Ce3þ–Mn2þ cooperative Ba9Y2Si6O24 orthosilicate phosphors

Sangmoon Park n

Center for Green Fusion Technology and 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 9 April 2014Accepted 19 July 2014Available online 1 August 2014

Keywords:OrthosilicateX-ray diffractionphosphorsPhotoluminescenceEnergy transfer

a b s t r a c t

Ce3þ–Mn2þ cooperative barium–yttrium–orthosilicate phosphors composed of Ba9�3m/2�nCemMnnY2Si6O24

(m¼0.005–0.4, n¼0–0.5) were prepared using a solid-state reaction. The X-ray diffraction patterns of theresultant phosphors were examined to index the peak positions. The photoluminescence (PL) excitation andemission spectra of the Ce3þ-activated phosphors were clearly monitored; furthermore, critical emissionquenching as a function of Ce3þ content in the Ba9�3m/2CemY2Si6O24 was observed at relatively lowconcentrations of the activator. After doping the host structure with Ce3þ and Mn2þ emitters, intense blueand red emission lights observed in the PL spectra under near-ultraviolet (NUV) excitation were monitored.The dependence of the luminescent intensity of the Mn2þ co-doped (n¼0, 0.1, 0.3, and 0.5) host lattices onCe3þ content (m¼0.025, 0.1, and 0.2) was also investigated. Co-doping Mn2þ into the Ce3þ-doped hoststructure enabled high energy transfer from Ce3þ to Mn2þ; this energy transfer mechanism is discussed.Using these phosphors, the desired CIE values including emissions throughout the blue and red regions ofthe spectra were achieved.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Since the fabrication of the first white light-emitting diodes(LEDs) based on a blue LED chip combined with a Y3Al5O12:Cephosphor in 1997, phosphor-converted white LEDs have becomeimportant sources of light, especially in applications such asillumination lamps, automobiles, imaging, agriculture, and medi-cine [1–3]. However, in generating warm white-light stimulatedby the blue LED chip, a paucity of red light in the spectral emissionof yellow-emitting YAG:Ce phosphors was observed [3,4]. There-fore, additional red phosphors are often required to improve colorrender indices. For this reason, red phosphors containing yellow-emitting phosphors such as Sr(Ca)S:Eu or quantum dots based onCdSe or CuInS2/ZnS in combination with a blue LED chip wereoften utilized [4]. Mixing emissions from red, green, and bluephosphors with ultraviolet (UV) LED chips also improved thewhite light in LEDs. Furthermore, single-phase white emittingphosphors pumped either by UV or near-ultraviolet (NUV) excita-tions were intensively studied owing to their excellent Ra valuesand color stability. Single-phase and color-tunable phosphors canbe prepared by harmonizing the concentration of two or morerare-earth activators via an energy-transfer mechanism. Thisenergy-transfer phenomenon consists of electrostatic and quan-tum mechanical exchange interactions. The electrostatic energytransfer appears to be a multi-pole interaction between the donor

and acceptor; on the other hand, the exchange energy transferoccurs when their wave functions overlap because of the proxi-mity (3–4 Å) between the donor and acceptor [5]. Studies oncolor-tunable and single-phase phosphors involving energy trans-fer between the donor and acceptor ions such as Eu2þ–Mn2þ orCe3þ–Mn2þ were performed in various hosts. A combination ofNUV LED chips with blue–green–red-emitting single-phase phos-phors such as KCaY(PO4)2:Eu2þ ,Mn2þ or Ca3Sc2Si3O12:Ce3þ ,Mn2þ can demonstrate white LEDs well [6–8]. A unique layeredorthosilicate Ba9Y2Si6O24, having a trigonal phase with an R-3Hspace group and containing a large Ba2þ as well as an octahedralY3þ and a tetrahedral Si4þ species, has been studied previously[9–11]. Moreover, either the Eu2þ–Mn2þ or Ce3þ–Tb3þ-dopedBa9Y2Si6O24 phosphors acting as efficient luminescent materials,and activated in the NUV region, have already been elucidatedpreviously. In this work, near-ultraviolet (NUV)-excitable andbarium–yttrium–silicate based optical materials composed ofBa9�3m/2�nCemMnnY2Si6O24 (m¼0.005–0.4, n¼0–0.5) were pre-pared and their X-ray diffraction patterns were characterized. Thephotoluminescence (PL) spectra, which exhibited an efficient blueemission assigned to the f–d transitions of a Ce3þ emitter in theorthosilicates, were analyzed. Moreover, the PL spectra of Ce3þ

and Mn2þ co-doped orthosilicate phosphors under NUV excitationwere monitored. The dependence of the luminescent intensity andenergy-transfer mechanism of the Mn2þ co-doped (n¼0.1–0.5)host lattices on Ce3þ content (m¼0.1, 0.3, 0.5) were also studied.Using these phosphors, the desired CIE values including emissionlights throughout the blue and red regions of the spectra wereachieved.

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.07.1340167-577X/& 2014 Elsevier B.V. All rights reserved.

n Tel.: þ82 51 999 5891; fax: þ82 51 999 5335.E-mail address: spark@silla.ac.kr

Materials Letters 135 (2014) 59–62

2. Experimental

Optical materials of Ba9�3m/2�nCemMnnY2Si6O24 (m¼0.005–0.4, n¼0–0.5) were prepared by heating the appropriate stoichio-metric amounts of BaCO3 (Alfa 99.8%), Y2O3 (Alfa 99.9%), SiO2 (Alfa99.5%), CeO2 (Alfa 99.9%), and MnO (Aldrich 99%) at 950 and1100 1C for 3 h, respectively, under the atmosphere using 4% H2/96% Ar. In each sample, an excess 2.5 wt% Li2CO3 (Alfa 99%) fluxwas added to generate rare-earth ion doped orthosilicate Ba9Y2-Si6O24 compounds. Phase identification was established using aShimadzu XRD-6000 powder diffractometer (Cu-Kα radiation). UVspectroscopy to measure the excitation and emission spectra ofthe optical materials was done using spectrofluorometers (SincoFluoromate FS-2).

3. Results and discussion

Fig. 1(a) depicts the X-ray diffraction (XRD) patterns of theBa9Y2Si6O24 (a¼9.994(9) Å, c¼22.130(2) Å) structure, which featuresa trigonal phase with an R-3H space group [9–12]. Ba9Y2Si6O24

orthosilicate host lattice comprises tetrahedral SiO4 and octahedralYO6 moieties arranged along the c-axis, with dodeca-coordinated Ba(1), nona-coordinated Ba(2), and deca-coordinated Ba(3) cations.Fig. 1(b)–(d) shows the XRD patterns of Ba9�3m/2�nCemMnnY2Si6O24

(m¼0.1; m¼0.4; m¼0.2, n¼0.5) orthosilicates. Phase identificationwas performed by studying the powder X-ray diffraction patternafter the Ce3þ and Mn2þ ions were replaced with Ba2þ ions in theBa9Y2Si6O24 host lattice. When the Ba2þ ions were substituted withsmaller Ce3þ ions, the resulting XRD patterns of Ba9�3m/2CemY2-Si

6O24 (m¼0.1, 0.4) showed indistinct shifts in the positions of the

various Bragg reflections at higher angles, as shown in Fig. 1(b) and(c). As reported before, when the Ba2þ ions were replaced by theCe3þ ions, the defect of barium ions (Ba2þm/2) in the obtainedphosphors were determined by calculating the appropriate stoichio-metric amount of Ba9�3m/2CemY2Si6O24 phosphors needed forcharge compensation [9]. The XRD pattern of Ce3þ , Mn2þ co-doped Ba9�3m/2�nCemMnnY2Si6O24 (m¼0.2, n¼0.5) as shown inFig. 1(d) presents clear deviations from the patterns of the Ce3þ

doped host lattices in peak positions of the reflections at higherangles owing to the addition of Mn2þ ions.

Fig. 2(a) shows the PL spectra of Ba9�3m/2CemY2Si6O24 (m¼0.005–0.4) phosphors that exhibit intense blue emission. There are two mainexcitation peaks around 290 and 320 nm due to the crystal fieldsplitting of the 5d levels of Ce3þ transitions. The maximum intensitypoints of the emission bands were monitored from 400 nm to 415 nmalong with an increase in the molar concentrations of Ce3þ . When theCe3þ content reached m¼0.1 (�1.1 mol%) in Ba9�3m/2CemY2Si6O24

phosphors, the relative luminescent intensity calculated using theintegrated emission was maximized. Once the maximum luminescentintensity reached the Ce3þ content corresponding to m¼0.1, a furtherincrease in the Ce3þ content in the orthosilicate phosphors led to aclear quenching of the relative intensity (I410 nm) of the blue emissionin Fig. 2(b). As the concentration of Ce3þ ions increased, the distancebetween the activators became smaller with the increasing energytransfer. A decrease in the emission intensity resulted in nonradiativeenergy transfer between the activators resulting from the electricdipole–dipole interaction. The critical distance (Rc) was calculatedusing the following formula:.

Rc ¼ 2½3V=4πxcN�1=3 ð1Þwhere V is unit cell volume, N is the number of available sites for thedopant in the unit cell, xc is the critical concentration of Ce3þ , and Rc isthe critical distance for energy transfer [13,14]. For the Ba9Y2Si6O24

host, N¼11 and V¼1914.1 Å3 [9]. For Ba9�3m/2CemY2Si6O24, Rc wasdetermined to be 36.5, 23.0, 18.3, 16.0, and 14.5 Å for m¼0.025, 0.1,0.2, 0.3, and 0.4, respectively.

Fig. 3(a) shows the PL spectra of Ba8.85Ce0.1Y2Si6O24 and Ba8.5M-n0.5Y2Si6O24 phosphors. The intensity of the Mn2þ-substituted phos-phor, which was multiplied by a factor of 10 as shown, was quite lowcompared to the intensity of the blue Ba8.85Ce0.1Y2Si6O24 phosphor[10]. The luminescent excitation centers of Mn2þ ions were assignedas 4E(D), 4T2(D), 4A1(4G)/4E(4G), and 4T1(4G) between 300 and 450 nm,and their emissions were due to the d–d transition [4T1(6G)-6A1(6S)]around 630 nm, which was consistent with previous reports [10,15]. Itis known that the corresponding electric dipole transition is forbidden;hence, the Mn2þ d–d transition was difficult to observe. However, thered emission corresponding to Mn2þ transitions in host lattices couldbe intensified by co-doping a Ce3þ sensitizer through energy transfer

Fig. 1. XRD patterns of (a) Ba9Y2Si6O24 (a¼9.994(9) Å, c¼22.130(2) Å), (b)–(d) Ba9�3m/2�nCemMnnY2Si6O24 (m¼0.1; m¼0.4; m¼0.2, n¼0.5) orthosilicatestructure.

Fig. 2. (a) The excitation and emission spectra of Ba9�3m/2CemY2Si6O24 (m¼0.005–0.4)phosphors and (b) the changes in the relative intensity (I410 nm) of blue PL emission inthe orthosilicate phosphors.

S. Park / Materials Letters 135 (2014) 59–6260

from Ce3þ to Mn2þ ions [7,16]. Fig. 3(b) shows the PL emissionspectra and relative intensity of the Ba9�3m/2�nCemMnnY2Si6O24

(m¼0.025, 0.1, 0.2, n¼0, 0.1, 0.3, and 0.5) phosphors. Co-dopingMn2þ into the Ce3þ-doped host structure enabled a high energytransfer from Ce3þ to Mn2þ . The Ce3þ blue emission ofBa8.9625Ce0.025MnnY2Si6O24 phosphor centered around 400 nm dis-tinctly decreased when the Mn2þ content corresponded to n¼0.5,whereas a steady increase in the Mn2þ red emission ofBa8.9625Ce0.025MnnY2Si6O24 phosphors was observed up to n¼0.5(Mn2þ content). In Fig. 3(c), analogous to the Ba8.9625Ce0.025MnnY2-Si6O24 phosphor, the Ce3þ luminescent intensities of Ba9�3m/

2�nCemMnnY2Si6O24 phosphors for Ce3þ content corresponding tom¼0.1 increased as the Mn2þ content increased up to n¼0.5.

However, the maximum Mn2þ red emission of Ba8.7�nCe0.2MnnY2-Si6O24 phosphor was observed when the Mn2þ content corre-sponded to n¼0.3. Any further increase in the Mn2þ content of thephosphors led to an apparent quenching of the relative intensity(I610 nm) of the red emission. In this Ba9�3m/2�nCemMnnY2Si6O24

host structure, a transfer of energy from Ce3þ to Mn2þ occurredthrough the absorption of Ce3þ . Moreover, the Ce3þ and Mn2þ ionsplayed the roles of sensitizer and activator, respectively.

The energy transfer efficiency (ηT) between the Ce3þ and Mn2þ

ions was calculated using the formula:

ηT ¼ 1–IS=ISO ð2Þ

Fig. 3. (a) The PL spectra of Ba8.85Ce0.1Y2Si6O24 and Ba8.5Mn0.5Y2Si6O24 phosphors,(b) the PL emission spectra and relative intensity of the Ba9�3m/2-nCemMnnY2Si6O24

(m¼0.025, 0.1, 0.2, n¼0, 0.1, 0.3, and 0.5) phosphors, and (c) the changes in therelative intensity (I610 nm) versus the content of co-doping Mn2þ ions in the Ce3þ-doped orthosilicate phosphors.

Fig. 4. (a) The energy transfer efficiency (ηT) from Ce3þ to Mn2þ in Ba9�3m/2-

nCemMnnY2Si6O24 phosphors and (b) and (c) the plot of IS/ISO versus CMnα/3 (α¼6, 8).

S. Park / Materials Letters 135 (2014) 59–62 61

where IS and ISO are the luminescence intensities of the sensitizerin the presence and absence of an activator, respectively [17,18].Using this expression, the energy transfer efficiency from Ce3þ toMn2þ was calculated as shown in Fig. 4(a). As the Mn2þ content inBa9�3m/2�nCemMnnY2Si6O24 phosphors (m¼0.025, 0.2) increasedfrom n¼0.1 to 0.3, the efficiency at each level was extensivelyenhanced by up to 80%. When the Mn2þ content in the orthosi-licate phosphors increased to n¼0.5, the efficiency of the energytransfer further increased to 90%. By keeping the Ce3þ concentra-tion corresponding to m¼0.1 in the Ba9�3m/2�nCemMnnY2Si6O24

phosphors, n was varied from 0.1 to 0.5; consequently, theefficiency of energy transfer to Mn2þ ions gradually increased,and became higher compared to those of the other energytransfers. For the Ce3þ concentration of m¼0.1 in Ba9�3m/

2�nCemMnnY2Si6O24 phosphors, the critical concentration wasabout 0.35 from the total concentration of the Ce3þ and Mn2þ

ions at which the energy-transfer efficiency is 0.5. From Eq. (1), thecritical distance Rc of the Ba9�3m/2�nCemMnnY2Si6O24 phosphor(m¼0.1, n¼0.25) was calculated to be about 15.2 Å. This energytransfer between Ce3þ and Mn2þ ions could be caused by anelectric multipolar interaction. This is because the critical distance,15.2 Å, is quite longer than 3–4 Å, indicating an energy transferthrough the exchange interaction mechanism. Furthermore,according to the Dexter theory, the energy transfer mechanismcan be expressed by the linear plots of IS/ISO versus CMn

α/3, where CMn

is concentration of Mn2þ ions, with α¼6, 8, or 10, whichcorresponds to dipole–dipole, dipole–quadrupole, and quadru-pole–quadrupole interactions, respectively. In Fig. 4(b) and (c),the linear plots show that the energy transfer from the Ce3þ toMn2þ ions in the case of Ba9�3m/2�nCemMnnY2Si6O24 phosphors(R2¼0.9951, m¼0.025 and R2¼0.9998, m¼0.2) is caused by anonradiative dipole–dipole (α¼ 6) interaction. Moreover, becausethe values of α that are determined from the linear plots forBa9�3m/2�nCemMnnY2Si6O24 phosphors (R2¼0.9968, m¼0.1) areclose to 8, it appears that the dipole–quadrupole interaction isinvolved in the energy transfer mechanism [10,17,18]. A color-tunable emission from blue to red owing to the presence of Ce3þ

to Mn2þ as a sensitizer and activator, respectively, was observed inthe Ba9Y2Si6O24 host lattice. As shown in Fig. 5, the chromaticitycoordinates, x and y, were in accordance with the CIE values of theblue, pink, and red orthosilicate phosphors, corresponding to Ba9�3m/

2�nCemMnnY2Si6O24 with m¼0.1, n¼0–0.5. The photographs of thePL light emission from blue to red in Ba8.85�nCe0.1MnnY2Si6O24

(n¼0.1–0.5) phosphors under 312 nm handheld lamps, respectively,are shown as insets of Fig. 5. The CIE values are summarized in theinsets of Fig. 5, along with the values obtained for the Ce3þ–Mn2þ

co-doped orthosilicate optical materials. The CIE coordinates near theblue and red regions of the CIE diagramwere observed to be x¼0.155and y¼0.067, x¼0.497 and y¼0.273 for m¼0.1 (A), m¼0.1, n¼0.1(D) in the Ba9�m�nEumMnnY2Si6O24 phosphors, respectively.

4. Conclusions

A single phase of Ce3þ , Mn2þ co-doped Ba9Y2Si6O24 phosphorswas successfully prepared using Li2CO3 flux below 1100 1C in areducing environment. By doping a Ce3þ emitter in the barium–

yttrium–orthosilicate host lattice, efficient blue emission wasachieved under NUV excitation. The maximum luminescent intensityof Ba9�3m/2CemY2Si6O24 phosphors was achieved when the Ce3þ

content corresponded to m¼0.1. The emission of Mn2þ d–d transi-tions in the orthosilicate host lattices was enhanced by co-doping aCe3þ sensitizer through energy transfer from Ce3þ to Mn2þ and theresulting PL spectra were monitored. As the Ce3þ content (m¼0.2) inBa9�(3m/2)�nCemMnnY2Si6O24 phosphors increased, a noticeablequenching of the relative intensity of the Mn2þ (n¼0.5) red emissionoccurred. The mechanism of the energy transfer from Ce3þ to Mn2þ

in Ba9Y2Si6O24 phosphors was expressed by the linear plots of IS/ISOof Ce3þ versus CMn

α/3. The desired CIE values including emissions in theBa9�3m/2�nCemMnnY2Si6O24 (m¼0.1, n¼0–0.5) phosphors through-out the blue and red regions of the spectra were achieved. The co-doping of Ce3þ–Mn2þ in the single-phase orthosilicate structureresulted in the emission of white light, which would be effective forapplications requiring NUV executable white LEDs.

Acknowledgments

This research was supported by KIST institutional funding(Project No. 2E23891).

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Fig. 5. The chromaticity coordinates with the CIE values of Ba9�3m/2�nCemMnnY2Si6O24

(m¼0.1, n¼0–0.5) phosphors and photographs of the PL light emission from blue, pink,and red in the orthosilicate phosphors under 312 nm handheld lamps (inset).

S. Park / Materials Letters 135 (2014) 59–6262