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Journal of Alloys and Compounds 663 (2016) 332e339

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Journal of Alloys and Compounds

journal homepage: http: / /www.elsevier .com/locate/ ja lcom

Blue-emitting Ca5(PO4)3Cl:Eu2þ phosphor for near-UV pumped lightemitting diodes: Electronic structures, luminescence properties andLED fabrications

Jianghui Zheng a, Shunqing Wu b, Guo Chen a, Sijia Dang c, Yixi Zhuang d, Ziquan Guo c,Yijun Lu c, Qijin Cheng a, **, Chao Chen a, b, c, *

a College of Energy, Xiamen University, Xiamen 361005, Chinab Department of Physics, Xiamen University, Xiamen 361005, Chinac Department of Electronic Science, Fujian Engineering Research Center for Solid-state Lighting, Xiamen University, Xiamen 361005, Chinad College of Materials, Xiamen University, Xiamen 361005, China

a r t i c l e i n f o

Article history:Received 19 August 2015Received in revised form7 December 2015Accepted 8 December 2015Available online 11 December 2015

Keywords:Optical materialsChemical synthesisElectronic propertiesOptical propertiesLuminescence

* Corresponding author. College of Energy, XiamenChina.** Corresponding author.

E-mail addresses: qijin.cheng@xmu.edu.cn (Q.(C. Chen).

http://dx.doi.org/10.1016/j.jallcom.2015.12.0540925-8388/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

A blue-emitting Ca5(PO4)3Cl:Eu2þ phosphor was prepared via a conventional high temperature solid-

state reaction method. Crystal and electronic structure properties of the Ca5(PO4)3Cl:Eu2þ phosphorwere investigated using X-ray diffraction and density functional theory (DFT), respectively. The micro-morphology, reflectance spectra, thermal stability and quantum efficiency of the Ca5(PO4)3Cl:Eu

2þ

phosphor were also studied. The optimum Eu2þ concentration in Ca5(PO4)3Cl was determined to be2.0 mol% and the concentration quenching mechanism can be explained by the dipoleedipole interac-tion. The emission intensity of the Ca5(PO4)3Cl:Eu

2þ phosphor was 58.2% of the initial value when themeasured temperature increased from 30 �C to 150 �C. The activation energy was determined to be0.254 eV, suggesting the good stability of this phosphor. A bright blue LED was fabricated using anInGaN-based near-UV LED chip (385 nm) and a Ca5(PO4)3Cl:Eu2þ phosphor, and has an excellent blue-emitting property with CIE coordinates of (0.1480, 0.0350). Furthermore, a bright near-UV warm whiteLED was fabricated using an InGaN-based near-UV LED chip (395 nm) in combination with the presentblue phosphor and the commercial green and red phosphors, which exhibits an excellent color-renderingindex (Ra ¼ 96.65) at a warm correlated color temperature of 3902 K with CIE coordinates of (0.3781,0.3879). All the results suggest that the Ca5(PO4)3Cl:Eu2þ phosphor is a potential blue-emitting candidatefor the application in the near-UV pumped blue and warm white LEDs.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, solid-state lighting based on the combination oflight-emitting diode (LED) chips and phosphors has attracted sig-nificant attention [1e3]. Due to the satisfactory characteristics ofenergy saving properties, long lifetime, high efficiency and highmaterial stability, the solid-state lighting for LEDs is considered tobe the next generation lighting source [4e6]. A typical commercialwhite LED (w-LED) is generated by pumping the blue InGaN based-

University, Xiamen 361005,

Cheng), cchen@xmu.edu.cn

LED chip on a yellow-emitting Y3Al5O12:Ce3þ (YAG:Ce3þ) phosphor.Nevertheless, this type of white LEDs suffers many disadvantagessuch as a high correlated color temperature (CCT > 4500 K) as wellas a low color-rendering index (CRI < 80) due to the lack of a redcomponent in the visible spectrum [7,8]. Another alternativeapproach for generating white light is by pumping tricolor phos-phors (blue, green and red) with near-UV light chips(350e420 nm). This approach produces white light with excellentCRI values and suitable CCT. The white light obtained from thisapproach is close to the nature light and thus some applicationareas can be broadened [7,9,10]. Therefore, phosphor materials playa very important role in these LEDs and it is vital to develop thehigh-performance tricolor phosphors which can be efficientlyexcited by the near-UV light.

As a highly efficient activator with the allowed 4f-5d transitions,

Fig. 1. XRD patterns of the representative Ca5(PO4)3Cl and Ca4.98(PO4)3Cl:0.02Eu2þ

samples. The standard data card ICSD#24237 of Ca5(PO4)3Cl is provided as a reference.Inset shows the FE-SEM image of Ca4.98(PO4)3Cl:0.02Eu2þ. Some possible secondaryphases might be formed and marked with asterisks (*).

J. Zheng et al. / Journal of Alloys and Compounds 663 (2016) 332e339 333

Eu2þ ion is the most used activator in the phosphor and has beenwidely investigated at present. The excitation and emission spectrain a specific host generally consist of broad bands due to the 4f-5dtransitions of Eu2þ ions [8]. Therefore, many Eu2þ ion-dopedphosphors for near-UV pumped LEDs have been developed, suchas Ba2Ca(PO4)2:Eu2þ [11], Sr2SiO4-3xNx:Eu2þ [12], SrB2O4:Eu2þ [13],BaMgSiO4 [14], Ba3Si6O12N3:Eu2þ [15], BaSi3Al3O4N5:Eu2þ [16], andso on. Recently, much attention has been drawn to the Eu2þ ion-activated phosphates due to the advantages of high physical andchemistry stability, low synthesis temperature, and high quantumefficiency [12,17e19]. Among the reported Eu2þ ion-activatedphosphates, Ca5(PO4)3Cl:Eu2þ has attracted particular attention.Some previous works reported the luminescent properties ofCa5(PO4)3Cl:Eu2þ [20e22], but to the best of our knowledge, thereis no report dedicated to the electronic structures and temperature-dependent luminescence properties of Ca5(PO4)3Cl:Eu2þ phosphorsas well as the fabrication of Ca5(PO4)3Cl:Eu2þ converted near-UVblue and white LEDs.

In this work, the electronic structures, reflectance spectra,thermal stability and quantum efficiency (QE) of the blue-emittingCa5(PO4)3Cl:Eu2þ phosphors have been studied in detail. Moreover,we have also successfully fabricated the near-UV blue and whiteLEDs, respectively. The blue LED was fabricated using an InGaN-based n-UV LED chip (385 nm) and a Ca5(PO4)3Cl:Eu2þ phosphor.Meanwhile, the near-UV white LED was fabricated using an InGaN-based n-UV LED chip (395 nm) in combination with the presentblue phosphor and the commercial green and red phosphors. Theresults suggest that the Ca5(PO4)3Cl:Eu2þ phosphor is a promisingblue-emitting material for the application in the near-UV pumpedblue and warm white light emitting diodes.

2. Materials and method

2.1. Synthesis

Samples with a general formula of Ca5-x(PO4)3Cl:xEu2þ (x ¼ 0,0.005, 0.01, 0.02, 0.03, 0.05 and 0.07) were prepared by solid statereaction method. NH4H2PO4 (A.R. grade), CaCO3 (A.R. grade), NH4Cl(A.R. grade) and Eu2O3 (99.99% purity) were used as starting ma-terials. The stoichiometric materials were weighed and groundtogether in an agate mortar. Then themixturewas put into a mufflefurnace and precalcined at 400 �C for 1 h, and subsequently furthersintered at 1000 �C for 5 h in the thermal carbon environment(weak CO reducing atmosphere). Finally, the furnace cooled downto room temperature naturally and the mixtures were ground in anagate mortar.

2.2. Characterization

The crystalline phase of the synthesized samples was carried outon an X-ray diffraction (XRD) diffractometer (Panalytical X-PertPRO diffractometer) with a Cu Ka (40.0 KV, 30.0 mA) radiation(l ¼ 1.5418 Å). Surface morphology of the as-prepared phosphorswas observed by Hitachi SU-70 field-emission scanning electronmicroscope (FE-SEM). The diffuse reflectance spectrum wasmeasured by Cary 5000 UV-VIS-NIR spectrophotometer. Photo-luminescence excitation (PLE) and emission (PL) spectra weremeasured by Hitachi F-7000 spectrofluorometer equipped with a150 W Xenon lamp as an excitation source. The internal QEs of therepresentative samples were measured using a standard EdinburghInstruments FLS980 spectrometer equipped with an integratingsphere attachment. The samples were placed in the integratingsphere and the Xe lamp was employed as the light source to pumpthe samples. The Keithley 2611 source meter was used to supply acurrent of 350 mA for the illumination of the fabricated LEDs and

the spectra of the fabricated LEDs were measured using the SP320spectrometer with an integrating sphere manufactured by Instru-ment Systems Inc. All the afore-mentioned measurements wereconducted at room temperature. The temperature-dependent PLspectra were measured by Hitachi F-4600 spectrofluorometer,which was equipped with a computer-controlled electric furnaceand a self-made heating attachment.

2.3. Computational details

To investigate the electronic properties of the Ca5(PO4)3Cl hostmatrix, the calculations on the electronic structures of Ca5(PO4)3Clhost matrix were performed using the projector-augmented wave(PAW) method within the density functional theory (DFT), asimplemented in the Vienna ab initio simulation package (VASP)[23e25]. The exchange and correlation function is treated asgeneralized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) formula [26]. The wave functions were expandedby using the plane waves up to a kinetic energy cutoff of 550 eV.Brillouin-zone integrations were approximated by using special k-point sampling of Monkhorst-Pack scheme with a 5 � 5 � 5 mesh.The lattice vectors (both the shape and the size of a unit cell) arefully relaxed together with atomic coordinates until the Hellmann-Feynman force on each atom is less than 0.01 eV/Å.

3. Results and discussion

3.1. Crystal structure and morphology characteristics of theCa5(PO4)3Cl:Eu

2þ phosphors

The XRD patterns of the representative Ca5(PO4)3Cl andCa4.98(PO4)3Cl:0.02Eu2þ samples are shown in Fig. 1. The diffractionpeaks fit well with the standard data card ICSD#24237 ofCa5(PO4)3Cl, indicating that the introducing of Eu2þ ions has notbrought obvious change of the structure of the Ca5(PO4)3Cl host.Some tiny impurity peaks marked with asterisks located at 27.78�

and 31.01� for the Ca5(PO4)3Cl and Ca4.98(PO4)3Cl:0.02Eu2þ samplesin Fig. 1 might be attributed to the presence of Ca3(PO4)2(ICSD#97550) due to the tiny additional reactions. The inset ofFig. 1 shows the representative FE-SEM image of the

Fig. 2. The crystal structure of Ca5(PO4)3Cl viewed along the c-axis (a) and the coordination environment of Ca2þ ions (b).

J. Zheng et al. / Journal of Alloys and Compounds 663 (2016) 332e339334

Ca4.98(PO4)3Cl:0.02Eu2þ sample. It can be found that theCa4.98(PO4)3Cl:0.02Eu2þ phosphor consists of irregular grains withan average grain size about 1e3 mm Fig. 2 (a) and (b) show thecrystal structure of Ca5(PO4)3Cl viewed along the c-axis and thecoordination environment of Ca2þ ions, respectively. One can seethat Ca5(PO4)3Cl has a phase of hexagonal structure with a spacegroup of P 63/m (176) and that there are two kinds of Ca2þ siteswith different coordination environments in the Ca5(PO4)3Cl crystal(the Ca1 atom is surrounded by six O2� ions and located at 4f site,while the Ca2 atom is surrounded by five O2� ions and two Cl� ionsand located at 6 h site.). The Eu2þ ions are expected to randomlysubstitute the Ca sites in the host lattice in this case since the radiiof Ca2þ (r ¼ 1.00 Å for CN ¼ 6 and r ¼ 1.06 Å for CN ¼ 7) and Eu2þ

(r ¼ 1.17 Å for CN ¼ 6 and 1.20 for CN ¼ 7) are similar.

Fig. 3. Calculated band structure of Ca5(PO4)3Cl. The Fermi level is set at zero energy.

3.2. Electronic structure of the Ca5(PO4)3Cl host

Fig. 3 shows the band structure of Ca5(PO4)3Cl along highsymmertry points of the Brillouin zone of the hexagonal crystal. It isclearly observed that Ca5(PO4)3Cl has a wide indirect band gap ofapproximately 5.30 eV and that the band gap is determined fromthe top of the valence band at M point to the bottom of the con-duction band at G point. It is worthwhile to mention that the Mpoint in the absorption edge of the valence band is very close to theG point in the absorption edge of the valence band in this case.Hence, this host may also feature a direct band gap property andduring some intrinsic disorder it may be transferred. As we konw, ahost matrix having a high band gap (generally more than 3eV) willbe suitable for luminescence materials to accommodate lumines-cent ions as an emitting center [27]. Thus, we can infer thatCa5(PO4)3Cl is a suitable host matrix to accommodate Eu2þ ions as

an emitting center based on the calculation result.To further figure out the detailed composition of the energy

bands, the density of states of Ca5(PO4)3Cl was also calculated usingDFT. The calculated total density of states of Ca5(PO4)3Cl and pro-jected density of states for Ca, P, O and Cl atoms are shown in Fig. 4.It can be found that the conduction band is mainly composed of Ca-3d, P-2p, O-2p, Cl-3s and Cl-3p states ranging from 4.1 to 8 eV andthat the valence band is mainly composed of O-2p and Cl-3p statesranging from �8 to 0 eV. Moreover, it can be observed that thebottom of the conduction band is dominated by Ca-3d states andthat the top of the valence band is dominated by O-2p state. From

Fig. 4. Total and partial (Ca, P, O, Cl) density of states for Ca5(PO4)3Cl.

J. Zheng et al. / Journal of Alloys and Compounds 663 (2016) 332e339 335

the calculation result, one can infer that the host absorption ofCa5(PO4)3Cl at the n-UV region can be explained by the chargetransition from the Ca-3d state to the O-2p state.

3.3. Luminescence properties of the Ca5(PO4)3Cl:Eu2þ phosphors

Fig. 5 shows the PLE (lem ¼ 456 nm) and PL (lex ¼ 273 and346 nm) spectra of the Ca4.98(PO4)3Cl:0.02Eu2þ phosphor. The PLEspectrum shows a broad emission band from 260 to 420 nm, whichis ascribed to the transition of Eu2þ ions from the 4f7 (8S7/2) groundstate to the 4f65 d1 excited state, indicating that this phosphor canbe well excited by n-UV light from a broad range between 260 and420 nm [28]. The PL spectra of the Ca4.98(PO4)3Cl:0.02Eu2þ phos-phor under the 237 and 346 nm excitation wavelengths present abroad blue emission band ranging from 400 to 530 nm peaking at456 nm, which is attributed to the transition from the lowestrelaxed 4f65 d1 level to the 4f7 (8S7/2) level of Eu2þ ions [10]. Theinset of Fig. 5 illustrates the energy level diagram of the Eu2þ ions inCa5(PO4)3Cl:Eu2þ, which reveals the probable transitions involvedin this phosphor.

Fig. 5. PLE (lem ¼ 456 nm) and PL (lex ¼ 273 and 346 nm) spectra of theCa4.98(PO4)3Cl:0.02Eu2þ phosphor; diffuse reflectance spectrum (DRS) of the pureCa4.98(PO4)3Cl:0.02Eu2þ phosphor. The inset illustrates the energy level diagram of theEu2þ ions in Ca5(PO4)3Cl:Eu2þ.

Additionally, we briefly discuss the effect of the presence of theadditional Ca3(PO4)2 phase on the PL property. According to thelatest reports about the luminescence property of Ca3(PO4)2:Eu2þ

[29], the Ca3(PO4)2:Eu2þ phosphor has a broad emission bandpeaking at 416 nm, which can be well excited by NUV light from250 to 380 nm. Based on the PL spectra of the Ca5(PO4)3Cl:Eu2þ

samples excited by 273 nm in our work, the additional emissionpeak was not found for the studied samples. Hence, we believe thatthe slight additional Ca3(PO4)2 phase does not affect the PL prop-erty of the synthesized samples in this work.

Since the CIE coordinates are essential for evaluating the per-formance of luminescence materials [30], the CIE chromaticity co-ordinates of Ca4.98(PO4)3Cl:0.02Eu2þ have been calculated based onthe emission spectra. The CIE chromaticity coordinates of theCa4.98(PO4)3Cl:0.02Eu2þ phosphor are calculated to be (0.1420,0.0530), which are located in the blue area. Meanwhile, Fig. 5 alsoshows a diffuse reflectance spectrum (DRS) of theCa4.98(PO4)3Cl:0.02Eu2þ phosphor. A strong broad absorptionranging from 230 to 450 nm can be found in the DRS of theCa5(PO4)3Cl:Eu2þ sample and matches well with the excitationspectrum. The result indicates that the Eu2þ ions are well incor-porated into the Ca5(PO4)3Cl host as an emitting center.

As mentioned previously, the Eu2þ ions have randomlysubstituted two Ca sites and the emission position of Eu2þ ions ishighly related to its local environment [31]. Hence, it is necessary tofigure out the effect of the different local environments of Eu2þ ionson the PL properties of the Ca5(PO4)3Cl:Eu2þ phosphor. Fig. 6 showsthe deconvoluted PL spectra of the Ca4.98(PO4)3Cl:0.02Eu2þ phos-phor using two Gaussian equations with reasonable fitting values.It can be found that two peaks (~452 nm and ~475 nm) are presentin the deconvolution of the emission band. According to the theoryof Van Uitert, the possible crystallographic sites can be deduced asfollows [1,7]:

E ¼ Q

"1�

�V4

�1=V

� 10�ðn$Ea$rÞ=80#

(1)

where Q stands for the position in energy for the lower d-band edgeof the free Eu2þ ions (in cm�1); V is the valence of the Eu2þ ion; n isthe number of anions in the immediate shell around the Eu2þ ion;Ea is the electron affinity of the anion atom (in eV); and r is theradius of the host cation substituted by the Eu2þ ion (in Å). In this

Fig. 6. Deconvoluted PL spectra using two Gaussian equations for theCa4.98(PO4)3Cl:0.02Eu2þ sample (lex ¼ 273 nm).

J. Zheng et al. / Journal of Alloys and Compounds 663 (2016) 332e339336

case, according to Eq. (1), the value of E has a positive relationshipwith the value of n*r since V and Ea have the same value for twosubstituted sites in the same host. Hence we can infer that thedeconvoluted Gaussian curve peaking at 452 nm (22124 cm�1) canbe assigned to the substitution of Eu2þ ions with seven coordina-tion numbers while the deconvoluted Gaussian curve peaking at475 nm (21052 cm�1) can be assigned to the substitution of Eu2þ

ions with six coordination numbers.The effect of the concentration of Eu2þ ions on the photo-

luminescence of the Ca5-x(PO4)3Cl:xEu2þ phosphors was alsoinvestigated. Fig. 7 shows the PL emission spectra of Ca5-x(PO4)3Cl:xEu2þ (x ¼ 0.005, 0.01, 0.02, 0.03, 0.05 and 0.07) as afunction of the concentration of Eu2þ ions under the 273 nm lightexcitation. It can be found that the emission intensity of the Ca5-x(PO4)3Cl:xEu2þ phosphors initially increases with the increase ofthe content of Eu2þ ions and reaches a maximum at x ¼ 0.02. Then,with a further increase of the content of Eu2þ ions, the emissionintensity decreases, which can be attributed to the concentrationquenching effect of Eu2þ ions. According to the concentrationquenching theory, the occurrence of concentration quenching at ahigher Eu2þ content can be explained by two kinds of mechanisms:one is the interaction between Eu2þ ions and the other is energymigration from a percolation bunch of Eu2þ ions to quenchingcenters [32,33]. Thus, the concentration quenching process ishighly dependent on the critical transfer distance (Rc) betweenEu2þ ions. The critical transfer distance can be calculated using thefollowing equation [34]:

Rc ¼ 2�

3V4pxcN

�1 =

3

(2)

where N is the number of cations in the unit cell; xc is the optimalconcentration; and V is the volume of the unit cell. For theCa5(PO4)3Cl host, the values ofN, xc and V are 10, 0.01 and 537.64Å3,respectively. Based on these values, Rc is determined to be 15.07 Åusing Eq. (2). Since the critical distance of the exchange interactionis generally less than 5 Å [8], the energy transfer process for theconcentration quenching of Eu2þ ions in this phosphor is domi-nated by electric multipolar interactions.

According to the Dexter theory [35], the interaction mechanismfor the Ca5(PO4)3Cl:Eu2þ phosphor can be inferred by the followingequation:

Fig. 7. PL emission spectra of Ca5-x(PO4)3Cl:xEu2þ (x ¼ 0.005, 0.01, 0.02, 0.03, 0.05 and0.07; lex ¼ 273 nm) as a function of the concentration of Eu2þ ions.

I=x ¼ Kh1þ bðxÞQ=3

i�1(3)

where x is the activator concentration; K and b are constants of thesame excitation condition for a given host crystal; Q ¼ 3 means theenergy transfer among the nearest-neighbor ions while Q ¼ 6, 8,and 10 is for dipoleedipole (ded), dipoleequadrupole (d-q), orquadrupoleequadrupole (qeq) interactions, respectively. When xexceeds the critical concentration, the Eq. (3) can be simplified asfollows [36e38]:

I=x ¼ K0hbðxÞQ=3

i�1(4)

where K0is a constant. Based on this equation, the value of Q can be

determined from the slope of the plot of lg(I/x) versus lg(x). Fig. 8shows the curve of lg(I/x) versus lg(x). From Fig. 8, one can seethat the relationship of lg(I/x) versus lg(x) is linear with a slopeof �1.786 and then the value of Q can be calculated to be 5.358,which is close to 6. This result indicates that the major concentra-tion quenching mechanism in the Ca5(PO4)3Cl phosphor is domi-nated by dipoleedipole interaction.

3.4. Thermal quenching properties of the Ca5(PO4)3Cl:Eu2þ

phosphors

Because the thermal stability of the phosphor is an importanttechnological parameter in evaluating its potential for the LEDapplication due to its influence on the service life, light output aswell as color rendering index [39,40], we have also investigated thethermal stability of the synthesized Ca5(PO4)3Cl:Eu2þ phosphors.Fig. 9 presents the emission spectra (lex ¼ 273 nm) of theCa4.98(PO4)3Cl:0.02Eu2þ phosphor within the temperature range of30 �Ce250 �C. It is noticed that the emission intensity of theCa5(PO4)3Cl:Eu2þ phosphor was 58.2% of the initial value when thetemperature increased from 30 �C to 150 �C. Simultaneously, it canbe found that the emission wavelength of the PL spectra shows ablue shift with the increase of temperature. This can be explainedthat the thermally active phonon-assisted excitation from theexcited states of the lower energy emission band to the excitedstates of the higher energy emission band for the excited states ofEu2þ ions [41]. This result indicates a certain role of the phononsubsystem in the PL properties of Eu2þ ions [42,43].

Fig. 8. Plot of lg(I/x) versus lg(x) in the Ca5-x(PO4)3Cl:xEu2þ phosphors (lex ¼ 273 nm).

Fig. 9. Emission spectra of Ca4.98(PO4)3Cl:0.02Eu2þ measured at different temperatures(lex ¼ 273 nm). The inset shows the relative emission intensity as a function of thetemperature.

Fig. 11. Electroluminescent spectrum of the near-UV blue LED fabricated using anInGaN-based n-UV LED chip (385 nm) and a Ca5(PO4)3Cl:Eu2þ phosphor driven using acurrent of 350 mA and a voltage of 3.25 V. The inset shows the photo of the fabricatednear-UV blue LED package.

J. Zheng et al. / Journal of Alloys and Compounds 663 (2016) 332e339 337

According to the Dorenbos theory, the main mechanismresponsible for the thermal quenching of Eu2þ luminescence in thehost is the ionization of the electron from the lowest energy level ofthe relaxed Eu2þ 4f65 d1 electronic configuration to the conductionband level of the host lattice [44,45]. Based on this theory, theactivation energy DE for thermal quenching, which is the energyrequired to raise the electron from the relaxed excited level into theconduction band of the host lattice, can be calculated using thefollowing equation [46,47]:

IðTÞ ¼ I0

1þ c exp�� DE

kT

� (5)

where I0 is the initial emission intensity at room temperature; I(T) isthe emission intensity measured at different temperatures; c is aconstant; k is the Boltzman's constant (8.617 � 10�5 eV/K); and DEis the activation energy for the thermal quenching. Fig. 10 showsthe plot of ln[(I0/I)-1] versus 1/(kT) and this curve can be linearlyfitted. From the slope of this straight line, the activation energy DE

Fig. 10. Arrhenius fitting of the emission intensity of Ca4.98(PO4)3Cl:0.02Eu2þ.

for thermal quenching is obtained to be about 0.254 eV. The rela-tively high activation energy achieved in this work indicates thatthe phosphor has a good thermal stability and can be used in thehigh-power LED application.

3.5. Quantum efficiency and electroluminescence properties of thefabricated white LEDs

Since quantum efficiency of the phosphor is also an importantfactor in evaluating its potential for the LED application [48,49], theinternal QE of the Ca4.98(PO4)3Cl:0.02Eu2þ phosphor was alsoinvestigated. Under the 385 and 395 nm near-UV light excitation,the internal QEs of the Ca4.98(PO4)3Cl:0.02Eu2þ phosphor wereestimated to be 32.45% and 25.56%, respectively. The internal QE isrelatively lower compared with the reported BAM commercialphosphor. The obtained internal QE can be further improved

Fig. 12. Electroluminescent spectrum of an InGaN-based near-UV LED chip (395 nm)comprising of a mixture of Ca5(PO4)3Cl:Eu2þ, green-emitting (Ba,Sr)2SiO4:Eu2þ andred-emitting CaAlSiN3:Eu2þ phosphors driven using a current of 350 mA and a voltageof 3.29 V. The inset shows the photo of the fabricated w-LED package.

Table 1Full set of the 14 CRIs and Ra of the fabricated w-LED.

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 Ra

98 99 98 99 99 96 94 86 67 98 96 96 98 97 96.65

J. Zheng et al. / Journal of Alloys and Compounds 663 (2016) 332e339338

through optimization of the preparation condition.To substantiate the potential application of the Ca5(PO4)3Cl:

Eu2þ phosphor, the n-UV blue and white LEDs were fabricated. Theblue LED was fabricated using an InGaN-based n-UV LED chip(385 nm) and a Ca5(PO4)3Cl:Eu2þ phosphor. Fig. 11 shows theelectroluminescent spectrum of the fabricated blue LED drivenusing an electrical current of 350 mA and a voltage of 3.25 V. TheCIE chromaticity coordinates are determined to be (0.1480, 0.0350).The inset shows the photo of the fabricated w-LED package. Thebright blue light can be observed by naked eyes.

Accordingly, the n-UVwhite LEDwas fabricated using an InGaN-based n-UV LED chip (395 nm) in combination with the presentCa5(PO4)3Cl:Eu2þ phosphor, the green-emitting (Ba,Sr)2SiO4:Eu2þ

phosphor, and the red-emitting CaAlSiN3:Eu2þ phosphor. Fig. 12shows the electroluminescent spectrum of the fabricated w-LED,which was driven using a current of 350 mA and a voltage of 3.29 V.The excellent color rendering index (Ra) of 96.65 (Ra of the LEDswas obtained from the full set of the 14 CRIs, which is shown inTable 1) at a CCT of 3902 K with CIE coordinates of (0.3952, 0.3790)was obtained for the fabricated near-UV pumped w-LED. All theresults suggest that the Ca5(PO4)3Cl:Eu2þ phosphor is an efficientblue-emitting phosphor for the application in the near-UV pumpedblue and warm white light emitting diodes.

4. Conclusions

In summary, a series of Eu2þ doped Ca5(PO4)3Cl blue-emittingphosphors under near-UV excitation has been investigated for thepotential application in the blue and warm LEDs. The electronicstructures of the Ca5(PO4)3Cl host matrix were calculated using DFTand a wide band gap (5.30 eV) of the Ca5(PO4)3Cl host matrix en-ables it to accommodate Eu2þ ions as an luminescent center. Theconcentration quenching of Eu2þ ions in the Ca5(PO4)3Cl host isdetermined to be 2.0 mol% and the mechanism of concentrationquenching can be explained by the dipoleedipole interaction. Theemission intensity of the Ca5(PO4)3Cl:Eu2þ phosphor was 58.2% ofthe initial value when the measured temperature increased from30 �C to 150 �C. The activation energy was determined to be0.254 eV, indicating the good stability of this phosphor. The CIEchromaticity coordinates of the near-UV blue LED fabricated usingan InGaN-based n-UV LED chip (385 nm) and a Ca5(PO4)3Cl:Eu2þ

phosphor are determined to be (0.1480, 0.0350). Meanwhile, theobtained warm white LED device exhibited an excellent color-rendering index (Ra ¼ 96.65) at a correlated color temperature of3902 K with CIE coordinates of (0.3781, 0.3873). Our resultsdemonstrate that the blue-emitting Ca5(PO4)3Cl:Eu2þ phosphor is apromising candidate for the application in the near-UV pumpedblue and warm white LEDs.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Grant No. 61076056 and No. 51308481),Natural Science Foundation of Fujian Province, China (Grant No.2013J05082), Fujian Provincial Department of Science & Technol-ogy (Grant No. 2015H0036), Fundamental Research Funds for theCentral Universities (Grant No. 2013121031 and 2013SH004), Pro-gram for New Century Excellent Talents in Fujian Province

University (NCETFJ), Scientific Research Foundation for theReturned Overseas Chinese Scholars, State Education Ministry,China. We gratefully thank Dr. Jian Chen (China University ofGeosciences, Beijing, China) for temperature-dependentmeasurements.

References

[1] S.P. Lee, T.S. Chan, T.M. Chen, ACS Appl. Mater. Interfaces 7 (2015) 40e44.[2] F.B. Xiong, D.S. Guo, H.F. Lin, L.J. Wang, H.X. Shen, W.Z. Zhu, J. Alloys Compd.

647 (2015) 1121e1127.[3] Y. Zhang, X. Li, K. Li, H. Lian, M. Shang, J. Lin, ACS Appl. Mater. Inter 7 (2015)

2715e2725.[4] Q. Liu, Z. Zheng, X. Zhang, Z. Bai, J. Alloys Compd. 628 (2015) 298e302.[5] X. Li, J.D. Budai, F. Liu, J.Y. Howe, J. Zhang, X.J. Wang, Z. Gu, C. Sun, R.S. Meltzer,

Z. Pan, Light Sci. Appl. 2 (2013) e50.[6] Y. Wang, X. Liu, Y. Li, L. Jing, J. Alloys Compd. 653 (2015) 315e320.[7] J. Zheng, Q. Cheng, S. Wu, Z. Guo, Y. Zhuang, Y. Lu, Y. Li, C. Chen, J. Mater.

Chem. C 3 (2015) 11219e11227.[8] J. Chen, Y. Liu, L. Mei, H. Liu, M. Fang, Z. Huang, Sci. Rep. 5 (2015) 9673.[9] D. Huang, Y. Zhou, W. Xu, K. Wang, Z. Liu, M. Hong, J. Alloys Compd. 653

(2015) 148e155.[10] C.Y. Wang, R.J. Xie, F. Li, X. Xu, J. Mater. Chem. C 2 (2014) 2735e2742.[11] Y. Zhang, Z. Xia, W. Wu, J. Liu, J. Am, Ceram. Soc. 96 (2013) 1043e1046.[12] W. Tian, K. Song, F. Zhang, P. Zheng, J. Deng, J. Jiang, J. Xu, H. Qin, J. Alloys

Compd. 638 (2015) 249e253.[13] J. Zheng, L. Ying, Q. Cheng, Z. Guo, L. Cai, Y. Lu, C. Chen, Mater. Res. Bull. 64

(2015) 51e54.[14] M. Peng, Z. Pei, G. Hong, Q. Su, J. Mater. Chem. 13 (2003) 1202e1205.[15] J. Tang, J. Chen, L. Hao, X. Xu, W. Xie, Q. Li, J. Lumin 131 (2011) 1101e1106.[16] J.Y. Tang, W.J. Xie, K. Huang, L.Y. Hao, X. Xu, R.J. Xie, Electrochem. Solid. St. 14

(2011) J45eJ47.[17] H.S. Kim, K.I. Machida, T. Horikawa, H. Hanzawa, J. Alloys Compd. 633 (2015)

97e103.[18] J. Chen, Y.G. Liu, L. Mei, Z. Wang, M. Fang, Z. Huang, J. Mater. Chem. C 3 (2015)

5516e5523.[19] Z. He, X. Huang, R. Zhou, W. Huang, J. Alloys Compd. 658 (2016) 36e40.[20] J. Yu, C. Guo, Z. Ren, J. Bai, Opt. Laser Technol. 43 (2011) 762e766.[21] M. Hwang, E. Lee, S.H. Hong, Y.B. Sun, Y. Kim, J. Electrochem. Soc. 156 (2009)

J185eJ188.[22] Y. Gao, Z. Wu, C. Shi, Chin. J. Inorg. Chem. 14 (1998) 190e193.[23] G. Kresse, J. Hafner, Phys. Rev. B 55 (1997) 7539.[24] G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169.[25] G. Kresse, J. Furthmüller, Comp. Mater. Sci. 6 (1996) 15e50.[26] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.[27] N. Zhang, C. Guo, J. Zheng, X. Su, J. Zhao, J. Mater. Chem. C 2 (2014)

3988e3994.[28] S.J. Gwak, P. Arunkumar, W.B. Im, J. Phys. Chem. C 118 (2014) 2686e2692.[29] H. Ji, Z. Huang, Z. Xia, M. Molokeev, V. Atuchin, M. Fang, Y. Liu, J. Phys. Chem. C

119 (2015) 2038e2045.[30] J. Zheng, Q. Cheng, W. Chen, Z. Guo, C. Chen, ECS J. Solid. State. Sci. 4 (2015)

R72eR77.[31] T. Kalpana, M.G. Brik, V. Sudarsan, P. Naresh, V.R. Kumar, L.V. Kityk,

N. Veeraiah, J. Non-Cryst. Solids 419 (2015) 75e81.[32] Z.C. Wu, S. Wang, J. Liu, J.H. Yin, S.P. Kuang, J. Alloys Compd. 644 (2015)

274e279.[33] Z. Xia, X. Wang, Y. Wang, L. Liao, X. Jing, Inorg. Chem. 50 (2011) 10134e10142.[34] G. Blasse, Phys. Lett. A 28 (1968) 444e445.[35] G. Blasse, J. Solid State Chem. 62 (1986) 207e211.[36] Y. Zhang, Z. Wu, D. Geng, X. Kang, M. Shang, X. Li, H. Lian, Z. Cheng, J. Lin, Adv.

Funct. Mater. 24 (2014) 6581e6593.[37] R. Yu, S. Zhong, N. Xue, H. Li, H. Ma, Dalton T 43 (2014) 10969e10976.[38] D. Wang, Q. Yin, Y. Li, M. Wang, J. Lumin 97 (2002) 1e6.[39] K. Shioi, N. Hirosaki, R.J. Xie, T. Takeda, Y.Q. Li, J. Mater. Sci. 45 (2010)

3198e3203.[40] Y.Q. Li, N. Hirosaki, R.J. Xie, T. Takeka, M. Mitomo, J. Solid State Chem. 182

(2009) 301e311.[41] C. Liu, Z. Xia, Z. Lian, Z. Lian, J. Zhou, Q. Yan, J. Mater. Chem. C 1 (2013)

7139e7147.[42] A.H. Reshak, G. Lakshminarayana, G. Proskurina, V.G. Yushanin, S. Calus,

M. Chmiel, R. Miedzinski, M.G. Brik, Opt. Commun. 283 (2010) 3049e3051.[43] O.V. Parasyuk, A.H. Reshak, T.L. Klymuk, I.I. Mazurets, O.V. Zamuruyeva,

G.L. Myronchuk, J. Owsik, Opt. Commun. 307 (2013) 1e4.[44] P. Dorenbos, J. Phys. Condens. Mat. 17 (2005) 8103.

J. Zheng et al. / Journal of Alloys and Compounds 663 (2016) 332e339 339

[45] S. Zhang, Y. Nakai, T. Tsuboi, Y. Huang, H.J. Jin Seo, Inorg. Chem. 50 (2011)2897e2904.

[46] A.M. Srivastava, H.A. Comanzo, S. Camardello, S.B. Chaney, M. Aycibin,U. Happek, J. Lumin 9 (2009) 919e925.

[47] T. Suehiro, N. Hirosaki, R.J. Xie, T. Sato, Appl. Phys. Lett. 95 (2009) 051903.

[48] J. Zheng, Q. Cheng, S. Wu, Y. Zhuang, Z. Guo, Y. Lu, C. Chen, Mater. Chem. Phys.165 (2015) 168e176.

[49] D. Geng, M. Shang, Y. Zhang, Z. Cheng, J. Lin, Eur. J. Inorg. Chem. 2013 (2013)2947e2953.