Luminescent property of Eu2+, Mn2+ co-doped Y7O6F9 phosphors

Luminescent property of Eu2+, Mn2+ co-doped Y7O6F9 phosphors

Journal of Luminescence 178 (2016) 463–469 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/loca...

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Journal of Luminescence 178 (2016) 463–469

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Full Length Article

Luminescent property of Eu2 þ , Mn2 þ co-doped Y7O6F9 phosphors Chu-Young Park, Sangmoon Park n Center for Green Fusion Technology and Division of Energy Convergence Engineering, Major in Energy & Applied Chemistry, Silla University, Busan 617-736, Republic of Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 30 November 2015 Received in revised form 17 May 2016 Accepted 14 June 2016 Available online 18 June 2016

Single-phase and near-ultraviolet (NUV)–excitable materials composed of Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (p ¼0.01, 0.05, q ¼ 0–0.1) were prepared via a flux-assisted method in reducing atmosphere. X-ray diffraction patterns of the obtained phosphors were examined to index the peak positions. After doping the host structure with Eu2 þ and Mn2 þ emitters, blue to intense orange emission lights that were observed in the photoluminescence spectra under NUV excitation were monitored. The dependence of the luminescent intensity of the Mn2 þ co-doped (q ¼ 0.005–0.1) host lattices on the fixed Eu2 þ content (p ¼ 0.01, 0.05) is also investigated. Co-doping Mn2 þ into the Eu2 þ -doped host structure enabled green and orange Mn2 þ transitions in different sites of Y7O6F9 host lattice was attained. Co-doping Mn2 þ into the Eu2 þ -doped host structure enabled clear energy transfer from Eu2 þ to Mn2 þ ; this energy transfer mechanism is discussed. With these phosphors, the desired CIE values including emissions from the blue to orange regions of the spectra were achieved. & 2016 Elsevier B.V. All rights reserved.

Keywords: X-ray diffraction patterns Photoluminescence Eu2 þ –Mn2 þ Energy transfer Phosphors

1. Introduction Yellow YAG (Y3Al5 O12: Ce) phosphor-converted visible blue– LED chip is commonly used for white–light LEDs as a high efficient light source in various applications [1,2]. Red–green– blue (RGB) phosphor combined with near-ultraviolet (NUV) LED chip is also used for white LEDs because their superiority of color-rendering index for red emission [2–4]. Mn2 þ ions can play an important role as orange/red emitting activators in RGB phosphors. However, the optical intensity of Mn2 þ ions is quite weak in the near–UV through visible region because the excited level of d5 is spin–forbidden [5]. The energy transfer from sensitizers such as Ce3 þ and Eu2 þ to Mn2 þ ions can provide efficient orange/red emission lights, which are attributed by Mn2 þ transitions in host lattices [4,6–10]. Mn2 þ activators can actually emit both green and orange/red luminescent lights at peak wavelength of 490 to 750 nm in inorganic compounds [5]. The green and orange/red emission lights of Mn2 þ transitions can be observed in different Mn2 þ activator sites by the crystal field symmetry in host lattices [5,11,12]. The green and red emission lights of Mn2 þ transitions under UV radiation can be achieved when Mn2 þ ions occupy in the tetrahedral or octahedral coordination in host lattice, respectively [11–14]. For n

Corresponding author. Tel.: þ 82 51 999 5891; fax: þ 82 51 999 5335. E-mail address: [email protected] (S. Park).

http://dx.doi.org/10.1016/j.jlumin.2016.06.035 0022-2313/& 2016 Elsevier B.V. All rights reserved.

example, Mn2 þ ions doped in Zn2SiO4 or CaF2 host lattices of their 4- or 8-coordinated sites emit green light. Furthermore, Mn2 þ ions also emit orange/red light when the coordination number of Mn2 þ is 6 in ZnF2 or KMgF3 hosts [5]. As studied previously, the vernier structure of Y 7O 6F9, space group Abm2, consists of one-dimensional superstructure of fluorite, which is slightly displaced from the ideal fluorite sites, with the unit cell a  7b  c. Moreover, the Y3 þ ions are particularly coordinated by two different sites, four O2  and three or four F  anions (YO4 F3, YO4 F4), and a single (YO)7 þ layer is sandwiched between F7  layers, as shown in Fig. 1. [15,16]. In this work, near–ultraviolet (NUV) –excitable and yttrium oxyfluoride based optical materials composed of Y 7(1  2(p þ q)/3) Eu7pMn7qO 6F9 (p ¼0.01, 0.05, q ¼0–0.1) were prepared, and their X-ray diffraction patterns were characterized. The photoluminescence (PL) spectra of Eu 2 þ and Mn2 þ co-doped Y7 O6F9 phosphors, which exhibit green and orange emission attributed to the Mn2 þ transitions placed in the 7- and 8-coordinated Y3 þ sites on each Eu2 þ content (p ¼ 0.01, 0.05) were analyzed. The dependence of the luminescent intensity and energy-transfer mechanism of the Mn 3 þ co-doped (q ¼0– 0.1) host lattices on Eu 2 þ content (p ¼ 0.01, 0.05) were also studied for Y7(1  2(p þ q) /3)Eu7pMn7qO6F9. With these phosphors, the desired CIE values including emission lights from the blue to orange regions of the spectra were achieved.

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Fig. 1. The structure of Y7O6F9 vernier phase.

2. Experimental Optical materials of Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (p ¼0.01, 0.05, q ¼0–0.1) were prepared by heating the appropriate stoichiometric amounts of Y2O3 (Alfa 99.9%), Eu2O3 (Alfa 99.9%), and MnO (Aldrich 99%), and NH4F (Alfa 99%) in pellets at 1050 °C for 2 h under the atmosphere using 4%H2/96%Ar. The vernier phosphors of Y7O6F9:Eu were previously studied at high temperature using NH4F flux and the flux-assist method was employed by the modified molar ratio of the ½Y2O3 precursors and NH4F flux at 1050 °C for the preparation of Y7O6F9 crystal structure [16]. Phase identification was established using a Shimadzu XRD-6000 powder diffractometer (Cu-Kα  radiation). UV spectroscopy to measure the excitation and emission spectra of the optical materials was done using spectrofluoro-meters (Sinco Fluoromate FS-2) at room temperature.

3. Results and discussion The phase of Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (p ¼0.01, 0.05, n ¼0–0.1) was identified based on powder X-ray diffraction (XRD) analysis after Eu2 þ and Mn2 þ ions were substituted for Y3 þ ions in Y7O6F9 host lattices. Fig. 2(a)–(d) shows the calculated XRD patterns of the Y2O3 (ICSD 27772), YF3 (ICSD 26595), YOF (ICSD 14282), and Y7O6F9 (ICSD 68951) structures. Fig. 2(e), (f) and (g) show the XRD patterns of Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (p ¼0.05, q ¼0, p ¼0.01, q ¼ 0.1 and p ¼ 0.05, q ¼0.1) synthesized at 1050 °C using 1:2 M ratio of Y3 þ (Eu2 þ , Mn2 þ ) ions to NH4F flux. When the molar ratio of the NH4F flux was doubled, the Y2O3 precursor reacted completely, as shown in Fig. 2(e)–(g). The mixture with a 1:2 M ratio of Y3 þ (Eu2 þ , Mn2 þ ) ions to NH4F flux clearly resulted in the Y7O6F9 vernier phase, whereas the negligible amount of YF3 and YOF structures was observed around 23–27θ and 28.5θ, respectively. The reductions of Eu2 þ as well as Mn2 þ ions in the host structures take place under 4% H2/96%Ar atmospheres, even if the heating temperature is high and annealing time is long. For the synthesis of host Y7O6F9 structure using Y2O3 and NH4F precursors, YOF and YF3 impurity phases occur owing to the vaporization of the NH4F flux with

Fig. 2. The XRD patterns of (a) Y2O3 (ICSD 27772), (b) YF3 (ICSD 26595), (c) YOF (ICSD 14282), (d) Y7O6F9 (ICSD 68951), Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (e) p ¼ 0.05, q ¼0, (f) p ¼ 0.01, q ¼ 0.1, and (g) p ¼0.05, q ¼ 0.1.

the loss of F  ions at high temperature and for long heating time. As mentioned above, the temperature (1050 °C) and annealing time (2 h) were experimentally optimized to generate the pure phases and maximum emissions of Y7(1  2(p þ q)/3) Eu7pMn7qO6F9 phosphors. When the Y3 þ ions (r ¼ 0.96 Å, CN ¼7 and r ¼1.019 Å, CN ¼8) were substituted with larger Eu2 þ ions (r ¼ 1.20 Å, CN ¼ 7 and r ¼1.25 Å, CN ¼8), the obtained XRD patterns of Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (p ¼ 0.05, q ¼ 0) showed clear shifts in the positions of the various Bragg reflections, for example (171) and (002), at lower angles, as shown in Fig. 2 (d) and (e). The XRD patterns of Eu2 þ and Mn2 þ co-doped Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (p ¼0.01, q ¼0.1 and p ¼0.05, q ¼ 0.1), shown in Fig. 2(f) and (g), show clear deviations from the patterns of Eu2 þ -doped host lattices in peak positions of the reflections at higher angles by adding smaller Mn2 þ ions (r ¼0.90 Å, CN ¼7 and r ¼0.96 Å, CN ¼ 8). Fig. 3(a) and (b) show photoluminescence (PL) spectra of Y7 (1  2(p þ q)/3) Eu 7pMn7qO6 F 9 (p ¼ 0.01, q ¼ 0–0.1 and p ¼0.05, q ¼0–0.1) phosphors, respectively. In the PL spectra of Y7(1  2 (p þ q)/3) Eu 7pMn7qO6 F 9 (p ¼0.01, q ¼ 0 and p ¼0.05, q ¼0) phosphors, two main spectral bands, centered around 300 and 370 nm, characterize the Eu2 þ transition, which represents the transition from the ground state 4f75d° to the excited state 4f65d1. The absorption peak centered near 370 nm became stronger than the other peak. Doping the Y7O6F9 host lattice with Eu2 þ ions can produce clear NUV excitation and blue emission band. The main blue emission peak attributed to the f– d electric dipole-allowed transitions of the Eu2 þ ions in the yttrium–oxyfluoride host centered at 470 nm is in the wavelength range 430–700 nm. The d–f transition of Eu2 þ ions

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Fig. 3. The excitation and emission spectra of Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (a) p ¼ 0.01, q ¼ 0–0.1 and (b) p ¼0.05, q ¼0–0.1 and the relative intensity as a function of the content Mn2 þ ions at 515 and 575 nm.

results in the emission light with wavelength ranging from the

weak excitation and emission peaks attributed to the f–f tran-

UV to the visible range owing to the crystal field effect of the

sitions of Eu3 þ ions were attained around 400 nm and 610 nm,

activator in the host structure [17,18]. In the Y7O6F9 host lattice,

respectively, in the Eu-doped Y7O6F9 phosphors. When less than

a weak crystal-field effect of the Eu2 þ activator occurs. The

5 mol% of Eu ions was substituted in the host lattice, a relatively

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small amount of unreduced Eu3 þ ions remained in Eu2 þ –doped Y7O6F9 phosphors. However, when Mn2 þ ions were replaced with Y3 þ ions in the optimized 1 and 5 mol % Eu ion doped Y7O6F9 host lattice, the Eu3 þ transition was faded away, as shown in Fig. 3(a) and (b), Y7(1  2p/3)Eu7pO6F9 (p ¼0.01 and 0.05). The non-reduction of the Eu3 þ ions (r ¼ 1.01 Å, CN ¼ 7 and r ¼1.066 Å, CN ¼8) can be certainly retained in the 7- or 8coordinated Y3 þ sites (r ¼0.96 Å, CN ¼7 and r ¼1.019 Å, CN ¼8) in Y7O6F9 host lattice owing to their similar atomic size with same valence state under reducing atmosphere. Eu3 þ was easily reduced to Eu2 þ under same reducing condition in the host structure when Mn2 þ ions (r ¼ 0.90 Å, CN ¼7 and r ¼0.96 Å, CN ¼8), which are small and have a low valence state, were replaced with Y3 þ ions [15,16]. As previous studies [9,19], the Mn2 þ d–d transitions, attributed to 4E(D), 4T2(D), 4 A1(4G)/4E(4G), and 4T1(4G) between 300 and 500 nm and 4 T1(6G)-6A1(6S) at around 600 nm, are difficult to achieve because the corresponding electric dipole is forbidden [5,11,12].

The PL spectrum of only Mn2 þ -substituted Y7O6F9 phosphor was barely observed, as compared with that of the blue Eu2 þ -doped phosphor. However, the green or red emission of Mn2 þ transitions in host lattices can be elevated by codoping a sensitizer such as Eu2 þ ion through energy transfer from Eu2 þ to Mn2 þ ions [4,9]. As shown in the PL emission spectra of the Eu2 þ , Mn2 þ co-doped (p ¼0.01, q ¼ 0–0.1 and p ¼0.05, q ¼0–0.1) Y7O6F9 phosphors in Fig. 3(a) and (b), codoping the Mn2 þ into the Eu2 þ -doped host structure enabled a high energy transfer from Eu2 þ to Mn2 þ in the vernier Y7O6F9 phase. Similar to the observations for the Y7(1  2(p þ q)/3) Eu7pMn7qO6F9 (p ¼0.01, q ¼0–0.1 and p ¼0.05, q ¼0–0.1) phosphor, the Eu2 þ luminescent intensities of Y7(1  2(p þ q)/3) Eu7pMn7qO6F9 phosphors for Eu2 þ content corresponding to p ¼0.01 and 0.05 decreased as the Mn2 þ content increased up to the level corresponding to q ¼0.1. The maximum Mn2 þ green emission of Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 phosphors was reached when the Mn2 þ content reached q ¼0.01. However, the

Fig. 4. The energy transfer efficiency (ηT) from Eu2 þ to Mn2 þ in Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 phosphors and the plot of ISO/IS versus CMnα/3 (α ¼3, 6). (a)(c)(e) p¼ 0.01, q¼ 0–0.1 and (b)(d)(f) p ¼ 0.05, q¼ 0–0.1.

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Fig. 4. (continued)

maximum Mn2 þ red emission of the phosphors was reached when the Mn2 þ content was q ¼0.01 and 0.05, respectively, and a further increase in the Mn2 þ content in the phosphors led to the apparent quenching of the relative intensity of the red emission, as shown in Fig. 3(c) and (d). The emission of Mn2 þ ion occurs from green to red regions owing to the coordination environment of the host structures. When Mn2 þ ions occupy 4 and 8-coordinated sites in host lattices, the green emission can usually be obtained owing to their weak crystal field, but other than the 4– and 8–fold environments of Mn2 þ ions in the host lattices, red emission light can be observed because of strong crystal field effect. The Y3 þ ions are coordinated by four O2  and three F  anions (YO4F3) and four O2  and four F  anions (YO4F4) in the structure of Y7O6F9, as shown in Fig. 1. The green emission was initially observed centered at 515 nm, and then, the orange emission was consecutively observed in the longer wavelength region near 575 nm, as shown in Fig. 3(a) and (b). When the Mn2 þ ions (r ¼0.90 Å, CN ¼7 and r ¼0.96 Å, CN ¼8) were gradually replaced by Y3 þ ions (r ¼0.96 Å, CN ¼7 and

r ¼1.019 Å, CN ¼8) in Eu2 þ doped Y7O6F9 structure, the 7coordinated Y3 þ site is expected to be a suitable site for the Mn2 þ ions with regard to the ionic size. However, from the beginning of substitution, the larger site of YO4F4 is preferentially occupied by Mn2 þ ions showing green light caused by a weak crystal field effect. After the concentration of Mn2 þ ions was moderately increased, the activators possibly occupy 7coordinated sites of YO4F3 in the phosphors, showing orange emission as a result of the strong crystal field effect. As shown in Fig. 3(a) and (b), a transfer of energy from Eu2 þ to Mn2 þ obviously occurred through the absorption of Eu2 þ centered at 468 nm in this Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (p ¼0.01, 0.05, q ¼ 0– 0.1) structure. The Eu2 þ and Mn2 þ ions played the roles of sensitizer and activator, respectively. In Fig. 4(a) and (b), the energy transfer efficiency (ηT) between the Ce3 þ and Mn2 þ ions in Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (p ¼ 0.01, 0.05, q ¼0–0.1) was calculated using the formula:

ηT ¼ 1–IS =ISO

ð1Þ

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Eu7pMn7qO6F9 phosphors, the critical concentrations from the total concentrations of the Eu2 þ and Mn2 þ ions, at which the energy-transfer efficiency is around 0.5, were 0.06 and 0.1. This energy transfer between Eu2 þ and Mn2 þ ions could be initiated by an electric multipolar interaction. According to the Dexter theory, the energy transfer mechanism can be expressed by the α linear plots of ISO/IS versus CMn /3, where CMn is the concentration of Mn2 þ ions, with α ¼3, 6, or 8, which corresponds to exchange, dipole–dipole, or dipole-quadrupole interactions, respectively. In Fig. 4(c) and (d), when α ¼ 3, the linear plots show the energy transfer from the Eu2 þ to Mn2 þ ions in the case of Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 phosphors (R2 ¼0.9996, p ¼0.01 and R2 ¼0.9955, p ¼0.05). When α ¼6, the linear plots show the energy transfer from the Eu2 þ to Mn2 þ ions in the case of Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 phosphors (R2 ¼0.9585, p ¼0.01 and R2 ¼0.9168, p ¼0.05) in Fig. 4(e) and (f). Therefore, the exchange interaction appears to be involved in this energy transfer mechanism [9,10,20,21]. As shown in Fig. 5, the chromaticity coordinates, x and y, are in accordance with the CIE values from the blue to orange vernier phosphors, corresponding to Y7(1 2(p þ q)/3)Eu7pMn7qO6F9 with p ¼0.01, 0.05 and q ¼ 0–0.1. Photographs of the PL light emission from blue to orange in the phosphors under 312 nm handheld lamps are shown in the insets of Fig. 5. The CIE values are also summarized in the insets in Fig. 5, along with the values obtained for the Eu2 þ – and Eu2 þ –Mn2 þ co-doped Y7O6F9 optical materials. The CIE coordinates near the blue, blue–white, and green–white regions of the CIE diagram were observed to be x ¼0.2279 and y ¼0.2558, x ¼0.2278 and y ¼0.2592, x ¼0.2408 and y ¼0.3174, x ¼ 0.2664 and y ¼0.3458 for p ¼ 0.01, 0.05, q ¼0 and p ¼0.01, 0.05, q ¼0.005 in the Y7(1 2(p þ q)/3) Eu7pMn7qO6F9 phosphors, respectively. When the concentration of Mn2 þ ions in the Y7(1 2(p þ q)/3)Eu7pMn7qO6F9 (p ¼ 0.01, 0.05) phosphors was further increased to q ¼0.01 through 0.1, the CIE coordinates exhibited a significant shift to intense orange regions.

4. Conclusions

Fig. 5. The chromaticity coordinates with the CIE values of Y7(1  2(p þ q)/3) Eu7pMn7qO6F9 (p ¼ 0.01, 0.05 and q ¼0–0.1) phosphors and photographs of the PL light emission from the blue to orange in the vernier phosphors under 312 nm handheld lamps (inset).

where IS and ISO are the luminescence intensities of the sensitizer in the presence and absence of an activator, respectively [6–10]. As the Mn2 þ content in Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (p ¼0.01, 0.05) phosphors increased from q ¼ 0.01 to 0.05, the efficiencies were noticeably enhanced by up to around 55 and 46%, respectively. When the Mn2 þ content in the phosphors moderately increased to q ¼0.1, the efficiency of the energy transfer reached to 74 and 62%, respectively. Consequently, the efficiency of energy transfer to Mn2 þ ions was maximized. For the Eu2 þ concentrations of p ¼ 0.01 and 0.05 in Y7(1  2(p þ q)/3)

Single phase of Eu2 þ , Mn2 þ co-doped Y7O6F9 phosphors was successfully prepared using a NH4F flux at 1050 °C in reducing environment. By doping a Eu2 þ emitter in the yttrium–oxyfluoride vernier host lattice, efficient blue emission was achieved under NUV excitation. The luminescent intensity of Y7(1  2p/3)Eu7pO6F9 phosphors was exploited when the Eu2 þ content corresponding to p ¼0.01, 0.05. The emission of Mn2 þ d–d transitions in the Y7O6F9 host lattices was raised by co-doping a Eu2 þ sensitizer by transferring energy from Eu2 þ to Mn2 þ ions, and their PL spectra were monitored. As the Mn2 þ contents in Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 (p ¼0.01, 0.05) phosphors increased, noticeable green and orange intensities of the Mn2 þ emission spectra consecutively appeared owing to the energy transfer with the crystal field effect in the vernier phase lattice. When α ¼3, as determined by the linear α plots of ISO/IS of Eu2 þ versus CMn /3, the mechanism of the exchange interaction energy-transfer from Eu2 þ to Mn2 þ in Y7O6F9 phosphors was clarified. The desired CIE values including emissions in Y7(1  2(p þ q)/3)Eu7pMn7qO6F9 phosphors throughout the blue to orange regions of the spectra were achieved.

Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the ministry of Education, Science and Technology (NRF2015R1D1A1A01059655).

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