Research article Received: 15 December 2013,
Revised: 26 February 2014,
Accepted: 27 April 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2708
Luminescence in Sr4Al14O25:Ce3+ aluminate phosphor G. N. Nikhare,a S. C. Gedamb* and S. J. Dhoblea ABSTRACT: Cerium-doped Sr4Al14O25 phosphor is prepared using a single-step combustion synthesis and its X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) and thermoluminescence (TL) properties are characterized. XRD reveals the formation of the desired phase in the prepared sample. SEM micrographs of the prepared Sr4Al14O25 phosphor show that the particle size is 10 μm. The prepared Sr4Al14O25, along with Sr4Al14O25:Cex (x = 0.5–5 mol%) shows a PL emission peak at 314 nm under UV excitation of 262 nm wavelength due to 5d → 4f transition. The phosphor is suitable for higher concentrations of Ce ions. The TL glow peak reveals three clearly visible distinct peaks at temperatures around 130, 231 and 336ºC. The three peaks are separated by deconvolution and kinetic parameters calculated using Chen’s peak shape method. The calculation shows that the reaction follows second-order kinetics with activation energy (E) values of 0.52, 0.81 and 1.12 eV, and frequency factor (s) values of 5.58 × 105, 4.53 × 107 and 4.57 × 108 s-1 for the three individual peaks. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: aluminates; phosphors; combustion synthesis; photoluminescence, thermoluminescence
Introduction
Luminescence 2014
Experimental Sr4Al14O25:Ce aluminates were produced using the urea combustion technique. For this, stoichiometric compositions of the metal nitrates (oxidizers) and urea (fuel) were calculated using the total oxidizing and reducing valences of the components, which serve as numerical coefficients so that the equivalence ratio is unity and the heat liberated during combustion is at a maximum. To prepare Sr4Al14O25:Cex, we used the constituents * Correspondence to: S. C. Gedam, K.Z.S. Science College, Kalmeshwar, Nagpur 441501, India. E-mail: [email protected] a
Department of Physics, RTM Nagpur University, Nagpur 440033 India
b
K.Z.S. Science College, Kalmeshwar, Nagpur 441501, India
Copyright © 2014 John Wiley & Sons, Ltd.
1
Cerium-doped single crystals have been extensively studied because of their promising applications as scintillators in medical imaging applications such as positron emission tomography (PET), single-photon emission computerized tomography (SPECT) or γ-ray imaging camera (1–6). Among these crystals, the yttrium aluminates YAlO3:Ce (YAP:Ce) (5) and Y3Al5O12:Ce (YAG:Ce) (6) have received special attention because of their good thermal, mechanical and chemical stability and efficient emissions in the near-UV spectral region (300–450 nm) with fast decay responses (30–70 ns). Despite their usefulness, a shortcoming of bulk single crystals lies in their usually difficult, expensive and time-consuming growth, not to mention the limited concentration of doping ions that can be incorporated in the crystal lattice prior to segregation. One alternative that has received increased attention is the use of Ce3+-doped ceramic powders for the fabrication of scintillator screens (7). The characteristic emission of Ce3+ in the near-UV spectral region originates from parity allowed electric dipole transitions between the excited 5d and ground 4f states. The 5d orbitals have their energy levels split by crystal field effects into at least two sublevels, 2E and 2T2. Via spin–orbit interactions, the lowest sublevels of 5d can be further split into new components, as is also the case for the ground state level 4f, which is split into the 2F5/2 and 2F7/2 components. The energy difference between the latter is 2000 cm-1 and is practically independent of the host due to shielding of the 4f orbitals by the more external 5s and 5p orbitals. However, this splitting is often not observed as a spectral feature, particularly at high Ce3+ concentrations (7). Depending on the strength of the crystal field, the splitting of the 5d levels might be so large that Ce3+ emission can also be observed in the visible range. Rare earth (RE) ions possess unique optical behavior when doped into materials and have paved the way for the development of optical amplifiers and phosphors. The optical value of
these ions results from electronic transitions occurring within the partially filled 4f energy shell of the lanthanide series. Ce3+ is a low-cost activator that can provide strong absorption of UV and an efficient conversion to longer wavelengths. In their report, van der Kolk et al. (8) selected Sr0.7La0.3Al11.7Mg0.3O19 on the basis of the photon cascade emission (PCE) observed in SrAl12O19 (9). The relationship between size and shape of the coordination polyhedron and the energy of the 4fn–15d1 crystal-field states of trivalent RE ions has recently been discussed by Dorenbos in a series of articles (10–12). From this work and the fact that the shapes and sizes of the coordination polyhedra around Sr2+ and La3+ in Sr0.7La0.3Al11.7Mg0.3O19 are identical to those for Sr2+ in SrAl12O19, PCE is expected in Sr0.7La0.3Al11.7Mg0.3O19. Considering that aluminates can easily be prepared by combustion synthesis and a large number of aluminates with a well-characterized structure are known, it was decided to study the emission of Ce3+ in aluminates. Typical Ce3+ emission bands were observed at 356 and 460 nm.
G. N. Nikhare et al. Sr(NO3)2, Al(NO)3,, (NH2)2CO and Ce(NO)3. The details of the reactions for all compounds are described below:
Result and discussion Photoluminescence in Sr4Al14O25:Ce
4SrðNO3 Þ2 þ 14AlðNOÞ3 þ CeðNOÞ3 þ ðNH2 Þ2 CO→Sr4 Al14 O25 : Cex þ N2 þ H2 O þ CO2
All the basic compounds were weighed using a Dhona monopan balance (0.01 mg). Known amounts of each nitrate and urea (all Analar grade) were added together and the mixture was crushed in a mortar for 30 min to form a thick paste. The resulting paste was transferred into a china crucible (3" J brand) and placed in a vertical cylindrical muffle furnace (90 cm high and 20 cm in diameter) maintained at 500 ± 10ºC. The mixture underwent dehydration and then decomposition with the liberation of NH3 and NOx. Being highly exothermic, the process continued and the liberated gases caused the mixture to reach a large volume. The high level of exothermicity resulted in a flame that chaged the mixture into the gaseous phase. A flame temperature of up to 1600ºC converted the vapour phase oxides into mixed aluminates. The flame persisted for ~ 30 s. The crucible was then taken out of the furnace and the foamy product was crushed into a fine power. The resultant polycrystalline mass was crushed to a fine powder and used in further study. The prepared host lattice was characterized for phase purity and crystallinity by X-ray powder diffraction (XRD) using a PANanalytical diffractometer (Cu-Kα radiation) at a scanning step of 0.01o and continuation time of 20 s, in the 2θ range from 10 to 60o. Photoluminescence (PL) measurements of the excitation and emission were recorded on a Shimadzu RF5301PC spectrofluorophotometer fitted with a sensitive photomultiplier tube. This spectrofluorophotometer provides corrected excitation and emission spectra in the 220–400 and 300–700 nm ranges, respectively, at room temperature. Samples of 2 g were used for each measurement. The excitation and emission spectra were recorded using a spectral slit width bandpass of 1.5 nm. The thermoluminescence glow curves were studied using a thermoluminescence reader. The Co60 radioisotope gamma dose employed for irradiation was 5 Gy. In order to reduce error, thermoluminescence (TL) glow curve measurements were carried out just after irradiation. The TL response was then measured up to 400ºC using a TL reader at a temperature rate of 5ºC/s.
The XRD pattern of the Sr4Al14O25 host sample is shown in Fig. 1. The crystalline phase of the prepared compound was confirmed by XRD. The as-prepared compound showed peaks that matched well with the JCPDS data file (JCPDs File Number- 52-1876). Some unwanted phases, SrAl2O4 and SrAl4O7, were also observed along with pure Sr4Al14O25, which might due to the high temperature of the exothermic combustion reaction. The literature reveals that this compound has been found in the pure Sr4Al14O25 phase at temperatures between 1200 and 1500ºC, at low temperatures the SrA4O7 phase is prominent, whereas at high temperatures Sr4Al14O25 decomposes into SrAl2O4 and SrAl12O19 (13). The host matrix in its pure form possess an orthorhombic crystal structure and is a primitive lattice; when Ce is doped into the host matrix, the Ce3+ ion is expected to occupy the Sr2+ site because they have comparative ionic radii. The SEM micrographs of the prepared phosphor Sr4Al14O25 are shown in Fig. 2. It is evident from these images that the particle size of the phosphor is 10 μm. The crystal structure of the phosphor is not visible because the particles are not regular and are aggregates of several crystal lattices. Figure 3
Figure 2. SEM of prepared Sr4Al14O25 phosphor.
2 Figure 1. X-Ray diffraction pattern of Sr4Al14O25.
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Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Luminescence in Sr4Al14O25:Ce3+ aluminate phosphor
3+
Figure 3. PL excitation and emission spectra of Sr4Al14O25:Cex , x = 0, 0.5, 1, 2 and 5 mol%.
3+
Figure 5. PL Intensity vs concentration of dopant Ce
in Sr4Al14O25:Ce phosphor.
Figure 4. Energy scheme of allowed 5d–4f excitation and emission transitions.
Luminescence 2014
Figure 6. TL glow curve of Sr4Al14O25:Cex for a γ-ray exposure of 5 Gy: (a) x = 0.5 mol%, (b) x = 1 mol%, (c) x = 2 mol%, (d) x = 5 mol%.
Thermoluminescence in Sr4Al14O25:Ce The TL glow curve of Sr4Al14O25:Ce for diffferent concentrations of Ce3+ from 0.5 to 5 mol% is shown in Fig. 6. Before taking the TL, the sample is annealed at 300ºC and then irradiated at a γ-ray dose of 5 Gy. To avoid the possibility of error, TL measurements are carried out soon after irradiation of the sample. The TL is observed up to 400ºC at a heating rate of 5ºC/s. The glow curve reaveals three clear and well-distinguished peaks at 130, 231 and 336ºC. An increase in the concentration of the dopant Ce3+
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3
shows the PL excitation and emission spectra of Sr4Al14O25:Cex phosphor with different amounts of Cex (x = 0, 0.5, 1, 2 and 5 mol%) ion and the emission peak position with respect to its relative intensity is shown. The excitation spectrum (monitoring at 314 nm emission) consists of a strong maximum at 262 nm. Selecting 262 nm excitation wavelengths, we recorded the emission spectra for the pure host as well as samples containing various concentrations of Ce3+ ions. As with the emission spectra, the emission of Ce3+ in this host is observed at relatively lower wavelengths (314 nm). The emission peak position in these phosphors is around 314 nm, is almost constant for all concentrations of Ce3+, and is assigned to the 5d→4f transition of Ce3+ ions. The schematic energy scheme is shown in Fig. 4. As shown in Fig. 5, as the concentration of Ce3+ ions increases, the intensity of the 314 nm peak increases and the maximum intensity is observed with 5 mol% of Ce3+ ion. This indicates that the Sr4Al14O25:Ce lattice is more suitable for PL with higher concentrations of Ce3+ ions.
G. N. Nikhare et al. in the aluminate host does not influence the position of the peak, but does affect the TL intensity. As seen in Fig. 5, no such decrease in intensity is observed in case of PL, moreover, the host also shows PL characteristics not observed in Thermally Stimulated Luminescence (TSL); this indicates that PL and TL are entirely different entities and they follow completely different mechanisms. The TL intensity of this sample is small compared with the standard Thermoluminescence Dosimetry (TLD) material, and therefore does not have any direct application in dosimetry. Therefore, in order to study the energy-level structure, the kinetic parameters of the defect centers are evaluated.
Table 1. Experimental peak shape parameters for three deconvoluted peaks for 2 mol% dopant Ce Peak Peak temp Half Half Full Geometric no. Tm (ºC) width (τ) width (δ) width (ω) factor (μg) 1 2 3
133.49 228.63 334.85
39.02 41.89 44.24
45.49 42.63 46.72
μg ¼ Study of kinetic parameters The TL glow curve shows the nature of traps present in the material and also gives us information about the energy absorbed by the material during irradiation. A study of kinetic parameters such as the activation energy (E) and frequency factor (s) provides information about the defect centers responsible for TL in the material (14,15). Therefore, we calculated the kinetic parameters of the prepared TL sample. The peak shape method of Chen (general order kinetics) was used to calculate the kinetic parameters, viz. the geometrical factor, μg, E and s. Three peaks are clearly visible in the TSL glow curve, therefore in order to calculate trapping parameters, the experimental curve with maximum intensity among the other glow curves is deconvoluted using a computerized curve fitting method (shown in Fig. 7). The result obtained is similar to the thermal cleaning method, wherein the sample is heated repeatedly and rapidly cooled to separate the peaks. The initial rise method is also in use; however, we used Chen’s method because it is applicable to any kinetic order and we found that the values of E and s were within the limits given by Chen. As shown in the Table 1, Tm is the peak temperature and; T1 and T2 are the temperatures at half intensity on either side of the peak. τ, δ and ω are the half width on the low temperature side, half width on the high temperature side and full width at half maximum, respectively. They are the temperature differences between Tm – T1, T2 – Tm and T2 – T1. The peak shape method is generally used to calculate the order of kinetics and is obtained from the geometric factor (peak shape parameter). The geometric factor can be calculated as
84.51 84.52 90.96
0.53 0.50 0.51
δ ω
(1)
The order of kinetics depends on the shape of the peak. The value of μg for first- and second-order kinetics is 0.42 and 0.52, respectively. The activation energy, E, and frequency factor, s, for any order kinetics (16,17) can be calculated as E ¼ cα
2 kT m bα ð2kT m Þ α
(2)
and " # βE 1 E 2kTm exp s¼ 2 kTm T m k 1 þ ðb 1Þ E
(3)
Where, α stands for τ, δ and ω, respectively, β is the heating rate, k is the Boltzmann constant and b is order of kinetics, which can be obtained from eqn (1). The constants in eqn (2) can be obtained using the expressions: cτ ¼ 1:51 þ 3:0 μg –0:42 ; bτ ¼ 1:58 þ 4:2 μg –0:42 cδ ¼ 0:976 þ 7:3 μg –0:42 ; bδ ¼ 0 cω ¼ 2:52 þ 10:2 μg –0:42 ; bω ¼ 1
The peak shape parameters τ, δ and ω were initially determined for temperatures T1, Tm and T2. These values are used to calculate the geometric factor μg from eqn (1), which determines the order of kinetics. The values of E and s were then calculated from eqns (2) and (3) and are given in Table 2. It is also concluded that studying the effect of different annealing temperatures on the trap depth and frequency factor may further reveal some more important aspects of TL. Table 2. Trap depth, E (eV) and frequency factor s (s-1) values for the three deconvoluted peaks at 2 mol% of dopant Ce Peak no.
4 Figure 7. Deconvuluted peaks for the calculation of trapping parameters.
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1 2 3
Peak temp Tm (ºC)
Order of kinetics (b)
Trap depth E (eV)
Freq factor s (s-1)
133.49 227.02 334.85
2 2 2
0.52 ± 0.04 0.81 ± 0.02 1.12 ± 0.01
5.58 × 105 4.53 × 107 4.57 × 108
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Luminescence in Sr4Al14O25:Ce3+ aluminate phosphor 3+
Conclusions Cerium-doped aluminate phosphor Sr4Al14O25:Ce with an orthorhombic structure was prepared using a single-step combustion synthesis in a normal atmosphere. Formation of the desired phase of the compound was verified using XRD analysis of the prepared sample. It was found that the pure Sr4Al14O25 host also shows PL along with the Ce3+-doped Sr4Al14O25, which shows high-intensity PL emission at 314 nm under 262 nm UV excitation wavelength due to 5d → 4f transition of the Ce3+ ion. It also shows fair TL characteristics, except that the pure aluminate host Sr4Al14O25 gives three distinct peaks at temperatures around 130, 231 and 336ºC. The three experimental peaks are separated using computerized deconvulution to evaluate the kinetic parameters. Calculation of the kinetic parameters for each individual peak using Chen’s peak shape method shows that it follows second-order kinetics with energy values between 0.52 and 1.12 eV and frequency factor values of 105–108 s-1.
4.
5.
6. 7. 8. 9. 10.
Acknowledgement
11.
SJD wishes to thank the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Govt of India, for providing financial assistance.
12. 13.
References
14.
1. Nikolopoulos D, Valais I, Kandarakis I, Cavouras D, Linardatos D, Sianoudis I, et al. Evaluation of the GSO: Ce scintillator in the energy range from 40 to 140 kV for possible applications in medical imaging. Nucl Instrum Meth A 2006;560:577–83. 2. Nikl M. Wide band gap scintillation materials: Progress in the technology and material understanding. Phys Status Solidi A 2000;178:595–620. 3. Mares JA, Nikl M, Soloviera N, D’Ambrosio C, Notaristefani FD, 3+ Blazek K, et al. Scintillation and spectroscopic properties of Ce -
15. 16. 17.
3+
doped YAlO3 and Lux(RE)1- xAlO3(RE=Y and Gd ) scintillators. Nucl Instrum Meth A 2003;498:312–27. Birowosuto MD, Dorenbos P. Novel γ-and X-ray scintillator research: on the emission wavelength, light yield and time 3+ response of Ce doped halide scintillators. Phys Status Solidi A 2009;206:9–20. Valias I, David S, Michail C, Nikolopoulos D, Sianoudis I, et al. Comparative study of luminescence properties of Lu2SiO5:Ce and YAlO3:Ce Single crystal scintillators for use in medical imaging. Nucl Instrum Meth A 2003;510:577–83. Moszynski M, Kapusta M, Wolski D, Klamra W, Cederwal B. Properties of YAP:Ce scintillator. Nucl Instrum Meth A 1994;345:157–62. Blasse G, Bril A. A new phosphor for flying-spot cathode-ray tubes 3+ for color television: yellow emitting Y3Al5O12-Ce . Appl Phys Lett 1967;11:53–57. van der Kolk E, Dorenbos P, van Eijk CWE. Vacuum ultraviolet excita1 3 3+ tion of S0 and P0 emission of Pr in Sr0.7La0.3Al11.7Mg0.3O19 and SrB4O7. J Phys Condens Matter 2001;13:5471. 3+ Srivastava AM, Beers WW. Luminescence of Pr in SrAl12O19: observation of two photon luminescence in oxide lattice. J Lumin 1997;71:285–90. 3+ Dorenbos P. 5d-level energies of Ce and the crystalline environment. I. Fluoride compounds. Phys Rev B 2000;62:15640. 3+ Dorenbos P. 5d-level energies of Ce and the crystalline environment. II. Chloride, bromide, and iodide compounds. Phys Rev B 2000;62:15650. n n~1 5d transitions of the trivalent lanthaDorenbos P. The 4f à4f nides in halogenides and chalcogenides. J Lumin 2000;91:91–106. Capron M, Fayon F, Massiot D, André D. Sr4Al14O25: Formation, stability, and 27Al high-resolution NMR characterization. Chem Mater 2003;15:575–79. Dhoble SJ, Moharil SV, Gundu Rao TK. Correlated ESR, PL and TL studies on Sr5(PO4)3Cl:Eu thermoluminescence dosimetry phosphor. J Lumin 2007;126:383–86. Dhoble SJ, Moharil SV, Gundu Rao TK. Correlated ESR, PL and TL studies on K3Na(SO4)2: Eu thermoluminescence dosimetry phosphor. J Lumin 2001;93:43–49. Chen RJ. On the calculation of activation energies and frequency factors from glow curves. Appl Phys 1969;40:570–85. Azorin J. Determination of thermoluminescence parameters from glow curves-I. J Int J Radiat Appl Instrum D 1986;II(3):159–66.
5
Luminescence 2014
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
Revised: 26 February 2014,
Accepted: 27 April 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2708
Luminescence in Sr4Al14O25:Ce3+ aluminate phosphor G. N. Nikhare,a S. C. Gedamb* and S. J. Dhoblea ABSTRACT: Cerium-doped Sr4Al14O25 phosphor is prepared using a single-step combustion synthesis and its X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) and thermoluminescence (TL) properties are characterized. XRD reveals the formation of the desired phase in the prepared sample. SEM micrographs of the prepared Sr4Al14O25 phosphor show that the particle size is 10 μm. The prepared Sr4Al14O25, along with Sr4Al14O25:Cex (x = 0.5–5 mol%) shows a PL emission peak at 314 nm under UV excitation of 262 nm wavelength due to 5d → 4f transition. The phosphor is suitable for higher concentrations of Ce ions. The TL glow peak reveals three clearly visible distinct peaks at temperatures around 130, 231 and 336ºC. The three peaks are separated by deconvolution and kinetic parameters calculated using Chen’s peak shape method. The calculation shows that the reaction follows second-order kinetics with activation energy (E) values of 0.52, 0.81 and 1.12 eV, and frequency factor (s) values of 5.58 × 105, 4.53 × 107 and 4.57 × 108 s-1 for the three individual peaks. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: aluminates; phosphors; combustion synthesis; photoluminescence, thermoluminescence
Introduction
Luminescence 2014
Experimental Sr4Al14O25:Ce aluminates were produced using the urea combustion technique. For this, stoichiometric compositions of the metal nitrates (oxidizers) and urea (fuel) were calculated using the total oxidizing and reducing valences of the components, which serve as numerical coefficients so that the equivalence ratio is unity and the heat liberated during combustion is at a maximum. To prepare Sr4Al14O25:Cex, we used the constituents * Correspondence to: S. C. Gedam, K.Z.S. Science College, Kalmeshwar, Nagpur 441501, India. E-mail: [email protected] a
Department of Physics, RTM Nagpur University, Nagpur 440033 India
b
K.Z.S. Science College, Kalmeshwar, Nagpur 441501, India
Copyright © 2014 John Wiley & Sons, Ltd.
1
Cerium-doped single crystals have been extensively studied because of their promising applications as scintillators in medical imaging applications such as positron emission tomography (PET), single-photon emission computerized tomography (SPECT) or γ-ray imaging camera (1–6). Among these crystals, the yttrium aluminates YAlO3:Ce (YAP:Ce) (5) and Y3Al5O12:Ce (YAG:Ce) (6) have received special attention because of their good thermal, mechanical and chemical stability and efficient emissions in the near-UV spectral region (300–450 nm) with fast decay responses (30–70 ns). Despite their usefulness, a shortcoming of bulk single crystals lies in their usually difficult, expensive and time-consuming growth, not to mention the limited concentration of doping ions that can be incorporated in the crystal lattice prior to segregation. One alternative that has received increased attention is the use of Ce3+-doped ceramic powders for the fabrication of scintillator screens (7). The characteristic emission of Ce3+ in the near-UV spectral region originates from parity allowed electric dipole transitions between the excited 5d and ground 4f states. The 5d orbitals have their energy levels split by crystal field effects into at least two sublevels, 2E and 2T2. Via spin–orbit interactions, the lowest sublevels of 5d can be further split into new components, as is also the case for the ground state level 4f, which is split into the 2F5/2 and 2F7/2 components. The energy difference between the latter is 2000 cm-1 and is practically independent of the host due to shielding of the 4f orbitals by the more external 5s and 5p orbitals. However, this splitting is often not observed as a spectral feature, particularly at high Ce3+ concentrations (7). Depending on the strength of the crystal field, the splitting of the 5d levels might be so large that Ce3+ emission can also be observed in the visible range. Rare earth (RE) ions possess unique optical behavior when doped into materials and have paved the way for the development of optical amplifiers and phosphors. The optical value of
these ions results from electronic transitions occurring within the partially filled 4f energy shell of the lanthanide series. Ce3+ is a low-cost activator that can provide strong absorption of UV and an efficient conversion to longer wavelengths. In their report, van der Kolk et al. (8) selected Sr0.7La0.3Al11.7Mg0.3O19 on the basis of the photon cascade emission (PCE) observed in SrAl12O19 (9). The relationship between size and shape of the coordination polyhedron and the energy of the 4fn–15d1 crystal-field states of trivalent RE ions has recently been discussed by Dorenbos in a series of articles (10–12). From this work and the fact that the shapes and sizes of the coordination polyhedra around Sr2+ and La3+ in Sr0.7La0.3Al11.7Mg0.3O19 are identical to those for Sr2+ in SrAl12O19, PCE is expected in Sr0.7La0.3Al11.7Mg0.3O19. Considering that aluminates can easily be prepared by combustion synthesis and a large number of aluminates with a well-characterized structure are known, it was decided to study the emission of Ce3+ in aluminates. Typical Ce3+ emission bands were observed at 356 and 460 nm.
G. N. Nikhare et al. Sr(NO3)2, Al(NO)3,, (NH2)2CO and Ce(NO)3. The details of the reactions for all compounds are described below:
Result and discussion Photoluminescence in Sr4Al14O25:Ce
4SrðNO3 Þ2 þ 14AlðNOÞ3 þ CeðNOÞ3 þ ðNH2 Þ2 CO→Sr4 Al14 O25 : Cex þ N2 þ H2 O þ CO2
All the basic compounds were weighed using a Dhona monopan balance (0.01 mg). Known amounts of each nitrate and urea (all Analar grade) were added together and the mixture was crushed in a mortar for 30 min to form a thick paste. The resulting paste was transferred into a china crucible (3" J brand) and placed in a vertical cylindrical muffle furnace (90 cm high and 20 cm in diameter) maintained at 500 ± 10ºC. The mixture underwent dehydration and then decomposition with the liberation of NH3 and NOx. Being highly exothermic, the process continued and the liberated gases caused the mixture to reach a large volume. The high level of exothermicity resulted in a flame that chaged the mixture into the gaseous phase. A flame temperature of up to 1600ºC converted the vapour phase oxides into mixed aluminates. The flame persisted for ~ 30 s. The crucible was then taken out of the furnace and the foamy product was crushed into a fine power. The resultant polycrystalline mass was crushed to a fine powder and used in further study. The prepared host lattice was characterized for phase purity and crystallinity by X-ray powder diffraction (XRD) using a PANanalytical diffractometer (Cu-Kα radiation) at a scanning step of 0.01o and continuation time of 20 s, in the 2θ range from 10 to 60o. Photoluminescence (PL) measurements of the excitation and emission were recorded on a Shimadzu RF5301PC spectrofluorophotometer fitted with a sensitive photomultiplier tube. This spectrofluorophotometer provides corrected excitation and emission spectra in the 220–400 and 300–700 nm ranges, respectively, at room temperature. Samples of 2 g were used for each measurement. The excitation and emission spectra were recorded using a spectral slit width bandpass of 1.5 nm. The thermoluminescence glow curves were studied using a thermoluminescence reader. The Co60 radioisotope gamma dose employed for irradiation was 5 Gy. In order to reduce error, thermoluminescence (TL) glow curve measurements were carried out just after irradiation. The TL response was then measured up to 400ºC using a TL reader at a temperature rate of 5ºC/s.
The XRD pattern of the Sr4Al14O25 host sample is shown in Fig. 1. The crystalline phase of the prepared compound was confirmed by XRD. The as-prepared compound showed peaks that matched well with the JCPDS data file (JCPDs File Number- 52-1876). Some unwanted phases, SrAl2O4 and SrAl4O7, were also observed along with pure Sr4Al14O25, which might due to the high temperature of the exothermic combustion reaction. The literature reveals that this compound has been found in the pure Sr4Al14O25 phase at temperatures between 1200 and 1500ºC, at low temperatures the SrA4O7 phase is prominent, whereas at high temperatures Sr4Al14O25 decomposes into SrAl2O4 and SrAl12O19 (13). The host matrix in its pure form possess an orthorhombic crystal structure and is a primitive lattice; when Ce is doped into the host matrix, the Ce3+ ion is expected to occupy the Sr2+ site because they have comparative ionic radii. The SEM micrographs of the prepared phosphor Sr4Al14O25 are shown in Fig. 2. It is evident from these images that the particle size of the phosphor is 10 μm. The crystal structure of the phosphor is not visible because the particles are not regular and are aggregates of several crystal lattices. Figure 3
Figure 2. SEM of prepared Sr4Al14O25 phosphor.
2 Figure 1. X-Ray diffraction pattern of Sr4Al14O25.
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Luminescence 2014
Luminescence in Sr4Al14O25:Ce3+ aluminate phosphor
3+
Figure 3. PL excitation and emission spectra of Sr4Al14O25:Cex , x = 0, 0.5, 1, 2 and 5 mol%.
3+
Figure 5. PL Intensity vs concentration of dopant Ce
in Sr4Al14O25:Ce phosphor.
Figure 4. Energy scheme of allowed 5d–4f excitation and emission transitions.
Luminescence 2014
Figure 6. TL glow curve of Sr4Al14O25:Cex for a γ-ray exposure of 5 Gy: (a) x = 0.5 mol%, (b) x = 1 mol%, (c) x = 2 mol%, (d) x = 5 mol%.
Thermoluminescence in Sr4Al14O25:Ce The TL glow curve of Sr4Al14O25:Ce for diffferent concentrations of Ce3+ from 0.5 to 5 mol% is shown in Fig. 6. Before taking the TL, the sample is annealed at 300ºC and then irradiated at a γ-ray dose of 5 Gy. To avoid the possibility of error, TL measurements are carried out soon after irradiation of the sample. The TL is observed up to 400ºC at a heating rate of 5ºC/s. The glow curve reaveals three clear and well-distinguished peaks at 130, 231 and 336ºC. An increase in the concentration of the dopant Ce3+
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shows the PL excitation and emission spectra of Sr4Al14O25:Cex phosphor with different amounts of Cex (x = 0, 0.5, 1, 2 and 5 mol%) ion and the emission peak position with respect to its relative intensity is shown. The excitation spectrum (monitoring at 314 nm emission) consists of a strong maximum at 262 nm. Selecting 262 nm excitation wavelengths, we recorded the emission spectra for the pure host as well as samples containing various concentrations of Ce3+ ions. As with the emission spectra, the emission of Ce3+ in this host is observed at relatively lower wavelengths (314 nm). The emission peak position in these phosphors is around 314 nm, is almost constant for all concentrations of Ce3+, and is assigned to the 5d→4f transition of Ce3+ ions. The schematic energy scheme is shown in Fig. 4. As shown in Fig. 5, as the concentration of Ce3+ ions increases, the intensity of the 314 nm peak increases and the maximum intensity is observed with 5 mol% of Ce3+ ion. This indicates that the Sr4Al14O25:Ce lattice is more suitable for PL with higher concentrations of Ce3+ ions.
G. N. Nikhare et al. in the aluminate host does not influence the position of the peak, but does affect the TL intensity. As seen in Fig. 5, no such decrease in intensity is observed in case of PL, moreover, the host also shows PL characteristics not observed in Thermally Stimulated Luminescence (TSL); this indicates that PL and TL are entirely different entities and they follow completely different mechanisms. The TL intensity of this sample is small compared with the standard Thermoluminescence Dosimetry (TLD) material, and therefore does not have any direct application in dosimetry. Therefore, in order to study the energy-level structure, the kinetic parameters of the defect centers are evaluated.
Table 1. Experimental peak shape parameters for three deconvoluted peaks for 2 mol% dopant Ce Peak Peak temp Half Half Full Geometric no. Tm (ºC) width (τ) width (δ) width (ω) factor (μg) 1 2 3
133.49 228.63 334.85
39.02 41.89 44.24
45.49 42.63 46.72
μg ¼ Study of kinetic parameters The TL glow curve shows the nature of traps present in the material and also gives us information about the energy absorbed by the material during irradiation. A study of kinetic parameters such as the activation energy (E) and frequency factor (s) provides information about the defect centers responsible for TL in the material (14,15). Therefore, we calculated the kinetic parameters of the prepared TL sample. The peak shape method of Chen (general order kinetics) was used to calculate the kinetic parameters, viz. the geometrical factor, μg, E and s. Three peaks are clearly visible in the TSL glow curve, therefore in order to calculate trapping parameters, the experimental curve with maximum intensity among the other glow curves is deconvoluted using a computerized curve fitting method (shown in Fig. 7). The result obtained is similar to the thermal cleaning method, wherein the sample is heated repeatedly and rapidly cooled to separate the peaks. The initial rise method is also in use; however, we used Chen’s method because it is applicable to any kinetic order and we found that the values of E and s were within the limits given by Chen. As shown in the Table 1, Tm is the peak temperature and; T1 and T2 are the temperatures at half intensity on either side of the peak. τ, δ and ω are the half width on the low temperature side, half width on the high temperature side and full width at half maximum, respectively. They are the temperature differences between Tm – T1, T2 – Tm and T2 – T1. The peak shape method is generally used to calculate the order of kinetics and is obtained from the geometric factor (peak shape parameter). The geometric factor can be calculated as
84.51 84.52 90.96
0.53 0.50 0.51
δ ω
(1)
The order of kinetics depends on the shape of the peak. The value of μg for first- and second-order kinetics is 0.42 and 0.52, respectively. The activation energy, E, and frequency factor, s, for any order kinetics (16,17) can be calculated as E ¼ cα
2 kT m bα ð2kT m Þ α
(2)
and " # βE 1 E 2kTm exp s¼ 2 kTm T m k 1 þ ðb 1Þ E
(3)
Where, α stands for τ, δ and ω, respectively, β is the heating rate, k is the Boltzmann constant and b is order of kinetics, which can be obtained from eqn (1). The constants in eqn (2) can be obtained using the expressions: cτ ¼ 1:51 þ 3:0 μg –0:42 ; bτ ¼ 1:58 þ 4:2 μg –0:42 cδ ¼ 0:976 þ 7:3 μg –0:42 ; bδ ¼ 0 cω ¼ 2:52 þ 10:2 μg –0:42 ; bω ¼ 1
The peak shape parameters τ, δ and ω were initially determined for temperatures T1, Tm and T2. These values are used to calculate the geometric factor μg from eqn (1), which determines the order of kinetics. The values of E and s were then calculated from eqns (2) and (3) and are given in Table 2. It is also concluded that studying the effect of different annealing temperatures on the trap depth and frequency factor may further reveal some more important aspects of TL. Table 2. Trap depth, E (eV) and frequency factor s (s-1) values for the three deconvoluted peaks at 2 mol% of dopant Ce Peak no.
4 Figure 7. Deconvuluted peaks for the calculation of trapping parameters.
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1 2 3
Peak temp Tm (ºC)
Order of kinetics (b)
Trap depth E (eV)
Freq factor s (s-1)
133.49 227.02 334.85
2 2 2
0.52 ± 0.04 0.81 ± 0.02 1.12 ± 0.01
5.58 × 105 4.53 × 107 4.57 × 108
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Luminescence in Sr4Al14O25:Ce3+ aluminate phosphor 3+
Conclusions Cerium-doped aluminate phosphor Sr4Al14O25:Ce with an orthorhombic structure was prepared using a single-step combustion synthesis in a normal atmosphere. Formation of the desired phase of the compound was verified using XRD analysis of the prepared sample. It was found that the pure Sr4Al14O25 host also shows PL along with the Ce3+-doped Sr4Al14O25, which shows high-intensity PL emission at 314 nm under 262 nm UV excitation wavelength due to 5d → 4f transition of the Ce3+ ion. It also shows fair TL characteristics, except that the pure aluminate host Sr4Al14O25 gives three distinct peaks at temperatures around 130, 231 and 336ºC. The three experimental peaks are separated using computerized deconvulution to evaluate the kinetic parameters. Calculation of the kinetic parameters for each individual peak using Chen’s peak shape method shows that it follows second-order kinetics with energy values between 0.52 and 1.12 eV and frequency factor values of 105–108 s-1.
4.
5.
6. 7. 8. 9. 10.
Acknowledgement
11.
SJD wishes to thank the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Govt of India, for providing financial assistance.
12. 13.
References
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doped YAlO3 and Lux(RE)1- xAlO3(RE=Y and Gd ) scintillators. Nucl Instrum Meth A 2003;498:312–27. Birowosuto MD, Dorenbos P. Novel γ-and X-ray scintillator research: on the emission wavelength, light yield and time 3+ response of Ce doped halide scintillators. Phys Status Solidi A 2009;206:9–20. Valias I, David S, Michail C, Nikolopoulos D, Sianoudis I, et al. Comparative study of luminescence properties of Lu2SiO5:Ce and YAlO3:Ce Single crystal scintillators for use in medical imaging. Nucl Instrum Meth A 2003;510:577–83. Moszynski M, Kapusta M, Wolski D, Klamra W, Cederwal B. Properties of YAP:Ce scintillator. Nucl Instrum Meth A 1994;345:157–62. Blasse G, Bril A. A new phosphor for flying-spot cathode-ray tubes 3+ for color television: yellow emitting Y3Al5O12-Ce . Appl Phys Lett 1967;11:53–57. van der Kolk E, Dorenbos P, van Eijk CWE. Vacuum ultraviolet excita1 3 3+ tion of S0 and P0 emission of Pr in Sr0.7La0.3Al11.7Mg0.3O19 and SrB4O7. J Phys Condens Matter 2001;13:5471. 3+ Srivastava AM, Beers WW. Luminescence of Pr in SrAl12O19: observation of two photon luminescence in oxide lattice. J Lumin 1997;71:285–90. 3+ Dorenbos P. 5d-level energies of Ce and the crystalline environment. I. Fluoride compounds. Phys Rev B 2000;62:15640. 3+ Dorenbos P. 5d-level energies of Ce and the crystalline environment. II. Chloride, bromide, and iodide compounds. Phys Rev B 2000;62:15650. n n~1 5d transitions of the trivalent lanthaDorenbos P. The 4f à4f nides in halogenides and chalcogenides. J Lumin 2000;91:91–106. Capron M, Fayon F, Massiot D, André D. Sr4Al14O25: Formation, stability, and 27Al high-resolution NMR characterization. Chem Mater 2003;15:575–79. Dhoble SJ, Moharil SV, Gundu Rao TK. Correlated ESR, PL and TL studies on Sr5(PO4)3Cl:Eu thermoluminescence dosimetry phosphor. J Lumin 2007;126:383–86. Dhoble SJ, Moharil SV, Gundu Rao TK. Correlated ESR, PL and TL studies on K3Na(SO4)2: Eu thermoluminescence dosimetry phosphor. J Lumin 2001;93:43–49. Chen RJ. On the calculation of activation energies and frequency factors from glow curves. Appl Phys 1969;40:570–85. Azorin J. Determination of thermoluminescence parameters from glow curves-I. J Int J Radiat Appl Instrum D 1986;II(3):159–66.
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