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Materials Chemistry and Physics 100 (2006) 329–332 Thermal annealing effect on Y 2 O 3 :Eu 3+ phosphor films prepared by yttrium 2-methoxyethoxide sol–gel precursor M.K. Chong , K. Pita, C.H. Kam Photonics Laboratory 1, S1-B3a-08, Photonics Research Centre, Microelectronics Division, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore Received 13 September 2005; received in revised form 13 December 2005; accepted 11 January 2006 Abstract In this work, we demonstrate that yttrium 2-methoxyethoxide is a convenient sol–gel precursor to synthesize the Y 2 O 3 :Eu 3+ phosphor films. The crystallization of Y 2 O 3 :Eu 3+ phosphor films prepared from the yttrium 2-methoxyethoxide occurs at about 550 C. We have also observed that our Y 2 O 3 :Eu 3+ phosphor films undergo crystal structure change above annealing temperature of 750 C which is not previously observed in the sol–gel fabrication method. The change of photoluminescent (PL) spectra is related to the evolution of Y 2 O 3 crystal structure. It is shown in this investigation that the post-annealing treatment will help to produce phosphor films of improved brightness. The reasons assigned are the effective elimination of OH impurities and the grain growth of phosphor films. © 2006 Elsevier B.V. All rights reserved. Keywords: Optical materials; Sol–gel growth; Photoluminescence spectroscopy 1. Introduction Lanthanides rare earth elements have significant industrial and commercial applications. In electronic display application alone, the phosphor materials contain lanthanides rare earth element as dopant have been widely used in cathode ray tubes displays (CRTs), electroluminescent displays (ELDs), field emission displays (FEDs), and plasma displays (PDs). The improved performance of these displays requires stable and high quality phosphors with sufficient luminescence. Oxide-based phosphors have recently been attracting much attention as an alternative to sulphide-based phosphors mainly attributed to its stability. Among the oxide-based phosphors, Y 2 O 3 -based phosphor films have certain advantageous such as low phonon energy and trivalent substitution site. The Y 2 O 3 -based phosphor films have been grown mainly using deposition methods such as sputtering [1], pulsed laser deposition [2], and electron beam evaporation [3]. Sol–gel process is distinguished from the afore- mentioned physical vapor deposition (PVD) techniques by its excellent material stoichiometry and relatively inexpensive raw materials [4]. Therefore, the sol–gel process has attracted much Corresponding author. Tel.: +65 6790 4036; fax: +65 6790 4161. E-mail address: [email protected] (M.K. Chong). research interests in the preparation of phosphor films. The hosts that have been developed by the sol–gel process include Zn 2 SiO 4 [5],Y 2 SiO 5 [6], and Y 3 Al 5 O 12 [7]. Surprisingly, the preparation of the Y 2 O 3 -based phosphor films with the sol–gel process is not much reported despite its excellent suitability as the host. The main reason is probably associated with the high reactivity of the yttrium alkoxide sol–gel precursor. Several approaches that have been examined to prepare the Y 2 O 3 -based sol–gel films include the preparation of yttrium hydroxide gel by using the ion exchange process [8] and the addition of diethylenetriamine (DETA) as chelating agent to yttrium acetate [9]. In this paper, we report our works in preparing the Y 2 O 3 :Eu 3+ phosphor films in a convenient way by using yttrium 2-methoxyethoxide as sol–gel precursor. The yttrium 2-methoxyethoxide precursor is more stable since the formation of chelat cycles has retarded the precursor reactivity. Thus, this approach requires only the precursor and solvent, without the need of complicated chemical modification processes or additional chelating agents. This is because the chelating agents might introduce more luminescent quenching species into the phosphor films. 2. Experimental The Y 2 O 3 :Eu 3+ phosphor films were prepared by the sol–gel process. The chemical mixing and spin coating processes were performed in a dry glove 0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.01.006

Thermal annealing effect on Y2O3:Eu3+ phosphor films prepared by yttrium 2-methoxyethoxide sol–gel precursor

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Page 1: Thermal annealing effect on Y2O3:Eu3+ phosphor films prepared by yttrium 2-methoxyethoxide sol–gel precursor

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Materials Chemistry and Physics 100 (2006) 329–332

Thermal annealing effect on Y2O3:Eu3+ phosphor films preparedby yttrium 2-methoxyethoxide sol–gel precursor

M.K. Chong ∗, K. Pita, C.H. KamPhotonics Laboratory 1, S1-B3a-08, Photonics Research Centre, Microelectronics Division, School of Electrical and Electronic Engineering,

Nanyang Technological University, Singapore 639798, Singapore

Received 13 September 2005; received in revised form 13 December 2005; accepted 11 January 2006

bstract

In this work, we demonstrate that yttrium 2-methoxyethoxide is a convenient sol–gel precursor to synthesize the Y2O3:Eu3+ phosphor films.he crystallization of Y2O3:Eu3+ phosphor films prepared from the yttrium 2-methoxyethoxide occurs at about 550 ◦C. We have also observed thatur Y O :Eu3+ phosphor films undergo crystal structure change above annealing temperature of 750 ◦C which is not previously observed in the

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ol–gel fabrication method. The change of photoluminescent (PL) spectra is related to the evolution of Y2O3 crystal structure. It is shown in thisnvestigation that the post-annealing treatment will help to produce phosphor films of improved brightness. The reasons assigned are the effectivelimination of OH impurities and the grain growth of phosphor films.

2006 Elsevier B.V. All rights reserved.

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eywords: Optical materials; Sol–gel growth; Photoluminescence spectroscopy

. Introduction

Lanthanides rare earth elements have significant industrialnd commercial applications. In electronic display applicationlone, the phosphor materials contain lanthanides rare earthlement as dopant have been widely used in cathode ray tubesisplays (CRTs), electroluminescent displays (ELDs), fieldmission displays (FEDs), and plasma displays (PDs). Themproved performance of these displays requires stable and highuality phosphors with sufficient luminescence. Oxide-basedhosphors have recently been attracting much attention as anlternative to sulphide-based phosphors mainly attributed tots stability. Among the oxide-based phosphors, Y2O3-basedhosphor films have certain advantageous such as low phononnergy and trivalent substitution site. The Y2O3-based phosphorlms have been grown mainly using deposition methods suchs sputtering [1], pulsed laser deposition [2], and electron beamvaporation [3]. Sol–gel process is distinguished from the afore-

entioned physical vapor deposition (PVD) techniques by its

xcellent material stoichiometry and relatively inexpensive rawaterials [4]. Therefore, the sol–gel process has attracted much

∗ Corresponding author. Tel.: +65 6790 4036; fax: +65 6790 4161.E-mail address: [email protected] (M.K. Chong).

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254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2006.01.006

esearch interests in the preparation of phosphor films. The hostshat have been developed by the sol–gel process include Zn2SiO45], Y2SiO5 [6], and Y3Al5O12 [7]. Surprisingly, the preparationf the Y2O3-based phosphor films with the sol–gel process is notuch reported despite its excellent suitability as the host. Theain reason is probably associated with the high reactivity of

he yttrium alkoxide sol–gel precursor. Several approaches thatave been examined to prepare the Y2O3-based sol–gel filmsnclude the preparation of yttrium hydroxide gel by using theon exchange process [8] and the addition of diethylenetriamineDETA) as chelating agent to yttrium acetate [9]. In this paper,e report our works in preparing the Y2O3:Eu3+ phosphorlms in a convenient way by using yttrium 2-methoxyethoxides sol–gel precursor. The yttrium 2-methoxyethoxide precursors more stable since the formation of chelat cycles has retardedhe precursor reactivity. Thus, this approach requires onlyhe precursor and solvent, without the need of complicatedhemical modification processes or additional chelating agents.his is because the chelating agents might introduce more

uminescent quenching species into the phosphor films.

. Experimental

The Y2O3:Eu3+ phosphor films were prepared by the sol–gel process. Thehemical mixing and spin coating processes were performed in a dry glove

Page 2: Thermal annealing effect on Y2O3:Eu3+ phosphor films prepared by yttrium 2-methoxyethoxide sol–gel precursor

3 istry and Physics 100 (2006) 329–332

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30 M.K. Chong et al. / Materials Chem

ox flushed with nitrogen at 15% relative humidity. Initially, suitable amountf anhydrous 2-methoxyethanol (Aldrich, 99.8%) and water were added intohe yttrium 2-methoxyethoxide (Alfa Aesar) precursor for hydrolysis and con-ensation reactions. Dopant chemical of europium(III) nitrate (Aldrich, 99.9%)as subsequently added into the sol mixture according to the chemical for-ula Y2O3:12 mol% Eu3+ [10]. The sol mixture was aged for about 15 h insealed condition at room temperature before the spin coating process. The

1 0 0) Silicon substrates were spun with a speed of 3500 rpm for 30 s afterhe sol deposition. The gel films were subsequently annealed in rapid thermalrocessor (RTP Jipelec Jetfirst) from 450 ◦C to 950 ◦C. The RTP annealingtmosphere was vacuum environment and the annealing time was 60 s [10]. Tonvestigate the post-annealing effect on the Y2O3:Eu3+ phosphor films, con-entional furnace (Thermolyne 47900) was used. The post-annealing treatmentas performed in ambient atmosphere at temperature 750 ◦C for 1–20 h. TheRD data were recorded using X-ray diffractometer (Siemens D5005, Cu K�

arget). Room temperature photoluminescent (PL) excitation and emission mea-urement were recorded with a Spectrofluorometer (SPEX Fluorolog-3). The PLeasurements were made at excitation wavelength of 254 nm. The OH impuri-

ies content in the annealed sample was identified with FTIR spectrophotometerPerkin-Elmer Spectrum 2000). The thickness of the phosphor films was about00 nm measured by a surface profiler (Sloan Dektak 3).

. Results and discussion

Fig. 1 shows the XRD pattern of Y2O3:Eu3+ phosphor filmss a function of RTP annealing temperature. It is seen that the2O3:Eu3+ phosphor films start to crystallize at about 550 ◦C.his is comparatively lower than Y2O3:Eu3+ phosphor films pre-ared from yttrium isopropoxide precursor which shows crys-allization at about 650 ◦C [10]. The Y2O3:Eu3+ phosphor filmsnnealed at 750 ◦C corresponds to body-centered cubic Y2O3. Inhe same figure, some changes in films crystallinity are observedhen the samples were annealed at annealing temperature above50 ◦C. Previously, it has been reported that the Y2O3:Eu3+

hosphor materials prepared from the sol–gel process either inhe physical form of thin film [8] or powder [11] did not undergouch crystal structure change in the annealing temperature rangef 400–1000 ◦C and 500–1250 ◦C, respectively. To compare our

esults with the results in Ref. [8] and [11], we therefore annealedur Y2O3:Eu3+ phosphor films at 750 ◦C, 950 ◦C, 1150 ◦C, and250 ◦C in oven for 2 h. When our Y2O3:Eu3+ phosphor filmsere annealed at different temperatures in oven, the evolution

ig. 1. XRD pattern of Y2O3:Eu3+ phosphor films annealed at different anneal-ng temperatures (450–950 ◦C) in rapid thermal processor.

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ig. 2. XRD pattern of Y2O3:Eu3+ phosphor films annealed in oven at temper-ture of 750 ◦C, 950 ◦C, 1150 ◦C, and 1250 ◦C.

f crystal structure was indeed observed. The change of crys-al structure is presented in Fig. 2. The additional diffractioneaks emerge at above annealing temperature of 750 ◦C could bendexed to monoclinic Y2O3 [12]. The evolution of Y2O3:Eu3+

hosphor films crystal structure shown in Fig. 2 is accompaniedy the change of PL emission spectra, as presented in Fig. 3. Theependence of PL emission spectra on the evolution of Y2O3rystal structure can be explained as follows. Yttrium atomsccupy two sites with different crystal symmetry in cubic Y2O3,amely C2 and S6 symmetry. 75% of the yttrium cation sites aren C2 symmetry whereas the other 25% are in S6 symmetry.n Y2O3:Eu3+ phosphor films, the Eu3+ ions substitute for Y3+

ons. The emission peak at 613 nm originates from the Eu3+

ons in C2 symmetry whereas the emission peak at 590 nm orig-nates from Eu3+ ions in S6 symmetry [13]. The ratio of S6 to

2 sites could be changed corresponds to the evolution of Y2O3rystal structure in this experiment. This eventually leads to thehange of PL emission spectra as presented in Fig. 3. The PL

mission peaks at 613 nm and 625–633 nm have been assignedo 5D0 → 7F2 transition of Eu3+ ions [14–16]. The PL emissioneak at 613 nm is known as hypersensitive transition in which theransition is sensitive to the local environment of the Eu3+ ions

ig. 3. PL emission spectra corresponds to the change of crystal structures fromnnealing temperature 750–1250 ◦C (λext = 254 nm).

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M.K. Chong et al. / Materials Chemistry and Physics 100 (2006) 329–332 331

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ig. 4. PL emission intensity ratio at 590–613 nm (R = I( D0 → F1)/I5D0 → 7F2)) corresponds to the change of crystal structures from annealingemperature 750–1250 ◦C (λext = 254 nm).

17,18]. Hence, the predominant 613 nm or 633 nm emissionn one material is related to the local environment of Eu3+ ions.he evolution of crystal structure as a function of annealing tem-erature could also change the local environment of Eu3+ ionsnd thus the predominant transition. The PL emission intensityatio (R = I(5D0 → 7F1)/I(5D0 → 7F2)) of Y2O3:Eu3+ phosphorlms as a function of annealing temperature is shown in Fig. 4.t is clear that the orange emission at 590 nm is increasinglyominating when the Y2O3:Eu3+ phosphor films were annealedrom 750 ◦C to 1250 ◦C. In view of the red, green, and blueRGB) are the three primary colors for phosphor materials, the

2O3:Eu3+ phosphor films with substantial mixture of orangemission above annealing temperature of 750 ◦C is not very use-ul for display applications. If the yttrium 2-methoxyethoxide issed as a sol–gel precursor, the annealing temperature should notar beyond 750 ◦C in order to obtain the Y2O3:Eu3+ phosphor

lms which give pure red emission.

Fig. 5 compares the PL emission spectra of Y2O3:Eu3+

hosphor films with and without post-annealing treatment at50 ◦C. The dominant emission peak at 613 nm is assigned to

ig. 5. PL emission spectra of Y2O3:Eu3+ phosphor films as a function of post-nnealing time (λext = 254 nm).

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ig. 6. FTIR spectra of Y2O3:Eu3+ phosphor films as a function of post-nnealing time.

he 5D0 → 7F2 transition of Eu3+ ions. In Fig. 5, it is observedhat the PL emission intensity of Y2O3:Eu3+ phosphor filmsncreases as a function of post-annealing time. With the post-nnealing treatment, the sol–gel films are further densified. Thiss accomplished by the elimination of organic and hydroxylroups that might still be entrapped in the phosphor films afterransient RTP annealing. It is because the sol–gel films formedhrough hydrolysis usually entrap some water, alcohol, and otherrganic groups [19]. The entrapped organic residuals that haveigh vibration frequencies can efficiently quench the radiativeransition of optically active ions through multi-phonon relax-tion. Hence, the entrapped organic residuals are detrimental tohe optical performance of phosphor films. This is also the keyonsideration why we prefer not to add chelating agents to sta-ilize the sol–gel precursor because this will tend to entrap moreydroxyl and organic groups in the sol–gel films.

Fig. 6 compares the FTIR spectra of Y2O3:Eu3+ phosphorlms with and without post-annealing treatment at 750 ◦C. TheTIR spectra show the disappearance of OH impurities at about600 cm−1 from the samples after post-annealing treatment,ompared with the negligible amount of OH impurities in the2O3:Eu3+ phosphor film which is only subjected to transientTP annealing at 750 ◦C. Since the luminescent intensity ofhosphor materials is inversely proportional to the OH impu-ities content, the phosphor films with less OH impurities willherefore show higher PL emission intensity. This explanations consistent with the study of PL emission intensity as a func-ion of post-annealing time as presented in Fig. 5. In Fig. 6, otherhan absorption peaks of Y2O3 at about 570 cm−1, there are threether absorption peaks exist after post-annealing treatment. Thebsorption peaks at about 450 cm−1, 810 cm−1, and 1070 cm−1

re ascribed to Si–O–Si bending, stretching, and asymmetrytretching, respectively [20]. These peaks are believed to origi-ate from the interfacial SiO2 layer that formed in between the2O3:Eu3+ phosphor films and the silicon wafer substrate.

Despite the efficient elimination of entrapped residue organic

olecules from the Y2O3:Eu3+ phosphor films after post-nnealing treatment for 1 h, the PL emission intensity stillncreases with longer post-annealing time. In our view, the

Page 4: Thermal annealing effect on Y2O3:Eu3+ phosphor films prepared by yttrium 2-methoxyethoxide sol–gel precursor

332 M.K. Chong et al. / Materials Chemistry

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ig. 7. XRD pattern of Y2O3:Eu3+ phosphor films as a function of post-nnealing time.

ncrease of the PL emission intensity with longer post-annealingime is probably due to enlargement of grain size. The increase ofhosphor PL emission intensity has been explained on the basisf improved phosphor films crystallinity [21]. Grain boundarys believed to be another non-radiative transition routes in phos-hor films because the defects are present at the grain boundaries.s a result, the density of grain boundaries and hence the densityf defects are reduced if the grains of the phosphor films growarger. As can be seen in Fig. 7, the crystallinity of phosphor filmsndeed improves with longer post-annealing time at constantemperature of 750 ◦C. According to Scherrer equation, the grainize is inversely related to the full width half maximum (FWHM)f the most dominant diffraction peak. In Fig. 8, the FWHM of2 2 2) peak decreases with post-annealing time and this impliesn increase of grain size. In other words, the decrease of FWHM

ndicates the reduction of defects at the grain boundaries. Con-equently, the PL emission intensity increases as a function ofost-annealing time, as previously presented in Fig. 5.

ig. 8. The full width half maximum (FWHM) of (2 2 2) peak decreases withhe increase of post-annealing time.

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and Physics 100 (2006) 329–332

. Conclusion

In conclusion, it is shown in this experiment that the yttrium-methoxyethoxide can be used as a convenient sol–gel pre-ursor to prepare Y2O3-based phosphor films. The Y2O3:Eu3+

hosphor films undergo crystallization at about annealing tem-erature of 550 ◦C. This can serve as an important property inhe selection of good host. In addition, the Y2O3:Eu3+ phosphorlms undergo crystal structure change after annealing temper-ture of 750 ◦C. This change has not been observed in other2O3-based films at such a low temperature. The evolution of2O3:Eu3+ phosphor films crystal structure leads to the changef PL emission spectra. This is attributed to the change of Eu3+

ons local environment. Post-annealing treatment appears to ben essential processing step to enhance the PL emission inten-ity by eliminating the OH impurities and enlarging the grainize of phosphor films.

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