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Photoluminescence enhancement in manganese-doped magnesium stannate phosphors synthesized by millimeter-wave irradiation
Radiation Measurements 45 (2010) 503–505
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Photoluminescence enhancement in manganese-doped magnesium stannate phosphors synthesized by millimeter-wave irradiation M. Kitaura a, *, S. Tani b, S. Mitsudo b, K. Fukui c a
Fukui National College of Technology, Sabae, Fukui 916-8507, Japan Research Center for Development of Far Infrared Region, University of Fukui, Fukui 910-8507, Japan c Department of Electrical and Electronics Engineering, University of Fukui, Fukui 910-8507, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2009 Received in revised form 20 December 2009 Accepted 14 January 2010
We have investigated photoluminescence (PL) of Mg2SnO4:Mn phosphors synthesized by the irradiation with millimeter-wave of 24 GHz. The PL spectrum at room temperature is dominated by a green band peaking at 500 nm, suggesting the existence of tetrahedral Mn2þ sites. When the phosphors are grown by the millimeter-wave heating, the 500 nm band is so much intense compared to those by the electric furnace heating. To clarify the origin of the PL enhancement, we have carried out the measurements of scanning electron microscope (SEM) and electron spin resonance (ESR). The SEM image exhibits the growth of small round particles with an average size of 1.6 mm. Such well-grown micron-sized particles were not observed under the electric furnace heating. The ESR spectra exhibit six prominent lines, the intensity of which becomes greater for the phosphors obtained by the millimeter-wave heating. From this observation, it is supposed that the PL enhancement is mainly due to the increase in the number of tetrahedral Mn2þ sites. Therefore, the millimeter-wave heating has an important advantage over the electric furnace heating in the synthesis of Mg2SnO4:Mn phosphors. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Millimeter-wave heating Photoluminescence ESR Mg2SnO4:Mn
1. Introduction Electromagnetic-wave heating is known to be a novel processing technique in ceramic materials (Rao et al., 1999). This method utilizes the inner exothermic reaction of materials, which is caused under irradiation with electromagnetic-wave in the frequency range of 0.3–300 GHz. The merit is to heat up materials rapidly and uniformly. Actually, a number of phosphors have been successfully synthesized under irradiation with the 2.45 GHz microwave for a few minutes (Dutta et al., 2002; Uematsu et al., 2004a, 2004b; Zhang et al., 2007). The heating time is surprisingly short compared to that by the electric furnace heating. This result is certainly interesting from the view of energy saving: however, it has not yet been cleared whether the electromagnetic-wave heating is effective for the improvement of photoluminescence (PL) properties in phosphors. Nowadays, the microwave heating is widely used, but it partially causes sudden raise in temperature owing to the formation of stationary wave in applicator. This problem is settled by heightening the frequency of electromagnetic-wave, that is, by using millimeter-wave. In the present study, we have synthesized the * Corresponding author. Tel.: þ81 778 62 1111; fax: þ81 778 62 2597. E-mail address: [email protected] (M. Kitaura). 1350-4487/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.01.024
phosphors of Mg2SnO4:Mn under the irradiation with millimeterwave. The PL properties of them were investigated at 300 K. Investigations using powder X-ray diffraction (XRD), scanning electron microscope (SEM) and electron spin resonance (ESR) were also performed to evaluate the character of phosphors. The experimental results for the phosphors obtained by the millimeter-wave heating are compared to those by the electric furnace heating. From these results, the advantage of the millimeter-wave heating in the synthesis of Mg2SnO4:Mn phosphors will be found. 2. Experimental procedure The synthesis of Mg2SnO4:Mn phosphors by millimeter-wave heating was carried out using the 24 GHz high-power electromagnetic-wave irradiation system at the Research Center for Development of Far Infrared Region in University of Fukui. The commercial powders of MgO (99.9%), SnO2 (99.99%), and MnCO3 (99.9%) were used as starting materials. The concentration of manganese ions was adjusted to be 0.5 mol% in preparation. The powders were mixed for 2 h using a planetary ball-mill, and were molded to the pellets with the size of 10 mm in diameter and 5 mm in height. The millimeter-wave heating and electric furnace heating of these pellets were conducted at 800 C for 20 min and at 1200 C for 3 h, respectively. A part of the pellets was pulverized to use in
504
M. Kitaura et al. / Radiation Measurements 45 (2010) 503–505
the measurements of XRD and ESR spectra. From the analysis of XRD spectra, it turn out that the powders thus obtained are almost the single phase of Mg2SnO4 with the inverse spinel structure. The remainders were used in the measurements of PL spectra and SEM images. The PL spectra were measured at 300 K with use of a xenon lamp. The ultraviolet and visible light from a xenon lamp was dispersed through a grating monochromator, and was used as an excitation light. The PL spectra shown here were corrected for the spectral response and sensitivity of the detecting system by a calibrated tungsten lamp. The SEM images were observed using a field emission type SEM apparatus. Palladium thin film was pre-coated at the surface of phosphors by argon sputtering, to eliminate the surface charging effect due to electron beam irradiation. The ESR spectra were measured at 300 K using an X-band spectrometer. The magnetic field modulation was performed at 100 kHz. The microwave power was set to be 0.5 mW in our measurement, because the ESR signals were not saturated at 0.5 mW in our samples. 3. Results and discussions Fig. 1(a) and 1(b) show PL spectra of Mg2SnO4:Mn synthesized by millimeter-wave heating and electric furnace heating, respectively. The PL spectra were measured under excitation with ultraviolet light at 270 nm. An emission band peaking at around 500 nm is predominantly observed in both PL spectra. This band has been reported by Kim et al. (2002) and Lei et al. (2006). An appearance of the 500 nm band suggests the existence of Mn2þ ions at tetrahedral sites (Tamatani, 2007). Another emission band also appears around 675 nm, but it is very weak. The 675 nm band has not been reported by Kim et al. (2002) and Lei et al. (2006). The origin of the 675 nm band is not clear at present. The 500 and 675 nm bands in Fig. 1(a) are much intense compared to those in Fig. 1(b). From this result, it
Fig. 1. PL spectra of Mg2SnO4:Mn phosphors synthesized by the millimeter-wave heating (a) and electric furnace heating (b), measured at room temperature under excitation with UV light at 270 nm.
is apparent that, in the synthesis of Mg2SnO4:Mn phosphors, the millimeter-wave heating has the advantage over the electric furnace heating. Fig. 2(a) and 2(b) show SEM images of Mg2SnO4:Mn phosphors synthesized by the millimeter-wave heating and electric furnace heating, respectively. In Fig. 2(a), well-grown round particles are observed. The particle size is roughly estimated to be 1.6 mm in average. Such micron-sized particles are not observed in Fig. 2(b). Therefore, it is evident that the crystalinity of Mg2SnO4:Mn phosphors becomes better when the phosphors are grown by the millimeter-wave heating. In Fig. 2(b), it also looks that small particles aggregate to form large lumps in places. The formation of such lumps would cause an increase in the scattering of excitation light, which leads to the decrease in the intensity of an emission band. Fig. 3(a) and 3(b) present ESR spectra of Mg2SnO4:Mn phosphors synthesized by the millimeter-wave heating and electric furnace heating, respectively. Both ESR spectra have six sharp lines. These lines are assigned to the allowed transitions (DMS ¼ 1, DMI ¼ 0) of MS ¼ 1/2 4 þ 1/2 split by the hyperfine interaction between the electron cloud of Mn2þ ion and 55Mn nucleus of spin I ¼ 5/2 (Tomita et al., 2004; Singh et al., 2007). The intensity of such six lines becomes greater for the phosphors synthesized by the millimeter-wave heating. Since this result is linked to the results of SEM observation in Fig. 2, we suppose that an improvement of crystalinity enables effective doping of Mn2þ ions at tetrahedral lattice sites. Therefore, it is reasonable to attribute the PL enhancement in Fig. 1 to the increase in the number of Mn2þ ions at tetrahedral surroundings.
Fig. 2. SEM images of Mg2SnO4:Mn phosphors synthesized by the millimeter-wave heating (a) and electric furnace heating (b).
M. Kitaura et al. / Radiation Measurements 45 (2010) 503–505
505
an indicative of the advantage of millimeter-wave heating in the synthesis of Mg2SnO4:Mn phosphors. It was revealed that the primary particles are well-grown in the phosphors synthesized by the millimeter-wave heating. This result is linked to high reactivity at the interface between particles, which enables an effective doping of activators and a good crystalline quality of host material. Therefore, the millimeter-wave heating has the potential as a fruitful method to prepare the ceramics of materials with high melting point. The present study should be extended to the preparation of transparent ceramic scintillators with high functionality. Acknowledgements This work was supported by JSPJ KAKENHI (20613013) and Nippon Sheet Glass Foundation for Materials Science and Engineering. References
Fig. 3. ESR spectra of Mg2SnO4:Mn phosphors synthesized by the millimeter-wave heating (a) and electric furnace heating (b), measured at room temperature.
4. Summary The phosphors of Mg2SnO4:Mn have been synthesized by the 24 GHz mm-wave heating and electric furnace heating. For the samples obtained by these methods, PL spectra were measured at 300 K under excitation with ultraviolet light at 270 nm. The PL spectra exhibit a dominant band peaking at 500 nm, suggesting the existence of tetrahedral Mn2þ sites. The 500 nm band becomes higher when the millimeter-wave hating is adopted. This result is
Dutta, S., Mohammad, Q., Manoharan, S.S., 2002. Luminescence properties of Eu3þ/ Y2O3 ultrafine powders prepared by rapid microwave-combustion route. J. Mater. Sci. Lett. 21, 1077–1079. Kim, K.N., Jung, H.K., Park, H.D., Kim, D., 2002. High luminance of new green emitting phosphor, M2SnO4:Mn. J. Lumi 99, 169–173. Lei, B., Li, B., Wang, X., Li, W., 2006. Green emitting long lasting phosphorescence (LLP) properties of Mg2SnO4:Mn2þ phosphor. J. Lumi 118, 173–178. Rao, K.J., Vaidhyanathan, B., Ganguli, M., Ramakrishnan, P.A., 1999. Synthesis of inorganic solids using microwaves. Chem. Mater. 11, 882–895. Singh, V., Chakradhar, R.P.S., Rao, J.L., Kim, D.K., 2007. Synthesis, characterization, photoluminescence and EPR investigations of Mn doped MgAl2O4 phosphors. J. Solid State Chem. 180, 2067–2074. Tamatani, S., 2007. Principal phosphor materials and their optical properties. In: William, M.Y., Shionoya, S., Yamamoto, H. (Eds.), Phosphor Handbook, second ed. CRC Press, Boca Raton, pp. 167–190. Tomita, A., Sato, T., Tanaka, K., Kawabe, Y., Shirai, M., Tanaka, K., Hanamura, E., 2004. Luminescence channels of manganese-doped spinel. J. Lumi 109, 19–24. Uematsu, K., Toda, K., Sato, M., 2004a. Intelligent reactions of inorganic phosphor materials with microwave heating. Chem. Lett. 33, 990–991. Uematsu, K., Toda, K., Sato, M., 2004b. Preparation of YVO4:Eu3þ phosphor using microwave heating method. J. Alloys Comp. 389, 209–214. Zhang, P., Xu, M., Zheng, Z., Sun, B., Zhang, Y., 2007. Rapid formation of red long afterglow phosphor Sr3Al2O6:Eu2þ, Dy3þ by microwave irradiation. Mater. Sci. Eng. B 136, 159–164.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Photoluminescence enhancement in manganese-doped magnesium stannate phosphors synthesized by millimeter-wave irradiation M. Kitaura a, *, S. Tani b, S. Mitsudo b, K. Fukui c a
Fukui National College of Technology, Sabae, Fukui 916-8507, Japan Research Center for Development of Far Infrared Region, University of Fukui, Fukui 910-8507, Japan c Department of Electrical and Electronics Engineering, University of Fukui, Fukui 910-8507, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2009 Received in revised form 20 December 2009 Accepted 14 January 2010
We have investigated photoluminescence (PL) of Mg2SnO4:Mn phosphors synthesized by the irradiation with millimeter-wave of 24 GHz. The PL spectrum at room temperature is dominated by a green band peaking at 500 nm, suggesting the existence of tetrahedral Mn2þ sites. When the phosphors are grown by the millimeter-wave heating, the 500 nm band is so much intense compared to those by the electric furnace heating. To clarify the origin of the PL enhancement, we have carried out the measurements of scanning electron microscope (SEM) and electron spin resonance (ESR). The SEM image exhibits the growth of small round particles with an average size of 1.6 mm. Such well-grown micron-sized particles were not observed under the electric furnace heating. The ESR spectra exhibit six prominent lines, the intensity of which becomes greater for the phosphors obtained by the millimeter-wave heating. From this observation, it is supposed that the PL enhancement is mainly due to the increase in the number of tetrahedral Mn2þ sites. Therefore, the millimeter-wave heating has an important advantage over the electric furnace heating in the synthesis of Mg2SnO4:Mn phosphors. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Millimeter-wave heating Photoluminescence ESR Mg2SnO4:Mn
1. Introduction Electromagnetic-wave heating is known to be a novel processing technique in ceramic materials (Rao et al., 1999). This method utilizes the inner exothermic reaction of materials, which is caused under irradiation with electromagnetic-wave in the frequency range of 0.3–300 GHz. The merit is to heat up materials rapidly and uniformly. Actually, a number of phosphors have been successfully synthesized under irradiation with the 2.45 GHz microwave for a few minutes (Dutta et al., 2002; Uematsu et al., 2004a, 2004b; Zhang et al., 2007). The heating time is surprisingly short compared to that by the electric furnace heating. This result is certainly interesting from the view of energy saving: however, it has not yet been cleared whether the electromagnetic-wave heating is effective for the improvement of photoluminescence (PL) properties in phosphors. Nowadays, the microwave heating is widely used, but it partially causes sudden raise in temperature owing to the formation of stationary wave in applicator. This problem is settled by heightening the frequency of electromagnetic-wave, that is, by using millimeter-wave. In the present study, we have synthesized the * Corresponding author. Tel.: þ81 778 62 1111; fax: þ81 778 62 2597. E-mail address: [email protected] (M. Kitaura). 1350-4487/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.01.024
phosphors of Mg2SnO4:Mn under the irradiation with millimeterwave. The PL properties of them were investigated at 300 K. Investigations using powder X-ray diffraction (XRD), scanning electron microscope (SEM) and electron spin resonance (ESR) were also performed to evaluate the character of phosphors. The experimental results for the phosphors obtained by the millimeter-wave heating are compared to those by the electric furnace heating. From these results, the advantage of the millimeter-wave heating in the synthesis of Mg2SnO4:Mn phosphors will be found. 2. Experimental procedure The synthesis of Mg2SnO4:Mn phosphors by millimeter-wave heating was carried out using the 24 GHz high-power electromagnetic-wave irradiation system at the Research Center for Development of Far Infrared Region in University of Fukui. The commercial powders of MgO (99.9%), SnO2 (99.99%), and MnCO3 (99.9%) were used as starting materials. The concentration of manganese ions was adjusted to be 0.5 mol% in preparation. The powders were mixed for 2 h using a planetary ball-mill, and were molded to the pellets with the size of 10 mm in diameter and 5 mm in height. The millimeter-wave heating and electric furnace heating of these pellets were conducted at 800 C for 20 min and at 1200 C for 3 h, respectively. A part of the pellets was pulverized to use in
504
M. Kitaura et al. / Radiation Measurements 45 (2010) 503–505
the measurements of XRD and ESR spectra. From the analysis of XRD spectra, it turn out that the powders thus obtained are almost the single phase of Mg2SnO4 with the inverse spinel structure. The remainders were used in the measurements of PL spectra and SEM images. The PL spectra were measured at 300 K with use of a xenon lamp. The ultraviolet and visible light from a xenon lamp was dispersed through a grating monochromator, and was used as an excitation light. The PL spectra shown here were corrected for the spectral response and sensitivity of the detecting system by a calibrated tungsten lamp. The SEM images were observed using a field emission type SEM apparatus. Palladium thin film was pre-coated at the surface of phosphors by argon sputtering, to eliminate the surface charging effect due to electron beam irradiation. The ESR spectra were measured at 300 K using an X-band spectrometer. The magnetic field modulation was performed at 100 kHz. The microwave power was set to be 0.5 mW in our measurement, because the ESR signals were not saturated at 0.5 mW in our samples. 3. Results and discussions Fig. 1(a) and 1(b) show PL spectra of Mg2SnO4:Mn synthesized by millimeter-wave heating and electric furnace heating, respectively. The PL spectra were measured under excitation with ultraviolet light at 270 nm. An emission band peaking at around 500 nm is predominantly observed in both PL spectra. This band has been reported by Kim et al. (2002) and Lei et al. (2006). An appearance of the 500 nm band suggests the existence of Mn2þ ions at tetrahedral sites (Tamatani, 2007). Another emission band also appears around 675 nm, but it is very weak. The 675 nm band has not been reported by Kim et al. (2002) and Lei et al. (2006). The origin of the 675 nm band is not clear at present. The 500 and 675 nm bands in Fig. 1(a) are much intense compared to those in Fig. 1(b). From this result, it
Fig. 1. PL spectra of Mg2SnO4:Mn phosphors synthesized by the millimeter-wave heating (a) and electric furnace heating (b), measured at room temperature under excitation with UV light at 270 nm.
is apparent that, in the synthesis of Mg2SnO4:Mn phosphors, the millimeter-wave heating has the advantage over the electric furnace heating. Fig. 2(a) and 2(b) show SEM images of Mg2SnO4:Mn phosphors synthesized by the millimeter-wave heating and electric furnace heating, respectively. In Fig. 2(a), well-grown round particles are observed. The particle size is roughly estimated to be 1.6 mm in average. Such micron-sized particles are not observed in Fig. 2(b). Therefore, it is evident that the crystalinity of Mg2SnO4:Mn phosphors becomes better when the phosphors are grown by the millimeter-wave heating. In Fig. 2(b), it also looks that small particles aggregate to form large lumps in places. The formation of such lumps would cause an increase in the scattering of excitation light, which leads to the decrease in the intensity of an emission band. Fig. 3(a) and 3(b) present ESR spectra of Mg2SnO4:Mn phosphors synthesized by the millimeter-wave heating and electric furnace heating, respectively. Both ESR spectra have six sharp lines. These lines are assigned to the allowed transitions (DMS ¼ 1, DMI ¼ 0) of MS ¼ 1/2 4 þ 1/2 split by the hyperfine interaction between the electron cloud of Mn2þ ion and 55Mn nucleus of spin I ¼ 5/2 (Tomita et al., 2004; Singh et al., 2007). The intensity of such six lines becomes greater for the phosphors synthesized by the millimeter-wave heating. Since this result is linked to the results of SEM observation in Fig. 2, we suppose that an improvement of crystalinity enables effective doping of Mn2þ ions at tetrahedral lattice sites. Therefore, it is reasonable to attribute the PL enhancement in Fig. 1 to the increase in the number of Mn2þ ions at tetrahedral surroundings.
Fig. 2. SEM images of Mg2SnO4:Mn phosphors synthesized by the millimeter-wave heating (a) and electric furnace heating (b).
M. Kitaura et al. / Radiation Measurements 45 (2010) 503–505
505
an indicative of the advantage of millimeter-wave heating in the synthesis of Mg2SnO4:Mn phosphors. It was revealed that the primary particles are well-grown in the phosphors synthesized by the millimeter-wave heating. This result is linked to high reactivity at the interface between particles, which enables an effective doping of activators and a good crystalline quality of host material. Therefore, the millimeter-wave heating has the potential as a fruitful method to prepare the ceramics of materials with high melting point. The present study should be extended to the preparation of transparent ceramic scintillators with high functionality. Acknowledgements This work was supported by JSPJ KAKENHI (20613013) and Nippon Sheet Glass Foundation for Materials Science and Engineering. References
Fig. 3. ESR spectra of Mg2SnO4:Mn phosphors synthesized by the millimeter-wave heating (a) and electric furnace heating (b), measured at room temperature.
4. Summary The phosphors of Mg2SnO4:Mn have been synthesized by the 24 GHz mm-wave heating and electric furnace heating. For the samples obtained by these methods, PL spectra were measured at 300 K under excitation with ultraviolet light at 270 nm. The PL spectra exhibit a dominant band peaking at 500 nm, suggesting the existence of tetrahedral Mn2þ sites. The 500 nm band becomes higher when the millimeter-wave hating is adopted. This result is
Dutta, S., Mohammad, Q., Manoharan, S.S., 2002. Luminescence properties of Eu3þ/ Y2O3 ultrafine powders prepared by rapid microwave-combustion route. J. Mater. Sci. Lett. 21, 1077–1079. Kim, K.N., Jung, H.K., Park, H.D., Kim, D., 2002. High luminance of new green emitting phosphor, M2SnO4:Mn. J. Lumi 99, 169–173. Lei, B., Li, B., Wang, X., Li, W., 2006. Green emitting long lasting phosphorescence (LLP) properties of Mg2SnO4:Mn2þ phosphor. J. Lumi 118, 173–178. Rao, K.J., Vaidhyanathan, B., Ganguli, M., Ramakrishnan, P.A., 1999. Synthesis of inorganic solids using microwaves. Chem. Mater. 11, 882–895. Singh, V., Chakradhar, R.P.S., Rao, J.L., Kim, D.K., 2007. Synthesis, characterization, photoluminescence and EPR investigations of Mn doped MgAl2O4 phosphors. J. Solid State Chem. 180, 2067–2074. Tamatani, S., 2007. Principal phosphor materials and their optical properties. In: William, M.Y., Shionoya, S., Yamamoto, H. (Eds.), Phosphor Handbook, second ed. CRC Press, Boca Raton, pp. 167–190. Tomita, A., Sato, T., Tanaka, K., Kawabe, Y., Shirai, M., Tanaka, K., Hanamura, E., 2004. Luminescence channels of manganese-doped spinel. J. Lumi 109, 19–24. Uematsu, K., Toda, K., Sato, M., 2004a. Intelligent reactions of inorganic phosphor materials with microwave heating. Chem. Lett. 33, 990–991. Uematsu, K., Toda, K., Sato, M., 2004b. Preparation of YVO4:Eu3þ phosphor using microwave heating method. J. Alloys Comp. 389, 209–214. Zhang, P., Xu, M., Zheng, Z., Sun, B., Zhang, Y., 2007. Rapid formation of red long afterglow phosphor Sr3Al2O6:Eu2þ, Dy3þ by microwave irradiation. Mater. Sci. Eng. B 136, 159–164.