence of additional decay channels that can shorten the lifetime of the excited state. In other words, the shortening in the luminescence decay lifetime of the sensitizer with the increasing concentration of the activator is a strong evidence of ET process from the former to the latter. Therefore, the energy transfer of Tb3+→Eu3+ is verified in MgIn2P4O14 Eux,Tb0.05 phosphors. Color-Tunable Luminescent Properties of MgIn2P4O14: Eu, Tb Phosphors. According to PL spectra excited by 254 nm radiation, the Commission International de I'Eclairage (CIE) chromaticity coordinates for Eu/Tb-doped MgIn2P4O14 phosphors were calculated. The CIE chromaticity diagram and relevant digital lighting photographs are shown in Fig. 10. Notably, the emission color of single-doped MgIn2P4O14: Tb phosphors vary from blue to green with increasing Tb doping content (point 1–9). This is consistent with the discussion above. When the Eu concentration is adjusted with Tb concentration fixed at 0.05, the emission color gradient varies from green to yellow to orangered (point 5, 10–15). The phosphor components and their corresponding CIE chromaticity coordinates are given in Table 1.
Fig. 9. Luminescence decay curve for Tb3+ ions in MgIn2P4O14: Eu0。01,Tb0.05 phosphor (λex = 254 nm, λem = 548 nm), the inset table lists the fitted decay times of MgIn2P4O14: Eux,Tb0.05 phosphors (x = 0, 0.01, 0.03 and 0.05).
radiation. As it can be seen from Fig. S1, the 5D0 → 7F0 emission of Eu3+ can be fitted into two peaks. Since no splitting occurs at 5D0 and 7F0 levels, the number of 5D0 → 7F0 peaks is equal to the number of lattice sites occupied by Eu3+, which indicates that the doping ions occupied two different sites in MgIn2P4O14 host. Thus, double exponential decay mode is applied and it can fit well with the decay curve [31]:
t t I (t ) = A1 exp ⎛− ⎞ + A2 exp ⎛− ⎞ τ τ 1 ⎝ ⎠ ⎝ 2⎠ ⎜
⎟
⎜
4. Conclusions
⎟
(3)
A series of Eu3+/Tb3+ doped MgIn2P4O14 phosphors were synthesized by conventional solid state reaction method. The Eu3+/Tb3+ doped MgIn2P4O14 phosphors are nearly transparent to visible light and show a strong absorption in the UV region. At low Tb concentration, the 5 D3 → 7Fj transitions of Tb3+ generate obvious emissions at wavelength before 450 nm, leading to a light-blue color of luminescence. At high Tb concentration, the cross-relaxation phenomenon leads to attenuation of blue emission, and then the green emission 5D4 → 7Fj becomes the absolute majority. With Eu3+ added in, the overlap between the 5D3→ 7 F6 emission of Tb3+ and the 7F0→5L7 absorption of Eu3+ gives birth to
where t is time, I(t) is the luminescence intensity at time t, A1 and A2 are weighting parameters, and τ1 and τ1 are decay lifetime components. The average decay life τav can be calculated by equation:
τav =
A1 τ12 + A2 τ22 A1 τ1 + A2 τ2
(4)
The values of τav of Tb emission for the MgIn2P4O14: Eux,Tb0.05 phosphors are calculated to be 3.88, 3.86, 3.78, and 3.75 ms, corresponding to x = 0, 0.01, 0.03, and 0.05, respectively. Obviously, the 3+
6
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Table 1 CIE coordination of MgIn2P4O14: Eux,Tby. Label
Components
(x, y)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Tb0.005 Tb0.01 Tb0.02 Tb0.04 Tb0.05 Tb0.06 Tb0.08 Tb0.09 Tb0.11 Eu0.003Tb0.05 Eu0.005Tb0.05 Eu0.01Tb0.05 Eu0.03Tb0.05 Eu0.05Tb0.05 Eu0.07Tb0.05
(0.2424, (0.2500, (0.2632, (0.2769, (0.2791, (0.2862, (0.2950, (0.2993, (0.3037, (0.3874, (0.4112, (0.4675, (0.5380, (0.5555, (0.5674,
on a new phosphate Ba3In4(PO4)6, J. Alloy. Compd. 797 (2019) 775–785. [8] J. Zhen, M.J. Xia, Blue-green tunable color of Ce3+/Tb3+ coactivated NaBa3La3Si6O20 phosphor via energy transfer, Sci. Rep. 6 (2016) 33283. [9] J. Yang, J. Dong, R. Wu, H. Wu, H. Song, S. Gan, L. Zou, A novel color-tunable phosphor, Na5Gd9F32: Ln3+ (Ln = Eu, Tb, Dy, Sm, Ho) sub-microcrystals: structure, luminescence and energy transfer properties, Dalton Trans. 47 (29) (2018) 9795–9803. [10] J. Yang, Z. Quan, D. Kong, X. Liu, J. Lin, Y2O3: Eu3+ Microspheres: solvothermal synthesis and luminescence properties, Cryst. Growth Des. 7 (4) (2007) 730–735. [11] X. Huang, B. Li, H. Guo, D. Chen, Molybdenum-doping-induced photoluminescence enhancement in Eu3+-activated CaWO4 red-emitting phosphors for white lightemitting diodes, Dyes Pigments 143 (2017) 86–94. [12] M.A. van de Haar, J. Werner, N. Kratz, T. Hilgerink, M. Tachikirt, J. Honold, M.R. Krames, Increasing the effective absorption of Eu3+-doped luminescent materials towards practical light emitting diodes for illumination applications, Appl. Phys. Lett. 112 (13) (2018) 132101. [13] Y. Xiao, Z. Hao, L. Zhang, W. Xiao, D. Wu, X. Zhang, G.-H. Pan, Y. Luo, J. Zhang, Highly efficient green-emitting phosphors Ba2Y5B5O17 with low thermal quenching due to fast energy transfer from Ce3+ to Tb3+, Inorg. Chem. 56 (8) (2017) 4538–4544. [14] T.O. Sales, R.J. Amjad, C. Jacinto, M.R. Dousti, Concentration dependent luminescence and cross-relaxation energy transfers in Tb3+ doped fluoroborate glasses, J. Lumin. 205 (2019) 282–286. [15] U. Caldiño, A. Speghini, M. Bettinelli, Optical spectroscopy of zinc metaphosphate glasses activated by Ce3+ and Tb3+ ions, J. Phys. Condens. Matter 18 (13) (2006) 3499–3508. [16] N. Jain, R.K. Singh, A. Srivastava, S.K. Mishra, J. Singh, Prominent blue emission through Tb3+ doped La2O3 nano-phosphors for white LEDs, Phys. B 538 (2018) 70–73. [17] X. Yan, G.R. Fern, R. Withnall, J. Silver, Effects of the host lattice and doping concentration on the colour of Tb3+ cation emission in Y2O2S: Tb3+ and Gd2O2S: Tb3+ nanometer sized phosphor particles, Nanoscale 5 (18) (2013) 8640–8646. [18] C. Zeng, H. Huang, Y. Hu, S. Miao, J. Zhou, A novel blue-greenish emitting phosphor Ba3LaK(PO4)3F: Tb3+ with high thermal stability, Mater. Res. Bull. 76 (2016) 62–66. [19] M. Li, L. Wang, W. Ran, Z. Deng, J. Shi, C. Ren, Tunable luminescence in Sr2MgSi2O7: Tb3+, Eu3+ phosphors based on energy transfer, Materials 10 (3) (2017) 227. [20] J. Zhang, G. Cai, L. Yang, Z. Ma, Z. Jin, Layered crystal structure, color-tunable photoluminescence, and excellent thermal stability of MgIn2P4O14 phosphate-based phosphors, Inorg. Chem. 56 (21) (2017) 12902–12913. [21] J. Zhang, G.X. Zhang, G.M. Cai, Z.P. Jin, Reduction of Ce(IV) to Ce(III) induced by structural characteristics and performance characterization of pyrophosphate MgIn2P4O14-based phosphors, J. Lumin. 203 (2018) 590–598. [22] J. Torrent, V. Barrón, Diffuse Reflectance Spectroscopy, Methods of Soil Analysis. Part 5, (2008), pp. 367–387. [23] S. Sinha, M.K. Mahata, K. Kumar, S.P. Tiwari, V.K. Rai, Dualistic temperature sensing in Er3+/Yb3+ doped CaMoO4 upconversion phosphor, Spectrochim. Acta A Mol. Biomol. Spectrosc. 173 (2017) 369–375. [24] J. Zhou, Y. Teng, S. Ye, Y. Zhuang, J. Qiu, Enhanced downconversion luminescence by Co-doping Ce3+ in Tb3+-Yb3+ doped borate glasses, Chem. Phys. Lett. 486 (4) (2010) 116–118. [25] T. Miyakawa, D.L. Dexter, Phonon sidebands, multiphonon relaxation of excited states, and phonon-assisted energy transfer between ions in solids, Phys. Rev. B 1 (7) (1970) 2961. [26] G. Blasse, Energy transfer in oxidic phosphors, Philips Res. Rep. 24 (2) (1969) 131–144. [27] M. Shi, D. Zhang, C. Chang, Tunable emission and concentration quenching of Tb3+ in magnesium phosphate lithium, J. Alloy. Compd. 627 (2015) 25–30. [28] M. Xu, L. Wang, D. Jia, F. Le, Luminescence properties and energy transfer investigations of Zn2P2O7: Ce3+, Tb3+ phosphor, J. Lumin. 158 (2015) 125–129. [29] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (5) (1953) 836–850. [30] D. Dexter, J.H. Schulman, Theory of concentration quenching in inorganic phosphors, J. Chem. Phys. 22 (6) (1954) 1063–1070. [31] F. Liu, Q. Liu, Y. Fang, N. Zhang, B. Yang, G. Zhao, White light emission from NaLa (PO3)4: Dy3+ single-phase phosphors for light-emitting diodes, Ceram. Int. 41 (1) (2015) 1917–1920.
λex (nm) 0.2462) 0.2803) 0.3602) 0.4026) 0.4122) 0.4235) 0.4622) 0.4925) 0.5210) 0.4289) 0.4181) 0.3623) 0.3417) 0.3431) 0.3492)
254 254 254 254 254 254 254 254 254 254 254 254 254 254 254
the ET process from Tb3+ to Eu3+ in MgIn2P4O14. The ET mechanism is d-d interaction, and phonon-assisted process also plays a role in it. By adjusting the doping concentrations of Tb3+ and Eu3+, reddish orange -yellow-green-blue color-tunable emission can be realized in Eu3+/ Tb3+ doped MgIn2P4O14 phosphors. Acknowledgements Financial supports by grants from the National Natural Science Foundation of China (No. 51772330 and No. 51472273), National Key Research and Development Plan (No. 2016YFB0701301), and BSRF (Beijing Synchrotron Radiation Facilities) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jlumin.2019.116732. References [1] S. Shionoya, W.M. Yen, H. Yamamoto, Phosphor Handbook, CRC Press, 2006. [2] W. Lü, Z. Hao, X. Zhang, Y. Luo, X. Wang, J. Zhang, Tunable full-color emitting BaMg2Al6Si9O30: Eu2+, Tb3+, Mn2+ phosphors based on energy transfer, Inorg. Chem. 50 (16) (2011) 7846–7851. [3] C.H. Huang, T.W. Kuo, T.M. Chen, Thermally stable green Ba3Y(PO4)3: Ce3+, Tb3+ and red Ca3Y(AlO)3(BO3)4: Eu3+ phosphors for white-light fluorescent lamps, Opt. Exp. 19 (1) (2011) A1–A6. [4] X. Li, B. Milićević, M.D. Dramićanin, X. Jing, Q. Tang, J. Shi, M. Wu, Eu3+Activated Sr3ZnTa2O9 single-component white light phosphors: emission intensity enhancement and color rendering improvement, J. Mater. Chem. C 7 (9) (2019) 2596–2603. [5] C.-H. Huang, T.-M. Chen, A novel single-composition trichromatic white-light Ca3Y (GaO)3(BO3)4: Ce3+, Mn2+, Tb3+ phosphor for UV-light emitting diodes, J. Phys. Chem. C 115 (5) (2011) 2349–2355. [6] V.G. Suchithra, P.P. Rao, B.A. Aswathy, Color-tunable phosphors in weberite type system La3SbO7: Bi3+, Eu3+ for near-UV LED applications, Angew. Chem. (2017), https://doi.org/10.1002/slct.201701043. [7] G.X. Zhang, J. Zhang, Y.J. Liu, J.Y. Si, X.M. Tao, G.M. Cai, Structure and luminescence properties of multicolor phosphors with excellent thermal stability based
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Red-green-blue-tunable emission from Eu3+ and Tb3+ codoped pyrophosphate phosphors J. Zhanga, Z.Y. Maa, J.G. Guob, G.M. Caia,c,∗, L. Mac, X.J. Wangc,∗∗ a
School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, PR China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, PR China c Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA b
A B S T R A C T
Color-tunable phosphors based on a single host are superior to traditional mixed primary-color phosphors in fluorescent lamps, due to the better consistency of different color components in service. In this work, a series of Eu3+/Tb3+ doped MgIn2P4O14 phosphors were synthesized by a conventional solid state reaction method. The phosphors exhibit strong absorption around 254 nm with negligible absorption in the visible light region. Under the joint action of the co-activation, cross-relaxation, sensitization, and phonon-assistance, color-tunable reddish-orange-yellow-green-blue emission is achieved in MgIn2P4O14: Eu, Tb phosphors by adjusting the doping content, which demonstrates great potential for the application in fluorescent lamps.
1. Introduction Three band fluorescent lamps are widely applied in household lighting because the three primary color components (red, green, and blue) of luminescence provide a high color rendering index (CRI). The stable performance of a fluorescent lamp requires a good match of the phosphors in some of their properties such as temperature dependence of emission and phosphor lifespan [1–3]. Since those properties are usually reliant on the structures of host compounds, it is a desirable way to integrate the three primary color components into a same host compound [4,5]. Therefore, color-tunable phosphors based on a single host have gained much attention in the past years [6–9]. Eu3+ and Tb3+ are prominent rare earth activators often jointly used in luminescent materials, due to their strong characteristic f-f emissions and the efficient energy transfer (ET) from Tb3+ to Eu3+10−11. The 5D0 → 7Fj transitions of Eu3+ usually generate emissions within the wavelength range of 580–700 nm, especially at 590 nm and 610 nm. As a result, Eu3+ is a commonly used red luminescent center [10–12]. Tb3+ serves as a green center because its dominant 5D4 → 7Fj transitions are normally located around 540 nm [13]. Meanwhile, the 5D3 → 7Fj transitions of Tb3+ in the blue region (370–450 nm) are generally quenched through the cross-relaxation between neighboring Tb3+ ions [1,14]. However, the cross-relaxation process, especially at low doping concentration, was found to be weak in some hosts, where the 5D3 → 7Fj blue emissions of Tb3+ may be strong, such as in Zn (PO3)2: Tb3+ [15], La2O3: Tb3+ [16], Y2O2S: Tb3+ [17], Ba3LaK
∗
(PO4)3F: Tb3+ [18] and Sr2MgSi2O7: Tb3+ [19]. In those cases, a supplementary, even major blue component could be added into the Tb3+ doped phosphors. In our previous reports, the monoclinic magnesium indium phosphate MgIn2P4O14 (S G.: C2/c, No.15) with a novel layered structure was found, and the Tm3+, Dy3+ or Ce3+ ions doped MgIn2P4O14 phosphors showed outstanding luminescent characteristics [20,21]. Recently, we found that 5D3→ 7Fj transitions of Tb3+ could produce blue emissions in MgIn2P4O14 host at low Tb3+ concentrations under UV excitation. This inspires us to fabricate three-primary-color phosphors for fluorescent lamps with MgIn2P4O14 host. In the present work, we focus on the Tb3+ activated luminescence based on the host and also investigate the simultaneous double activation of Eu3+ and Tb3+ in this compound. We have obtained multicolor emissions over red-yellowgreen-blue region by adjusting the doping concentrations of Eu3+ and Tb3+ in the MgIn2P4O14: Eu3+, Tb3+ phosphors. Energy transfer, absorption competition, and cross relaxation play the roles for the tunable emissions. 2. Experimental 2.1. Sample preparations Conventional high-temperature solid-state reaction method was adopted for the syntheses of a series of Eu3+/Tb3+ single- and co-doped Mg(In1-x-yEuxTby)2P4O14 (MgIn2P4O14: Eux, Tby, x = 0–0.07,
Corresponding author. School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, PR China. Corresponding author. E-mail addresses: [email protected] (G.M. Cai), [email protected] (X.J. Wang).
∗∗
https://doi.org/10.1016/j.jlumin.2019.116732 Received 29 June 2019; Received in revised form 11 July 2019; Accepted 4 September 2019 0022-2313/ © 2019 Published by Elsevier B.V.
Please cite this article as: J. Zhang, et al., Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.116732
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In order to achieve higher service efficiency, luminescent devices usually require phosphors to have strong absorption in the ultraviolet region to ensure that they can be effectively stimulated; secondly, phosphors should have high transmittance in the visible light to prevent the energy loss caused by the re-absorption of light emitted by themselves. From the UV–Vis diffuse reflection spectra in Fig. 3, it can be seen that the reflectance curves of all the doped and undoped samples present approximate horizontally with an R value of ~90% in the visible-near infrared area. This deviation from complete transmission is caused by limited test conditions (incomplete transmittance of quartz slice). The small peaks at around 390 nm and 465 nm for the MgIn2P4O14: Eu curve belong to the characteristic f-f transition 7F0→5L6 and 7F0→5D2, respectively. By applying formula A = log(1/R), in which A refers to absorbance and R reflectance, absorbance of those samples can be obtained [22] and shown as the inset of Fig. 3. Clearly, after Eu3+/Tb3+ doping, the absorbance of MgIn2P4O14 before 400 nm is significantly enhanced, which is advantageous to the luminescence properties.
y = 0.005–0.11). Firstly, stoichiometric amount of starting materials MgO (Aladdin, 99.99%), In2O3 (Aladdin, 99.99%), NH4H2PO4 (Xilong Scientific Co., LTD., 99.5%), Eu2O3 (Aladdin, 99.99%) and Tb4O7 (Aladdin, 99.99%) were ground in an agate mortar for 30 min to form homogeneous mixtures. Then, the mixtures were heated in corundum crucibles at 600 °C for 12 h to get the precursors. Subsequently, the precursors underwent two 24-hour calcinations at 1000 °C after being ground for each time. Finally, the products were gradually cooled to room temperature in the furnace and were ground again into homogeneous powders for the following analyses. 2.2. Characterization X-ray Rigaku diffractometer D/MAX-2500 were used to record powder XRD patterns. The field-emission scanning electron microscope (FE-SEM, Nova Nano SEM 230, FEI, Czech Republic), equipped with an energy dispersive spectroscopy (EDS), was used to observe the morphology and conduct elemental analysis. UV–Vis diffuse reflectance (UV-DR) spectra were recorded on a Shimadzu UV-2600 spectrophotometer. The regular photoluminescence excitation and emission (PLE/PL) spectra were obtained on a Hitachi F-7000 fluorescence spectrophotometer. Time-resolved emission decay behavior were evaluated on an Edinburgh Instruments Ltd. FLS920 spectrometer with μF900 lamps as the excitation source and a R928-PA photo multiplier as the signal detector. The vacuum ultraviolet (VUV) spectra were obtained at Beam line 4B8 in Beijing Synchrotron Radiation Facilities.
3.2. Photoluminescence properties of single-doped MgIn2P4O14: Tb UV excited luminescence properties of MgIn2P4O14: Tb phosphors. Under the monitoring of the optimum emission wavelength of Tb3+ at 548 nm, a wide absorption band appears in the excitation spectrum (PLE) in the range of 200–275 nm, which is the characteristic d-f absorption of trivalent Tb ions, i.e., the inter-configuration 4f8→ 4f75d1 transition (see Fig. 4(a)). The absorption peaks around 223 and 264 nm correspond to two different transition types of 4f8→4f75d1, i.e., spin-allowed and spin-forbidden transitions, respectively. The inset in Fig. 4(a) presents the changes of peak intensity (~223 nm) versus Tb3+ doping concentration with each data point labeled with peak position value. It can be noticed that a slight red shift occurs with Tb3+ content increasing. This is because the crystal lattice expands and the band gap gets narrow, as more In3+ ions (0.80 Å) are substituted by Tb3+ ions (0.92 Å) in host matrix. In contrast, the inter-configuration 4f8→4f8 absorption band of Tb3+ is much weaker (within the wavelength range of 275–380 nm), and its intensity increases with the increase of Tb3+ concentration, while the peak position does not shift. Excited by optimal d-f absorption, the PL spectra of the MgIn2P4O14: Tb phosphors (Fig. 4(b)) exhibit a set of typical peaks within 370–650 nm, which are the characteristic f-f emission of Tb3+. Apparently, the relative peak intensity varies with Tb3+ concentration. Peaks at wavelength below 470 nm can be attributed to the transitions from 5D3 to 7Fj (j = 6, 5 and 4) level of Tb3+ ions and the intensity of the emission from the 5D3 level decreases gradually with the increase in Tb3+ concentration. For those peaks corresponding to 5D4→7Fj (j = 6, 5, 4 and 3) at wavelength longer than 470 nm, the peak intensity rises with increasing Tb3+ content. The energy difference between the 5D3 and 5D4 excited states matches approximately the energy difference between the 7F6 ground state and higher 7F0 state, which promotes the cross-relaxation process between neighboring Tb3+ ions: 5D3 (Ion I) + 7F6 (Ion II) → 5D4 (Ion I) + 7F0 (Ion II). At large Tb–Tb distances (low Tb content), the probability of cross-relaxation is low, and the emissions from both the 5D3 and 5D4 excited states are observed in the PL spectra. At high Tb concentration, cross-relaxation quenches the emission from the 5D3 level, and the emission peaks on the PL spectra are mainly from the 5D4 level.
3. Results and discussion 3.1. Phase analysis Previously, the crystal structure of MgIn2P4O14 compound was analyzed and refined based on the single-phase XRD diffraction data [20]. Fig. 1 compares the XRD patterns of several representative Eu3+/ Tb3+ doped MgIn2P4O14 phosphors and un-doped MgIn2P4O14 host compound. By contrast, the XRD spectra of the doped ones were consistent with those of the matrix without doping, and no redundant diffraction peaks were observed, indicating that the substitution of Eu and Tb for In was successful and no second phase was produced. Fig. 2 presents the SEM images of MgIn2P4O14: Eu0.005,Tb0.05 sample and the EDS elemental maps from the corresponding area. The EDS elemental mapping results corroborate that Eu and Tb disperse homogeneously in the MgIn2P4O14 matrix.
3.3. Photoluminescence properties of Eu/Tb co-doped MgIn2P4O14 phosphors Possible energy transfer (ET) of MgIn2P4O14: EuTb phosphors. Fig. 5 depicts the PLE and PL spectra of Eu3+ and Tb3+ single doped MgIn2P4O14 phosphors. It is possible that, in certain energy conditions, ET process from Tb3+ to Eu3+ appears due to the proximity between the 5D3→7F6 emission of Tb3+ and the 7F0→5L7 absorption of Eu3+ as
Fig. 1. XRD patterns of representative MgIn2P4O14: Eu, Tb phosphors. 2
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Fig. 2. SEM images (a) and SEM-EDS elemental mapping of Mg, In, P, O, Eu, and Tb (b–g) for MgIn2P4O14: Eu0.005,Tb0.05 sample.
ET mechanism of MgIn2P4O14: EuTb phosphors. Fig. 6(a) depicts the PL spectra of MgIn2P4O14: Eu0.03Tby phosphor excited at 254 nm. The intensity of Eu3+ characteristic emission peaks increases slightly when a small amount of Tb was doped into the MgIn2P4O14: Eu0.03 phosphor. This reveals the sensitizing effect of Tb3+ to Eu3+, namely, the ET process from Tb3+ to Eu3+. As the content of Tb further increases to 0.03 and above, the Eu3+ characteristic emission declines gradually because of the competition from Tb3+ absorption. At low Tb concentration, the influence of Tb3+ absorption on Eu3+ absorption is limited, so the contribution of ET to Eu3+ emission exceeds the competition from Tb3+ absorption. In contrast, at high Tb concentration, a considerable portion of incident light is absorbed by Tb3+, and the ET process cannot compensate the decrease in Eu3+ absorption. With Tb content increasing, the Tb3+ characteristic emission is intensified gradually. However, the intensity of Tb3+ emission still cannot match that of Eu3+ emission. This is because absorption of Eu3+ is much stronger than that of Tb3+ at 254 nm in MgIn2P4O14 host (according to Fig. 5) and the emission of Eu3+ is strengthened by ET from Tb3+ to Eu3+ (seen from inset of Fig. 6(a)). Fig. 6(b) shows the PL spectra of MgIn2P4O14: EuxTb0.05 phosphors excited at 254 nm. With the Eu content increasing from 0 to 0.07, the characteristic emission of Eu3+ (at above 580 nm) is intensified linearly (inset in Fig. 6(b)). However, the addition of Eu, even at low concentrations, causes drastic decline in the intensity of characteristic Tb3+ emission (at around 540 nm). This is also attributed to the energy transfer from Tb3+ to Eu3+ and the competition of absorption between the two species. In order to further verify the process of Tb3+→Eu3+ energy transfer, vacuum ultraviolet (VUV) fluorescence properties were
Fig. 3. UV–Vis diffuse reflection spectra of MgIn2P4O14, MgIn2P4O14: Eu0.05 and MgIn2P4O14: Tb0.03. The inset presents the absorbance of MgIn2P4O14, MgIn2P4O14: Eu0.05 and MgIn2P4O14: Tb0.03.
shown in the shaded area of Fig. 5. Besides, the significant overlapped region within 200–300 nm between the PLE spectra of Eu3+ and Tb3+ single doped MgIn2P4O14 phosphors indicate that simultaneous double activation is feasible. Thus, a set of Eu, Tb double-doped MgIn2P4O14 phosphors were prepared with luminescence investigated. 3
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Fig. 4. (a) PLE spectra of MgIn2P4O14: Tb phosphors monitored at 548 nm. Inset is the plot of peak intensity versus Tb content; (b) PL spectra of MgIn2P4O14: Tb. Inset is schematic energy level diagram illustrating characteristic excitation and emission of Tb3+ in MgIn2P4O14 (NRT and CR denotes non-radiative transition and crossrelaxation, respectively).
MgIn2P4O14: Eu, MgIn2P4O14: Tb and MgIn2P4O14: Eu,Tb. Monitored at the characteristic emission of Eu3+ (592 nm), it is obvious that the PLE spectra profile of MgIn2P4O14: Eu, Tb before 325 nm contains the characteristics of 4f-5d absorption of Tb3+ and CTB band of Eu3+ ions, while the absorption peak located after 325 nm is a combination of the 4f-4f transitions of Eu3+ and Tb3+. The appearance of Tb3+ characteristic absorption under Eu3+ monitoring confirms the occurrence of Tb3+→Eu3+ ET process. Meanwhile, the 5D4 → 7F4 emission of Tb3+ ions are also located at around 592 nm (see Fig. 4(b)), which makes it plausible to obtain the Tb3+ excitation peak by monitoring the emission of Eu3+. Fig. 7(b) and (c) show the PLE spectra of MgIn2P4O14: Eu0.01,Tb0.05 phosphor excited by 540 and 592 nm wavelengths measured at various low temperatures within 27–293 K ranges, respectively. Both spectra exhibit an absorption peak at ~224 nm, which is ascribed to the characteristic 4f8 → 4f75d1 transition of Tb3+. Fig. 7(d) presents the VUV PL spectra of MgIn2P4O14: Eu0.01,Tb0.05 excited by 224 nm at different temperatures within 25–298 K. This practically presents complete results of strong thermal quenching effect at room temperature. By the way, it can be seen from Fig. 7(d) that the ratio of Tb3+ emission intensity to that of Eu3+ increases with the decrease of temperature, indicating that the energy transferred from Tb3+ to Eu3+ is temperature-dependent. This is consistent with the theory that the ET process requires thermal energy to provide additional phonon assistance (lattice vibration) [23–25]. VUV-Excited Luminescence Properties of MgIn2P4O14: Eu0.01,Tb0.05 Phosphor. Under general PL measurement, only the luminescence performance after 200 nm can be obtained. Here, we studied the vacuum ultraviolet fluorescence properties of the co-doped
Fig. 5. Comparison of both excitation and emission spectra of Eu and Tb singly doped MgIn2P4O14.
investigated since the synchrotron radiation source has higher intensity and resolution level, which can provide information not only at the wavelength range before 200 nm but also at low temperatures within 27–293 K. Fig. 7(a) shows the comparison of the VUV PLE spectra of
Fig. 6. PL spectra of MgIn2P4O14: Eu0.03Tby (a) and MgIn2P4O14: EuxTb0.05 (b) phosphors with different Eu/Tb doping concentrations under the excitation of 254 nm. 4
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Fig. 7. (a) The VUV PLE spectra of Eu/Tb single doped and Eu, Tb co-doped MgIn2P4O14 phosphors; (b), (c) and (d) are the vacuum ultraviolet (VUV) PLE and PL spectra of MgIn2P4O14: Eu0.01,Tb0.05 measured at various low temperatures within 27–293 K ranges.
MgIn2P4O14: Eu0.01,Tb0.05 phosphor under synchrotron radiation. Fig. 7(a) and (b) show the PLE spectra of MgIn2P4O14: Eu0.01,Tb0.05 phosphor excited by 540 and 592 nm wavelengths measured at various low temperatures within 27–293 K ranges, respectively. Both spectra exhibit an absorption peak at ~224 nm. Fig. 7(c) presents the vacuum ultraviolet PL spectra of MgIn2P4O14: Eu0.01,Tb0.05 phosphor excited by 224 nm measured at different temperatures from 25 K to 298 K. This practically presents complete results of strong thermal quenching effect at room temperature. Similar to the general UV PLE spectra in Fig. 4(a), the VUV PLE spectra of MgIn2P4O14: Tb in Fig. 7(d) exhibit a peak at ~224 nm, which is also ascribed to the characteristic 4f8 → 4f75d1 transition of Tb3+. As is shown in Fig. 7(b), when the monitoring wavelength is changed to the characteristic emission of Eu3+ (592 nm), the absorption peaks of Tb3+ ions are also present in the PLE spectra. There are two reasons for this phenomenon. One is that the Tb3+→ Eu3+ ET process takes place in the Eu/Tb co-doped MgIn2P4O14 phosphor, which makes it plausible to obtain the Tb3+ excitation peak by monitoring the emission of Eu3+. The other is that the 5D4 → 7F4 emission peaks of Tb3+ ions are also located at around 592 nm (see Fig. 4(b)). Therefore, the 224 nm excitation peaks of Tb3+ can be obtained by using 592 nm monitoring wavelength. Here, further discussion of the mechanism of Tb3+→Eu3+ energy transfer in MgIn2P4O14 will be carried out. The resonant-type energy transfer is generally performed through exchanged interaction or multipolar interaction. The former type occurs only when the critical distance Rc between sensitizers and activators shorter enough (usually < 5 Å), which can be calculated by the equation proposed by G. Blasse [26–28]:
V=
4 R 3 π ⎛ c ⎞ ⋅x c ⋅N , 3 ⎝2⎠
(1)
where N stands for the number of lattice sites that can be occupied by activator ions in unit cell and V is the volume of unit cell (12 and 1032.56 Å3, respectively). For the case Rc < 5 Å, xc is calculated to be larger than 1, which is obviously unreasonable. Therefore, the ET mechanism in MgIn2P4O14: Eux,Tb0.05 phosphors is governed by the multipolar interaction. In the light of Dexter's energy transfer theory and Reisfeld's approximation, details of multipolar interaction can be revealed on the basis of relation [29,30]:
IS0 θ ∝ C3, IS
(2) 3+
where IS0 and IS represents the emission intensity of Tb ions in the absence and presence of Eu3+ ions, respectively, C is the sum of doping ions concentration and θ is an indicator of the ET mechanism type. θ = 6, 8, and 10 corresponds to dipole-dipole (d-d), dipole-quadrupole (d-q), and quadrupole-quadrupole (q-q) interaction, respectively. The θ
dependences of Is0/ Is on C 3 are all fitted with linear relations in Fig. 8 and the best fit appears at θ = 6 with an R-squared factor of 0.9993. This clearly indicates that the ET process from Tb3+ to Eu3+ follows the non-radiative dipole-dipole interaction mechanism. Luminescence decay properties of MgIn2P4O14: Eu, Tb phosphors. Luminescence decay time measurements were performed to further analyze the ET process. Fig. 9 shows the typical luminescence decay curve of 5D4 → 7F5 emission of the Tb3+ at 548 nm for the MgIn2P4O14: Eu0.01,Tb0.05 phosphors under the excitation of 254 nm 5
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Fig. 8. Energy transfer efficiencies from Tb3+ to Eu3+ in MgIn2P4O14: Eux,Tb0.05 phosphor.
Fig. 10. CIE chromaticity diagram and digital photographs of Tb single-doped MgIn2P4O14 and Eu/Tb co-doped MgIn2P4O14 phosphors under the excitation of 254 nm.
increase in the Eu3+ doping concentration results in a slight downturn in the decay time of Tb3+ emission, which is in consistence with the literature results [31]. In the case of energy transfer, the decay lifetime of a sensitizer will be shortened, because of the presence of additional decay channels that can shorten the lifetime of the excited state. In other words, the shortening in the luminescence decay lifetime of the sensitizer with the increasing concentration of the activator is a strong evidence of ET process from the former to the latter. Therefore, the energy transfer of Tb3+→Eu3+ is verified in MgIn2P4O14 Eux,Tb0.05 phosphors. Color-Tunable Luminescent Properties of MgIn2P4O14: Eu, Tb Phosphors. According to PL spectra excited by 254 nm radiation, the Commission International de I'Eclairage (CIE) chromaticity coordinates for Eu/Tb-doped MgIn2P4O14 phosphors were calculated. The CIE chromaticity diagram and relevant digital lighting photographs are shown in Fig. 10. Notably, the emission color of single-doped MgIn2P4O14: Tb phosphors vary from blue to green with increasing Tb doping content (point 1–9). This is consistent with the discussion above. When the Eu concentration is adjusted with Tb concentration fixed at 0.05, the emission color gradient varies from green to yellow to orangered (point 5, 10–15). The phosphor components and their corresponding CIE chromaticity coordinates are given in Table 1.
Fig. 9. Luminescence decay curve for Tb3+ ions in MgIn2P4O14: Eu0。01,Tb0.05 phosphor (λex = 254 nm, λem = 548 nm), the inset table lists the fitted decay times of MgIn2P4O14: Eux,Tb0.05 phosphors (x = 0, 0.01, 0.03 and 0.05).
radiation. As it can be seen from Fig. S1, the 5D0 → 7F0 emission of Eu3+ can be fitted into two peaks. Since no splitting occurs at 5D0 and 7F0 levels, the number of 5D0 → 7F0 peaks is equal to the number of lattice sites occupied by Eu3+, which indicates that the doping ions occupied two different sites in MgIn2P4O14 host. Thus, double exponential decay mode is applied and it can fit well with the decay curve [31]:
t t I (t ) = A1 exp ⎛− ⎞ + A2 exp ⎛− ⎞ τ τ 1 ⎝ ⎠ ⎝ 2⎠ ⎜
⎟
⎜
4. Conclusions
⎟
(3)
A series of Eu3+/Tb3+ doped MgIn2P4O14 phosphors were synthesized by conventional solid state reaction method. The Eu3+/Tb3+ doped MgIn2P4O14 phosphors are nearly transparent to visible light and show a strong absorption in the UV region. At low Tb concentration, the 5 D3 → 7Fj transitions of Tb3+ generate obvious emissions at wavelength before 450 nm, leading to a light-blue color of luminescence. At high Tb concentration, the cross-relaxation phenomenon leads to attenuation of blue emission, and then the green emission 5D4 → 7Fj becomes the absolute majority. With Eu3+ added in, the overlap between the 5D3→ 7 F6 emission of Tb3+ and the 7F0→5L7 absorption of Eu3+ gives birth to
where t is time, I(t) is the luminescence intensity at time t, A1 and A2 are weighting parameters, and τ1 and τ1 are decay lifetime components. The average decay life τav can be calculated by equation:
τav =
A1 τ12 + A2 τ22 A1 τ1 + A2 τ2
(4)
The values of τav of Tb emission for the MgIn2P4O14: Eux,Tb0.05 phosphors are calculated to be 3.88, 3.86, 3.78, and 3.75 ms, corresponding to x = 0, 0.01, 0.03, and 0.05, respectively. Obviously, the 3+
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Table 1 CIE coordination of MgIn2P4O14: Eux,Tby. Label
Components
(x, y)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Tb0.005 Tb0.01 Tb0.02 Tb0.04 Tb0.05 Tb0.06 Tb0.08 Tb0.09 Tb0.11 Eu0.003Tb0.05 Eu0.005Tb0.05 Eu0.01Tb0.05 Eu0.03Tb0.05 Eu0.05Tb0.05 Eu0.07Tb0.05
(0.2424, (0.2500, (0.2632, (0.2769, (0.2791, (0.2862, (0.2950, (0.2993, (0.3037, (0.3874, (0.4112, (0.4675, (0.5380, (0.5555, (0.5674,
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λex (nm) 0.2462) 0.2803) 0.3602) 0.4026) 0.4122) 0.4235) 0.4622) 0.4925) 0.5210) 0.4289) 0.4181) 0.3623) 0.3417) 0.3431) 0.3492)
254 254 254 254 254 254 254 254 254 254 254 254 254 254 254
the ET process from Tb3+ to Eu3+ in MgIn2P4O14. The ET mechanism is d-d interaction, and phonon-assisted process also plays a role in it. By adjusting the doping concentrations of Tb3+ and Eu3+, reddish orange -yellow-green-blue color-tunable emission can be realized in Eu3+/ Tb3+ doped MgIn2P4O14 phosphors. Acknowledgements Financial supports by grants from the National Natural Science Foundation of China (No. 51772330 and No. 51472273), National Key Research and Development Plan (No. 2016YFB0701301), and BSRF (Beijing Synchrotron Radiation Facilities) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jlumin.2019.116732. References [1] S. Shionoya, W.M. Yen, H. Yamamoto, Phosphor Handbook, CRC Press, 2006. [2] W. Lü, Z. Hao, X. Zhang, Y. Luo, X. Wang, J. Zhang, Tunable full-color emitting BaMg2Al6Si9O30: Eu2+, Tb3+, Mn2+ phosphors based on energy transfer, Inorg. Chem. 50 (16) (2011) 7846–7851. [3] C.H. Huang, T.W. Kuo, T.M. Chen, Thermally stable green Ba3Y(PO4)3: Ce3+, Tb3+ and red Ca3Y(AlO)3(BO3)4: Eu3+ phosphors for white-light fluorescent lamps, Opt. Exp. 19 (1) (2011) A1–A6. [4] X. Li, B. Milićević, M.D. Dramićanin, X. Jing, Q. Tang, J. Shi, M. Wu, Eu3+Activated Sr3ZnTa2O9 single-component white light phosphors: emission intensity enhancement and color rendering improvement, J. Mater. Chem. C 7 (9) (2019) 2596–2603. [5] C.-H. Huang, T.-M. Chen, A novel single-composition trichromatic white-light Ca3Y (GaO)3(BO3)4: Ce3+, Mn2+, Tb3+ phosphor for UV-light emitting diodes, J. Phys. Chem. C 115 (5) (2011) 2349–2355. [6] V.G. Suchithra, P.P. Rao, B.A. Aswathy, Color-tunable phosphors in weberite type system La3SbO7: Bi3+, Eu3+ for near-UV LED applications, Angew. Chem. (2017), https://doi.org/10.1002/slct.201701043. [7] G.X. Zhang, J. Zhang, Y.J. Liu, J.Y. Si, X.M. Tao, G.M. Cai, Structure and luminescence properties of multicolor phosphors with excellent thermal stability based
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