- Email: [email protected]
Synthesis and luminescence of SrZnO2 phosphors
ARTICLE IN PRESS Journal of Luminescence 130 (2010) 1690–1693
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and luminescence of SrZnO2 phosphors D.R. Taikar a, C.P. Joshi a, S.V. Moharil b,n, P.L. Muthal c, S.M. Dhopte c a b c
Physics Department, Shri Ramdeobaba K.N. Engineering College, Katol Road, Nagpur 440 013, India Department of Physics, Nagpur University, Nagpur 440010, India National Environmental Engineering Research Institute, C.S.I.R., Nagpur, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 September 2009 Received in revised form 24 March 2010 Accepted 26 March 2010 Available online 1 April 2010
SrZnO2 phosphors have been synthesized by two new methods viz. carbonate decomposition at 1000 1C and combustion synthesis. Phosphors activated with Pb2 + , Sm3 + , Tb3 + , Bi3 + and Pr3 + could be prepared in one step using the combustion synthesis. Characteristic emission and excitation were observed for Bi3 + . For the remaining activators excitation spectra always contained a band at 283 nm. Presence of this band for all these different types of activators was interpreted as host sensitization. & 2010 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Oxide phosphors Combustion synthesis
1. Introduction Crystal structure of SrZnO2 was reported as early as in 1960 [1]. There were not many luminescence studies made until recently. Kubota et al. [2] reported luminescence of Mn2 + . In the following years, luminescence of various activators Pb2 + [3], Ce3 + [4], Pr3 + [5], Sm3 + [6,7], Tb3 + [8,9], Eu3 + [10–12], etc. in this host was studied. Use of these phosphors in applications like solid state lighting was also suggested [6]. Most of the investigators used the conventional solid state reaction route, but some novel methods like sol–gel [5] and gel-combustion [6] have been used recently. Solid state reaction involves heating the reactants at high temperatures, around 1200 1C. Some simpler preparation methods will be desirable. In this paper we report the preparation of SrZnO2 phosphors containing various activators by two different routes including combustion synthesis.
2. Experimental Two methods were used for preparation of SrZnO2 phosphors. In the first method, analytical reagent grade Sr(NO3)2 and Zn(NO3)2 6H2O were dissolved in double distilled water. The solutions were mixed. Desired quantity of activator in the form of nitrate was added to this solution. (NH4)2CO3 solution was then slowly added. The precipitate thus obtained was separated by filtering, dried and thoroughly crushed to yield fine powders. It was decomposed by heating in air at 1000 1C for 1 h to yield SrZnO2 phosphor containing the desired activator. n
Corresponding author. Tel.: + 91 712 2283708; fax: +91 712 2249875. E-mail address: [email protected] (S.V. Moharil).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.03.033
In the second procedure, modified combustion synthesis was used [13]. Exothermic reaction between NH4NO3 and urea is used to prepare mixed metal oxides. Sr(NO3)2 6H2O, ZnO, NH4NO3 and urea in proportions 1:1:10:10 were thoroughly mixed. The china dish containing the mixture was inserted in a furnace preheated to 500 1C. Within minutes the mixture swelled, foamed and finally ignited, yielding a white fluffy powder, which was collected after removing the dish from the furnace. Activator in the form of nitrate was added to the combustion mixture during the mixing. Activator concentration was varied between 0.1 and 5 mol%. However, maximum PL intensity was observed for 1 mol%. Hence results corresponding to this concentration are only presented. X-ray diffraction patterns were recorded on a Philips PANalytical X’pert Pro diffractometer. Particles in the range 63–100 mm were sieved for PL measurements. Particle size distribution was studied using CILAS particle size analyser Model L/D 1180. Optical micrographs were obtained with the help of Olympus optical microscope model B-41 with a CCD camera. Photoluminescence (PL) spectra in the spectral range 220–700 nm were recorded at room temperature on a Hitachi F-4000 spectro-fluorimeter with spectral slit widths of 1.5 nm. Various SrZnO2 phosphors were found to be stable; no change in the PL properties was observed for the phosphors stored for several months. Samples were also found to be stable against UV irradiation that was used for the PL measurements. No changes in spectral positions or intensities were observed during several, successive PL runs.
3. Results and discussions SrZnO2 crystallizes in the orthorhombic system in the space group Pnma. Schnering and Hoppe [1] reported the crystal
ARTICLE IN PRESS D.R. Taikar et al. / Journal of Luminescence 130 (2010) 1690–1693
structure. The crystal structure had been determined by threedimensional Patterson and Fourier syntheses. In SrZnO2, [ZnO2] arrangement is a previously unknown two-dimensional one, and the ZnO4 tetrahedra share edges with each other and form waved layers. The Sr atoms are located at the cavity between the layer. There is one site for the Sr atom, one for the Zn atom and two for the O atoms. Seven oxygen atoms surround one strontium atom and all Sr sites are crystallographically equivalent. Fig. 1 shows diffraction data obtained for the SrZnO2 samples prepared by the combustion method. The patterns are compared with the major lines in the ICDD data file 41-0551. An excellent match is seen. Similar results were obtained for the powders prepared by carbonate decomposition. SrZnO2 is thus formed by two new procedures described above. It is remarkable that SrZnO2 is formed in one step by the combustion procedure. No high temperature annealing was required. XRD pattern of the precipitated powders, on the other hand, was very noisy, most probably due to poor crystallinity of the precipitates. Few lines could be made out superposed on the noisy background around 2.7475 and 3.2365 A, which could be due to ZnCO3 (83–1765) and SrCO3 (74–1624), respectively. There is no mention of the double carbonate SrZn(CO3)2 in the ICDD database. Fig. 2 shows the particle size distribution. For powders prepared by carbonate decomposition and 1000 1C annealing, the distribution is rather broad; particles of size ranging from 3 to 300 mm are formed. Maximum is observed around 125 mm. Powders prepared by combustion synthesis are finer. The distribution is still broader and maximum is observed around 20 mm. Optical micrographs of some selected particles are presented in Fig. 3.
Fig. 1. XRD results for SrZnO2 diffraction pattern of SrZnO2 are compared with the stick pattern of ICDD file 41-0551.
Fig. 2. Typical particle size distribution phosphors prepared by (a) carbonate decomposition method, (b) combustion synthesis.
1691
Fig. 3. Optical micrographs of SrZnO2: (a) sample prepared by combustion synthesis and (b) sample prepared by precipitation followed by thermal decomposition.
The luminescence results, which are described next, were also similar for the phosphors prepared by two methods.
3.1. ns2 activators For ns2 type impurities, the ground level is 1S0 arising from the S2 configuration and the lowest excited levels are 3P0, 3P1 and 1P1 derived from the excited sp configuration. The absorption spectra of ns2 ions in solids consist of three main bands labeled A, B and C in order of increasing energy corresponding to transitions 1 S0-3P0, 3P1 and 1P1, respectively. The C band corresponds to allowed transition, whereas transitions corresponding to A and B bands are only partly allowed by spin–orbit coupling and vibronic coupling, respectively. The A band lies in the UV range. It is markedly sensitive to the environment [14,15]. The lower-lying 3 P0 and 3P1 excited state levels are responsible for the luminescence features. The transition from the 3P0 to the ground 1S0 state is J–J forbidden and only weakly allowed by phonon interactions. It is thus characterized by long radiative lifetime, of the order of milliseconds. The transition from the 3P1 to the ground 1S0 state is partially allowed due to the spin–orbit coupling, which mixes the spin-allowed 1P1 level with 3P1 level. Such mixing results in radiative lifetime of the order of hundreds of nanoseconds [16]. The 3P0 level is usually called metastable, while the 3P1 level is often referred to as radiative. The 3P1 and 3P0 levels are close enough to obtain the thermal population of the 3P1 level from 3P0 at higher temperatures. Fig. 4 shows PL emission (curve a) and excitation (curve b) spectra for Pb2 + doped (1 mol%) SrZnO2. Due to matching size, Pb2 + is expected to occupy Sr2 + substitutional site. Emission of moderate intensity is observed in the form of a rather broad band around 450 nm. In the excitation spectrum a broad band is observed around 283 nm. A second excitation band around 315 nm is much narrower. Pb2 + has been studied in several hosts (for a short review see Ref. [17]). No systematic dependence of the position of Pb2 + band on the physico-chemical properties was noticed, and hence Pb2 + emission is said to be difficult to predict [18]. In several of these hosts the quenching temperature is quite low. Considering this, the emission of even moderate intensity in SrZnO2 is encouraging. The positions of the emission and excitation bands are in excellent agreement with the results of Ref. [3]. Manavbasi and LaCombe [3] attributed 283 and
ARTICLE IN PRESS 1692
D.R. Taikar et al. / Journal of Luminescence 130 (2010) 1690–1693
Fig. 4. PL spectra for SrZnO2 activated with some ns2 impurities. (a) Pb2 + emission for 283 nm excitation. (b) Excitation for 450 nm emission of SrZnO2:Pb2 + . (c) Bi3 + emission in SrZnO2 for 375 nm excitation. (d) Excitation for 513 nm emission of SrZnO2:Bi3 + .
Fig. 5. PL spectra for SrZnO2 doped with lanthenide activators. (a) Tb3 + emission for 283 nm excitation, (b) Excitation for 543 nm emission of SrZnO2:Tb3 + , (c) Sm3 + emission in SrZnO2 for 415 nm excitation, (d) Excitation for 606 nm emission of SrZnO2:Sm3 + .
315 nm excitation bands to the Pb2 + ion trapped exciton and A band (1S0-3P1), respectively. Fig. 4 also includes PL spectra for another ns2 ion viz. Bi3 + , which was added in concentration of 1 mol% to SrZnO2. Very broad band ranging from 400 to 600 nm and peaking around 515 nm is observed in the emission spectrum (Fig. 4, curve c). The excitation band is much narrower (Fig. 4, curve d). It is located at 377 nm with a shoulder around 325 nm. Unlike Pb2 + there is no host sensitized emission in Bi3 + as we do not observe the band around 283 nm in the excitation spectrum for Bi3 + . There are no previous studies available on Bi3 + emission in SrZnO2 host for comparison.
(112 pm) and Zn2 + (76 pm). With incorporation of a cation vacancy, Sm3 + may occupy Sr2 + substitutional site or Zn2 + substitutional site. The former lacks centre of inversion, while the latter possesses such symmetry. The forced electric dipole transition 4G5/2-6H9/2 of Sm3 + , responsible for 651 nm emission in SrZnO2, is highly sensitive to site symmetry. It can be observed only when Sm3 + ion is at the site lacking an inversion symmetry [19]. The transition 4G5/2-6H7/2 of Sm3 + , responsible for 607 nm emission in SrZnO2, is observed when Sm3 + ion is at the site possessing inversion symmetry. Tentatively, we assign 607 nm emission band to Sm3 + at Zn2 + substitutional site and the 651 nm emission band to Sm3 + at Sr2 + substitutional site. These observations are consistent with the literature results [6]. The excitation spectrum (Fig. 5, curve d) shows strong bands around 410 and 283 nm. There is also a prominent shoulder around 250 nm, which appears to be characteristic of Sm3 + . The 410 nm excitation band can be assigned to 6H5/2-4L13/2 transition of Sm3 + , while the 250 nm absorption is due to charge transfer (CT) band. The 283 nm excitation band is related to the host. Intense UV excited luminescence of Pr3 + can be observed only if f–d excitation band is located in this region or if there is energy transfer from host, which absorbs in the UV region to one of the levels of 4f2 configuration of Pr3 + . Pr3 + emission can be observed in UV, blue, green or red region depending on the position of the f–d excitation band [15]. In most cases this band is situated in the VUV region. For SrZnO2:Pr3 + (0.5%), intense white emission was observed (Fig. 6, curve a). The most intense, double humped peak was observed in the blue green region with maxima at 494 and 503 nm. Several other peaks of much smaller intensities were observed around 533, 560, 600 and 620 nm. These can be identified with 3Pj-3H4, 3Pj-3H5, 3Pj-3H6,3F2 transitions of Pr3 + . The prominent excitation around 283 nm (Fig. 6, curve b) can be attributed to the host absorption, while the excitation band in the form of a shoulder around 230 nm to the f–d excitation. PL spectra for Ce3 + activated SrZnO2 are also included in Fig. 6 (curves c and d). The emission is in the form of a broad band extending from 374 to 609 nm with a peak at around 466 nm under 290 nm excitation. This is in good agreement with the results of Manavbasi and LaCombe [4]. The excitation spectrum consists of a prominent band around 289 nm with a shoulder on the long wavelength side around 340 nm. These may be attributed to the transitions from 2F7/2 state of 4f1 configuration to the states of 5d1 configuration. There are several shoulders on the short wavelength side around 233, 240, 260 and 282 nm. The last two appear to be quite similar to those observed for the other rare earth ions. It is quite likely that host excitation is overlapping the Ce3 + absorption.
3.2. 4fn activators Terbium shows strong excitation corresponding to allowed transition between 7F6 ground state of 4f8 configuration to the levels of 4f75d1 configuration. Quite often, this falls in deep UV or VUV region of the spectrum. From the excited state the ion relaxes in several steps to 5Dj levels of 4f8 configuration. Line emission corresponding to f–f transitions is observed. Commonly observed, most intense lines are around 485 and 543 nm corresponding to 5 D4-7F6 and 5D4-7F5 transitions, respectively. At low concentrations blue emission is dominant. Near UV emission around 385 nm corresponding to 5D3-7F6 transition is also observable. For high concentrations, these emissions are quenched by cross relaxation and the green emission becomes dominant. Fig. 5(a) shows emission spectrum for SrZnO2:Tb3 + (1%). Broad emission lines are located at 413, 438, 481, 491, 544, 590 and 620 nm. The first two of these, which are rather weak, can be identified with the transitions 5D3-7F5, 7F4. The remaining ones are due to 5D4-7FJ transitions. The most intense line corresponding to the transition 5D4-7F5 is split into two components. The excitation spectrum (Fig. 5, curve b) contains a strong band around 283 nm. Since this band is observed for most of the activators, it cannot be attributed to the intra-configurational transition 4f8 (7F6)-4f75d1. This excitation band is related to the host and previously attributed to the exciton absorption near the activator [3]. PL spectra for Sm activated (1%) SrZnO2 are also included in Fig. 5. One may expect Sm to enter as divalent ion at Sr2 + substitutional site. However, no emission that can be attributed to Sm2 + was observed. Broad emission lines are observed around 570, 607 and 651 nm (Fig. 5, curve c), which can be attributed to 4 G5/2-6H5/2, 4G5/2-6H7/2 and 4G5/2-6H9/2 transitions of Sm3 + . Ionic radius of Sm3 + (96 pm) is intermediate to those for Sr2 +
ARTICLE IN PRESS D.R. Taikar et al. / Journal of Luminescence 130 (2010) 1690–1693
1693
there is strong excitation attributable to the host in addition to the characteristic f–d excitation. Reasons for absence of the 283 nm excitation for Bi3 + are not yet clear. However, it should not be surprising. Host sensitized luminescence cannot be observed for all the activators. Several conditions are to be satisfied. More studies on SrZnO2 will be required to clarify this point.
4. Conclusions SrZnO2 containing various activators viz. Pb2 + , Ce3 + , Sm3 + , Tb , Bi3 + and Pr3 + could be successfully prepared by carbonate decomposition method as well as combustion synthesis. The latter would provide a fast, one step method for preparing these phosphors. Characteristic excitation and emission were observed for Bi3 + , while for the remaining activators a band around 283 nm is always present in the excitation spectrum. This excitation band had been observed in most of the previous works but neglected quite often. Notwithstanding previous interpretations, this has been attributed to the host. The luminescence of Pb2 + , Sm3 + , Tb3 + and Pr3 + in SrZnO2 is host sensitized. The mechanism of energy transfer from host to the activator is not yet clear and more studies like the lifetime measurements, low temperature measurements, other activators, etc. will be needed to get the details. 3+
Fig. 6. PL spectra for SrZnO2 doped with Pr3 + or Ce3 + . (a) Pr3 + emission for 283 nm excitation. (b) Excitation for 500 nm emission of SrZnO2:Pr3 + . (c) Ce3 + emission in SrZnO2 for 290 nm excitation. (d) Excitation for 466 nm emission of SrZnO2:Ce3 + .
It is thus seen that appreciable luminescence is observed in SrZnO2 for several activators. Except for Bi, all the activators studied in this work show strong 283 nm excitation band. Manavbasi and LaCombe [3] found that the 283 nm absorption band is related to the host absorption. There is compelling evidence to suggest that the 283 nm excitation band is related to this absorption. However, in the previous work [3] it has been argued that emission in SrZnO2:Pb2 + is not due to host to activator energy transfer. This was based on the observation that there is no emission from the SrZnO2 host lattice, which overlaps with the absorption spectrum of the Pb2 + . In fact Yu et al. [8] found that SrZnO2 has no characteristic excitation and emission bands.. They observed excitations around 250, 262 and 280 nm for SrZnO2:Tb3 + . They totally ignored the last one and attributed the first two to the transitions from 7F6 (4f8) level to the levels of 4f75d1 configuration. A shoulder around 280 nm is also observed in the excitation spectrum of Eu3 + activated SrZnO2 [10,11], which was again neglected by the authors during discussions. We thus see that arguments for rejecting the host to activator energy transfer in SrZnO2 are rather weak and based on PL studies related to a specific activator. On the other hand we find 283 nm excitation band for a variety of activators like Pb2 + , Ce3 + , Sm3 + , Tb3 + and Pr3 + . It is too much of a coincidence that all these activators will have their characteristic excitation at this wavelength. In fact for several hosts Tb3 + and Pr3 + f–d excitations have been observed at much shorter wavelengths [20]. For Tb3 + -doped compounds the first allowed 4f8–4f75d1 transition is usually located at 14 000 cm 1 higher energy than if Ce is the dopant [21]. Pr3 + f–d excitations will appear at still shorter wavelengths [20]. These observations suggest that it will not be prudent to assign the 283 nm excitation for both the activators, Tb3 + and Pr3 + , to f–d excitations. It is thus clear that luminescence of Pb2 + , Sm3 + , Tb3 + and Pr3 + in SrZnO2 is host sensitized. Even for Ce3 + ,
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
H.G. von Schnering, R. Hoppe, Die Naturwissenschaften 47 (1960) 467. S. Kubota, T. Oyama, H. Yamane, M. Shimada, Chem. Mater. 15 (2003) 3403. A. Manavbasi, J.C. LaCombe, J. Lumin. 128 (2008) 129. A. Manavbasi, J.C. LaCombe Mater., Res. Soc. Symp. Proc. 916 (2006) 41. X.H. He, J. Zhou, C.L. Bi, W.H. Li, Rengong Jingti Xuebao/J. Synthetic Cryst. 36 (2007) 1408. L. Yang, X. Yu, S. Yang, C. Zhou, P. Zhou, W. Gao, P. Ye, Mater. Lett. 62 (2008) 907. X. He, Xiyou Jinshu, Cailiao Yu, Gongcheng/Rare Metal Mater. Eng. 36 (2007) 1574. X. Yu, X. Xu, P. Zhou, X. Peng, S. Yang, Mater. Lett. 59 (2005) 1178. S.P. Khatkar, V.B. Taxak, D. Kumar, S.D. Han, C.H. Han, G. Sharma, J. Korean Phys. Soc. 48 (2006) 1355. Y. Wang, H. Gao, J. Solid State Chem. 179 (2006) 1870. X. Yu, X. Xu, C. Zhou, J. Tang, X. Peng, S. Yang, Mater. Res. Bull. 41 (2006) 1578. X.H. He, J. Zhou, W.H. Li, Q.F. Zhou, Chin. J. Inorg. Chem. 22 (2006) 1706. D.S. Thakare, S.K. Omanwar, S.V. Moharil, S.M. Dhopte, P.L. Muthal, V.K. Kondawar, Opt. Mater. 29 (2007) 1731–1735. F. Agullo-Lopez, J.M. Calleja, F. Cuso, F. Jaque, F.J. Lopez, Prog. Mater. Sci. 30 (1986) 187. G. Blasse Prog, Solid State Chem. 18 (1988) 79. A. Ranfagni, D. Mugnai, M. Bacci, G. Viliani, M.P. Fontana, Adv. Phys. 32 (1983) 823. C.P. Joshi, S.V. Moharil, Phys. Status Solidi (B) 220 (2000) 985. A. Van Dijken, H.F. Folkerts, G. Blasse, J. Lumin. 72/74 (1997) 660. Y. Tamura, A. Shibukawa, Jpn. J. Appl. Phys. 32 (1993) 3187. P. Dorenbos, J. Lumin. 91 (2000) 91. G. Blasse, J.P.M. van Vliet, J.W.M. Verwey, R. Hoogendam, M. Wiegel, J. Phys. Chem. Solids 50 (1989) 583.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and luminescence of SrZnO2 phosphors D.R. Taikar a, C.P. Joshi a, S.V. Moharil b,n, P.L. Muthal c, S.M. Dhopte c a b c
Physics Department, Shri Ramdeobaba K.N. Engineering College, Katol Road, Nagpur 440 013, India Department of Physics, Nagpur University, Nagpur 440010, India National Environmental Engineering Research Institute, C.S.I.R., Nagpur, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 September 2009 Received in revised form 24 March 2010 Accepted 26 March 2010 Available online 1 April 2010
SrZnO2 phosphors have been synthesized by two new methods viz. carbonate decomposition at 1000 1C and combustion synthesis. Phosphors activated with Pb2 + , Sm3 + , Tb3 + , Bi3 + and Pr3 + could be prepared in one step using the combustion synthesis. Characteristic emission and excitation were observed for Bi3 + . For the remaining activators excitation spectra always contained a band at 283 nm. Presence of this band for all these different types of activators was interpreted as host sensitization. & 2010 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Oxide phosphors Combustion synthesis
1. Introduction Crystal structure of SrZnO2 was reported as early as in 1960 [1]. There were not many luminescence studies made until recently. Kubota et al. [2] reported luminescence of Mn2 + . In the following years, luminescence of various activators Pb2 + [3], Ce3 + [4], Pr3 + [5], Sm3 + [6,7], Tb3 + [8,9], Eu3 + [10–12], etc. in this host was studied. Use of these phosphors in applications like solid state lighting was also suggested [6]. Most of the investigators used the conventional solid state reaction route, but some novel methods like sol–gel [5] and gel-combustion [6] have been used recently. Solid state reaction involves heating the reactants at high temperatures, around 1200 1C. Some simpler preparation methods will be desirable. In this paper we report the preparation of SrZnO2 phosphors containing various activators by two different routes including combustion synthesis.
2. Experimental Two methods were used for preparation of SrZnO2 phosphors. In the first method, analytical reagent grade Sr(NO3)2 and Zn(NO3)2 6H2O were dissolved in double distilled water. The solutions were mixed. Desired quantity of activator in the form of nitrate was added to this solution. (NH4)2CO3 solution was then slowly added. The precipitate thus obtained was separated by filtering, dried and thoroughly crushed to yield fine powders. It was decomposed by heating in air at 1000 1C for 1 h to yield SrZnO2 phosphor containing the desired activator. n
Corresponding author. Tel.: + 91 712 2283708; fax: +91 712 2249875. E-mail address: [email protected] (S.V. Moharil).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.03.033
In the second procedure, modified combustion synthesis was used [13]. Exothermic reaction between NH4NO3 and urea is used to prepare mixed metal oxides. Sr(NO3)2 6H2O, ZnO, NH4NO3 and urea in proportions 1:1:10:10 were thoroughly mixed. The china dish containing the mixture was inserted in a furnace preheated to 500 1C. Within minutes the mixture swelled, foamed and finally ignited, yielding a white fluffy powder, which was collected after removing the dish from the furnace. Activator in the form of nitrate was added to the combustion mixture during the mixing. Activator concentration was varied between 0.1 and 5 mol%. However, maximum PL intensity was observed for 1 mol%. Hence results corresponding to this concentration are only presented. X-ray diffraction patterns were recorded on a Philips PANalytical X’pert Pro diffractometer. Particles in the range 63–100 mm were sieved for PL measurements. Particle size distribution was studied using CILAS particle size analyser Model L/D 1180. Optical micrographs were obtained with the help of Olympus optical microscope model B-41 with a CCD camera. Photoluminescence (PL) spectra in the spectral range 220–700 nm were recorded at room temperature on a Hitachi F-4000 spectro-fluorimeter with spectral slit widths of 1.5 nm. Various SrZnO2 phosphors were found to be stable; no change in the PL properties was observed for the phosphors stored for several months. Samples were also found to be stable against UV irradiation that was used for the PL measurements. No changes in spectral positions or intensities were observed during several, successive PL runs.
3. Results and discussions SrZnO2 crystallizes in the orthorhombic system in the space group Pnma. Schnering and Hoppe [1] reported the crystal
ARTICLE IN PRESS D.R. Taikar et al. / Journal of Luminescence 130 (2010) 1690–1693
structure. The crystal structure had been determined by threedimensional Patterson and Fourier syntheses. In SrZnO2, [ZnO2] arrangement is a previously unknown two-dimensional one, and the ZnO4 tetrahedra share edges with each other and form waved layers. The Sr atoms are located at the cavity between the layer. There is one site for the Sr atom, one for the Zn atom and two for the O atoms. Seven oxygen atoms surround one strontium atom and all Sr sites are crystallographically equivalent. Fig. 1 shows diffraction data obtained for the SrZnO2 samples prepared by the combustion method. The patterns are compared with the major lines in the ICDD data file 41-0551. An excellent match is seen. Similar results were obtained for the powders prepared by carbonate decomposition. SrZnO2 is thus formed by two new procedures described above. It is remarkable that SrZnO2 is formed in one step by the combustion procedure. No high temperature annealing was required. XRD pattern of the precipitated powders, on the other hand, was very noisy, most probably due to poor crystallinity of the precipitates. Few lines could be made out superposed on the noisy background around 2.7475 and 3.2365 A, which could be due to ZnCO3 (83–1765) and SrCO3 (74–1624), respectively. There is no mention of the double carbonate SrZn(CO3)2 in the ICDD database. Fig. 2 shows the particle size distribution. For powders prepared by carbonate decomposition and 1000 1C annealing, the distribution is rather broad; particles of size ranging from 3 to 300 mm are formed. Maximum is observed around 125 mm. Powders prepared by combustion synthesis are finer. The distribution is still broader and maximum is observed around 20 mm. Optical micrographs of some selected particles are presented in Fig. 3.
Fig. 1. XRD results for SrZnO2 diffraction pattern of SrZnO2 are compared with the stick pattern of ICDD file 41-0551.
Fig. 2. Typical particle size distribution phosphors prepared by (a) carbonate decomposition method, (b) combustion synthesis.
1691
Fig. 3. Optical micrographs of SrZnO2: (a) sample prepared by combustion synthesis and (b) sample prepared by precipitation followed by thermal decomposition.
The luminescence results, which are described next, were also similar for the phosphors prepared by two methods.
3.1. ns2 activators For ns2 type impurities, the ground level is 1S0 arising from the S2 configuration and the lowest excited levels are 3P0, 3P1 and 1P1 derived from the excited sp configuration. The absorption spectra of ns2 ions in solids consist of three main bands labeled A, B and C in order of increasing energy corresponding to transitions 1 S0-3P0, 3P1 and 1P1, respectively. The C band corresponds to allowed transition, whereas transitions corresponding to A and B bands are only partly allowed by spin–orbit coupling and vibronic coupling, respectively. The A band lies in the UV range. It is markedly sensitive to the environment [14,15]. The lower-lying 3 P0 and 3P1 excited state levels are responsible for the luminescence features. The transition from the 3P0 to the ground 1S0 state is J–J forbidden and only weakly allowed by phonon interactions. It is thus characterized by long radiative lifetime, of the order of milliseconds. The transition from the 3P1 to the ground 1S0 state is partially allowed due to the spin–orbit coupling, which mixes the spin-allowed 1P1 level with 3P1 level. Such mixing results in radiative lifetime of the order of hundreds of nanoseconds [16]. The 3P0 level is usually called metastable, while the 3P1 level is often referred to as radiative. The 3P1 and 3P0 levels are close enough to obtain the thermal population of the 3P1 level from 3P0 at higher temperatures. Fig. 4 shows PL emission (curve a) and excitation (curve b) spectra for Pb2 + doped (1 mol%) SrZnO2. Due to matching size, Pb2 + is expected to occupy Sr2 + substitutional site. Emission of moderate intensity is observed in the form of a rather broad band around 450 nm. In the excitation spectrum a broad band is observed around 283 nm. A second excitation band around 315 nm is much narrower. Pb2 + has been studied in several hosts (for a short review see Ref. [17]). No systematic dependence of the position of Pb2 + band on the physico-chemical properties was noticed, and hence Pb2 + emission is said to be difficult to predict [18]. In several of these hosts the quenching temperature is quite low. Considering this, the emission of even moderate intensity in SrZnO2 is encouraging. The positions of the emission and excitation bands are in excellent agreement with the results of Ref. [3]. Manavbasi and LaCombe [3] attributed 283 and
ARTICLE IN PRESS 1692
D.R. Taikar et al. / Journal of Luminescence 130 (2010) 1690–1693
Fig. 4. PL spectra for SrZnO2 activated with some ns2 impurities. (a) Pb2 + emission for 283 nm excitation. (b) Excitation for 450 nm emission of SrZnO2:Pb2 + . (c) Bi3 + emission in SrZnO2 for 375 nm excitation. (d) Excitation for 513 nm emission of SrZnO2:Bi3 + .
Fig. 5. PL spectra for SrZnO2 doped with lanthenide activators. (a) Tb3 + emission for 283 nm excitation, (b) Excitation for 543 nm emission of SrZnO2:Tb3 + , (c) Sm3 + emission in SrZnO2 for 415 nm excitation, (d) Excitation for 606 nm emission of SrZnO2:Sm3 + .
315 nm excitation bands to the Pb2 + ion trapped exciton and A band (1S0-3P1), respectively. Fig. 4 also includes PL spectra for another ns2 ion viz. Bi3 + , which was added in concentration of 1 mol% to SrZnO2. Very broad band ranging from 400 to 600 nm and peaking around 515 nm is observed in the emission spectrum (Fig. 4, curve c). The excitation band is much narrower (Fig. 4, curve d). It is located at 377 nm with a shoulder around 325 nm. Unlike Pb2 + there is no host sensitized emission in Bi3 + as we do not observe the band around 283 nm in the excitation spectrum for Bi3 + . There are no previous studies available on Bi3 + emission in SrZnO2 host for comparison.
(112 pm) and Zn2 + (76 pm). With incorporation of a cation vacancy, Sm3 + may occupy Sr2 + substitutional site or Zn2 + substitutional site. The former lacks centre of inversion, while the latter possesses such symmetry. The forced electric dipole transition 4G5/2-6H9/2 of Sm3 + , responsible for 651 nm emission in SrZnO2, is highly sensitive to site symmetry. It can be observed only when Sm3 + ion is at the site lacking an inversion symmetry [19]. The transition 4G5/2-6H7/2 of Sm3 + , responsible for 607 nm emission in SrZnO2, is observed when Sm3 + ion is at the site possessing inversion symmetry. Tentatively, we assign 607 nm emission band to Sm3 + at Zn2 + substitutional site and the 651 nm emission band to Sm3 + at Sr2 + substitutional site. These observations are consistent with the literature results [6]. The excitation spectrum (Fig. 5, curve d) shows strong bands around 410 and 283 nm. There is also a prominent shoulder around 250 nm, which appears to be characteristic of Sm3 + . The 410 nm excitation band can be assigned to 6H5/2-4L13/2 transition of Sm3 + , while the 250 nm absorption is due to charge transfer (CT) band. The 283 nm excitation band is related to the host. Intense UV excited luminescence of Pr3 + can be observed only if f–d excitation band is located in this region or if there is energy transfer from host, which absorbs in the UV region to one of the levels of 4f2 configuration of Pr3 + . Pr3 + emission can be observed in UV, blue, green or red region depending on the position of the f–d excitation band [15]. In most cases this band is situated in the VUV region. For SrZnO2:Pr3 + (0.5%), intense white emission was observed (Fig. 6, curve a). The most intense, double humped peak was observed in the blue green region with maxima at 494 and 503 nm. Several other peaks of much smaller intensities were observed around 533, 560, 600 and 620 nm. These can be identified with 3Pj-3H4, 3Pj-3H5, 3Pj-3H6,3F2 transitions of Pr3 + . The prominent excitation around 283 nm (Fig. 6, curve b) can be attributed to the host absorption, while the excitation band in the form of a shoulder around 230 nm to the f–d excitation. PL spectra for Ce3 + activated SrZnO2 are also included in Fig. 6 (curves c and d). The emission is in the form of a broad band extending from 374 to 609 nm with a peak at around 466 nm under 290 nm excitation. This is in good agreement with the results of Manavbasi and LaCombe [4]. The excitation spectrum consists of a prominent band around 289 nm with a shoulder on the long wavelength side around 340 nm. These may be attributed to the transitions from 2F7/2 state of 4f1 configuration to the states of 5d1 configuration. There are several shoulders on the short wavelength side around 233, 240, 260 and 282 nm. The last two appear to be quite similar to those observed for the other rare earth ions. It is quite likely that host excitation is overlapping the Ce3 + absorption.
3.2. 4fn activators Terbium shows strong excitation corresponding to allowed transition between 7F6 ground state of 4f8 configuration to the levels of 4f75d1 configuration. Quite often, this falls in deep UV or VUV region of the spectrum. From the excited state the ion relaxes in several steps to 5Dj levels of 4f8 configuration. Line emission corresponding to f–f transitions is observed. Commonly observed, most intense lines are around 485 and 543 nm corresponding to 5 D4-7F6 and 5D4-7F5 transitions, respectively. At low concentrations blue emission is dominant. Near UV emission around 385 nm corresponding to 5D3-7F6 transition is also observable. For high concentrations, these emissions are quenched by cross relaxation and the green emission becomes dominant. Fig. 5(a) shows emission spectrum for SrZnO2:Tb3 + (1%). Broad emission lines are located at 413, 438, 481, 491, 544, 590 and 620 nm. The first two of these, which are rather weak, can be identified with the transitions 5D3-7F5, 7F4. The remaining ones are due to 5D4-7FJ transitions. The most intense line corresponding to the transition 5D4-7F5 is split into two components. The excitation spectrum (Fig. 5, curve b) contains a strong band around 283 nm. Since this band is observed for most of the activators, it cannot be attributed to the intra-configurational transition 4f8 (7F6)-4f75d1. This excitation band is related to the host and previously attributed to the exciton absorption near the activator [3]. PL spectra for Sm activated (1%) SrZnO2 are also included in Fig. 5. One may expect Sm to enter as divalent ion at Sr2 + substitutional site. However, no emission that can be attributed to Sm2 + was observed. Broad emission lines are observed around 570, 607 and 651 nm (Fig. 5, curve c), which can be attributed to 4 G5/2-6H5/2, 4G5/2-6H7/2 and 4G5/2-6H9/2 transitions of Sm3 + . Ionic radius of Sm3 + (96 pm) is intermediate to those for Sr2 +
ARTICLE IN PRESS D.R. Taikar et al. / Journal of Luminescence 130 (2010) 1690–1693
1693
there is strong excitation attributable to the host in addition to the characteristic f–d excitation. Reasons for absence of the 283 nm excitation for Bi3 + are not yet clear. However, it should not be surprising. Host sensitized luminescence cannot be observed for all the activators. Several conditions are to be satisfied. More studies on SrZnO2 will be required to clarify this point.
4. Conclusions SrZnO2 containing various activators viz. Pb2 + , Ce3 + , Sm3 + , Tb , Bi3 + and Pr3 + could be successfully prepared by carbonate decomposition method as well as combustion synthesis. The latter would provide a fast, one step method for preparing these phosphors. Characteristic excitation and emission were observed for Bi3 + , while for the remaining activators a band around 283 nm is always present in the excitation spectrum. This excitation band had been observed in most of the previous works but neglected quite often. Notwithstanding previous interpretations, this has been attributed to the host. The luminescence of Pb2 + , Sm3 + , Tb3 + and Pr3 + in SrZnO2 is host sensitized. The mechanism of energy transfer from host to the activator is not yet clear and more studies like the lifetime measurements, low temperature measurements, other activators, etc. will be needed to get the details. 3+
Fig. 6. PL spectra for SrZnO2 doped with Pr3 + or Ce3 + . (a) Pr3 + emission for 283 nm excitation. (b) Excitation for 500 nm emission of SrZnO2:Pr3 + . (c) Ce3 + emission in SrZnO2 for 290 nm excitation. (d) Excitation for 466 nm emission of SrZnO2:Ce3 + .
It is thus seen that appreciable luminescence is observed in SrZnO2 for several activators. Except for Bi, all the activators studied in this work show strong 283 nm excitation band. Manavbasi and LaCombe [3] found that the 283 nm absorption band is related to the host absorption. There is compelling evidence to suggest that the 283 nm excitation band is related to this absorption. However, in the previous work [3] it has been argued that emission in SrZnO2:Pb2 + is not due to host to activator energy transfer. This was based on the observation that there is no emission from the SrZnO2 host lattice, which overlaps with the absorption spectrum of the Pb2 + . In fact Yu et al. [8] found that SrZnO2 has no characteristic excitation and emission bands.. They observed excitations around 250, 262 and 280 nm for SrZnO2:Tb3 + . They totally ignored the last one and attributed the first two to the transitions from 7F6 (4f8) level to the levels of 4f75d1 configuration. A shoulder around 280 nm is also observed in the excitation spectrum of Eu3 + activated SrZnO2 [10,11], which was again neglected by the authors during discussions. We thus see that arguments for rejecting the host to activator energy transfer in SrZnO2 are rather weak and based on PL studies related to a specific activator. On the other hand we find 283 nm excitation band for a variety of activators like Pb2 + , Ce3 + , Sm3 + , Tb3 + and Pr3 + . It is too much of a coincidence that all these activators will have their characteristic excitation at this wavelength. In fact for several hosts Tb3 + and Pr3 + f–d excitations have been observed at much shorter wavelengths [20]. For Tb3 + -doped compounds the first allowed 4f8–4f75d1 transition is usually located at 14 000 cm 1 higher energy than if Ce is the dopant [21]. Pr3 + f–d excitations will appear at still shorter wavelengths [20]. These observations suggest that it will not be prudent to assign the 283 nm excitation for both the activators, Tb3 + and Pr3 + , to f–d excitations. It is thus clear that luminescence of Pb2 + , Sm3 + , Tb3 + and Pr3 + in SrZnO2 is host sensitized. Even for Ce3 + ,
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
H.G. von Schnering, R. Hoppe, Die Naturwissenschaften 47 (1960) 467. S. Kubota, T. Oyama, H. Yamane, M. Shimada, Chem. Mater. 15 (2003) 3403. A. Manavbasi, J.C. LaCombe, J. Lumin. 128 (2008) 129. A. Manavbasi, J.C. LaCombe Mater., Res. Soc. Symp. Proc. 916 (2006) 41. X.H. He, J. Zhou, C.L. Bi, W.H. Li, Rengong Jingti Xuebao/J. Synthetic Cryst. 36 (2007) 1408. L. Yang, X. Yu, S. Yang, C. Zhou, P. Zhou, W. Gao, P. Ye, Mater. Lett. 62 (2008) 907. X. He, Xiyou Jinshu, Cailiao Yu, Gongcheng/Rare Metal Mater. Eng. 36 (2007) 1574. X. Yu, X. Xu, P. Zhou, X. Peng, S. Yang, Mater. Lett. 59 (2005) 1178. S.P. Khatkar, V.B. Taxak, D. Kumar, S.D. Han, C.H. Han, G. Sharma, J. Korean Phys. Soc. 48 (2006) 1355. Y. Wang, H. Gao, J. Solid State Chem. 179 (2006) 1870. X. Yu, X. Xu, C. Zhou, J. Tang, X. Peng, S. Yang, Mater. Res. Bull. 41 (2006) 1578. X.H. He, J. Zhou, W.H. Li, Q.F. Zhou, Chin. J. Inorg. Chem. 22 (2006) 1706. D.S. Thakare, S.K. Omanwar, S.V. Moharil, S.M. Dhopte, P.L. Muthal, V.K. Kondawar, Opt. Mater. 29 (2007) 1731–1735. F. Agullo-Lopez, J.M. Calleja, F. Cuso, F. Jaque, F.J. Lopez, Prog. Mater. Sci. 30 (1986) 187. G. Blasse Prog, Solid State Chem. 18 (1988) 79. A. Ranfagni, D. Mugnai, M. Bacci, G. Viliani, M.P. Fontana, Adv. Phys. 32 (1983) 823. C.P. Joshi, S.V. Moharil, Phys. Status Solidi (B) 220 (2000) 985. A. Van Dijken, H.F. Folkerts, G. Blasse, J. Lumin. 72/74 (1997) 660. Y. Tamura, A. Shibukawa, Jpn. J. Appl. Phys. 32 (1993) 3187. P. Dorenbos, J. Lumin. 91 (2000) 91. G. Blasse, J.P.M. van Vliet, J.W.M. Verwey, R. Hoogendam, M. Wiegel, J. Phys. Chem. Solids 50 (1989) 583.