Luminescence of BaAl2O4:Mn2+,Ce3+ phosphor

Luminescence of BaAl2O4:Mn2+,Ce3+ phosphor

ARTICLE IN PRESS Journal of Luminescence 127 (2007) 483–488 www.elsevier.com/locate/jlumin Luminescence of BaAl2O4:Mn2+,Ce3+ phosphor N. Suriyamurth...

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ARTICLE IN PRESS

Journal of Luminescence 127 (2007) 483–488 www.elsevier.com/locate/jlumin

Luminescence of BaAl2O4:Mn2+,Ce3+ phosphor N. Suriyamurthya, B.S. Panigrahib, a

Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India Technical Services Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India

b

Received 10 July 2006; received in revised form 7 February 2007; accepted 8 February 2007 Available online 25 February 2007

Abstract Powder samples of barium aluminate doped with Mn2+ and Ce3+ were prepared by solid-state reaction method and their photoluminescence and thermoluminescence properties were studied. Substitution of Ca/Sr in place of Ba resulted in enhanced emission from Ce3+ ions without changing the spectral profile. Cerium efficiently sensitized the manganese luminescence in barium aluminate. Photoluminescence and thermo luminescence observations have indicated the presence of V3+ defects in undoped barium aluminate. k However, Barium aluminate (either undoped or doped with manganese) did not exhibit long afterglow. r 2007 Elsevier B.V. All rights reserved. Keywords: Barium aluminate; Energy transfer; Ce3+/Mn2+; Luminescence

1. Introduction Oxide matrices are attractive host materials for the study of development of advanced phosphors due to their ease of synthesis and stability. Rare-earth-doped aluminates serve as an important class of phosphor for fluorescent lamp and plasma display applications and phosphorescence. Recently, Sohn et al. [1] have studied the photo physical properties of BaAl12O19:Eu2+ and Mn2+. Their study shows that this material can serve as an alternative to the well-known green emitting plasma display phosphor material Zn2SiO4:Mn2+. Crystals of strontium aluminate (SrAl2O4:Eu2+,Dy3+) and barium aluminate (BaAl2O4: Eu2+,Dy3+) are promising host materials for long afterglow phosphorescence [2,3]. While the afterglow properties of rare-earth-doped SrAl2O4 have been extensively studied but the BaAl2O4 matrix is not. Recently, BaAl2O4 doped with Ce3+ with long-lasting afterglow for more than 10 h was reported [4]. The afterglow was attributed to cationic defects [5] there. Luminescence from Mn2+ plays an important role in several inorganic compounds including silicates, aluminates and sulfides wherein it emits in the visible region. The emission can be either in green or red region depending on the host matrix because of the Corresponding author. Tel.: +91 44 27480119; fax: +91 44 27480336.

E-mail address: [email protected] (B.S. Panigrahi). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.02.038

sensitivity of the d–d transition in Mn2+ to the crystal field [6]. To the best of our knowledge, the luminescence properties Mn2+ in BaAl2O4 and the effect of Ce3+ codopant on it are not reported. The Ce3+ with 4f1 electronic configuration undergoes 5d–4f transitions that are parity and electric dipole allowed transitions. It is well known that such transitions are sensitive to crystal field environment because of the nearest excited state of Ce3+ that lies in the d-shell. In most of the host materials Ce3+ absorbs in the ultraviolet (UV) region [7]. Ce3+ acts as an efficient sensitizer by transferring a better part of its excitation energy to co-activators like Tb3+, Eu2+ [8,9] and Mn2+ [10]. In the present work, the photoluminescence and thermostimulated luminescence (TSL) properties of Ba0.99Al2O4: Mn2+ and Ba0.98Al2O4:Mn2+,Ce3+ were studied. The trap parameters such as trap depth and trap density were calculated and energy transfer between Ce3+ and Mn2+ has also been explored. BaAl2O4 has a stuffed tridymite structure. The divalent Mn2+ with ionic radius of 89 pm is expected to preferentially occupy the site of bigger ions, i.e. Ba2+ ions (rBa2þ ¼ 134 and rAl3þ ¼ 67 pm) in the BaAl2O4. 2. Experimental Powder samples of Ba0.99Al2O4:Mn2+ and Ba0.98Al2O4: Mn2+,Ce3+ were synthesized by conventional solid-state

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reactions. The starting materials: BaO2, Al2O3, MnSO4  H2O and CeO2 were of analytical grade reagents. The reactants were intimately mixed using mortar and pestle and prefired at 1000 1C for 5 h under ambient air atmosphere. Subsequently, the samples were again pulverized and pressed into 10 mm size pellets before sintering in a tubular furnace for 5 h under reducing atmosphere of 2% H2/98% N2 mixture at an elevated temperature of 1300 1C. CeO2 and MnSO4 H2O were added as dopants. A weakly reducing atmosphere was necessary to prevent the oxidation of Ce3+ to Ce4+. It has been reported that addition of boric acid dramatically increases the photoluminescence (PL) intensity in aluminate matrix [11]. In this study also, about 2 mol% of boric acid (H3BO3) was added as hightemperature flux to enhance the phase formation and subsequently a significant enhancement of luminescence intensity was observed. The formation of single- phased barium aluminate was confirmed by recording XRD pattern of the synthesized sample using a Siemens D500 XRD spectrometer (Cu Ka: 1.541 A˚) and comparing with JCPDS 82-1350. Excitation and emission measurements were recorded with a Jobin-Yvon Fluorolog-3 Spectrofluorimeter equipped with 450 W xenon lamp as excitation source. All the excitation and emission spectra presented here are uncorrected. The relative intensities have been compared only when the experimental conditions and spectral profiles are identical. The luminescence decay was recorded using the phosphorimeter setup of Fluorolog-3. TSL measurements were carried out using a TLD reader (Nucleonix product) in the temperature range of 40–410 1C under linear heat rating of 10 1C/s. Before TSL measurements, samples were irradiated with a 4 W UV lamp (365 nm) for 5 min. Every TSL measurement was repeated thrice to ensure reproducibility.

3. Results and discussion 3.1. Photoluminescence 3.1.1. Ba0.99Al2O4:Mn2+ The d–d transition in Mn2+ is both spin and parity forbidden [12]. The luminescence observed from Mn2+ is due to the admixture of parity between 3d and 4p configurations lifting the spin selection rule and also possible electron–phonon coupling. The excitation and emission spectra of Mn2+ in Ba0.99Al2O4 are shown in Fig. 1a and c. The excitation spectrum was recorded by monitoring the emission at 512 nm. In the excitation spectrum, four excitation peaks at 361 nm (27700 cm1), 383 nm (26109 cm1), 425 nm (23529 cm1) and 450 nm (22222 cm1) were observed. The band at 383 nm corresponds to transition of 6A1-4T2 (4D) [13]. The 425 nm peak remains unchanged irrespective of host lattice because the two energy levels 4A1 (4G) and 4E (4G) are having same energy and also parallel to the ground state [14]. The peaks

Fig. 1. Excitation spectra of (a) BaAl2O4:Mn2+; lemi ¼ 512 nm, (b) BaAl2O4:Ce3+; lemi ¼ 415 nm, and emission spectra of (c) BaAl2 O4:Mn2+; lexci ¼ 425 nm and (d) BaAl2O4:Ce3+; lexci ¼ 350 nm.

at 425 and 450 nm are ascribed 6A1-4A1 (4G) and 4E (4G) and 6A1-4T2 (4G) transitions, respectively [15]. In the emission spectrum, the emission profile (peaking at 512 nm) remains unchanged for all excitations and the emission bandwidth (FWHM) was 32 nm. This emission at 512 nm is the characteristic emission of Mn2+ due to 4T1 (4G)-6A1 (6S) transition [13]. Divalent manganese emission critically depends on two factors, i.e. the size of crystallographic cation site where Mn2+ is likely to occupy and the coordination number of Mn2+ in the host matrix. The Mn2+ ion emits green light when it is tetrahedrally coordinated (CN ¼ 4) in the lattice and red light in octahedral coordination (CN ¼ 6) [16]. In the present case, observation of green emission around 512 nm suggests that Mn2+ probably occupies a tetrahedral site in BaAl2O4 matrix. The concentration dependence of PL intensity with respect to Mn2+ doped in barium aluminate phosphor was studied to find out the optimum concentration of Mn2+. As the concentration of Mn2+ increased, the PL intensity also increased and attained the maximum intensity at 0.015 M. Beyond this, any further increase in concentration reduced the Mn2+ luminescence intensity. With increase in concentration, the increased mutual interaction between the two nearby Mn2+ results in killing its luminescence. This could be the concentration quenching due to migration of excitation energy from one activator center to another that acts as an energy sink [17]. The increase in Mn2+ concentration in barium aluminate was not accompanied with the shift in the peak position as reported in case of MgAl2O4:Mn2+ [19]. In the present study, the manganese concentration used was probably low enough to cause any peak shift. The excitation and emission spectra of BaAl2O4:Mn2+ were also recorded at 77 K and

ARTICLE IN PRESS N. Suriyamurthy, B.S. Panigrahi / Journal of Luminescence 127 (2007) 483–488

the change in intensity was not very significant. This apart, there was no change in the spectral profile of Mn2+ compared to room temperature. Luminescence decay from BaAl2O4:Mn2+ was recorded at room temperature using pulsed xenon flash lamp (3 ms FWHM). The luminescence decay time of Mn2+ is reported in literature to be in milliseconds [20]. In the present study, the Mn2+ concentration was varied from 0.005 to 0.025 M to study the effect of concentration on lifetime of Mn2+ in BaAl2O4 matrix. The lifetime was found to be decreasing from 4.8 ms (0.005 M) to 4.5 ms (0.025 M) as the Mn2+ concentration was increased (Fig. 2). A similar observation was made by Ronda and Amrein [18] in Zn2SiO4; the shortening of decay time was attributed to magnetic interaction between manganese neighboring ions with increasing manganese concentration. 3.1.2. Ba0.99Al2O4:Ce3+ The excitation and emission spectra of Ce3+-doped barium aluminate are given in the Fig. 1b and d. The excitation spectrum shows a broad band with maximum at 350 nm besides a shoulder at 298 nm. It is reported [4] that in barium aluminate host, there are two Ba2+ sites with average bond distance Ba–O is 2.85 and 2.94 A˚, respectively. Therefore, the ligand field strength at one site is expected to be stronger than that of at the other site resulting in difference in the intensities of their emission as suggested earlier by Jia et al. [4]. In this study, the presence of shoulder near 298 nm besides main peak at 350 nm is probably due to the presence of Ce3+ at two different crystallographic sites. On excitation at 350 nm, the emission spectrum exhibits a well-defined broadband emission (FWHM ¼ 102 nm) with maximum at 415 nm. The emission occurs from lowest component of 5d1 to the ground state of Ce3+ ions. The bandwidth of Ce3+ emission is 102 nm whereas only 32 nm for Mn2+ emission.

Fig. 2. Lifetime decay of BaAl2O4:Mn2+ (a) 0.005 M, (b) 0.01 M, (c) 0.015 M, (d) 0.02 M and (e) 0.025 M.

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In BaAl2O4:Eu2+ phosphor the emission band of Eu2+ due to f–d transition was reportedly shifting as a result of substitution by the divalent calcium ions [21]. The effect of substitution of Sr2+/Ca2+ in place of Ba2+ was studied here to understand the influence of environment on BaAl2O4:Ce3+ as f–d transition in Ce3+ is susceptible to crystal field strength. Samples were prepared with 0.02 M Ce3+ and varying Sr2+/Ca2+ concentration. The ionic radius of Ca2+, Sr2+ and Ba2+ are 106, 127 and 134 pm, respectively. Therefore, substitution of Ba2+ by another element with a different ionic radius (Ca2+/Sr2+) may change the symmetry at microlevel and also the crystal field experienced by the activator. In Fig. 3, the emission spectra of Sr and Ca substituted Ba1x(Ca/Sr)xAl2O4:Ce3+ phosphors for x ¼ 0.02 M compositions of Sr2+/Ca2+ are shown. These spectra do not indicate any shift in the emission band from 415 nm but the Ce3+ intensity was enhanced significantly on substitution. The enhancement was five and three times in case of Ca2+ and Sr2+, respectively. The reason for such enhancement is not clear. In the study of BaAl2O4:Eu2+ [21], 9% of Ba was substituted by Ca/Sr whereas in the present study only 2% Ba ions are substituted and this low concentration substitution may be the reason for not observing any shift in the emission band. If more Ba2+ ions are substituted, there is a possibility of phase segregation and hence we have not opted for that. 3.1.3. Ba0.98Al2O4:Mn2+,Ce3+ The luminescence properties of pairs of ions can be different from that of single ions due to energy transfer between the pair. Several activators like Ce3+, Pb2+, Sb3+,

Fig. 3. Emission spectra of (a) BaAl2O4:Ce3+, (b) Ba1xSrxAl2O4:Ce3+, x ¼ 0.02 M and (c) Ba1xCaxAl2O4:Ce3+, x ¼ 0.02 M, lexci ¼ 350 nm.

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and Eu2+ having efficient absorption in UV region are reported to sensitize the Mn2+ emission [1]. In this report, we have studied the energy transfer between Ce3+ and Mn2+ in barium aluminate as to the best of our knowledge; no such study in this matrix has been reported in literature. Fig. 1 shows that there is a remarkable overlapping between emission band of Ce3+ (donor) and absorption band of Mn2+ (acceptor) in BaAl2O4 indicating a good possibility of energy transfer from Ce3+ to Mn2+. Fig. 4 shows the luminescence intensities of Mn2+ (0.01 M) in the presence and absence of Ce3+ (0.01 M) in barium aluminate. Fig. 4c and b shows the manganese emission by exciting at 350 nm (maximum of Ce3+ excitation) and 425 nm (maximum of Mn2+ excitation) in BaAl2O4:Mn2+. Fig. 4a shows the manganese emission by exciting at 350 nm in BaAl2O4:Mn2+,Ce3+. The sensitization of Mn2+ emission in presence of Ce3+ is obvious. The excitation spectra of BaAl2O4:Mn2+, BaAl2O4:Ce3+ and BaAl2O4:Mn2+,Ce3+ are shown in Fig. 5a–c, respectively. These spectra were recorded by monitoring the Mn2+ emission at 512 nm. A comparison of excitation spectra of BaAl2O4:Mn2+ and BaAl2O4:Mn2+,Ce3+ shows that in presence of Ce3+ all the excitation bands of Mn2+ are enhanced in addition to the strong band at 350 nm. This was due to the fact that the Ce3+ emission band overlaps all the four excitation bands of Mn2+ as seen in Fig. 1. In addition to enhancement in Mn2+ emission, Fig. 5 also shows that in the presence of Mn2+, the luminescence of Ce3+ is quenched; thus confirming the energy transfer from Ce3+ to Mn2+. The enhancement of Mn2+ emission is

Fig. 5. Excitation spectra of (a) BaAl2O4:Mn2+; lemi ¼ 512 nm, (b) BaAl2O4:Mn2+,Ce3+; lemi ¼ 512 nm, (c) BaAl2O4:Ce3+; lemi ¼ 415 nm and (d) BaAl2O4:Mn2+,Ce3+; lemi ¼ 512 nm (77 K).

observed to be more than 100 times in presence of Ce3+. Fig. 5 also shows the excitation spectrum of BaAl2O4: Mn2+,Ce3+ recorded at 77 K. This figure shows that the energy transfer is a bit more efficient at low temperature on excitation at 350 nm as this peak seems more intense compared to that of at room temperature. The energy transfer efficiency (Z) from a donor to acceptor can be calculated using the expression Z ¼ 1[IS/ISo] [22], where IS and ISo are the corresponding intensities of sensitizer (Ce3+) in presence and absence of acceptor (Mn2+). In case of BaAl2O4:Mn2+,Ce3+, the energy transfer from Ce3+ sensitizer to Mn2+ acceptor in barium aluminate was calculated using the above-mentioned relationship and the transfer was found to be about 55%. However, when the sensitizer concentration was reduced to 0.05 M the energy transfer efficiency was only 5%. This could be due to increased distance between sensitizer and activator at low concentrations of the activators as more is the distance between the sensitizer and acceptor, less is the energy transfer efficiency. 3.2. Thermo-stimulated luminescence

Fig. 4. Emission spectra of (a) BaAl2O4:Mn2+,Ce3+; lexci ¼ 350 nm, (b) BaAl2O4:Mn2+; lexci ¼ 425 nm and (c) BaAl2O4:Mn2+; lexci ¼ 350 nm.

In TSL spectrum the shape, position and intensity of the glow peaks are considered as fingerprints of the trapping state parameters responsible for TL emission. These parameters include the order of kinetics b, the activation energy E (eV) and the frequency factor S (s–1). Fig. 6 shows the TSL spectrum of BaAl2O4 without any dopant (a), BaAl2O4:Ce3+,Mn2+ (b), BaAl2O4:Mn2+ (c) and BaAl2 O4:Ce3+ (d) recorded after irradiating the sample for 5 min with UV light. Jia and Yen [23] in their recent study of MgAl2O4 have observed the presence of V3+ (a hole trapped at a divalent k site) centers [5] that exhibited luminescence at 520 nm with

ARTICLE IN PRESS N. Suriyamurthy, B.S. Panigrahi / Journal of Luminescence 127 (2007) 483–488

Fig. 6. TSL spectra of (a) BaAl2O4, (b) BaAl2O4:Mn2+,Ce3+; (c) BaAl2O4:Mn2+ and (d) BaAl2O4:Ce3+.

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a glow peak at 94 1C. This shows that doping with Ce3+ and/or Mn2+ probably assists in generating more V3+ or k similar centers. Jia et al. had observed the enhancement (with respect to both intensity and time) of the 520 nm long glow luminescence in presence of cerium and proposed a different mechanism for the long afterglow based on the V3+ centers. In case of barium aluminate, we have not k observed any enhancement of the 487 nm luminescence in presence of cerium or manganese. Moreover, neither the undoped barium aluminate nor the cerium/manganesedoped barium aluminate exhibited any afterglow in the present study, suggesting the mechanism of the afterglow in aluminates is yet to be clearly understood. The shape of Ce3+-doped barium aluminate is symmetrical at 103 1C and in case of manganese doped and cerium/manganese doped the shapes are nearly symmetrical. The symmetry factor, trap depth and trap density were calculated [24–26] using the following equations: mg ¼ (d/d+t), Et ¼ 3.5(kT2m/o)2kTm and no ¼ oIm/ {b[2.52+10.2(mg0.52)]}, where Tm is the glow peak temperature, o is the FWHM of peak, k is Boltzman constant (1.38E23 J/K), d is the high-temperature halfwidth and t is the low-temperature half-width and Im is the TL intensity of the glow peaks (Fig. 6). The symmetry factor was found to be 0.52 for BaAl2O4:Ce3+, and for BaAl2O4:Mn2+ the value was 0.53. The typical value for second-order process is (mgE0.49–0.52) and for the firstorder kinetics the value is mgE0.39–0.42 [24]. The calculated trap depth of Mn2+, Ce3+ and Mn2+/Ce3+ doped samples are 0.56, 0.69 and 0.53 eV and their corresponding trap density (no) are 1212, 731 and 475 cm3, respectively. 4. Conclusion

Fig. 7. Excitation (a) spectra of BaAl2O4; lemi ¼ 487 nm and emission spectra of BaAl2O4; lexci ¼ 346 nm.

weak excitation band between 300 and 400 nm and glow peaks at 41 and 238 1C. In our study, PL at 487 nm with excitation peak at 346 nm and a glow peak at 103 1C were observed with the undoped barium aluminate (Fig. 7). Probably these observations are indicative of the presence of similar V3+ centers in barium aluminates. Barium k aluminate doped with cerium exhibited two glow peaks at 103 and 185 1C. In case of BaAl2O4:Mn2+, the prominent glow peak was seen at 115 1C along with two shoulders at 135 and 195 1C. Similarly, BaAl2O4:Ce3+,Mn2+ exhibited

Barium aluminate doped with cerium or manganese exhibited strong photoluminescence. Substitution of barium by calcium or strontium has remarkably enhanced the cerium luminescence. Cerium sensitized the manganese luminescence and the luminescence enhancement of manganese was more than two orders of magnitude. The photoluminescence and thermoluminescence observed in undoped barium aluminate suggests the presence of V3+ k centers. However, Barium aluminate without dopant or with cerium and/or manganese dopants did not exhibit any afterglow suggesting presence of V3+ centers may not be a k prerequisite factor for observing long after glow in aluminates. References [1] K.-S. Sohn, F.s. Park, C.H. Kim, H.D. Park, J. Electrochem. Soc. 147 (11) (2000) 4368. [2] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670. [3] Y. Lin, Z. Zhang, Z. Tang, J. Zhang, Z. Zheng, X. Lu, Mater. Chem. Phys. 70 (2001) 156.

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