Journal of Luminescence 143 (2013) 215–218

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Journal of Luminescence

0022-23http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/jlumin

Preparation and luminescent properties of Tb3+ and Tb3+–Ce3+ dopedBa9Y2Si6O24 phosphors

Seul Lee, Sangmoon Park n

Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 617-736, Republic of Korea

a r t i c l e i n f o

Article history:Received 8 October 2012Received in revised form10 April 2013Accepted 3 May 2013Available online 13 May 2013

Keywords:OrthosilicatePhosphorsCross-relaxationEnergy transfer

13/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jlumin.2013.05.008

esponding author. Tel.: +82 51 999 5891; fax:ail address: spark@silla.ac.kr (S. Park).

a b s t r a c t

Luminescent materials composed of Ba9−3m/2TbmY2Si6O24 (m¼0.005–0.6) were prepared via a solid-statereaction. The dependence of the luminescent intensity of the Tb3+-doped orthosilicates on the Tb3+

content (m¼0.005–0.06) is also investigated. The photoluminescence properties of Tb3+ in Ba9−3m/2

TbmY2Si6O24 orthosilicate phosphors were elucidated. Using these phosphors, the desired CIE valuesincluding emissions throughout the blue, blue-green, and green regions of the spectra were achieved. Co-doping Ce3+ into the Tb3+-doped host structure enabled a high-energy charge-transfer from Ce3+ to Tb3+;accordingly, the Tb3+-Ce3+-doped green phosphors will be useful for applications involving white-lightnear-UV light-emitting-diodes.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

For applications involving UV excitation for the generation of avariety of display panels, the rare-earth Tb3+ ion is a well-knownactivator that effectively emits green light in diverse host lattices[1,2]. Green emission is an important component of white light,which is generated by red/green/blue emission at a 3:6:1 ratio.Many intense green-emitting Tb3+ containing phosphors such asY3Al5O12:Tb, Y2SiO5:Tb, Y2O2S:Tb, Gd2O2S:Tb, LaOBr:Tb, YBO3:Tb,and Sr3AlO4F:Tb have been thoroughly studied [1–6]. Of theknown rare-earth activators, Tb3+ has a relatively simple energylevel structure that consists of 7FJ (J¼6–0), 5D4, and 5D3 states.Usually, the 5D4–

7F5 transition leads to the predominantly greenemission of the Tb3+-activated phosphors. Furthermore, a dynamicchange in the emission spectra from blue to green can be achievedvia a cross-relaxation process that alters the population of the 5D3

and 5D4 transitions in Tb3+-doped compounds [1–7]. Further, theefficient luminescent properties that are activated by Tb3+ ions areenhanced by the addition of Ce3+ ions via an energy transfer fromCe3+ to Tb3+; this process involves excitation from the ground stateto the 5d state of the Ce3+ ions followed by energy transfer to the5D3 and other levels of the Tb3+ ions [8–10].

In a previous reports, Ba9Sc2Si6O24 compounds were prepared asnovel luminescent host materials [11,12]. The barium–yttrium–

orthosilicate host lattice comprises three Ba2+ sites and an Sc3+ sitewith isolated silicate tetrahedrons that do not share oxygen atoms.This kind of orthosilicate (or nesosilicate) subclass shows greatpotential in commercial LED phosphors, and includes YAG:Ce and

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+82 51 999 5335.

Sr2SiO4:Eu [13,14]. In this paper, we focus on the excitation andemission photoluminescence (PL) spectra of Tb3+- and Tb3+–Ce3+-doped examples of this new orthosilicate host structure, Ba9Y2Si6O24,when activated with ultraviolet (UV) and near-UV (NUV) light atroom temperature. The cross-relaxation of the Tb3+ ions as a functionof their content and the energy transfer from Ce3+ to Tb3+ in this hostlattice are described. We also show that orthosilicate phosphors withdesired CIE values including blue and green emission can be madevia substitution with the appropriate rare-earth ion.

2. Experimental

Samples of Ba9−3m/2TbmY2Si6O24 (m¼0.005–0.6) and Ba8.25−3n/2Tb0.5CenY2Si6O24 (n¼0.005–0.05) were prepared by heating theappropriate stoichiometric amounts of BaCO3 (Alfa 99.8%), Tb4O11

(Alfa 99.9%), CeO2 (Alfa 99.9%), Y2O3 (Alfa 99.9%), and SiO2 (Alfa99.5%) at 1100 1C in air. In each sample, the optimized 2.5 wt% Li2CO3

(Alfa 99%) was added as flux. The as-made samples were annealed at1000 and 1100 1C for 3 h each under the reducing atmosphereusing 5%H2/95%Ar. Phase identification was established using aShimadzu XRD-6000 powder diffractometer (Cu-Kα radiation). UVspectroscopy to measure the excitation and emission spectra of thephosphor materials was done using spectrofluoro-meters (SincoFluoromate FS-2).

3. Results and discussion

As reported previously, Fig. 1 shows the Ba9Sc2Si6O24 orthosi-licate structure (a¼9.8716(2) Å, c¼21.932(7) Å), which is a trigo-nal crystal with R-3H space group [11,12]. This structure contains

Ba(1)

Ba(2)

Ba(3)

Sc(Y)O6

SiO4

a

c

Fig. 1. The structure of Ba9Sc2Si6O24 [15].

Fig. 2. XRD patterns of (a) calculated Ba9Sc2Si6O24 (ICSD 75175), (b), (c), (d) observedBa9−3m/2TbmY2Si6O24 (m¼0.005, 0.05, 0.5) and (e) Ba8.2125Tb0.5Ce0.025Y2Si6O24

phosphors.

Fig. 3. (a) The excitation and emission spectra, (b) the relative emission intensitiesas a function of Tb3+ content (m), and (c) the CIE diagram and coordinates (inset) ofBa9−3m/2TbmY2Si6O24 (m¼0.005–0.6) phosphors.

S. Lee, S. Park / Journal of Luminescence 143 (2013) 215–218216

dodeca-coordinated Ba(1), non-coordinated Ba(2), deca-coordi-nated Ba(3), hexa-coordinated Sc and tetra-coordinated Si cationsites [11,12]. The silicon tetrahedra do not share oxygen atoms, asshown in Fig. 1. When Sc3+ ions were replaced by larger Y3+ ions, anew single-phase Ba9Y2Si6O24 (a¼9.994(9) Å, c¼22.130(2) Å)structure was materialized as a luminescent host compound inprevious study [15]. The Tb3+ ions (1.235 Å, CN¼9) properlyoccupy the non-fold Ba(2) (1.61 Å) sites. Phase identification wasperformed after the Tb3+ ions were replaced with Ba2+ ions in theBa9Y2Si6O24 host lattice via powder XRD patterns. Fig. 2(a) depictsthe XRD patterns of the calculated Ba9Sc2Si6O24 (ICSD 50736)structure., Single-phase orthosilicate Ba8.25Tb0.5Y2Si6O24 phos-phors were observed when the Tb3+ content reached �7 mol%in the Ba(2) site in Fig. 2(b)–(d). Furthermore, there was noapparent impurity in Ce3+–Tb3+ co-doped Ba8.2125Tb0.5Ce0.025Y2-Si6O24 phosphor as well in Fig. 2(e). In Fig. 2(b)–(d), the XRDpatterns of Ba9−3m/2TbmY2Si6O24 (m¼0.005, 0.05, or 0.5) show noclear shifts in the positions of the various Bragg reflections to

higher angles upon substitution of the unit cells with the smallerTb3+ ions. Accordingly, when the Ba2+ ions were replaced by Tb3+

ions, the Ba2+ defects in the obtained phosphors were determinedby calculating the appropriate stoichiometric amount of Ba9−3m/2

TbmY2Si6O24 phosphors required for charge compensation. How-ever, once the Ba2+ ions in the orthosilicate host lattice werepartially substituted with Tb3+ions, the relative intensity indexed(1 1 −6) gradually increased relative to the (3 0 0) phase.

We observed intense blue, blue-green, and green emissions whenthe Ba9−3m/2TbmY2Si6O24 (m¼0.005−0.6) phosphors were excitedwith 254 nm UV light, as shown in Fig. 3(a). The major blue emissionpeak of the Tb3+-substituted barium–yttrium–orthosilicate phosphorin the range of 360 to 460 nm is at 379 nm and is assigned tothe 5D3–

7F6 transition. Additionally, 5D3–7F5, 5D3–

7F4, and 5D3–7F3

transitions are observed between 400 and 460 nm. The foremost

Fig. 4. (a) The excitation spectra of Ba8.25−3n/2Tb0.5CenY2Si6O24 (n¼0.005–0.05)phosphors and (b) the excitation and emission spectra of Ba8.2125Tb0.5Ce0.025Y2-Si6O24 phosphors.

S. Lee, S. Park / Journal of Luminescence 143 (2013) 215–218 217

green emission peaks of the phosphors in the range of 460–640 nmare at 543 and 551 nm; these are assigned to the 5D4–

7F5 transition.The further transitions monitored include 5D4–

7F6, which appearsbelow 530 nm, and 5D4–

7F4 and 5D4–7F3, which both appear above

560 nm. When the content of Tb3+ ions increased such that m¼0.05in Ba9−3m/2TbmY2Si6O24, the relative intensity of the 5D3-

7FJ transi-tions was maximized. On the other hand, the relative intensity of the5D4-

7FJ transitions noticeably increased up tom¼0.3 in the Ba9−3m/2

TbmY2Si6O24 phosphors. This process is known as cross-relaxation,which depends on an interaction between two proximate Tb3+ ions[1–7]. This concentration-dependent process can be depicted by Tb3+(5D3)+Tb3+(7F6)-Tb3+(5D4)+Tb3+(7F0). The emissions of the ortho-silicate phosphors dynamically changed from blue to blue-green andgreen with increasing Tb3+ concentration; this phenomenon issimilar to that of other Tb3+-doped materials [1–7]. Fig. 3(b) showsthe change in the relative emission intensities (i.e., I377 nm andI542 nm) of the 5D3–

7F6 and 5D4–7F5 transitions with Tb3+ content

(m) in Ba9−3m/2TbmY2Si6O24 with excitation at 377 and 542 nm. Themaximum intensities of the 5D3–

7F6 and 5D4–7F5 transitions appear at

concentrations of �0.6 and 3 mol%, respectively. Strong excitationpeaks also appear between 250 and 300 nm in the Ba9−3m/2TbmY2-Si

6O24 PL spectra; these peaks are assigned to the 4f8–4f75d1 transi-

tions of Tb3+. As shown in Fig. 3(c), the x and y chromaticitycoordinates are in accordance with the CIE values of the orthosilicatephosphors, where Ba9−3m/2TbmY2Si6O24 with m¼0.005 and 0.01,m¼0.05–0.2, and m¼0.3–0.6 emitting near 254 nm are blue, blue-green, and green emitters, respectively. The CIE values are summar-ized in Table (inset) along with those of the Tb3+-doped orthosilicatehost in Fig. 3(c). The CIE coordinates are x¼0.188 and y¼0.151 forBa8.9925Tb0.005Y2Si6O24, x¼0.257 and y¼0.437 for Ba8.85Tb0.1Y2Si6O24,and x¼0.296 and y¼0.597 for Ba8.25Tb0.5Y2Si6O24, which are nearthe blue, blue-green, and green regions of the CIE diagram, respec-tively. Therefore, when the Tb3+ content reaches 3 mol%, it is worthinvestigating the CIE coordinates of Ba9−3m/2TbmY2Si6O24, which shiftquite significantly from the blue to green region.

Fig. 4(a) depicts the excitation spectra of co-doped Ba8.25−3n/2Tb0.5CenY2Si6O24 phosphors as a function of the Ce3+-concentration(n¼0.005–0.05). The energy transfer from Ce3+ to Tb3+ is a well-known process that has been reported for Ba2Ga2Si4O13:Ce3+,Tb3+ andmany other materials [7–9]. The energy transfer process occurs fromthe 5d state of the Ce3+ ions to the 5D3 level of the Tb3+ ions followedby nonradiative relaxation to the 5D4 level. This indicates that theelectrons at the 5d level that were excited from the 2FJ transition of theCe3+ ions can be effectively transferred to the 5D3 level of the Tb3+ ionsdue to their similar energy levels. When the Ce3+ content reachedn¼0.025 (0.3 mol%) in the Ba8.25−3n/2Tb0.5CenY2Si6O24 phosphors, themaximum 5d transition of Ce3+ ions excited at around 340 nm wasobserved. Fig. 4(b) shows the excitation and emission spectra of Tb3+-and Ce3+-co-doped orthosilicate Ba8.2125Tb0.5Ce0.025Y2Si6O24 phos-phors. There are two main excitation peaks in the excitation spectrum(EXEM¼543 nm) around 256 and 336 nm, which are caused by the4f–4f5d transitions of the Tb3+ and Ce3+ ions under 543 nm emissionlight, respectively. The blue-green emissions (EMEX¼304 nm andEMEX¼336 nm) of the co-doped Ba8.2125Tb0.5Ce0.025Y2Si6O24 phosphorsin the PL spectra under 304 and 336 nm excitation reveal a broademission band of the 5d-4f transition of the Ce3+ ion around 420 nmand intense f–f transitions of the Tb3+ ions that are primarily caused byenergy transfer from Ce3+ to Tb3+ in the broad range of 360–640 nm.Furthermore, the relatively broad emission peaks of the 5D4–

7F5transitions in the orthosilicate phosphors were monitored. Under256 nm excitation, sharp and prominent green emission peaks appearin the PL spectrum of the co-doped orthosilicate phosphors; thesepeaks correspond to the 5D3–

7FJ and 5D4-7FJ transitions of the Tb3+

ions. When Tb3+ and Ce3+ are co-doped into orthosilicates, a con-centrated greenish emission is evident near the UV region. The CIEcoordinates of x¼0.291 and y¼0.586 (EMEX¼256 nm), x¼0.257 and

y¼0.418 (EMEX¼304 nm), and x¼0.268 and y¼0.505 (EMEX¼336 nm) ofthe Ba8.2125Tb0.5Ce0.025Y2Si6O24 phosphors are near the green andblue-green regions of the CIE diagram.

4. Conclusions

Tb3+-substituted Ba9−3m/2TbmY2Si6O24 (m¼0.005–0.6) orthosi-licate phosphors were prepared in air at 1100 1C with 2.5 wt%Li2CO3 flux. When the Tb3+ content reached about 0.6 and 3 mol%,the 5D3-

7FJ and 5D4-7FJ transitions, respectively, of the Tb3+ ions

were maximized due to the cross-relaxation process. The CIEcoordinates of the Tb3+-doped orthosilicate phosphors werelocated in the blue, blue-green, and green regions. Tb3+ and Ce3+

co-doped orthosilicates show effective energy transfer from Ce3+

to Tb3+. When 0.3 mol% Ba2+ ions were replaced by Ce3+ in theTb3+-doped orthosilicate phosphors, the excitation intensity of the5d level of Ce3+ was enhanced and a concentrated green emissionresulted. The intense green emission of the Tb3+ ions due toenergy transfer from Ce3+ ions excited near the NUV region will beuseful for applications that require white-light NUV-LEDs.

Acknowledgments

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) funded bythe Ministry of Education, Science and Technology (2011-0010756).

S. Lee, S. Park / Journal of Luminescence 143 (2013) 215–218218

References

[1] A. Lakshmananá, Luminescence and Display Phosphors: Phenomena andApplications, Nova Science Publishers Inc., New York, 2008.

[2] W.M. Yen, S. Shionoya, Phosphor Handbook, CRC Press, Boca Raton, 1999.[3] D.J. Robbins, B. Cockayne, B. Lent, J.L. Glasper, Solid State Commun. 20 (1976)

673.[4] W.F. van der Weg, Th.J.A. Popma, A.T. Vink, J. Appl. Phys. 57 (1985) 5450.[5] Z. Hao, J. Zhang, X. Zhang, S. Lu, X. Wang, J. Electrochem. Soc. 156 (2009) H193.[6] A. Podhorodecki, M. Banski, J. Misiewicz, J. Serafinczuk, N.V. Gaponenko,

J. Electrochem. Soc. 157 (2010) H628.

[7] S. Park, T. Vogt, J. Lumin. 129 (2009) 952.[8] H. Guo, H. Zhang, J.J. Li, F. Li, Opt. Express 18 (2010) 27257.[9] K.Y. Jung, H.W. Lee, J. Lumin. 126 (2007) 469.[10] H. You, X. Wu, H. Cui, G. Hong, J. Lumin. 104 (2003) 223.[11] L.H. Wang, L.F. Schneemeyer, R.J. Cava, T. Siegrist, J. Solid State Chem. 113

(1994) 211.[12] T. Nakano, Y. Kawakami, K. Uematsu, T. Ishigaki, K. Toda, M Sato, J. Lumin. 129

(2009) 1654.[13] Y. Shimizu et. al. Patent US 5998925K. Ota et. al. Patent US 6943380.[14] S. Ye, F. Xiao, Y.X. Ma, Q.Y. Zhang, Mater. Sci. Eng. R 71 (2010) 1.[15] Y. Kim and S. Park, submitted for publication.