Luminescence properties and energy transfer of host sensitized CaMoO4:Tb3+ green phosphors

JOURNAL OF RARE EARTHS, Vol. 31, No. 7, July 2013, P. 655

Luminescence properties and energy transfer of host sensitized CaMoO4:Tb3+ green phosphors ZHOU Xianju (周贤菊), YANG Xiaodong (杨小东), XIAO Tengjiao (肖腾蛟), ZHOU Kaining (周凯宁), CHEN Tianyu (陈天宇), YAN Hao (闫 浩), WANG Zhongqing (汪仲清) (Department of Mathematics and Physics, Chongqing University of Posts and Telecommunications, Chongqing 400065, China) Received 27 December 2012; revised 2 May 2013

Abstract: A series of CaMoO4:xTb3+ (x=0.01, 0.03, 0.05, 0.07, 0.09, 0.15 and 0.20) phosphors in pure phase were prepared via high temperature solid-state reaction approach. The crystal structure of the phosphors was investigated by X-ray diffraction (XRD), and the optical properties were investigated by Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-Vis) and photoluminescence (PL) spectroscopy. The PL spectra illustrated that these phosphors could be efficiently excited by the charge transfer band of the host and the energy transfer efficiency from the host to the doped activator reached 60% when the doping concentration of the activator Tb3+ was 20 mol.%. The concentration quenching occurred at x=10 mol.%, from which the critical distance of activator was calculated to be about 1.14 nm. The CIE coordinates were estimated to be close to the standard green value. The host sensitized samples had potential application as green phosphors. Keywords: luminescence; host sensitization; energy transfer; green phosphor; rare earths

White light emitting diodes (WLEDs) are considered as the new generation of solid state lighting thanks to its high efficiency, energy saving, environment friendly and long working lifetime advantages[1,2]. Accordingly, tricolor phosphors for WLEDs have attracted much attention in recent years[3–5]. At present, common commercial green phosphors are sulfide-base materials such as CaGaS2:Eu2+ or ZnS:Cu+/Al3+ [6,7], which face serious problems of stability and efficiency. Firstly, the sulfide hosts are likely to decompose at high temperature and produce harmful gas. Secondly, divalent europium ions could be oxidized easily to trivalent and cause color drift and degradation[8]. Therefore, new green phosphors of improved stability and efficiency are needed urgently. Intensive studies have been carried out on Tb3+ doped green phosphors[9–11]. It is known that Tb3+ has sharp excitation peaks at about 350–390 nm due to characteristic f-f transitions of Tb3+ [12–15]. Therefore, most of the reported Tb3+ activated green phosphors are excited by the narrow bands[9–11]. However, the f-f transitions are only partially allowed induced electric dipole transitions and its optical oscillator strength is small[16,17], resulting in weak excitation of the phosphors. Furthermore, the full width at half maximum (FWHM) of the line-like lanthanide absorption is too narrow to contain the tiny emission wavelength shift of LED chips[18]. The combination of the strong and broad charge transfer band of the host and

the 4f-4f sharp line emission of Tb3+ would possibly overcome the drawbacks mentioned above. In this study, green-emitting phosphors of Tb3+ doped CaMoO4 were synthesized by solid state reaction, and these phosphors could be efficiently excited by the broad charge transfer band of the host, the energy transfer efficiency from MoO42– group to Tb3+ activator reached 60%.

1 Experimental A series of phosphors CaMoO4:xTb3+(x=0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.15 and 0.20) were synthesized by high temperature solid-state method. Stoichiometric amount of starting materials including CaCO3 (analytical reagent, AR), MoO3 (AR), and Tb4O7 (99.99%) were well ground in an agate mortar for 20 min, and then heated at 800 ºC for 4 h in a corundum crucible in a muffle furnace with appropriate amount of active carbon grains to get reductive atmosphere. After being cooled to room temperature naturally, the final product was ground again. The X-ray powder diffraction (XRD) experiment was carried out on XD-2 (Beijing Purkinje General Instrument Co., Ltd.) X-ray diffractometer using Cu Kα radiation (λ=0.154056 nm) operating at 36 kV and 20 mA, with the scan range from 10° to 80° with 2 (°)/min. The Fourier transform infrared (FT-IR) spectrum of the sample was recorded from 400 to 4000 cm–1 with KBr pellet

Foundation item: Project supported by National Natural Science Foundation of China (20903123), Key Project of Chinese Ministry of Education (211154) and Natural Science Foundation Project of Chongqing (KJ110532, CSTCjjA1425) * Corresponding author: ZHOU Xianju (E-mail: [email protected]; Tel.: +86-23-62471347) DOI: 10.1016/S1002-0721(12)60337-8

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technique using a Spectrum 65 (Perkin Elmer Co.) spectrophotometer. The ultraviolet-visible reflectance spectrum was obtained by using TU-1901 (Beijing Purkinje General Instrument Co., Ltd.) double-beam spectrometer equipped with a Ф60 mm integrating sphere whose inner surface is coated with BaSO4 and BaSO4 is used as the standard during the measurement. The photoluminescence (PL), photoluminescence excitation (PLE) spectra and the decay curves were measured by an FLSP920 spectrometer (Edinburgh Instrument) equipped with a 450 W xenon lamp as light source and Shimidazu R9287 photomultiplier as the detector. All the measurements were carried out at room temperature.

2 Results and discussion Fig. 1 presents the XRD patterns of CaMoO4:xTb3+ (x= 0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.15 and 0.20) samples. The position and the relative intensity of the diffraction peaks of all the samples are well consistent with the standard Joint Committee on Powder Diffraction Standards (JCPDS No. 29-0351) and no impurity phase is observed. CaMoO4 is of scheelite structure with I41/a space group, and the crystal parameters are a=b=0.5226 nm, and c=1.1430 nm. In a unit cell, the central Mo atom is coordinated by four O atoms in tetrahedral symmetry[13], and the calcium atom is coordinated to eight oxygen atoms[14]. The XRD results illustrate that the doped Tb3+ ions are introduced to the crystal lattice without changing the crystal structure obviously. Fig. 2 is the selected FT-IR spectra of CaMoO4: 0.03Tb3+ powder (the spectra of all samples are extremely alike, for clarity, Fig. 2 presents the result of one sample only). The IR spectrum of the inorganic phosphor is simple and clear with only four absorption peaks. The most intense one is the asymmetric stretching vibration of Mo–O bond at 815 cm–1. Correspondingly, the sym-

Fig. 1 X-rays diffraction patterns of CaMoO4:xTb3+ (x=0.01−0.2) phosphors sintered at 800 ºC for 4 h

JOURNAL OF RARE EARTHS, Vol. 31, No. 7, July 2013

Fig. 2 FT-IR spectrum of CaMoO4:0.03Tb3+ with KBr pellet technique

metric stretching vibration of Mo–O is the weak peak locating at 434 cm–1 [19]. These two peaks are assigned to Mo–O ν3 (F2) and ν4 (F2) vibration mode, respectively[14,15]. The characteristic peaks at 1638 and 3436 cm–1 originate from the bending vibration and the stretching vibration of H–O–H bonds, respectively[16,17]. The water molecules are most likely caused by the absorption of the moisture in air during KBr disc preparation[18,20]. The ultraviolet-visible reflectance spectra of the solidstate green phosphors recorded by integrating sphere are shown in Fig. 3. The reflectivity changes dramatically in the range of 400–230 nm and reveals a strong broad absorption peak for every sample. This peak corresponds to the charge transfer band of Mo–O. In addition, the intensity of the charge transfer band increases with increasing concentration of Tb3+ activators. On the other hand, this charge transfer band shifts to long wavelength with increasing of Tb3+ doping concentration as well. The redshift is probably due to change of Mo–O covalence, which is caused by the charge compensation as a result of the replacement of Ca2+ with Tb3+ ions. Shift of Mo–O charge transfer band has also been observed previously[21,22]. The excitation and emission spectra of CaMoO4:xTb3+ (x=0, 0.07) are presented in Fig. 4. The solid and dot curves correspond to the samples of x=0 and x=0.07, re-

Fig. 3 Ultraviolet-visible reflectance spectra of CaMoO4:xTb3+ (x=0.01−0.09) phosphors

ZHOU Xianju et al., Luminescence properties and energy transfer of host sensitized CaMoO4:Tb3+ green phosphors

Fig. 4 PLE and PL spectra of CaMoO4 (solid curves) and CaMoO4:0.07Tb3+ (dotted curves)

spectively. The PL spectra were excited at 299 nm, and PLE spectra were recorded monitoring 505 and 544 nm (Tb3+:5D4→7F5) for the undoped and doped phosphors respectively. In the PLE spectra, the broad peak centralizing at 299 nm belongs to the charge transfer of [MoO4]2– cluster of O2– (p electron)→Mo6+ (d orbit), which is consistent with the result of UV-vis reflectance spectra (see Fig. 3). The sharp but weak peak at 485 nm in Tb3+ doped sample (dot curve) is due to the intraconfigurational f-f transition of Tb3+ 7F6→5D4. Obviously, the observation of the charge transfer band of Mo–O in the excitation spectrum of Tb3+ doped phosphor monitoring the emission of Tb3+ illustrates the presence of the energy transfer from host to activator Tb3+. In addition, from the comparison of the relative intensity of the charge transfer band and f-f transition, the Tb3+ doped green phosphor is expected to be excited by CT band more efficiently than by f-f transition. And it is verified by the PL spectra (data not shown here). The broad strong emission centralizing at 505 nm origin from the host disappears entirely in the CaMoO4:0.07Tb3+ sample, and only the characteristic f-f transitions of Tb3+ located at 488, 544, 587 and 622 nm belong to the transition 5 D4→ 7FJ (J=6, 5, 4 and 3)[23] are observed. The emission spectra of the green phosphors with various doping concentration under the excitation wavelength of 299 nm are presented in Fig. 5(a), and the details of the emission spectra in the range of 450–530 nm are shown in Fig. 5(b). Similar results with the dot curve in Fig. 4 are obtained. The four characteristic transitions of Tb3+ are clearly observed in all the samples. But the broad green-yellow emission of the host from 400 to 700 nm overlaps with the f-f transition of Tb3+ in the low-concentration doped phosphors. The intensity of the broad band decreases with increasing of Tb3+ until it entirely disappears in concentrated doped sample. This is the evidence of the energy transfer from the host to activators. Moreover, the energy-transfer efficiency can be calculated using τ d o ped η = 1(1) τ u nd o p ed

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where τdoped and τundoped are defined as the lifetimes of doped and undoped samples, respectively[24,25]. The florescence decay curves of the host and Tb3+ doped samples are presented in Fig. 6, with the excitation at 299 nm and monitoring emission at 515 nm. The energy transfer efficiency η of these phosphors CaMoO4:xTb3+ (x=0.01, 0.03, 0.05, 0.07, 0.09, 0.15 and 0.20) are calculated to be 18%, 29%, 34%, 37%, 38%, 51% and 60%, respectively. Fig. 5(c) plots the relationship between the emission intensity of Tb3+ 5D4→7F5 at 544 nm and activator doping concentration at the excitation of host-sensitization 299 nm and Tb3+ characteristic excitation of 485 nm respectively. Just as expected from the excitation spectra in Fig. 4, the host-sensitization is more efficient than Tb3+ characteristic excitation band at 485 nm to excite these phosphors. Furthermore, under 485 nm excitation, the emission intensity increases monotonically without concentration quenching until the Tb3+ doped concentration reaches 20 mol.%. But under 299 nm excitation, the optimal concentration is 10 mol.%. Additionally, there appears a quenching concentration at Tb3+ 10 mol.% upon 299 nm excitation, but no quenching is observed for 485 nm excitation in our study. The explanation might relate to the different excitation paths for these two excitation sources. The first one is: charge transfer band of

Fig. 5 (a) Emission spectra of CaMoO4:xTb3+ under excitation of 299 nm; (b) the details of the emission spectra in the range of 450–530 nm; (c) the emission intensity of 544 nm as a function of Tb3+ doping concentration

Fig. 6 Luminescence decay curves of CaMoO4:xTb3+ phosphors

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the host, exactly the charge transfer of [MoO4]2– cluster of O2– (p electron) to Mo6+ (d orbit), is excited upon 299 nm, and then the energy is transferred to the doped Tb3+ activators. The second one: the 5D4 state of the Tb3+ f orbit is directly excited by 485 nm. The quenching behavior is different for these two paths: the former is sensitive to concentration of the activators, and concentration quenching occurs. However, the latter is not so sensitive; therefore, no concentration is observed in the studied concentration range. According to Dexter’s energy transfer theory, the main contribution of quenching of concentration of Tb3+ is the cross relaxation process between 5D3→5D4 and 7F6→ 7 F0–2[26]. The cross relaxation rate depends on the distance between activators. The critical distance (Rc) of quenching can be expressed by the following formula (2). 2 Rc ≈ （

1

3V 4πxc N

(2)

)3

where V is the volume of unit cell, N is the number of host cations which are occupied by activators, and xc is the critical concentration[27,28]. For the host-sensitized green phosphor in this work, V is 0.312165 nm3, N is 4, and xc is 0.10. Therefore the critical distance for quenching of concentration is estimated to be about 1.14 nm. The Commission International de L’Eclairage (CIE) 1931 chromaticity coordinates of the host-sensitized green phosphors have been calculated from the spectra in Fig. 5 under excitation of 299 nm and are listed in Table 1. Table 1 CIE coordinates of the phosphors Sample

X coordinate

Y coordinate

1%

0.280

0.460

3%

0.300

0.510

5%

0.290

0.530

7%

0.290

0.550

9%

0.290

0.560

15%

0.300

0.600

20%

0.300

0.610

Fig. 7 CIE diagram of CaMoO4:xTb3+ samples

The CIE diagram of the phosphors is presented in Fig. 7 and the corresponding CIE coordinates are inserted in the figure. It shows that the CIE chromaticity coordinates of the phosphors obtained in this work are approaching the standard value of green (x=0.300, y=0.600) of Cold Cathode Fluorescent Lamp (CCFL) with increasing Tb3+ doped concentration.

3 Conclusions CaMoO4:xTb3+ (x=0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.15, 0.20) phosphors were synthesized via high temperature solid state reaction approach. X-ray diffraction (XRD), ultraviolet visible (UV-vis), Fourier transform infrared (FTIR), photoluminescence (PL) spectra, energy transfer and CIE coordinate of the phosphors were investigated. The XRD results revealed that the phosphors synthesized in this work were of pure phase. There were two excitation peaks located at 299 and 486 nm which were assigned to O→Mo charge transfer band of the host and 7 F6→5D4 transition of Tb3+ respectively. In addition, phosphors could be excited more effectively by the excitation of charge transfer band than that by the characteristic excitation of Tb3+, which illustrated efficient energy transfer from the host to the doped activators Tb3+ ions. Energy transfer efficiency changed from 18% to 60% with the concentration of the dopant increasing from 1 mol.% to 20 mol.%. The most intense emission was located at 544 nm and was due to 5D4→7F5 transition of Tb3+. The CIE coordinate approached to pure green region with increasing Tb3+ doping concentration.

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