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Luminescence of Eu2+ and Eu2+-Mn2+ in sodium scandium diphosphate NaScP2O7 crystal
JOURNAL OF RARE EARTHS, Vol. 35, No. 5, May 2017, P. 453
Luminescence of Eu2+ and Eu2+-Mn2+ in sodium scandium diphosphate NaScP2O7 crystal ZHANG Xinmin (张新民)1,2,*, ZHENG Wei (郑 薇)1, ZHANG Hongzhi (张宏志)1, CHEN Cuili (陈翠丽)3, SEO Hyo Jin (徐孝镇)3 (1. School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China; 2. Hunan Collaborative Innovation Center for Effective Utilizing of Wood & Bamboo Resource, Changsha 410018, China; 3. Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea) Received 3 August 2016; revised 14 October 2016
Abstract: In this work, the photoluminescence (PL) of NaScP2O7:Eu2+ and NaScP2O7:Eu2+,Mn2+ was investigated. Phase purity was checked using X-ray powder diffractometry (XRD). PL excitation and emission spectra were recorded to elucidate the PL properties of NaScP2O7:Eu2+ and NaScP2O7:Eu2+,Mn2+. Furthermore, fluorescence lifetime measurements were performed. PL and lifetime measurements were carried out from 10 to 525 K. Moreover, the Eu2+ site occupation was discussed. It turned out that the incorporated Eu2+ ions substituted for Na+ site and occupied two different sites. Temperature dependent PL measurements indicated the emission intensity decreased with increasing temperature due to temperature quenching in NaScP2O7:Eu2+. Fluorescence lifetimes of Eu2+ in NaScP2O7:Eu2+ almost did not change with a decay constant τ=~0.53 μs in the temperature range of 10–280 K, and then shortened due to temperature quenching. The luminescent lifetime reached ~0.05 μs at T=525 K. Finally, it was found that energy transfer occurred from Eu2+ to Mn2+ in co-doped NaScP2O7:Eu2+,Mn2+. Keywords: phosphors; decay; site occupancy; luminescence; rare earths
Luminescence properties of Eu2+ ions have been studied in a lot of host lattices[1,2]. When Eu2+ is incorporated into a compound as an impurity ion, it will substitute for a specific site. For example, the doped Eu2+ should occupy the crystallographic sites of alkaline earth metal ions in the system (Sr,Ca)AlSiN3:Eu2+ [3–5]. If Eu2+ substitutes for a cation with a unit positive charge, charge compensation is necessary. The optical properties of Eu2+ in the alkali halides have been investigated by Capelletti, Hernandez et al., and other groups[6–8]. The doped Eu2+ ions dissolve in alkali halides matrix as impurity-vacancy dipoles. Recently, several subjects of lanthanide ions doped Li+/Na+/K+ and Sc3+ related crystals have been reported. For the systems NaScSi2O6 and Na3ScSi3O9, Kakihana and his co-workers regard that Eu2+ ions in the hosts occupy the Na+ sites in consideration with the similarity in the ionic radii of Na+ and Eu2+ as well as the case of NaAlSiO4:Eu2+ [9]. However, the presence of Eu2+ dimers occupying both Na+ and Sc3+ sites with the self charge compensation as reported for KLuS2:Eu2+ could not be excluded due to the large Stokes shift and anomalously broad emission in the host of NaScSi2O6[10]. Xia et al. thinks that the Sc3+ site is very small for Eu2+ and the Eu2+ ion can penetrate more easily into the Na+ site than into the Sc3+ site in the hosts of NaScSi2O6 and
Foundation item: Project supported by the Science and Technology Program of Hunan Province (2010FJ3092) * Corresponding author: ZHANG Xinmin (E-mail: [email protected]; Tel.: +86-731-85623303) DOI: 10.1016/S1002-0721(17)60933-5
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Li3Sc2(PO4)3[11,12]. The existed vacancies ( VLi , VNa ) with a positive charge will compensate the charge balance. In general, matrices with Eu2+ ions on different sites often exhibit more than one emission band. Recently, Nikl and his co-workers developed a new interesting group of phosphors, RE- doped ALnS2 (A=Na, K, Rb; Ln=La, Gd, Lu, Y) sulfides, and used electron paramagnetic resonance (EPR) technique to determine the Eu2+ ions site occupation[13–18]. In addition, the study of radiationless transfer of optical excitation energy from one optical center to another has been paid more attention due to the importance of developing new phosphors which can be used for solid state lighting[19,20]. For example, Mn2+ luminescence is known to occur in a lot of inorganic compounds[21–24]. However, Mn2+ luminescence has the disadvantage that the optical absorption intensity is weak since all the transitions from the ground sextet to every excited level are spin-forbidden. To solve this problem, energy transfer mechanisms are applied to sensitize Mn2+ luminescence. Some sensitizers (Sb3+, Pb2+, Sn2+, Ce3+ and Eu2+) have been found to enhance the Mn2+ luminescence effectively[25–39]. White LEDs packaged using near-UV chips combined with a blend of red-, green- and blue-emitting phosphors have attracted more attention due to tunable correlated color temperature, tunable CIE chromaticity coordinates,
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and excellent color rendering index. It is valuable to search for new phosphors for near-UV LEDs application. Phosphates have been used as hosts of lamp phosphors for many years due to their low cost, easy synthesis, and reasonable stability. Recently, Xia and his co-workers reviewed the recent developments in the new inorganic solid state LED phosphors[40]. Among them, many promising phosphate LED phosphors have been reported. The crystal structure of NaScP2O7 has been previously determined by Cempirek et al.[41]. We investigated Ce3+ luminescence in NaScP2O7 crystal recently[42]. The results indicate that Ce3+ ions occupy multiple sites with different optical properties due to changes in local charge compensation. Few reports have been found on divalent lanthanide luminescence in this crystal. In the present paper, we reported a detailed investigation of the NaScP2O7:Eu2+ and NaScP2O7:Eu2+,Mn2+ crystals using PL and luminescence decay techniques. The results indicated that doped Eu2+ ions occupy two different sites in the NaScP2O7 crystal. Moreover, the phenomenon of energy transfer from Eu2+ to Mn2+ ions was also observed. The excitation spectra of NaScP2O7:Eu2+ exhibited a broad band in the near-UV range matching with the emission of near-UV chip, so they could be a white LEDs phosphor.
JOURNAL OF RARE EARTHS, Vol. 35, No. 5, May 2017
Fig. 1 XRD patterns of (2) NaScP2O7:1 mol.%Eu2+, (3) NaScP2O7:2 mol.%Eu2+, (4) NaScP2O7:10 mol.%Eu2+, and (5) NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ (The reference pattern of (1) monoclinic NaScP2O7 (calculated from CIF file) is included for comparison)
1 Experimental The phosphor samples were synthesized by solid state reaction as described in Ref. [42]. The raw materials Na2CO3, Sc2O3, NH4H2PO4, Eu2O3 and MnCO3 were bought from Aldrich. All samples of phase purity were checked by XRD. Excitation spectra, emission spectra and decay curves were recorded down to 10 K. For more details of the measurement see Ref. [43].
2 Results and discussion XRD patterns of samples (NaScP2O7 doped with different concentrations of Eu2+ and Eu2+, Mn2+ co-doped NaScP2O7) as well as the reference pattern of host lattice are shown in Fig. 1. The XRD patterns show that all the samples form the monoclinic NaScP2O7 phase and are consistent with the reference peaks of NaScP2O7 calculated from CIF file. No obvious change in the host structure is observed. PL excitation and emission spectra of NaScP2O7: 1 mol.%Eu2+ are presented in Fig. 2. The excitation spectra are broad band corresponding to the 4f7→4f65d transition of Eu2+. The excitation spectra show no clear dependence on the emission wavelength. The emission spectrum shows two obvious emission bands at about 385 and 450 nm, corresponding to the 4f65d→4f7 transition of Eu2+. Since the peak position of Eu2+ emission is strongly dependent on the ligand field around Eu2+ ions, the two emission
Fig. 2 PL excitation and emission spectra of NaScP2O7:1 mol.%Eu2+
peaks should correspond to different Eu2+ emission centers. According to the reported data of ionic radii[44], Eu2+ ion is too large to incorporate into Sc3+ site. Therefore, one kind of Eu2+ ion should substitute for Na+ ion. However, Cempirek et al. reported that the Na atoms are 9-fole coordinate by O atoms in a distorted environment, in other words, only one Na site exists in NaScP2O7 crystal (NaO9)[41]. Where is the second kind of Eu2+ ion located within the NaScP2O7 crystal? Incorporating Eu2+ on a Na+ site could result in multiple Eu2+ sites with different luminescence behaviors because of variations in local charge compensation[45]. The mechanism of charge compensation can be expressed by Kröger-Vink notation: ×
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2Na Na → Eu Na + VNa'
(1) 2+
In this case, we assume that one kind of the Eu centers can be assigned to a Eu2+ ion on a Na+ site without a nearby charge compensator; while another kind of Eu2+ center is ascribed to Eu2+ ions associated with a cation vacancy[46]. In addition, the presence of Eu2+ dimers occupying both Na+ and Sc3+ sites with the self charge compensation as reported for KLuS2:Eu2+ could not be
ZHANG Xinming et al., Luminescence of Eu2+ and Eu2+-Mn2+ in sodium scandium diphosphate NaScP2O7 crystal
excluded[10]. Fig. 3 shows the PL emission spectra of NaScP2O7: xEu2+ samples with different Eu2+ concentrations. All the emission spectra exhibit two emission bands and the emission intensity has not displayed great change. In addition, the Eu3+ f→f transition emission becomes more and more obvious with increasing Eu concentration. The reason could be due to the fact that the radius of Eu3+ is less than that of Eu2+ and the doped Eu3+ ions are more stable than Eu2+ ions. Recently, Nikl and his coworkers observed Eu3+ 5D0→7FJ emission lines at lower temperatures in Eu-doped ternary sulfides ALnS2 (A=Na, K, Rb; Ln=La, Gd, Lu, Y) and assumed that both Eu2+ and Eu3+ ions occupy the Ln3+ position in these crystals[17]. We guess that the Eu3+ would occupy the Na+ site in NaScP2O7 crystal. Fig. 4 presents the temperature-dependent of the emission spectra of NaScP2O7:1 mol.%Eu2+ under 266 nm UV excitation in the range of 10–525 K. As can be seen from Fig. 4, the emission spectrum at 10 K exhibits two separated bands peaking at 390 and 490 nm. The emission intensity decreases slowly with increasing temperature due to temperature quenching. At the same time, the band peaking at 490 nm shifts towards higher energy. At 300 K, it situates at about 465 nm. In general, the emission often redshifts with temperature increasing and this redshift can be explained by Varshini equation[47,48]. In this case, the blueshift could be attributed to the thermal population of the higher 7FJ levels within the lowest excited 5d state[49]. The decay curves of the luminescence signal (390 and 480 nm) of NaScP2O7:1 mol.%Eu2+ obtained upon 266 nm excitation at 10 K are presented in Fig. 5. Both of them show a mono-exponential behavior in the recorded time range. The obtained lifetimes are 0.40 and 0.54 μs for 390 and 480 nm emissions, respectively. The different lifetimes further confirm the two emission bands resulting from different Eu2+ centers. As discussed above, the luminescent intensity of NaScP2O7:1 mol.%Eu2+ decreases gradually with increasing temperature. This kind of decrease is called
Fig. 3 PL emission spectra of NaScP2O7:xEu2+
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Fig. 4 Temperature-dependent emission spectra of NaScP2O7: 1 mol.%Eu2+ (a) T=10–280 K; (b) T=300–525 K (λex=266 nm)
Fig. 5 Luminescence decay curves of NaScP2O7:1 mol.%Eu2+ under UV excitation (λex=266 nm) at 10 K (The solid lines indicate exponential fits to the respective data)
“thermal quenching” and it can be explained by the configuration coordinate diagram[43]. The f-d transition of Eu2+ is an allowed transition; the decay time will shorten with thermal quenching taking place. The PL decay curves as a function of emission wavelengths (λem=480 nm) for the NaScP2O7:1 mol.%Eu2+ phosphor at temperatures between 10 and 525 K were measured and the results are shown in Fig. 6. All the decay curves show a mono-exponential behavior in the recorded time range. The luminescent lifetime almost does not change with a
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Fig. 6 Luminescence decay curves of the Eu2+ emission in NaScP2O7:1 mol.%Eu2+ (for λex=266 nm and λem=480 nm) as a function of temperature
decay constant τ=~0.53 μs in the temperature range of 10–280 K. Then, the lifetime shortens due to temperature quenching. The luminescent lifetime reaches ~0.05 μs at T=525 K. Some models have been suggested to explain the intensity and decay time decrease[43,50]. Recently, Nikl, Mihokova and Dorenbos demonstrate that the ionization of the 5d electron to conduction band states can act as a quenching mechanism[51,52]. In this case, we regard that the thermal activation of an electron from the Eu2+ excited state to the conduction band can not be excluded to explain the thermal quenching behavior in NaScP2O7:Eu2+ system. The obtained lifetime of 480 nm emission for NaScP2O7:1 mol.%Eu2+ phosphor as a function of temperature is depicted in Fig. 7. In most cases, an increase in non- radiative relaxation rate plays an important role in the lifetime of excited state becoming short with increasing temperature[43,50]. The activation energy (ΔE) for thermal quenching of the Eu2+ emission in NaScP2O7: 1 mol.%Eu2+ phosphor can be derived through fitting the temperature-dependent lifetimes. The Arrhenius equation is used to calculate the activation energy as follows[53,54]:
τ =
τr
1 + [τ r / τ nr ] exp( −ΔE / kT )
(2)
in this case, τr is the radiative lifetime of Eu2+; τnr is the non-radiative lifetime of Eu2+; ΔE is activation energy of thermal quenching; k is the Boltzmann constant. The fitted radiative and non-radiative lifetimes of the Eu2+ excited state are ~5.4×10–7 and 1.19×10–10 s. The activation energy is ~0.30 eV. Investigation on the Eu2+→Mn2+ energy transfer has been paid more attention recently[4,22–24,30,31,55–58]. Usually, the Mn2+ 6A1→4T2 and 6A1→4A1,4E absorptions situate at about 400 and 430 nm, and the appearance of the excitation spectrum corresponding to these absorptions is nearly the same for other host materials. One can see clearly from Fig. 2 that the Eu2+ exhibits luminescence in the wavelength range of 350–500 nm. It is evident that
Fig. 7 Temperature dependence of the decay time of the Eu2+ emission (480 nm) derived from the curves in Fig. 6 (The red solid line is a fit to Eq. (2))
there exists an overlap region between the Eu2+ emission and the Mn2+ 6A1→4T2 and 6A1→4A1,4E absorptions. Therefore, the existence of energy transfer between Eu2+ and Mn2+ in NaScP2O7 crystals is expected. Fig. 8 illustrates the PL excitation and emission spectra of Eu2+, Mn2+ co-doped NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ sample. Under 258 nm excitation, besides the emission bands originating from Eu2+, the emission spectrum of NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ shows another separated emission band peaking at about 572 nm, which can be assigned to electronic transition of Mn2+ (4T1→6A1). PL excitation spectra of NaScP2O7: 1 mol.%Eu2+,4 mol.%Mn2+ were recorded, monitoring the 385, 450 nm emissions of Eu2+ as well as the 572 nm emission of Mn2+. These excitation spectra appear similar to that of singly doped NaScP2O7:1 mol.%Eu2+ monitoring the 385 and 455 emissions of Eu2+. The excitation spectra of NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ indicate that energy transfer occurs from Eu2+ to Mn2+. In order to further confirm energy transfer from divalent Eu and Mn ions, we measured the excitation spectrum of Mn2+ single doped NaScP2O7:4 mol.%Mn2+ sample without any europium. No obvious absorption is observed in the range of 250–300 nm for Mn2+ single doped NaScP2O7:4 mol.%Mn2+
Fig. 8 PL excitation and emission spectra of NaScP2O7:1 mol.% Eu2+,4 mol.%Mn2+
ZHANG Xinming et al., Luminescence of Eu2+ and Eu2+-Mn2+ in sodium scandium diphosphate NaScP2O7 crystal
sample; therefore, when excited by 258 nm, the Mn2+ emission should be contributed to energy transfer from Eu to Mn. In order to further testify the 572 nm emission band of NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ resulting from Mn2+, we measured the decay curves of 575 nm emission and the result is depicted in Fig. 9. The decay curve is single exponential. The decay time is about 16 ms. The long decay time of 16 ms for the Mn2+ emission in this lattice is caused by the parity and spin forbidden character of the Mn2+ transition (4T1→6A1). The effect of varying Eu2+ and Mn2+ concentrations on the luminescent properties and the mechanism of energy transfer from Eu2+ to Mn2+ are under investigation.
Fig. 9 Decay curve of Mn2+ emission in NaScP2O7:1 mol.% Eu2+,4 mol.%Mn2+ (λex=266 nm, λem=575 nm) (The solid red line indicates exponential fit to the experimental data)
3 Conclusions NaScP2O7:Eu2+ and NaScP2O7:Eu2+,Mn2+ powder samples were prepared via high temperature solid state synthesis. Excited by UV radiation, NaScP2O7:Eu2+ showed two emission bands located at about 385 and 450 nm originating from two different Eu2+ centers. One kind of the Eu2+ centers could be assigned to a Eu2+ ion on a Na+ site without a nearby charge compensator; while the other kind of Eu2+ center was ascribed to Eu2+ ions associated with a cation vacancy. Temperature dependent PL measurements indicated that the emission intensity decreased slowly with increasing temperature due to temperature quenching in NaScP2O7:Eu2+. Fluorescence lifetimes of Eu2+ in NaScP2O7:Eu2+ almost did not change with a decay constant τ=~0.53 μs in the temperature range of 10–280 K, and then shortened due to temperature quenching. The luminescent lifetime reached ~0.05 μs at T=525 K. The calculated radiative and non-radiative lifetimes of the Eu2+ excited state were ~5.4×10–7 and 1.19×10–10 s. The activation energy was ~0.30 eV. The PL excitation and emission spectra of Eu2+, Mn2+ co-doped NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ sample
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confirmed the existence of energy transfer from Eu2+ to Mn2+ in co-doped NaScP2O7:Eu2+,Mn2+.
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Ce3+, Pr3+ and Nd3+ activated RbCl. Opt. Mater., 2011, 33: 347. Grinberg M, Barzowska J, Shen Y, Bray K L. Inhomogeneous broadening of Cr3+ luminescence in doped LiTaO3. Phys. Rev. B, 2001, 63: 214104. Kim J S, Park Y H, Kim S M, Choi J C, Park H L. Temperature-dependent emission spectra of M2SiO4:Eu2+ (M= Ca, Sr, Ba) phosphors for green and greenish white LEDs. Solid State Commun., 2005, 133: 445. Varshni Y P. Temperature dependence of the energy gap in semiconductors. Physica, 1967, 34: 149. Suta M, Wickleder C. Photoluminescence of CsMI3:Eu2+ (M=Mg, Ca, and Sr)- a spectroscopic probe on structural distortions. J. Mater. Chem. C, 2015, 3: 5233. Chartier C, Barthou C, Benalloul P, Frigerio J M. Photoluminescence of Eu2+ in SrGa2S4. J. Lumin., 2005, 111: 147. Mihokova E, Nikl M. Luminescent materials: probing the excited state of emission centers by spectroscopic methods. Meas. Sci. Technol., 2015, 26: 012001. Dorenbos P. Thermal quenching of Eu2+ 5d-4f luminescence in inorganic compounds. J. Phys. Condens. Matter, 2005, 17: 8103. Zhao C L, Xia Z G, Yu S X. Thermally stable lumines-
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Luminescence of Eu2+ and Eu2+-Mn2+ in sodium scandium diphosphate NaScP2O7 crystal ZHANG Xinmin (张新民)1,2,*, ZHENG Wei (郑 薇)1, ZHANG Hongzhi (张宏志)1, CHEN Cuili (陈翠丽)3, SEO Hyo Jin (徐孝镇)3 (1. School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China; 2. Hunan Collaborative Innovation Center for Effective Utilizing of Wood & Bamboo Resource, Changsha 410018, China; 3. Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea) Received 3 August 2016; revised 14 October 2016
Abstract: In this work, the photoluminescence (PL) of NaScP2O7:Eu2+ and NaScP2O7:Eu2+,Mn2+ was investigated. Phase purity was checked using X-ray powder diffractometry (XRD). PL excitation and emission spectra were recorded to elucidate the PL properties of NaScP2O7:Eu2+ and NaScP2O7:Eu2+,Mn2+. Furthermore, fluorescence lifetime measurements were performed. PL and lifetime measurements were carried out from 10 to 525 K. Moreover, the Eu2+ site occupation was discussed. It turned out that the incorporated Eu2+ ions substituted for Na+ site and occupied two different sites. Temperature dependent PL measurements indicated the emission intensity decreased with increasing temperature due to temperature quenching in NaScP2O7:Eu2+. Fluorescence lifetimes of Eu2+ in NaScP2O7:Eu2+ almost did not change with a decay constant τ=~0.53 μs in the temperature range of 10–280 K, and then shortened due to temperature quenching. The luminescent lifetime reached ~0.05 μs at T=525 K. Finally, it was found that energy transfer occurred from Eu2+ to Mn2+ in co-doped NaScP2O7:Eu2+,Mn2+. Keywords: phosphors; decay; site occupancy; luminescence; rare earths
Luminescence properties of Eu2+ ions have been studied in a lot of host lattices[1,2]. When Eu2+ is incorporated into a compound as an impurity ion, it will substitute for a specific site. For example, the doped Eu2+ should occupy the crystallographic sites of alkaline earth metal ions in the system (Sr,Ca)AlSiN3:Eu2+ [3–5]. If Eu2+ substitutes for a cation with a unit positive charge, charge compensation is necessary. The optical properties of Eu2+ in the alkali halides have been investigated by Capelletti, Hernandez et al., and other groups[6–8]. The doped Eu2+ ions dissolve in alkali halides matrix as impurity-vacancy dipoles. Recently, several subjects of lanthanide ions doped Li+/Na+/K+ and Sc3+ related crystals have been reported. For the systems NaScSi2O6 and Na3ScSi3O9, Kakihana and his co-workers regard that Eu2+ ions in the hosts occupy the Na+ sites in consideration with the similarity in the ionic radii of Na+ and Eu2+ as well as the case of NaAlSiO4:Eu2+ [9]. However, the presence of Eu2+ dimers occupying both Na+ and Sc3+ sites with the self charge compensation as reported for KLuS2:Eu2+ could not be excluded due to the large Stokes shift and anomalously broad emission in the host of NaScSi2O6[10]. Xia et al. thinks that the Sc3+ site is very small for Eu2+ and the Eu2+ ion can penetrate more easily into the Na+ site than into the Sc3+ site in the hosts of NaScSi2O6 and
Foundation item: Project supported by the Science and Technology Program of Hunan Province (2010FJ3092) * Corresponding author: ZHANG Xinmin (E-mail: [email protected]; Tel.: +86-731-85623303) DOI: 10.1016/S1002-0721(17)60933-5
•
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Li3Sc2(PO4)3[11,12]. The existed vacancies ( VLi , VNa ) with a positive charge will compensate the charge balance. In general, matrices with Eu2+ ions on different sites often exhibit more than one emission band. Recently, Nikl and his co-workers developed a new interesting group of phosphors, RE- doped ALnS2 (A=Na, K, Rb; Ln=La, Gd, Lu, Y) sulfides, and used electron paramagnetic resonance (EPR) technique to determine the Eu2+ ions site occupation[13–18]. In addition, the study of radiationless transfer of optical excitation energy from one optical center to another has been paid more attention due to the importance of developing new phosphors which can be used for solid state lighting[19,20]. For example, Mn2+ luminescence is known to occur in a lot of inorganic compounds[21–24]. However, Mn2+ luminescence has the disadvantage that the optical absorption intensity is weak since all the transitions from the ground sextet to every excited level are spin-forbidden. To solve this problem, energy transfer mechanisms are applied to sensitize Mn2+ luminescence. Some sensitizers (Sb3+, Pb2+, Sn2+, Ce3+ and Eu2+) have been found to enhance the Mn2+ luminescence effectively[25–39]. White LEDs packaged using near-UV chips combined with a blend of red-, green- and blue-emitting phosphors have attracted more attention due to tunable correlated color temperature, tunable CIE chromaticity coordinates,
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and excellent color rendering index. It is valuable to search for new phosphors for near-UV LEDs application. Phosphates have been used as hosts of lamp phosphors for many years due to their low cost, easy synthesis, and reasonable stability. Recently, Xia and his co-workers reviewed the recent developments in the new inorganic solid state LED phosphors[40]. Among them, many promising phosphate LED phosphors have been reported. The crystal structure of NaScP2O7 has been previously determined by Cempirek et al.[41]. We investigated Ce3+ luminescence in NaScP2O7 crystal recently[42]. The results indicate that Ce3+ ions occupy multiple sites with different optical properties due to changes in local charge compensation. Few reports have been found on divalent lanthanide luminescence in this crystal. In the present paper, we reported a detailed investigation of the NaScP2O7:Eu2+ and NaScP2O7:Eu2+,Mn2+ crystals using PL and luminescence decay techniques. The results indicated that doped Eu2+ ions occupy two different sites in the NaScP2O7 crystal. Moreover, the phenomenon of energy transfer from Eu2+ to Mn2+ ions was also observed. The excitation spectra of NaScP2O7:Eu2+ exhibited a broad band in the near-UV range matching with the emission of near-UV chip, so they could be a white LEDs phosphor.
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Fig. 1 XRD patterns of (2) NaScP2O7:1 mol.%Eu2+, (3) NaScP2O7:2 mol.%Eu2+, (4) NaScP2O7:10 mol.%Eu2+, and (5) NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ (The reference pattern of (1) monoclinic NaScP2O7 (calculated from CIF file) is included for comparison)
1 Experimental The phosphor samples were synthesized by solid state reaction as described in Ref. [42]. The raw materials Na2CO3, Sc2O3, NH4H2PO4, Eu2O3 and MnCO3 were bought from Aldrich. All samples of phase purity were checked by XRD. Excitation spectra, emission spectra and decay curves were recorded down to 10 K. For more details of the measurement see Ref. [43].
2 Results and discussion XRD patterns of samples (NaScP2O7 doped with different concentrations of Eu2+ and Eu2+, Mn2+ co-doped NaScP2O7) as well as the reference pattern of host lattice are shown in Fig. 1. The XRD patterns show that all the samples form the monoclinic NaScP2O7 phase and are consistent with the reference peaks of NaScP2O7 calculated from CIF file. No obvious change in the host structure is observed. PL excitation and emission spectra of NaScP2O7: 1 mol.%Eu2+ are presented in Fig. 2. The excitation spectra are broad band corresponding to the 4f7→4f65d transition of Eu2+. The excitation spectra show no clear dependence on the emission wavelength. The emission spectrum shows two obvious emission bands at about 385 and 450 nm, corresponding to the 4f65d→4f7 transition of Eu2+. Since the peak position of Eu2+ emission is strongly dependent on the ligand field around Eu2+ ions, the two emission
Fig. 2 PL excitation and emission spectra of NaScP2O7:1 mol.%Eu2+
peaks should correspond to different Eu2+ emission centers. According to the reported data of ionic radii[44], Eu2+ ion is too large to incorporate into Sc3+ site. Therefore, one kind of Eu2+ ion should substitute for Na+ ion. However, Cempirek et al. reported that the Na atoms are 9-fole coordinate by O atoms in a distorted environment, in other words, only one Na site exists in NaScP2O7 crystal (NaO9)[41]. Where is the second kind of Eu2+ ion located within the NaScP2O7 crystal? Incorporating Eu2+ on a Na+ site could result in multiple Eu2+ sites with different luminescence behaviors because of variations in local charge compensation[45]. The mechanism of charge compensation can be expressed by Kröger-Vink notation: ×
•
2Na Na → Eu Na + VNa'
(1) 2+
In this case, we assume that one kind of the Eu centers can be assigned to a Eu2+ ion on a Na+ site without a nearby charge compensator; while another kind of Eu2+ center is ascribed to Eu2+ ions associated with a cation vacancy[46]. In addition, the presence of Eu2+ dimers occupying both Na+ and Sc3+ sites with the self charge compensation as reported for KLuS2:Eu2+ could not be
ZHANG Xinming et al., Luminescence of Eu2+ and Eu2+-Mn2+ in sodium scandium diphosphate NaScP2O7 crystal
excluded[10]. Fig. 3 shows the PL emission spectra of NaScP2O7: xEu2+ samples with different Eu2+ concentrations. All the emission spectra exhibit two emission bands and the emission intensity has not displayed great change. In addition, the Eu3+ f→f transition emission becomes more and more obvious with increasing Eu concentration. The reason could be due to the fact that the radius of Eu3+ is less than that of Eu2+ and the doped Eu3+ ions are more stable than Eu2+ ions. Recently, Nikl and his coworkers observed Eu3+ 5D0→7FJ emission lines at lower temperatures in Eu-doped ternary sulfides ALnS2 (A=Na, K, Rb; Ln=La, Gd, Lu, Y) and assumed that both Eu2+ and Eu3+ ions occupy the Ln3+ position in these crystals[17]. We guess that the Eu3+ would occupy the Na+ site in NaScP2O7 crystal. Fig. 4 presents the temperature-dependent of the emission spectra of NaScP2O7:1 mol.%Eu2+ under 266 nm UV excitation in the range of 10–525 K. As can be seen from Fig. 4, the emission spectrum at 10 K exhibits two separated bands peaking at 390 and 490 nm. The emission intensity decreases slowly with increasing temperature due to temperature quenching. At the same time, the band peaking at 490 nm shifts towards higher energy. At 300 K, it situates at about 465 nm. In general, the emission often redshifts with temperature increasing and this redshift can be explained by Varshini equation[47,48]. In this case, the blueshift could be attributed to the thermal population of the higher 7FJ levels within the lowest excited 5d state[49]. The decay curves of the luminescence signal (390 and 480 nm) of NaScP2O7:1 mol.%Eu2+ obtained upon 266 nm excitation at 10 K are presented in Fig. 5. Both of them show a mono-exponential behavior in the recorded time range. The obtained lifetimes are 0.40 and 0.54 μs for 390 and 480 nm emissions, respectively. The different lifetimes further confirm the two emission bands resulting from different Eu2+ centers. As discussed above, the luminescent intensity of NaScP2O7:1 mol.%Eu2+ decreases gradually with increasing temperature. This kind of decrease is called
Fig. 3 PL emission spectra of NaScP2O7:xEu2+
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Fig. 4 Temperature-dependent emission spectra of NaScP2O7: 1 mol.%Eu2+ (a) T=10–280 K; (b) T=300–525 K (λex=266 nm)
Fig. 5 Luminescence decay curves of NaScP2O7:1 mol.%Eu2+ under UV excitation (λex=266 nm) at 10 K (The solid lines indicate exponential fits to the respective data)
“thermal quenching” and it can be explained by the configuration coordinate diagram[43]. The f-d transition of Eu2+ is an allowed transition; the decay time will shorten with thermal quenching taking place. The PL decay curves as a function of emission wavelengths (λem=480 nm) for the NaScP2O7:1 mol.%Eu2+ phosphor at temperatures between 10 and 525 K were measured and the results are shown in Fig. 6. All the decay curves show a mono-exponential behavior in the recorded time range. The luminescent lifetime almost does not change with a
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Fig. 6 Luminescence decay curves of the Eu2+ emission in NaScP2O7:1 mol.%Eu2+ (for λex=266 nm and λem=480 nm) as a function of temperature
decay constant τ=~0.53 μs in the temperature range of 10–280 K. Then, the lifetime shortens due to temperature quenching. The luminescent lifetime reaches ~0.05 μs at T=525 K. Some models have been suggested to explain the intensity and decay time decrease[43,50]. Recently, Nikl, Mihokova and Dorenbos demonstrate that the ionization of the 5d electron to conduction band states can act as a quenching mechanism[51,52]. In this case, we regard that the thermal activation of an electron from the Eu2+ excited state to the conduction band can not be excluded to explain the thermal quenching behavior in NaScP2O7:Eu2+ system. The obtained lifetime of 480 nm emission for NaScP2O7:1 mol.%Eu2+ phosphor as a function of temperature is depicted in Fig. 7. In most cases, an increase in non- radiative relaxation rate plays an important role in the lifetime of excited state becoming short with increasing temperature[43,50]. The activation energy (ΔE) for thermal quenching of the Eu2+ emission in NaScP2O7: 1 mol.%Eu2+ phosphor can be derived through fitting the temperature-dependent lifetimes. The Arrhenius equation is used to calculate the activation energy as follows[53,54]:
τ =
τr
1 + [τ r / τ nr ] exp( −ΔE / kT )
(2)
in this case, τr is the radiative lifetime of Eu2+; τnr is the non-radiative lifetime of Eu2+; ΔE is activation energy of thermal quenching; k is the Boltzmann constant. The fitted radiative and non-radiative lifetimes of the Eu2+ excited state are ~5.4×10–7 and 1.19×10–10 s. The activation energy is ~0.30 eV. Investigation on the Eu2+→Mn2+ energy transfer has been paid more attention recently[4,22–24,30,31,55–58]. Usually, the Mn2+ 6A1→4T2 and 6A1→4A1,4E absorptions situate at about 400 and 430 nm, and the appearance of the excitation spectrum corresponding to these absorptions is nearly the same for other host materials. One can see clearly from Fig. 2 that the Eu2+ exhibits luminescence in the wavelength range of 350–500 nm. It is evident that
Fig. 7 Temperature dependence of the decay time of the Eu2+ emission (480 nm) derived from the curves in Fig. 6 (The red solid line is a fit to Eq. (2))
there exists an overlap region between the Eu2+ emission and the Mn2+ 6A1→4T2 and 6A1→4A1,4E absorptions. Therefore, the existence of energy transfer between Eu2+ and Mn2+ in NaScP2O7 crystals is expected. Fig. 8 illustrates the PL excitation and emission spectra of Eu2+, Mn2+ co-doped NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ sample. Under 258 nm excitation, besides the emission bands originating from Eu2+, the emission spectrum of NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ shows another separated emission band peaking at about 572 nm, which can be assigned to electronic transition of Mn2+ (4T1→6A1). PL excitation spectra of NaScP2O7: 1 mol.%Eu2+,4 mol.%Mn2+ were recorded, monitoring the 385, 450 nm emissions of Eu2+ as well as the 572 nm emission of Mn2+. These excitation spectra appear similar to that of singly doped NaScP2O7:1 mol.%Eu2+ monitoring the 385 and 455 emissions of Eu2+. The excitation spectra of NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ indicate that energy transfer occurs from Eu2+ to Mn2+. In order to further confirm energy transfer from divalent Eu and Mn ions, we measured the excitation spectrum of Mn2+ single doped NaScP2O7:4 mol.%Mn2+ sample without any europium. No obvious absorption is observed in the range of 250–300 nm for Mn2+ single doped NaScP2O7:4 mol.%Mn2+
Fig. 8 PL excitation and emission spectra of NaScP2O7:1 mol.% Eu2+,4 mol.%Mn2+
ZHANG Xinming et al., Luminescence of Eu2+ and Eu2+-Mn2+ in sodium scandium diphosphate NaScP2O7 crystal
sample; therefore, when excited by 258 nm, the Mn2+ emission should be contributed to energy transfer from Eu to Mn. In order to further testify the 572 nm emission band of NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ resulting from Mn2+, we measured the decay curves of 575 nm emission and the result is depicted in Fig. 9. The decay curve is single exponential. The decay time is about 16 ms. The long decay time of 16 ms for the Mn2+ emission in this lattice is caused by the parity and spin forbidden character of the Mn2+ transition (4T1→6A1). The effect of varying Eu2+ and Mn2+ concentrations on the luminescent properties and the mechanism of energy transfer from Eu2+ to Mn2+ are under investigation.
Fig. 9 Decay curve of Mn2+ emission in NaScP2O7:1 mol.% Eu2+,4 mol.%Mn2+ (λex=266 nm, λem=575 nm) (The solid red line indicates exponential fit to the experimental data)
3 Conclusions NaScP2O7:Eu2+ and NaScP2O7:Eu2+,Mn2+ powder samples were prepared via high temperature solid state synthesis. Excited by UV radiation, NaScP2O7:Eu2+ showed two emission bands located at about 385 and 450 nm originating from two different Eu2+ centers. One kind of the Eu2+ centers could be assigned to a Eu2+ ion on a Na+ site without a nearby charge compensator; while the other kind of Eu2+ center was ascribed to Eu2+ ions associated with a cation vacancy. Temperature dependent PL measurements indicated that the emission intensity decreased slowly with increasing temperature due to temperature quenching in NaScP2O7:Eu2+. Fluorescence lifetimes of Eu2+ in NaScP2O7:Eu2+ almost did not change with a decay constant τ=~0.53 μs in the temperature range of 10–280 K, and then shortened due to temperature quenching. The luminescent lifetime reached ~0.05 μs at T=525 K. The calculated radiative and non-radiative lifetimes of the Eu2+ excited state were ~5.4×10–7 and 1.19×10–10 s. The activation energy was ~0.30 eV. The PL excitation and emission spectra of Eu2+, Mn2+ co-doped NaScP2O7:1 mol.%Eu2+,4 mol.%Mn2+ sample
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confirmed the existence of energy transfer from Eu2+ to Mn2+ in co-doped NaScP2O7:Eu2+,Mn2+.
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