Effect of particle morphology and coating thickness on ... - Springer Link

J Mater Sci: Mater Electron (2015) 26:6744–6749 DOI 10.1007/s10854-015-3279-6

Effect of particle morphology and coating thickness on fluorescent behavior of Ce doped yttrium aluminium garnet phosphor screens Z. Asghar1 • G. H. Zahid1 • E. Ahmad1 • Rafi ud Din1 • Muhammad Noaman ul Haq2 T. Subhani3 • Z. Hussain1 • S. Badshah4

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Received: 10 March 2015 / Accepted: 27 May 2015 / Published online: 13 June 2015 Springer Science+Business Media New York 2015

Abstract Cerium activated yttrium aluminium garnet (Y3Al5O12; YAG) powder was synthesized by co-precipitation method using aluminium nitrate, yttrium nitrate and cerium nitrate as starting materials and ammonium carbonate as precipitant. The concentration of cerium (Ce) was varied from 0.02 to 0.1 mol. The precursor of Ce doped YAG was calcined at 1250 C for 1–12 h for achieving different morphology of particles. X-ray diffraction analysis was carried out to confirm the formation of YAG phase. Electrophoretic deposition was used for uniform coating of synthesized YAG:Ce powder on glass substrate. The deposition time was varied from 1 to 9 min for achieving different thickness of coating. Photoluminescence property of coating was investigated as a function of dopant concentration, particle morphology and coating thickness. The excitation wavelength used in this investigation was 430 nm. YAG:Ce showed peak emission at *530 nm. The maximum emission intensity was achieved at Ce content of 0.04 mol in YAG with spherical morphology of particles having 300 nm particle size and 11 lm coating thickness.

& Z. Asghar [email protected] 1

Materials Division, Directorate of Technology, PINSTECH, P. O. Nilore, Islamabad, Pakistan

2

Optics Laboratories, P.O. Box 1021, Islamabad, Pakistan

3

Materials Science and Engineering, IST, Islamabad, Pakistan

4

Department of Mechanical Engineering, International Islamic University, Islamabad, Pakistan

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1 Introduction Polycrystalline yttrium aluminium garnet (Y3Al5O12; YAG) activated with trivalent cerium (Ce), has attracted great interest in numerous applications such as solid state lighting, imaging and display devices [1–8] due to broad emission spectrum at green–yellow region, broad excitation band at blue region, excellent thermal quenching behavior, high quantum efficiency, excellent chemical stability and fast spontaneous emission rate [2]. During the recent years, a considerable attention has been paid in developing YAG:Ce fine particles for improving the brightness and resolution of the displays [9]. YAG:Ce powder with \2 lm size and spherical morphology are replaced to improve the luminescent performance [10]. YAG:Ce powders of desired size and morphology can be synthesized by soft chemistry techniques such as combustion synthesis, sol gel method, spray pyrolysis, solvothermal, polyacrylamide gel, heterogeneous and co-precipitation method [11–14]. Comparing with other techniques, co-precipitation is a relatively simple method, requiring low sintering temperature and mixing at molecular level [15–18]. Different synthesis techniques for YAG:Ce powder have been investigated and it has been found that YAG:Ce powder synthesized by co-precipitation method exhibits the highest emission intensity [9]. Ce doped YAG powder is generally employed in the form of coating or films. There are a variety of techniques available to produce phosphor coatings such as pulse laser deposition, liquid phase epitaxy, precursor plasma spraying, RF magnetron sputtering, metal–organic chemical vapor deposition, electrophoretic deposition, slurry method, settling method and sol–gel processing [19–21]. Among these, electrophoretic deposition (EPD) is relatively rapid, simple, cost effective, reproducible and has

J Mater Sci: Mater Electron (2015) 26:6744–6749

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Fig. 1 XRD patterns of undoped and Ce doped (x = 0.02–0.1) Y1-xAl5O12:Cex powders

Fig. 3 a Emission spectra of Ce doped (X = 0.04) YAG phosphor coatings with calcinations time, b emission intensity with calcination time

Fig. 2 a Emission spectra of undoped and Ce doped (X = 0.02–0.1) YAG phosphor coatings, b emission intensity with Ce doping concentration

the potential to use in a continuous process [22–25]. A comparison of three different coating techniques in literature reveals that the coatings produced by EPD exhibit better uniformity and higher packing density compared to those produced by slurry and settling methods. Furthermore, it was found that the thickness of coating affects the emission spectrum of Y3Al5O12:Ce0.05 phosphor [21]. The highest emission efficiency of phosphor coatings depends on the purity of YAG phase, dopant concentration, particles size, packing density and thickness of coatings [8, 11, 16, 17, 20–25]. In the present work, co-precipitation method is used to synthesize YAG:Ce phosphor powders and EPD is employed for the deposition of synthesized powders on glass substrate. The objective of this work is to optimize the luminescence behavior of phosphor coating as a function of dopant concentration, particle morphology and coating thickness.

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Fig. 4 Microstructures of YAG:Ce (X = 0.04) phosphor particles calcined at 1250 C for a 1 h, b 4 h, c 8 h, d 12 h

2 Experimental 2.1 Co-precipitation Yttrium nitrate (Y(NO3)36H2O), aluminium nitrate (Al(NO3)3 9H2O), cerium nitrate (Ce(NO3)36H2O) from Merck and ammonium carbonate ((NH4)2CO3) from Sigma-Aldrich were used as starting materials. The nitrates were completely dissolved in distilled water separately. The dissolved nitrate solutions were mixed dropwise according to the stoichiometric proportions (Y1-xAl5O12:Cex with X = 0.02, 0.04, 0.06, 0.08 and 0.1) and heated up to 70 C. On the other side, 1.8 M (NH4)2CO3 solution was added dropwise into multi-cation solutions. As soon as, the pH reached at 8, precipitation was stopped. The precipitated slurry was agitated for 18 h to achieve a sufficient reaction, and then the suspensions were filtered and washed three times with distilled water and one time with ethanol to remove residual ammonia and nitric ions. After filtration, the precursors were oven dried at 120 C for 12 h to remove superfluous water. The dried precursors were ground with mortar and pestle to fine particles. Finally, the precursors

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were calcined at 1250 C for 1, 4, 8 and 12 h in a muffle furnace to obtain YAG:Ce phosphors. X-ray diffraction using Cu Ka radiation was used for phase identification in the calcined samples.

2.2 Electrophoretic deposition EPD was carried out in a bath consisting of 2 g/l YAG: Ce phosphor powder suspended in 300 ml isopropanol, 0.039 g Mg(NO3)26H2O, 3 ml water and 6 ml glycerin. The deposition bath was agitated with magnetic stirrer for 30 min before placing the EPD cell, which consists of two parallel Teflon plates held at a fixed distance of 3 cm by two Teflon cylinders. The electrodes were embedded in the center of Teflon plates. The anode was a 3 9 3 cm2 steel plate while cathode was a 3 9 3 cm2 glass substrate coated with 13.6 nm thick gold by sputter deposition. Both electrodes were connected to a high voltage power supply and a specific voltage of 800 V was applied for 1, 3, 5, 7 and 9 min. After the deposition, the phosphor coated glass substrate was


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removed from the cell and dried in air. The post annealing treatment was carried out for 1 h at 425 C in air for improving the adhesive strength of the coatings. The surface morphology of coatings was investigated by field emission scanning electron microscope (FESEM). The thickness and surface roughness of coatings were examined by optical profilometer (NANOVEA PS50). The photoluminescence characteristics of coatings were investigated by fluorescence spectrophotometer (Hitachi F-4000) equipped with a 150 W xenon lamp. The excitation wavelength was 430 nm and the scan speed was 600 nm/min.

3 Results and discussion

Fig. 5 a Emission spectra of Ce doped (X = 0.04) YAG phosphor coatings for different coating time, b emission intensity and coating thickness as a function of coating time

Fig. 6 Photograph of YAG:Ce (X = 0.04) phosphor deposit on gold coated glass substrate

Figure 1 shows XRD patterns of undoped and Ce-doped (with different contents) YAG powders. The diffraction pattern of undoped YAG exhibit that all the peaks are corresponding to the YAG phase (JCPDF: 33-0040) and no other crystalline phase is detected although three stable phases in the Y2O3– Al2O3 binary systems have been reported: YAG (Y3Al5O12: cubic) YAM (Y4Al2O9: monoclinic) and YAP (YAlO3: perovskite). YAM and YAP phases deteriorate the luminescent properties [16]. The undoped and Ce doped YAG powders show similar diffraction patterns except line broadening in the latter, which increased with the increase in doping concentration. This is due to difference in the size of ionic crystal radii of dopant (Ce3? = 0.101 nm) and host (Y3? = 0.090 nm), which produces microstrains in the lattice. Figure 2a shows photoluminescence (PL) emission spectra of undoped and Ce-doped (with different contents) YAG coatings. There is no emission spectra observed in undoped YAG coatings while emission peak at *530 nm wavelength is observed in Ce doped YAG coatings, which is due to the 5d to 4f transitions of Ce3? [4, 6]. The same wavelength (*530 nm) for emission peak of Ce doped YAG nanophosphors have been reported but 2.75 % lower than bulk phosphors. This is due to the decrease in equilibrium distance of Ce3? with its ligands [8]. Figure 2b shows emission intensity with dopant concentration. The emission intensity is increased with increase in dopant concentration, which reaches maximum at X = 0.04 mol. Beyond the value of 0.04 mol, the emission intensity was found to decrease. This is due to the effect of concentration quenching [13]. When the doping of Ce concentration is increases, the distance between Ce ions is decreases. As a result, the immigration of excitation energy via resonant energy transfer occurs between Ce ions, resulting in the decrease of PL emission intensity [26]. Hence, the doping concentration of Ce (x = 0.04) corresponding to the highest luminescence was selected for further investigations in this study. Figure 3a shows the effect of calcination time of YAG:Ce phosphors on the emission intensity.

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Fig. 7 a SEM photograph and b roughness profile of Y2.96Ce0.04Al5O12 phosphor deposit by EPD

The emission intensity increased with calcination up to 4 h and then decreased as shown in Fig. 3b This may be associated with the morphology of YAG:Ce phosphors particles as shown in Fig. 4. The morphology of particles produced after calcination at 1250 oC for 1 h is spherical with *200 nm diameter (Fig. 4a). The size of the particles is increased to *300 nm with increase in calcination time from 1 to 4 h (Fig. 4b). Further increase in time to 8 h changed the morphology of particles from spherical to nonspherical by neck formation at the point of contacts (Fig. 4c). Still further increase in time to 12 h increased the length of attached particles at the cost of their diameter, as clearly visible in Fig. 4d. The lower emission intensity in YAG:Ce phosphor calcined for 1 h may be due to large surface defects as reported elsewhere [8]. The surface

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defects such as incomplete garnet lattice surrounding and nonbridging oxygen decreased with increase in size, which mitigates emission quenching effect. Therefore, the emission intensity is increased with calcination time up to 4h as shown in Fig. 3b. The spherical morphology of YAG:Ce particles may responsible for reducing internal scattering, which is lost with further increase in calcination time as shown in Fig. 4c, d. Figure 5a shows the effect of coating time on the emission intensity. The emission intensity increased with increase in coating time and reached to maximum at 300 s. The further increase in coating time resulted in a decrease in the emission intensity. Figure 5b shows the correlation of coating time with emission intensity and coating thickness. The maximum emission intensity of Ce doped YAG


phosphor screen is achieved at a coating thickness of 11 lm with packing density of 50 % corresponding to a coating time of 300 s. After attaining maximum emission intensity, further increase in thickness resulted in a decrease in emission intensity, which may be due to process of self absorption and scattering [27]. It has been reported elsewhere that emission intensity of Ce:YAG coatings by sedimentation under X-ray excitation (100 kVp) increases with thickness up to 23 lm and followed by the decrease. The optimum thickness available in literature is higher than found in the present study, which may be due to differences in concentration of doping and morphology of phosphor particles. Figure 6 shows photograph of Y2.96Al5O12:Ce0.04 phosphor coating on glass substrate, which exhibits uniform coverage of glass substrate except the edges. This is due to high electric field at the substrate edge. Figure 7a shows SEM photograph of Y2.96Al5O12:Ce0.04 phosphor coating on glass substrate. The uniformity of phosphor coating across 1 cm length is shown in Fig. 7b. The mean roughness (Ra) is 2.4 lm, which is due to the presence of aggregates in the slurry.

4 Conclusions Cerium (Ce) doped yttrium aluminium garnet (Y3Al5O12; YAG) powder was synthesized by co-precipitation method and coated on glass substrate by electrophoretic deposition. Following conclusions are drawn: • • •

The maximum emission intensity is achieved for x = 0.04 mol (Y3-xAl5O12 Cex). The 11 lm thick and 50 % densly packed Ce doped YAG coating showed maximum emission intensity. The 4 h calcination time is most favorable for achieving spherical morphology with 300 nm particle size for maximum emission intensity.

Acknowledgments The authors are grateful to Dr. Maqsood Ahmad (LINAC) for the financial support for this R & D work. The authors acknowledge with thanks the help provided by Mr. Liaqat (O-Labs) for photoluminescence spectrometry and Dr. Ahmad Nawaz Khan (SCME, NUST) for profilometric measurement. Last but not the least, thanks are due to the staff of Powder Processing Group for their efforts and cooperation in carrying out this work. Conflict of interest of interest.

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The authors declare that they have no conflict

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