Photoluminescence due to efficient energy transfer from Ce3+ to Tb3+ and Mn2+ in Sr3Al10SiO20

Materials Chemistry and Physics 91 (2005) 524–531

Photoluminescence due to efficient energy transfer from Ce3+ to Tb3+ and Mn2+ in Sr3Al10SiO20 Abanti Nag, T.R.N. Kutty∗ Materials Research Centre, Indian Institute of Science, Bangalore 560012, India Received 18 August 2004; received in revised form 7 December 2004; accepted 20 December 2004

Abstract Sr3 Al10 SiO20 , doped with Ce3+ , Tb3+ , Eu2+ , (Ce3+ + Tb3+ ), (Ce3+ + Mn2+ ), (Eu2+ + Tb3+ ) and (Eu2+ + Mn2+ ) are prepared by wet-chemical route and energy transfer from Ce3+ to Tb3+ or Mn2+ are investigated. The spectrum of Sr3 Al10 SiO20 :Ce3+ shows single emission band at blue–violet region indicating the weak spin-orbit coupling of the ground state of Ce3+ . Tb3+ doped Sr3 Al10 SiO20 yields both the blue emission 5 D3 –7 FJ (J = 3, 4, 5, 6) and green emission 5 D4 –7 FJ (J = 3, 4, 5, 6) of Tb3+ . However, the relative intensity of 5 D4 –7 FJ transitions increases with concentration due to cross-relaxation between two interacting Tb3+ centers. The green emission of Tb3+ is remarkably enhanced due to energy transfer from Ce3+ to Tb3+ when present together in Sr3 Al10 SiO20 . The energy transfer phenomenon is also observed for Ce3+ and Mn2+ co-doped Sr3 Al10 SiO20 indicating that Ce3+ ion acts as a sensitizer in Sr3 Al10 SiO20 independent of the activator. Sr3 Al10 SiO20 :Eu2+ shows broad emission band with the maximum in the blue–green region. However, Eu2+ does not act as a sensitizer of luminescence when co-doped with Tb3+ or Mn2+ . This might be because of the existence of Eu2+ emission level at lower energy than the absorption energy level of Tb3+ as well as Mn2+ which does not allow the spectral overlap between the emission spectra of Eu2+ and the absorption spectra of Mn2+ or Tb3+ to induce sensitized luminescence. © 2004 Elsevier B.V. All rights reserved. Keywords: Inorganic compounds; Precipitation; Photoluminescence spectroscopy; Luminescence

1. Introduction Recently, considerable attention has been paid to research on new phosphor materials for application in advanced display technologies, including plasma and field emission [1]. Field emission displays (FEDs) can be thought of as the flat panel analogue of the familiar cathode ray tube (CRT). FED technology offers the same excellent display characteristics as the CRT, high brightness and contrast, wide viewing angle and fast response time. Further, FED phosphors are required to operate at lower voltages and higher current densities compared to CRT screen pigments. In addition, low voltage FEDs have a number of potential advantages over high voltage CRTs, including less complexity and consequently, lower manufacturing costs. However, the high energy surface excitation on the phosphor particles in the case of ∗

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0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.12.020

low voltage FEDs causes degradation of sulfides, leaving the oxide hosts as the only favorable choice. Low voltage FEDs require the development phosphors that are bright and efficient under low energy excitation, are stable at high currents and have good saturation characteristics. Although the photophysical properties leading to luminescence are relatively well understood, the specific spectral properties, luminescence efficiencies and operational lifetimes depend on complex interaction between the excitation sources, host lattice, sensitizers, activators, defects and interfaces, all of which are strongly dependent on the compositions [2,3]. Oxides are attractive host materials for the development of advanced phosphors due to their ease of synthesis and stability. For example, the red emitting SrTiO3 :Al3+ , Pr3+ has been investigated for its application in display devices, the properties of which arise from the in situ formed transitional nanophase of SrAl12 O19 and/or TiO2 and the associated defects therefrom [4]. The research work has been extended to develop other semicoherent solid solutions where the

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luminescence may arise from the in situ formed transitional nanophases and the associated interfacial defects leading to the red phosphor SrAl12 O19 :Pr3+ , Ti4+ for FEDs [5]. Kubota et al., have reported a new luminescent host, Sr3 Al10 SiO20 , for application in FEDs [6]. Sr3 Al10 SiO20 :Tb3+ shows efficient green emission under vaccum ultraviolet (VUV) excitation, the relative intensity of which is higher than that of other Tb3+ -doped strontium aluminum silicates, e.g. Sr2 Al2 SiO7 and SrAl2 Si2 O8 as well as SrAl12 O19 [7]. Similarly, a moderately efficient blue emission is obtained from Sr3 Al10 SiO20 :Eu2+ with relative intensity of 60% of commercial phosphor BaMgAl10 O17 :Eu2+ (BAM) under VUV excitation [8]. Presently, we report the efficient blue–violet emission from Ce3+ doped Sr3 Al10 SiO20 , having a single band around 390 nm. Often, energy transfer in terms of multipolar interactions between two ions is used for the sensitization of one of the ions to achieve brighter emissions [9]. As it is well known that Ce3+ is an efficient sensitizer, especially for Tb3+ , an approach is made to enhance the green emission of Tb3+ via energy transfer from Ce3+ to Tb3+ in Sr3 Al10 SiO20 . Further, the reduction process of Eu3+ to Eu2+ in Sr3 Al10 SiO20 is investigated because the conversion is found to be incomplete even under the atmosphere of low partial pressure of oxygen (pO2 ) at higher temperatures. Applications using the concept of energy transfer are not limited to rare-earth ions, rather it has also been observed in (rare earth + transition metal) codoped materials. Hence, our experiments are extended to study the energy transfer processes in (Ce3+ + Mn2+ ), (Eu2+ + Mn2+ ) and (Eu2+ + Tb3+ ) codoped Sr3 Al10 SiO20 .

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Phase identification of the powders was carried out by X-ray diffraction using a Philips PW3179 Powder Diffrac˚ radiation. tometer with Ni-filtered Cu K␣ (λ = 1.5406 A) Photoluminescence (PL) spectra were obtained at room temperature using JASCO-FP-6500/6600 spectrofluorimeter fitted with a 350 W xenon flash lamp and Hamamatsu R928 photomultiplier detector.

3. Results and discussion 3.1. X-ray diffraction analyses Fig. 1 shows X-ray diffraction pattern of Sr3 Al10 SiO20 , annealed at 1450 ◦ C, with different dopants. All the samples are identified as phase-pure monoclinic Sr3 Al10 SiO20 with space group C2/m, irrespective of the dopants as well as the reduction treatment [6]. With Sr3 Al10 SiO20 :Eu, the reduction of Eu3+ to Eu2+ could not be completely realized even when the annealing is carried out in strong reducing atmosphere of 100% H2 at 1450 ◦ C. The four conditions, reported [10] to be necessary for the reduction of Eu3+ → Eu2+ are: (i) absence of oxidizing ions in the host compound; (ii) the dopant trivalent Eu3+ should replace a divalent cation in the host; (iii) the substituted cation should have similar radius to the divalent Eu2+ ion; (iv) the host compound should have an appropriate structure based upon the tetrahedral anion groups (BO4 , SO4 or PO4 ). In Sr3 Al10 SiO20 , there is no oxidizing ion present in the host lattice which satisfies the condition (i). Further, Eu3+ occupies Sr2+ sites because the ionic radii of rare-earth

2. Experimental Bulk samples of single-phase Sr3 Al10 SiO20 :A (A = Ce3+ , Eu2+ or Mn2+ ) phosphor samples were synthesized by a wet-chemical method, referred to as gel–carbonate composite precipitation, to yield high-purity ultrafine powders with predictable dopant level and a high degree of compositional homogeneity. The starting materials were SrCl2 ·6H2 O (99.98%, Merck, Germany), AlCl3 ·6H2 O (99.99%), aerosol–SiO2 (99.99%, Chemplast, India), Tb4 O7 (99.99%, Indian Rare-earths), (NH4 )2 Ce(NO3 )6 (99.99%, Fluka, Switzerland), Eu2 O3 (99.99%, Indian Rare-earths) and MnCl2 ·4H2 O (99.9%, Lancaster, England). During the course of preparation, the gels of hydrated hydroxides of Al3+ (Al2 O3 ·xH2 O, 30 < x < 70), Ce3+ (CeO2 ·xH2 O, 40 < x < 75) and Tb3+ were co-precipitated along with SrCO3 by the addition of 1 M ammonium carbonate to the corresponding mixed chloride solutions at 60 ◦ C until the pH was around 8. The precipitates were then washed free of anions and ammonium ions, dispersed with stoichiometric amounts of aerosol–SiO2 , dried at 110 ◦ C for 12 h, calcined at 950 ◦ C and then annealed at 1450 ◦ C in a reducing atmosphere (90% N2 + 10% H2 ). Both Ce3+ and Tb3+ concentrations were varied from 0.5 to 5 at.%. Tb3+ ,

Fig. 1. X-ray diffraction patterns of (a) (Sr0.90 Tb0.05 Ce0.05 )3 Al10 SiO20 , (b) (Sr0.993 Eu0.007 )3 Al10 SiO20 after reduction and (c) (Sr0.993 Eu0.007 )3 Al10 SiO20 with 5% B2 O3 after reduction.

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Table 1 ˚ Comparison of effective ionic radii (A) Ions

CN

Sr2+

Tb3+

Ce3+

Eu3+

Eu2+

Effective ionic radii in different coordination

VIII

1.26

1.04

1.14

1.066

1.25

X

1.36

–

1.25

–

1.35

ions almost coincides with that of Sr2+ (Table 1), satisfy conditions (ii) and (iii) [11]. In order to fulfill the condition (iv), ≤5 mol% B2 O3 is added to Sr3 Al10 SiO20 :Eu. The addition of B2 O3 does not alter the phase content of Sr3 Al10 SiO20 [Fig. 1(c)]. Comparing Fig. 1(b) and (c), it can be concluded that boron will occupy the tetrahedral position as BO4 and (Al,Si,B)O4 group will facilitate the reduction process of Eu. 3.2.

27 Al

MAS NMR spectra

Fig. 2 shows the 27 Al MAS NMR spectrum of Sr3 Al10 SiO20 :Ce3+ , Tb3+ /Mn2+ at the magnetic field of 7.05 T. An important characteristic of 27 Al solid-state NMR is the dependence of isotropic chemical shifts (δiso ) of the +1/2|↔|−1/2 transition on the local coordination of aluminium: AlO4 ∼ 80–45 ppm, AlO5 ∼ 40–20 ppm and AlO6 ∼ 20 to −20 ppm. The ranges of δiso (shielding) correlate with the chemical composition of the second order coordination sphere, i.e. the next-nearest-neighbor (NNN) and the bond angles between the aluminium ions as well as the ions from the nearest-neighbor (NN) and NNN sites [12]. Fig. 2(a) shows an intense asymmetric signal at δ = 80.92 ppm and another low intensity asymmetric signal at δ = 2.46 ppm. Similarly, Fig. 2(b) shows two signals, one at δ = 80.89 ppm and another at δ = 2.81 ppm. The signal around δ ≈ 80 ppm is originating from AlO4 -tetrahedra whereas the same around δ ≈ 2 ppm is arising from AlO6 -octahedra. The asymmetric nature of the spectrum clearly indicates the presence of several overlapping resonances in the frequency ranges of

Fig. 2. 27 Al MAS NMR spectra of Sr3 Al10 SiO20 (a) Sr3 Al10 SiO20 :Ce3+ , Tb3+ and (b) Sr3 Al10 SiO20 :Ce3+ , Mn2+ recorded at 7.05 T.

tetrahedral AlO4 sites and octahedral AlO6 sites. The difference in intensities between AlO4 and AlO6 signals indicates that in Sr3 Al10 SiO20 there are more AlO4 sites than AlO6 . Further, the broadened asymmetric lineshape of AlO4 signal indicates the structure disorder due to the Al/Si substitution in the tetrahedral sites. Similarly, asymmetric nature of AlO6 signal indicated the presence of two AlO6 resonances corresponding to the two non-equivalent octahedral sites of the structure which is in good agreement with the site multiplicities. Hence, 27 Al MAS NMR gives an overview of the site occupancies of Al3+ in Sr3 Al10 SiO20 . 3.3. Photoluminescence properties 3.3.1. Sr3 Al10 SiO20 :Ce3+ Fig. 3(a) shows the excitation and emission spectra of (Sr0.95 Ce0.05 )3 Al10 SiO20 . The excitation spectrum, monitored at 390 nm, shows a broad band which can be resolved into multiple components. On an energy scale, this broad band can be separated into four Gaussians with maxima at about 231 nm (43,290 cm−1 ), 244 nm (40,983 cm−1 ), 271 nm (36,900 cm−1 ) and 320 nm (31,250 cm−1 ), respectively, with crystal-field splitting of ∼12,000 cm−1 . The emission spectrum, recorded at λexc = 320 nm, shows a broad band (FWHM = 3095 cm−1 ) with the maximum in

Fig. 3. (a) Excitation (EXC; λm = 390 nm) and emission (EM; λexc = 320 nm) of (Sr0.95 Ce0.05 )3 Al10 SiO20 and (b) schematic illustration of the energy level diagram for (Sr1−y Cey )3 Al10 SiO20 . The solid and dotted arrows indicate radiative and non-radiative processes, respectively. The upward and downward solid arrows indicate excitation and emission processes, respectively.

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the blue–violet region with extended emission into longer wavelength. The feature of the emission spectrum remains unchanged when recorded at λexc = 231, 244 or 271 nm except for the difference in intensity. The excitation spectra can be accounted in terms of the multiple site occupancy of Ce3+ ions. The crystal structure of Sr3 Al10 SiO20 [6] consists of one-dimensional pillars of edge-sharing AlO6 octahedra parallel to the c-axis and sixmembered rings of corner sharing (Al,Si)O4 tetrahedra in the ‘b–c’ plane. The Sr2+ cations are located in the tunnels running parallel to the c-axis. There are two crystallographically independent Sr atoms with different coordination environments, namely 10-coordinated Sr(1) and 8-coordinated Sr(2). Sr2+ (1) ions are surrounded by pillars of edge-sharing AlO6 octahedra as well as three chains of six-membered rings of (Al,Si)O4 arrays so that Sr(1) is located on a mirror plane. Sr2+ (2) ions are surrounded by two AlO6 pillars and two chains of six-membered (Al,Si)O4 rings and have the site symmetry of 2/m [6]. Ce3+ can occupy both the Sr2+ sites with different coordination number because of the clossness in ionic radii of Ce3+ with that of Sr2+ (Table 1) [11]. The appearance of multiple excitation maxima can be attributed to arise from two different structural sites of Ce3+ . The excitation bands can be assigned to the transitions from the ground 4f state of Ce3+ to the excited 5d states of Ce3+ . The 4f–5d transition of Ce3+ is highly dependent on the crystal field symmetry of the host lattice, which causes the splitting of the excited state involving the 5d orbital. The appearance of multiple maxima indicate the lowering of local site symmetry of Ce3+ from cubic to C2v or CS which causes the splitting of 5d orbital to A1 , A2 , B1 and B2 , respectively, into which the excitation takes place. Generally, the Ce3+ emission band shows doublet structure due to spin-orbit splitting of the ground state (2 F7/2 and 2 F5/2 ) with the energy difference upto ∼2000 cm−1 . However, the presence of a single band emission indicates weak spin-orbit coupling of the ground state of Ce3+ . The slight asymmetry towards longer wavelength may be due to strong crystal field at the Ce3+ site which dominates over the spin-orbit coupling. In Sr3 Al10 SiO20 , the crystal field splitting of the 4f state into a2u , t2u and t1u dominates as it is found in the case of elpasolites (Rb2 NaYF6 ) [13]. In view of this more extensive splitting, the emission consists of unresolved broad band of Ce3+ . This phenomenon is attractive for tunable solid-state laser. Further, though the Stokes shift is not large (5609 cm−1 ), the spectral overlap integral between excitation and emission spectra is very small (0.1 eV−1 ) indicating less interaction between adjacent Ce3+ centers. Hence, (Sr1−y Cey )3 Al10 SiO20 shows broad range of substitution (0.005 ≤ y ≤ 0.7) without concentration quenching. The PL spectra remain unchanged with changing concentrations of Ce3+ . The total process is schematically illustrated in Fig. 3(b). 3.3.2. Sr3 Al10 SiO20 :Tb3+ Fig. 4 shows the excitation-emission spectra of (Sr1−x Tbx )3 Al10 SiO20 with different Tb3+ concentrations.

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Fig. 4. Excitation (EXC) and emission (EM) spectra of (a) (Sr0.995 Tb0.005 )3 Al10 SiO20 (λexc = 226 nm; λm = 437 nm) and (b) (Sr0.95 Tb0.05 )3 Al10 SiO20 (λexc = 226 nm; λm = 542 nm).

The excitation spectrum of Tb3+ contains an intense band at 226 nm (44,247 cm−1 ) with shoulders at 205 nm (48,780 cm−1 ) and 270 nm (37,370 cm−1 ), which are due to the transitions from the ground state (7 F6 ) of the Tb3+ (4f8 ) to the different excited states of the 4f7 5d configuration. These broad excitation bands could not be related to Tb3+ ← O2− charge-transfer (CT) transition because Tb3+ ← O2− CT state is located at much higher energy (∼60,000 cm−1 ) than 5d states of Tb3+ [14]. Further, the forbidden f–f transitions of Tb3+ cannot be observed due to weak intensity with respect to that of the allowed 4f8 –4f7 5d transition. The excitation spectrum of the samples with different Tb content is the same. Excitation into the 4f8 –4f7 5d band at 226 nm yields characteristic blue emission 5 D3 –7 FJ (J = 3, 4, 5, 6) and green emission 5 D4 –7 FJ (J = 3, 4, 5, 6) of Tb3+ . However, the relative intensity of 5 D3 –7 FJ and 5 D4 –7 FJ transitions is strongly dependent on the Tb concentration. The emission intensity of 5 D3 –7 FJ transitions decreases with Tb3+ concentration, whereas the emission intensity of 5 D4 –7 FJ transitions increases. Fig. 5(a) shows the integrated emission intensity ratio of the transitions of 5 D3 –7 FJ to that of 5 D4 –7 FJ as a function of Tb concentration. It is evident that the higher-energy level emission (5 D3 –7 FJ ) is quenched in favour of the lower-energy level emission (5 D4 –7 FJ ). The occurrence of this situation can be explained by any of the following mechanisms [2]: (i) multiphonon emission and (ii) cross-relaxation. However, the multiphonon emission depends on the phonon vibration and it occurs if the energy difference between the levels involved (5 D3 and 5 D4 ) is less than about five times the highest vibrational frequency of the host lattice. Further, multiphonon emission is independent of the concentration of the luminescent centers. Hence, the quenching of the higher-energy level emission can be explained by cross-relaxation between two interacting Tb3+ centers. In concentrated terbium systems, the average distance between Tb3+ ion is small and 5 D3 –7 FJ emission is quenched by transferring the energy difference

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Fig. 6. Excitation (EXC) and emission (EM) spectra of (Sr0.90 Tb0.05 Ce0.05 )3 Al10 SiO20 (solid line) and (Sr0.94 Tb0.01 Ce0.05 )3 Al10 SiO20 (dotted line): (a) λm = 390 nm and λexc = 320 nm and (b) λm = 542 nm and λexc = 226 nm.

Fig. 5. (a) Variation of the relative intensities (IR ) of 5 D3 –7 F4 transition with Tb3+ concentrations in (Sr1−x Tbx )3 Al10 SiO20 . IR is the ratio between the I(5 D3 −7 F4 ) and (b) schematic configurational coordiI(5 D3 −7 F4 )+I(5 D4 −7 F5 ) nate diagram for Tb3+ in Sr3 Al10 SiO20 showing quenching of the higher level emission by cross-relaxation: the 5 D3 emission on ion 1 is quenched by transferring the energy difference 5 D3 –5 D4 to ion 2 which is promoted to 7 F0 level.

intensities

5D

5D

non-radiatively to another Tb3+ ion which is promoted to the 7 F0 level via cross-relaxation process such as, 3–

4

Tb3+ (5 D3 ) + Tb3+ (7 F0 ) → Tb3+ (5 D4 ) + Tb3+ (7 F0 ) As a result, the 5 D3 emission decreases (concentration quenching), whereas the 5 D4 emission increases. The crossrelaxation process is schematically illustrated in Fig. 5(b). 3.3.3. Sr3 Al10 SiO20 :Tb3+ , Ce3+ Sr3 Al10 SiO20 , co-doped with Ce3+ and Tb3+ shows green luminescence with enhanced intensity than that of (Sr1−x Tbx )3 Al10 SiO20 under UV excitation. Fig. 6 shows the excitation-emission spectra of (Sr0.95−x Tbx Ce0.05 )3 Al10 SiO20 with different Tb3+ concentration. The excitation spectra, monitored at λm = 390 nm emission of Ce3+ , shows broad band which has already been assigned to 4f–5d transition of Ce3+ [Fig. 6(a)]. The excitation spectra, monitored at λm = 542 nm emission of Tb3+ (5 D4 –7 F5 ) seem to contain a significant additional contribution from Ce3+ absorption along with the 226 nm band [Fig. 6(b)]. Excitation spectra remains unchanged with Tb3+ concentrations (solid and dotted

curves). Similarly, excitation into Ce3+ band at 320 nm yields both the emissions of Ce3+ (390 nm) and Tb3+ (5 D3 –7 FJ and 5 D –7 F ) where most of the 5 D –7 F emission lines are over4 J 3 J lapping with the broad Ce3+ emission band [Fig. 6(a)]. The Ce3+ emission dominates at low Tb3+ concentration (dotted line) whereas both Ce3+ (390 nm) and Tb3+ (542 nm) emissions have comparable intensity at higher Tb3+ concentration (solid line). However, excitation into Tb3+ band at 226 nm yields both the 5 D3 –7 FJ and 5 D4 –7 FJ emissions of Tb3+ with complete absence of Ce3+ emission [Fig. 6(b)]. Similar to (Sr1−x Tbx )3 Al10 SiO20 , the 5 D3 –7 FJ emission dominates at low Tb3+ concentration (dotted line) whereas 5 D4 –7 FJ becomes the major emission at high Tb3+ concentration (solid line). These results indicate that energy transfer from Ce3+ to Tb3+ occurs. The 5d–4f transition of Ce3+ is electric-dipole allowed and is several orders of magnitude stronger than 4f intraconfiguration transitions. Therefore, Ce3+ can strongly absorb UV radiation and efficiently transfer its energy to Tb3+ . Hence, the Ce3+ ions act as sensitizer, i.e. energy donors and the Tb3+ ions as activators, i.e. energy acceptors. The conditions for the energy transfer [2] from broad band emitter to line absorber are (i) the absorbing ions should be the nearest neighbors in the crystal lattice so that there is an orbital wavefunction overlap between sensitizer and activator and (ii) an overlap between the energy level of the sensitizer and activator. As already mentioned, in Sr3 Al10 SiO20 , there are two crystallographically independent Sr atoms (Sr(1) and Sr(2)) with different coordination environment where Tb3+ as well as Ce3+ can be present statistically. However, Kubota et al. has revealed that Tb3+ ions were located only at Sr(1) sites whereas the possibility that the Tb3+ ion located at the Sr(2) site or at both sites was rejected by the negative occupancy parameters of the Tb3+ ions at the Sr(2) site [7]. Therefore, Ce3+ occupying the Sr(2) sites at the next-nearest-neighbor

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Table 2 The energy transfer efficiency ηET from Ce3+ to Tb3+ and integrated emission intensities of Ce3+ and Tb3+ in (Sr0.95−x Tbx Ce0.05 )3 Al10 SiO20 x

ICe3+ ITb3+ (5 D4 − 7 FJ ) ηET (%)

0

0.005

0.01

0.05

1761

920 94 47.7

218 312 87.6

167 605 90.5

position of Tb3+ will have strong electrostatic interaction with Tb3+ ions. However, the overlap between the emission spectra of Ce3+ and the absorption of Tb3+ is very minimal in the case of (Sr1−x−y Tbx Cey )3 Al10 SiO20 . Now, at low Tb3+ concentration, emission spectrum [Fig. 6, dotted line] consists of very strong Ce3+ emission and weak Tb3+ emission, including both 5 D3 –7 FJ and 5 D4 –7 FJ ; while at high Tb3+ concentrations, the emission spectrum [Fig. 6, solid line] is composed of very weak Ce3+ emission and very strong Tb3+ emission containing mainly 5 D4 –7 FJ , where the 5 D3 –7 FJ emission is quenched due to the cross-relaxation effect. This suggests that the energy transfer efficiency from Ce3+ to Tb3+ depends strongly on their doping concentrations, i.e. number of nextnearest occupants in Sr3 Al10 SiO20 :Ce3+ , Tb3+ . The energy transfer efficiency from a donor (Ce3+ ) to an acceptor (Tb3+ ) can be calculated according to the formula [15] ηET = 1 −

IS IS0

(1)

where, IS and IS0 are the corresponding luminescence intensities of the donor (Ce3+ ) in the presence or absence of the acceptor (Tb3+ ), respectively, for the same donor (Ce3+ ) concentration. The energy transfer efficiencies from Ce3+ to Tb3+ in (Sr0.95−x Tbx Ce0.05 )3 Al10 SiO20 (x = 0.005–0.07) is investigated systematically and the results are listed in Table 2. Clearly (from Table 2) it is evident that the energy transfer efficiencies from Ce3+ to Tb3+ increase gradually with Tb3+ concentration. This is because the energy transfer probability from Ce3+ to Tb3+ is proportional to R−6 (R is the average distance between Ce3+ and Tb3+ ) [16]. The strongest emission was observed for (Sr0.9 Ce0.05 Tb0.05 )3 Al10 SiO20 with an energy transfer efficiency of 90.5% from Ce3+ to Tb3+ . 3.3.4. Sr3 Al10 SiO20 :Ce3+ , Mn2+ In order to understand the sensitizer activity of Ce3+ , experiment was also carried out with sample containing Ce3+ and Mn2+ . Fig. 7(a) shows the excitation and emission spectra of (Sr0.99 Ce0.005 Mn0.005 )3 Al10 SiO20 . The excitation spectrum, monitored at 390 nm, shows a broad band which can be resolved into multiple components. Comparing with Ce-only sample, this broad band can be assigned to 4f–5d transitions of Ce3+ . The emission spectrum, recorded at λexc = 320 nm, shows a broad band with the maximum in the blue–violet region with extended emission into longer wavelength which has already been attributed to emission from Ce3+ . However, while recorded at λexc = 263 nm, the emission spectra shows

Fig. 7. Excitation (EXC) and emission (EM) spectra of (Sr0.99 Ce0.005 Mn0.005 )3 Al10 SiO20 : (a) λm = 390 nm; λexc = 320 nm and (b) λm = 516 nm; λexc = 263 nm.

two maxima, one at 390 nm due to Ce3+ emission and another at 516 nm [Fig. 7(b)]. Mn2+ has the 3d5 configuration and from Tanabe–Sugano diagram it follows that the ground level is 6 A1 . Emission arises from 4 T1 (4 G) level, which shifts to lower energies for higher crystal field strengths. All optical absorption transitions are parity and spin forbidden. Generally, Mn2+ -activated phosphors are divided into two classes: those with green emission and those with orange–red emission. In octahedral surroundings with large crystal field, the emission is usually red; in tetrahedral surroundings with much smaller crystal field, the emission is usually green [17]. Sr3 Al10 SiO20 , codoped with Ce3+ and Mn2+ , shows broad band emission with maximum at 516 nm which can be attributed to 4 T1 → 6 A1 transition of Mn2+ . The green emission indicates that Mn2+ occupies tetrahedral Al3+ sites in framework of Sr3 Al10 SiO20 . This is further confirmed from 27 Al MAS NMR study which shows more Al(4) sites than Al(6) sites. The excitation spectra, monitored at λm = 516 nm shows broad band maximizing at 263 nm [Fig. 7(b)]. This broad band cannot be assigned to arising from Mn2+ because Sr3 Al10 SiO20 :Mn2+ does not show any PL spectra. Hence, this could be attributed to 4f–5d transition of Ce3+ . However, Sr3 Al10 Sio20 :Ce3+ does not show excitation band at 263 nm. By comparing with Sr3 Al10 SiO20 :Ce3+ (Fig. 3), it can be seen that the excitation spectra of Sr3 Al10 SiO20 :Ce3+ , Mn2+ does not show excitation band at 320 nm arising from A2 crystal field level of excited 5d state of Ce3+ . Hence, the presence of Mn2+ at the next-nearest-neighbor position modify the site symmetry of Ce3+ which, in turn, modify the crystal field of 5d excited state. The spectral study concludes the efficient energy transfer from Ce3+ to Mn2+ present at Sr2+ sites in the next-nearest position of Mn2+ . 3.3.5. Energy transfer mechanism of Sr3 Al10 SiO20 :Ce3+ , Tb3+ /Mn2+ In Sr3 Al10 SiO20 , Ce3+ acts as a sensitizer to yield sensitized luminescence from Tb3+ and Mn2+ . Table 3 shows

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Table 3 Relative efficiencies of Sr3 Al10 SiO20 phosphors Phosphor :Tb3+

Sr3 Al10 SiO20 Sr3 Al10 SiO20 :Tb3+ , Ce3+ LaPO4 :Ce3+ , Tb3+ Sr3 Al10 SiO20 :Ce3+ , Mn2+ BaMgAl10 O17 :Eu2+ , Mn2+

λmax (nm)

QE (%)

542 542 542 516 520

80 90 95 75 90

the relative efficiencies of Sr3 Al10 SiO20 phosphors. Tb3+ shows green emission in absence of Ce3+ as well. Whereas, the presence of (Ce3+ + Tb3+ ) enhances the intensity of green emission. On the other hand, Sr3 Al10 SiO20 :Mn2+ does not show any emission under UV excitation indicating the presence of Mn2+ levels at high energy. However, (Ce3+ + Mn2+ ) co-doped Sr3 Al10 SiO20 shows intense green emission resulting from efficient energy transfer from Ce3+ to Mn2+ . The transfer of energy from sensitizer to activator may be accomplished through any of the following mechanism [2]: (i) emission–reabsorption; (ii) resonance radiationless; (iii) non-resonance radiationless. The radiationless processes indicate that there will not be any emission of sensitizer seen in the emission spectrum. The energy absorbed by the sensitizer is transferred to activator via resonance radiationless process occurring by dipole–dipole or by dipole–quadrupole interaction or via non-resonance radiationless process through exchange interaction [16]. In the case of Sr3 Al10 SiO20 :Ce3+ , Tb3+ , however, the emission spectra show both Ce3+ and Tb3+ emission when recorded at λexc = 320 nm [Fig. 6(a)] even at higher Tb3+ concentration. Further, the PL spectra of Sr3 Al10 SiO20 :Ce3+ , Tb3+ show the energy overlap of Ce3+ emission band with the 5 D -excitation band of Tb3+ which is the other condition for 3 emission–reabsorption type energy transfer to occur. This indicates that energy transfer from Ce3+ to Tb3+ takes place by emission–reabsorption mechanism rather than radiationless processes. In emission–reabsorption process, both the ions act as independent system. The emission energy of Ce3+ is reabsorbed by the Tb3+ 5 D3 -energy level leading to 5 D4 –7 FJ emissions of Tb3+ with enhanced intensity. However, in the case of Sr3 Al10 SiO20 :Ce3+ , Mn2+ , the energy-transfer process cannot be interpreted by emission–reabsorption type mechanism. This is because of the fact that the condition necessary for this way of energy transfer to manifest is the closeness of the emission energy of one ion to the absorption energy of the other. However, Fig. 7 clearly shows that there is no Mn2+ energy level overlapping with the absorption level of Ce3+ . Hence, the energy transfer from Ce3+ to Mn2+ occurs by radiationless mechanism. 3.3.6. Sr3 Al10 SiO20 :Eu2+ Fig. 8(a) shows the excitation-emission spectra of (Sr0.993 Eu0.007 )3 Al10 SiO20 before reduction. The excitation spectrum, monitored at λm = 618 nm, shows broad band with maximum at 263 nm which corresponds to the Eu3+ ← O2− charge-transfer transition. The emission

Fig. 8. Excitation (EXC) and emission (EM) spectra of (a) (Sr0.993 Eu0.007 )3 Al10 SiO20 before reduction; λm = 618 nm; λexc = 263 nm, (b) (Sr0.993 Eu0.007 )3 Al10 SiO20 after reduction; λm = 450 nm (solid line); λexc = 350 nm (solid line); λexc = 280 nm (dotted line) and (c) (Sr0.993 Eu0.007 )3 Al10 SiO20 with 5% B2 O3 after reduction; λm = 450 nm (solid line); λexc = 350 nm (solid line); λexc = 280 nm (dotted line).

spectrum, recorded at λexc = 263 nm, shows sharp lines in the region of 550–700 nm which could be assigned to 5 D –7 F transition of Eu3+ . In Fig. 8(b), the excitation spec0 J trum of reduced (Sr0.993 Eu0.007 )3 Al10 SiO20 , monitored at λm = 450 nm, shows broad band which can be resolved into the multiple maxima as 275, 286, 350 and 395 nm. Eu2+ ions occupy Sr2+ sites having C2v or CS symmetry which causes the splitting of 5d state of Eu2+ to A1 , A2 , B1 and B2 into which excitation takes place. This broad excitation band is different from Eu3+ ← O2− charge-transfer band and could be assigned to 4f7 (8 S7/2 ) → 4f6 5d transition of Eu2+ . The corresponding emission spectra show broad band with the maximum in the blue–green region. In fact, there are two maxima; one at 450 nm and another at 470 nm. These emission bands could be attributed to the 4f7 → 4f6 5d transition of Eu2+ . The 450 nm band has comparatively higher intensity when recorded at λexc = 350 nm, whereas 470 nm band dominates at λexc = 280 nm. Further, the broad emission band of Eu2+ is superimposed with sharp line in the region of 550–700 nm range when recorded at λexc = 280 nm. The sharp lines have already been attributed to 5 D0 –7 FJ transition of Eu3+ indicating the presence of residual Eu3+ in (Sr1−x Eux )3 Al10 SiO20 after reduction. Hence, the reduction process remains incomplete for (Sr0.993 Eu0.007 )3 Al10 SiO20 in 100% H2 atmosphere at elevated temperatures. Fig. 8(c) shows the excitation-emission spectra of 5% B2 O3 added (Sr0.993 Eu0.007 )3 Al10 SiO20 . The emission spectra do not show any peaks (line spectra) corresponding to Eu3+ eventhough recorded at λexc = 280 nm indicating that the reduction process is totally complete in presence 5% B2 O3 added to (Sr1−x Eux )3 Al10 SiO20 as a flux. From the crystal structure, it is expected that the Eu2+ will occupy the Sr2+ sites and consequently generates two types of

A. Nag, T.R.N. Kutty / Materials Chemistry and Physics 91 (2005) 524–531

Eu2+ emission centers in Sr3 Al10 SiO20 . The doublet nature of emission spectra is due to the presence of Eu2+ in two different Sr2+ sites. The distinct difference in relative intensities of the two sets of emission lines of (Sr1−x Eux )3 Al10 SiO20 :Eu2+ depending upon the excitation wavelength, is due to the varying site symmetry for two different Sr2+ positions in which Eu2+ is substitutively present. The splitting of 5d excitation levels of the Eu2+ ion in solids strongly depends upon the strength of crystal field which, in turn, is influenced by the coordination number. Higher the coordination number, larger will be the ionicity and higher will be the emission energy. Hence, the 470 nm (21,276 cm−1 ) emission is ascribed to Eu2+ ions on 8-coordinated Sr(2) sites whereas the emission at 450 nm (22,222 cm−1 ) is due to Eu2+ ion on 10-coordinated Sr(1) sites. Multiple site symmetry and lower ionicity will account for the fact that, Eu3+ present in Sr(2) sites does not undergo complete reduction in absence of B2 O3 . However, it is evident from Table 1 that the ionic radius of Eu2+ is larger than that of Eu3+ and is better suited to Sr2+ sites. Hence, the incomplete reduction of Eu3+ to Eu2+ cannot be explained due to mismatch of ionic radii. Rather, it can be due to shielding of Sr2+ sites by the framework of AlO6 octahedra and (Al,Si)O4 tetrahedra which inhibits the electron transfer process responsible for reduction of Eu3+ substitutively present on Sr2+ sites. However, addition of boron modify the framework of Sr3 Al10 SiO20 by forming BO4 tetrahedra along with (Al,Si)O4 tetrahedra. The BO4 5− anionic tetrahedra are more rigid in comparison to (Al,Si)O4 tetrahedra and are believed to play an important role of electron transfer in the reduction of Eu3+ to Eu2+ and the network of BO4 5− tetrahedra may also act as a shield to prevent reoxidation and hence favors the complete reduction of Eu3+ to Eu2+ in Sr(2) sites. 3.3.7. Sensitization luminescence involving Eu2+ Like Ce3+ , Eu2+ also shows broad emission spectrum in the blue–green region. Hence, it was expected that Eu2+ will also show energy transfer phenomenon. However, Eu2+ does not act as a sensitizer of luminescence when codoped with Tb3+ or Mn2+ . This might be because of the fact that the existence of Eu2+ emission level at lower energy than the absorption energy level of Tb3+ as well as Mn2+ . Hence, there will not be any spectral overlap between the emission spectra of Eu2+ and the absorption spectra of Mn2+ or Tb3+ which is the reason for the absence of energy transfer phenonema in (Eu2+ + Tb3+ ) or (Eu2+ + Mn2+ ) systems.

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4. Conclusions The energy transfer phenomenon of Ce3+ → Tb3+ /Mn2+ and Eu2+ → Tb3+ /Mn2+ is investigated in Sr3 Al10 SiO20 . Ce3+ shows efficient energy transfer to Tb3+ through emission–reabsorption type mechanism, whereas energy transfer to Mn2+ occurs through radiationless mechanism. However, Eu2+ does not act as a sensitizer. This is attributed to the existence of Eu2+ level at lower energy than the absorption level of Tb3+ or Mn2+ which does not allow sufficient spectral overlap. The enhanced emission intensity of Tb3+ as well as Mn2+ due to energy transfer from Ce3+ yields as efficient material suitable for application in FEDs.

Acknowledgement One of the authors (AN) is grateful to CSIR (Council of Scientific and Industrial Research), New Delhi, for providing financial support throughout the research work.

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