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Luminescent properties and energy transfer of double-emitting Na2SrMgP2O8:Eu2+, Mn2+ phosphor
Optics Communications 307 (2013) 106–109
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Luminescent properties and energy transfer of double-emitting Na2SrMgP2O8:Eu2+, Mn2+ phosphor Tang Wanjun n, Hu Shanshan, Zhang Fen Hubei Key Laboratory for Catalysis and Material Science, College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan 430074, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 May 2013 Received in revised form 5 June 2013 Accepted 10 June 2013 Available online 27 June 2013
Olgite-type Na2SrMgP2O8 doped with Eu2+ and Mn2+ solely or doubly were prepared by a combustionassisted synthesis method. The phase formation was confirmed by X-ray powder diffraction measurement. Upon excitation of 352 nm ultraviolet (UV) light, two intense broad bands have clearly been observed due to the allowed 5d–4f transition of Eu2+ and the forbidden 4T1–6A1 transition of Mn2+, respectively. On the basis of the luminescence spectra and fluorescence decay curves, we confirm that the energy transfer process from the Eu2+ to Mn2+ ions occur in the codoped Na2SrMgP2O8:Eu2+, Mn2+ phosphor. The composition-optimized Na2Sr0.99MgP2O8:0.01Eu2+ and Na2Sr0.99Mg0.9P2O8: 0.01Eu2+, 0.1Mn2+ phosphors exhibits superior external quantum efficiency (87.2% and 69.3%, respectively). Based on the principle of energy transfer, the relative intensities of blue and red emission could be tuned by adjusting the contents of Eu2+ and Mn2+. & 2013 Elsevier B.V. All rights reserved.
Keywords: Optical materials and properties Luminescence Energy transfer Olgite-type phosphate
1. Introduction Mn2+ is an extensively investigated emission center, the emission of which originates from 3d5 intra-shell transition. The Mn2+ ions doped in a host lattice show a broad band emission due to the 4 T1-6A1 transition within the 3d shell in which the electrons are strongly coupled to lattice vibration and affected by crystal field strength and site symmetry [1–4]. The different crystal field strength on Mn2+ tunes the emission color, which varies from green (weak crystal field) to orange/red (strong crystal field). However, the d–d absorption transition is difficult to pump since the transitions within the 3d5 electron configuration in Mn2+ ions are forbidden by spin and parity selection rules. One of the commonly used methods to enhance the Mn2+ emission is codoping of sensitizer which facilitates the intra-shell transition through resonance energy transfer. In order to achieve efficient energy transfer, large spectral overlap is required between the excitation bands of Mn2+ ions and the emission bands of sensitizer. Eu2+ and Ce3+ ions are well known sensitizers for Mn2+ emission because their broadband 5d–4f emissions increase the spectral overlap. The increase of emission intensity of Mn2+-singly-doped host matrix via codoping with Ce3+ or Eu2+ has already been proposed in the literatures [5–9]. Rare earth and transition-metal-ion-doped orthophosphate luminescent materials can be used in displays and lighting due to
n
Corresponding author. Tel.: +86 27 67842 752. E-mail address: [email protected] (T. Wanjun).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.06.017
their excellent luminescent properties [10–12]. Orthophosphatebased materials have attracted much attention because of their potential application in solid-state lighting substituting the traditional fluorescent lamp and incandescent lamp in the near future. Recently, oligite-type layered phosphates Na2AMgP2O8 (A¼ Ca, Sr Ba) were developed by Yonesaki et al. [13,14] Eu2+-doped oligitetype Na2AMgP2O8 phosphors have been reported to emit blue light under ultraviolet (UV) excitation. Bright red luminescence is observed from Mn2+-doped Na2BaMgP2O8 with the aid of Eu2+ or Ce3+ sensitizer [15]. However, the luminescence properties of Na2SrMgP2O8:Eu2+, Mn2+ have not been investigated yet. In this contribution, we prepared a series of Eu2+ single-doped, Mn2+ single-doped, and Eu2+/Mn2+-codoped Na2SrMgP2O8 samples. Energy transfer behavior in Eu2+/Mn2+ codoped Na2SrMgP2O8 has been investigated.
2. Experiments Eu2+ single-doped and Eu2+, Mn2+-codoped Na2SrMgP2O8 samples were prepared as follows. First, NaH2PO4 2H2O (A.R.), Mg(NO3)2 6H2O (A.R.), and Sr(NO3)2 (A.R.) were weighed according to the stoichiometric ratio of each cation, and an appropriate amount of CO(NH2)2 (A.R.) was added as fuel. Second, Eu2O3 and MnCO3 with purity of 99.99% were dissolved in HNO3 in order to obtain the corresponding nitrate. Then, these reagents were dissolved in deionized water and introduced into a muffle furnace maintained at 600 1C for 5 min. Finally, the mixtures were fired at
T. Wanjun et al. / Optics Communications 307 (2013) 106–109
107
900 1C for 3 h under a 5%H2/95%N2 mixture gas to form crystalline phosphates. X-ray powder diffractions (XRD) of the samples were done on a Bruker D8 diffraction system with Cu Kα (λ¼ 1.54056 Å) radiation to identify the crystal phase. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded on a JASCO FP-6500 fluorescence spectrophotometer equipped with a 150W xenon lamp as the excitation light source. The PL decay curves of Eu2+ were measured with an FLSP920 Combined Steady State and Lifetime Spectrometer (Edinburgh Instrument) with a nanosecond flash lamp as a light source. The quantum efficiency (QE) was analyzed with a PL quantum-efficiency measurement system attached with FLSP920. For comparison, all measurements were performed at room temperature with the identical instrumental parameters.
3. Results and discussion 3.1. X-ray diffraction pattern Measurements of X-ray diffraction (XRD) of all powder samples were performed to verify the phase purity and to check the crystal structure. Shown in Fig. 1 is the comparison of power XRD patterns of pristine, singly, and doubly doped Na2SrMgP2O8:Eu2+, Mn2+ phosphors. All XRD patterns were found to agree well with those reported [13], indicating that the doped Eu2+ or codoped Eu2+/Mn2 + ions did not generate any impurity or induce significant changes in the host structure [16]. 3.2. Eu2+ luminescence in Na2SrMgP2O8 phosphors The excitation and emission spectra of the Eu2+ singly doped Na2SrMgP2O8 were investigated. Fig. 2 shows the excitation and emission spectra of Na2Sr0.99MgP2O8:0.01Eu2+, the excitation spectra show a broad absorption band within the 250–380 nm ultraviolet (UV) range, ascribed to transition from the 8S7/2(4f 7) ground state to the 4f65d1 excited state of Eu2+. A blue–purple emission band at about 400 nm corresponds to the 4f5d transition of the Eu2+ under UV excitation (λex ¼352 nm). The emission band is asymmetric and can be resolved into two Gaussian bands centered at about 396 and 430 nm. The spectrum does not show any sharp 4f-4f transitions and thus indicates that no Eu3+ ions are present in Na2Sr0.99MgP2O8:0.01Eu2+. It is well known that the emission wavelength of Eu2+ depends on the strength of the crystal field around the Eu2+ ion, i.e., it depends on the host material. The appearance of two emission bands is due to the fact that there are two different cation sites to be substituted by Eu2+
Fig. 2. Excitation and emission spectra of Na2Sr0.99MgP2O8:0.01Eu2+; inset shows dependence of Na2Sr1−xMg P2O8:xEu2+ emission intensity (λex ¼ 352 nm) on the Eu2 + contents (x).
ions in Na2SrMgP2O8 [13]. However, Na2SrMgP2O8 has only one independent Sr-site in the unit cell. Na2SrMgP2O8 has glaseritetype monoclinic layered structure (S.G., P21/c), in which interlayer A-site and layer-embedded B-site are occupied by Sr2+ and Na+, respectively [13]. The two emission peaks corresponded to the Eu2 + ions at both A- and B-sites in the monoclinic Na2SrMgP2O8. Probably, the 396-nm emission is attributed to Eu2+ ions at A-site and the longer-wavelength emission to Eu2+ ions at B-site from the following reason: according to Shannon [17], the ionic radii are arranged as Mg2+ oNa+ oEu2+≈Sr2+. The Eu2+ ions at A-site may substitute Sr2+ ions because Sr2+ is roughly the size of Eu2+. The Eu2+ ions may also substitute probably the Na+ ions at B-site sites because the compact four- and five-fold coordination of the Mg2+ ions with short Mg–O distance in Na2SrMgP2O8 offers too small a site for the Eu2+ ions. A big rare earth ion substituting Na+ ions is possible in a crystal. For example, the substitutions of Li+ by Eu2+ in LiMgPO4 [18] and the substitutions of K+ by Eu2+ in KMgPO4 [19]. Eu2+ ions showing different emission bands in different crystallographic sites have been observed in other phosphor systems, such as Ca2B5O9Cl:Eu2+ [20], Ba1.6Ca0.4P2O7:Eu2+ [21], Sr6BP5O20:Eu2+ [22], etc. The excitation spectrum of Na2Sr0.99MgP2O8:0.01Eu2+ exhibits a broadband and extends from 220 to 380 nm, which can be assigned to the 4f-5d transition of Eu2+ ions as reported [13]. The intensity of the absorption band in the UV region for Na2Sr1−xMgP2O8:xEu2+ (x¼ 0.0025–0.02) samples is enhanced for higher Eu2+ concentration. The inset in Fig. 2 shows the change of the emission intensity as a function of Eu2+ concentration (x). The luminescence intensity increases with Eu2+ doping increasing until a maximum intensity at x¼ 0.01 is reached, and then it decreases because of conventional concentration quenching process. 3.3. Mn2+ luminescence in Na2SrMgP2O8 phosphors
Fig. 1. XRD patterns of pristine, Eu2+-doped, and Eu2+/Mn2+ codoped Na2SrMgP2O8.
The emission and excitation spectra of Na2SrMg0.925P2O8:0.075Mn2+ are shown in Fig. 3a. Mn2+ single-doped samples do not show intense emission or absorption bands of Mn2+ in the UV–visible region. The excitation spectrum of Na2SrMg0.925P2O8:0.075Mn2+ shows an absorption band in the range of 320– 500 nm, corresponding to the transitions of the Mn2+ ion from the ground state 6A1 to the excited 4A1, 4E, and 4T2 levels. Under excitation at 411 nm, a broad red emission band is predominant around 627 nm. This emission band can be assigned to the spinforbidden transition of the Mn2+ ion from the lowest excited level
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Fig. 3. Emission and excitation spectra of Na2SrMg0.925P2O8:0.075Mn2+ (a) and Na2Sr0.99Mg0.925P2O8:0.01Eu2+, 0.075Mn2+ (b).
of 4T1 to the ground state 6A1. It is well known that divalent Mn2+ has a broad band, the position of which depends strongly on the crystal field of the host materials. Generally, tetrahedrally coordinated Mn2+ ions (weak crystal field) exhibit a green emission; octahedrally coordinated Mn2+ ions (strong crystal field) give an orange to red emission. [1,23]. For the case of Na2SrMgP2O8:Mn2+ phosphor, the spectral distribution of the emission spectra exhibits a broad band locating at around 627 nm. Mn2+ is roughly the size of Mg2+ [17] while Sr2+ ion has an ionic radius much larger than Mn2+. This fact indicates that the Mn2+ ion occupies an octahedral site (Mg site), which is in agreement with the crystal structure of Mg2+ in Na2SrMgP2O8 [13]. Since the d–d transitions of Mn2+ ions are spin and parity forbidden, the intensities of the excitation bands of Mn2+ itself are very weak. Therefore, the emission intensities of Mn2+-doped phosphors are generally very weak for direct excitation of Mn2+ excitation levels. Generally, the energy-transfer phenomenon is attributed to the considerable overlap between the excitation spectrum of the Mn2+ and the emission spectrum of sensitizer [24,25]. An obviously spectral overlap between the Eu2+ emission band of Na2Sr0.99MgP2O8:0.01Eu (Fig. 2) and the Mn2+ excitation band of Na2SrMg0.925P2O8:0.075Mn2+ (Fig. 3a) is observed, hence, energy transfer is expected to occur from Eu2+ to Mn2+ in Na2SrMgP2O8 crystal.
Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ (y¼0, 0.025, 0.05, 0.075, 0.1, 0.125, 0.15) were synthesized. Fig. 4 shows the Mn2+-doped concentration dependence of the emission spectra of Na2Sr0.99Mg1 2+ 2+ −yP2O8:0.01Eu , yMn . Excited at 352 nm, the emission intensity of the Mn2+ ions increases with increasing Mn2+ content, while the intensity of the Eu2+ ions was found to decrease simultaneously. This could be an evidence that the effective energy transfer exists in the Eu2+ and Mn2+ co-activated Na2SrMgP2O8 systems. The red emission intensity increases reaching a maximum at y¼0.075, and thereafter the intensity decreases. The decrease in the emission intensity occurs as a result of concentration quenching due to the nonradiative energy transfer among Mn2+ ions. Similar results have been reported in other Eu2+, Mn2+ codoped systems [26–29]. The energy transfer from Eu2+ to Mn2+, which results in the improvement of Mn2+ emission intensity, is strongly evidenced by the decay behavior of the Eu2+ emission in co-doped samples. The PL decay curves of the Eu2+ ions in Na2Sr0.99Mg1 2+ 2+ phosphors were measured with excitation −yP2O8:0.01Eu , yMn at 352 nm and monitored at 400 nm, and are depicted in Fig. 5. The lifetime decay curves have been analyzed by curve-fitting and all the decay curves can be fitted successfully based on the following double-exponential equation: IðtÞ ¼ I 0 þ A1 expð−t=τ1 Þ þ A2 expð−t=τ2 Þ
ð1Þ
where I and I0 are the luminescence intensities at times t and 0, A1 and A2 are fitting constants, and τ1 and τ2 are the short and long lifetimes for exponential components, respectively.
3.4. Energy transfer from Eu2+to Mn2+ in Na2SrMgP2O8 phosphors The Eu2+ or Mn2+ singly doped Na2SrMgP2O8 phosphor emits blue or red light, so it is possible to obtain a phosphor with double color emission by the introduction of Eu2+ and Mn2+ if energy transfer exists. To prove our design, we investigate the systems of Eu2 + and Mn2+ co-doped Na2SrMgP2O8 phosphor. The excitation and emission spectra of the Na2SrMgP2O8:Eu,Mn phosphor are compared in Fig. 3b. Blue and red double color emission bands are observed in the emission spectra of Eu2+ and Mn2+ co-doped Na2SrMgP2O8 phosphors. Under excitation at 352 nm, the emission from Eu2+ remains at 396 and 430 nm, and a new band appears at 627 nm due to emission from Mn2+. The excitation spectra monitored at 400 and 627 nm both have strong absorption band in UV range. It is clearly shown that the two excitation spectra are the same except the relative intensity. The excitation spectra remain at almost identical wavelengths with that of the Eu2+ single-doped Na2SrMgP2O8 sample, which indicates that the Mn2+ emissions are excited almost exclusively via the Eu2+ 4f-5d absorption. The identity of the excitation spectra for these two emissions confirms that the energy transfer occurs from Eu2+ to Mn2+ in Na2SrMgP2O8 crystal. The variations in the emission intensities of Eu2+ and Mn2+ with the content of Mn2+ are investigated. A series of samples
Fig. 4. PL emission spectra of Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ (λex ¼352 nm).
Fig. 5. PL decay curves of Eu2+ in Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ (excited at 352 nm, monitored at 400 nm).
T. Wanjun et al. / Optics Communications 307 (2013) 106–109
Table 1 Decay lifetime for exponential components of Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ phosphors excited at 352 nm with the emission monitored at 400 nm. Sample
I0
A1
τ1/ns
A2
τ2/ns
y ¼0 y ¼0.025 y ¼0.05 y ¼0.1 y ¼0.15
2.9 3.7 4.3 8.2 8.5
1324.3 1336.9 1321.4 1396.5 1352.1
553 504 494 444 369
314.5 924.5 1570.0 6696.2 12886.0
81 70 63 45 39
109
corresponding color tone of the phosphor shifts from blue to red purple. Thus, the emitting is tunable in a large color gamut by adjusting the doping content of Mn2+ ions. The color coordinates of Na2SrMgP2O8: Eu2+, Mn2+ phosphors can be adjusted and can be combined with other phosphors to obtain white light easily.
4. Conclusions In conclusion, a series of phosphors Eu2+ and Mn2+ singly or doubly doped Na2SrMgP2O8 were synthesized and their optical properties were studied. Under excitation at 352 nm, Na2SrMgP2O8: Eu2+, Mn2+ phosphor shows an intense blue emission band which peaks at 400 nm and a red emission band peaked at 627 nm. The relative intensity of blue and red emission could be tuned by adjusting the concentrations of Eu2+ and Mn2+. Thus, Eu2+ and Mn2+ co-doped Na2SrMgP2O8 is a promising double colors emission phosphor. The efficient energy transfer from the Eu2+ to Mn2+ ions was confirmed by the luminescence spectra and the fluorescence decay measurements. Na2Sr0.99Mg1−yP2O8: Eu2+, Mn2+ phosphors have relatively appropriate QE and could be used as phosphors in solid-state lighting.
Acknowledgment Authors acknowledge the financial support from the Natural Science Foundation of Hubei Province (No. 2011CDB421) and the Natural Science Foundation of the State Ethnic Affairs Commission (No. CMZY13001). References
Fig. 6. CIE chromaticity coordinates of Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ phosphors with the changes of Mn content (y¼0, 0.025, 0.05, 0.075, 0.1, 0.125, and 0.15, labeled as Mn_0, Mn_1, Mn_2, Mn_3, Mn_4, Mn_5, and Mn_6, respectively). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
[1] [2] [3] [4] [5] [6] [7] [8]
The values of I0, τ1, τ2, A1 and A2 are analyzed, determined, and summarized in Table 1, which indicates that in solely Eu2+-activated system, the average decay time is long. However, in the Eu2+/Mn2+ co-doped system, the average decay time was found to be shortened with increasing doped Mn2+ content. Similar observations could be attributed to the formation of paired Mn2+ centers with faster decay than single Mn2+ centers, as proposed by Ruelle et al. [30]. 3.5. QE and CIE chromaticity of Na2SrMgP2O8: Eu2+,Mn2+ phosphors The quantum efficiency (QE) of phosphors is an important parameter for their potential application in solid-state lighting. So the QE of the obtained phosphors was also measured. For Na2Sr0.99MgP2O8:0.01Eu2+ and Na2Sr0.99Mg0.9P2O8:0.01Eu2+, 0.1Mn2+ phosphors, their external QEs were determined to be 87.2% and 69.3% upon excitation at 352 nm. These data indicate that Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ samples have good QEs and could be used as phosphors in solid-state lighting. With increasing Mn2+ doping concentration, the ratio of emission intensity between Eu2+ and Mn2+ changes owing to energy transfer from Eu2+ to Mn2+. The calculated color coordinates (X,Y) of Na2Sr0.91Mg1−yP2O8:0.01Eu, yMn (y¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125, 0.15, labeled as Mn_0, Mn_1, Mn_2, Mn_3, Mn_4, Mn_5, and Mn_6, respectively) phosphor under 352 nm UV excitation are shown in Fig. 6. As the value of y increases from 0 to 0.1 in the Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ phosphors system, the
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Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Luminescent properties and energy transfer of double-emitting Na2SrMgP2O8:Eu2+, Mn2+ phosphor Tang Wanjun n, Hu Shanshan, Zhang Fen Hubei Key Laboratory for Catalysis and Material Science, College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan 430074, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 May 2013 Received in revised form 5 June 2013 Accepted 10 June 2013 Available online 27 June 2013
Olgite-type Na2SrMgP2O8 doped with Eu2+ and Mn2+ solely or doubly were prepared by a combustionassisted synthesis method. The phase formation was confirmed by X-ray powder diffraction measurement. Upon excitation of 352 nm ultraviolet (UV) light, two intense broad bands have clearly been observed due to the allowed 5d–4f transition of Eu2+ and the forbidden 4T1–6A1 transition of Mn2+, respectively. On the basis of the luminescence spectra and fluorescence decay curves, we confirm that the energy transfer process from the Eu2+ to Mn2+ ions occur in the codoped Na2SrMgP2O8:Eu2+, Mn2+ phosphor. The composition-optimized Na2Sr0.99MgP2O8:0.01Eu2+ and Na2Sr0.99Mg0.9P2O8: 0.01Eu2+, 0.1Mn2+ phosphors exhibits superior external quantum efficiency (87.2% and 69.3%, respectively). Based on the principle of energy transfer, the relative intensities of blue and red emission could be tuned by adjusting the contents of Eu2+ and Mn2+. & 2013 Elsevier B.V. All rights reserved.
Keywords: Optical materials and properties Luminescence Energy transfer Olgite-type phosphate
1. Introduction Mn2+ is an extensively investigated emission center, the emission of which originates from 3d5 intra-shell transition. The Mn2+ ions doped in a host lattice show a broad band emission due to the 4 T1-6A1 transition within the 3d shell in which the electrons are strongly coupled to lattice vibration and affected by crystal field strength and site symmetry [1–4]. The different crystal field strength on Mn2+ tunes the emission color, which varies from green (weak crystal field) to orange/red (strong crystal field). However, the d–d absorption transition is difficult to pump since the transitions within the 3d5 electron configuration in Mn2+ ions are forbidden by spin and parity selection rules. One of the commonly used methods to enhance the Mn2+ emission is codoping of sensitizer which facilitates the intra-shell transition through resonance energy transfer. In order to achieve efficient energy transfer, large spectral overlap is required between the excitation bands of Mn2+ ions and the emission bands of sensitizer. Eu2+ and Ce3+ ions are well known sensitizers for Mn2+ emission because their broadband 5d–4f emissions increase the spectral overlap. The increase of emission intensity of Mn2+-singly-doped host matrix via codoping with Ce3+ or Eu2+ has already been proposed in the literatures [5–9]. Rare earth and transition-metal-ion-doped orthophosphate luminescent materials can be used in displays and lighting due to
n
Corresponding author. Tel.: +86 27 67842 752. E-mail address: [email protected] (T. Wanjun).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.06.017
their excellent luminescent properties [10–12]. Orthophosphatebased materials have attracted much attention because of their potential application in solid-state lighting substituting the traditional fluorescent lamp and incandescent lamp in the near future. Recently, oligite-type layered phosphates Na2AMgP2O8 (A¼ Ca, Sr Ba) were developed by Yonesaki et al. [13,14] Eu2+-doped oligitetype Na2AMgP2O8 phosphors have been reported to emit blue light under ultraviolet (UV) excitation. Bright red luminescence is observed from Mn2+-doped Na2BaMgP2O8 with the aid of Eu2+ or Ce3+ sensitizer [15]. However, the luminescence properties of Na2SrMgP2O8:Eu2+, Mn2+ have not been investigated yet. In this contribution, we prepared a series of Eu2+ single-doped, Mn2+ single-doped, and Eu2+/Mn2+-codoped Na2SrMgP2O8 samples. Energy transfer behavior in Eu2+/Mn2+ codoped Na2SrMgP2O8 has been investigated.
2. Experiments Eu2+ single-doped and Eu2+, Mn2+-codoped Na2SrMgP2O8 samples were prepared as follows. First, NaH2PO4 2H2O (A.R.), Mg(NO3)2 6H2O (A.R.), and Sr(NO3)2 (A.R.) were weighed according to the stoichiometric ratio of each cation, and an appropriate amount of CO(NH2)2 (A.R.) was added as fuel. Second, Eu2O3 and MnCO3 with purity of 99.99% were dissolved in HNO3 in order to obtain the corresponding nitrate. Then, these reagents were dissolved in deionized water and introduced into a muffle furnace maintained at 600 1C for 5 min. Finally, the mixtures were fired at
T. Wanjun et al. / Optics Communications 307 (2013) 106–109
107
900 1C for 3 h under a 5%H2/95%N2 mixture gas to form crystalline phosphates. X-ray powder diffractions (XRD) of the samples were done on a Bruker D8 diffraction system with Cu Kα (λ¼ 1.54056 Å) radiation to identify the crystal phase. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded on a JASCO FP-6500 fluorescence spectrophotometer equipped with a 150W xenon lamp as the excitation light source. The PL decay curves of Eu2+ were measured with an FLSP920 Combined Steady State and Lifetime Spectrometer (Edinburgh Instrument) with a nanosecond flash lamp as a light source. The quantum efficiency (QE) was analyzed with a PL quantum-efficiency measurement system attached with FLSP920. For comparison, all measurements were performed at room temperature with the identical instrumental parameters.
3. Results and discussion 3.1. X-ray diffraction pattern Measurements of X-ray diffraction (XRD) of all powder samples were performed to verify the phase purity and to check the crystal structure. Shown in Fig. 1 is the comparison of power XRD patterns of pristine, singly, and doubly doped Na2SrMgP2O8:Eu2+, Mn2+ phosphors. All XRD patterns were found to agree well with those reported [13], indicating that the doped Eu2+ or codoped Eu2+/Mn2 + ions did not generate any impurity or induce significant changes in the host structure [16]. 3.2. Eu2+ luminescence in Na2SrMgP2O8 phosphors The excitation and emission spectra of the Eu2+ singly doped Na2SrMgP2O8 were investigated. Fig. 2 shows the excitation and emission spectra of Na2Sr0.99MgP2O8:0.01Eu2+, the excitation spectra show a broad absorption band within the 250–380 nm ultraviolet (UV) range, ascribed to transition from the 8S7/2(4f 7) ground state to the 4f65d1 excited state of Eu2+. A blue–purple emission band at about 400 nm corresponds to the 4f5d transition of the Eu2+ under UV excitation (λex ¼352 nm). The emission band is asymmetric and can be resolved into two Gaussian bands centered at about 396 and 430 nm. The spectrum does not show any sharp 4f-4f transitions and thus indicates that no Eu3+ ions are present in Na2Sr0.99MgP2O8:0.01Eu2+. It is well known that the emission wavelength of Eu2+ depends on the strength of the crystal field around the Eu2+ ion, i.e., it depends on the host material. The appearance of two emission bands is due to the fact that there are two different cation sites to be substituted by Eu2+
Fig. 2. Excitation and emission spectra of Na2Sr0.99MgP2O8:0.01Eu2+; inset shows dependence of Na2Sr1−xMg P2O8:xEu2+ emission intensity (λex ¼ 352 nm) on the Eu2 + contents (x).
ions in Na2SrMgP2O8 [13]. However, Na2SrMgP2O8 has only one independent Sr-site in the unit cell. Na2SrMgP2O8 has glaseritetype monoclinic layered structure (S.G., P21/c), in which interlayer A-site and layer-embedded B-site are occupied by Sr2+ and Na+, respectively [13]. The two emission peaks corresponded to the Eu2 + ions at both A- and B-sites in the monoclinic Na2SrMgP2O8. Probably, the 396-nm emission is attributed to Eu2+ ions at A-site and the longer-wavelength emission to Eu2+ ions at B-site from the following reason: according to Shannon [17], the ionic radii are arranged as Mg2+ oNa+ oEu2+≈Sr2+. The Eu2+ ions at A-site may substitute Sr2+ ions because Sr2+ is roughly the size of Eu2+. The Eu2+ ions may also substitute probably the Na+ ions at B-site sites because the compact four- and five-fold coordination of the Mg2+ ions with short Mg–O distance in Na2SrMgP2O8 offers too small a site for the Eu2+ ions. A big rare earth ion substituting Na+ ions is possible in a crystal. For example, the substitutions of Li+ by Eu2+ in LiMgPO4 [18] and the substitutions of K+ by Eu2+ in KMgPO4 [19]. Eu2+ ions showing different emission bands in different crystallographic sites have been observed in other phosphor systems, such as Ca2B5O9Cl:Eu2+ [20], Ba1.6Ca0.4P2O7:Eu2+ [21], Sr6BP5O20:Eu2+ [22], etc. The excitation spectrum of Na2Sr0.99MgP2O8:0.01Eu2+ exhibits a broadband and extends from 220 to 380 nm, which can be assigned to the 4f-5d transition of Eu2+ ions as reported [13]. The intensity of the absorption band in the UV region for Na2Sr1−xMgP2O8:xEu2+ (x¼ 0.0025–0.02) samples is enhanced for higher Eu2+ concentration. The inset in Fig. 2 shows the change of the emission intensity as a function of Eu2+ concentration (x). The luminescence intensity increases with Eu2+ doping increasing until a maximum intensity at x¼ 0.01 is reached, and then it decreases because of conventional concentration quenching process. 3.3. Mn2+ luminescence in Na2SrMgP2O8 phosphors
Fig. 1. XRD patterns of pristine, Eu2+-doped, and Eu2+/Mn2+ codoped Na2SrMgP2O8.
The emission and excitation spectra of Na2SrMg0.925P2O8:0.075Mn2+ are shown in Fig. 3a. Mn2+ single-doped samples do not show intense emission or absorption bands of Mn2+ in the UV–visible region. The excitation spectrum of Na2SrMg0.925P2O8:0.075Mn2+ shows an absorption band in the range of 320– 500 nm, corresponding to the transitions of the Mn2+ ion from the ground state 6A1 to the excited 4A1, 4E, and 4T2 levels. Under excitation at 411 nm, a broad red emission band is predominant around 627 nm. This emission band can be assigned to the spinforbidden transition of the Mn2+ ion from the lowest excited level
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Fig. 3. Emission and excitation spectra of Na2SrMg0.925P2O8:0.075Mn2+ (a) and Na2Sr0.99Mg0.925P2O8:0.01Eu2+, 0.075Mn2+ (b).
of 4T1 to the ground state 6A1. It is well known that divalent Mn2+ has a broad band, the position of which depends strongly on the crystal field of the host materials. Generally, tetrahedrally coordinated Mn2+ ions (weak crystal field) exhibit a green emission; octahedrally coordinated Mn2+ ions (strong crystal field) give an orange to red emission. [1,23]. For the case of Na2SrMgP2O8:Mn2+ phosphor, the spectral distribution of the emission spectra exhibits a broad band locating at around 627 nm. Mn2+ is roughly the size of Mg2+ [17] while Sr2+ ion has an ionic radius much larger than Mn2+. This fact indicates that the Mn2+ ion occupies an octahedral site (Mg site), which is in agreement with the crystal structure of Mg2+ in Na2SrMgP2O8 [13]. Since the d–d transitions of Mn2+ ions are spin and parity forbidden, the intensities of the excitation bands of Mn2+ itself are very weak. Therefore, the emission intensities of Mn2+-doped phosphors are generally very weak for direct excitation of Mn2+ excitation levels. Generally, the energy-transfer phenomenon is attributed to the considerable overlap between the excitation spectrum of the Mn2+ and the emission spectrum of sensitizer [24,25]. An obviously spectral overlap between the Eu2+ emission band of Na2Sr0.99MgP2O8:0.01Eu (Fig. 2) and the Mn2+ excitation band of Na2SrMg0.925P2O8:0.075Mn2+ (Fig. 3a) is observed, hence, energy transfer is expected to occur from Eu2+ to Mn2+ in Na2SrMgP2O8 crystal.
Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ (y¼0, 0.025, 0.05, 0.075, 0.1, 0.125, 0.15) were synthesized. Fig. 4 shows the Mn2+-doped concentration dependence of the emission spectra of Na2Sr0.99Mg1 2+ 2+ −yP2O8:0.01Eu , yMn . Excited at 352 nm, the emission intensity of the Mn2+ ions increases with increasing Mn2+ content, while the intensity of the Eu2+ ions was found to decrease simultaneously. This could be an evidence that the effective energy transfer exists in the Eu2+ and Mn2+ co-activated Na2SrMgP2O8 systems. The red emission intensity increases reaching a maximum at y¼0.075, and thereafter the intensity decreases. The decrease in the emission intensity occurs as a result of concentration quenching due to the nonradiative energy transfer among Mn2+ ions. Similar results have been reported in other Eu2+, Mn2+ codoped systems [26–29]. The energy transfer from Eu2+ to Mn2+, which results in the improvement of Mn2+ emission intensity, is strongly evidenced by the decay behavior of the Eu2+ emission in co-doped samples. The PL decay curves of the Eu2+ ions in Na2Sr0.99Mg1 2+ 2+ phosphors were measured with excitation −yP2O8:0.01Eu , yMn at 352 nm and monitored at 400 nm, and are depicted in Fig. 5. The lifetime decay curves have been analyzed by curve-fitting and all the decay curves can be fitted successfully based on the following double-exponential equation: IðtÞ ¼ I 0 þ A1 expð−t=τ1 Þ þ A2 expð−t=τ2 Þ
ð1Þ
where I and I0 are the luminescence intensities at times t and 0, A1 and A2 are fitting constants, and τ1 and τ2 are the short and long lifetimes for exponential components, respectively.
3.4. Energy transfer from Eu2+to Mn2+ in Na2SrMgP2O8 phosphors The Eu2+ or Mn2+ singly doped Na2SrMgP2O8 phosphor emits blue or red light, so it is possible to obtain a phosphor with double color emission by the introduction of Eu2+ and Mn2+ if energy transfer exists. To prove our design, we investigate the systems of Eu2 + and Mn2+ co-doped Na2SrMgP2O8 phosphor. The excitation and emission spectra of the Na2SrMgP2O8:Eu,Mn phosphor are compared in Fig. 3b. Blue and red double color emission bands are observed in the emission spectra of Eu2+ and Mn2+ co-doped Na2SrMgP2O8 phosphors. Under excitation at 352 nm, the emission from Eu2+ remains at 396 and 430 nm, and a new band appears at 627 nm due to emission from Mn2+. The excitation spectra monitored at 400 and 627 nm both have strong absorption band in UV range. It is clearly shown that the two excitation spectra are the same except the relative intensity. The excitation spectra remain at almost identical wavelengths with that of the Eu2+ single-doped Na2SrMgP2O8 sample, which indicates that the Mn2+ emissions are excited almost exclusively via the Eu2+ 4f-5d absorption. The identity of the excitation spectra for these two emissions confirms that the energy transfer occurs from Eu2+ to Mn2+ in Na2SrMgP2O8 crystal. The variations in the emission intensities of Eu2+ and Mn2+ with the content of Mn2+ are investigated. A series of samples
Fig. 4. PL emission spectra of Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ (λex ¼352 nm).
Fig. 5. PL decay curves of Eu2+ in Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ (excited at 352 nm, monitored at 400 nm).
T. Wanjun et al. / Optics Communications 307 (2013) 106–109
Table 1 Decay lifetime for exponential components of Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ phosphors excited at 352 nm with the emission monitored at 400 nm. Sample
I0
A1
τ1/ns
A2
τ2/ns
y ¼0 y ¼0.025 y ¼0.05 y ¼0.1 y ¼0.15
2.9 3.7 4.3 8.2 8.5
1324.3 1336.9 1321.4 1396.5 1352.1
553 504 494 444 369
314.5 924.5 1570.0 6696.2 12886.0
81 70 63 45 39
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corresponding color tone of the phosphor shifts from blue to red purple. Thus, the emitting is tunable in a large color gamut by adjusting the doping content of Mn2+ ions. The color coordinates of Na2SrMgP2O8: Eu2+, Mn2+ phosphors can be adjusted and can be combined with other phosphors to obtain white light easily.
4. Conclusions In conclusion, a series of phosphors Eu2+ and Mn2+ singly or doubly doped Na2SrMgP2O8 were synthesized and their optical properties were studied. Under excitation at 352 nm, Na2SrMgP2O8: Eu2+, Mn2+ phosphor shows an intense blue emission band which peaks at 400 nm and a red emission band peaked at 627 nm. The relative intensity of blue and red emission could be tuned by adjusting the concentrations of Eu2+ and Mn2+. Thus, Eu2+ and Mn2+ co-doped Na2SrMgP2O8 is a promising double colors emission phosphor. The efficient energy transfer from the Eu2+ to Mn2+ ions was confirmed by the luminescence spectra and the fluorescence decay measurements. Na2Sr0.99Mg1−yP2O8: Eu2+, Mn2+ phosphors have relatively appropriate QE and could be used as phosphors in solid-state lighting.
Acknowledgment Authors acknowledge the financial support from the Natural Science Foundation of Hubei Province (No. 2011CDB421) and the Natural Science Foundation of the State Ethnic Affairs Commission (No. CMZY13001). References
Fig. 6. CIE chromaticity coordinates of Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ phosphors with the changes of Mn content (y¼0, 0.025, 0.05, 0.075, 0.1, 0.125, and 0.15, labeled as Mn_0, Mn_1, Mn_2, Mn_3, Mn_4, Mn_5, and Mn_6, respectively). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
[1] [2] [3] [4] [5] [6] [7] [8]
The values of I0, τ1, τ2, A1 and A2 are analyzed, determined, and summarized in Table 1, which indicates that in solely Eu2+-activated system, the average decay time is long. However, in the Eu2+/Mn2+ co-doped system, the average decay time was found to be shortened with increasing doped Mn2+ content. Similar observations could be attributed to the formation of paired Mn2+ centers with faster decay than single Mn2+ centers, as proposed by Ruelle et al. [30]. 3.5. QE and CIE chromaticity of Na2SrMgP2O8: Eu2+,Mn2+ phosphors The quantum efficiency (QE) of phosphors is an important parameter for their potential application in solid-state lighting. So the QE of the obtained phosphors was also measured. For Na2Sr0.99MgP2O8:0.01Eu2+ and Na2Sr0.99Mg0.9P2O8:0.01Eu2+, 0.1Mn2+ phosphors, their external QEs were determined to be 87.2% and 69.3% upon excitation at 352 nm. These data indicate that Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ samples have good QEs and could be used as phosphors in solid-state lighting. With increasing Mn2+ doping concentration, the ratio of emission intensity between Eu2+ and Mn2+ changes owing to energy transfer from Eu2+ to Mn2+. The calculated color coordinates (X,Y) of Na2Sr0.91Mg1−yP2O8:0.01Eu, yMn (y¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125, 0.15, labeled as Mn_0, Mn_1, Mn_2, Mn_3, Mn_4, Mn_5, and Mn_6, respectively) phosphor under 352 nm UV excitation are shown in Fig. 6. As the value of y increases from 0 to 0.1 in the Na2Sr0.99Mg1−yP2O8:0.01Eu2+, yMn2+ phosphors system, the
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