Article pubs.acs.org/JPCC
Tunable Blue-Green Color Emission and Energy Transfer of Ca2Al3O6F:Ce3+,Tb3+ Phosphors for Near-UV White LEDs Zhiguo Xia*,† and Ru-Shi Liu*,‡ †
School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
‡
ABSTRACT: A series of new luminescent emission-tunable phosphors Ca2Al3O6F:Ce3+,Tb3+ have been synthesized by a high temperature solid-state reaction. The UV−vis reflectance, photoluminescence emission and excitation spectra, the lifetime, and the effect of Tb3+ concentration are investigated in detail. The intense green emission is realized in the Ca2Al3O6F:0.08Ce3+,0.05Tb3+ phosphors on the basis of the highly efficient energy transfer from Ce3+ to Tb3+ with an efficiency of over 90%. The energy transfer mechanism from Ce3+ to Tb3+ in the Ca2Al3O6F host was ascribed to the exchange interactions, and the formation of the Ce−Ce clusters and Ce−Tb clusters should be the reason for the high energy transfer efficiency. The critical distance of the energy transfer has also been calculated by the concentration-quenching method. These results indicate that the Ca2Al3O6F:Ce3+,Tb3+ phosphors have potential applications as a near UV-convertible phosphor for white light-emitting diodes because of its broad excitation in the near-ultraviolet range and the efficient green emission light.
1. INTRODUCTION
spectral region of 250−450 nm and the efficient energy transfer between Ce3+ and Tb3+.11,12 Recently, a family of anion ordered oxyfluorides with the general composition Sr3‑xAxMO4F (with A = Ca, Ba and 0 ≤ x ≤ 1 and M = Ga1‑zAlz and 0 ≤ z ≤ 1) show photoluminescence when doped with rare-earth activators.13−15 This novel Sr-based oxyfluorides have been investigated by several research groups based on their good stability, cheap raw materials, simple synthesis conditions and good luminescence properties.13−16 Here in this work, we reported for the first time a novel Cabased oxyfluoride host, Ca2Al3O6F. Rare earth ions Ce3+, Tb3+ singly doped and Ce3+/Tb3+ codoped Ca2Al3O6F phosphors were synthesized, and their luminescence and energy transfer properties were also investigated. Owing to the different valence states and the possible charge compensation behavior of the doping Ce3+/Tb3+ ions and Ca2+ ion in the present Ca2Al3O6F host, the nonstoichiometry of this host lattice (Ca2−3x/2RExAl3O6F) was performed in actual experiment without adding the charge compensator ions, however, the formula in the form of Ca2Al3O6F:Ce3+,Tb3+ was still as written hereafter. The as-prepared Ca2Al3O6F:Ce3+,Tb3+ phosphor shows good excitation and absorption profile for the practical n-UV LED, and the highly efficient energy transfer from Ce3+ to Tb3+ with an efficiency of over 90% is also found and studied in detail.
Due to their higher energy efficiency compared to both traditional incandescent and currently implemented fluorescent lamps, white light-emitting diodes (w-LEDs) have received lots of attention in solid-state lighting area.1,2 Presently the w-LEDs based on YAG:Ce3+ phosphor face problems of thermal quenching and poor color rendition, so that a near UV (nUV) LED (350−420 nm) coated with tricolor phosphors was introduced, which can provide superior color uniformity with a high color rendering index and excellent quality of light.3,4 Accordingly, w-LEDs fabricated with n-UV LED chips and tricolor (red, green and blue) phosphors are expected to dominate the market in the near future.5 Therefore, the exploration of novel phosphor materials plays an important role in the development of w-LEDs. It is well-known that the Tb3+ ion is frequently used as an activator of green emitting luminescent materials due to its predominant 5 D 4 − 7 F 5 transition peak at around 545 nm.6,7 However, the intensities of the Tb3+ absorption peaks in the n-UV region are very weak and their widths are very narrow due to the strictly forbidden 4f−4f transitions.6 The energy transfer from sensitizer to activator by rare earth ions has been investigated in many inorganic hosts, such as fluorides, silicates, phosphates, and borates.8−10 As a promising sensitizer for Tb3+ ions, Ce3+ has been widely used in many hosts. 10,11 Therefore, the luminescence originating from Ce3+/Tb3+ couples can act as the green component for the n-UV LED since the 4f−5d transition of the Ce3+ ion shows superior absorption in the © 2012 American Chemical Society
Received: May 15, 2012 Revised: July 2, 2012 Published: July 3, 2012 15604
dx.doi.org/10.1021/jp304722z | J. Phys. Chem. C 2012, 116, 15604−15609
The Journal of Physical Chemistry C
Article
2. EXPERIMENTAL PROCEDURES The Ca2Al3O6F:Ce3+,Tb3+ phosphors were synthesized by a high temperature solid-state reaction. The starting materials used for the studied phosphors were CaCO3 (Aldrich, 99.9%), Al2O3 (Aldrich, 99.9%), CaF2 (Aldrich, 99.9%), Eu2O3 (Aldrich, 99.995%), and Tb4O7 (Aldrich, 99.995%), and they were mixed and ground according to the given stoichiometric ratio. Some excessive CaF2 (5%) is necessary for loss of F source at high temperature. After all the materials were ground thoroughly in an agate mortar, the mixture was placed into an alumina crucible and then was fired at 1250 °C in a reducing (N2/H2 = 95:5) atmosphere for 4 h. The phase structure determination of the Ca 2 Al 3 O 6 F:Ce 3+,Tb 3+ phosphor was checked by X-ray diffraction (XRD) using a Bruker D2 PHASER diffractometer with Cu Kα radiation (λ = 1.5418 Å). Diffuse reflection spectra were measured on a UV−vis−NIR spectrophotometer (SHIMADZU UV-3600) attached to an integral sphere. BaSO4 was used as a reference standard. Room temperature excitation and emission spectra were measured on a JOBIN YVON FluoroMax-3 fluorescence spectrophotometer with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp used as the excitation lamp. The lifetimes were recorded on a spectro-fluorometer (HORIBA, JOBIN YVON FL3-21), and the 370 nm pulse laser radiation (nano-LED) was used as the excitation source. All of the measurements were performed at room temperature.
be ascribed to the successful introduction of lanthanide ions, such as Ce3+ (1.03 Ǻ ) and Tb3+ (0.92 Ǻ ), into the Ca2+ (0.99 Ǻ ) sites in the Ca2Al3O6F structure, which in turn lead to the slight expansion of the lattice volume. Furthermore, the lattice parameters of the selected Ca2Al3O6F:0.08Ce3+,0.05Tb3+ sample were calculated by the UnitCell program18 in the hexagonal system based on the given XRD data in Figure 1, and the corresponding lattice constants are as follows: a = b = 17.35 Å, c = 7.00 Å, and V = 1824.20 Å3. The above results indicate that the Ce3+ and/or Tb3+ ions are completely dissolved in the Ca2Al3O6F host. Figure 2a shows the PLE and PL spectra of Ca2Al3O6F:0.08Ce3+. The PLE spectrum monitored at 413
3. RESULTS AND DISCUSSIONS Figure 1 shows the typical powder XRD patterns of Ca2Al3O6F:0.08Ce3+ and Ca2Al3O6F:0.08Ce3+,0.05Tb3+ sam-
Figure 2. PLE (left) and PL (right) spectra of Ca2Al3O6F:0.08Ce3+ (a), Ca2Al3O6F:0.05Tb3+ (b), and Ca2Al3O6F:0.08Ce3+,0.05Tb3+ (c) samples. The corresponding monitoring wavelengths are also given in the figure.
nm exhibits two distinct excitation bands at 294 and 366 nm, which is assigned to the 4f−5d transitions of Ce3+. The latter demonstrates a broad excitation band from 322 to 387 nm, which is very important for the excitation absorption of the nUV LED chip. Under excitation of n-UV light (λex = 350 nm), the PL spectrum exhibits an asymmetric blue emission band that extends from 365 to 500 nm with a maximum at about 413 nm. The typical doublet bands due to the transition of the Ce3+ ions from the 5d excited state to the 2F5/2 and 2F7/2 ground states cannot be distinguished directly. However, the emission band can be decomposed into two well-separated Gaussian components (the dotted lines in Figure 2a) with maxima at 397 nm (25 188 cm−1) and 429 nm (23 310 cm−1) on an energy scale with an energy difference of about 1878 cm−1, which is in agreement with the theoretical difference between the 2F5/2 and 2 F7/2 levels (about 2000 cm−1).19,20 As for the Tb3+ singly doped Ca2Al3O6F sample, its PLE and PL spectra are presented in Figure 2b. It is found that only some narrow f−f transition lines (300−450 nm) of Tb3+ in the excitation spectrum could be located at the excitation range of n-UV LED. It is accepted that the f−f absorption transitions of Tb3+ ions are forbidden transition and the ions are difficult to pump, as shown in the PLE spectrum in Figure 2b. Furthermore, the PL spectrum under the excitation of 350 nm only displays a series of weak emissions at 488, 542, 584, and 619 nm, due to the 5D4−7FJ (J = 6, 5, 4, and 3) characteristic transitions of Tb3+ ions. In order to enhance the absorption intensity in the n-UV region for the Tb3+ emission, Ce3+ ions can be codoped as sensitizers to transfer excitation energy to Tb3+ ions. We can also observe
Figure 1. XRD patterns of Ca 2 Al 3 O 6 F:0.08Ce 3+ (a) and Ca2Al3O6F:0.08Ce3+,0.05Tb3+ (b) samples. The standard data for Ca2Al3O6F (JCPDS card no. 17-0107) is shown as a reference.
ples. The XRD patterns of as-prepared Ca2Al3O6F compounds accompanying with different doping rare earth ions are in good agreement with the reported Ca2Al3O6F phase (JCPDS card no. 17-0107), and no other impurity phase can be detected. As we know, the Ca2Al3O6F compound was first reported by Leary in 1962, and it has hexagonal structure with a = b = 17.29 Å, c = 7.01 Å, the cell volume V = 1814.84 Å3, and N = 12.17 In Figure 1, we can also find that the diffraction peaks of Ca2Al3O6F:0.08Ce3+ and Ca2Al3O6F:0.08Ce3+,0.05Tb3+ samples shift toward the smaller 2θ diffraction direction. It should 15605
dx.doi.org/10.1021/jp304722z | J. Phys. Chem. C 2012, 116, 15604−15609
The Journal of Physical Chemistry C
Article
from Figure 2a and Figure 2b that there is an overlap between the emission band of Ce3+ and the f−f absorptions of Tb3+ indicating the possible resonance type energy transfer from Ce3+ to Tb3+ in Ca2Al3O6F host. This can be further confirmed by the PLE spectra in Figure 2c. The excitation spectrum of Ca2Al3O6F:0.08Ce3+,0.05Tb3+ monitored with 413 nm (Ce3+ emission) is similar to that monitored with 542 nm (Tb3+ emission) except for the difference of the relative intensity. The presence of the broad-band transition from Ce3+ ions in the PLE spectrum monitored at the 5D4-7F5 transition of Tb3+ proves the occurrence of energy transfer from Ce3+ to Tb3+.11,20 Under the excitation of 350 nm, both the emission of Ce3+ and Tb3+ can be observed in the PL spectrum of the codoped sample, which means that Ca2Al3O6F:Ce3+,Tb3+ can serve as the green emitting phosphor for n-UV LEDs through energy transfer from the Ce3+ intense broad band absorption. The reflectance spectra of Ca2Al3O6F host, Ca2Al3O6F:0.08Ce3+, and Ca2Al3O6F:0.08Ce3+,0.05Tb3+ phosphors were further shown in Figure 3. The Ca2Al3O6F host shows energy
Figure 4. PL spectra for Ca2Al3O6F:0.08Ce3+,yTb3+ phosphors on Tb3+ doping content (y).
due to the 5D4−7FJ (J = 6, 5, 4, and 3) characteristic transitions of Tb3+ ions, and the green emission line at 542 nm from 5 D4−7F5 transitions dominates the whole spectrum. Accordingly, we can observe strong green emission in this series of samples. We propose that clusters can be formed owing to the existence of Ca vacancy for charge compensation, which will further promote clusters of Ce, Tb, and the Ca vacancy.21,22 Since clustering is important in the present phosphor system, the relative intensities will change in Ce3+/Tb3+ codoped samples between high Ce concentrations where all the Tb ions would be clustered with Ce ions and low Ce concentrations where there are insufficient Ce ions to create mixed clusters. Accordingly, we can find substantial differences of the Tb3+ emission in the single doped and Ce3+/Tb3+ coped samples. Furthermore, when the Tb concentration becomes higher than the Ce concentration, the Tb must cluster with themselves, so that we can find the variation of the Tb3+ emission for the Ca2Al3O6F:0.08Ce3+,yTb3+ phosphors. Figure 5 illuminates the dependence of the intensities of the 4f−5d transition of Ce3+ at 413 nm and 5D4−7F5 transition of
Figure 3. Reflectance spectra of Ca 2 Al 3 O 6 F host (a), Ca2Al3O6F:0.08Ce3+ (b), and Ca2Al3O6F:0.08Ce3+,0.05Tb3+ (c) samples.
absorption in the
