Crystal structure of blue–white–yellow color-tunable Ca
4
Si
2
O
7
F
2
:Eu
2+
,Mn
2+
phosphor and investigation of color tunability through energy transfer for
single-phase white-light near-ultraviolet LEDs†
Chien-Hao Huang,*
adTing-Shan Chan,
bWei-Ren Liu,
cDe-Yin Wang,
dYi-Chen Chiu,
aYao-Tsung Yeh
aand Teng-Ming Chen*
dReceived 18th May 2012, Accepted 25th July 2012 DOI: 10.1039/c2jm33160h
The crystal structure of Ca4Si2O7F2:Eu2+,Mn2+was refined and determined from X-ray diffraction (XRD) profiles obtained using a synchrotron light source by the Rietveld refinement method. It was found to crystallize into a monoclinic structure with the P21/c(14) space group. On examining the Mn2+-concentration-dependent photoluminescence properties, we found that the emission colors could be tuned from blue (0.152, 0.112) to white-light (0.351, 0.332) and eventually to yellow (0.430, 0.423) through energy transfer by changing the Eu2+/Mn2+ratio. Moreover, energy transfer from a sensitizer Eu2+to an activator Mn2+occurs via a resonance-type dipole–quadrupole interaction mechanism, and the critical distances of the energy transfer were calculated to be 11.66 A and 12.61 A using
concentration quenching and spectral overlap methods, respectively. Combining a 400 nm near-ultraviolet (NUV) chip and a single-phase white-emitting (Ca0.96Eu0.01Mn0.03)4Si2O7F2phosphor produced a white-light NUV LED with CIE chromaticity coordinates of (0.347, 0.338) and a warm color temperature of 4880 K.
1.
Introduction
In recent years, phosphor-converted white light-emitting diodes (LEDs) have been in high demand for use in solid-state lighting technology applications, because of their high efficiency, good material stability, long operational lifetime, and environmentally friendly characteristics.1–3 Nowadays, the majority of white
LEDs use a combination of a blue InGaN chip and a yellow-emitting Y3Al5O12:Ce3+phosphor. However, the disadvantages of this method are low color-rendering index and high correlated color temperature (CCT). These disadvantages can be attributed to the deficiency of red emission in the visible spectrum.4It is also
possible to produce white-light by means of adopting two, three, or even four phosphors in near-ultraviolet (NUV)/UV LED chips;5 however, poor luminous efficiency attributed to
reabsorption has commonly been encountered. Thus, white LEDs fabricated using NUV/UV LED chips with a single-composition phosphor are considered to be potentially useful because they exhibit low color aberration, low cost, high color-rendering indexes, good color tone, and tunability of the CIE color coordinates and the CCT. A single-composition white-light phosphor can be produced by co-doping a sensitizer and an activator into the same crystalline matrix, using the principle of energy transfer from the sensitizer to the activator. Mn2+-doped luminescent materials have been known to have wide-range emissions from 500 to 700 nm depending on the crystal field of the host material.6–8They could be good candidates from green
(weak crystal field) to red (stronger crystal field) phosphors, but the disadvantage of the Mn2+ions is that their d–d absorp-tion transiabsorp-tion is difficult to pump, since it is both parity and spin forbidden. As a promising sensitizer for the Mn2+ion, Eu2+ has been widely applied in many Mn2+-doped hosts, such as Ca9Y(PO4)7,9 Ba3MgSi2O8,10 Na(Sr,Ba)PO4,11 Ca3Al 2-Si2O8Cl4,12SrMgB6O11,13Ca2SiO3Cl2,14Sr3Y(PO4)3(ref. 15) and Ca9Gd(PO4)7,16to improve the emission intensity of Mn2+.
In this work, we report the preparation and investigation of a series of single-composition emission-tunable Ca4Si2O7 -F2:Eu2+,Mn2+ phosphors, including their crystal structure, reflectance spectra, and luminescence properties. The energy transfer mechanism between Eu2+and Mn2+in the Ca4Si2O7F2 host matrix was also studied, and the critical distance of the energy transfer from the sensitizer Eu2+to the activator Mn2+
aMaterial and Chemical Research Laboratories, Industrial Technology
Research Institute, Hsinchu, Taiwan 30011, R.O.C. E-mail: [email protected]; Tel: +86-886-3-5732438
bNational Synchrotron Radiation Research Center, Hsinchu Science Park,
Hsinchu, Taiwan 30076, R.O.C
cDepartment of Chemical Engineering, Chung Yuan Christian University,
Chungli, Taiwan 32023, R.O.C
dPhosphors Research Laboratory and Department of Applied Chemistry,
National Chiao Tung University, Hsinchu, Taiwan 30010, R.O.C. E-mail: [email protected]; Tel: +86-886-35731695
† Electronic supplementary information (ESI) available. CCDC 883884. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2jm33160h
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 20210
www.rsc.org/materials
PAPER
was calculated by concentration quenching and spectra overlap methods. Finally, we have also successfully demonstrated the fabrication and examined the optical properties of a white phosphor-converted LED by adopting a white-emitting Ca4Si2O7F2:Eu2+,Mn2+ phosphor with a 400 nm NUV LED chip.
2.
Experimental
2.1. Materials and synthesis
A series of rare earth-doped (Ca0.99xEu0.01Mnx)4Si2O7F2(x¼ 0–0.1 mol) phosphors were synthesized by a high-temperature solid-state reaction in which the constituent raw materials CaCO3(99.99%, Aldrich), CaF2(99.99%, Aldrich), SiO2(99.6%, Aldrich), EuF2(99.9%, Alfa) and MnO (99.9%, Aldrich) were weighed in stoichiometric proportions. The powder reactants were blended and ground thoroughly in an agate mortar, and the homogeneous mixture was transferred to an alumina crucible and calcined in a furnace at 1373 K for 8 h under a reducing atmosphere of 15% H2/85% N2. The products were then cooled to room temperature in the furnace, ground, and pulverized for further measurements.
2.2. Materials characterization
The crystal structure of the as-synthesized samples identified by using synchrotron XRD patterns with l¼ 0.774908 A (16 keV) were recorded with a large Debye–Scherrer camera installed at beam line 01C2 of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan; the GSAS program17was used for
the structural refinements. The diffuse reflectance (DR) spectra were measured with a Hitachi 3010 double-beam UV-vis spectrometer (Hitachi Co., Tokyo, Japan). The photo-luminescence (PL) and photophoto-luminescence excitation (PLE) spectra of the samples were analyzed by using a Spex Fluorolog-3 Spectrofluorometer equipped with a 450 W Xe light source. The Commission International de I’Eclairage (CIE) chromaticity coordinates for all samples were measured by a Laiko DT-101 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan). The specification for the 400 nm NUV chip: (AOT Product no: DC0004CAA, Spec: 370U02C, wavelength peak: 395–400 0.6 nm, chip size: 40 40 mil2, forward voltage: 3.8–4.0 0.02 V, power: 10–20 0.21 mW).
3.
Results and discussion
3.1. Crystal structure
Fig. 1 shows the observed (crosses) and calculated (solid line) synchrotron X-ray diffraction (XRD) profiles and their differ-ence (bottom) for the Rietveld refinement of (Ca0.99Eu0.01)4 -Si2O7F2and (Ca0.89Eu0.01Mn0.1)4Si2O7F2at room temperature with l¼ 0.774908 A. These results indicate that when Eu2+or Eu2+/Mn2+is doped in the Ca4Si2O7F2 host, it remains single phased. Ca4Si2O7F2:Eu2+,Mn2+ crystallizes into a monoclinic structure with the space group P21/c(14).18 Fig. 1a shows the results of the Rietveld refinement of the (Ca0.99Eu0.01)4Si2O7F2 crystal; the lattice parameters were determined to be a ¼ 7.5624(1) A, b ¼ 10.5722(2) A, c ¼ 10.9451(2) A, b ¼
109.5984(11), V¼ 824.37(9) A3, and Z¼ 4 and the refinement finally converged to Rp¼ 5.23%, Rwp¼ 7.49% and c2¼ 1.96 (Table 1). Since Ca2+ in the (Ca0.99Eu0.01)4Si2O7F2 structure was replaced by a small Mn2+ ion, the lattice parameters
Fig. 1 Observed (crosses) and calculated (solid line) synchrotron XRD profiles and their difference (bottom) for the Rietveld refinement of (a) (Ca0.99Eu0.01)Si2O7F2 and (b) (Ca0.89Eu0.01Mn0.1)4Si2O7F2 at room
temperature with l¼ 0.774908 A (16 keV).
Table 1 Rietveld refinement and crystal data of (Ca0.99Eu0.01)4Si2O7F2
and (Ca0.89Eu0.01Mn0.1)4Si2O7F2phosphors
Formula (Ca0.99Eu0.01)4Si2O7F2 (Ca0.89Eu0.01Mn0.1)4Si2O7F2
Radiation type (A) 0.7749 0.7749 2q range (deg.) 5–40 5–40 Temperature (K) 298 298 Formula weight 371.00 376.94 Symmetry Monoclinic Monoclinic Space group P21/c(14) P21/c(14) a (A) 7.5624(1) 7.5463(2) b (A) 10.5722(2) 10.5532(4) c (A) 10.9451(2) 10.9381(4) a ¼ g (deg.) 90 90 b (deg.) 109.5984(11) 109.5782(20) Volume (A3) 824.37(9) 820.72(5) Z 4 4 Rp 5.23% 7.05% Rwp 7.49% 10.30% c2 1.96 3.09
of (Ca0.89Eu0.01Mn0.1)4Si2O7F2 became a ¼ 7.5463(2) A, b ¼ 10.5532(4) A, c ¼ 10.9381(4) A, b ¼ 109.5782(20), V ¼ 820.72(5) A3, and Z¼ 4 and the refinement finally converged to Rp¼ 7.05%, Rwp¼ 10.30% and c2¼ 3.09, in good agreement with the previous result, as shown in Fig. 1b. There is a negligible amount of impurities in phase at 2q¼ 7.96, 16.43 and 16.82 site, as the (Ca0.99Eu0.01)4Si2O7F2is doped with higher concentrations of Mn2+(>5 mol%). The final refined positions of all atoms and the lattice parameters for the (Ca0.89Eu0.01Mn0.1)4Si2O7F2 phosphors are listed in Table S1.†
The insets of Fig. 1a show the crystal structure of Ca4Si2O7F2. Each cation has several different coordination environments: Ca(1) atom is eight-coordinated by five O atoms with an average Ca(1)–O distance of 2.53571 A and three F atoms with an average Ca(1)–F distance of 2.41412 A, Ca(2) atom is sur-rounded by four O and three F atoms at average distances of 2.49490 and 2.33007 A, Ca(3) atom is six-coordinated by five O atoms at an average Ca(3)–O distance of 2.36809 A and one F atom at an average Ca(3)–F distance of 2.29127 A, Ca(4) atoms are seven-coordinated by six O atoms at an average Ca(4)–O distance of 2.45061 A and one F atom at a Ca(4)–F distance of 2.37536 A, and Si(1) and Si(2) atoms are tetrahedrally coordi-nated by four O atoms at average distances of 1.6398 and 1.64253
A. The bond distance is shown in Table S2.† The ionic radii for the eight- and six-coordinated Ca2+ions are 1.12 and 1.00 A, respectively. Similarly, the ionic radii for the eight- and six-coordinated Eu2+ions are 1.25 and 1.17 A and those for Mn2+ are 0.96 and 0.83 A, respectively. Based on the effective ionic radii of cations with different coordination numbers and electric charge balances, we propose that the Eu2+and Mn2+ions are expected to randomly occupy the Ca2+ ion sites in the Ca4Si2O7F2crystal structure.
3.2. Luminescence properties
Fig. 2 illustrates the reflectance spectra of the pure Ca4Si2O7F2 host and Eu2+-doped Ca4Si2O7F2 and the photoluminescence (PL)/photoluminescence excitation (PLE) spectra of the Ca4Si2O7F2:Eu2+ phosphors. The reflectance spectrum of the Ca4Si2O7F2 host exhibits an absorption band from 240 to 350 nm that corresponds to the host lattice absorption. The
absorption edge of the host material was estimated to be about 4.2 eV (295 nm, 33 898 cm1).19,20Strong absorption occurred in
the range from UV to blue (240–500 nm) for the Ca4Si2O7 -F2:Eu2+ matrix, which mainly resulted from the transition of Eu2+from the 4f ground state to a 5d excited state. The PLE spectrum (lem¼ 460 nm) of Ca4Si2O7F2:Eu2+shows a broad-band excitation due to the 4f–5d dipole-allowed electronic transitions of Eu2+ to an electronic configuration of 4f65d1, which is consistent with the absorption observed in the reflec-tion spectrum. The strong broadband absorpreflec-tion of the Ca4Si2O7F2:Eu2+phosphor matches well with the emission of the NUV chips for applications in white-light NUV LEDs. A broad, asymmetric band was observed in the PL spectrum (lex¼ 400 nm) of Ca4Si2O7F2:Eu
2+
in the wavelength range of 420–530 nm with a strong blue emission centered at 460 nm, which corresponds to the allowed 4f65d1 / 4f7 electronic transitions of Eu2+ions.21The broad and asymmetric emission bands of Ca4Si2O7F2:Eu2+may be attributed to the transitions of Eu2+ions occupying four crystallographically distinct Ca2+ sites in the host structure. The PL spectrum of the Ca4Si2O7 -F2:Eu2+ phosphor can be decomposed by Gaussian deconvo-lution into four Gaussian profiles with peaks centered at 449 nm (22 272 cm1), 463 nm (21 598 cm1), 484 nm (20 661 cm1), and 513 nm (19 493 cm1). These peaks can be ascribed to four different emission sites, which could be identified as the four different coordination environments of the Ca2+ ion sites occupied by Eu2+ions.
The quantum efficiency of a phosphor is an important parameter to be considered for practical applications. To deter-mine the absolute quantum efficiency of photo-conversion for (Ca0.99Eu0.01)4Si2O7F2 phosphor, we applied the integrated sphere method for the measurements of optical absorbance (A) and quantum efficiency (F) of phosphor samples. The optical absorbance and quantum efficiency were calculated by using the following equations:22
A¼L0ðlÞ LiðlÞ
L0ðlÞ (1)
F ¼EiðlÞ ð1 AÞ$E0ðlÞ
LeðlÞ$A (2)
where L0(l) is the integrated excitation profile when the sample is diffusely illuminated by the integrated sphere’s surface; Li(l) is the integrated excitation profile when the sample is directly excited by the incident beam; E0(l) is the integrated luminescence of the sample excited by indirect illumination from the sphere; and Ei(l) is the integrated luminescence of the sample upon direct excitation. The term Le(l) is the integrated excitation profile obtained from the empty integrated sphere (without the sample). The internal (hi) and external (ho) quantum efficiencies (A F) were calculated based on the equations previously reported by Hirosaki et al.23Upon excitation at 400 nm, the optical absor-bance (A) of (Ca0.99Eu0.01)4Si2O7F2 and BaMgAl10O17:Eu2+ (BAM:Eu2+) phosphors (commercial product KX661 from Kasei Optonix Ltd) was calculated to be 61.2% and 55.4%, the internal quantum efficiencies were 29.3% and 87.1%, and the corresponding external quantum efficiencies were 17.9% and 48.3%, respectively.
Fig. 2 Reflectance spectra of Ca4Si2O7F2and Ca4Si2O7F2:Eu2+and PL/
PLE spectra of Ca4Si2O7F2:Eu 2+
phosphor.
As indicated in Fig. 3a, Eu2+ of Ca4Si2O7F2:Eu 2+
shows broadband emission from 420 to 530 nm centered at 460 nm, which was attributed to the 4f65d1/ 4f7transition. The Mn2+ excitation of Ca4Si2O7F2:Mn2+, on the other hand, contains several bands centered at 343, 357, and 461 nm, corresponding to the transitions from the 6A1(
6
S) ground state to the 4E(4D), 4T
2(4D), and 4T1(4G) excited states, respectively.24 We have observed significant spectral overlap between the emission band centered at 460 nm of Ca4Si2O7F2:Eu2+and the excitation band centered at 461 nm of Ca4Si2O7F2:Mn2+. Therefore, the spectral overlap is matched for Eu2+ and Mn2+, and energy can be transferred from Eu2+to Mn2+. Thus, effective resonance-type energy transfer from a sensitizer Eu2+to an activator Mn2+is expected to occur in Ca4Si2O7F2:Eu2+,Mn2+.25Fig. 3b shows the emission spectra of (Ca0.99xEu0.01Mnx)4Si2O7F2 phosphors under 400 nm NUV excitation. For (Ca0.99x Eu0.01-Mnx)4Si2O7F2samples, two broad emission bands were observed centered at 460 (4f65d1/ 4f7transition of Eu2+) and 576 nm (4T1(4G)/6A1(6S) transition of Mn2+). The emission intensity of Eu2+at 460 nm was found to decrease with increasing Mn2+ content x, and the emission intensity of Mn2+at 576 nm was found to increase with increasing Mn2+content until the emis-sion intensity of Mn2+became saturated when x reached 0.1. Concentration quenching occurred when the Mn2+ dopant content x was greater than 0.07, which is related to the energy
transfer probability from Eu2+ to Mn2+. The concentration quenching can be attributed to energy reabsorption among the nearest Eu2+or Mn2+ions.26
Fig. 4 shows the energy transfer efficiency hTand the relative emission intensity of Eu2+ as a function of Mn2+ molar concentration for (Ca0.99xEu0.01Mnx)4Si2O7F2phosphors. The relative emission intensity of Eu2+ decreased with increasing Mn2+doping content x in (Ca0.99xEu0.01Mnx)4Si2O7F2 phos-phors, and the results indicate continuous energy transfer from the sensitizer Eu2+to the activator Mn2+. According to Paulose et al.,27the energy transfer efficiency (h
T) from the sensitizer Eu 2+ to the activator Mn2+can be expressed as
hT¼ 1 IS
IS0 (3)
where IS0is the luminescence intensity of the sensitizer Eu 2+
in the sample in the absence of Mn2+, and I
Sis the luminescence intensity of Eu2+ in the presence of Mn2+. hT is the energy transfer efficiency from the sensitizer Eu2+to the activator Mn2+ in (Ca0.99xEu0.01Mnx)4Si2O7F2calculated as a function of x, as shown by the black line in Fig. 4. More precisely, the hTvalues were determined to be 0%, 5.87%, 13.5%, 25.3%, 32.0%, 46.3%, 66.1% and 82.3% for (Ca0.99–xEu0.01Mnx)4Si2O7F2with x¼ 0, 0.005, 0.01, 0.02, 0.03, 0.05, 0.07 and 0.10, respectively. As a consequence, the energy transfer efficiency from the sensitizer Eu2+ to the activator Mn2+ in cuspidine phosphors increases gradually with increasing Mn2+doping concentration. When the doped Mn2+ion content was 0.1, h
Twas observed to be above 82.3%.
Fig. 5 and Table 2 show the CIE chromaticity diagram and chromaticity coordinates (x, y) of the single-phase emission-tunable phosphor (Ca0.99xEu0.01Mnx)4Si2O7F2 under 400 nm excitation. The chromaticity coordinates (x, y) for (Ca0.99xEu0.01Mnx)4Si2O7F2 phosphors were measured to be (0.152, 0.112), (0.195, 0.191), (0.268, 0.274), (0.310, 0.288), (0.351, 0.332), (0.378, 0.366), (0.419, 0.411) eventually to (0.430, 0.423) with x¼ 0, 0.005, 0.01, 0.02, 0.03, 0.05, 0.07, and 0.1. These results indicate that the color can be tuned from blue (solely 0.01Eu2+, point 1) through white-light (0.01Eu2+/ 0.03Mn2+, point 5) and eventually to yellow (0.01Eu2+/0.1Mn2+,
Fig. 3 (a) Spectral overlap between the Ca4Si2O7F2:Eu2+ emission
spectrum and the Ca4Si2O7F2:Mn2+excitation spectrum and (b) emission
spectra of (Ca0.99xEu0.01Mnx)4Si2O7F2(x¼ 0.005–0.1) phosphors under
400 nm NUV excitation.
Fig. 4 Energy transfer efficiency hTand the relative emission intensity of
Eu2+ as a function of Mn2+molar concentration for (Ca
0.99xEu0.01
-Mnx)4Si2O7F2(x¼ 0–0.1) phosphors.
point 8) in the visible spectral region by systematically adjusting the relative Mn2+ dopant concentrations. The insets of Fig. 5 show photographs of (Ca0.99xEu0.01Mnx)4Si2O7F2 phosphors with different Mn2+contents x in a 365 nm UV lamp box.
3.3. Energy transfer mechanism and critical distance
The energy transfer mechanism for multipolar interactions has been discussed by many authors and can be determined using the relation28
IS0 IS
fCa=3 (4)
where C is the concentration of Mn2+; I
S0and ISare the lumi-nescence intensities of the sensitizer Eu2+ in the absence and
presence of the activator Mn2+. IS0/ISf Ca/3with a¼ 6, 8, and 10 corresponds to dipole–dipole, dipole–quadrupole, and quad-rupole–quadrupole interactions, respectively. The linear (red line) and polynomial (blue line) fits to the relationship between IS0/ISand Ca/3based on the above equation are illustrated in Fig. 6a–c. For Fig. 6a and c, the R2of the linear fit is less than that of the polynomial fit. The R2values for Fig. 6b of the linear and polynomial fits were calculated to be 0.9980 and 0.9964, which means that the relationship is closest to a linear behavior when a¼ 8. Therefore, the energy absorbed by Eu2+is trans-ferred to Mn2+ via a nonradiative dipole–quadrupole mecha-nism. The above results indicate that the energy transfer occurs from the sensitizer Eu2+ to the activator Mn2+in the (Ca0.99–xEu0.01Mnx)4Si2O7F2 phosphor and that the relative intensity of blue and yellow emissions can be tuned by adjusting the relative concentrations of Eu2+and Mn2+, respectively.
According to Dexter and Schulman,29concentration quench-ing in many cases is due to energy transfer from one activator to another until an energy sink in the lattice is reached. Blasse suggested that the critical distance REu–Mnof energy transfer can be calculated by30
Fig. 5 CIE chromaticity diagram of (Ca0.99xEu0.01Mnx)4Si2O7F2
phosphors under 400 nm excitation: (1) x¼ 0, (2) x ¼ 0.005, (3) x ¼ 0.01, (4) x¼ 0.02, (5) x ¼ 0.03, (6) x ¼ 0.05, (7) x ¼ 0.07, and (8) x ¼ 0.1. (a) blue InGaN chip. (b) white-light Y3Al5O12:Ce3+. (c) Y3Al5O12:Ce3+
phosphor. The insets show (Ca0.99xEu0.01Mnx)4Si2O7F2 phosphors
irradiated under a 365 nm UV lamp box.
Table 2 The CIE coordinates and relative emission intensity of Eu2+/
Mn2+ for (Ca0.99xEu0.01Mnx)4Si2O7F2 phosphors under 400 nm
excitation CIE diagram sites Doped Mn2+ molar conc. Eu2+/Mn2+ relative intensity CIE chromaticity coordinates x y 1 x¼ 0.000 100/0 0.152 0.112 2 x¼ 0.005 94/16 0.195 0.191 3 x¼ 0.010 86/38 0.268 0.274 4 x¼ 0.020 75/52 0.310 0.288 5 x¼ 0.030 68/71 0.351 0.332 6 x¼ 0.050 54/78 0.378 0.366 7 x¼ 0.070 34/86 0.419 0.411 8 x¼ 0.100 18/53 0.430 0.423 a Blue InGaN chip 0.144 0.030 b White light Y3Al5O12:Ce3+ 0.291 0.300
c Y3Al5O12:Ce 3+ phosphor 0.429 0.553 Fig. 6 Dependence of IS0/ISof Eu 2+ on (a) C6/3, (b) C8/3, and (c) C10/3.
REu--Mn¼ 2 3V 4pxN 1=3 (5) where x is the total concentration of Eu2+and Mn2+, N is the number of Z ions in the unit cell, and V is the volume of the unit cell. For the cuspidine crystal, the analytical and experimental values were N¼ 4 and V ¼ 802.92 A3. Thus, the R
Eu–Mnvalues of (Ca0.99xEu0.01Mnx)4Si2O7F2 were determined to be 18.56, 16.86, 14.73, 13.38, 11.69, 10.62, and 9.55 A for x¼ 0.005, 0.01, 0.02, 0.03, 0.05, 0.07, and 0.1, respectively. The critical doping content xcat which the luminescence intensity of Eu
2+
is half that of the sample in the absence of Mn2+is 0.2416 mol. Therefore, the critical distance (Rc) of energy transfer was calculated to be 11.66 A.
According to Caldi~no, the dipole–quadrupole probability (WDQsa ) can be given in terms of the critical distance of energy transfer from the sensitizer Eu2+to the activeator Mn2+as31
WDQ sa ¼ 3h-4c4f ql2sQa 4pn4s0 sfdR8sa UðFs; FaÞ (6) where h
-, c-,p, and n are constants; Qais the integrated absorption coefficient of Mn2+the acceptor; lsis the emission wavelength of the sensitizer Eu2+; fdand fqare the oscillator strengths of the activator dipole and quadrupole electrical transitions;s0
s is the intrinsic lifetime of the sensitizer; Rsais the distance between the ions involved in the transfer and U(Fs, Fa)¼ÐFs(E)Fa(E)E4dE represents the spectral overlap between the Eu2+emission Fs(E) and Mn2+ absorption Fa(E) shown in Fig. 3a. The critical distance Rcof the energy transfer from the sensitizer Eu
2+ to the activator Mn2+is defined as the distance for which the proba-bility of transfer equals the probaproba-bility of radiative emission of Eu2+, i.e., the distance for which WDQsa s0s¼ 1. Therefore, Rccan be obtained from the following simplified equation:32
R8 c¼ 0:63 10 28fql2sQa fdE4 s ð FsðEÞFaðEÞdE (7) where Qa¼ 4.8 1016, fdis the absorption coefficient of Mn2+; fd¼ 107and fq¼ 1010are the oscillator strengths of dipole and quadrupole electrical absorption transitions for Mn2+; ls(in A) and E (in eV) are the emission wavelength and emission energy of Eu2+ and ÐFs(E)Fa(E)E4dE expresses the spectral overlap between Eu2+and Mn2+, which was estimated to be 5.2983 eV1. Therefore, the critical distance for a dipole–quadrupole type energy transfer was calculated to be 12.61 A, which is similar to those obtained for (Ca, Mg, Sr)9Y(PO4)7:Eu2+/Mn2+(11.09 A)33
and Ca9La(PO4)7:Eu 2+
/Mn2+ (11.36 A)34 and this result is in
good agreement with that obtained using the concentration quenching method. These results imply that the emission inten-sity (peaking at 576 nm) of Mn2+ions increases with decreasing Eu2+–Mn2+distance (or increasing Mn2+content) until it reaches saturation. Furthermore, when the Mn2+content exceeded 0.01 (i.e., the Eu2+–Mn2+distance was shorter than Rc), the Mn
2+ emission intensity began to decrease, which was attributed to the occurrence of energy reabsorption among the nearest Mn2+ions. 3.4. EL spectrum of white-light LED lamps
To demonstrate the potential application of Ca4Si2O 7-F2:Eu2+,Mn2+phosphors, a LED lamp was fabricated from a
400 nm NUV LED chip and a single-phase white-emitting composition-optimized (Ca0.96Eu0.01Mn0.03)4Si2O7F2 phosphor under a forward bias of 350 mA. The electroluminescence (EL) spectrum in Fig. 7a clearly shows a NUV band at around 400 nm, a blue-emitting band centered at 460 nm attributable to the 4f7 / 4f65d1 transition of the Eu2+ions, and yellow-emitting bands at around 576 nm that correspond to the 4T1(4G) / 6
A1(6S) transition of Mn2+. The optical properties of the white-light LED show a warm CCT of 4880 K and CIE color coordinates of (0.347, 0.338). For comparison, YAG:Ce3+ pumped with an InGaN blue chip was also considered, and this system was found to emit white-light with a CCT of 7272 K and CIE color coordinates of (0.302, 0.315), as shown in Fig. 7b. The insets of Fig. 7a and b show a well-packaged single-composition LED lamp and a white-light emission LED driven by a 350 mA current. These results demonstrate that our single-composition (Ca0.96Eu0.01Mn0.03)4Si2O7F2 phosphor-converted white-light NUV LED produced warm white-light and lower CCT values than a white-light LED fabricated with a YAG:Ce3+ phosphor pumped with a blue InGaN chip. Therefore, the Ca4Si2O7F2:Eu
2+
,Mn2+phosphors are promising candidates for application in single-phase color-tunable white-light NUV LEDs.
Fig. 7 EL spectrum of white LEDs fabricated using (a) a 400 nm NUV chip combined with a single-phase white-emitting (Ca0.96Eu0.01Mn0.03)
4-Si2O7F2phosphor and (b) an InGaN chip pumped with a YAG:Ce 3+
phosphor under a forward bias of 350 mA. The insets show photographs of the packaged white-light LEDs.
4.
Conclusions
In summary, we have synthesized a series of single-composition emission-tunable Ca4Si2O7F2:Eu2+,Mn2+phosphors and inves-tigated their luminescence properties and crystal structure. We have demonstrated that the generation of white-light can be achieved using these phosphors because of the effective reso-nance-type energy transfer from a sensitizer Eu2+to an activator Mn2+via a dipole–quadrupole mechanism. The critical distance REu–Mnhas also been evaluated by both concentration quenching and spectral overlap methods. Because of the energy transfer, the emission hue can be varied from blue (0.152, 0.112) to white-light (0.351, 0.332) and eventually to yellow (0.430, 0.423) by tuning the Eu2+/Mn2+ ratio. The fabricated single-phase white-light LED shows a warm white CCT of 4880 K and CIE color coor-dinates of (0.347, 0.338). These results indicate that our novel single-composition color-tunable Ca4Si2O7F2:Eu2+,Mn2+ phos-phor is superior to a YAG:Ce3+ phosphor-converted LED pumped with a blue LED chip, which had a CCT of 7272 K and CIE color coordinates of (0.302, 0.315). Therefore, Ca4Si2O 7-F2:Eu2+,Mn2+ is a promising candidate for application in phosphor-converted white-light NUV LEDs.
Acknowledgements
This research was supported by the Industrial Technology Research Institute under contract no. B301AR4850 (C. H. H.) and in part by the National Science Council of Taiwan under contract no. NSC98-2113-M-009-005-MY3 (T. M. C.) and NSC-101-2218-E-033-001 (W. R. L.)
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