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Synthesis and photoluminescence of nanocrystalline lutetium pyrosilicate doped with Ce3+
Optical Materials 31 (2009) 826–830
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and photoluminescence of nanocrystalline lutetium pyrosilicate doped with Ce3+ Jerzy Sokolnicki *, Malgorzata Guzik Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie Street, 50-383 Wroclaw, Poland
a r t i c l e
i n f o
Article history: Received 19 June 2008 Received in revised form 8 September 2008 Accepted 23 September 2008 Available online 5 November 2008
a b s t r a c t The nanocrystalline Lu2Si2O7:Ce3+ (LPS:Ce) phosphor was obtained as a result of a solid state reaction at 1250 °C of Lu2O3 and SiO2. Compared to the monocrystals grown by the Czochralski method (below 0.5%) the procedure of the synthesis allows much higher level of the activator doping (5%). Two samples, one in air (LPS:Ce–O) and the second one in N2/H2 atmosphere (LPS:Ce–R) were synthesized for comparison. X-ray diffraction analysis confirmed crystallization of a single phase of LPS in the indicated temperature. The UV excited optical luminescence spectrum of LPS:Ce–O at 10 K consists of two wide bands centered at 380 and 407 nm. The positions of the bands and the decay time of 35 ns characteristic for the parity allowed 5d–4f fast emission of the Ce3+ ion. The overlap between a self-trapped (STE) emission of the host (between 320 and 470 nm) and 4f–5d absorption of Ce3+ (centered at 350 nm) allows the energy transfer, which explains the scintillation efficiency of LPS:Ce. Two luminescence mechanisms were detected for the LPS:Ce–R sample. The decay times of 30 ns and 200 ls cannot have the same origin, as they defer too much. According to the tentative explanation the fast emission is a result of the 5d–4f transition of Ce3+ and the slow component of the Ce4+ charge transfer luminescence associated with defects caused by reducing atmosphere. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Development of new phosphors for scintillator application requires a better understanding of the fundamental luminescence mechanisms in current materials [1]. In recent years, considerable attention has been given to the family of Ce3+ doped silicates, Lu2SiO5 (LSO) and Lu2Si2O7 (LPS). In particular, inorganic scintillator Ce3+ doped Lu2Si2O7 has been studied due to promising performance for applications such as scintillators, emission tomography or oil well logging [2,3]. LPS:Ce3+ single crystals display high light yield (about 26,000 ph/MeV) with a fast decay time of 38 ns, without afterglow and high-temperature luminescence efficiency [4]. Yan et al. [5] compared optical properties of LSO:Ce3+ and LSP: Ce3+ indicating some advantages of the latter: only one luminescence mechanism, larger segregation coefficient, lower melting point and lower cost of synthesis. Lu2Si2O7 crystallizes in a monoclinic structure, with the space group C2/m (No. 12) [6]. The structure may be considered as a distorted hexagonal packing of oxygens containing the lutetium atoms in octahedral sites and the silicon ions in tetrahedral ones in alternating layers [7]. Three types of oxygen can be distinguished: the first (O1) is bridging oxy* Corresponding author. E-mail address: [email protected] (J. Sokolnicki). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.09.006
gen between the two silicon ions of the [Si2O7]6 group and it is not a lutetium compound. The other two oxygen ions (O2 and O3) are terminal oxygen of the [Si2O7]6 group and they are involved in lutetium surrounding. The lattice offers only one crystallographic site for the doping lanthanide ion. Nanoscale materials are known to exhibit changed physical properties of the host, which affects the luminescence and dynamics of an optically active dopant. Investigation and comprehension of these materials is essential for optimizing their optical properties intended for technological applications in communication and display devices. Preparing lanthanide doped nanostructures also gives a possibility to develop and study transparent composite materials. Nanometer-sized particles exhibit reduced optical scattering, allowing the preparation and use of nanocrystals embedded in an amorphous matrix in applications such as laser and amplifiers, which usually require high quality crystals or glasses. Such ceramics in the form of thin films [8,9] and as a bulk [10], obtained by the sol–gel method, have recently proved to be very promising for scintillation application. This paper is aimed at the spectroscopic investigation of nanostructured LPS:Ce3+ phosphors obtained as a result of a solid state reaction in reducing and oxidizing atmospheres. It is has been indicated that at 1250 oC the synthesis of pyrosilicate is completed and optical properties of the material strongly depend on reducing oxidizing conditions of heat treatment.
J. Sokolnicki, M. Guzik / Optical Materials 31 (2009) 826–830
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2. Experimental 2.1. Samples preparation Lu2O3:Ce3+ containing 5 mol% of active ions with respect to Lu have been prepared using solution combustion synthesis involving a mixed fuel containing glutamic acid and glycine [11]. Stoichiometric quantities of the metal nitrates and fuel (glycine/glutamic acid) were dissolved in a small amount of water in a beaker. Subsequently, the solution has been dried at 150 °C for about 3 h. The obtained material was transferred into a furnace preheated up to 600 °C (Carbolite chamber furnace RHF 16/15) and fired for 2 h in air in order to decompose the residual nitrate ions. Silica sol was obtained by acid-catalysed hydrolysis of tetraethoxysilane (TEOS) with deionized water and an addition of C2H5OH so as to allow the dissolution of TEOS. The pH were adjusted to 2 with diluted nitric acid. The molar ratios [TEOS]: [H2O]: [ethanol] were 1:4:8. The sol was mixed with of Lu2O3:Ce3+ in stoichiometric conditions directly after the completion of hydrolysis. Subsequently, the sol was allowed to gelate in a polyethylene container under a parafilm cover by three days and then dried in a drier at 150 °C for 6 h. The sample was thermally treated up to 1250 °C in H2/N2 atmosphere. For comparison, the second sample obtained in the same way was calcined in air. 2.2. Apparatus Emission spectra were measured using SpectraPro 750 monochromator, equipped with Hamamatsu R928 photomultiplier and 1200 l/mm grating blazed at 500 nm. The 450 W xenon arc lamp was used as an excitation source. It was coupled with 275 mm excitation monochromator which used a 1800 l/mm grating blazed at 250 nm. Excitation spectra have been corrected for the excitation light intensity while emission spectra were not corrected for the instrument response. The measurements were done at room (RT) and 77 K temperatures. The emission and emission excitation spectra of the Ce3+ sample involving synchrotron radiation have been performed at Superlumi station of HASYLAB at DESY, Hamburg (Germany) [12]. The spectra have been corrected for the instrument response and light intensity. Decay kinetics were measured with F900 Spectrometer from Edinburgh Instruments. X-ray analysis were performed with a DRON-1 diffractometer, using Cu Ka radiation(k = 1.5418 Å) filtered with Ni. The diffractograms were recorded with a step of 2h = 0.1° for the range of 10–120°. The Scherrer’s relation, Eq. (1),was used to estimate the crystallites size
D¼
0:9k qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi cos h b2 b20
Fig. 1. X-ray patterns of the sample obtained and theoretical pattern of LPS.
peaks indicates a crystallite average dimension in the range 35– 40 nm (planes 110, 201 and 131). Fig. 2 presents the VUV excited luminescence (A) and excitation (B) spectra at 10 K of Lu2Si2O7:Ce3+ (LPS:Ce–O) nanocrystals calcined in air atmosphere. The emission spectrum (Fig. 2A) consists of a wide band with two maximums at 380 and 407 nm assigned to allowed transitions from the lowest sublevel of the 5d state to the 2F7/2 and 2F5/2 manifolds of the 4f configuration in Ce3+. The position of these peaks is largely in accordance with those reported by Pidol et al. (378 and 405 nm) [13] and Yan et al. (384 and 407 nm) [5]. Oxidizing atmosphere used in the preparation method suggests the presence of a considerable amount of tetravalent cerium in the sample. The spectrum proves however, that the Ce3+ content is still sufficient to record its efficient emission. The excitation spectrum of LPS:Ce–O at 10 K monitoring emission at 380 nm (Fig. 2B) consists of two bands centered at 304 and 240 nm, corresponding to the transition from the 4f ground state to the 5d sublevels of Ce3+ and bands centered at 182 and
ð1Þ
In this equation D is an average crystallite size, k denotes the X-ray radiation wavelength, b a full-width at half maximum of a diffraction line located at h and b represents a scan aperture of the diffractometer. 3. Results and discussion The X-ray patterns of the synthesized samples were almost identical and are depicted in Fig. 1 in comparison with the theoretical pattern of Lu2Si2O7. All the crystalline samples were single phase, and the diffraction patterns obtained could be indexed according to the following card number: Lu2Si2O7 – JCPDS card 35-0326. A quantitative line-broadening analysis of the diffraction
Fig. 2. VUV excited luminescence (A) and excitation (B) spectra at 10 K of LPS:Ce–O. The insert presents STE emission of pure LPS excited at 182 nm.
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Fig. 3. VUV excited luminescence (A) and excitation (B) spectra at 10 K of LPS:Ce–R.
171 nm assigned to self-trapped exciton (STE) and the host absorption respectively [13]. The spectrum reveals STE role in energy transfer to the 4f states of Ce3+. The insert presents STE emission of pure LPS excited at 182 nm. It displays a wide band centered at around 360 nm, thus in the range of Ce3+ absorption. The efficient scintillation of LPS:Ce is due to this overlap. Fig. 3 presents the VUV excited luminescence (A) and excitation (B) spectra at 10 K of calcined in reducing, H2/N2 atmosphere Lu2Si2O7:Ce3+ (LPS:Ce–R) nanocrystals. A wide emission band (Fig. 3A) generally overlap that of LPS:Ce3+–O emission, but has no well-defined maximums and, as compared, is broadened at the low energetic side. It is clear that a source of different optical behavior of both samples lies in different preparation conditions. Reducing atmosphere allows us to assume that the cerium ions are maintained at the 3+ oxidation state, and eventual content of Ce4+ is negligibly small. Moreover, when the opportunity occurs, some defects (oxygen vacancies) can be created in the lattice. For example, the defect structure in Y2SiO5:Ce3+ was confirmed by Aitasalo et al. [14]. Therefore it is necessary to consider more than one luminescence mechanism in the sample, including charge transfer (CT)
luminescence of Ce4+ and emission from defects in the lattice. It is well known that radiative decay from charge transfer state to the ground state is possible in some cases [15–18]. Defect luminescence in oxide compounds was also reported [17,19,20]. The excitation spectrum at 10 K monitoring emission at 380 nm (Fig. 3B) displays the same bands as those recorded for LPS:Ce3+–O, but differs in their intensities ratio. In this case the key role of STE is also visible. Please note that Superlumi station setup limitations allow to detect only a part of Ce3+ excitation spectrum (50–335 nm). In order to detect all of the absorption transition in the samples, measurements were performed using also spectrofluorometer equipped with a Xe arc lamp. Fig 4 shows the 77 K excitation spectra of LPS:Ce–R as compared to LPS:Ce–O. The excitation spectrum monitoring the emission at 380 nm (Fig. 4A) consists of two bands centered at 350 and 302 nm, corresponding to transitions from the 4f ground state to the lowest 5d sublevels of Ce3+. The LPS:Ce–O nanocrystals display similar two bands excitation spectrum with maximums at 352 and 304 nm, thus shifted to lower energies. The intensities ratio of the bands is different too. It means that in both samples the active ions occupied crystallographic sites having slightly different symmetry. It is probably due to a different Ce3+/Ce4+ ratio in the samples, resulting from the treatment atmosphere, as well as the presence of defects in LPS:Ce–R. The excitation spectrum (Fig. 4B) monitoring the emission at the tail of the emission band (470 nm) consists of two bands peaking at 350 and 302 nm. At the high energetic side of the latter band a distinct inflection centered at about 280 nm is observed, which is not detected at room temperature. Lutetium pyrosilicate crystallizes in C2/m symmetry group [6] and the lattice offers only one crystallographic site for the doping lanthanide ion. The presence of Ce3+ in more then one crystallographic site is however possible and likely associated with lattice perturbations induced by substitution of the larger Ce3+ ion (1.034 Å) for the host Lu3+ ion (0.848 Å). Previously, we had reported two crystallographic centers in LPS:Eu3+ [21], whose existence was explained by a high concentration of the activator (10%) as well as by the differences in the ionic radii of Eu3+ and Lu3+. The concentration of Ce3+ is 5% mol, thus over order of magnitude higher than segregation coefficient for Ce3+ in a single crystal (0.38 [6]). However, the emission originated from the second crystallographic site of Ce3+ would then be recordable also at room temperature, which is not the case. This
Fig. 4. Excitation spectra at 77 K of LPS:Ce–R and LPS:Ce–O.
J. Sokolnicki, M. Guzik / Optical Materials 31 (2009) 826–830
Fig. 5. Emission spectra of LPS:Ce–R excited with Xe arc lamp.
Table 1 Emission decay times for LPS:Ce3+–O and LPS:Ce3+–R LPS:Ce-O
LPS:Ce-R
kexc
kobs
352 304
380 380
s (ns) RT
77 K
26.10 25.95
34.88 34.90
kexc
kobs
350 280
380 470
s (ns) RT
77 K
19.90
29.80 2.01 105
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observation does not exclude the possibility of existence of two Ce3+ crystallographic centers. Fast energy transfer between them can make it impossible to resolve both emissions. Decay kinetics measured for both emissions of LPS:Ce–R will be decisive in their ascribing to the proper emitting center. The 77 K emission spectra of LPS:Ce–R, excited with Xe arc lamp, are presented in Fig. 5. As can be observed, the spectrum depends on the excitation wavelength and it is possible to distinguish at least two different emissions. The spectrum excited at 350 nm displays typical two bands emission of Ce3+ located between 350 and 475 nm. The spectrum is similar to that obtained for LPS:Ce– O (Fig. 2A) and most likely originates from the similar crystallographic center of Ce3+ replacing the host Lu3+ ion. Excitation at 280 nm leads to a wide band with its maximum at 407 nm and a broadening on the low energetic side, which disappears at room temperature. The component with the maximum at 380 nm is visible as an inflection on the high energetic side of the band. This band is similar to that recorded under STE excitation (Fig. 3A). The Stokes shift for the wide band luminescence determined between the excitation maximum (280 nm), not ascribed to the Ce3+ emission, and the emission maximum deduced from the spectra (470 nm) is 14438 cm1, thus falling in the range expected for a CT transition on RE ion (typically from 4000 up to 17000 cm1). Taking into account the above said and the fact that the emission is quenched at elevated temperature, it can be assumed that the luminescence can result from the CT transition in which the Ce4+ ion is involved. However, a definite assignment will be possible after correlation of the emission profiles with their decay kinetics, which will make it possible to rule out the other possible sources of the broad band luminescence. Table 1 collects the decay times for LPS:Ce3+–O and LPS:Ce3+–R at room and 77 K temperature. Luminescence decay curves of the LPS:Ce–R luminescence are shown in Fig. 6. The decay time of 35 ns, detected for LPS:Ce3+–O, is typical of fully allowed 4fn15d ? 4fn transitions and was observed to decrease with increasing temperature (26 ns). For LPS:Ce3+–R two different decay times were measured. One of them is of nanosecond order, 30 ns at 77 K (20 ns at RT) and can be related to the Ce3+crystallographic site. Shorter decay times determined for the
Fig. 6. Luminescence decay curves of the LPS:Ce–R emission. The solid lines represent the one-exponential fits to the decay curves.
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sample obtained in reducing atmosphere are probably due to the higher concentration of the active ions. The second one is 200 ls, thus two orders of magnitude longer. It is apparent that the slow component cannot be related to the Ce3+ emission and the other source of that luminescence must be considered. It is observed that firing atmosphere is a main factor influencing the LPS:Ce3+ luminescence. LPS:Ce–O contains a large amount of Ce4+, but no related luminescence was observed. As LPS:Ce–R was prepared in reducing atmosphere in order to prevent the oxidation of Ce3+ to Ce4+, the eventual content of Ce4+ should be negligibly small. However, it is safe to assume that some oxygen vacancies are created in the LPS lattice [14]. The vacancies are positive when compared to the O2 site and can electrostatically attract electrons from the surrounding ions forming F centers. The Ce3+ ions can serve as a source of electrons, as they are known to exist in the tetravalent state. In this way the creation of both lattice defects and Ce4+ can be realized and charge imbalance is stabilized. For electrostatic reasons the defect and Ce4+ center are close to each other and this proximity can result in the CT (Ce4+ ? Ce3+) luminescence by an electron transfer from the F center [22]. The role of defects in LPS:Ce–R can be deduced from a comparison with LPS:Ce–O that contains a considerable amount of Ce4+, although defects (oxygen vacancies) are not very likely to occur. Consequently, the Ce4+ ions that were (eventually) created by oxidizing the Ce3+ ions (oxygen is present in the system even under reducing conditions) according to the proposed mechanism are not involved in the CT emission either in the oxidized or in the reduced sample. Literature survey showed that for the dipole allowed CT transition the decay times were one or two orders of magnitude longer than in the case of d–f transitions. For example, the typical decay times for the Yb3+ CT emission fall in the range between 100 and 200 ns [15–17]. For strontium cerate, Sr2CeO4 a decay time of 65 ls was measured [23]. The decay times of 700 and 800 ls were observed by Aitasalo et al. [22] for delayed luminescence of Y2SiO5:Ce3+(1%). Such a slow mechanism was explained by a rather small concentration of Ce3+ in the sample. As our samples contain higher concentration of the Ce3+ ions (5 mol%) and consequently that of Ce4+ can be considered higher, the value of 200 ls seems to be reasonable. In conclusion, the tentative explanation, consistent though with all the results obtained, relates the broad band emission to the CT transition of the Ce4+ centers associated with the F defects. 4. Conclusions The nanocrystalline LPS:Ce phosphor with relatively high content of cerium ions (5 mol%) can be obtained by a solid state reac-
tion at 1250 oC of Lu2O3 and SiO2. The firing atmosphere, oxidizing or reducing, influences the optical behavior of the samples. While LPS:Ce–O exhibits the typical fast (35 ns) Ce3+ emission, LPS:Ce–R emits both from Ce3+ and the CT state of Ce4+ associated with defect created by reducing atmosphere. The assignment of the latter emission was based on its decay time (200 ls) and its occurrence only at low temperature. The nature of the slow emission and the defect structure in LPS:Ce will be investigated in the future by the time-resolved spectroscopy. Acknowledgements The work was funded by the Polish Committee for Scientific Research (KBN, Grant No. 20401331/0289) and European Union, Grant No. I-20060244 EC for measurements at Superlumi station at Hasylab, Desy, Hamburg. References [1] M.J. Weber, J. Lumin. 100 (2002) 35. [2] D. Pauwels et al., US Patent Specification (2002) 6437336. [3] L. Pidol, A. Kahn-Harari, B. Viana, B. Ferrand, P. Dorenbos, J.T.M. de Haas, C.W.E. Van Eijk, E. Virey, J. Phys.: Condens. Matter 15 (2003) 2091. [4] L. Pidol, B. Viana, A. Kahn-Harari, B. Ferrand, P. Dorenbos, C.W.E. Van Eijk, Nucl. Instrum. Methods A 537 (2005) 256. [5] C. Yan, G. Zhao, Y. Hang, L. Zhang, J. Xu, J. Cryst. Growth 281 (2005) 411. [6] C. Yan, G. Zhao, Y. Hang, L. Zhang, J. Xu, Mater. Lett. 60 (2006) 1960. [7] D. Pauwels et al., IEEE Trans. Nucl. Sci. 51 (2004) 1084. [8] A. Garcia-Murillo, C. Le Luyer, C. Dujardin, T. Martin, C. Garapon, C. Pedrini, J. Mugnier, Nucl. Instrum. Methods A 486 (2002) 181. [9] A. Garcia-Murillo, C. Le Luyer-Urlacher, C. Dujardin, C. Pedrini, J. Mugnier, J. Sol–Gel Sci. Technol. 26 (2003) 957. [10] D. Hreniak, E. Zych, L. Ke˛pin´ski, W. Stre˛k, J. Phys. Chem. Solid 64 (2003) 111. [11] M. Daldosso, J. Sokolnicki, L. Kepinski, J. Legendziewicz, A. Speghini, M. Bettinelli, J. Lumin. 122–123 (2007) 858. [12] G. Zimmerer, Nucl. Instrum. Methods, Phys. Res. Sect. A 308 (1991) 178. [13] L. Pidol, B. Viana, A. Kahn-Harari, A. Bessire, P. Dorenbos, Nucl. Instrum. Methods A 537 (2005) 125. [14] T. Aitasalo, J. Holsa, M. Lastusaari, J. Niittykoski, F. Pelle´, Phys. Stat. Sol. C 2 (1) (2005) 272. [15] E. Nakazawa, Chem. Phys. Lett. 56 (1978) 161. [16] E. Nakazawa, J. Lumin. 18/19 (1979) 272. [17] L. Van Pieterson, M. Heeroma, E. De Heer, A. Meijerink, J. Lumin. 91 (2000) 177. [18] J. Legendziewicz, J. Sokolnicki, J. Alloy Compds 451 (2008) 600. [19] W.H. Green, K.P. Le, J. Grey, T.T. Au, A.J. Sailor, Science 276 (1997) 1826. [20] J.A. Garcia, A. Remon, J. Pigueras, Phys. Stat. Sol. A 89 (1985) 237. [21] J. Sokolnicki, J. Solid State Chem. 180 (2007) 2400. [22] T. Aitasalo, J. Holsa, M. Lastusaari, J. Legendziewicz, J. Niittykoski, F. Pelle, Opt. Mater. 26 (2004) 107. [23] L. van Pieterson, S. Soverna, A. Meijerink, J. Electrochem. Soc. 147 (2000) 4688.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and photoluminescence of nanocrystalline lutetium pyrosilicate doped with Ce3+ Jerzy Sokolnicki *, Malgorzata Guzik Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie Street, 50-383 Wroclaw, Poland
a r t i c l e
i n f o
Article history: Received 19 June 2008 Received in revised form 8 September 2008 Accepted 23 September 2008 Available online 5 November 2008
a b s t r a c t The nanocrystalline Lu2Si2O7:Ce3+ (LPS:Ce) phosphor was obtained as a result of a solid state reaction at 1250 °C of Lu2O3 and SiO2. Compared to the monocrystals grown by the Czochralski method (below 0.5%) the procedure of the synthesis allows much higher level of the activator doping (5%). Two samples, one in air (LPS:Ce–O) and the second one in N2/H2 atmosphere (LPS:Ce–R) were synthesized for comparison. X-ray diffraction analysis confirmed crystallization of a single phase of LPS in the indicated temperature. The UV excited optical luminescence spectrum of LPS:Ce–O at 10 K consists of two wide bands centered at 380 and 407 nm. The positions of the bands and the decay time of 35 ns characteristic for the parity allowed 5d–4f fast emission of the Ce3+ ion. The overlap between a self-trapped (STE) emission of the host (between 320 and 470 nm) and 4f–5d absorption of Ce3+ (centered at 350 nm) allows the energy transfer, which explains the scintillation efficiency of LPS:Ce. Two luminescence mechanisms were detected for the LPS:Ce–R sample. The decay times of 30 ns and 200 ls cannot have the same origin, as they defer too much. According to the tentative explanation the fast emission is a result of the 5d–4f transition of Ce3+ and the slow component of the Ce4+ charge transfer luminescence associated with defects caused by reducing atmosphere. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Development of new phosphors for scintillator application requires a better understanding of the fundamental luminescence mechanisms in current materials [1]. In recent years, considerable attention has been given to the family of Ce3+ doped silicates, Lu2SiO5 (LSO) and Lu2Si2O7 (LPS). In particular, inorganic scintillator Ce3+ doped Lu2Si2O7 has been studied due to promising performance for applications such as scintillators, emission tomography or oil well logging [2,3]. LPS:Ce3+ single crystals display high light yield (about 26,000 ph/MeV) with a fast decay time of 38 ns, without afterglow and high-temperature luminescence efficiency [4]. Yan et al. [5] compared optical properties of LSO:Ce3+ and LSP: Ce3+ indicating some advantages of the latter: only one luminescence mechanism, larger segregation coefficient, lower melting point and lower cost of synthesis. Lu2Si2O7 crystallizes in a monoclinic structure, with the space group C2/m (No. 12) [6]. The structure may be considered as a distorted hexagonal packing of oxygens containing the lutetium atoms in octahedral sites and the silicon ions in tetrahedral ones in alternating layers [7]. Three types of oxygen can be distinguished: the first (O1) is bridging oxy* Corresponding author. E-mail address: [email protected] (J. Sokolnicki). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.09.006
gen between the two silicon ions of the [Si2O7]6 group and it is not a lutetium compound. The other two oxygen ions (O2 and O3) are terminal oxygen of the [Si2O7]6 group and they are involved in lutetium surrounding. The lattice offers only one crystallographic site for the doping lanthanide ion. Nanoscale materials are known to exhibit changed physical properties of the host, which affects the luminescence and dynamics of an optically active dopant. Investigation and comprehension of these materials is essential for optimizing their optical properties intended for technological applications in communication and display devices. Preparing lanthanide doped nanostructures also gives a possibility to develop and study transparent composite materials. Nanometer-sized particles exhibit reduced optical scattering, allowing the preparation and use of nanocrystals embedded in an amorphous matrix in applications such as laser and amplifiers, which usually require high quality crystals or glasses. Such ceramics in the form of thin films [8,9] and as a bulk [10], obtained by the sol–gel method, have recently proved to be very promising for scintillation application. This paper is aimed at the spectroscopic investigation of nanostructured LPS:Ce3+ phosphors obtained as a result of a solid state reaction in reducing and oxidizing atmospheres. It is has been indicated that at 1250 oC the synthesis of pyrosilicate is completed and optical properties of the material strongly depend on reducing oxidizing conditions of heat treatment.
J. Sokolnicki, M. Guzik / Optical Materials 31 (2009) 826–830
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2. Experimental 2.1. Samples preparation Lu2O3:Ce3+ containing 5 mol% of active ions with respect to Lu have been prepared using solution combustion synthesis involving a mixed fuel containing glutamic acid and glycine [11]. Stoichiometric quantities of the metal nitrates and fuel (glycine/glutamic acid) were dissolved in a small amount of water in a beaker. Subsequently, the solution has been dried at 150 °C for about 3 h. The obtained material was transferred into a furnace preheated up to 600 °C (Carbolite chamber furnace RHF 16/15) and fired for 2 h in air in order to decompose the residual nitrate ions. Silica sol was obtained by acid-catalysed hydrolysis of tetraethoxysilane (TEOS) with deionized water and an addition of C2H5OH so as to allow the dissolution of TEOS. The pH were adjusted to 2 with diluted nitric acid. The molar ratios [TEOS]: [H2O]: [ethanol] were 1:4:8. The sol was mixed with of Lu2O3:Ce3+ in stoichiometric conditions directly after the completion of hydrolysis. Subsequently, the sol was allowed to gelate in a polyethylene container under a parafilm cover by three days and then dried in a drier at 150 °C for 6 h. The sample was thermally treated up to 1250 °C in H2/N2 atmosphere. For comparison, the second sample obtained in the same way was calcined in air. 2.2. Apparatus Emission spectra were measured using SpectraPro 750 monochromator, equipped with Hamamatsu R928 photomultiplier and 1200 l/mm grating blazed at 500 nm. The 450 W xenon arc lamp was used as an excitation source. It was coupled with 275 mm excitation monochromator which used a 1800 l/mm grating blazed at 250 nm. Excitation spectra have been corrected for the excitation light intensity while emission spectra were not corrected for the instrument response. The measurements were done at room (RT) and 77 K temperatures. The emission and emission excitation spectra of the Ce3+ sample involving synchrotron radiation have been performed at Superlumi station of HASYLAB at DESY, Hamburg (Germany) [12]. The spectra have been corrected for the instrument response and light intensity. Decay kinetics were measured with F900 Spectrometer from Edinburgh Instruments. X-ray analysis were performed with a DRON-1 diffractometer, using Cu Ka radiation(k = 1.5418 Å) filtered with Ni. The diffractograms were recorded with a step of 2h = 0.1° for the range of 10–120°. The Scherrer’s relation, Eq. (1),was used to estimate the crystallites size
D¼
0:9k qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi cos h b2 b20
Fig. 1. X-ray patterns of the sample obtained and theoretical pattern of LPS.
peaks indicates a crystallite average dimension in the range 35– 40 nm (planes 110, 201 and 131). Fig. 2 presents the VUV excited luminescence (A) and excitation (B) spectra at 10 K of Lu2Si2O7:Ce3+ (LPS:Ce–O) nanocrystals calcined in air atmosphere. The emission spectrum (Fig. 2A) consists of a wide band with two maximums at 380 and 407 nm assigned to allowed transitions from the lowest sublevel of the 5d state to the 2F7/2 and 2F5/2 manifolds of the 4f configuration in Ce3+. The position of these peaks is largely in accordance with those reported by Pidol et al. (378 and 405 nm) [13] and Yan et al. (384 and 407 nm) [5]. Oxidizing atmosphere used in the preparation method suggests the presence of a considerable amount of tetravalent cerium in the sample. The spectrum proves however, that the Ce3+ content is still sufficient to record its efficient emission. The excitation spectrum of LPS:Ce–O at 10 K monitoring emission at 380 nm (Fig. 2B) consists of two bands centered at 304 and 240 nm, corresponding to the transition from the 4f ground state to the 5d sublevels of Ce3+ and bands centered at 182 and
ð1Þ
In this equation D is an average crystallite size, k denotes the X-ray radiation wavelength, b a full-width at half maximum of a diffraction line located at h and b represents a scan aperture of the diffractometer. 3. Results and discussion The X-ray patterns of the synthesized samples were almost identical and are depicted in Fig. 1 in comparison with the theoretical pattern of Lu2Si2O7. All the crystalline samples were single phase, and the diffraction patterns obtained could be indexed according to the following card number: Lu2Si2O7 – JCPDS card 35-0326. A quantitative line-broadening analysis of the diffraction
Fig. 2. VUV excited luminescence (A) and excitation (B) spectra at 10 K of LPS:Ce–O. The insert presents STE emission of pure LPS excited at 182 nm.
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Fig. 3. VUV excited luminescence (A) and excitation (B) spectra at 10 K of LPS:Ce–R.
171 nm assigned to self-trapped exciton (STE) and the host absorption respectively [13]. The spectrum reveals STE role in energy transfer to the 4f states of Ce3+. The insert presents STE emission of pure LPS excited at 182 nm. It displays a wide band centered at around 360 nm, thus in the range of Ce3+ absorption. The efficient scintillation of LPS:Ce is due to this overlap. Fig. 3 presents the VUV excited luminescence (A) and excitation (B) spectra at 10 K of calcined in reducing, H2/N2 atmosphere Lu2Si2O7:Ce3+ (LPS:Ce–R) nanocrystals. A wide emission band (Fig. 3A) generally overlap that of LPS:Ce3+–O emission, but has no well-defined maximums and, as compared, is broadened at the low energetic side. It is clear that a source of different optical behavior of both samples lies in different preparation conditions. Reducing atmosphere allows us to assume that the cerium ions are maintained at the 3+ oxidation state, and eventual content of Ce4+ is negligibly small. Moreover, when the opportunity occurs, some defects (oxygen vacancies) can be created in the lattice. For example, the defect structure in Y2SiO5:Ce3+ was confirmed by Aitasalo et al. [14]. Therefore it is necessary to consider more than one luminescence mechanism in the sample, including charge transfer (CT)
luminescence of Ce4+ and emission from defects in the lattice. It is well known that radiative decay from charge transfer state to the ground state is possible in some cases [15–18]. Defect luminescence in oxide compounds was also reported [17,19,20]. The excitation spectrum at 10 K monitoring emission at 380 nm (Fig. 3B) displays the same bands as those recorded for LPS:Ce3+–O, but differs in their intensities ratio. In this case the key role of STE is also visible. Please note that Superlumi station setup limitations allow to detect only a part of Ce3+ excitation spectrum (50–335 nm). In order to detect all of the absorption transition in the samples, measurements were performed using also spectrofluorometer equipped with a Xe arc lamp. Fig 4 shows the 77 K excitation spectra of LPS:Ce–R as compared to LPS:Ce–O. The excitation spectrum monitoring the emission at 380 nm (Fig. 4A) consists of two bands centered at 350 and 302 nm, corresponding to transitions from the 4f ground state to the lowest 5d sublevels of Ce3+. The LPS:Ce–O nanocrystals display similar two bands excitation spectrum with maximums at 352 and 304 nm, thus shifted to lower energies. The intensities ratio of the bands is different too. It means that in both samples the active ions occupied crystallographic sites having slightly different symmetry. It is probably due to a different Ce3+/Ce4+ ratio in the samples, resulting from the treatment atmosphere, as well as the presence of defects in LPS:Ce–R. The excitation spectrum (Fig. 4B) monitoring the emission at the tail of the emission band (470 nm) consists of two bands peaking at 350 and 302 nm. At the high energetic side of the latter band a distinct inflection centered at about 280 nm is observed, which is not detected at room temperature. Lutetium pyrosilicate crystallizes in C2/m symmetry group [6] and the lattice offers only one crystallographic site for the doping lanthanide ion. The presence of Ce3+ in more then one crystallographic site is however possible and likely associated with lattice perturbations induced by substitution of the larger Ce3+ ion (1.034 Å) for the host Lu3+ ion (0.848 Å). Previously, we had reported two crystallographic centers in LPS:Eu3+ [21], whose existence was explained by a high concentration of the activator (10%) as well as by the differences in the ionic radii of Eu3+ and Lu3+. The concentration of Ce3+ is 5% mol, thus over order of magnitude higher than segregation coefficient for Ce3+ in a single crystal (0.38 [6]). However, the emission originated from the second crystallographic site of Ce3+ would then be recordable also at room temperature, which is not the case. This
Fig. 4. Excitation spectra at 77 K of LPS:Ce–R and LPS:Ce–O.
J. Sokolnicki, M. Guzik / Optical Materials 31 (2009) 826–830
Fig. 5. Emission spectra of LPS:Ce–R excited with Xe arc lamp.
Table 1 Emission decay times for LPS:Ce3+–O and LPS:Ce3+–R LPS:Ce-O
LPS:Ce-R
kexc
kobs
352 304
380 380
s (ns) RT
77 K
26.10 25.95
34.88 34.90
kexc
kobs
350 280
380 470
s (ns) RT
77 K
19.90
29.80 2.01 105
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observation does not exclude the possibility of existence of two Ce3+ crystallographic centers. Fast energy transfer between them can make it impossible to resolve both emissions. Decay kinetics measured for both emissions of LPS:Ce–R will be decisive in their ascribing to the proper emitting center. The 77 K emission spectra of LPS:Ce–R, excited with Xe arc lamp, are presented in Fig. 5. As can be observed, the spectrum depends on the excitation wavelength and it is possible to distinguish at least two different emissions. The spectrum excited at 350 nm displays typical two bands emission of Ce3+ located between 350 and 475 nm. The spectrum is similar to that obtained for LPS:Ce– O (Fig. 2A) and most likely originates from the similar crystallographic center of Ce3+ replacing the host Lu3+ ion. Excitation at 280 nm leads to a wide band with its maximum at 407 nm and a broadening on the low energetic side, which disappears at room temperature. The component with the maximum at 380 nm is visible as an inflection on the high energetic side of the band. This band is similar to that recorded under STE excitation (Fig. 3A). The Stokes shift for the wide band luminescence determined between the excitation maximum (280 nm), not ascribed to the Ce3+ emission, and the emission maximum deduced from the spectra (470 nm) is 14438 cm1, thus falling in the range expected for a CT transition on RE ion (typically from 4000 up to 17000 cm1). Taking into account the above said and the fact that the emission is quenched at elevated temperature, it can be assumed that the luminescence can result from the CT transition in which the Ce4+ ion is involved. However, a definite assignment will be possible after correlation of the emission profiles with their decay kinetics, which will make it possible to rule out the other possible sources of the broad band luminescence. Table 1 collects the decay times for LPS:Ce3+–O and LPS:Ce3+–R at room and 77 K temperature. Luminescence decay curves of the LPS:Ce–R luminescence are shown in Fig. 6. The decay time of 35 ns, detected for LPS:Ce3+–O, is typical of fully allowed 4fn15d ? 4fn transitions and was observed to decrease with increasing temperature (26 ns). For LPS:Ce3+–R two different decay times were measured. One of them is of nanosecond order, 30 ns at 77 K (20 ns at RT) and can be related to the Ce3+crystallographic site. Shorter decay times determined for the
Fig. 6. Luminescence decay curves of the LPS:Ce–R emission. The solid lines represent the one-exponential fits to the decay curves.
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sample obtained in reducing atmosphere are probably due to the higher concentration of the active ions. The second one is 200 ls, thus two orders of magnitude longer. It is apparent that the slow component cannot be related to the Ce3+ emission and the other source of that luminescence must be considered. It is observed that firing atmosphere is a main factor influencing the LPS:Ce3+ luminescence. LPS:Ce–O contains a large amount of Ce4+, but no related luminescence was observed. As LPS:Ce–R was prepared in reducing atmosphere in order to prevent the oxidation of Ce3+ to Ce4+, the eventual content of Ce4+ should be negligibly small. However, it is safe to assume that some oxygen vacancies are created in the LPS lattice [14]. The vacancies are positive when compared to the O2 site and can electrostatically attract electrons from the surrounding ions forming F centers. The Ce3+ ions can serve as a source of electrons, as they are known to exist in the tetravalent state. In this way the creation of both lattice defects and Ce4+ can be realized and charge imbalance is stabilized. For electrostatic reasons the defect and Ce4+ center are close to each other and this proximity can result in the CT (Ce4+ ? Ce3+) luminescence by an electron transfer from the F center [22]. The role of defects in LPS:Ce–R can be deduced from a comparison with LPS:Ce–O that contains a considerable amount of Ce4+, although defects (oxygen vacancies) are not very likely to occur. Consequently, the Ce4+ ions that were (eventually) created by oxidizing the Ce3+ ions (oxygen is present in the system even under reducing conditions) according to the proposed mechanism are not involved in the CT emission either in the oxidized or in the reduced sample. Literature survey showed that for the dipole allowed CT transition the decay times were one or two orders of magnitude longer than in the case of d–f transitions. For example, the typical decay times for the Yb3+ CT emission fall in the range between 100 and 200 ns [15–17]. For strontium cerate, Sr2CeO4 a decay time of 65 ls was measured [23]. The decay times of 700 and 800 ls were observed by Aitasalo et al. [22] for delayed luminescence of Y2SiO5:Ce3+(1%). Such a slow mechanism was explained by a rather small concentration of Ce3+ in the sample. As our samples contain higher concentration of the Ce3+ ions (5 mol%) and consequently that of Ce4+ can be considered higher, the value of 200 ls seems to be reasonable. In conclusion, the tentative explanation, consistent though with all the results obtained, relates the broad band emission to the CT transition of the Ce4+ centers associated with the F defects. 4. Conclusions The nanocrystalline LPS:Ce phosphor with relatively high content of cerium ions (5 mol%) can be obtained by a solid state reac-
tion at 1250 oC of Lu2O3 and SiO2. The firing atmosphere, oxidizing or reducing, influences the optical behavior of the samples. While LPS:Ce–O exhibits the typical fast (35 ns) Ce3+ emission, LPS:Ce–R emits both from Ce3+ and the CT state of Ce4+ associated with defect created by reducing atmosphere. The assignment of the latter emission was based on its decay time (200 ls) and its occurrence only at low temperature. The nature of the slow emission and the defect structure in LPS:Ce will be investigated in the future by the time-resolved spectroscopy. Acknowledgements The work was funded by the Polish Committee for Scientific Research (KBN, Grant No. 20401331/0289) and European Union, Grant No. I-20060244 EC for measurements at Superlumi station at Hasylab, Desy, Hamburg. References [1] M.J. Weber, J. Lumin. 100 (2002) 35. [2] D. Pauwels et al., US Patent Specification (2002) 6437336. [3] L. Pidol, A. Kahn-Harari, B. Viana, B. Ferrand, P. Dorenbos, J.T.M. de Haas, C.W.E. Van Eijk, E. Virey, J. Phys.: Condens. Matter 15 (2003) 2091. [4] L. Pidol, B. Viana, A. Kahn-Harari, B. Ferrand, P. Dorenbos, C.W.E. Van Eijk, Nucl. Instrum. Methods A 537 (2005) 256. [5] C. Yan, G. Zhao, Y. Hang, L. Zhang, J. Xu, J. Cryst. Growth 281 (2005) 411. [6] C. Yan, G. Zhao, Y. Hang, L. Zhang, J. Xu, Mater. Lett. 60 (2006) 1960. [7] D. Pauwels et al., IEEE Trans. Nucl. Sci. 51 (2004) 1084. [8] A. Garcia-Murillo, C. Le Luyer, C. Dujardin, T. Martin, C. Garapon, C. Pedrini, J. Mugnier, Nucl. Instrum. Methods A 486 (2002) 181. [9] A. Garcia-Murillo, C. Le Luyer-Urlacher, C. Dujardin, C. Pedrini, J. Mugnier, J. Sol–Gel Sci. Technol. 26 (2003) 957. [10] D. Hreniak, E. Zych, L. Ke˛pin´ski, W. Stre˛k, J. Phys. Chem. Solid 64 (2003) 111. [11] M. Daldosso, J. Sokolnicki, L. Kepinski, J. Legendziewicz, A. Speghini, M. Bettinelli, J. Lumin. 122–123 (2007) 858. [12] G. Zimmerer, Nucl. Instrum. Methods, Phys. Res. Sect. A 308 (1991) 178. [13] L. Pidol, B. Viana, A. Kahn-Harari, A. Bessire, P. Dorenbos, Nucl. Instrum. Methods A 537 (2005) 125. [14] T. Aitasalo, J. Holsa, M. Lastusaari, J. Niittykoski, F. Pelle´, Phys. Stat. Sol. C 2 (1) (2005) 272. [15] E. Nakazawa, Chem. Phys. Lett. 56 (1978) 161. [16] E. Nakazawa, J. Lumin. 18/19 (1979) 272. [17] L. Van Pieterson, M. Heeroma, E. De Heer, A. Meijerink, J. Lumin. 91 (2000) 177. [18] J. Legendziewicz, J. Sokolnicki, J. Alloy Compds 451 (2008) 600. [19] W.H. Green, K.P. Le, J. Grey, T.T. Au, A.J. Sailor, Science 276 (1997) 1826. [20] J.A. Garcia, A. Remon, J. Pigueras, Phys. Stat. Sol. A 89 (1985) 237. [21] J. Sokolnicki, J. Solid State Chem. 180 (2007) 2400. [22] T. Aitasalo, J. Holsa, M. Lastusaari, J. Legendziewicz, J. Niittykoski, F. Pelle, Opt. Mater. 26 (2004) 107. [23] L. van Pieterson, S. Soverna, A. Meijerink, J. Electrochem. Soc. 147 (2000) 4688.