Persistent luminescence behavior of materials doped with Eu<sup>2+</sup> and Tb<sup>3+</sup>

doi:10.1364/OME.2.000382

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Persistent luminescence behavior of materials doped with Eu²⁺ and Tb³⁺

Lucas C. V. Rodrigues, Hermi F. Brito, Jorma Hölsä, and Mika Lastusaari »View Author Affiliations

Optical Materials Express, Vol. 2, Issue 4, pp. 382-390 (2012)
http://dx.doi.org/10.1364/OME.2.000382

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– Abstract
1. Introduction
2. Experimental
3. Results and discussion
4. Conclusions
– References and links

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Abstract

In this work, the persistent luminescence mechanisms of Tb³⁺ (in CdSiO₃) and Eu²⁺ (in BaAl₂O₄) based on solid experimental data are compared. The photoluminescence spectroscopy shows the different nature of the inter- and intraconfigurational transitions for Eu²⁺ and Tb³⁺, respectively. The electron is the charge carrier in both mechanisms, implying the presence of electron acceptor defects. The preliminary structural analysis shows a free space in CdSiO₃ able to accommodate interstitial oxide ions needed by charge compensation during the initial preparation. The subsequent annealing removes this oxide leaving behind an electron trap. Despite the low band gap energy for CdSiO₃, determined with synchrotron radiation UV-VUV excitation spectroscopy of Tb³⁺, the persistent luminescence from Tb³⁺ is observed only with UV irradiation. The need of high excitation energy is due to the position of ⁷F₆ level deep below the bottom of the conduction band, as determined with the 4f⁸→4f⁷5d¹ and the ligand-to-metal charge-transfer transitions. Finally, the persistent luminescence mechanisms are constructed and, despite the differences, the mechanisms for Tb³⁺ and Eu²⁺ proved to be rather similar. This similarity confirms the solidity of the interpretation of experimental data for the Eu²⁺ doped persistent luminescence materials and encourages the use of similar models for other persistent luminescence materials.

1. Introduction

Persistent luminescence materials can emit light for several hours after the removal of the irradiation source. These materials have received special attention lately due to their significant applications in emergency signalization, micro defect sensing, optoelectronics for image storage, detectors of high energy radiation and thermal sensors [1

T. Aitasalo, J. Hölsä, H. Jungner, M. Lastusaari, and J. Niittykoski, “Thermoluminescence study of persistent luminescence materials: Eu²⁺- and R³⁺-doped calcium aluminates, CaAl₂O₄:Eu²⁺,R^3+.,” J. Phys. Chem. B 110(10), 4589–4598 (2006). [CrossRef] [PubMed]

T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, “A new long phosphorescent phosphor with high brightness, SrAl₂O₄:Eu²⁺,Dy³⁺,” J. Electrochem. Soc. 143(8), 2670–2673 (1996). [CrossRef]

]. The persistent luminescence phenomenon is considered a special case of thermally stimulated luminescence, where the excitation energy is stored to traps and then released, induced by thermal energy available at room temperature [1

]. The advance in the persistent luminescence materials research arose with the discovery of the phosphor SrAl₂O₄:Eu²⁺,Dy³⁺ [2

] in 1996. This breakthrough was followed by the finding of new efficient persistent luminescence materials based on different stable aluminates CaAl₂O₄:Eu²⁺,Nd³⁺ [1

], Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ [3

Y. Lin, Z. Tang, Z. Zhang, and C. W. Nan, “Anomalous luminescence in Sr₄Al₁₄O₂₅:Eu,Dy phosphors,” Appl. Phys. Lett. 81(6), 996–998 (2002). [CrossRef]

] and silicate materials like Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ [4

Y. Lin, Z. Tang, Z. Zhang, X. Wang, and J. Zhang, “Preparation of a new long afterglow blue-emitting Sr₂MgSi₂O₇-based photoluminescent phosphor,” J. Mater. Sci. Lett. 20(16), 1505–1506 (2001). [CrossRef]

]. Presently, the best materials can continue emitting light in excess of 24 h in the dark.

One may observe that the majority of published works are about Eu²⁺ doped materials. Despite this majority, other materials containing different dopants, such as Tb³⁺ [5

L. C. V. Rodrigues, H. F. Brito, J. Hölsä, R. Stefani, M. C. F. C. Felinto, M. Lastusaari, M. Malkamäki, and L. A. O. Nunes, “Discovery of the persistent luminescence mechanism of CdSiO₃:Tb³⁺,” J. Phys. Chem. C (to be published).

–8

C. Liu, G. Che, Z. Xu, and Q. Wang, “Luminescence properties of a Tb³⁺ activated long-afterglow phosphor,” J. Alloys Compd. 474(1-2), 250–253 (2009). [CrossRef]

], Eu³⁺ [9

J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, J. Niittykoski, and E. Zych, “Effect of Mg²⁺ and Ti^IV doping on the luminescence of Y₂O₂S:Eu³⁺,” Opt. Mater. 31(12), 1791–1793 (2009). [CrossRef]

,10

J. Fu, “Orange and red emitting long-lasting phosphors MO:Eu³⁺ (M = Ca, Sr, Ba),” Electrochem. Solid-State Lett. 3(7), 350–351 (1999). [CrossRef]

], Mn²⁺ [11

T. Aitasalo, A. Hietikko, D. Hreniak, J. Hölsä, M. Lastusaari, J. Niittykoski, and W. Stręk, “Luminescence properties of BaMg₂Si₂O₇:Eu²⁺,Mn²⁺,” J. Alloys Compd. 451(1-2), 229–231 (2008). [CrossRef]

,12

X. Qu, L. Cao, W. Liu, G. Su, and P. Wang, “Luminescence properties of CdSiO₃: Mn²⁺,RE³⁺ (RE = Sm, Dy, Eu) phosphors,” J. Alloys Compd. 487(1-2), 387–390 (2009). [CrossRef]

] or Ti [13

Y. Cong, B. Li, B. Lei, and W. Li, “Long lasting phosphorescent properties of Ti doped ZrO₂,” J. Lumin. 126(2), 822–826 (2007). [CrossRef]

,14

J. M. Carvalho, L. C. V. Rodrigues, J. Hölsä, M. Lastusaari, L. A. O. Nunes, M. C. F. C. Felinto, O. L. Malta, and H. F. Brito, “Influence of titanium and lutetium on the persistent luminescence of ZrO₂,” Opt. Mater. Express 2(3), 331–340 (2012). [CrossRef]

] also show efficient persistent luminescence. Among these materials, those doped with Tb³⁺ ions perform the best though only very recently the first persistent luminescence mechanism based on solid experimental data for a Tb³⁺ doped material was developed [5

In order to improve the luminescence intensity and duration of the persistent phosphors it is important to know profoundly the mechanism of the phenomenon. The mechanism of Eu²⁺ persistent luminescence has attracted much attention lately [1

,15

K. Van den Eeckhout, P. F. Smet, and D. Poelman, “Persistent luminescence in Eu²⁺-doped compounds: a review,” Materials 3(4), 2536–2566 (2010). [CrossRef]

] and is close to achieving general acceptance though some minor details are still contested [1

]. Comparing the Eu²⁺ persistent luminescence materials with those doped with different dopants e.g. Tb³⁺ may still contribute significantly to the understanding of the persistent luminescence mechanisms.

In this work, the persistent luminescence mechanisms for Eu²⁺ (in BaAl₂O₄) [1

,16

L. C. V. Rodrigues, R. Stefani, H. F. Brito, M. C. F. C. Felinto, J. Hölsä, M. Lastusaari, T. Laamanen, and M. Malkamäki, “Thermoluminescence and synchrotron radiation studies on the persistent luminescence of BaAl₂O₄:Eu²⁺,Dy³⁺,” J. Solid State Chem. 183(10), 2365–2371 (2010). [CrossRef]

] and Tb³⁺ (in CdSiO₃) [5

] are discussed. Both mechanisms were developed based on the experimental results derived from photoluminescence, synchrotron radiation (SR) UV-VUV spectroscopy and thermoluminescence measurements. The positions of the ground state of the R²⁺ and R³⁺ ions (R: La-Nd, Sm-Lu) in the hosts’ band structure were determined using the 4f→5d and the ligand-to-metal charge-transfer (LMCT) transitions following the ideas of the previous empirical model [17

P. Dorenbos, “Locating lanthanide impurity levels in the forbidden band of host crystals,” J. Lumin. 108(1-4), 301–305 (2004). [CrossRef]

2. Experimental

2.1 Materials preparation

The BaAl₂O₄:Eu²⁺,Dy³⁺ materials were prepared via combustion synthesis, where the metal nitrates and urea were used as reactants and fuel, respectively. The precursors were dissolved into the smallest possible amount of distilled water. A silica capsule filled with the homogeneous solution was inserted into a furnace pre-heated at 500 °C [16

,18

Z. Qiu, Y. Zhou, M. Lu, A. Zhang, and Q. Ma, “Combustion synthesis of long-persistent luminescent MAl₂O₄:Eu²⁺, R³⁺ (M = Sr, Ba, Ca, R = Dy, Nd and La) nanoparticles and luminescence mechanism research,” Acta Mater. 55(8), 2615–2620 (2007). [CrossRef]

–20

B. M. Mothudi, O. M. Ntwaeaborwa, J. R. Botha, and H. C. Swart, “Photoluminescence and phosphorescence properties of MAl₂O₄:Eu²⁺,Dy³⁺ (M=Ca, Ba, Sr) phosphors prepared at an initiating combustion temperature of 500 °C,” Physica B 404(22), 4440–4444 (2009). [CrossRef]

]. The reaction began after ca. 5 minutes after the introduction of the capsule into the furnace. The mixture was then self-ignited with a white flame and produced a white powder. After the completion of the reaction, the furnace was turned off and was allowed to cool freely. The products were removed from the oven when the temperature had decreased to ca. 25 °C. The nominal concentrations of Eu²⁺ and Dy³⁺ were 1 and 2 mole-% (of the Ba²⁺ amount), respectively.

The polycrystalline CdSiO₃:R³⁺ (R: Eu and Tb) materials were prepared with a conventional solid state reaction. Cadmium acetate (Cd(CH₃COO)₂⋅2H₂O), fumed silica (SiO₂) and rare earth nitrates (R(NO₃)₃·6H₂O) in stoichiometric amounts were ground intimately. The mixtures were then heated in air at 950 °C for seven hours in aluminosilicate crucibles. The nominal concentration of R³⁺ (of the Cd²⁺ amount) was 1 mole-%.

For both materials, the europium, terbium and dysprosium nitrates used for doping were obtained from the respective oxides with a reaction with concentrated nitric acid.

2.2 Apparatus

The photoluminescence measurements on the CdSiO₃:Tb³⁺ and BaAl₂O₄:Eu²⁺,Dy³⁺ materials were carried out at room temperature with a SPEX-Fluorolog-2 spectrofluorometer equipped with two 0.22 m SPEX 1680 double grating monochromators. A 450 W Xenon lamp was used as the excitation source. The excitation, emission and persistent luminescence spectra were collected at an angle of 22.5 ° (front face). All spectra were recorded using automatic detector mode correction.

The time-resolved excitation spectra of the BaAl₂O₄:Eu²⁺,Dy³⁺ material were recorded with a SPEX 1934D phosphorimeter accessory coupled to the SPEX-Fluorolog-2 spectrofluorometer. A 50 W pulsed Xenon lamp was used as the excitation source and the spectra were recorded with a 1 ms delay.

The UV-VUV excitation spectra of the CdSiO₃:R³⁺ (R: Eu, Tb) and BaAl₂O₄:Eu²⁺,Dy³⁺ materials were recorded between 80 and 330 nm by using the UV-VUV synchrotron radiation facility at the SUPERLUMI beamline I of HASYLAB (Hamburger Synchrotronstrahlungs-labor) at DESY (Deutsches Elektronen-Synchrotron, Hamburg, Germany) [21

HASYLAB, Beamline I: SUPERLUMI, http://hasylab.desy.de/facilities/doris_iii/beamlines/i_superlumi (accessed on Dec. 20, 2011).

]. The polycrystalline materials were mounted on the cold finger of a liquid He flow cryostat and the spectra were recorded at selected temperatures between 10 and 298 K. The setup consisted of a 2-m McPherson type primary (excitation) monochromator with a resolution up to 0.02 nm. The UV-VUV excitation spectra were corrected for the variation in the incident flux of the excitation beam using the excitation spectrum of sodium salicylate as a standard.

3. Results and discussion

3.1 Photoluminescent properties

The emission spectrum of CdSiO₃:Tb³⁺ was obtained under excitation in the host at 247 nm at room temperature (Fig. 1 , left). The spectrum is composed almost exclusively of the line emission due to the intraconfigurational 4f⁸ transitions of Tb³⁺, ⁵D₃→⁷F_J and ⁵D₄→⁷F_6-2. The most intense transition of Tb³⁺, ⁵D₄→⁷F₅, peaks at ca. 545 nm and yields the green emission color. For practical applications, this is close to the optimal sensitivity range of the human eye though at low illumination conditions blue emission might be even better. Due to the low site symmetry (C₁) of the three Cd²⁺ sites occupied by Tb³⁺ in CdSiO₃ (and to the charge compensation defects), the appearance of the Tb³⁺ emission is more or less band-like due to many superimposed Tb³⁺ emission lines. It has been reported [22

Y. Liu, J. Kuang, B. Lei, and C. Shi, “Color-control of long-lasting phosphorescence (LLP) through rare earth ion-doped cadmium metasilicate phosphors,” J. Mater. Chem. 15(37), 4025–4031 (2005). [CrossRef]

] that the CdSiO₃:Tb³⁺ emission, as many of CdSiO₃ doped with the R³⁺ ions with less strong luminescence in visible (e.g. Nd³⁺ and Gd³⁺ just to mention a few), contains also a broad band centered at ca. 400 nm. In the present investigation, this band emission is either very weak or entirely absent, indicating that the energy transfer process from the host to Tb³⁺ is effective. Furthermore, the almost total absence of the ⁵D₃→⁷F_J transitions from CdSiO₃:Tb³⁺ in the blue (and near UV) range is rather unusual taken into account the low dopant concentration, only 1 mole-%. This may be interpreted in a way that the emission from the ⁵D₃ level(s) is quenched and the excitation energy ends to the ⁵D₄ level(s) instead. The absence of the ⁵D₃ emission indicates that the ⁵D₃ levels are located in the conduction band (CB) of CdSiO₃ and the ⁵D₃ emission is quenched to the lowest emitting levels, ⁵D₄ via the host’s CB. The 77 K emission spectrum shows that the ⁵D₃ emission is completely quenched at low temperatures, in agreement with the position of ⁵D₃ close to the CB bottom.

Fig. 1 UV excited and persistent luminescence spectra of CdSiO₃:Tb³⁺ (left) and BaAl₂O₄:Eu²⁺,Dy³⁺ (right).

The emission spectrum of BaAl₂O₄:Eu²⁺,Dy³⁺ (Fig. 1, right) presents a broad band assigned to the interconfigurational 4f⁶5d¹→4f⁷ transition of Eu²⁺. Two maxima, peaking at 435 and 500 nm, were observed due to the transitions from the lowest ²D level of the excited 4f⁶5d¹ configuration to the ground ⁸S_7/2 level of the 4f⁷ configuration of Eu²⁺. The existence of two bands can be due to the presence of two Ba²⁺ sites in the BaAl₂O₄ structure or to the creation of a new Ba²⁺ site due to the effect of water exposure on BaAl₂O₄:Eu²⁺ [23

A. V. S. Lourenço, L. C. V. Rodrigues, C. A. Kodaira, R. Stefani, H. F. Brito, M. C. F. C. Felinto, and J. Hölsä, “Persistent luminescence BaAl₂O₄:Eu²⁺,Dy³⁺ phosphor encapsulated in silica: water resistance,” in Proc. Int. Conf. Adv. Mater. (Braz. Mater. Res. Soc.), Rio de Janeiro-RJ, Brazil, September 20–25, 2009, BB556. http://www.sbpmat.org.br/icam2009/pdf/BB556.pdf.

The persistent emission spectra, measured 120 s after ceasing the irradiation, are very similar to the conventional ones. For the Tb³⁺ material, the ⁵D₃ persistent emission is even less intense than the conventional one (when compared to the ⁵D₄ one) but the differences are too small to justify a different quenching mechanism. The Tb³⁺ persistent luminescence is also observed under excitation on the ⁵D₃ level (378 nm), indicating that this level is indeed located in host’s CB. In the case of the Eu²⁺ doped material, the only difference is the absence of the 435 nm band in the persistent luminescence spectrum, indicating that probably one of the Ba²⁺ sites does not participate on the persistent luminescence process.

One of the most critical points in the proposed mechanisms of persistent luminescence for Eu²⁺ [1

] is the transfer of the charge (electron) from the 4f⁶5d¹ levels to the host’s conduction band: both the energetics (position of the 4f⁶5d¹ levels vis-à-vis CB) and the time of transfer (transition probability) seem to matter. The lifetime of the Laporte allowed though spin-forbidden 4f⁶5d¹→4f⁷ emission of Eu²⁺ is ca. 1 µs, which seems to be largely sufficient to allow efficient electron transfer to CB. The case with the Tb³⁺ doped materials is somewhat similar to the Eu²⁺ doped ones: the excitation occurs to the 4f⁷5d¹ levels which are located, at least partially, in host’s CB. This requires a host with low band gap energy, however, because the ⁷F₆ ground level of Tb³⁺ is considerably farther away from the host’s CB than the ⁸S_7/2 ground level of Eu²⁺. To perhaps balance this disadvantage, the lifetime of the Laporte forbidden ⁵D₄→⁷F_J transitions is much longer, in the range of several ms, thus enabling the capture of electrons to CB in a manner even more efficient than for the Eu²⁺ ion.

3.2 Trap nature and structure

The persistent luminescence phenomenon occurs due to the presence of charge carriers (electrons or holes) trapped in defects in the host structure. In the mechanisms studied, it is suggested that the electron (e^-) is the charge carrier, implying the presence of electron acceptor defects (e.g. oxygen vacancy

V_{O}^{••}

) [1

]. The creation of the oxide vacancies can occur in both cases studied by the evaporation of metal oxides (MO, M: Cd, Ba) at the high preparation temperatures together with a cation vacancy (

V_{M}^{''}

). Furthermore, the lattice defects are created by the charge compensation necessary when a R³⁺ ion replaces a M²⁺ host cation (

R_{M}^{•}

), either creating a metal vacancy

V_{M}^{''}

or an interstitial oxide ion

O_{i}^{''}

. All symbols are according to the Kröger-Vink notation [24

F. A. Kröger and H. J. Vink, in Proc. Intern. Colloq. “Semiconductors and phosphors” (Interscience Publishers, Inc., New York, 1958), p. 17.

]. In the Eu²⁺ doped materials, R³⁺ co-doping frequently increases the persistent luminescence efficiency, since R³⁺ may also act as traps. However, in the Tb³⁺ doped CdSiO₃, there is no need for co-doping because Tb³⁺ acts both as a luminescence center and a creator of the energy storing defects.

When studying more closely the CdSiO₃ structure [25

M. Weil, “Parawollastonite-type Cd₃[Si₃O₉],” Acta Crystallogr. Sect. E Struct. Rep. Online 61(6), i102–i104 (2005). [CrossRef]

] as presented in the DIAMOND [26

K. Brandenburg, Diamond v.3.2g, Crystal Impact: Bonn, Germany, 2011.

] view (Fig. 2 ), it is observed that the Cd-Cd distance between the terminal Cd ions belonging to different three-membered ribbons (only two shown) is ca. 5.2 Å whilst within the ribbons composed of three CdO₆ octahedra the distances are rather short, ca. 3.4 Å. There is thus a considerable free space that may accommodate an interstitial oxide ion needed to compensate the charge (Tb³⁺ vs. Cd²⁺) mismatch. In fact, the distance of 5.2 Å is just sufficient to form two Tb-O bonds of the length 2.5-2.6 Å. The position of the charge compensation defect immediately adjacent to the two Tb³⁺ ions (

{Tb}_{Cd}^{•}

) gives a rather infrequent clear possibility for clustering of the defects as follows:

{Tb}_{Cd}^{•}

O_{i}^{''}

{Tb}_{Cd}^{•}

. This should be studied further with e.g. synchrotron radiation EXAFS methods.

Fig. 2 A DIAMOND [26

K. Brandenburg, Diamond v.3.2g, Crystal Impact: Bonn, Germany, 2011.

] view of the crystal structure of CdSiO₃ drawn with data from [25

M. Weil, “Parawollastonite-type Cd₃[Si₃O₉],” Acta Crystallogr. Sect. E Struct. Rep. Online 61(6), i102–i104 (2005). [CrossRef]

The interstitial oxide may well be accommodated in the regular 2b (½, ½, ½) site in the space group P2₁/c, yet to be studied by DFT (Density Functional Theory) calculations, giving the Tb-O distances of 2.54 Å. This value is few tenths of Å longer than the optimal bond distance of Tb-O but well within the bond range found for the Tb-O bond. However, this yields one short (2.1 Å) O-O distance between

O_{i}^{''}

and one oxygen in the SiO₄ groups, much shorter than the O-O distances in the SiO₄ group (2.56 – 2.80 Å) in e.g. CdSiO₃. Because of this increased oxygen-oxygen repulsion the interstitial oxide may have reduced stability, and may be replaced by trapped electrons and thus may have an important role to play in the energy storing in the persistent luminescence. An analogous situation has been reported [27

P. G. Radaelli, J. D. Jorgensen, A. J. Schultz, J. L. Peng, and R. L. Greene, “Evidence of apical oxygen in Nd₂CuO_y determined by single-crystal neutron diffraction,” Phys. Rev. B Condens. Matter 49(21), 15322–15326 (1994). [CrossRef] [PubMed]

] to arise in the cuprate superconducting phases. Taken into account that the crystallographic position of the interstitial oxide is only an approximate one, the Tb-O and O-O distances may well be modified with the optimization of the atomic positions. This optimization is being carried out with the DFT calculations but is out of the scope of the present work.

Similar free spaces may probably exist in other hosts as MAl₂O₄ accepting interstitial oxygens when co-doped with R³⁺. The analysis of their structure can help the understanding of the effect of the co-dopants in the persistent luminescence of Eu²⁺ doped materials, too. Furthermore, the reduction mechanism of Eu³⁺ to Eu²⁺ without the use of reducing atmosphere proposed for the BaAl₂O₄ material [16

] involves the release of

O_{i}^{''}

. Therefore, the reduced stability of

O_{i}^{''}

may be related to this spontaneous reduction, and further studies with other hosts should be carried out.

3.3 Band gap structure

The Synchrotron Radiation (SR) excitation spectra of the materials (Fig. 3 ) were measured at 10 K in order to avoid the persistent luminescence which may smudge the excitation spectrum, at least at room temperature. A sharp edge is observed at ca. 235 nm (5.28 eV) in the excitation spectra of the Tb³⁺ doped CdSiO₃. The edge is the excitation from the top of the valence (VB) to the bottom of the conduction band, i.e. the band gap energy (E_g). In the excitation spectra of the Eu²⁺ doped BaAl₂O₄, the corresponding edge is at a much higher energy, at ca. 190 nm (6.5 eV). As assumed above, the band gap should be smaller for the Tb³⁺ doped materials to enable persistent luminescence at all or, at least, to let the persistence to be excited with low energy UV radiation (or blue light) and thus improve the efficiency. However, despite the low band gap energy, the persistent luminescence from Tb³⁺ in CdSiO₃ is observed only with UV irradiation.

Fig. 3 The SR UV-VUV excitation spectra of CdSiO₃:Tb³⁺ and BaAl₂O₄:Eu²⁺,Dy³⁺ at 10 K.

One may observe in the UV-VUV excitation spectrum of CdSiO₃:Tb³⁺ (Fig. 3) the weakness of the Tb³⁺ 4f⁸→4f⁷5d¹ transition bands even at 10 K indicating the easy delocalization of electrons to the conduction band of CdSiO₃. This was originally shown for the Ce³⁺ doped Y₃Al₅O₁₂ (YAG) single crystals [28

T. Tomiki, H. Akamine, M. Gushiken, Y. Kinjoh, M. Miyazato, T. Miyazato, N. Toyokawa, M. Hiraoka, N. Hirata, Y. Ganaha, and T. Futemma, “Ce³⁺ centres in Y₃Al₅O₁₂ (YAG) single crystals,” J. Phys. Soc. Jpn. 60(7), 2437–2445 (1991). [CrossRef]

]. The 4fⁿ→4f^n-15d¹ transitions can thus lose their superior intensity relative to the 4f-4f transitions when they are within the host’s CB. The 4f⁷5d¹ levels of Tb³⁺ are thus probably entirely inside CB, whilst the 4f⁶5d¹ levels of Eu²⁺ are only partially inside the CB since some Eu²⁺ 4f⁷→4f⁶5d¹ transition bands have rather high intensity.

In order to determine the R^2+/3+ (R: La-Nd, Sm-Lu) 4f ground state positions in the hosts’ band structure, it is important to start with their LMCT transitions [17

P. Dorenbos, “Locating lanthanide impurity levels in the forbidden band of host crystals,” J. Lumin. 108(1-4), 301–305 (2004). [CrossRef]

]. The most accessible CT transition in the whole Rⁿ⁺ doped CdSiO₃ system is for CdSiO₃:Eu³⁺. This information is obtained by the SR UV-VUV excitation spectrum of Eu³⁺ (Fig. 4 , left), where a broad band is observed between 230 to 270 nm (5.4 to 4.7 eV, respectively). The band was assigned to the LMCT O^2-(2p)→Eu³⁺ transition. The CT energy gives the position of the Eu²⁺ ground state in the band gap (4.7 eV above the top of the host's VB). The energy difference between the ground levels of the same rare earth but in a different oxidation state, i.e. Eu²⁺ and Eu³⁺, depends on the host. This difference is quite high for a free ion (18 eV) but is lowered in solid state, e.g. in fluorides, oxides and sulfides down to >7, 6-7 and <6 eV, respectively [29

P. Dorenbos, “Lanthanide charge transfer energies and related luminescence, charge carrier trapping, and redox phenomena,” J. Alloys Compd. 488(2), 568–573 (2009). [CrossRef]

]. For the Eu²⁺/Eu³⁺ pair in CdSiO₃, a value of 6.1 eV was used for this energy difference due to the softness of the CdSiO₃ host. Once the position of the ground level of Eu³⁺ is known, following the host independent evolution of the R³⁺ energy levels [30

P. Dorenbos, “Systematic behaviour in trivalent lanthanide charge transfer energies,” J. Phys. Condens. Matter 15(49), 8417–8434 (2003). [CrossRef]

], it is possible to determine the position of the ground levels of all other R³⁺’s. For example, the energy of the Tb³⁺ ground level (⁷F₆) is at 3.5 eV above that of Eu³⁺ (⁷F₀). The Tb³⁺ (⁷F₆) ground level position in CdSiO₃ was thus found at ca. 2.1 eV above the VB top. The position of the ⁷F₆ ground level deep in the band gap explains the presence of persistent luminescence only with UV irradiation.

Fig. 4 The SR UV-VUV excitation spectra of CdSiO₃:Eu³⁺ (left) and time resolved UV excitation spectrum of BaAl₂O₄:Eu²⁺,Dy³⁺ (right).

For the BaAl₂O₄ system, the trivalent Eu³⁺ doped material is not easily obtained, since the spontaneous reduction from Eu³⁺ to Eu²⁺ occurs even in non-reducing atmospheres [16

,31

M. Peng and G. Hong, “Reduction from Eu³⁺ to Eu²⁺ in BaAl₂O₄:Eu phosphor prepared in an oxidizing atmosphere and luminescent properties of BaAl₂O₄:Eu,” J. Lumin. 127(2), 735–740 (2007). [CrossRef]

]. Furthermore, the 4f-4f transitions of Eu³⁺ are absent in the steady-state spectra due to non-radiative processes (quenching) by Eu²⁺, whose 4f⁶5d¹→4f⁷ transitions are allowed, having a much shorter lifetime (ca. 1 μs) than the 4f→4f transitions of Eu³⁺ (several ms) and thus larger transition probability. Consequently, in order to determine the Eu³⁺ LMCT in BaAl₂O₄, the time-resolved excitation spectra of BaAl₂O₄:Eu²⁺,Dy³⁺ were measured at 77 K with a delay of 1 ms to avoid both the persistent luminescence and the Eu²⁺ emission (Fig. 4, right). A broad band is observed between 220 to 300 nm (5.6 to 4.1 eV) readily assigned to the Eu³⁺ O^2-(2p)→Eu³⁺ LMCT transition, though overlapped by several Eu^{3+ 7}F₀→^2S+1L_J bands. The higher energy position of the Eu²⁺ ground state in the BaAl₂O₄ band gap, as in the majority of the hosts [17

P. Dorenbos, “Locating lanthanide impurity levels in the forbidden band of host crystals,” J. Lumin. 108(1-4), 301–305 (2004). [CrossRef]

] allows the excitation of persistent luminescence with blue light even in wide band gap hosts.

3.4 The mechanisms of persistent luminescence

Once the collection of vital information about the energy level systems in both the Tb³⁺ doped CdSiO₃ and the Eu²⁺ doped BaAl₂O₄ was completed, the mechanisms of the persistent luminescence could be constructed. As a first observation, despite some differences in the photoluminescence processes, band gap energy and ground state positions of the luminescence centers, the mechanisms for Tb³⁺ and Eu²⁺ are quite similar. The duration of the persistent luminescence is comparable in both cases, longer than 1 h.

In general, the charging mechanisms for both the Eu²⁺ and Tb³⁺ doped materials consider that (Fig. 5 ): i) under irradiation of the material, in addition to conventional luminescence, some electrons escape to the host’s conduction band, along with a simultaneous formation of the pairs Eu²⁺−h⁺ or Tb³⁺−h⁺; ii) some of these electrons are trapped from CB to the defects created by charge compensation and the release of the unstable interstitial oxide, thus storing part of the excitation energy. The reverse decharging process of freeing the electrons from the traps to the excited states via CB precedes the radiative relaxation of the system back to the ground states of Eu²⁺ or Tb³⁺ via the excited states 4f⁶5d¹ (Eu²⁺) or ⁵D_3,4 (Tb³⁺) − thus creating the persistent luminescence.

Fig. 5 Persistent luminescence mechanisms of Tb³⁺ in CdSiO₃ (left) and Eu²⁺ in BaAl₂O₄ (right). Please note the different energy scales. The trap depth values were obtained from thermoluminescence measurements [5

,16

4. Conclusions

In closing the present work, plausible mechanisms coherent with experimental evidence could be constructed for the persistent luminescence from both Eu²⁺ and Tb³⁺. The mechanisms were found rather similar, despite many seemingly different basic properties of the systems, including the nature of the inter- and intraconfigurational transitions. In both systems, the electrons were found as the charge carriers and the nature of the traps is thus similar (electron traps). Nevertheless, the R³⁺ co-dopants may act in different ways in the two systems. For CdSiO₃, the structural analysis showed that the interstitial oxide ions created by charge compensation can be accommodated in the free space. The low stability of these interstitial oxide ions can cause their substitution by trapped electrons thus acting as electron traps creators. More profound structural analyses should be carried out to find if the possible free space for

O_{i}^{''}

exists in other hosts as MAl₂O₄. Only sparse data is available about the general properties for the Tb³⁺ doped persistent luminescence materials, thus more detailed studies are needed on these systems, too.

Acknowledgments

Financial support is acknowledged from the Coimbra Group, Turku University Foundation, Academy of Finland (Finland, contract #137333/2010), FAPESP, CAPES, CNPq, inct-INAMI and Nanobiotec-Brasil RH-INAMI (all Brazil). The synchrotron radiation studies were supported by the European Community – Research Infrastructure Action under the FP6 Structuring the European Research Area Programme, RII3-CT-2004-506008 (IA-SFS). Dr. Aleksei Kotlov (HASYLAB) is gratefully acknowledged for his assistance during the synchrotron measurements.

References and links

1.	T. Aitasalo, J. Hölsä, H. Jungner, M. Lastusaari, and J. Niittykoski, “Thermoluminescence study of persistent luminescence materials: Eu²⁺- and R³⁺-doped calcium aluminates, CaAl₂O₄:Eu²⁺,R^3+.,” J. Phys. Chem. B 110(10), 4589–4598 (2006). [CrossRef] [PubMed]
2.	T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, “A new long phosphorescent phosphor with high brightness, SrAl₂O₄:Eu²⁺,Dy³⁺,” J. Electrochem. Soc. 143(8), 2670–2673 (1996). [CrossRef]
3.	Y. Lin, Z. Tang, Z. Zhang, and C. W. Nan, “Anomalous luminescence in Sr₄Al₁₄O₂₅:Eu,Dy phosphors,” Appl. Phys. Lett. 81(6), 996–998 (2002). [CrossRef]
4.	Y. Lin, Z. Tang, Z. Zhang, X. Wang, and J. Zhang, “Preparation of a new long afterglow blue-emitting Sr₂MgSi₂O₇-based photoluminescent phosphor,” J. Mater. Sci. Lett. 20(16), 1505–1506 (2001). [CrossRef]
5.	L. C. V. Rodrigues, H. F. Brito, J. Hölsä, R. Stefani, M. C. F. C. Felinto, M. Lastusaari, M. Malkamäki, and L. A. O. Nunes, “Discovery of the persistent luminescence mechanism of CdSiO₃:Tb³⁺,” J. Phys. Chem. C (to be published).
6.	J. Trojan-Piegza, E. Zych, J. Hölsä, and J. Niittykoski, “Spectroscopic properties of persistent luminescence phosphors: Lu₂O₃:Tb³⁺,M²⁺ (M = Ca, Sr, Ba),” J. Phys. Chem. C 113(47), 20493–20498 (2009). [CrossRef]
7.	T. Kinoshita, M. Yamazaki, H. Kawazoe, and H. Hosono, “Long lasting phosphorescence and photostimulated luminescence in Tb-ion-activated reduced calcium aluminate glasses,” J. Appl. Phys. 86(7), 3729–3733 (1999). [CrossRef]
8.	C. Liu, G. Che, Z. Xu, and Q. Wang, “Luminescence properties of a Tb³⁺ activated long-afterglow phosphor,” J. Alloys Compd. 474(1-2), 250–253 (2009). [CrossRef]
9.	J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, J. Niittykoski, and E. Zych, “Effect of Mg²⁺ and Ti^IV doping on the luminescence of Y₂O₂S:Eu³⁺,” Opt. Mater. 31(12), 1791–1793 (2009). [CrossRef]
10.	J. Fu, “Orange and red emitting long-lasting phosphors MO:Eu³⁺ (M = Ca, Sr, Ba),” Electrochem. Solid-State Lett. 3(7), 350–351 (1999). [CrossRef]
11.	T. Aitasalo, A. Hietikko, D. Hreniak, J. Hölsä, M. Lastusaari, J. Niittykoski, and W. Stręk, “Luminescence properties of BaMg₂Si₂O₇:Eu²⁺,Mn²⁺,” J. Alloys Compd. 451(1-2), 229–231 (2008). [CrossRef]
12.	X. Qu, L. Cao, W. Liu, G. Su, and P. Wang, “Luminescence properties of CdSiO₃: Mn²⁺,RE³⁺ (RE = Sm, Dy, Eu) phosphors,” J. Alloys Compd. 487(1-2), 387–390 (2009). [CrossRef]
13.	Y. Cong, B. Li, B. Lei, and W. Li, “Long lasting phosphorescent properties of Ti doped ZrO₂,” J. Lumin. 126(2), 822–826 (2007). [CrossRef]
14.	J. M. Carvalho, L. C. V. Rodrigues, J. Hölsä, M. Lastusaari, L. A. O. Nunes, M. C. F. C. Felinto, O. L. Malta, and H. F. Brito, “Influence of titanium and lutetium on the persistent luminescence of ZrO₂,” Opt. Mater. Express 2(3), 331–340 (2012). [CrossRef]
15.	K. Van den Eeckhout, P. F. Smet, and D. Poelman, “Persistent luminescence in Eu²⁺-doped compounds: a review,” Materials 3(4), 2536–2566 (2010). [CrossRef]
16.	L. C. V. Rodrigues, R. Stefani, H. F. Brito, M. C. F. C. Felinto, J. Hölsä, M. Lastusaari, T. Laamanen, and M. Malkamäki, “Thermoluminescence and synchrotron radiation studies on the persistent luminescence of BaAl₂O₄:Eu²⁺,Dy³⁺,” J. Solid State Chem. 183(10), 2365–2371 (2010). [CrossRef]
17.	P. Dorenbos, “Locating lanthanide impurity levels in the forbidden band of host crystals,” J. Lumin. 108(1-4), 301–305 (2004). [CrossRef]
18.	Z. Qiu, Y. Zhou, M. Lu, A. Zhang, and Q. Ma, “Combustion synthesis of long-persistent luminescent MAl₂O₄:Eu²⁺, R³⁺ (M = Sr, Ba, Ca, R = Dy, Nd and La) nanoparticles and luminescence mechanism research,” Acta Mater. 55(8), 2615–2620 (2007). [CrossRef]
19.	S. Ekambaram, K. C. Patil, and M. Maaza, “Synthesis of lamp phosphors: facile combustion approach,” J. Alloys Compd. 393(1-2), 81–92 (2005). [CrossRef]
20.	B. M. Mothudi, O. M. Ntwaeaborwa, J. R. Botha, and H. C. Swart, “Photoluminescence and phosphorescence properties of MAl₂O₄:Eu²⁺,Dy³⁺ (M=Ca, Ba, Sr) phosphors prepared at an initiating combustion temperature of 500 °C,” Physica B 404(22), 4440–4444 (2009). [CrossRef]
21.	HASYLAB, Beamline I: SUPERLUMI, http://hasylab.desy.de/facilities/doris_iii/beamlines/i_superlumi (accessed on Dec. 20, 2011).
22.	Y. Liu, J. Kuang, B. Lei, and C. Shi, “Color-control of long-lasting phosphorescence (LLP) through rare earth ion-doped cadmium metasilicate phosphors,” J. Mater. Chem. 15(37), 4025–4031 (2005). [CrossRef]
23.	A. V. S. Lourenço, L. C. V. Rodrigues, C. A. Kodaira, R. Stefani, H. F. Brito, M. C. F. C. Felinto, and J. Hölsä, “Persistent luminescence BaAl₂O₄:Eu²⁺,Dy³⁺ phosphor encapsulated in silica: water resistance,” in Proc. Int. Conf. Adv. Mater. (Braz. Mater. Res. Soc.), Rio de Janeiro-RJ, Brazil, September 20–25, 2009, BB556. http://www.sbpmat.org.br/icam2009/pdf/BB556.pdf.
24.	F. A. Kröger and H. J. Vink, in Proc. Intern. Colloq. “Semiconductors and phosphors” (Interscience Publishers, Inc., New York, 1958), p. 17.
25.	M. Weil, “Parawollastonite-type Cd₃[Si₃O₉],” Acta Crystallogr. Sect. E Struct. Rep. Online 61(6), i102–i104 (2005). [CrossRef]
26.	K. Brandenburg, Diamond v.3.2g, Crystal Impact: Bonn, Germany, 2011.
27.	P. G. Radaelli, J. D. Jorgensen, A. J. Schultz, J. L. Peng, and R. L. Greene, “Evidence of apical oxygen in Nd₂CuO_y determined by single-crystal neutron diffraction,” Phys. Rev. B Condens. Matter 49(21), 15322–15326 (1994). [CrossRef] [PubMed]
28.	T. Tomiki, H. Akamine, M. Gushiken, Y. Kinjoh, M. Miyazato, T. Miyazato, N. Toyokawa, M. Hiraoka, N. Hirata, Y. Ganaha, and T. Futemma, “Ce³⁺ centres in Y₃Al₅O₁₂ (YAG) single crystals,” J. Phys. Soc. Jpn. 60(7), 2437–2445 (1991). [CrossRef]
29.	P. Dorenbos, “Lanthanide charge transfer energies and related luminescence, charge carrier trapping, and redox phenomena,” J. Alloys Compd. 488(2), 568–573 (2009). [CrossRef]
30.	P. Dorenbos, “Systematic behaviour in trivalent lanthanide charge transfer energies,” J. Phys. Condens. Matter 15(49), 8417–8434 (2003). [CrossRef]
31.	M. Peng and G. Hong, “Reduction from Eu³⁺ to Eu²⁺ in BaAl₂O₄:Eu phosphor prepared in an oxidizing atmosphere and luminescent properties of BaAl₂O₄:Eu,” J. Lumin. 127(2), 735–740 (2007). [CrossRef]

OCIS Codes
(160.2540) Materials : Fluorescent and luminescent materials
(160.2900) Materials : Optical storage materials
(160.5690) Materials : Rare-earth-doped materials
(260.3800) Physical optics : Luminescence
(300.2140) Spectroscopy : Emission

ToC Category:
Fluorescent and Luminescent Materials

History
Original Manuscript: January 3, 2012
Revised Manuscript: March 2, 2012
Manuscript Accepted: March 2, 2012
Published: March 6, 2012

Virtual Issues
Persistent Phosphors (2012) Optical Materials Express

Citation
Lucas C. V. Rodrigues, Hermi F. Brito, Jorma Hölsä, and Mika Lastusaari, "Persistent luminescence behavior of materials doped with Eu²⁺ and Tb³⁺," Opt. Mater. Express 2, 382-390 (2012)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-2-4-382

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References

T. Aitasalo, J. Hölsä, H. Jungner, M. Lastusaari, and J. Niittykoski, “Thermoluminescence study of persistent luminescence materials: Eu2+- and R3+-doped calcium aluminates, CaAl2O4:Eu2+,R3+.,” J. Phys. Chem. B110(10), 4589–4598 (2006). [CrossRef] [PubMed]
T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, “A new long phosphorescent phosphor with high brightness, SrAl2O4:Eu2+,Dy3+,” J. Electrochem. Soc.143(8), 2670–2673 (1996). [CrossRef]
Y. Lin, Z. Tang, Z. Zhang, and C. W. Nan, “Anomalous luminescence in Sr4Al14O25:Eu,Dy phosphors,” Appl. Phys. Lett.81(6), 996–998 (2002). [CrossRef]
Y. Lin, Z. Tang, Z. Zhang, X. Wang, and J. Zhang, “Preparation of a new long afterglow blue-emitting Sr2MgSi2O7-based photoluminescent phosphor,” J. Mater. Sci. Lett.20(16), 1505–1506 (2001). [CrossRef]
L. C. V. Rodrigues, H. F. Brito, J. Hölsä, R. Stefani, M. C. F. C. Felinto, M. Lastusaari, M. Malkamäki, and L. A. O. Nunes, “Discovery of the persistent luminescence mechanism of CdSiO3:Tb3+,” J. Phys. Chem. C (to be published).
J. Trojan-Piegza, E. Zych, J. Hölsä, and J. Niittykoski, “Spectroscopic properties of persistent luminescence phosphors: Lu2O3:Tb3+,M2+ (M = Ca, Sr, Ba),” J. Phys. Chem. C113(47), 20493–20498 (2009). [CrossRef]
T. Kinoshita, M. Yamazaki, H. Kawazoe, and H. Hosono, “Long lasting phosphorescence and photostimulated luminescence in Tb-ion-activated reduced calcium aluminate glasses,” J. Appl. Phys.86(7), 3729–3733 (1999). [CrossRef]
C. Liu, G. Che, Z. Xu, and Q. Wang, “Luminescence properties of a Tb3+ activated long-afterglow phosphor,” J. Alloys Compd.474(1-2), 250–253 (2009). [CrossRef]
J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, J. Niittykoski, and E. Zych, “Effect of Mg2+ and TiIV doping on the luminescence of Y2O2S:Eu3+,” Opt. Mater.31(12), 1791–1793 (2009). [CrossRef]
J. Fu, “Orange and red emitting long-lasting phosphors MO:Eu3+ (M = Ca, Sr, Ba),” Electrochem. Solid-State Lett.3(7), 350–351 (1999). [CrossRef]
T. Aitasalo, A. Hietikko, D. Hreniak, J. Hölsä, M. Lastusaari, J. Niittykoski, and W. Stręk, “Luminescence properties of BaMg2Si2O7:Eu2+,Mn2+,” J. Alloys Compd.451(1-2), 229–231 (2008). [CrossRef]
X. Qu, L. Cao, W. Liu, G. Su, and P. Wang, “Luminescence properties of CdSiO3: Mn2+,RE3+ (RE = Sm, Dy, Eu) phosphors,” J. Alloys Compd.487(1-2), 387–390 (2009). [CrossRef]
Y. Cong, B. Li, B. Lei, and W. Li, “Long lasting phosphorescent properties of Ti doped ZrO2,” J. Lumin.126(2), 822–826 (2007). [CrossRef]
J. M. Carvalho, L. C. V. Rodrigues, J. Hölsä, M. Lastusaari, L. A. O. Nunes, M. C. F. C. Felinto, O. L. Malta, and H. F. Brito, “Influence of titanium and lutetium on the persistent luminescence of ZrO2,” Opt. Mater. Express2(3), 331–340 (2012). [CrossRef]
K. Van den Eeckhout, P. F. Smet, and D. Poelman, “Persistent luminescence in Eu2+-doped compounds: a review,” Materials3(4), 2536–2566 (2010). [CrossRef]
L. C. V. Rodrigues, R. Stefani, H. F. Brito, M. C. F. C. Felinto, J. Hölsä, M. Lastusaari, T. Laamanen, and M. Malkamäki, “Thermoluminescence and synchrotron radiation studies on the persistent luminescence of BaAl2O4:Eu2+,Dy3+,” J. Solid State Chem.183(10), 2365–2371 (2010). [CrossRef]
P. Dorenbos, “Locating lanthanide impurity levels in the forbidden band of host crystals,” J. Lumin.108(1-4), 301–305 (2004). [CrossRef]
Z. Qiu, Y. Zhou, M. Lu, A. Zhang, and Q. Ma, “Combustion synthesis of long-persistent luminescent MAl2O4:Eu2+, R3+ (M = Sr, Ba, Ca, R = Dy, Nd and La) nanoparticles and luminescence mechanism research,” Acta Mater.55(8), 2615–2620 (2007). [CrossRef]
S. Ekambaram, K. C. Patil, and M. Maaza, “Synthesis of lamp phosphors: facile combustion approach,” J. Alloys Compd.393(1-2), 81–92 (2005). [CrossRef]
B. M. Mothudi, O. M. Ntwaeaborwa, J. R. Botha, and H. C. Swart, “Photoluminescence and phosphorescence properties of MAl2O4:Eu2+,Dy3+ (M=Ca, Ba, Sr) phosphors prepared at an initiating combustion temperature of 500 °C,” Physica B404(22), 4440–4444 (2009). [CrossRef]
HASYLAB, Beamline I: SUPERLUMI, http://hasylab.desy.de/facilities/doris_iii/beamlines/i_superlumi (accessed on Dec. 20, 2011).
Y. Liu, J. Kuang, B. Lei, and C. Shi, “Color-control of long-lasting phosphorescence (LLP) through rare earth ion-doped cadmium metasilicate phosphors,” J. Mater. Chem.15(37), 4025–4031 (2005). [CrossRef]
A. V. S. Lourenço, L. C. V. Rodrigues, C. A. Kodaira, R. Stefani, H. F. Brito, M. C. F. C. Felinto, and J. Hölsä, “Persistent luminescence BaAl2O4:Eu2+,Dy3+ phosphor encapsulated in silica: water resistance,” in Proc. Int. Conf. Adv. Mater. (Braz. Mater. Res. Soc.), Rio de Janeiro-RJ, Brazil, September 20–25, 2009, BB556. http://www.sbpmat.org.br/icam2009/pdf/BB556.pdf .
F. A. Kröger and H. J. Vink, in Proc. Intern. Colloq. “Semiconductors and phosphors” (Interscience Publishers, Inc., New York, 1958), p. 17.
M. Weil, “Parawollastonite-type Cd3[Si3O9],” Acta Crystallogr. Sect. E Struct. Rep. Online61(6), i102–i104 (2005). [CrossRef]
K. Brandenburg, Diamond v.3.2g, Crystal Impact: Bonn, Germany, 2011.
P. G. Radaelli, J. D. Jorgensen, A. J. Schultz, J. L. Peng, and R. L. Greene, “Evidence of apical oxygen in Nd2CuOy determined by single-crystal neutron diffraction,” Phys. Rev. B Condens. Matter49(21), 15322–15326 (1994). [CrossRef] [PubMed]
T. Tomiki, H. Akamine, M. Gushiken, Y. Kinjoh, M. Miyazato, T. Miyazato, N. Toyokawa, M. Hiraoka, N. Hirata, Y. Ganaha, and T. Futemma, “Ce3+ centres in Y3Al5O12 (YAG) single crystals,” J. Phys. Soc. Jpn.60(7), 2437–2445 (1991). [CrossRef]
P. Dorenbos, “Lanthanide charge transfer energies and related luminescence, charge carrier trapping, and redox phenomena,” J. Alloys Compd.488(2), 568–573 (2009). [CrossRef]
P. Dorenbos, “Systematic behaviour in trivalent lanthanide charge transfer energies,” J. Phys. Condens. Matter15(49), 8417–8434 (2003). [CrossRef]
M. Peng and G. Hong, “Reduction from Eu3+ to Eu2+ in BaAl2O4:Eu phosphor prepared in an oxidizing atmosphere and luminescent properties of BaAl2O4:Eu,” J. Lumin.127(2), 735–740 (2007). [CrossRef]

Aitasalo, T.
Akamine, H.
Aoki, Y.
Botha, J. R.
Brito, H. F.
Cao, L.
Carvalho, J. M.
Che, G.
Cong, Y.
Dorenbos, P.
Ekambaram, S.
Felinto, M. C. F. C.
Fu, J.
Futemma, T.
Ganaha, Y.
Greene, R. L.
Gushiken, M.
Hietikko, A.
Hiraoka, M.
Hirata, N.
Hölsä, J.
Hong, G.
Hosono, H.
Hreniak, D.
Jorgensen, J. D.
Jungner, H.
Kawazoe, H.
Kinjoh, Y.
Kinoshita, T.
Kuang, J.
Laamanen, T.
Lastusaari, M.
Lei, B.
Li, B.
Li, W.
Lin, Y.
Liu, C.
Liu, W.
Liu, Y.
Lu, M.
Ma, Q.
Maaza, M.
Malkamäki, M.
Malta, O. L.
Matsuzawa, T.
Miyazato, M.
Miyazato, T.
Mothudi, B. M.
Murayama, Y.
Nan, C. W.
Niittykoski, J.
Ntwaeaborwa, O. M.
Nunes, L. A. O.
Patil, K. C.
Peng, J. L.
Peng, M.
Poelman, D.
Qiu, Z.
Qu, X.
Radaelli, P. G.
Rodrigues, L. C. V.
Schultz, A. J.
Shi, C.
Smet, P. F.
Stefani, R.
Strek, W.
Su, G.
Swart, H. C.
Takeuchi, N.
Tang, Z.
Tomiki, T.
Toyokawa, N.
Trojan-Piegza, J.
Van den Eeckhout, K.
Wang, P.
Wang, Q.
Wang, X.
Weil, M.
Xu, Z.
Yamazaki, M.
Zhang, A.
Zhang, J.
Zhang, Z.
Zhou, Y.
Zych, E.

Acta Crystallogr. Sect. E Struct. Rep. Online

M. Weil, “Parawollastonite-type Cd3[Si3O9],” Acta Crystallogr. Sect. E Struct. Rep. Online61(6), i102–i104 (2005). [CrossRef]

Acta Mater.

Z. Qiu, Y. Zhou, M. Lu, A. Zhang, and Q. Ma, “Combustion synthesis of long-persistent luminescent MAl2O4:Eu2+, R3+ (M = Sr, Ba, Ca, R = Dy, Nd and La) nanoparticles and luminescence mechanism research,” Acta Mater.55(8), 2615–2620 (2007). [CrossRef]

Appl. Phys. Lett.

Y. Lin, Z. Tang, Z. Zhang, and C. W. Nan, “Anomalous luminescence in Sr4Al14O25:Eu,Dy phosphors,” Appl. Phys. Lett.81(6), 996–998 (2002). [CrossRef]

Electrochem. Solid-State Lett.

J. Fu, “Orange and red emitting long-lasting phosphors MO:Eu3+ (M = Ca, Sr, Ba),” Electrochem. Solid-State Lett.3(7), 350–351 (1999). [CrossRef]

J. Alloys Compd.

T. Aitasalo, A. Hietikko, D. Hreniak, J. Hölsä, M. Lastusaari, J. Niittykoski, and W. Stręk, “Luminescence properties of BaMg2Si2O7:Eu2+,Mn2+,” J. Alloys Compd.451(1-2), 229–231 (2008). [CrossRef]
X. Qu, L. Cao, W. Liu, G. Su, and P. Wang, “Luminescence properties of CdSiO3: Mn2+,RE3+ (RE = Sm, Dy, Eu) phosphors,” J. Alloys Compd.487(1-2), 387–390 (2009). [CrossRef]
C. Liu, G. Che, Z. Xu, and Q. Wang, “Luminescence properties of a Tb3+ activated long-afterglow phosphor,” J. Alloys Compd.474(1-2), 250–253 (2009). [CrossRef]
S. Ekambaram, K. C. Patil, and M. Maaza, “Synthesis of lamp phosphors: facile combustion approach,” J. Alloys Compd.393(1-2), 81–92 (2005). [CrossRef]
P. Dorenbos, “Lanthanide charge transfer energies and related luminescence, charge carrier trapping, and redox phenomena,” J. Alloys Compd.488(2), 568–573 (2009). [CrossRef]

J. Appl. Phys.

T. Kinoshita, M. Yamazaki, H. Kawazoe, and H. Hosono, “Long lasting phosphorescence and photostimulated luminescence in Tb-ion-activated reduced calcium aluminate glasses,” J. Appl. Phys.86(7), 3729–3733 (1999). [CrossRef]

J. Electrochem. Soc.

T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, “A new long phosphorescent phosphor with high brightness, SrAl2O4:Eu2+,Dy3+,” J. Electrochem. Soc.143(8), 2670–2673 (1996). [CrossRef]

J. Lumin.

P. Dorenbos, “Locating lanthanide impurity levels in the forbidden band of host crystals,” J. Lumin.108(1-4), 301–305 (2004). [CrossRef]
Y. Cong, B. Li, B. Lei, and W. Li, “Long lasting phosphorescent properties of Ti doped ZrO2,” J. Lumin.126(2), 822–826 (2007). [CrossRef]
M. Peng and G. Hong, “Reduction from Eu3+ to Eu2+ in BaAl2O4:Eu phosphor prepared in an oxidizing atmosphere and luminescent properties of BaAl2O4:Eu,” J. Lumin.127(2), 735–740 (2007). [CrossRef]

J. Mater. Chem.

Y. Liu, J. Kuang, B. Lei, and C. Shi, “Color-control of long-lasting phosphorescence (LLP) through rare earth ion-doped cadmium metasilicate phosphors,” J. Mater. Chem.15(37), 4025–4031 (2005). [CrossRef]

J. Mater. Sci. Lett.

Y. Lin, Z. Tang, Z. Zhang, X. Wang, and J. Zhang, “Preparation of a new long afterglow blue-emitting Sr2MgSi2O7-based photoluminescent phosphor,” J. Mater. Sci. Lett.20(16), 1505–1506 (2001). [CrossRef]

J. Phys. Chem. B

T. Aitasalo, J. Hölsä, H. Jungner, M. Lastusaari, and J. Niittykoski, “Thermoluminescence study of persistent luminescence materials: Eu2+- and R3+-doped calcium aluminates, CaAl2O4:Eu2+,R3+.,” J. Phys. Chem. B110(10), 4589–4598 (2006). [CrossRef] [PubMed]

J. Phys. Chem. C

L. C. V. Rodrigues, H. F. Brito, J. Hölsä, R. Stefani, M. C. F. C. Felinto, M. Lastusaari, M. Malkamäki, and L. A. O. Nunes, “Discovery of the persistent luminescence mechanism of CdSiO3:Tb3+,” J. Phys. Chem. C (to be published).
J. Trojan-Piegza, E. Zych, J. Hölsä, and J. Niittykoski, “Spectroscopic properties of persistent luminescence phosphors: Lu2O3:Tb3+,M2+ (M = Ca, Sr, Ba),” J. Phys. Chem. C113(47), 20493–20498 (2009). [CrossRef]

J. Phys. Condens. Matter

P. Dorenbos, “Systematic behaviour in trivalent lanthanide charge transfer energies,” J. Phys. Condens. Matter15(49), 8417–8434 (2003). [CrossRef]

J. Phys. Soc. Jpn.

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2009, Qu, J. Alloys Compd.
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Persistent luminescence behavior of materials doped with Eu2+ and Tb3+

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Abstract

1. Introduction

2. Experimental

2.1 Materials preparation

2.2 Apparatus

3. Results and discussion

3.1 Photoluminescent properties

3.2 Trap nature and structure

3.3 Band gap structure

3.4 The mechanisms of persistent luminescence

4. Conclusions

Acknowledgments

References and links

References

Acta Crystallogr. Sect. E Struct. Rep. Online

Acta Mater.

Appl. Phys. Lett.

Electrochem. Solid-State Lett.

J. Alloys Compd.

J. Appl. Phys.

J. Electrochem. Soc.

J. Lumin.

J. Mater. Chem.

J. Mater. Sci. Lett.

J. Phys. Chem. B

J. Phys. Chem. C

J. Phys. Condens. Matter

J. Phys. Soc. Jpn.

J. Solid State Chem.

Materials

Opt. Mater.

Opt. Mater. Express

Phys. Rev. B Condens. Matter

Physica B

Other

Cited By

Figures

Persistent luminescence behavior of materials doped with Eu²⁺ and Tb³⁺