Photoluminescence and electron-beam excitation induced cathodoluminescence properties of novel green-emitting Ba4La6O(SiO4)6:Tb3+phosphors

Ceramics International 42 (2016) 11099–11103

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Photoluminescence and electron-beam excitation induced cathodoluminescence properties of novel green-emitting Ba4La6O(SiO4)6:Tb3 þ phosphors G. Seeta Rama Raju, E. Pavitra, Jae Su Yu n Department of Electronics and Radio Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 10 March 2016 Received in revised form 30 March 2016 Accepted 4 April 2016 Available online 4 April 2016

Tb3 þ ions activated Ba4La6O(SiO4)6 (BLSO:Tb3 þ ) phosphors were synthesized by a citrate sol-gel method. The X-ray diffraction pattern confirmed their oxyapatite structure. The field-emission scanning electron microscope image established that the BLSO:Tb3 þ phosphor particles were closely-packed and acquired irregular shapes. The photoluminescence (PL) excitation spectra of BLSO:Tb3 þ phosphors showed intense f–d transitions along with low intense peaks corresponding to the f–f transitions of Tb3 þ ions in the lower energy region. The PL emission spectra displayed the characteristic emission bands of Tb3 þ ions, and the optimized concentrations were found to be at 1 and 6 mol% for blue and green emission peaks, respectively. The cathodoluminescece (CL) spectra exhibited a similar behavior that was observed in the PL spectra except the intensity variations in the blue and green regions. The CL spectra of the BLSO:6 mol% Tb3 þ phosphor unveiled accelerating voltage induced luminescent properties. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Sol–gel process B. X-ray diffraction C. Optical properties

1. Introduction Recently, the research has been focused on the multifunctional sources for lighting (incandescent light bulbs, compact fluorescent lamps, and phosphors and non-phosphors based semiconductor light-emitting diodes (LEDs)) and display (cathode ray tubes, fieldemission displays (FEDs), and plasma display panels) applications [1–5]. Among the lighting sources, increasing attention has been paid towards the development of phosphor converted LEDs (pcLEDs) because these devices provide an energy efficient and reliable source for indoor and outdoor lighting along with LEDs based display devices [1]. It is fortunate that the ultraviolet (UV) LED sources with different emission wavelengths in the UV-A (315– 400 nm), UV-B (280–315 nm) and UV-C (100–280 nm) regions of the electromagnetic spectrum have been developed in recent years [6–11]. These UV sources are useful to provide tri-band (blue, green and red) based pc-white LEDs (pc-WLEDs), and the tri-band emissions not only cover the entire visible region but also provide high color-rendering index (CRI) and color purity [6]. In the case of the tri-band WLEDs, it is well known that the oxides based inorganic phosphors would be one of the best candidates in terms of both chemical stability and luminescence efficiency [12]. n

Corresponding author. E-mail address: [email protected] (J.S. Yu).

http://dx.doi.org/10.1016/j.ceramint.2016.04.011 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Nowadays, the oxide phosphors have been also used to fabricate efficient FED systems [3]. Therefore, the availability of phosphors operating under UV excitation as well as low voltage electronbeam excitation with better performance is of prime importance for such LEDs and FEDs. In previous work, we have developed and reported efficient red phosphors for promising multifunctional applications [13]. Now, our efforts have been focused on the development of green phosphors for LED and FED applications. In this paper, we report on the Tb3 þ ions doped Ba4La6O(SiO4)6 phosphors (referred to it as BLSO:Tb3 þ ) by a sol-gel process for the first time. This phosphor exhibited efficient photoluminescence (PL) properties of the Tb3 þ ions under the excitation of UV wavelength region. The cathodoluminescence (CL) properties were explained under electron-beam excitations and exhibited the accelerating voltage induced emission properties. The obtained results suggested that the BLSO:Tb3 þ phosphors are a promising novel green-emitting material for multifunctional applications.

2. Experimental Ba4La6(1  x)O(SiO4)6:6xTb3 þ phosphors were synthesized by a citrate sol-gel process by taking the appropriate amounts of barium nitrate [Ba(NO3)2 ( Z99%)], lanthanum nitrate hexahydrate [La(NO3)3  6H2O (99.99%)], terbium nitrate pentahydrate [Tb (NO3)3  5H2O (99.9%)], tetraethyl orthosilicate (TEOS) [Si(OC2H5)4

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(98%)], and citric acid [HOC(COOH)(CH2COOH)2 (Z 99.5%)]. Initially, the solution was prepared by dissolving 4 mM of barium nitrate, 6(1-x) mM of lanthanum nitrate, and 6x mM of terbium nitrate in 200 ml of de-ionized (DI) water and 32 mM citric acid was added to the solution (metal ions and citric acid ratio of 1:2). The silicate source was prepared by dissolving the appropriate amount of TEOS in 20 ml of 2-PrOH. The two solutions were stirred individually using a magnetic stirrer until the homogeneous solutions were formed. Finally, the two solutions were mixed and the magnetic stirring was continued until the homogeneous solution formed. The mixed solution was heated on a hot plate and the solution temperature was maintained at 80 °C. The beaker was wrapped with a polythene cap for 1 h to get a homogeneous heating throughout the solution and then the cap was removed. The solution was evaporated gradually until the yellowish wet gel was formed. The obtained sol-gel was then dried in an oven at 120 °C for 24 h. The obtained xerogel was decomposed on further heating at 500 °C for 4 h, thus producing blackcolored flakes with highly fine particles. The resulting powders were further annealed at 1400 °C for 12 h. The X-ray diffraction (XRD) patterns of BLSO:Tb3 þ phosphors were recorded on a Mac Science (M18XHF-SRA) X-ray powder diffractometer with CuKα ¼1.5406 Å. The morphological features were observed by field-emission scanning electron microscope (FE-SEM: CARL ZEISS, SUPRA) image. The room-temperature PL spectra were measured by using a Scinco Fluromate FS-2. The CL properties were measured by a Gatan (UK) MonoCL3 system attached with the SEM (Hitachi S-4300 SE).

3. Results and discussion Fig. 1 shows the XRD pattern of the BLSO:6Tb3 þ phosphor after annealed at 1400 °C in reduced atmosphere. The XRD pattern was in good agreement with the standard JCPDS No. 27–0037 for oxyapatite hexagonal structure and space group P63/m (176). The crystallite size has been estimated by the well-known Scherrer equation [14], Dhkl ¼kλ/β cos θ, where D is the average grain size, k (0.9) is a shape factor, λ is X-ray wavelength (1.5406 Å), β is the full width at half maximum (FWHM) and θ is the diffraction angle of an observed peak, respectively. The strongest diffraction peaks

Fig. 1. XRD pattern of the BLSO:6Tb3 þ phosphor. Inset shows the SEM image of the corresponding sample.

Fig. 2. (a) PLE spectrum BLSO:6Tb3 þ phosphor.

and

(b)

Gaussian

fitting

curves

of

the

were used to calculate the crystallite size of CGZO: Tb3 þ nanophosphors, which yields an average value of 98.8 nm. The calculated lattice constants are a¼ 9.7732 Å, c ¼7.3015 Å, and v¼ 603.97 Å3. The inset of Fig. 1 shows the SEM image of the BLSO:6Tb3 þ phosphor with irregular morphology and closelypacked particles. Fig. 2a shows the PL excitation (PLE) spectrum of the BLSO:6Tb3 þ phosphor by monitoring the emission wavelength of 543 nm. The PLE spectrum consists of a broad excitation band between 200 and 300 nm and narrow bands due to 4f intra-configurational transitions. It is clearly observed that the broad band is due to the inter-cnfigurational f–d transitions of Tb3 þ ions. The f– d transition appeared between 210 and 300 nm with a band maximum at 240 nm [15]. However, the f–d transitions are composed of low spin (LS) and high spin (HS) 4f8-4f75d1 transitions [15,16]. For more clarity, the PLE spectrum was deconvoled by Gaussian fitting and exhibited eleven excitation peaks in the total spectrum (Fig. 2b). The bands centered at 241, 258 and 271 nm are related to the spin allowed (LS) and spin forbidden (HS) transitions, respectively. The position of these f–d transitions depends upon the nature of the host lattice [15,16]. The other group of

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explained by the large values of the reduced matrix element at J¼5 and the Judd–Ofelt theory [20,21]. In the close observation, the intensity of the blue emission from the 5D3-7FJ transition decreased with increasing the Tb3 þ ion concentration at above 1 mol% (i.e., 1Tb) due to the cross-relaxation process [22,23] and almost disappeared at higher Tb3 þ ion concentration, indicating that the energy transfer between the activator ions in the visible region increases by increasing the cross-relaxation efficiency. Furthermore, the 5D4-7Fj emission intensities of Tb3 þ ions increased gradually upto 6 mol% of Tb3 þ ion concentration in BLSO host lattice while the emission intensities decreased when the Tb3 þ ion concentration increased over 6 mol% due to the concentration quenching effect, as shown in Fig. 3(b). It can also be established that the concentration quenching is somewhat higher than some of the previously reported results in other host lattices [18,24]. The reason is that the emission intensity is associated with the average distance between luminescent centers. When the concentration of dopant ions increases, the distance between them decreases. The interaction between the dopant ions may cause the concentration quenching when the distance is short enough. Therefore, the homogeneous distribution of active ions in host lattice is essential to acquire highly doped phosphors without any concentration quenching. The observed results suggested that the sol-gel process provides a better environment for homogeneous distribution of dopant ions in the BLSO host lattice. To explore the concentration quenching mechanism, the critical distance (Rc) between the Tb3 þ ions was calculated from the Blasse proposed equation [25,26]: 1

⎡ 3V ⎤ 3 R c≈2⎢ ⎥ ⎣ 4π X c N ⎦

Fig. 3. (a) PL emission spectra and (b) blue (5D3-7F4) and green (5D4-7F5) emission intensity variations of BLSO:Tb3 þ phosphors as a function of Tb3 þ ion concentration (in mol%).

excitation bands related to the 4f intra-configurational transitions of Tb3 þ ions located in the higher wavelength region. The bands located at 284, 303, 319, 339, 353, 371, 378, and 484 nm are ascribed to the intra-configurational 4f electronic transitions of (7F6-5F4), (7F6-5H5), (7F6-5D0), (7F6-5L7), (7F6-5G4), (7F6-5L10), (7F6-5G6), and 376 nm (7F6-5D4), respectively [17]. The f–f transitions in the higher energy side of the excitation spectrum is unusual, but the detailed explanation about this unusual appearance was shown in earlier reports [18,19]. Based on the obtained results, the 241 nm of wavelength was selected as the excitation wavelength for PL measurements. Fig. 3a shows the PL emission spectra of BLSO:Tb3 þ phosphors as a function of Tb3 þ ion concentration under 241 nm excitation. It is well known that the emission peak positions of Tb3 þ ions are hardly influenced due to the protection of 4f electrons by its outer electron cloud. The PL emission spectra of BLSO:Tb3 þ phosphors exhibited very weak intensities at 414 and 438 nm due to the 5 D3-7F5 and 5D3-7F4 electronic transitions, respectively. However, the spectra of BLSO:Tb3 þ phosphors revealed the emissions at 489, 543, 591 and 622 nm corresponding to the electronic transitions of 5D4-7F6, 5D4-7F5, 5D4-7F4, and 5D4-7F3, respectively [17]. Among the various emission transitions of BLSO: Tb3 þ phosphors, green color producing magnetic dipole transition (5D4-7F5) at 543 nm exhibited the highest intensity, which can be

where V is the volume of the unit cell, N is the number of cationic sites in the host lattice unit cell, and Xc is the optimized concentration of activator ions. For the BLSO:6Tb3 þ , V ¼603.97 Å3, N ¼10 and the optimum concentration of Tb3 þ ions in the BLSO host lattice is Xc ¼0.06. Therefore, the Rc was found to be about 12.44 Å. To further examine the suitability of phosphors for FED applications, the CL measurements were carried out for the BLSO:6Tb3 þ sample as a function of accelerating voltage and filament current. The CL spectra revealed almost similar behavior as the PL spectra, but they exhibited a tremendous difference in the emission intensity in the blue wavelength region, as shown in Fig. 4a and b. The reason is that the fast and energetic electrons are used in CL process while the photons with the energy of only 4–6 eV are used in PL process. The detailed explanation were shown in our earlier report [18]. Briefly, fast energetic electrons are tuned from few electron volts to thousands of electron volts. So, the excitation energy on the dopant ion is much larger in CL than that in PL. Also, the fast energetic electrons always prefer to excite the host lattice. After being penetrated into the host lattice of luminescent materials, the fast primary electron will cause repeated ionization to create numerous secondary electrons. These secondary electrons excite the host lattice and create many electron–hole (exciton) pairs [3]. Therefore, the high energy electron beam easily excites the host absorption band or charge transfer (CT) bands of Tb3 þ (O2  Tb3 þ (E152 nm)) and (O2  -Gd3 þ (E 155 nm)), which may be overlapped with each other. Due to the strong interaction with the crystal lattice, the excited electrons tend to relax from the CT states via a multiple-phonon emission process to the lowest levels of 4f–5d states. Hence, at higher accelerating voltage, the transition probability from the 4f–5d configuration to 5D3, and 5D4 metastable states is expected to be large as compared to the PL process. It is also clear that the BLSO:6Tb3 þ phosphor exhibited strong emission bands in the blue wavelength region at the

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Fig. 4. CL spectra of the BLSO:6Tb3 þ phosphor at the accelerating voltages of (a) 5 kV and (b) 10 kV and (c and d) CL emission intensity variations of the BLSO:6Tb3 þ phosphor as a function of accelerating voltage and filament current, respectively.

accelerating voltage of 5 kV (Fig. 4(a)). However, the blue emission decreased with increasing the accelerating voltage (Fig. 4b and c) due to the deeper penetration depth by the recombination of the increased excitons [27]. Furthermore, the intensity of green emission was increased with increasing the accelerating voltage (Fig. 4c) or filament current (Fig. 4d). Because of the deeper penetration depth, more activator ions were excited at almost all parts of the particles including boundaries, surfaces and inside of particles [3]. Therefore, the cross-relaxation process may occur between the Tb3 þ ions in the BLSO host lattice at higher accelerating voltages, causing the reduced intensity of blue emission. Fig. 4c and d shows the emission intensity variations as a function of accelerating voltage and filament current. The CL results suggest that the BLSO:Tb3 þ phosphors provide accelerating voltage induced tunable emissions. The Commission International de I’Eclairage (CIE) chromaticity coordinates from PL and CL data of the BLSO:6Tb3 þ phosphor were calculated, as shown in Fig. 5. The PL spectrum exhibited the CIE chromaticity coordinates of (0.303, 0.571). From the CL spectra, the CIE chromaticity coordinates at fixed filament current of 55 μA were (0.279, 0.212), and (0.253, 0.457) at 5 and 10 kV of accelerating voltage, respectively. The calculated CIE chromaticity coordinates from the PL spectrum of the BLSO:6Tb3 þ phosphor showed almost similar green color purity with the European Broadcasting Union (EBU) provided green (0.29, 0.60) coordinates [28]. However, the CIE chromaticity coordinates varied from blue to green region, which depends upon the energy of electron excitation.

Fig. 5. Calculated CIE chromaticity coordinates from PL and CL spectra of the BLSO:6Tb3 þ phosphor and EBU green coordinates.

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4. Conclusion BLSO:Tb3 þ phosphors were successfully synthesized by a facile citrate sol-gel method. The XRD patterns confirmed their oxyapatite structure, and the FE-SEM measurements showed that the particles were closely packed with irregular shapes. The PLE spectra of BLSO:Tb3 þ phosphors displayed the strong f–d transitions between the wavelengths 200 and 300 nm. The PL emission spectra revealed that the intense green emission band at 543 nm due to the 5D4-7F5 transition, but the CL spectra revealed the intense blue and green emissions at lower accelerating voltages. The optimum concentration of Tb3 þ ions in the BLSO host lattice was found to be 1 and 6 mol% for blue and green emissions, respectively. The CL spectra exhibited accelerating voltage induced tunable emissions by increasing the green emission intensity due to the improved cross-relaxation process at higher accelerating voltages. Also, the green emission intensity increased without any saturation by increasing the filament current from 33 to 55 μA. Because of their strong PL and CL intensities, good CIE chromaticity coordinates with tunable emissions and low-cost raw materials, the BLSO:Tb3 þ phosphors are expected to have potential applications in the development of novel LED and FED devices.

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea Government (MSIP) (No. 2015R1A5A1037656).

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