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Synthesis of Y 2 O 3 :(Li,Eu) films using phosphor powders coated with SiO 2 nano particles In-Gyu Kim a , Sangmoon Park a , Seong-Gu Kang b , Jung-Chul Park a,n a Center for Green Fusion Technology & Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 617-736, Korea b Department of Chemical Engineering, Hoseo University, Chungnam 336-795, Korea article info Article history: Received 20 October 2009 Received in revised form 16 March 2010 Accepted 22 March 2010 Available online 27 March 2010 Keywords: Coatings Glass Nano-particles Y 2 O 3 :(Li,Eu) films abstract Y 1.9x Li 0.1 Eu x O 3 (x ¼0.02, 0.05, 0.08, and 0.12) films were fabricated by spin-coating method. A colloidal silica suspension with Y 1.9 x Li 0.1 Eu x O 3 phosphor powder was exploited to obtain the highly stable and effective luminescent films onto the glass substrate. After heating as-prepared Y 1.9x Li 0.1 Eu x O 3 films at 700 1C for 1 h, the phosphor films exhibit a high luminescent brightness as well as a strong adhesiveness on the glass substrate. The emission spectra of spin-coated and pulse-laser deposited Y 1.82 Li 0.1 Eu 0.08 O 3 films were compared. The cathodoluminescence of the phosphor films was carried out at the anode voltage 1 kV. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Phosphor films play an important role in the display devices such as electroluminescence panel, field emission display, and plasma display panel. The high resolution displays require a phosphor film exhibiting high contrast and ruggedness as well as high degree of uniformity and adhesion. Properly grown phosphor films have good thermal contact with the substrate and can withstand high temperatures and high electric charges. Dielectric yttrium oxide Y 2 O 3 film has attracted much attention as a host material for rare earth ions, particularly Eu 3+ ion [1,2].Y 2 O 3 films have been prepared using electron-beam evaporation [3], radio frequency sputtering [4], pulsed laser deposition (PLD) [1,2], or sol–gel techniques [5]. All these methods have both specific advantages and limitations depending on the type of potential application. It is generally accepted that thin film phosphor have some advantages over bulk-type phosphor, such as better thermal stability, better cohesion, and good uniformity on substrate surface. However, there still remains some fundamental problems in the application of thin film phosphors : (i) low brightness of thin film compared with that of bulk-type powder phosphor and (ii) the difficulty of thin film fabrication with large area. Recently, there has been a significant progress in luminescent efficiency of Li-doped Gd 2 x Y x O 3 :Eu 3+ powder phosphor and thin film [6–8]. It is well known that even in very small amounts, the Li + co- activators frequently play an important role in the enhancement of the luminescent efficiency of phosphor [9,10]. Yeh and Su [11] recently reported that the mixing of LiF to Gd 2 O 3 :Eu can greatly increase both of its photoluminescence (PL) and thermolumines- cence. They suggested that LiF simply acts as a lubricant for the complete incorporation of Eu 2 O 3 into Gd 2 O 3 during the solid- solution in the synthetic process. However, we suggested that the co-doping of Li + ions into the phosphor lattice should be considered in addition to the previous flux effect of LiF on the crystallinity of Gd 2 O 3 . In this work, we present a technique to prepare highly efficient films of Y 1.9 x Li 0.1 Eu x O 3 red phosphors, which are highly adhesive to the glass substrate. In order to increase the cohesion strength and the coating homogeneity, sample powders were suspended in the SiO 2 colloidal solution before spin-coating. It is interesting that the films obtained after annealing at 700 1C in the vicinity of the softening temperature of the glass, show high stability against any stress such as tapping and scratching without any loss of photo- and cathodoluminescent brightness. These results are quite desirable for a phosphor film because of many advantages compared with other techniques, for instance, the low-cost in the equipments setup, the simple preparation of the larger sized films, and the high cohesion strength of films to the glass substrate. Such advantages of the film easily fabricated from SiO 2 coated phosphor powders provide a promising candidate for application to display devices. 2. Experimental details Y 1.9 x Li 0.1 Eu x O 3 (x ¼0.02, 0.05, 0.08, 0.12) powders were prepared by sol–gel method. The appropriate mixtures of ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.03.023 n Corresponding author. Tel.: + 82 51 999 5469; fax: + 82 51 999 5806. E-mail address: [email protected] (J.-C. Park). Journal of Luminescence 130 (2010) 1521–1524

Synthesis of Y2O3:(Li,Eu) films using phosphor powders coated with SiO2 nano particles

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Journal of Luminescence 130 (2010) 1521–1524

Contents lists available at ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Synthesis of Y2O3:(Li,Eu) films using phosphor powders coated with SiO2

nano particles

In-Gyu Kim a, Sangmoon Park a, Seong-Gu Kang b, Jung-Chul Park a,n

a Center for Green Fusion Technology & Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 617-736, Koreab Department of Chemical Engineering, Hoseo University, Chungnam 336-795, Korea

a r t i c l e i n f o

Article history:

Received 20 October 2009

Received in revised form

16 March 2010

Accepted 22 March 2010Available online 27 March 2010

Keywords:

Coatings

Glass

Nano-particles

Y2O3:(Li,Eu) films

13/$ - see front matter & 2010 Elsevier B.V. A

016/j.jlumin.2010.03.023

esponding author. Tel.: +82 51 999 5469; fax

ail address: [email protected] (J.-C. Park).

a b s t r a c t

Y1.9�xLi0.1EuxO3 (x¼0.02, 0.05, 0.08, and 0.12) films were fabricated by spin-coating method. A colloidal

silica suspension with Y1.9�xLi0.1EuxO3 phosphor powder was exploited to obtain the highly stable and

effective luminescent films onto the glass substrate. After heating as-prepared Y1.9�xLi0.1EuxO3 films at

700 1C for 1 h, the phosphor films exhibit a high luminescent brightness as well as a strong

adhesiveness on the glass substrate. The emission spectra of spin-coated and pulse-laser deposited

Y1.82Li0.1Eu0.08O3 films were compared. The cathodoluminescence of the phosphor films was carried out

at the anode voltage 1 kV.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Phosphor films play an important role in the display devicessuch as electroluminescence panel, field emission display, andplasma display panel. The high resolution displays require aphosphor film exhibiting high contrast and ruggedness as well ashigh degree of uniformity and adhesion. Properly grown phosphorfilms have good thermal contact with the substrate and canwithstand high temperatures and high electric charges. Dielectricyttrium oxide Y2O3 film has attracted much attention as a hostmaterial for rare earth ions, particularly Eu3 + ion [1,2]. Y2O3 filmshave been prepared using electron-beam evaporation [3], radiofrequency sputtering [4], pulsed laser deposition (PLD) [1,2], orsol–gel techniques [5]. All these methods have both specificadvantages and limitations depending on the type of potentialapplication.

It is generally accepted that thin film phosphor have someadvantages over bulk-type phosphor, such as better thermalstability, better cohesion, and good uniformity on substratesurface. However, there still remains some fundamental problemsin the application of thin film phosphors : (i) low brightness ofthin film compared with that of bulk-type powder phosphor and(ii) the difficulty of thin film fabrication with large area. Recently,there has been a significant progress in luminescent efficiency ofLi-doped Gd2�xYxO3:Eu3 + powder phosphor and thin film [6–8]. Itis well known that even in very small amounts, the Li+ co-activators frequently play an important role in the enhancement

ll rights reserved.

: +82 51 999 5806.

of the luminescent efficiency of phosphor [9,10]. Yeh and Su [11]recently reported that the mixing of LiF to Gd2O3:Eu can greatlyincrease both of its photoluminescence (PL) and thermolumines-cence. They suggested that LiF simply acts as a lubricant for thecomplete incorporation of Eu2O3 into Gd2O3 during the solid-solution in the synthetic process. However, we suggested that theco-doping of Li+ ions into the phosphor lattice should beconsidered in addition to the previous flux effect of LiF on thecrystallinity of Gd2O3.

In this work, we present a technique to prepare highly efficientfilms of Y1.9�xLi0.1EuxO3 red phosphors, which are highly adhesiveto the glass substrate. In order to increase the cohesion strengthand the coating homogeneity, sample powders were suspended inthe SiO2 colloidal solution before spin-coating. It is interestingthat the films obtained after annealing at 700 1C in the vicinity ofthe softening temperature of the glass, show high stability againstany stress such as tapping and scratching without any loss ofphoto- and cathodoluminescent brightness. These results arequite desirable for a phosphor film because of many advantagescompared with other techniques, for instance, the low-cost in theequipments setup, the simple preparation of the larger sizedfilms, and the high cohesion strength of films to the glasssubstrate. Such advantages of the film easily fabricated from SiO2

coated phosphor powders provide a promising candidate forapplication to display devices.

2. Experimental details

Y1.9�xLi0.1EuxO3 (x¼0.02, 0.05, 0.08, 0.12) powders wereprepared by sol–gel method. The appropriate mixtures of

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(f)

(e)4

6

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)(4

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(631

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)(620

)(6

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(521

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I.-G. Kim et al. / Journal of Luminescence 130 (2010) 1521–15241522

Y(NO3)36(H2O), Eu(NO3)35(H2O), and Li(NO3) (100% excess) weredissolved in citric acid solution (the molar ratio of citric acid/metal¼3). NH4OH solution was then added into the mixture untilthe pH of all solutions was adjusted to be 4–5. The transparent gelgradually became the dark-gray powders during the heating.Well-ground dark-gray powder was fired at 650 1C for 10 h, andthen fired again at various temperatures (750 1C, 850 1C, 950 1C,1050 1C, and 1150 1C) for 5 h. In order to modify the surface ofY1.9�xLi0.1EuxO3 particles, the colloidal silica solution (E30 wt%)was used. The powder sample (E0.8 g) was added into 25 ml ofcolloidal silica suspension and stirred for 2 h. The phosphorpowder coated with the SiO2 nano-particles was obtained afterwashing and drying. For Y1.9�xLi0.1EuxO3 films, the glass sub-strates (pyrex, 1�1 cm2) were etched by the 10% HF solution for3 min and dried. The coating pastes were prepared by mixing0.15 g of SiO2 nano-particle coated phosphor powder and 0.15 mlof glycerin. These pastes were spin-coated on the glass substrateat the rate of 500 rpm for 10 s and then 1500 rpm for 20 s. Thecoated film was dried at 150 1C for 1 h in a drying oven. Finally,the film was treated at 700 1C for 1 h in an electrical furnace.Li-doped Y2O3:Eu3 + luminescent film has been grown on sapphiresubstrates using PLD [8].

The formation of a single phase of films was confirmed byX-ray diffraction (XRD) with Cu-Ka radiation using ShimadzuXRD-6000. The morphologies of phosphor particles after SiO2

nano-particle coating were observed by the Field EmissionScanning Electron Microscopy (FE-SEM) using Hitachi S-4200(15 kV). Specimens for electron microscope were coated with Pt-Rh for 180 s under vacuum. The PL intensity of phosphor filmswas measured at room temperature using a Shimadzu RF-5301PCfluorometer with a xenon flash lamp. The cathodoluminescence(CL) spectra were obtained under cathode ray excitation at0.5–1 kV and 10 mA/cm2. The spectral variation of CL wasmeasured by a fiber-optic coupled spectrometer with a wave-length resolution of 1 nm.

10

(d)

(c)

(b)

(a)

0

2

2 Theta (degree)20 30 40 50 60 70

Fig. 1. Powder X-ray diffraction patterns of the Y1.82Li0.1Eu0.08O3 powders

synthesized at various temperatures: (a) 650 1C for 10 h, (b) 750 1C for 5 h,

(c) 850 1C for 5 h, (d) 950 1C for 5 h, (e) 1050 1C for 5 h, and (f) 1150 1C for 5 h.

3. Results and discussion

We have previously reported Eu-doped Y2O3 red phosphorswith the incorporation of Li enabling the significant reduction ofthe crystallization temperature and the control of the particle sizeand shape [6–8]. The powdered samples of Y1.82Li0.1Eu0.08O3 weresynthesized at low temperature (650 1C) by using sol–geltechnique, which gives the cubic phase of Y2O3. Fig. 1 shows theX-ray diffraction patterns of Y1.82Li0.1Eu0.08O3 powder at varioustemperatures (650–1150 1C). X-ray diffraction patterns revealbetter crystallinity with increasing the heating temperature. Allthe Bragg’s angles and intensities are quite similar to those of

Fig. 2. SEM images of Y1.82Li0.1Eu0.08O3 powders synthesized at 105

Y2O3 and indexed on the basis of the Ia3 space group [12–14].When we increase the synthetic temperature from 650 to 1050 1C,the particle size of Y1.82Li0.1Eu0.08O3 was enlarged from 0.1 to1.0 mm and their particle shapes were quite similar such asspherical form. It appears in irregular flake shapes after wecontinuously increase the synthetic temperature up to 1150 1C. Itis generally accepted that in the fabrication of films the dualcharacters for phosphor powder, which are the morphology andluminescent efficiency, should be considered. The spherical-shapeparticles of Y1.9�xLi0.1EuxO3 prepared at 1050 1C were adopted asphosphors for coating pastes in the preparation of films. Fig. 2ashows SEM images of Y1.82Li0.1Eu0.08O3 particles synthesized at1050 1C. The particles are regular and spherical with average sizeabout 1 mm. It can be seen in Fig. 2b that SiO2 nano-particlesaround 30 nm are well covered on the surface of Y1.82Li0.1Eu0.08O3

particles via colloidal silica suspension. Si element was identifiedby using energy dispersive X-ray spectroscopy (EDS).

The pastes for spin coating were prepared by mixingY1.82Li0.1Eu0.08O3 covered with SiO2 nano-particles and glycerin.

0 1C for 5 h: (a) before and (b) after SiO2 nano-particle coating.

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Fig. 3. SEM images of the Y1.82Li0.1Eu0.08O3 films: (a) surface of 1st coated film and (b) surface of 3rd coated film.

560

(c)

(b)

(a)

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nsity

(arb

. uni

ts)

Wavelength (nm)580 600 620 640

Fig. 4. PL emission spectra of the Y1.82Li0.1Eu0.08O3 films : (a) film synthesized

by PLD method, (b) 1st-coated film synthesized by spin-coating method and

(c) 3rd-coated film synthesized by spin-coating method.

I.-G. Kim et al. / Journal of Luminescence 130 (2010) 1521–1524 1523

The pastes finally were spinned on the glass substrate and dried at150 1C for 1 h in a drying oven. The films were heated at 700 1Cfor 1 h in an electrical furnace. SEM images of films werepresented in Fig. 3. The top view of the films shows theY1.82Li0.1Eu0.08O3 particles on the glass substrate. The particleswere quite homogeneously distributed on the glass substrateafter 1st and 3rd times of successive spin coating. The thickness offilms was calculated to be E0.2 mm, which is not shown in thispaper. After the final heat treatment at 700 1C in the vicinity of theglass softening temperature, the enhanced cohesion strengthbetween glass substrate and phosphor powder particles wasexhibited due to the fused SiO2 nano-particle coated onto thesurface of phosphor powders. As a consequence, the cohesionstrength of film significantly increases. When the scratchhardness was measured using the Wolff-Wilborn pencil tester,the films prepared in this study showed stronger than 9 H pencilhardness. It was almost impossible to make a scratch on thecoated surface with a surgical knife. Such an effect persists afterfilm coating and subsequent heat treatment at 700 1C for 1 hwhen the powder synthesized below 1050 1C was used.Meanwhile, the luminescent emission intensity of films wasabruptly decreased when the bulk powder prepared at 1150 1Cwas used. This unfavorable decrease in brightness of phosphorevidently stems from the irregular shape of agglomeratedparticles after high temperature treatment. It is evidentlymentioned that the phosphor powder synthesized at 1050 1C iswell-defined precursor for the fabrication of films, which resultsin the strong adhesive and better luminescent brightness ofphosphor films. Therefore, it is suggested that the morphologyand particle size of powder before film fabrication should becontrolled to obtain high emission efficiency of phosphor film. Inthis respect, the incorporation of Li is quite important because itcontrols the particle size and shape to E1 mm size of sphericalparticles. Without Li incorporation, the small particle size andsevere aggregation of irregular-sized particle, result in the poorSiO2 coating density and then poorly adhesive film. Remarkably,the PL intensity of thick film is stronger than that ofY1.82Li0.1Eu0.08O3 thin film fabricated by PLD technique. As ourfilm fabrication method uses phosphor particles as-made, so theluminescent property of phosphors could be preserved. Fig. 4shows the photoluminescence spectra of SiO2-coatedY1.82Li0.1Eu0.08O3 film fabricated by spin-coating and Y2O3:Eufilm by PLD technique. Well-known intense emission at 612 nm isassociated with the 5D0 – 7F2 transition of the Eu3 + ion [15].

As previously reported, Li incorporation into a phosphor latticegives rise to a remarkable increase of PL and CL efficiency for bulkpowder [7,14]. Fig. 5 shows the variation of CL intensity forY1.9�xLi0.1EuxO3 film as a function of Eu content (x) at the anodevoltage 1 kV. The maximum CL intensity is observed with the Eu

content of 8–12 mol%. Relative emission intensity is also muchstronger than that of the same composition coated by PLDtechnique. These results indicate that the strong increase ofluminescence intensity for Li-doped Y2O3:Eu red phosphorpowder does not weaken or disappear after SiO2 nano-particlecoating on the surface of phosphor particle and heating the spin-coated films up to 700 1C.

4. Conclusion

In conclusion, SiO2 nano-particles coated on the surface ofphosphor particle remarkably improve the cohesion strength offilm particularly on the glass substrate. This enhancement isclosely related to embedded SiO2 particle into the substrate nearthe glass softening temperature. The scratch hardness of phos-phor film fabricated by typical spin-coating in this work iscomparable with that by PLD technique. Strong increase in theemission intensity of bulk phosphor by Li-doping is also retainedafter spin-coating and successive heat treatment. This simple andlow-cost method to get Y1.9�xLi0.1EuxO3 phosphor films would bea promising one for applications to display devices.

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(c)

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Fig. 5. (A) CL spectra of the Y1.9�xLi0.1EuxO3 films as a function of Eu contents: (a) x¼0.02, (b) 0.05, (c) 0.08, and (d) 0.12. (B) Depending on anode voltage, comparison of

the relative CL intensity of the Y1.9�xLi0.1EuxO3 films as a function of Eu contents: (a) x¼0.02, (b) 0.05, (c) 0.08, and (d) 0.12 (This phosphor powder is synthesized at 1050 1C

for 5 h).

I.-G. Kim et al. / Journal of Luminescence 130 (2010) 1521–15241524

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