Embed Size (px)
Citation preview
DaltonTransactions
PAPER
Cite this: Dalton Trans., 2014, 43,7917
Received 13th January 2014,Accepted 13th February 2014
DOI: 10.1039/c4dt00076e
www.rsc.org/dalton
Orangish-yellow-emitting Ca3Si2O7:Eu2+ phosphor
for application in blue-light based warm-whiteLEDs
Chien-Hao Huang,*a Wei-Ren Liu,b Ting-Shan Chanc and Yuan-Tai Lai*d
A Eu2+-activated Ca3Si2O7:Eu2+ orangish-yellow-emitting phosphor with strong luminescence was syn-
thesized and its crystal structure was determined on the basis of XRD profiles using synchrotron radiation.
The crystal structure was refined by the Rietveld refinement method. The excitation and emission spectra
of the Ca3Si2O7:Eu2+ phosphor show broad excitation bands in the range of 240–550 nm and a broad
yellow emission band centered at 603 nm, depending on the concentration of Eu2+. The optimized con-
centration of Eu2+ in the Ca3Si2O7:Eu2+ phosphor was determined to be 0.015 mol. The critical distance
and average decay time were found to be short and fast, respectively, ranging from 19.74 Å to 13.69 Å and
from 2.56 μs to 2.34 μs on increasing the Eu2+ doping content. Warm-white light-emitting diodes (LEDs)
fabricated using an InGaN-based blue LED chip combined with the Ca3Si2O7:0.015Eu2+ phosphor gave
color rendering indices between 76.0 and 38.9, correlated color temperatures between 1924 K and
4992 K, and tuned CIE chromaticity coordinates in the range from orangish-yellow (0.543, 0.389) to
reddish purple (0.333, 0.219). The color coordinates and emission intensity of a Ca3Si2O7:0.015Eu2+-
based white LED display were slightly yellow-shifted and the intensity increased on increasing the
forward-bias current. These results indicate that orangish-yellow-emitting Ca3Si2O7:0.015Eu2+ can serve
as a promising candidate for applications in warm-white LEDs.
Introduction
White light-emitting diodes (LEDs) are considered a promisingtechnology for next-generation solid-state lighting systemsbecause they are environmentally friendly and have severaladvantages, such as a long operational lifetime, highefficiency, compactness, good material stability, and resultantenergy savings.1–3 Nowadays, the majority of white LEDs uses acombination of a blue InGaN chip and a yellow-emittingY3Al5O12:Ce
3+ (YAG:Ce3+) phosphor.4 However, the disadvan-tages of this method are a cool white light with correspondingchromaticity coordinates of (0.292, 0.325), according to theCommission internationale de l’eclairage (CIE), poor color ren-dering indices (CRI) of 75 and a high correlated color tempera-ture (CCT) of 7756 K.5 These disadvantages can be attributedto a lack of red-light6 contributions which restricts the use of
current white LEDs in more vivid applications.7 Therefore, thedevelopment of orangish-yellow or red phosphors to improvethe CRI and CCT of YAG:Ce3+ white LEDs is very important.Many orangish-yellow or red phosphors with Eu2+ ions for acti-vation in InGaN-based LEDs have been reported, including Ca-α-SiAlON:Eu2+,8–10 Sr8Al12O24S2:Eu
2+,11 Sr2Si7Al3ON13:Eu2+,12
CaSrSiO4:Eu2+,13 Li2SrSiO4:Eu
2+,14 α-SrNCN:Eu2+,15 Sr3B2O6−3/2x-Nx:Eu
2+,16 CaAlSiN3:Eu2+,17 and Ca4(PO4)2O:Eu
2+.18 Li et al.19
used an excitation wavelength range of 300–490 nm andobtained emission in the visible range from blue to deep red,centered at 580 nm by the use of the yellow Ba0.93Eu0.07Al2O4
phosphor and a blue LED to fabricate warm-white LEDs withCCT < 4000 K and CRI > 80. Vitta et al.20 described the Ce3+-activated orange-emitting Y3Mg2AlSi2O12:Ce
3+ phosphor.Y3Mg2AlSi2O12:xCe
3+ phosphors exhibit a broad band oforange emission centered at about 600 to 606 nm on increas-ing the Ce3+ content from x = 0.015 to 0.06. A warm-white LEDwas fabricated by combining a 483 nm sky-blue InGaN LEDchip and the orange-emitting Y2.985Ce0.015Mg2AlSi2O12 phos-phor with CIE chromaticity coordinates of (0.4338, 0.4030) andCCT of 3000 K. Hong et al.21 reported an orange-yellow-emit-ting Sr3SiO5:Eu
2+ phosphor, in which the emission spectraexhibit a strong orange-yellow band centered at 581 nm. Theoptical properties of white LEDs fabricated using the Sr3SiO5:Eu2+ phosphor in combination with a 460 nm InGaN blue LED
aMaterial and Chemical Research Laboratories, Industrial Technology Research
Institute, Hsichu, Taiwan 30011, Republic of China. E-mail: [email protected] of Chemical Engineering, Chung Yuan Christian University, Chungli,
Taiwan 32023, Republic of ChinacNational Synchrotron Radiation Research Center, Hsinchu Science Park, Hsinchu,
Taiwan 30076, Republic of ChinadCenter for General Education, Chang Gung University, Tao-Yuan, Taiwan 33302,
Republic of China. E-mail: [email protected]
This journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 43, 7917–7923 | 7917
Publ
ishe
d on
09
Apr
il 20
14. D
ownl
oade
d by
UN
IVE
RSI
TY
OF
BR
IGH
TO
N o
n 07
/07/
2014
23:
08:1
9.
View Article OnlineView Journal | View Issue
chip show CIE color coordinates of x = 0.3582 and y = 0.3110and a CCT of 4193.4 K, which is much lower than that ofInGaN-based YAG:Ce3+ LEDs (7416 K).
In this work, we report on the preparation and investigationof a series of orangish-yellow-emitting Ca3Si2O7:Eu
2+ phos-phors, including the study of their crystal structures using syn-chrotron XRD profiles, luminescence properties, reflectancespectra, decay times, LED packages, and current tunablewarm-white LEDs. White LEDs fabricated using Ca3Si2O7:Eu
2+
exhibit a low CCT. These results demonstrate that Ca3Si2O7:Eu2+ is a promising orangish-yellow-emitting phosphor suit-able for applications in InGaN-based warm-white LEDs.
Experimental sectionMaterials and synthesis
A series of Eu2+-doped Ca3Si2O7:xEu2+ phosphors were syn-
thesized by a high temperature solid-state reaction, startingfrom a mixture containing CaCO3 (A. R. 99.9%), SiO2
(A. R. 99.99%), and Eu2O3 (A. R., 99.99%) in the followingnominal molar ratios: 3(1 − x) : 2 : 3x/2 (x = 0.01–0.03 mol).The powder reactants and 3 weight% of NaF as a flux wereblended and ground thoroughly in an agate mortar, and thehomogeneous mixture was transferred to an alumina crucibleand calcined in a furnace at 1250 °C for 8 h under a reducingatmosphere of 15% H2/85% N2.
Materials characterization
The crystal structures of the as-synthesized samples wererefined and determined using synchrotron XRD profiles withλ = 0.774908 Å, recorded with a large Debye–Scherrer camerainstalled at beam line 01C2 of the National Synchrotron Radi-ation Research Center (NSRRC) in Hsinchu, Taiwan. The GSASprogram22 was used for the structural refinements. The diffusereflectance (DR) spectra were measured with a Hitachi 3010double-beam UV-Vis spectrometer (Hitachi Co., Tokyo, Japan).The measurements of the PL and PLE spectra were performedusing a Spex Fluorolog-3 Spectrofluorometer (Instruments S.A.,NJ, U.S.A.) equipped with a 450 W Xe light source and doubleexcitation monochromators. The powder samples were com-pacted and excited under a 45° incidence angle and theemitted fluorescence was detected by a Hamamatsu PhotonicsR928 type photomultiplier perpendicular to the excitationbeam. Time-resolved measurements were performed with atunable nanosecond optical-parametric-oscillator/Q-switch-pumped YAG:Nd3+ laser system (NT341/1/UV, Ekspla). Emis-sion transients were collected with a nanochromater (Spectra-Pro-300i, ARC), detected with a photomultiplier tube (R928HA,Hamamatsu), connected to a digital oscilloscope (LT372,LeCrop) and transferred to a computer for kinetics analysis.The Commission International de I’Eclairage (CIE) chroma-ticity coordinates for all the samples were measured by a LaikoDT-101 color analyzer equipped with a CCD detector (LaikoCo., Tokyo, Japan).
Results and discussionCrystal structure
Fig. 1 shows the observed (crosses) and calculated (solid line)synchrotron XRD profiles and their difference (bottom) result-ing from the Rietveld refinement of Ca3Si2O7:0.015Eu
2+ atroom temperature with λ = 0.774908 Å. These results indicatethat doping the Ca3Si2O7 lattice with 0.015 mol of Eu2+ neithergenerated impurities nor induced significant structuralchanges. Ca3Si2O7:0.015Eu
2+ crystallizes in a monoclinic unitcell with the space group P121/a1 (14) and lattice constants ofa = 10.5775(4) Å, b = 8.90263(31) Å, c = 7.87443(27) Å, α = γ =90°, β = 119.5942(16)°, a corresponding cell volume of V =644.78(4) Å3, and Z = 4. All atomic positions, fraction factors,and thermal vibrational parameters were refined and con-verged. The reflection conditions were well satisfied, asreflected in reliability factors of Rp = 6.09%, Rwp = 10.38%, andχ2 = 2.608. All the atomic positions and the lattice parametersof the Ca2.955Eu0.045Si2O7 phosphors are shown in Table 1.
Fig. 2 shows the crystal structure of Ca3Si2O7, which wasreported by Saburi et al.23 Arrays of Si2O7 groups are parallel tothe c-axis and the Si2O7 groups are linked together by Caatoms in this structure. The Si2O7 groups are situated aroundseven-coordinate Ca atoms. In the structure of Ca3Si2O7, theCa(1), Ca(2), and Ca(3) sites all have seven oxygen atoms asnearest neighbors. Each coordination polyhedron forms theshape of a distorted decahedron, but there is a regularity in thestructure of distortion among these three decahedrons. The Ca(1)/Eu(1)–O bond lengths are in the range of 2.21294(5)–2.99504(6)Å, Ca(2)/Eu(2)–O are in the range of 2.26632(6)–2.57457(7) Å,Ca(3)/Eu(3)–O are in the range of 2.28021(8)–2.63772(8) Å, Si(1)–O are in the range of 1.62060(4)–1.67492(6) Å, and Si(2)–O are inthe range of 1.60951(4)–1.66578(5) Å. The atomic coordinatesand the equivalent isotropic displacement parameters areshown in Table 2. The seven-coordinated Ca(1), Ca(2), and Ca(3)
Fig. 1 Observed (crosses) and calculated (solid line) synchrotron XRDprofiles and their difference (bottom), resulting from the Rietveld refine-ment of the Ca3Si2O7:0.015Eu
2+ phosphor.
Paper Dalton Transactions
7918 | Dalton Trans., 2014, 43, 7917–7923 This journal is © The Royal Society of Chemistry 2014
Publ
ishe
d on
09
Apr
il 20
14. D
ownl
oade
d by
UN
IVE
RSI
TY
OF
BR
IGH
TO
N o
n 07
/07/
2014
23:
08:1
9.
View Article Online
Ca2+ cations have an ionic radius of 1.06 Å. However, the ionicradius for the seven-coordinated Eu2+ cation is 1.2 Å. Therefore,based on the effective ionic radii of the cations and electriccharge balance, we propose that Eu2+ is randomly distributedamong the three Ca2+ sites of the Ca3Si2O7 host structure.
Photoluminescence properties
Fig. 3 illustrates the reflectance spectra of the Ca3Si2O7 host,Ca3Si2O7:Eu
2+ phosphor, and the photoluminescence/photo-luminescence emission (PL/PLE) spectra of the Ca3Si2O7:Eu
2+
phosphor. The Ca3Si2O7 host material shows energy absorp-tion in the region of λ ≤ 335 nm,24 and the band gap was esti-mated to be about 4.133 eV (300 nm, black short-dashed line).As Eu2+ ions were doped into the Ca3Si2O7 host material, astrong, broad absorption appeared in the range of240–550 nm, corresponding to the UV–green range, which wasassigned to the 4f7→4f65d1 transition of the Eu2+ ions. Theinsets of Fig. 3 show photographs of the orangish-yellow-emit-ting Ca3Si2O7:Eu
2+ phosphor and the Ca3Si2O7:Eu2+ phosphor
under 365 nm UV irradiation. By Gaussian deconvolution, thePL spectra of the Ca3Si2O7:Eu
2+ phosphor can be decomposedinto three Gaussian profiles with peaks centered at 584 nm(17 123 cm−1), 631 nm (15 848 cm−1), and 667 nm(14 992 cm−1), as shown by the green solid lines in Fig. 3.These peaks can be ascribed to three different emission sites,which could be identified as the different coordinationenvironments of the Ca2+ ions in the cases where the Ca sitesare partially and randomly occupied by Eu2+ ions.25 The PLEspectra showed a broad absorption between 240 and 550 nm,which was attributed to the 4f7→4f65d1 transition of Eu2+. Thebroad excitation band matches well with the reflection spectraand UV–blue chips for application in white LEDs.
The concentration dependence of the PL/PLE intensity ofCa3Si2O7:xEu
2+ (x = 0.01–0.03) is demonstrated in Fig. 4. Theemission spectra showed a strong and broad orangish-yellowemission band in the range of 500–800 nm, centered at603 nm, which is typically attributed to the 4f65d1→4f7 electricdipole transitions of Eu2+ ions. The excitation spectra show abroad distribution of excitations over a range of 240–550 nm,centered at 272, 311, 345, 397, and 463 nm. This band of
Fig. 2 Crystal structure of Ca3Si2O7.
Table 2 Selected bond lengths (Å) of the Ca2.955Eu0.045Si2O7
phosphors
Bonds Lengths (Å) Bonds Lengths (Å)
Ca(1)/Eu(1)/–O(1) 2.44663(6) Ca(3)/Eu(3)/–O(2) 2.29362(5)Ca(1)/Eu(1)/–O(1) 2.31513(6) Ca(3)/Eu(3)/–O(3) 2.51188(6)Ca(1)/Eu(1)/–O(2) 2.41220(6) Ca(3)/Eu(3)/–O(3) 2.28021(8)Ca(1)/Eu(1)/–O(4) 2.51889(6) Ca(3)/Eu(3)/–O(4) 2.57450(7)Ca(1)/Eu(1)/–O(6) 2.21294(5) Ca(3)/Eu(3)/–O(5) 2.37468(7)Ca(1)/Eu(1)/–O(7) 2.66022(9) Ca(3)/Eu(3)/–O(6) 2.63772(8)Ca(1)/Eu(1)/–O(7) 2.99504(6) Ca(3)/Eu(3)/–O(7) 2.56470(8)
Ca(2)/Eu(2)/–O(1) 2.42545(6) Si(1)–O(4) 1.67237(5)Ca(2)/Eu(2)/–O(2) 2.51111(6) Si(1)–O(5) 1.67492(6)Ca(2)/Eu(2)/–O(3) 2.45692(6) Si(1)–O(6) 1.62060(4)Ca(2)/Eu(2)/–O(5) 2.26632(6) Si(1)–O(7) 1.64632(6)Ca(2)/Eu(2)/–O(5) 2.44389(9)Ca(2)/Eu(2)/–O(6) 2.30703(6) Si(2)–O(1) 1.62169(5)Ca(2)/Eu(2)/–O(7) 2.57457(7) Si(2)–O(2) 1.63195(4)
Si(2)–O(3) 1.60951(4)Si(2)–O(4) 1.66578(5)
Table 1 Atomic positions and lattice parameters of theCa2.955Eu0.045Si2O7 phosphors
Atoms Posit. x y z Occup. Uiso × 100
Ca(1) 4e 0.0061(5) 0.0571(4) 0.2907(6) 0.985 4.21(13)Ca(2) 4e 0.1690(5) 0.5723(4) 0.2109(6) 0.985 3.34(12)Ca(3) 4e 0.3407(4) 0.9062(4) 0.2833(6) 0.985 3.10(12)Eu(1) 4e 0.0061(5) 0.0571(4) 0.2907(6) 0.015 4.21(13)Eu(2) 4e 0.1690(5) 0.5723(4) 0.2109(6) 0.015 3.34(12)Eu(3) 4e 0.3407(4) 0.9062(4) 0.2833(6) 0.015 3.10(12)Si(1) 4e 0.0909(7) 0.2145(7) 0.9843(9) 1 3.68(18)Si(2) 4e 0.2961(6) 0.2317(6) 0.4308(9) 1 3.91(19)O(1) 4e 0.3568(12) 0.3973(14) 0.4227(16) 1 4.96(40)O(2) 4e 0.1775(11) 0.2415(11) 0.5058(15) 1 2.83(34)O(3) 4e 0.4115(12) 0.1003(15) 0.5466(15) 1 3.63(39)O(4) 4e 0.1957(11) 0.1525(13) 0.2126(16) 1 3.10(35)O(5) 4e 0.0955(12) 0.4017(14) 0.9666(15) 1 3.70(39)O(6) 4e 0.1421(11) 0.1364(12) 0.8422(17) 1 4.14(37)O(7) 4e 0.9265(11) 0.1666(13) 0.9376(14) 1 3.13(36)
Fig. 3 Reflectance spectra of the Ca3Si2O7 host (black short-dashedline), Ca3Si2O7:Eu
2+ phosphor (black solid line), and PL/PLE spectra ofthe Ca3Si2O7:Eu
2+ phosphor. The insets show the Ca3Si2O7:Eu2+ phos-
phor (a) exterior and (b) irradiated by 365 nm UV light.
Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 43, 7917–7923 | 7919
Publ
ishe
d on
09
Apr
il 20
14. D
ownl
oade
d by
UN
IVE
RSI
TY
OF
BR
IGH
TO
N o
n 07
/07/
2014
23:
08:1
9.
View Article Online
excitations comprises further unresolved transitions due to thesplitting of the 5d energy level of Eu2+ in the excited state.With the increase in the Eu2+ doping concentration, the exci-tation edge showed a red shift owing to enhanced Eu2+–Eu2+
interactions. The broad band of excitations again matches wellwith the spectra in the NUV–blue wavelength range of chips usedfor application in white LEDs. The optimal Eu2+ concentration (x)was determined to be 0.015 mol, since a further increase in con-centration resulted in concentration quenching, which, in turn,caused a decrease in the emission intensity. Concentrationquenching may be enabled by interactions between Eu2+ ions,resulting in energy reabsorption among neighboring Eu2+ ions inthe rare-earth sublattice and an energy transfer from a percolat-ing cluster of Eu2+ ions to killer centers.26
The critical distance, Rc, between neighboring Eu2+ ionswas calculated using the concentration quenching methodand is given by the relation:27
Rc � 23V
4πxcN
� �1=3ð1Þ
where V is the volume and N is the number of host cations inthe unit cell. The values of V and N are 644.78 Å3 and 4. The
critical Eu2+–Eu2+ distance, REu–Eu, in Ca3Si2O7:xEu2+ was
determined to be 19.74, 18.33, 17.25, 15.67, 14.55, and 13.69 Åfor x = 0.01, 0.0125, 0.015, 0.02, 0.025, and 0.03, respectively.The aforementioned result implies that on increasing the Eu2+
concentration, REu–Eu decreased and the crystal-field splittingof the 5d levels of the Eu2+ ions increased. The critical concen-tration (xc) of Eu2+ in the Ca3Si2O7 host was found to be0.015 mol. Therefore, the Rc value for energy transfer was cal-culated to be 17.25 Å, which is in good agreement with theresults reported in the literature.28
Luminescence decay times
The decay process of the Ca3Si2O7:xEu2+ (x = 0.01–0.03) phos-
phors excited at 460 nm and monitored at 603 nm weremeasured and are presented in Fig. 5. The correspondingluminescence decay times can be best fit by a third-order expo-nential decay model described by the following equation:29
I ¼ A1expð�t=τ1Þ þ A2expð�t=τ2Þ þ A3expð�t=τ3Þ ð2Þwhere I is the luminescence intensity; A1, A2, and A3 are con-stants; t is time; and τ1, τ2, and τ3 are the rapid, medium, andslow lifetimes of the exponential components. The values ofthe parameters A1, A2, A3 and of the lifetimes τ1, τ2, and τ3 ofCa3Si2O7:xEu
2+ (x = 0.01–0.03) were determined and are sum-marized in Table 3. The lifetime τ1 becomes longer from33.6 ns to 37.5 ns, and τ2 and τ3 become shorter from 924 nsto 844 ns and from 2.74 μs to 2.61 μs, respectively. The con-stants A1 and A2 increase and A3 decreases on increasing theEu2+ content. Using these parameters, the average decay times(τ*) can be determined as follows:30
kτ*l ¼Ð10 tIðtÞdtÐ10 IðtÞdt ¼
Piaiτi2P
iaiτi
¼ A1τ12 þ A2τ22 þ A3τ32
A1τ1 þ A2τ2 þ A3τ3ð3Þ
The average decay times (τ*) of the Ca3Si2O7:xEu2+ phos-
phors were calculated to be 2.56, 2.53, 2.48, 2.41, 2.37, and2.34 μs for x = 0.01, 0.0125, 0.015, 0.02, 0.025, and 0.03,
Fig. 4 Concentration dependence of the (a) PLE and (b) PL intensity asa function of Eu2+ content for the Ca3Si2O7:xEu
2+ (x = 0.01–0.03) phos-phors. The inset shows the relative emission intensity as a function ofEu2+ content under excitation at 460 nm.
Fig. 5 Decay curves of the Eu2+ emission in the Ca3Si2O7:xEu2+ (x =
0.01–0.03) phosphors, excited at 460 nm and monitored at 603 nm.
Paper Dalton Transactions
7920 | Dalton Trans., 2014, 43, 7917–7923 This journal is © The Royal Society of Chemistry 2014
Publ
ishe
d on
09
Apr
il 20
14. D
ownl
oade
d by
UN
IVE
RSI
TY
OF
BR
IGH
TO
N o
n 07
/07/
2014
23:
08:1
9.
View Article Online
respectively. As is evident, the average decay time was found todecrease on increasing the doping content of the Eu2+ ions.The faster average decay can be attributed to nonradiativeenergy transfer among the Eu2+ ions at higher Eu2+ concen-trations. The fitting results obtained by the exponential decayemploying a third-order component indicate that the Eu2+ ionsoccupy three different Ca2+ sites with different coordinationenvironments in the Ca3Si2O7 lattice.
31
White LED-lamp fabrication and electroluminescence (EL)spectra
White-light LED lamps were fabricated by dropping a mixtureof a transparent silicone resin and the orangish-yellow-emit-ting Ca3Si2O7:0.015Eu
2+ or yellow-emitting YAG:Ce3+ phosphoronto an InGaN-based blue LED chip and heat-treating it at120 °C for 10 h afterwards. Fig. 6 shows the EL spectra of thewarm-white LEDs composed of (a) an InGaN blue-light chipcombined with the Ca3Si2O7:0.015Eu
2+ phosphor, (b) a blue-light chip pumped by a YAG phosphor, driven by a current of350 mA. The EL spectra clearly show blue bands originatingfrom the InGaN blue-light chip at around 460 nm, and orang-ish-yellow bands corresponding to the Ca3Si2O7:0.015Eu
2+
phosphor at around 621 nm (Fig. 6a), or to the yellow band ofYAG:Ce3+ at around 550 nm (Fig. 6b). By tuning the weightratio of the Ca3Si2O7:0.015Eu
2+ phosphor to silicone, the CIEcolor coordinates, CCT, and CRI of the warm-white LEDs canbe tuned from (0.543, 0.389) to (0.333, 0.219), 1733 K to4992 K, and from 76 to 39, respectively, as shown in Fig. 6a.The optical properties of YAG-based white LEDs, whose spec-trum is shown in Fig. 6b, are described by CIE color coordi-nates of (0.302, 0.315) at a CCT of 7272 K and CRI of 78.4. Theinset shows photographs of the LED lamp package under aforward bias of 350 mA and witout. A comparison of the YAG-and Ca3Si2O7:0.015Eu
2+-based white LEDs shows CCT valuesof CCT = 1733–4992 K for Ca3Si2O7:0.015Eu
2+ and 7272 K forYAG, along with poor CRI values of CRI = 76–39 forCa3Si2O7:0.015Eu
2+ and 78.4 for YAG. Therefore, the Ca3Si2O7:Eu2+ phosphors are promising materials for application inwarm-light LEDs.
As shown in Fig. 7 and Table 4, the CIE chromaticitydiagram, CIE chromaticity coordinates, CCT, and CRI weredetermined for the Ca3Si2O7:0.015Eu
2+-based white LEDsdriven by a current of 350 mA. By weight-ratio tuning, theoutputs of the silicone resin and the Ca3Si2O7:0.015Eu
2+ phos-phor were found to systematically shift toward the blue regionon increasing the silicone-resin weight ratio. The color tone
can be tuned from orangish-yellow through warm-white up toreddish purple with corresponding chromaticity coordinates(x, y) at six different weight ratios varying from (point 1,(0.543, 0.389)) via (point 2, (0.517, 0.371)), (point 3, (0.481,0.342)), (point 4, (0.435, 0.308)) to (point 5, (0.388, 0.266))and, finally, to (point 6, (0.333, 0.219)). Accordingly, CCT andCRI can be tuned from (1733 K, 76.0) via (1815 K, 73.6),(1924 K, 70.9), (2157 K, 65.8), (2494 K, 55.4), to (4992 K, 38.9),respectively. It is thus observed that CCT and CRI graduallyincrease and decrease, respectively, on increasing the silicone-resin weight ratio. However, the disadvantage of theseCa3Si2O7:0.015Eu
2+ phosphor-based white LEDs is the low CRI
Table 3 Decay times of the Ca3Si2O7:xEu2+ phosphors, excited at 460 nm whereas the emission peak is monitored at 603 nm
Sample τ1 A1 τ2 A2 τ3 A3 τ*
x = 0.0100 3.36 × 10−8 0.69 9.24 × 10−7 0.17 2.74 × 10−6 0.63 2.56 × 10−6
x = 0.0125 3.44 × 10−8 0.70 9.02 × 10−7 0.18 2.73 × 10−6 0.61 2.53 × 10−6
x = 0.0150 3.46 × 10−8 0.72 8.93 × 10−7 0.18 2.68 × 10−6 0.58 2.48 × 10−6
x = 0.0200 3.61 × 10−8 0.75 8.62 × 10−7 0.20 2.64 × 10−6 0.55 2.41 × 10−6
x = 0.0250 3.68 × 10−8 0.77 8.53 × 10−7 0.22 2.62 × 10−6 0.53 2.37 × 10−6
x = 0.0300 3.75 × 10−8 0.80 8.44 × 10−7 0.23 2.61 × 10−6 0.51 2.34 × 10−6
Fig. 6 EL spectra of warm-white LEDs fabricated using an InGaN-based blue LED chip combined with (a) the Ca3Si2O7:0.015Eu
2+ and (b)the YAG:Ce3+ phosphor under a forward bias of 350 mA. The insetshows a photograph of the LED lamps package.
Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 43, 7917–7923 | 7921
Publ
ishe
d on
09
Apr
il 20
14. D
ownl
oade
d by
UN
IVE
RSI
TY
OF
BR
IGH
TO
N o
n 07
/07/
2014
23:
08:1
9.
View Article Online
value of CRI = 76.0–38.9, owing to the lack of a green spectralcontribution. To improve the poor CRI of LEDs based on anInGaN chip in combination with an orangish-yellow-emittingCa3Si2O7:0.015Eu
2+ phosphor, we could add the green-emitting(Ba,Sr)2SiO4:Eu
2+, (Ca,Sr,Ba)Si2O2N2 or LuAG phosphor.Fig. 8 shows the EL spectra of the white LEDs fabricated
using an InGaN-based blue LED chip combined with theCa3Si2O7:0.015Eu
2+ phosphor under different forward-bias cur-rents in the range of 150–850 mA. It is observed that the colorcoordinates of the fabricated LED were slightly yellow-shifted,and the emission intensity of the Ca3Si2O7:0.015Eu
2+-basedwhite LED display increased on increasing the forward-bias
current, which indicates that the Ca3Si2O7:0.015Eu2+ phosphor
did not reach saturation in the EL spectrum, thus turningthem into promising materials for application in warm-whiteLEDs.32 Upon increasing the current from 150 mA to 850 mA,the chromaticity coordinates yellow-shifted slightly from(0.466, 0.333) to (0.382, 0.344), the CCT decreased from 2006 Kto 1924 K, and the CRI value increased from 69.21 to 71.8, asshown in Table 5, respectively.
Conclusions
A warm-white LED device with a low CCT has been fabricatedusing a blue LED chip combined with an orangish-yellow-emit-ting Ca3Si2O7:0.015Eu
2+ phosphor, and its luminescence pro-perties, reflectance spectra, luminescent decay times, and ELperformance have been investigated. The excitation and reflec-tance spectra of this phosphor show broad excitation bandsand absorption in the wavelength/color range of 240–550 nm/UV–green, indicating a high application potential in blueInGaN-based white LEDs. Upon excitation at 460 nm, the Eu2+-doped Ca3Si2O7 phosphors showed strong orangish-yellowemission centered at 603 nm, which could be ascribed to the
Fig. 7 CIE chromaticity diagram of an InGaN-based blue LED chipcombined with the Ca3Si2O7:0.015Eu
2+ phosphor (point 1–6), and YAG:Ce3+ (point w).
Table 4 CIE chromaticity coordinates, CCT, and CRI of InGaN-basedwarm-white LEDs at a current of 350 mA
Site
CIE (x, y)
CCT (K) CRIx y
1 0.543 0.389 1733 76.02 0.517 0.371 1815 73.63 0.481 0.342 1924 70.94 0.435 0.308 2157 65.85 0.388 0.266 2494 55.46 0.333 0.219 4992 38.9Y 0.425 0.454 — —B 0.144 0.030 — —W 0.302 0.315 7272 78.4
Fig. 8 EL spectra of the InGaN-based white LEDs at various driving cur-rents (150–850 mA). The inset shows the variation of the CIE chroma-ticity coordinates of the white LEDs operated at different currents.
Table 5 CIE chromaticity coordinates, CCT, and CRI of InGaN-basedwarm-white LEDs at various currents (150–850 mA)
Drive current I (mA)
CIE (x, y)
CCT (K) CRIx y
150 0.466 0.333 2006 69.21250 0.471 0.336 1969 69.77350 0.475 0.338 1947 70.10450 0.477 0.339 1940 70.38550 0.478 0.341 1938 70.74650 0.480 0.342 1936 71.08750 0.480 0.343 1931 71.28850 0.482 0.344 1924 71.38
Paper Dalton Transactions
7922 | Dalton Trans., 2014, 43, 7917–7923 This journal is © The Royal Society of Chemistry 2014
Publ
ishe
d on
09
Apr
il 20
14. D
ownl
oade
d by
UN
IVE
RSI
TY
OF
BR
IGH
TO
N o
n 07
/07/
2014
23:
08:1
9.
View Article Online
4f65d1→4f7 transitions of Eu2+. The optimal doping concen-tration of Eu2+ in Ca3Si2O7 was determined to be 0.015 mol.The optical properties in terms of color chromaticity coordi-nates (x, y), CCT, and CRI of the warm-white LEDs can betuned from orangish-yellow-emitting ((0.543, 0.389), 1733 K,76.0) through warm-white ((0.481, 0.342), 1924 K, 70.9) up toreddish purple ((0.333, 0.219), 4992 K, 38.9). Therefore, ourorangish-yellow-emitting Ca3Si2O7:Eu
2+ phosphor may serve asa key material for phosphor-converted warm-light LEDs.
Acknowledgements
This research was supported by the Industrial TechnologyResearch Institute under contract no. D354DC3200 (S. M. J.)and in part by the National Science Council of Taiwan undercontract no. NSC 100-2113-M-182-001-MY3.
Notes and references
1 M. S. Shur and A. Žukauskas, Proc. IEEE, 2005, 93, 1691–1703.
2 M. R. Krames, O. B. Shchekin, R. Mueller-Mach,G. O. Mueller, L. Zhou, G. Harbers and M. G. Craford,J. Disp. Technol., 2007, 3, 160–175.
3 M. H. Crawford, IEEE J. Sel. Top. Quantum Electron., 2009,15, 1028–1040.
4 V. Bachmann, C. Ronda and A. Meijerink, Chem. Mater.,2009, 21, 2077–2084.
5 H. S. Jang, Y. H. Won and D. Y. Jeon, Appl. Phys. B: LasersOpt., 2009, 95, 715–720.
6 M. Batentschuk, A. Osvet, G. Schierning, A. Klier,J. Schneider and A. Winnacker, Radiat. Meas., 2004, 38,539–543.
7 A. A. Setlur, W. J. Heward, Y. Gao, A. M. Srivastava,R. G. Chandran and M. V. Shankar, Chem. Mater., 2006, 18,3314–3322.
8 R. J. Xie, M. Mitomo, K. Uheda, F. F. Xu and Y. Akimune,J. Am. Ceram. Soc., 2002, 85, 1229–1234.
9 R. J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto andM. Mitomo, Appl. Phys. Lett., 2004, 84, 5404–5406.
10 J. W. H. van Krevel, J. W. T. van Rutten, H. Mandal,H. T. Hintzen and R. Metselaar, J. Solid State Chem., 2002,165, 19–24.
11 T. W. Kuo, C. H. Huang and T. M. Chen, Opt. Express, 2010,18, A231–A236.
12 Y. Fukuda, A. Okada and A. K. Albessard, Appl. Phys.Express, 2012, 5, 062102.
13 S. Tezuka, Y. Sato, T. Komukai, Y. Takatsuka, H. Kato andM. Kakihana, Appl. Phys. Express, 2013, 6, 072101.
14 T. G. Kim, H. S. Lee, C. C. Lin, T. Kim, R. S. Liu, T. S. Chanand S. J. Im, Appl. Phys. Lett., 2010, 96, 061904.
15 M. Krings, G. Montana, R. Dronskowski and C. Wickleder,Chem. Mater., 2011, 23, 1694–1699.
16 S. H. Jung, D. S. Kang and D. Y. Jeon, J. Cryst. Growth, 2011,326, 116–119.
17 K. Uheda, N. Hirosaki, Y. Yamamoto, A. Naoto,T. Nakajima and H. Yamamoto, Electrochem. Solid-StateLett., 2006, 9, H22–H25.
18 D. Deng, H. Yu, Y. Li, Y. Hua, G. Jia, S. Zhao, H. Wang,L. Huang, Y. Li, C. Li and S. Xu, J. Mater. Chem. C, 2013, 1,3194–3199.
19 X. F. Li, J. D. Budai, F. Liu, J. Y. Howe, J. Zhang, X. J. Wang,Z. Gu, C. Sun, R. S. Meltzer and Z. Pan, Light: Sci. Appl.,2013, 2, e50.
20 A. Katelnikovas, T. Bareika, P. Vitta, T. Justel, H. Winkler,A. Kareiva, A. Ž. Ukauskas and G. Tamulaitis, Opt. Mater.,2010, 32, 1261–1265.
21 E. H. Kang, S. W. Choi, S. E. Chung, J. Jang, S. Kwon andS. H. Hong, J. Electrochem. Soc., 2011, 158, J330–J333.
22 A. C. Larson and R. B. Von Dreele, Generalized StructureAnalysis System (GSAS), Los Alamos National LaboratoryReport LAUR, 1994, 86–748.
23 S. Saburi, I. Kusachi, C. Henmi, A. Kawahara, K. Henmiand I. Kawada, Mineral. J., 1976, 8, 240–246.
24 C. H. Huang, T. S. Chan, W. R. Liu, D. Y. Wang, Y. C. Chiu,Y. T. Yeh and T. M. Chen, J. Mater. Chem., 2012, 22, 20210–20216.
25 X. Zhang, H. Chen, W. Ding, H. Wu and J. Kim, J. Am.Ceram. Soc., 2009, 92, 429–432.
26 V. A. Vyssotsy, S. B. Gordon, H. L. Frisch andJ. M. Hammersley, Phys. Rev., 1961, 123, 1566–1567.
27 C. H. Huang, Y. C. Chen, T. M. Chen, T. S. Chan andH. S. Sheu, J. Mater. Chem., 2011, 21, 5645–5649.
28 X. Zhang, Z. Lu, F. Meng, F. Lu, L. Hu, X. Xu and C. Tang,Mater. Lett., 2012, 66, 16–18.
29 M. C. Chang, J. W. Petrich, D. B. McDonald andG. R. Fleming, J. Am. Chem. Soc., 1983, 105, 3819–3824.
30 M. Shang, G. Li, D. Geng, D. Yang, X. Kang, Y. Zhang,H. Lian and J. Lin, J. Phys. Chem. C, 2012, 116, 10222–10231.
31 C. H. Huang and T. M. Chen, Inorg. Chem., 2011, 50, 5725–5730.
32 H. S. Jang, H. Yang, S. W. Kim, J. Y. Han, S. G. Lee andD. Y. Jeon, Adv. Mater., 2008, 20, 2696–2702.
Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 43, 7917–7923 | 7923
Publ
ishe
d on
09
Apr
il 20
14. D
ownl
oade
d by
UN
IVE
RSI
TY
OF
BR
IGH
TO
N o
n 07
/07/
2014
23:
08:1
9.
View Article Online
