A novel tunable blue-green-emitting CaGdGaAl2O7:Ce(3+),Tb(3+) phosphor via energy transfer for UV-excited white LEDs.

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Dalton Transactions View Article Online

DOI: 10.1039/C4DT03999H

via energy transfer for UV-excited white LEDs Chao Liang, Hongpeng You, Yibing Fu, Xiaoming Teng, Kai Liu, and Jinhua He* Jiangsu Bree Optronics Co. Ltd, Nanjing 211103, China

ABSTRACT: CaGdGaAl2O7 and CaGdGaAl2O7:Ce3+,Tb3+ have been synthesized by traditional solid state reaction for the first time. The Rietveld refinement confirmed that CaGdGaAl2O7 belongs to tetragonal crystal system with the space group P421m. The photoluminescence properties exhibit that the obtained phosphors can be efficiently excited in the range from 330 to 400 nm, which matches perfectly with the commercial UV LED chips. Tunable blue-green emitting CaGdGaAl2O7:Ce3+,Tb3+ phosphor has been obtained by codoping Ce3+ and Tb3+ ions into the host and varying their relative ratios, which may be good candidates for blue-green components in UV white LEDs. The luminescent properties and lifetimes reveal an efficient energy transfer from the Ce3+ to Tb3+ ions. The energy transfer is demonstrated to be a dipole-quadrupole mechanism, and the critical distances for Ce3+ to Tb3+ calculated by the concentration quenching is 12.25 Å.

∗

Author to whom correspondence should be addressed.

Email: [email protected]

1

Dalton Transactions Accepted Manuscript

Published on 18 March 2015. Downloaded by ECOLE POLYTECHNIC FED DE LAUSANNE on 24/03/2015 13:50:35.

A novel tunable blue-green-emitting CaGdGaAl2O7:Ce3+,Tb3+ phosphor

Dalton Transactions

Page 2 of 23 View Article Online

DOI: 10.1039/C4DT03999H

White light emitting diodes had been regarded as the next generation lighting source due to their merits of long lifetime, environmentally friendly and high efficiency, comparing with the conventional incandescent and fluorescence lamps.1-5 The main strategy to obtain WLEDs is the combination of blue or near ultraviolet (n-UV) LEDs with one or more phosphors, known as phosphor converted-LEDs (pc-LEDs). Now, typical white LED lamp are based on phosphor of blue InGaN LEDs by Y3Al5O12:Ce3+(YAG:Ce)-based yellow phosphors.6,7 However, this method suffers the problems of low color rendering index (Ra < 80) and high correlated color temperature (Tc > 4500 K) because of insufficient red-light contribution.8,9 Another approach to obtain white light is the combination of UV or near UV LEDs chips with tricolor (red, green and blue) phosphors. WLEDs fabricated in this way can overcome the above problems and produce an excellent color rendering index and easily controlled emission color properties.10-14 In this regard, it is essential to find novel phosphors which can be efficiently excited by the UV LED chips. It has been well known that Ce3+ ion exhibits good performances, due to its special luminescent properties of a broad band emission ranging from green to red region which depends on the surroundings of hosts, such as Ce3+ activated Y3Al5O12:Ce3+ (YAG:Ce3+),15 Ca3Sc2Si3O12:Ce3+,16 and CaAlSiN3:Ce3+.17 Moreover, the Ce3+ ion acts as an excellent sensitizer, transferring a part of its energy to activator ion such as Tb3+ in some coactivated materials.18-27 In these systems, the luminescent efficiency of the Tb3+ singly doped phosphor is very low upon UV/blue light excitation because of the 4f–4f weak absorption for Tb3+ ions. However, the luminescent intensity of the Tb3+ ions was efficient increased by the energy transfer from the 5d level of the Ce3+ ions to the 5D3,4 level of the Tb3+ ions. Although the energy transfer

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Introduction

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develop new and highly efficient green emitting phosphors used as green components in whitelight LEDs. Recently, many investigations of the preparation and the crystal chemistry of rare-earth doped melilite families have been performed. For example, Bao et al. reported the luminescent properties of nanoparticles CaGdAl3O7:RE3+ (RE= Eu, Tb).28 Kodama and Park et al. have studied the Ce3+ and Eu3+ doped CaYAl3O7 using for long-lasting phosphorescence and LED application.29,30 However, there has been no study on CaGdGaAl2O7 host lattice with compounds for LED application based on the energy transfer from Ce3+ to Tb3+. Herein, we report a new compound CaGdGaAl2O7 and its luminescence properties of Ce3+/Tb3+ activated CaGdGaAl2O7 phosphors. Though the energy transfer, the CaGdGaAl2O7:Ce3+,Tb3+ exhibits not only as a strong blue band of the Ce3+ ions but also as a strong green band of the Tb3+ ions at the excitation of 365 nm. Moreover, the energy transfer mechanism between the Ce3+ and Tb3+ ions has been investigated systematically. Experimental Section Materials and synthesis A series of CaGd1-x-yGaAl2O7:xCe3+, yTb3+ phosphors were synthesized by conventional high temperature solid-state reaction. The starting materials were CaCO3 (A.R.), Gd2O3 (A.R.), Al2O3 (A.R.), Ga2O3 (99.99%), CeO2 (99.99%), and Tb4O7 (99.99%). They were weighed in a proper stoichiometric ratio and thoroughly mixed by grinding in an agate mortar. After mixing and grinding in an agate mortar for 30 min, the mixture was placed in a crucible and then sintered

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from Ce3+ to Tb3+ ions in different hosts have been extensively investigated, it is required to

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DOI: 10.1039/C4DT03999H

were cooled to room temperature and reground for further measurements. Measurements and Characterization The powder X-ray diffraction (XRD) measurements were performed on D8 Focus diffractometer (Bruker) operating at 40 kV and 40 mA with Cu Kα radiation (λ = 1.5418 Å). The scanning rate was fixed at 10°/ min with 2θ ranges from 10° to 70°. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured by a Hitachi F-4500 spectrophotometer equipped with a 150W Xenon lamp as the excitation source. The morphology and size of the as-prepared samples were inspected with a field emission scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi, Japan). In fluorescence lifetime measurements, a tunable laser (pulse width = 4 ns, gate = 50 nm) was used as an excitation source, and the signals were detected with a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz). The above measurements were performed at room temperature. Results and Discussion

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at 1350 ºC for 4h in a reductive atmosphere (10% H2 + 90% N2). Finally, the prepared phosphors

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Figure 1. XRD patterns of typical prepared samples and standard data for CaGd0.96(JCPDS card no.50-1808).

All the prepared samples were characterized by the powder X-ray diffraction to verify their phase

purity.

Figure

3+ 3+ yGaAl2O7:0.04Ce ,yTb

1

shows

the

typical

powder

XRD

patterns

of

CaGd0.96-

samples. It can be seen that all the XRD patterns are well-matched

with the CaGdAl3O7 (JCPDS 50-1808) phase except that the diffraction peaks shift to smaller angles owing to the larger ionic radius of Ga3+ ion compared with that of Al3+ ions. No other phase was observed, confirming the formation of the single-phase nature. When Ce3+ and Tb3+ ions are incorporated into the crystal structure of CaGdGaAl2O7, in the view of effective ionic radii and ion valence , Ce3+ (1.14 Å), Tb3+ (1.04 Å), Gd3+ (1.05 Å) and Ca2+ (1.12 Å), it is reasonable to propose that Ce3+ and Tb3+ are expected to randomly occupy the Gd3+ sites in the host structure. The doping of lanthanide ions does not induce any other distinct impurities phases, indicating the successful incorporation of these activators into the CaGdGaAl2O7 host. Figure S1 presents the typical SEM images of samples. The morphology of the sample exhibits irregular spheroidicity with particle sizes below 5 µm. These results show that well-crystallized CaGd1-x3+ yGaAl2O7:xCe ,

yTb3+ powders have been obtained. The corresponding EDX spectrum analysis

(Figure S2) indicates that the product has a chemical composition of Ca, Gd, Ga, Al and O, and that no impurity elements are present.

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3+ 3+ yGaAl2O7:0.04Ce ,yTb

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Figure 2. Experimental (crosses), calculated (solid line), and their difference results (bottom) XRD profiles for the Rietveld refinement of CaGd0.96GaAl2O7:0.04Ce3+. The blue solid lines represent the Bragg reflection positions

Table 1. Final refined structural parameters for CaGd0.96GaAl2O7:0.04Ce3+ derived from Rietveld refinement of X-ray diffraction data at room temperature.

Atoms

Site

x

y

z

Occ.

Uiso(Å2)

Ca Gd

4e 4e

0.159000 0.159000

0.659000 0.659000

0.507200 0.507200

0.5000 0.5000

0.2500 0.2500

Al/Ga

2a

0.000000

0.000000

0.000000

1.0000

0.2500

Al/Ga

4e

0.357000

0.857000

0.959500

1.0000

0.2500

O1

4e

0.358000

0.858000

0.294000

1.0000

0.2500

O2

8f

0.337500

0.411700

0.197200

1.0000

0.2500

O3

2c

0.000000

0.500000

0.180000

1.0000

0.2500

z

3

a = b = 7.7691 Å, c = 5.1204 Å, V=309.06 Å ; Z = 2; space group,

P421m; weighted-profile agreement index Rwp = 4.74%, profile residual agreement index 3.10%, χ2 = 3.893.

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CaGdGaAl2O7 was performed using the general structure analysis system (GSAS) program.31 Figure 2 depicts the results of Rietveld refinement pattern of the CaGd0.96GaAl2O7:0.04Ce3+. In the refinement, the previously reported crystallographic data of CaLaAl3O7 (JCPDS 81-1797) which crystalizes in a tetragonal unit cell with the space group P421m was employed as initial structure model. Table 1 shows the final refined structural parameters and reliability factors for the CaGd0.96GaAl2O7:0.04Ce3+ phosphor. The material formed a single phase without impurity phases and the as-obtained goodness of fit parameters are χ2 = 3.893, Rwp = 4.74%, Rp = 3.10%, indicating that all atom positions, fraction factors and temperature factors well satisfy the reflection condition. The cell parameters were determined to be a = b = 7.7691 Å, c = 5.1204 Å, V=309.06 Å3. Figure 2 inset shows a schematic of the CaGdGaAl2O7 crystal structure, Ca and Gd are both coordinated by eight oxygen atoms.

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To further study the structure of the obtained samples, Rietveld structure refinement of

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Figure 3. (a)PL and PLE spectra of CaGd0.98GaAl2O7:0.02Ce3+ phosphor. The inset shows the variation of emission intensity as a function of doped Ce3+ molar concentration. (b) Dependence of log(I/m) on log(m) in CaGd1-xGaAl2O7:xCe3+ phosphors. Figure 3 depicts the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the as-prepared CaGd0.98GaAl2O7:0.02Ce3+ phosphor. The sample shows excitation band from 250 to 300 nm and a broad intense band from 330 to 400 nm with a maxium at 365 nm due to the 4f7→4f65d1 transition of the Ce3+ ions. At the excitation of 360 nm, the PL spectrum shows an intense blue emission band attributed to the 4f65d1→4f7 transition of the Ce3+ ion. With different excitation wavelengths of 360 and 380 nm, there are little changes in the green emission except the emission intensity. This result indicates that Ce3+ ions substitute only one site in this system. In order to investigate the effect of doping concentration on luminescence properites, a series of CaGd1-xGaAl2O7:xCe3+ (x = 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06) phosphors were synthesized. Figure 3 inset shows the intensity of PL spectra of CaGd13+ xGaAl2O7:xCe

with different doping contents. With an increasing Ce3+ doping concenration,

the blue emission of the Ce3+ increases gradually and reaches a maximum at x = 0.04. With further increment of Ce3+ concentration, the emission intensity begins to decrease due to

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equation given by Blasse:32

 3V  RC ≈ 2   4πxN 

1/3

(1)

where V is the volume of the unit cell, N is the number of sites that lanthanide ion can occupy in per unit cell, and x is the critical concentration of doped ions. For the CaGdGaAl2O7 host, N = 2, V=309.06 Å3, and x is 4% for Ce3+; therefore, the critical distance (RC) was calculated to be about 19.46 Å. In oxide phosphor, non-radiative energy transfer usually occurs as a result of exchange interaction or multipole-multipole interaction.33 Since the exchange interaction comes into effect only when the distance between activators is shorter than 5Å, the concentration quenching mechanism of the Ce3+ in the phosphor is dominated by the multipole-multipole interaction. According to Van Uiter’s report, the emission intensity (I) per activator ion follows -1

q 3 the equation: I / m = k 1 + b ( m )  Where m is the activator concentration, I/m is the emission  

intensity per activator concentration, k and β are constants for a given host in the same excitation condition, θ = 6, 8 and 10 represent the dipole–dipole, dipole–quadrupole, and quadrupole– quadrupole interactions, respectively. By modifying the equation, log(I/m) acts a liner function of log(m) with a slop of (-θ/3). To get the value of θ, the relationship between log(I/m) and log(m) is plotted with m ranging from 0.05 to 0.08. From Figure 6, the θ/3 is determined to be 2.03 and accordingly, θ is calculated to be 6.09 which is close to 6. The result indicates that the concentration quenching mechanism of the Ce3+ emission in CaGd1-xGaAl2O7:xCe3+ host is dominated by the dipole-dipole interaction.

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concentration quenching. The critical distance RC between Ce3+ ions can be estimated using the

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Figure 4. PL and PLE spectra of CaGd0.95GaAl2O7:0.05Tb3+ and dependence of the luminescent intensity of the 5D3-7FJ and 5D4-7FJ transition emissions on the concentration of the Tb3+ ions. Figure 4 illustrates the PL and PLE spectra of the CaGd0.95GaAl2O7:0.05Tb3+ phosphor and the dependence of luminescent intensity of 5D3-7FJ and 5D4-7FJ transition emissions on the concentration of the Tb3+ ions. The PLE spectrum monitored at 550 nm exhibits a broad band and several peaks. The broad band from 200 to 260 nm centered at 240 nm is ascribed to the f-d transitions of the Tb3+ ions, while the excitation peaks at 275, 307 and 313 nm originated from the transitions of 8S7/2→6I7/2, 8S7/2→6P5/2, and 8S7/2→6P7/2 of the Gd3+ ions. The other peaks in the wavelength ranging from 330 to 500 nm are due to the intra-4f8 transitions of the Tb3+ ions. The appearance of strong excitation sharp bands of the Gd3+ ions confirms that there is an efficient energy transfer from the Gd3+ to Tb3+ ions. At the excitation of 275 nm, the as-prepared CaGd0.95GaAl2O7:0.05Tb3+ phosphor emits two sets of emissions. The emission peaks at 490, 543, 590, and 620 nm are assigned to the 5D4-7FJ (J = 6, 5, 4, 3) transitions, while the emission peaks at 380, 415, and 436 nm are due to the 5D3-7FJ transitions. One can note from Figure 4 that the spectral energy distribution of the Tb3+ emission depends strongly on Tb3+ concentration. It is clear that the emission spectra show different ratio between the 5D3 and the 5D4 emissions at

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the increase of Tb3+ doping concentration. The increase in intensity ratio can be assigned to the concentration quenching of 5D3 emission, resulting from cross relaxation process between neighboring Tb3+ ions.34

Figure 5. PL and PLE spectra of CaGd0.9GaAl2O7: 0.05Ce3+,0.05Tb3+ phosphor. Inset: The schematic energy levels of Ce3+, Tb3+ and the energy transfer processes. Since there is an overlap between the PL spectra of CaGdGaAl2O7:Ce3+ and PLE spectra of CaGdGaAl2O7:Tb3+ phosphors, an effective energy transfer from the sensitizer Ce3+ to activator Tb3+

can

be

expected.

Figure

5

shows

the

PLE

and

PL

spectra

of

the

CaGd0.9GaAl2O7:0.05Ce3+,0.05Tb3+ phosphor. At the irradiation of 365 nm, the PL spectrum exhibits both the Ce3+ and the typical Tb3+ emissions. Monitored at 545 nm of the Tb3+ ions, one can find that the PLE spectrum in the range from 325 to 425 nm is similar to that monitored at 425 nm of the Ce3+ ions. These results on the PLE and PL spectra of the CaGd0.9GaAl2O7:0.05Ce3+,0.05Tb3+ phosphor prove the occurrence of the energy transfer from the Ce3+ to Tb3+ ions. The corresponding energy levels scheme of Ce3+ and Tb3+ and the possible optical transition involved in the energy transfer processes are schematically depicted in Fig. 5

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lower and higher Tb3+ concentration. The intensity ratio of 5D4 to 5D3 increases dramatically with

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emitting blue light but also by transferring to Tb3+ ions, which finally exhibits a green emission of Tb3+ ions.

Figure 6. PL spectra of CaGd0.95-xGaAl2O7: xCe3+,0.05Tb3+ phosphors with various Ce3+contents. To observe the luminescent sensitizing of the Tb3+ ions by the Ce3+ ions, the emission and excitation spectra of the Ce3+ and Tb3+ codoped CaGd0.9GaAl2O7 with various Ce3+ concentrations (0.01, 0.03, 0.05, 0.07 and 0.09, respectively.) are measured (Figure 6). For the Tb3+ single-doped sample, one can see the very weak emission of the Tb3+ ions since it has little absorption at 365 nm. Although the content of the Tb3+ ions is fixed, the emission of the Tb3+ ions dramatically increased with the doping of the Ce3+ ions. This is because the increase in donor concentration enhances the energy diffusion between donors, which speeds up the average transfer rate to the Tb3+ acceptors.

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inset. When Ce3+ ions absorb UV light, the excitation energy could be released not only by

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Figure 7. PL spectra of CaGd0.96-yGaAl2O7:0.04Ce3+,yTb3+ phosphors with various Tb3+ contents. In addition, Figure 7 depicts the PL spectra of the CaGd0.96-yGaAl2O7:0.04Ce3+,yTb3+ (y = 0.00 – 0.20) phosphors. It can be seen that the Ce3+ emission intensity is significantly decreased with the increase of the added Tb3+ ions due to the energy transfer. In many cases, concentration quenching is due to the energy transfer from one activator to another until energy sink in the lattice is reached. Blasse suggested that the average separation RCe-Tb of the energy transfer can be also estimated from the Eq. (1), here x is the total concentration of Ce3+ and Tb3+ ions. The critical concentration xC, where the luminescence intensity of sensitizer (Ce3+) is half that in the sample in the absence of activator (Tb3+). The critical distance of energy transfer RC is estimated to be about 12.25 Å from the total concentration of the Ce3+ (= 0.04) and Tb3+ (= 0.12).

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DOI: 10.1039/C4DT03999H

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DOI: 10.1039/C4DT03999H

(excited at 340 nm, monitored at 420 nm). The fluorescence decay curves of the Ce3+ ions in CaGd0.96-yGaAl2O7:0.04Ce3+,yTb3+ phosphors were measured and showed in Figure 8. The decay curve for CaGd0.96GaAl2O7:0.04Ce can be well fitted into single-exponential function, revealing that the Ce3+ ions have only one luminescent certer. This result is agreement with that the Ce3+ ion occupy the Gd3+ ion in host. The fluorescence lifetime can be obtaied by the following formula I = I0 exp( −t / τ )

(2)

where I0 and I are the luminescence intensities at time 0 and t, respectively, and τ is the decay lifetime. The fluorescence lifetime was obtained to be 29.64 ns. When the Tb3+ ions were doped into the host, the decay curves deviate from a single-exponential rule. This deviation is more evident with the increase of the Tb3+-concentration, confirming the existence of the energy transfer from the Ce3+ to Tb3+ ions. Owing to nonexponential decay of donor fluorescence intensity ID(t) in the presence of the Tb3+ ions, an average fluorescence lifetime

can be

determined by ∞

< τ D > = ∫ I D (t )dt 0

(3)

where ID(t) is normalized to its initial intensity. On the basis of equation (3), the fluorescence lifetimes of the Ce3+ ions are determined to be 29.64, 28.31, 27.13, 27.76, 26.14, 24.36, 24.35, 23.37 and 22.67 ns for the CaGd0.96-yGaAl2O7:0.04Ce3+,yTb3+(y=0.00–0.20) phosphors. The energy transfer efficiency ηT between the Ce3+ and Tb3+ ions was also obtained from the decay lifetime by using the equation:

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Figure 8. Decay curves for the luminescence of the CaGd0.96-yGaAl2O7:0.04Ce3+,yTb3+ samples.

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T

=1−

τ τ0

(4)

where the τ and τ0 are the fluorescence lifetimes of sensitizer (Ce3+) ion with and without the presence of activator (Tb3+), respectively. The lifetimes and energy transfer efficiencies are plotted as a function of the Tb3+ concentration and shown in Figure 9. The average lifetimes decrease monotonously while the energy transfer efficiency increase gradually with the increment of the Tb3+ ions. The value of ηT reaches the maximum of 23.5% when y = 0.20.

Figure 9. Dependence of the fluorescence lifetime of the Ce3+ and energy transfer efficiency on doped Tb3+ ions. On the basis of Dexter’s energy-transfer expression of multipolar interaction and Reisfeld’s approximation, the energy-transfer behavior from the Ce3+ to Tb3+ ions can take place via electric-multipole interactions, which can be given as34,35 I0 ∝ C n/3 I

(5)

where I0 is the intrinsic luminescence intensity of the Ce3+ ions, I is the luminescence intensity of the Ce3+ ions with the activator (Tb3+) present, and C is the total concentration of

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η

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DOI: 10.1039/C4DT03999H

dipole, dipole-quadrupole and quadrupole-quadrupole interactions. Figure 10 illustrates the relationships between (I0/I) versus Cn/3, and a liner relation is observed when n = 8. This result clearly indicates that the energy transfer mechanism from the Ce3+ to Tb3+ ions is a dipolequadrupole reaction.

Figure 10. Experimental data plots of I/I0 versus Cn/3. The red lines indicate the fitting behaviors. To further investigate the characteristics of multipolar interactions, such as dipole-dipole, dipole-quadrupole, and higher order interactions, the energy transfer probability PSA (in s−1) for each multipolar interaction was considered. The dipole-dipole energy transfer probability (PSA) from a sensitizer to an acceptor, and the relationship of the transfer probability between Pdq of the dipole-quadrupole interaction and Pdd of the dipole-dipole interaction are as follows:33,36 PCedd−Tb = 3.024 × 1012

2 P dq  λS  f q ≈  P dd  R  f d

fd R τS 6

∫

FS ( E ) FA ( E )dE E4

(6) (7)

where fd and fq are the oscillator strengths of the electric dipole and quadrupole transitions, respectively. τS is the radiative decay time of the sensitizer (in seconds), R is the sensitizer-

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Ce3+ and Tb3+ ions. The plots of (I0/I) versus Cn/3 with n= 6, 8, and 10 correspond to dipole-

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∫F

S

( E ) FA ( E ) / E 4 dE

represents the spectral overlap between the normalized shapes of the Ce3+

emission FS(E) and the Tb3+ excitation FA(E), and in our case it is calculated to be about 0.0042 eV−5. λS (in angstroms) is the wavelength position of the sensitizer’s emission, R is the sensitizeracceptor average distance (in angstroms). The critical distance (Rc) of the energy transfer from the sensitizer to the acceptor is defined as the distance for which the probability of transfer equals the probability of radiative emission of donor, the distance for which PCe–TbτS0 = 1. Using Eqs (6) and (7), we obtain RC8 = 3.024 × 1012 λ2S f q ∫

FS ( E ) FA ( E )dE E4

(8)

The oscillator strength of the Tb3+ quadrupole transition (fq) has not obtained up to now. However, it was suggest by Verstegen et al. that the ratio fq/fd is about 10-3-10-2. The oscillator strength of the Tb3+ electric dipole transition (fd) is on the order of 10-6. Using these values and the calculated spectral overlap, the critical distance RCdq for a dipole-quadrupole interaction mechanism is 11.01 – 14.68 Å, which agrees approximately with that obtained by using the concentration quenching method (12.25 Å), further supporting the result obtained by equation (5).

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acceptor average distance (in angstroms), E is the energy involved in the transfer (in eV), and

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DOI: 10.1039/C4DT03999H

together with a series of digital photos of the selected phosphors under a 365 nm UV lamp. Figure 11 shows the Commission International de L’Eclairage (CIE) chromaticity diagram for several typical CaGd0.96-yGaAl2O7: 0.04Ce3+,yTb3+ (y = 0.00–0.20) samples, together with a series of digital photos of the selected phosphors. It can be seen that the emitting color of the phosphors can be easily modulated from blue to green by simply varying the value of y from 0 to 0.20. Accordingly, the corresponding CIE coordinates of CaGd0.96-yGaAl2O7:0.04Ce3+,yTb3+ (y = 0.00 – 0.20) change from (0.154, 0.050) to (0.218, 0.381), due to the different emission compositions of the Ce3+ and Tb3+ ions. Thus, blue-green emitting phosphors which can be efficiently excited by the UV chips are obtained via the energy transfer from the Ce3+ to Tb3+ ions and the CaGd0.96-yGaAl2O7:0.04Ce3+,yTb3+ phosphor have potential value as blue-greenish phosphors used for UV white LEDs.

Conclusions In summary, a series of CaGd0.96-yGaAl2O7: 0.04Ce3+,yTb3+ (y = 0.00 – 0.20) phosphors had been synthesized. At the excitation of 365 nm, the CaGd0.96-yGaAl2O7: Ce3+ phosphors can emit intense blue light with an optimal concentration of the Ce3+ being 0.04. We observed the energy transfer from the Ce3+ to Tb3+ ions in CaGd0.96-yGaAl2O7. The dipole-quadrupole mechanism should be responsible for the energy transfer from Ce3+ to Tb3+ in CaGd0.963+ 3+ yGaAl2O7:0.04Ce ,yTb

phosphors. For the codoped samples, tunable colors from blue to green

can be realized by singly varying the doping concentration of the Tb3+ ion at the irradiation of 365 nm. The experimental results indicate that the CaGd0.96-yGaAl2O7: 0.04Ce3+,yTb3+ phosphor may have promising application in UV white LEDs.

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Figure 11. CIE chromaticity diagram for CaGd0.96-yGaAl2O7: 0.04Ce3+,yTb3+(y=0.00–0.20),

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DOI: 10.1039/C4DT03999H

This

work

is

Innovation Fund for Technology Based Firms

financially

supported

(14C26213201204),

by

the

Jiangsu

National Provincial

Achievement transformation project (BA2011016), Nanjing intellectual property development project

(201106032),

and

Nanjing

Science

and

technology

project

(201202047).

19



Acknowledgements:

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DOI: 10.1039/C4DT03999H

1

C. C. Lin and R. S. Liu, J. Phys. Chem. Lett., 2011, 2, 1268.

2

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Page 23 of 23


DOI: 10.1039/C4DT03999H

A tunable blue-green-emitting CaGdGaAl2O7:Ce3+, Tb3+ phosphor via energy transfer for UV-excited white LEDs

By tuning the relative content of the doped ions, tunable blue-green emission can be obtained by the irradiation at 365 nm.

23



Table of Content

The energy transfer and effect of doped Mg2+ in Ca3Sc2Si3O12:Ce3+, Pr3+ phosphor for white LEDs.

Mn-coactivated Mg2Al4Si5O18 as a potential phosphor for white-light LEDs.

Tb(3+) coactivated NaBa3La3Si6O20 phosphor via energy transfer.

A single-component white-emitting CaSr2Al2O6:Ce3+, Li+, Mn2+ phosphor via energy transfer.

Warm white light generation from a single phase Dy3+ doped Mg2Al4Si5O18 phosphor for white UV-LEDs.

tunable emissions in a single-phased X2-type Y2SiO5:Eu(3+),Bi(3+) phosphor for ultraviolet converted LEDs.

Tb3+ phosphor for white light emitting diodes.

Single-component and warm-white-emitting phosphor NaGd(WO4)2:Tm3+, Dy3+, Eu3+: synthesis, luminescence, energy transfer, and tunable color.

Sr7.3Ca2.7(PO4)6F2:Eu(2+),Mn(2+): a novel single-phased white light-emitting phosphor for NUV-LEDs.

Luminescence and energy transfer of color-tunable Li6Gd(BO3)3:Ce(3+), Tb(3+) phosphor.

High efficient blue emission of Ce3+ activated Ca4P2O9 phosphor for white LEDs.

Combustion Synthesis and Photoluminescence Properties of Red-Emitting CaAlSiN₃:Eu2+ Phosphor for White-LEDs.

Phosphor-converted LEDs with low circadian action for outdoor lighting.

Tunable blue-green-emitting wurtzite ZnS:Mg nanosheet-assembled hierarchical spheres for near-UV white LEDs.

A red phosphor BaTiF(6):Mn(4+): reaction mechanism, microstructures, optical properties, and applications for white LEDs.

Energy transfer in Eu³⁺ doped scheelites: use as thermographic phosphor.

Thermoresponsive scattering coating for smart white LEDs.

A highly efficient white light (Sr3,Ca,Ba)(PO4)3Cl:Eu2+, Tb3+, Mn2+ phosphor via dual energy transfers for white light-emitting diodes.

Spectral optimization of color temperature tunable white LEDs with excellent color rendering and luminous efficacy.

Surface-Tunable Bioluminescence Resonance Energy Transfer via Geometry-Controlled ZnO Nanorod Coordination.

Enhanced luminescence and white light emission from Eu(3+) -co-doped K3 Ca2 (SO4 )3 Cl:Dy(3+) phosphor with near visible ultraviolet excitation for white LEDs.

White thin-film flip-chip LEDs with uniform color temperature using laser lift-off and conformal phosphor coating technologies.

GaN nano-ring LEDs via nano-sphere lithography.

Synthesis of Sr2Si5N8:Ce3+ phosphors for white LEDs via an efficient chemical vapor deposition [corrected].