Photoluminescence and Cathodoluminescence Properties of a Novel 3+ Calaga3o7:Dy Phosphor

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Materials Science March 2012 Vol.57 No.7: 827831 doi: 10.1007/s11434-011-4938-5

Photoluminescence and cathodoluminescence properties of a novel 3+ CaLaGa3O7:Dy phosphor

ZHAO WenYu1,2*, AN ShengLi1, FAN Bin1, LI SongBo1 & DAI YaTang3

1 School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China; 2 School of Chemistry and Chemical Engineering, Inner Mongolia University of Science and Technology, Baotou 014010, China; 3 School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621000, China

Received July 14, 2011; accepted October 19, 2011; published online December 27, 2011

3+ A single host white emitting phosphor, CaLaGa3O7:Dy , was synthesized by chemical co-precipitation. Field emission scanning electron microscopy, X-ray diffraction, laser particle size analysis, and photoluminescence and cathodoluminescence spectra were used to investigate the structural and optical properties of the phosphor. The phosphor particles were composed of microspheres with a slight tendency to agglomerate, and an average diameter was of about 1.0 m. The Dy3+ ions acted as luminescent centers, 3+ and substituted La ions in the single crystal lattice of CaLaGa3O7 where they were located in Cs sites. Under excitation with 3+ 3+ ultraviolet light and a low voltage electron beam, the CaLaGa3O7:Dy phosphor exhibited the characteristic emission of Dy 4 6 4 6 ( F9/2- H15/2 and F9/2- H13/2 transitions) with intense yellow emission at about 573 nm. The chromaticity coordinates for the phosphor were in the white region. The relevant luminescence mechanisms of the phosphor are investigated. This phosphor may be applied in both field emission displays and white light-emitting diodes.

phosphor, cathodoluminescence, photoluminescence, field emission display

3+ Citation: Zhao W Y, An S L, Fan B, et al. Photoluminescence and cathodoluminescence properties of a novel CaLaGa3O7:Dy phosphor. Chin Sci Bull, 2012, 57: 827831, doi: 10.1007/s11434-011-4938-5

Recently, rare-earth-doped phosphors have been used widely mentally friendly, relatively cheap and easy to prepare [6,7]. in field emission displays (FEDs) and white light-emitting Calcium lanthanum gallium oxide CaLaGa3O7 forms te-

diodes (LEDs) because of advantages such as excellent lu- tragonal crystals that belong to the space group P 4 21m. minescent characteristics, good stability, high luminescence This crystal structure is composed of GaO4 layers formed in efficiency, corrosion-free gas emission under electron bom- the ab plane. Between the layers, divalent Ca and trivalent bardment and a strong excitation band in the blue or UV re- La cations are distributed randomly in eight coordinate sites gion [1,2]. Among these phosphors, much attention has been with Cs symmetry [8]. However, this material has not been focused on sulfide, nitride, oxide and oxynitride phosphors. reported as a host material for phosphor applications. In However, sulfide phosphors decompose readily and emit addition, most phosphors are prepared by traditional high sulfide gases during continuous electron beam irradiation. temperature solid-state reaction methods. However, because This not only causes cathodes to deteriorate but also lowers of insufficient mixing and the low reactivity of the raw ma- the luminous efficiency of the phosphors [3,4]. Nitride and terials, the prepared product usually possesses an irregular oxynitride phosphors require rigorous preparation conditions morphology or tendency to agglomerate with serious reun- [5]. Therefore, oxide-based phosphors doped with rare-earth ion and high hardness. In addition, luminescence efficiency ions have been extensively studied because of their high sta- of the product will be descended after milling. Use of chemical bility and luminescence efficiency as well as being environ- co-precipitation can overcome these disadvantages. Dy3+ ions, which luminesce in the 470–500 nm region because of 4 6 *Corresponding author (email: [email protected]) the F9/2– H15/2 transition and in the 570–600 nm region be-

4 6 cause of the F9/2– H13/2 transition, have attracted much attention because of their white light emission [9,10]. In this 3+ paper, a novel CaLaGa3O7:Dy phosphor was prepared by chemical co-precipitation. The photoluminescence (PL) and cathodoluminescence (CL) properties of this novel phosphor are investigated in detail.

1 Experimental

3+ 1.1 Synthesis of CaLaGa3O7:Dy

3+ The phosphor CaLaGa3O7:Dy was prepared by chemical co-precipitation. Oxalic acid was used as a precipitation 3+ agent. Based on single factor experiments, the optimum dop- Figure 1 FE-SEM micrograph of the CaLaGa3O7:5%Dy phosphor. ing concentration for Dy3+ was found to be 5 mol%. In a typical procedure, a nitrate solution was formed by adding Ca(NO3)2 (1 mol/L, 5 mL), La(NO3)3 (1 mol/L, 4.95 mL), Ga(NO3)3 (1 mol/L, 5 mL) and Dy(NO3)3(0.1 mol/L, 0.5 mL) to a 100 mL beaker. Oxalic acid solution (1 mol/L, 25 mL) was added dropwise to the nitrate solution under magnetic stirring. The pH was adjusted to 10–12 by adding ammonia solution (25%, w/v) dropwise. After 4 h, the resulting white powder was filtered, washed with distilled water and absolute ethanol several times, and then dried under vacuum at 80°C for 4 h. Finally, the phosphor CaLa- 3+ Ga3O7:5%Dy was obtained by calcining the above powder at 1100°C for 3 h in air. 3+ For comparison with CaLaGa3O7:5%Dy , the phosphor LaOCl:2%Dy3+ was prepared by the Pechini-type sol-gel Figure 2 Particle size distribution of the CaLaGa O :5%Dy3+ phosphor. method [11]. 3 7

3+ size distribution of the CaLaGa3O7:5%Dy particles, re- 1.2 Characterization 3+ spectively. The CaLaGa3O7:5%Dy sample consisted of The morphology of the sample was observed using a JEOL microspheres exhibiting slight agglomeration. The particles JSM-6700F field emission scanning electron microscope had a rather narrow size distribution, with an average diam- (FE-SEM). The particle size distribution of the sample was eter of about 1.0 m. According to our experience, these examined using a Mastersizer 2000 laser diffractometer. properties are suitable for making phosphor screens in FEDs. X-ray diffraction (XRD) measurements were performed on Figure 3 shows the XRD pattern of the CaLaGa3O7: a Rigaku D/max-IIIB diffractometer using Cu K radiation 5%Dy3+ phosphor and the Joint Committee for Powder Dif-

(= 0.15405 nm). An accelerating voltage and emission fraction Standards (JCPDS) card 39-1127 of CaLaGa3O7 as current of 35 kV and 60 mA, respectively, were used. PL a reference. All of the diffraction peak positions of the sam- measurements were performed on a Hitachi F-4500 spec- ple were consistent with those of the JCPDS card, with trophotometer equipped with a 150 W Xenon lamp as the three primary peaks at 2 of 30.283°, 35.655° and 50.069°. excitation source. CL measurements were performed in an These peaks were ascribed to the (311), (420) and (422) 4 ultrahigh-vacuum chamber (<8.0×10 Pa), where the phos- diffraction planes of the CaLaGa3O7 phase, respectively. No phors were excited by an electron beam at a voltage range additional peaks of other phases were detected, indicating of 1.0–6.0 kV and different filament currents. CL spectra the Dy3+ ions have been effectively doped into the CaLa- were recorded by a spectrometer (Hitachi F-4500) using a Ga3O7 host lattice. It is generally believed that impurity ions charge-coupled device camera through an optical fiber. All always take the place of the ions of similar radii and the 3+ the measurements were performed at room temperature. same valence. In the crystal structure of CaLaGa3O7, La cations are surrounded by eight anions in an octahedral fashion, and La3+ ions occupy sites with point symmetry of 2 Results and discussion 3+ 3+ Cs. As a result, the Dy ions substitute La with a point symmetry Cs because of the small difference in ionic radii Figures 1 and 2 show an FE-SEM micrograph and the particle and identical valence state (+3) between Dy3+ (102.7 pm) Zhao W Y, et al. Chin Sci Bull March (2012) Vol.57 No.7 829

in UV- and blue chips to generate visible light. Figure 5 shows the emission spectrum of CaLaGa3O7: 3+ 5%Dy (ex=349 nm). The emission spectrum only con- tains bands from f-f transitions from a Dy3+ 4f9 electron 4 6 configuration, i.e., F9/2→ H15/2 (476, 487, 495 nm) in the 4 6 blue region, and F9/2→ H13/2 (573 nm) in the yellow region. 4 6 It is well known that the F9/2→ H15/2 transition is predomi- nantly magnetically allowed and hardly varies with the crystal field strength around the Dy3+ ion. Moreover, the 4 6 F9/2→ H13/2 transition belongs to a hypersensitive transition

with the selection rule J = 2, which is a forced electric di- pole transition that is only allowed at low symmetry with no inversion center [15,16]. So, according to Figure 5, Dy3+ is 3+ Figure 3 XRD pattern of CaLaGa3O7:5%Dy . The standard data for located at low-symmetry local sites or lacks an inversion 3+ tetragonal phase CaLaGa3O7 (JCPDS No. 39-1127) is presented for com- center. As XRD analysis showed, the Dy ions occupy sites parison. with a point symmetry of Cs in the host CaLaGa3O7. Con- 3+ sequently, the symmetry of the Dy point sites remains Cs. 3+ and La (116 pm) ions in eightfold coordination [12,13]. In addition, the octahedron is distorted because the radius of According to the chekcell software (Collaborative Compu- the Dy3+ ions is smaller than that of the La3+ ions, which has tational Project Number 14 (CCP14); http://www.ccp14.ac. been shown by XRD analysis. The different radii of La3+ uk/ccp/web-mirrors/lmgp-laugier-bochu/), the lattice con- and Dy3+ reduce the symmetry of Dy3+. Reduced symmetry 3+ 3+ stants of the CaLaGa3O7:5%Dy sample were a=11.249 Å, of the coordination environment around the Dy ions will c=5.251 Å, V=664.5 Å3. So, compared with the lattice con- 4 6 enhance the yellow emission from the F9/2→ H13/2 transi- stants of tetragonal CaLaGa3O7 (JCPDS No. 39-1127, a= 3+ 3 tion. This evidence confirms that the Dy ions are mainly 11.252 Å, c=5.273 Å, V=667.2 Å ), the sample presents located in sites of the crystal lattice without inversion sym- slightly smaller lattice constants and unit cell volume be- 3+ 3+ metry. cause of the substitution of La by Dy with smaller ionic 3+ Figure 6 shows CL spectra of the CaLaGa3O7:5%Dy radius. and LaOCl:2%Dy3+ phosphors. The CL spectrum obtained Figure 4 shows the excitation spectrum of the CaLa- for CaLaGa O :5%Dy3+ consists of a broad blue band with 3+ 3 7 Ga3O7:5%Dy sample (em=573 nm). There are a number a maximum at 385 nm and two more intense emission bands of peaks with the highest intensity peak at 349 nm. Clearly, in the blue (470–500 nm) and yellow regions (560–600 nm). 3+ 9 these peaks are caused by the f-f transitions of Dy in a 4f The blue emission at 488 nm and the yellow emission at configuration, and can be assigned to the electronic transi- 576 nm are characteristic emissions of Dy3+, and are caused tions (6H →5P ) at 324 nm, (6H →6M2 ) at 349 nm, 4 6 4 6 15/2 3/2 15/2 1/2 by the transitions F9/2→ H15/2 and F9/2→ H13/2, respectively. 6 6 6 4 6 ( H15/2→ P7/2) at 362 nm, ( H15/2→ I13/2) at 386 nm, ( H15/2 The CL spectra are similar to the PL spectra of CaLa- 4 6 4 3+ → G11/2) at 426 nm, and ( H15/2→ I15/2) at 452 nm [14]. This Ga3O7:5%Dy except for the broad band at 385 nm and the 4 6 large range of excitation peaks is appropriate for application different intensity ratio of the F9/2→ H15/2,13/2 band. The

3+ 3+ Figure 4 Excitation spectrum of CaLaGa3O7:5%Dy . Figure 5 Emission spectrum of CaLaGa3O7:5%Dy . 830 Zhao W Y, et al. Chin Sci Bull March (2012) Vol.57 No.7

duce its characteristic emission. Moreover, it causes elec- 4 4 trons to transition from the A2 to the T2B energy level of Ga3+ and causes a band at 385 nm. As a result, the influence of crystal field splitting and the level splitting of energy for the Dy3+ ions are not important in the CL process. According to Figure 6, when the filament current is fixed at 50 mA, the CL intensity increases with raising the accelerating voltage from 1.0 to 6.0 kV. This can be attributed to the deeper penetration of the electrons into the phosphor. The electron penetration depth can be estimated using the empirical formula L(Å)=250(A/)(E/Z1/2)n, where n=1.2/(1– 0.29 log10 Z). A is the atomic or molecular weight of the material,  is its bulk density, Z is the atomic number or the number of electrons per molecule in the case of compounds,

3+ and E is the accelerating voltage (kV) [20]. For the CaLa- Figure 6 CL spectra of the CaLaGa3O7:5%Dy phosphor under low 3+ voltage electron beam excitation (accelerating voltage: 1, 3, 5 and 6 kV) Ga3O7:5%Dy sample, Z = 226, A = 500.15, and = 4.974 3 and the LaOCl:2%Dy3+ phosphor under low voltage electron beam excita- g/cm , so the electron penetration depths at anode voltages tion (accelerating voltage: 3 kV). of 1.0, 3.0, 5.0 and 6.0 kV are estimated to be 0.9, 5.7, 39.1 and 78.0 nm, respectively. Hence, the increase in CL inten- broad band at 385 nm can be attributed to the transition of a sity with increasing applied voltage is caused by an increase self-activated optical center related to octahedrally coordi- in the number of excited luminescent centers resulting from an increase in electron penetration depth. nated GaO6 groups [3] and/or corresponds to an energy re- 4 4 In addition, for CL, quantum efficiency (q) is irrelevant. laxation of electrons transiting from T2B to A2 energy lev- els of the distorted Ga-octahedral structure [17]. The dif- The radiant efficiency () is defined as the emitted power to ferent branch ratio of the 4F →6H band may relate to the power of the electron beam falling on the luminescent 9/2 15/2,13/2 material. The luminous efficiency (L) is defined in a similar differences in the responses of the apparatus and/or the ex- manner as for PL [21]. L is the ratio of the luminous flux citation mechanisms involved in CL and PL. In PL, UV and/or visible light is used to excite lumines- ( (v)) emitted by the material to the absorbed power. (v) cent materials. The energy of these photons is only around in the general case is given by the folding or weighting of 4–6 eV. Luminescent centers are directly excited by UV the spectral power distribution  () with the normalized photons, such as light with a wavelength of 349 nm. After photopic response curve V() and the constant factor Km 4 excitation, most of the energy transfers to the F9/2 level of [22]. () is directly proportional to the CL intensity (in 3+ 3+ 3+ 3+ Dy , which results in the characteristic transitions of Dy , height). So, for the CaLaGa3O7:Dy and LaOCl:Dy phos- as shown in Figure 5. The influence of crystal field splitting phors,  can be also compared roughly by considering their and the level splitting of energy for Dy3+ cannot be ne- CL emission peak areas [3,11]. L can be also compared glected in this process. However, under electron-beam irra- roughly by the weighting of their CL intensity (in height) diation during CL, luminescent centers in phosphors are with V(). It can be seen from Figure 6 that the emission 3+ excited by either direct or indirect excitation [18]. The en- peak area of CaLaGa3O7:5%Dy (3 kV, curve (b)) is higher ergy of electrons under acceleration from anode voltage can than that of LaOCl:2%Dy3+ (3 kV, curve (c)), but the be tuned from a few thousand to thousands of eV. When a weighting of CL intensity (in height) with V(λ) of CaL- 3+ 3+ high-energy electron reaches the surface of a phosphor, it aGa3O7:5%Dy is smaller than that of LaOCl:2%Dy . So 3+ may penetrate into it. After penetrating into the host lattice,  of CaLaGa3O7:5%Dy might be higher than that of 3+ 3+ the fast primary electrons will cause ionization. This ioniza- LaOCl:2%Dy , but L of CaLaGa3O7:5%Dy might be tion creates many secondary electrons. These secondary smaller than that of LaOCl:2%Dy3+. electrons can also cause ionization and create further sec- Figure 7 (A–E) shows the chromaticity coordinates (X, Y) 3+ ondary electrons [19]. When lower energy secondary elec- of the CaLaGa3O7:5%Dy phosphor excited at 349 nm, and trons reach the luminescent centers, they may directly excite 1.0, 3.0, 5.0 and 6.0 kV, respectively, in the Commission the luminescent centers. Also, these secondary electrons Internationale de L’Eclairage (CIE) 1931 chromaticity dia- generate many electron-hole pairs (EHs) by collision with gram. It is obvious that almost the whole emission of the 3+ lattice ions. Generated EHs do not directly recombine in the CaLaGa3O7:5%Dy phosphor is in the white light region. lattice. They act as mobile carriers in the particles, and re- Therefore, according to the PL spectra, the CaLaGa3O7: combine at luminescent centers or Ga3+ ions. These excitons, 5%Dy3+ phosphor is suited for use as a color converter of if they form, will decay nonradiatively through a resonant or white LEDs by combining a UV- or blue LED chip with quasi-resonant transfer to the 4f shell of Dy3+ ions and in- this phosphor. Compared with the yellow phosphor LaOCl: Zhao W Y, et al. Chin Sci Bull March (2012) Vol.57 No.7 831

search and Development Program of China (2008AAXXX0310) and the Innovation Fund of Inner Mongolia University of Science and Technology (2010NC026).

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