Journal ofMaterials Chemistry C

PAPER

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aSchool of Materials Science and Technology

100083, P. R. China. E-mail: xiazg@cugb.edbDepartment of Chemistry, Tsinghua Univer

Cite this: J. Mater. Chem. C, 2013, 1,7139

Received 22nd July 2013Accepted 7th September 2013

DOI: 10.1039/c3tc31423e

www.rsc.org/MaterialsC

This journal is ª The Royal Society of

Structure and luminescence properties of green-emitting NaBaScSi2O7:Eu

2+ phosphors for near-UV-pumped light emitting diodes

Chengying Liu,a Zhiguo Xia,*a Zhipeng Lian,b Jun Zhoua and Qingfeng Yanb

Green-emitting phosphor Eu2+ doped NaBaScSi2O7 was synthesized by a solid-state reaction, and the

photoluminescence properties were investigated in conjunction with a structural analysis. The

crystallographic occupancy of Eu2+ in the Sc silicate NaBaScSi2O7 matrix was studied based on the

Rietveld refinements results and the crystal chemistry rules. The optimum concentration of Eu2+ in the

NaBaScSi2O7 phosphor was about 10 mol%, and the concentration quenching mechanism was verified

to be the dipole–dipole interaction. Upon excitation at 365 nm, the composition-optimized

NaBaScSi2O7:Eu2+ exhibited strong green light peaking at 501 nm with the CIE chromaticity (0.0706,

0.5540) and a high internal quantum efficiency of about 65%. The thermally stable luminescence

properties were also studied and compared with those of the commercial green phosphors. A white

light emitting diode (w-LED) lamp was finally fabricated by using the present green phosphor and the

commercial blue and red phosphors, which exhibited a high color rendering index (Ra) of 86.5 at a

correlated color temperature of 2528 K with CIE coordinates of x ¼ 0.353, y ¼ 0.324. These results

suggest that NaBaScSi2O7:Eu2+ is a potential green phosphor candidate for near-UV-pumped w-LEDs.

1. Introduction

White light emitting diodes (w-LEDs) are hoped to be thefourth-generation lighting source owing to their promisingfeatures such as high brightness, low power consumption,compact lamp structure, and so on.1,2 At present, a combinationof a near ultraviolet (n-UV) LED chip (350–420 nm) with red,green, and blue-emitting phosphors has become the hot issueto realize the practical white light emission.3 There have beenmany reports describing the development of the tri-coloremission phosphors for n-UV LED chips, and green-emittingphosphors possess a large proportion in it.4 The presentcommercial green phosphor host compounds for w-LEDs canbe mainly classied as nitrides/oxynitrides represented bySiAlON:Eu2+ and Si2Si2O2N2:Eu

2+, and silicates represented byCa3Sc2Si3O12:Ce

3+ and Ba2SiO4:Eu2+.5,6 However, there are some

obvious disadvantages in the use of presently used phosphors.A well-known defect is that the synthesis of nitride phosphorsneeds a critical condition, such as high temperature and highpressure of N2.7 Some reported silicate phosphors generallyhave lower thermal stability. Therefore, it is essential to explorenew green-emitting phosphors for n-UV pumped LEDs.

Since the number of the existing silicate compounds in theinorganic phase database is large, great attention is also paid to

, China University of Geosciences, Beijing

u.cn; Tel: +86-10-82332247

sity, Beijing 100084, China

Chemistry 2013

the green-emitting silicate phosphors for the application in n-UV pumped LEDs, and some luminescence properties can beexpected to be optimized.8 Therefore, many Eu2+ and Ce3+

doped silicate-based green phosphors can be found from thepresent references, such as Ca3Sc2Si3O12:Ce

3+,6 Ba2SiO4:Eu2+,9

Li2CaSiO4:Eu2+,10 BaHfSi3O9:Eu

2+,11 Ba5SiO4(F,Cl):Eu2+.12

Among them, scandium (Sc) silicates comprise the frameworkstructures made of octahedra and tetrahedra, and the alkali andalkaline-earth atoms occupy voids in the resulting three-dimensional frameworks.13 As we can nd from the excellentphotoluminescence properties of the commercial Ca3Sc2-Si3O12:Ce

3+ phosphor, it is believed that such a group of Scsilicates can act as the suitable and efficient phosphor hosts forLED application.8 This is ascribed to that Sc silicates comprisethe rich crystal chemistry environment for the doped rare earthions and the stable physical and chemical properties comingfrom the rigid synthesis conditions in the Sc-enrichedenvironments.13

A new Sc silicate phosphor, NaBaScSi2O7:Eu2+, was rst

proposed by Ray in 2012,14 however, in this paper, the structureand luminescence properties, especially the crystallographicoccupancy of Eu2+ in the NaBaScSi2O7 matrix, as well as theapplication properties of the NaBaScSi2O7:Eu

2+ phosphor forLEDs were studied in detail. The NaBaScSi2O7 compound in theform of a single crystal was reported in 2010, which has amonoclinic system with a space-group symmetry of P21/m.13 Ourpresent studies have paid more attention to the coordinationenvironment of cations, Na+, Ba2+ and Sc3+ in NaBaScSi2O7, and

J. Mater. Chem. C, 2013, 1, 7139–7147 | 7139

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the cation sites occupied by Eu2+ have been analyzed by theRietveld analysis and the crystal chemistry rules. The relation-ship between the crystal structure and the luminescence prop-erties of Eu2+ in this NaBaScSi2O7 host was also investigated indetail, and the high luminescence efficiency and high thermalquenching temperature have been determined. A w-LED lampwas nally fabricated by using the present phosphor, whichveried that green-emitting NaBaScSi2O7:Eu

2+ could act as anexcellent wavelength-conversion phosphor for use in w-LEDs.

2. Experimental section2.1 Synthesis

NaBaScSi2O7:Eu2+ phosphors were synthesized by a high

temperature solid-state reaction. The starting materials wereNa2CO3 (99.95%), BaCO3 (99.95%), Sc2O3 (99.995%), SiO2

(99.95%) and Eu2O3 (99.995%), which were used directlywithout any further treatment. The raw powder materials wereweighed stoichiometrically and the above powder reactantswere mixed and ground thoroughly in an agatemortar. Then themixed samples were sintered at 1250 �C for 5 h in a CO reducingatmosphere, followed by an additional grinding. Aer this, thesamples were furnace-cooled to room temperature, and thegreen-emitting phosphors were nally obtained.

Fig. 1 Crystal structure of the NaBaScSi2O7 compound showing the existingpolyhedron and coordination environment of the Na+, Ba2+ and Sc3+ cations (a),and the corresponding coordination surroundings around the Si2O7 dimer units(b).

2.2 Characterizations

X-ray diffraction (XRD) patterns were collected using X-raydiffraction (SHIMADZU, XRD-6000) with Cu Ka radiation (l ¼0.15406 nm) at 40 kV, 30 mA. The powder diffraction data wereanalyzed using a computer soware General Structure AnalysisSystem (GSAS) package.15 The diffuse reection spectrum wasrecorded using a UV-Vis-NIR spectrophotometer (SHIMADZUUV-3600) using the white BaSO4 powder as a reference standard.Photoluminescence excitation (PLE) and emission (PL) spectrawere recorded using a uorescence spectrophotometer (F-4600,HITACHI, Japan) equipped with a photomultiplier tube oper-ating at 400 V, and a 150 W Xe lamp was used as the excitationlamp. The decay curves were recorded on a spectrouorometer(HORIBA, JOBIN YVON FL3-21), and the 370 nm pulse laserradiation (nano-LED) was used as the excitation source. Thetemperature-dependent luminescence properties weremeasured on an F-4600 spectrophotometer, which was equip-ped with a computer-controlled electric furnace. The quantumefficiency (QE) was measured using the integrating sphere onthe FLS920 uorescence spectrophotometer (EdinburghInstruments Ltd., UK), and a Xe900 lamp was used as an exci-tation source and white BaSO4 powder as a reference. Thesignals were collected using a Hamamatsu R928P photo-multiplier tube. A w-LED lamp was nally fabricated by using anear-UV LED chip (lmax ¼ 395 nm) with green-emittingNaBaScSi2O7:Eu

2+ phosphors and the commercial blue and redphosphors. Optical properties such as luminescence spectra,color-rendering index (Ra), and Commission International deI'Eclairage (CIE) color coordinates for samples were recordedusing a HAAS-2000 (Everne, China) light and radiationmeasuring instrument.

7140 | J. Mater. Chem. C, 2013, 1, 7139–7147

3. Results and discussion3.1 Crystal structure and the cation site analysis

Fig. 1 presents the crystal structure emphasizing the coordina-tion of Sc3+, Ba2+ and Na+ ions. In NaBaScSi2O7, Na

2+ and Ba2+

cations are located in voids in a Na–Ba–Na–Ba sequence parallelto the c axis, which also consists of the alternating layers of[ScO6] octahedra and [SiO4] tetrahedra.13 The versatile structureof NaBaScSi2O7 offers three types of sites for the occupancy ofvarious cations, type I with trivalent Sc sites in ScO6 polyhedra;type II with divalent Ba sites in BaO9 polyhedra and type III withmonovalent Na sites having 8-fold coordination. In this struc-ture, the existing different types of sites are available for theoccupancy of guest cations with different charges. In thepresent work, the ionic radii of Sc3+, Ba2+ and Na+ are 0.74, 1.47and 1.18 A, and the ionic radii for the six-, eight- and nine-coordinated Eu2+ are 1.17, 1.25 and 1.30 A.16 Therefore, basedon the comparison of the effective ionic radii of cations withdifferent coordination numbers, it could offer the possibleopportunity for Eu2+ doping ions to enter into the Sc3+, Ba2+ andNa+ ion sites, and the detailed analysis of the crystallographicoccupancy is given below.

This journal is ª The Royal Society of Chemistry 2013

Fig. 2 Experimental (crosses) and calculated (red solid line) XRD patterns andtheir difference (blue solid line) for the Rietveld fits of NaBaScSi2O7:0.03Eu

2+ (a)and Na0.97Eu0.03BaScSi2O7 (b) by the GSAS program. The short vertical lines showthe position of Bragg reflections of the calculated patterns.

Table 2 Interatomic distances of Sc–O and Na–O in the NaBaScSi2O7 Sample

Vector Length (A) Vector Length (A)

Sc–O1(�2) 2.052 Na–O1(�2) 2.906Sc–O2 2.196 Na–O3 2.518Sc–O4 2.039 Na–O4 2.457Sc–O5(�2) 2.139 Na–O4(�2) 2.907Avg (Sc–O) 2.10 Na–O5(�2) 2.672

Avg (Na–O) 2.74

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In order to seek a better understanding of the occupancy ofEu2+ at Sc3+, Ba2+ or Na+ ion sites in the NaBaScSi2O7 system,Rietveld structure renements for NaBaScSi2O7:xEu

2+ (x ¼ 0.03,0.1, and 0.2) were performed based on the GSAS program usingthe powder XRD data. Firstly we wonder what happens to theunit cell volume with the increase of Eu2+ doping concentration.So we ignored Eu atom occupancy in the initial parameter andjust rened the NaBaScSi2O7 framework structure on the basisof the original data of NaBaScSi2O7:xEu

2+ (x¼ 0.03, 0.1, and 0.2)

Table 1 Refinement, crystallographic and structure parameters of the NaBaScSi2O

Sample NaBaScSi2O7 (ICSD-166998) NaBaScSi2

Diffractometer — SHIMADZRenement range —a (A) 6.845 6.8542b (A) 5.626 5.6179c (A) 8.819 8.8155V (A3) 320.47 320.65Rwp (%) — 6.32Rp (%) — 4.19c2 — 3.17

This journal is ª The Royal Society of Chemistry 2013

samples. Fig. 2a shows the Rietveld t example of the XRDpattern of NaBaScSi2O7:0.03Eu

2+, and the nal rened unit cellparameters and residual factors of NaBaScSi2O7:xEu

2+ (x¼ 0.03,0.1, and 0.2) are summarized in Table 1. The unit cell param-eters of the reported NaBaScSi2O7 host were also given as areference. As shown in Table 1, one can see that the unit cellvolume becomes slightly larger with the increase of Eu2+ dopingconcentration, indicating that Eu2+ ions should enter into theSc3+ or Na+ ion sites with a comparatively small radius inthe NaBaScSi2O7 host lattice, and the possibility of Ba2+ sitescan be excluded in the present case although the chargenumber is the same for Ba2+ and Eu2+ ions.

Furthermore, the intermediate bond distance of Eu3+–O is2.31(5) A (ranging from 2.227 A to 2.356 A). The ionic radius ofthe Eu2+ ion is much higher than that of the Eu3+ ion because ofthe additional electron, thus the Eu2+–O bond distance shouldbe more than 2.31 A.17 As listed in Table 2, the ScO6 polyhedronexhibits six Sc–O bond distances from 2.039 to 2.196 A with anaverage value of 2.10 A (less than the Eu2+–O bond distance),which means that Sc3+ ion sites are too small for Eu2+ to occupy.The distances between Na and O are in the range of 2.457–2.907A, and the Na–O average distance is 2.74 A, which suggests thatNaO8 polyhedron provides a reasonable crystallographic latticefor Eu2+ ion incorporation. Therefore we propose that Eu2+ ionsenter into Na+ ion sites. Fig. 2b presents the Rietveld t of theas-prepared Na0.97Eu0.03BaScSi2O7 sample, and the renedpositions of all atoms and the lattice parameters ofNa0.97Eu0.03BaScSi2O7 are listed in Table 3. From the aboverened data, it can be concluded that it is reasonable for theoccupancy of Eu2+ at Na+ sites.

In order to further verify the occupancy of the cation sites,the following eqn (1) has been used to build up the relationshipbetween the coordination environment and emission peaks,which is proposed by Van Uitert and successfully used to

7 host and NaBaScSi2O7:xEu2+ (x ¼ 0.03, 0.1, and 0.2)

O7:0.03Eu2+ NaBaScSi2O7:0.1Eu

2+ NaBaScSi2O7:0.2Eu2+

U, XRD-6000, Cu Ka radiation with l ¼ 0.15406 nm5–130�

6.8624 6.82385.6136 5.60368.8198 8.8238321.13 321.767.87 10.125.86 6.872.40 3.24

J. Mater. Chem. C, 2013, 1, 7139–7147 | 7141

Table 3 The refined positions of all atoms and the lattice parameters ofNa0.97Eu0.03BaScSi2O7

Sample Na0.97Eu0.03BaScSi2O7

Atom x y z Occupancy Uiso

Ba 0.70541(24) 0.75000(0) 0.53641(17) 1.000(0) 0.02156Na 0.2637(12) 0.25000(0) 0.9959(8) 0.970(0) 0.03900Eu 0.2637(12) 0.25000(0) 0.9959(8) 0.030(0) 0.03900Sc 0.6968(6) 0.25000(0) 0.2630(5) 1.000(0) 0.01904Si(1) 0.9421(10) 0.25000(0) 0.6798(7) 1.000(0) 0.03050Si(2) 0.6376(9) 0.25000(0) 0.8454(8) 1.000(0) 0.02724O(1) 0.0820(10) 0.4815(14) 0.6964(8) 1.000(0) 0.03550O(2) 0.7498(17) 0.25000(0) 0.5306(14) 1.000(0) 0.02393O(3) 0.8745(17) 0.25000(0) 0.8520(14) 1.000(0) 0.02938O(4) 0.6408(19) 0.25000(0) 0.0244(16) 1.000(0) 0.05112O(5) 0.5367(12) 0.0093(15) 0.7636(9) 1.000(0) 0.04234

Fig. 3 XRD patterns of the as-prepared NaBaScSi2O7 host, and NaBaSc-Si2O7:0.005Eu

2+ and NaBaScSi2O7:0.1Eu2+ phosphors, and the ICSD card

(166998) of the NaBaScSi2O7 compound is also given as a comparison.

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explain many structure–property relationship studies in Eu2+ orCe3+ doped systems.18

E ¼ Q

�1�

�V

4

� 1V

10�near80

�(1)

where E is the position of the rare-earth ion emission peak(cm�1), Q represents the position in energy for the lower d-bandedge for the free ion (Q ¼ Eu2+, 34 000 cm�1 for Eu2+), V is thevalence of the “active” cation (V¼ 2 for Eu2+), n is the number ofanions in the immediate shell around the “active” cation, and ris the radius of the host cation occupied by the Eu2+ ion (A). ea isthe electron affinity of the atoms that form anions (eV).Considering that the ea value in the present NaBaScSi2O7 host isvery complex, we can hardly obtain the specic ea value tocalculate an accurate emission band position. However, it isreasonable that the ea value should be within an expected rangeof approximately 2.0–2.5 eV considering many reported refer-ences.19,20 According to eqn (1), E is proportional to the productof n and r. The three crystallographically independent Sc3+, Ba2+

and Na+ cation sites in NaBaScSi2O7 provide different crystalelds for the rare-earth ion Eu2+ emission. All three of theseconditions were calculated based on eqn (1) respectively, andthe results are listed in Table 4. According to the data in Table 4,when the Na+ ion is the occupancy site, the emission wavelengthis calculated to be 458–499 nm, which is closest to the measuredvalue (peaking at 501 nm). Compared with the calculatedvalues obtained from Sc3+ sites (603–649 nm) and Ba2+ sites(405–439 nm), it can be concluded that the Na+ ions can act asthe optimal host cations for Eu2+ ion doping, which is alsoconsistent with the analysis of crystallographic occupancybased on the above Rietveld renement results.

Table 4 The calculated results of the Eu2+ emission peak position for Sc3+, Ba2+

and Na+ occupancy cation sites respectively based on eqn (1)

Occupancysite Ea (eV) n r (A)

Ecalculated(cm�1)

lcalculated(nm)

lmeasured

(nm)

Sc3+ 2.0–2.5 6 0.74 15413–16571 649–603501Ba2+ 2.0–2.5 9 1.47 22774–24720 439–405

Na+ 2.0–2.5 8 1.18 20037–21811 499–458

7142 | J. Mater. Chem. C, 2013, 1, 7139–7147

On the basis of the above analysis, Eu2+ ions enter into Na+

ion sites in the NaBaScSi2O7 host, and the nal chemicalcomposition of Na1�xBaScSi2O7:xEu

2+ should be marked.However, we will still write as NaBaScSi2O7:xEu

2+ phosphorshereaer for the convenience of the expression, and the existingvacancy will compensate the charge balance induced by theEu2+–Na+ couples in this system. Accordingly, the phase purityof the as-prepared samples has been checked carefully. Fig. 3shows the XRD patterns of the NaBaScSi2O7 host, andNaBaScSi2O7:0.005Eu

2+ and NaBaScSi2O7:0.1Eu2+ phosphors,

and the ICSD card (166998) of the NaBaScSi2O7 compound isalso given as a comparison. The peak positions and relativeintensities of the observed XRD patterns conrm that allsamples are identied as the single phase by using the reportedstandard XRD patterns, and the structure of the NaBaScSi2O7

host lattice was unchanged upon the doping of Eu2+ ions as alsodiscussed previously by the Rietveld analysis.

3.2 Reectance spectra and photoluminescence properties

The photoluminescence excitation (PLE), photoluminescenceemission (PL) and ultraviolet-visible diffuse reection (UV-Vis)spectra of the selected NaBaScSi2O7:0.1Eu

2+ are shown in Fig. 4.As seen from the reection spectrum, Eu2+ doped NaBaScSi2O7

shows a strong and broad absorption band from 275 to 500 nmin the n-UV and blue region, which was attributed to the 4f /5d transition of Eu2+.21 Fig. 4 further displays the PLE/PL spectraof the NaBaScSi2O7:0.1Eu

2+ phosphor. The PLE spectrum indi-cates that the sample exhibits a broad band from 250–460 nm,which is consistent with the UV-Vis spectrum. The resultsveried that the phosphor can match well with the emission ofthe n-UV chip. Furthermore, the PL spectrum shows that theNaBaScSi2O7:0.1Eu

2+ phosphor exhibits a broad-band greenemission with a peak wavelength at 501 nm and a FWHMspectral width of 55 nm. The inset of Fig. 4 displays the digitalphotograph of the NaBaScSi2O7:0.1Eu

2+ phosphor under thedaylight, the green body color of the as-obtained sample can bealso clearly observed by the naked eye, which corresponds to theabove spectral results. It is believed that the NaBaScSi2O7:Eu

2+

This journal is ª The Royal Society of Chemistry 2013

Fig. 4 UV-Vis Diffuse reflectance, PLE (lem ¼ 501 nm) and PL (lex ¼ 365 nm)spectra of the as-prepared NaBaScSi2O7:0.1Eu

2+ phosphor, and the inset showsthe image of the phosphor in the daylight.

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phosphor has the potential as a promising green-emittingphosphor system for n-UV pumped LEDs.

Fig. 5 show the Eu2+ concentration dependent PL spectra ofNaBaScSi2O7:xEu

2+ phosphors (x ¼ 0.5, 3, 5, 8, 10, 15 and 20 mol%). Under 365 nm light excitation, this series of samples allproduce a strong and broad-band green emission with a peak ataround 501 nm, which corresponds to the 4f65d1 / 4f7 transi-tion of the Eu2+ ion. And the emission spectra have no obviouschanges in the spectra conguration except for the emissionintensity, which veries the previous results on the only one Eu2+

emission centre at Na+ sites. Furthermore, the inset shows theEu2+ content dependent PL intensities, and Eu2+ concentration(x) in NaBaScSi2O7:xEu

2+ reaches a saturation point at x ¼ 0.1.Aer that, the PL intensity begins to decrease with increasingEu2+ concentration due to the concentration quenching effect.

It is accepted that energy quenching can be ascribed to theenergy transfer between Eu2+ ions followed by energy transfer totraps or quenching sites. Therefore, in order to further conrm

Fig. 5 Eu2+ concentration dependent PL spectra of NaBaScSi2O7:xEu2+ phos-

phors, and the inset shows the corresponding dependence of PL intensity.

This journal is ª The Royal Society of Chemistry 2013

the process of energy transfer between Eu2+ ions in theNaBaScSi2O7:Eu

2+ phosphor, the interaction type betweensensitizers or between the sensitizer and activator can becalculated by the following equation:22

I

x¼ K

�1þ bðxÞ

q3

��1

(2)

where K and b are constants for each type of interaction for agiven host lattice; q is an indication of the electric multipolarcharacter. At the moment, q¼ 6, 8, 10 for dipole–dipole, dipole–quadrupole, quadrupole–quadrupole interactions, respectively.In this equation x is the activator concentration which is notless than the critical concentration. The dependence of log(I/x)on log(x) was found to be relatively linear, and it yields a straightline with a slope equal to �q/3. As shown in Fig. 6, the slope is�1.6369 for Eu2+ in NaBaScSi2O7:xEu

2+ phosphors. The value ofq can be calculated to be 4.9107, which is close to 6, indicatingthat in NaBaScSi2O7:Eu

2+ phosphors the quenching is domi-nated by the dipole–dipole interaction.

If we consider energy transfer between two identical centers,the critical distance (Rc) is dened as the distance for which theprobability of energy transfer equals the probability of radiativeemission of Eu2+. There are two common methods for thedetermination of Rc. First, the critical distance (Rc1) betweenEu2+ ions for the energy transfer process can be calculated byusing the concentration quenching method proposed byBlasse,23

Rcz2

�3V

4pXcN

�13

(3)

where V is the volume of the unit cell, Xc is the criticalconcentration of the activator ion, and N is the number ofchemical formula in the unit cell.2 In the present case, Xc ¼ 0.1,V ¼ 320.47 A3, and N ¼ 2. Therefore, Rc1 is determined to be14.51 A. In addition, the critical distance Rc2 could be obtainedfrom Dexter eqn (3) for energy transfer by the dipole–dipoleinteraction, Rc2 can be given as

Fig. 6 The fitting line of log(I/x) vs. log(x) in NaBaScSi2O7:xEu2+ phosphors

beyond the quenching concentration.

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Fig. 7 Decay curves of Eu2+ emission in NaBaScSi2O7:xEu2+ phosphors and the

fitted lifetimes are also given in it.

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Rc6 ¼ �

0:63� 1028 � 4:8� 10�16P��

E4

ðfsðEÞfaðEÞdE (4)

where P is the oscillator strength of the Eu2+ ion. E is the energyof the maximum spectral overlap and

ÐfsðEÞfaðEÞdE is the

spectral overlap integral from the normalized excitation andemission spectrum of NaBaScSi2O7:Eu

2+. P is 0.01 for an allowed4f–5d transition, and the values of E and

ÐfsðEÞfaðEÞdE were

determined to be 5.014 eV and 0.492 eV�1 from the normalizedexcitation and emission spectrum. It turns out that Rc2 shouldbe 16.93 A, which is approximate to the value that obtained byusing the concentration quenching method. This result furtherconrms that the energy transfer mechanism from Eu2+ to Eu2+

via a dipole–dipole interaction.23,24

3.3 Photoluminescence lifetime analysis

It is well known that the lifetime of phosphors applied in theeld of displays and lights should be suitable in order to avoidthe superimposition of images. Fig. 7 depicts the normalizeddecay curves for NaBaScSi2O7:xEu

2+ phosphors (x ¼ 3, 5, 8, 10and 15 mol%) under the excitation of 370 nm. These decaycurves are analyzed at the maximum of Eu2+ emission at 501 nmand the data are plotted as a semi-logarithmic plot. All the decaycurves can be well tted based on the following formula,25

Fig. 8 (a) The PL spectra (lex¼ 365 nm) of the NaBaScSi2O7:0.1Eu2+ phosphor unde

emission intensities and emission peaks as a function of temperature of the NaBaScSicomparison.

7144 | J. Mater. Chem. C, 2013, 1, 7139–7147

I(t) ¼ A1 exp(�t/s) (5)

where I and A1 correspond to the luminescence intensity at timet, and s is the lifetime. On the basis of eqn (5) and the measureddecay curves, the lifetime of Eu2+ decreases with increasing Eu2+

concentration. The values decreased from 0.612 ms to 0.384 mswhen the doped Eu2+ concentration increased from 3 to 15 mol%. Furthermore, we can nd that the lifetimes are reasonablefor the 5d/ 4f allowed transition of Eu2+, and it is also suitablefor solid-state lighting when used as w-LED phosphors.

3.4 Thermally stable luminescence properties

The thermal stability of phosphors is one of the importantparameters for the application in high performance LEDs wherethe operation temperature increases up to 300 �C. Temperature-dependent emission spectra of the selected NaBaSc-Si2O7:0.1Eu

2+ under 365 nm excitation are given in Fig. 8a. Thevariations of the relative emission intensities and emissionpeaks as a function of temperature are displayed in Fig. 8b. Ascan be seen, the relative PL intensity decreases slowly withincreasing temperature from room temperature (RT) up to300 �C. In addition, the PL intensity at 150 �C drops to 88.7% ofthe initial value at RT, while the emission intensity of thecommercial BaSiO4:Eu

2+ phosphor decreases to 77.8% (at150 �C) of the initial value, as also shown in Fig. 8b.19 Simul-taneously, the emission wavelength shows a slight blue shiingwith raising temperature, which can be ascribed to the ther-mally active phonon-assisted excitation from the excited statesof the lower-energy emission band to the higher-energy emis-sion band in the excited states of Eu2+.25a

To better understand the thermal quenching phenomena,the activation energy for the thermal quenching was tted usingthe Arrhenius equation26

I(T) ¼ I0/[1 + c exp(�E/kT)] (6)

where I0 is the initial PL intensity of the phosphor at 30 �C, I(T)is the PL intensity at different temperatures, c is a constant, E isthe activation energy for thermal quenching, and k is theBoltzmann constant (8.617 � 10�5 eV k�1). According to theequation, the plot of ln[(I0/IT) � 1] vs. 1/kT yields a straight line,

r different temperatures in the range of 30–300 �C. (b) The variations of the relative

2O7:0.1Eu2+ phosphor, and the data of the commercial phosphor are also given for

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Fig. 9 A ln[(I0/IT) � 1] vs. 1/kT activation energy graph for thermal quenching ofthe NaBaScSi2O7:0.1Eu

2+ phosphor.Fig. 11 Excitation line of BaSO4 and the emission spectrum of the NaBaSc-Si2O7:0.1Eu

2+ phosphor collected by using an integrating sphere. The inset showsa magnification of the emission spectrum.

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and the activation energy E is obtained from the slope of theplot. As shown in Fig. 9, a tting line of the ln[(I0/IT)� 1] vs. 1/kTactivation energy graph for thermal quenching is given with aslope of �0.102 and therefore we have obtained the activationenergy E value of the NaBaScSi2O7:0.1Eu

2+ phosphor to be about0.081 eV, which is higher than that of some recently reportedsilicate phosphors.24

3.5 Luminescence quantum efficiency, CIE parameter andelectroluminescence properties

The CIE color coordinate of the Eu2+-concentration optimizedNaBaScSi2O7:0.1Eu

2+ phosphor was calculated to be (0.0706,0.5540) under 365 nm UV excitation, as is marked in the CIEchromaticity diagram in Fig. 10. The inset of Fig. 10 shows thedigital photograph of the NaBaScSi2O7:0.1Eu

2+ phosphor under

Fig. 10 Color coordinates of NaBaScSi2O7:0.1Eu2+ in the CIE chromaticity

diagram, and the inset shows a digital photograph of the green-emittingphosphor.

This journal is ª The Royal Society of Chemistry 2013

a 365 nm UV lamp, which shows a strong green emission.Moreover, according to the reported method, we have alsomeasured the internal quantum efficiency (QE) of the NaBaSc-Si2O7:0.1Eu

2+ phosphor.16–18 From the measured results given inFig. 11, the internal QE value can be calculated by the followingequation (eqn (7)),27

hQE ¼ðLS

�ðER �

ðES

�(7)

where LS is the luminescence emission spectrum of the sample;ES is the spectrum of the light used for exciting the sample; ER isthe spectrum of the excitation light without the sample in thesphere; and all the spectra were collected using the integratingsphere. The measured internal QE values of the NaBaSc-Si2O7:0.1Eu

2+ phosphor are determined to be about 65% and46% under 365 nm and 400 nm excitation, respectively. It isbelieved that such values can be further improved by the opti-mized experimental processing. All the above results indicatethat the NaBaScSi2O7:Eu

2+ phosphor can be used as a potentialgreen phosphor for w-LED application.

Fig. 12 EL spectrum of blending three blue-emitting BaMgAl10O17:Eu2+, green-

emitting NaBaScSi2O7:Eu2+, and red-emitting CaAlSiN3:Eu

2+ phosphors on a 370nm emitting n-UV chip, and the insets show the photos of the LED package.

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Accordingly, the potential of the green-emitting NaBaSc-Si2O7:Eu

2+ phosphor for the application in n-UV LED-pumpedwhite-emitting LEDs is investigated. Fig. 12 shows the electro-luminescence (EL) spectrum of the fabricated white-light n-UVLED under a driving current of 25 mA, and the inset shows thephotographs of the fabricated WLED lamp and its practicalemission. The EL spectrum combines an n-UV band ascribedto chip emission around 370 nm, a blue-emission band ofBaMgAl10O17:Eu

2+ around 450 nm, and a green-emission bandcorresponding to NaBaScSi2O7:Eu

2+ around 500 nm, and a red-emission band of CaAlSiN3:Eu

2+ around 610 nm. The colorrendering index (Ra) of 86.5 at a correlated color temperature of2528 K (warm light) with CIE coordinates of x¼ 0.353, y¼ 0.324was found for the fabricated n-UV LED-pumped white-emittingLEDs. These results indicate that NaBaScSi2O7:Eu

2+ is a poten-tial green phosphor candidate for near-UV-pumped light emit-ting diodes.

4. Conclusions

In summary, a novel Sc silicate NaBaScSi2O7:Eu2+ phosphor was

reported in this paper, and the crystallographic occupancy,photoluminescence properties and application properties werestudied in detail. Eu2+ in the NaBaScSi2O7 matrix has beenfound to be located at Na+ sites by the Rietveld analysis and thecrystal chemistry rules. The green-emitting NaBaScSi2O7:Eu

2+

phosphor, which can be efficiently excited over a wide range of250 to 450 nm, shows a high luminescence efficiency of about65% under 365 nm excitation and a high thermal quenchingtemperature (88.7% at 150 �C of the initial intensity) superior tothat of the commercial Ba2SiO4:Eu

2+ phosphor. The w-LEDlamp fabricated by using the present green phosphor and thecommercial blue and red phosphors demonstrated the poten-tial of the NaBaScSi2O7:Eu

2+ phosphor for use in w-LEDs.

Acknowledgements

This present work was supported by the National NaturalScience Foundations of China (Grant no. 51002146, no.51272242), the Natural Science Foundations of Beijing(2132050), the Program for New Century Excellent Talents in theUniversity of Ministry of Education of China (NCET-12-0950),the Fundamental Research Funds for the Central Universities(2011YYL131), and Zhiguo Xia was supported by the BeijingNova Program (Z131103000413047). We thank Dr Zhiyong Mao(Shanghai Institute of Ceramics, Chinese Academy of Sciences)for the help in the fabrication of w-LED lamps.

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