Thin Solid Films 518 (2010) 4817–4820

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Electro-deposition of Y2O3:Eu3+ thin film phosphor and its luminescent properties

L. Wang, L. Shi, N. Liao, H. Jia, X. Yu, P. Du, Z. Xi, D. Jin ⁎Materials Engineering Center, Zhejiang Sci-Tech University, Hangzhou, Zhejiang Province, 310018, China

⁎ Corresponding author. Tel.: +86 571 86843265; faxE-mail address: jindl@zstu.edu.cn (D. Jin).

0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.01.059

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 March 2009Received in revised form 20 January 2010Accepted 28 January 2010Available online 10 February 2010

Keywords:Y2O3:Eu3+

Thin film phosphorElectro-depositionPhotoluminescenceConcentration quenching

Y2O3:Eu3+ red-emitting thin film phosphor was prepared by a two-step process: the cathodical deposition ofthin film of yttrium hydroxide and europium hydroxide followed by an annealing process to achieve Eu3+

doped Y2O3 film. It is found that the atomic content of Eu3+ can be well controlled by simply adjusting thevolume ratio of Y(NO3)3 to Eu(NO3)3 solutions. Dependence of the photoluminescence intensity on theatomic content of Eu3+ in Y2O3 was also studied. The best photoluminescence performance of Y2O3:Eu3+

thin film phosphor was achieved as atomic content of Eu3+ equal to 1.85 at.%.

: +86 571 86843266.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Due to the excellent thermal stability and light output, RE (rareearth) doped oxide phosphor has been widely studied for displaydevices such as CRT (cathode ray tube), LED (light emitting diode)and fluorescence lamp [1–5]. Also oxide phosphor activated bydifferent RE elements may cover the whole visible light spectrum:Y2O3:Eu3+ red [6–8], Gd2O3:Eu3+ red [9,10], YAG:Ce3+ yellow[11,12], Y2O3:Tb3+ green [13] and Y2O3:Tm3+ blue [13,14]. There-fore, RE doped oxide phosphors have drawn great interests for thelast several decades.

Required by the increasing demand of the flat panel display andsimilar field emission display devices, preparation and characteriza-tion of thin film phosphor (TFP) have been an important researchtopic. Compared with powder phosphor, TFP has advantages of betterthermal stability, better adhesion to substrates, better uniformity andhigher resolution. Y2O3:Eu3+ phosphor is one of the best redphosphors used for more than 40 years since 1964 [15]. Y2O3:Eu3+

TFP can be prepared by different methods such as PLD (pulsed laserdeposition) [16], sputtering [17], CVD (chemical vapor deposition)[18,19], ALD (atomic layer deposition) [20], sol–gel and spin coating[8,21], and spray-pyrolysis [13,22]. Compared with PLD, CVD and ALDmethods, electro-deposition needs no expensive and complicatedinstruments. Films prepared by electro-deposition can achieve auniform distribution of Eu3+. Brossard et al. [23] reported physicalproperties of the electro-deposited Y2O3 films. Gaikwad et al. [24]reported the electro-deposition of Eu and Eu2O3 films. However,

photoluminescence property of Y2O3:Eu3+ TFP prepared by electro-deposition was barely reported. In this work, Y2O3:Eu3+ TFP wasprepared by a two-step process: the cathodical deposition of yttriumhydroxide and europium hydroxide film followed by an annealingprocess. Microstructure and photoluminescence properties of theprepared samples were studied and results indicate that Eu3+

distributes uniformly in the Y2O3 lattice and the emission intensitystrongly depends on the atomic content of Eu3+.

2. Experimental details

All samples were electro-deposited in a conventional three-electrode cell, of which working electrode was FTO (fluorine dopedtin oxide film) coated glass (sheet resistance: 20 Ω/□), counterelectrode was a platinum foil, and reference electrode was Ag/AgCl/saturated KCl (Fig. 1). Y(NO3)3 and Eu(NO3)3 solutions with concen-tration of 0.1 M were made by dissolving Y(NO3)3∙6H2O and Eu(NO3)3∙6H2O in deionized water. Then Y(NO3)3 and Eu(NO3)3 solutionswere mixed with different R values (volume ratio of Y(NO3)3 solutionto Eu(NO3)3 solution) for electro-deposition. Applied potential was−1.2 V versus Ag/AgCl/saturated KCl. Deposition temperature was60 °C and deposition time for each samplewas 30 min. After deposition,samples were cleaned and then annealed in air at 600 °C for 2 h. X-raydiffraction (XRD) was performed by a Thermo ARL X'TRA X-ray powderdiffractometer with Cu Kα radiation (1.54 Å). Surface morphology ofthe prepared samples was characterized by scanning electron micro-scope (SEM, Model: JSM 5610LV) with an incident electron energy of 5to 10 kV. The coupled electron dispersive spectroscopy (EDS) was usedto study the composition and element distribution of the samples. Theoperation voltage was 10 kV and scanning area was 48 μm×60 μm.Before characterizing the samples, the system was first calibrated with

Fig. 1. The schematic structure of the 3-electrode cell for electro-deposition.

Fig. 2. (a) XRD patterns of samples deposited from solutions with different R values.(b) Calculated lattice constants of samples deposited from solutions with different Rvalues.

Fig. 3. SEM image of the sample with atomic content of Eu3+ equal to 5.07 at.% and theinserted image is a magnified view.

4818 L. Wang et al. / Thin Solid Films 518 (2010) 4817–4820

the standard silicon sample at an operation voltage of 10 kV.Photoluminescence spectra were taken at room temperature on aHitachi F-4600 fluorescence-spectrophotometer equipped with an150W xenon lamp as the excitation source.

3. Results and discussions

Y2O3:Eu3+ films were prepared by a two-step process: formationsof yttrium hydroxides and europium hydroxides followed by anannealing process to achieve oxide films. As the applied potential wasnegative enough, NO3

− ions in the solution around the workingelectrode would be reduced according to the following equation:

NO−3 þ H2O þ 2e

−→NO−2 þ 2OH

−: ð1Þ

Reduction of NO3− would generate OH− and cause increase of pH in

the solution around the working electrode. Y3+ and Eu3+ wouldprecipitate with OH−, which in turn forms a layer of yttrium hydroxideand europium hydroxide film on the working electrode. The preparedhydroxide films were then annealed to achieve Y2O3:Eu3+ film. Theelectro-deposited samples have similar thickness of around6.56 μmandthe average deposition rate is around 0.219 μm/min.

XRD patterns, as shown in Fig. 2 (a), indicate that all samples arecomposed of poly-crystalline Y2O3 with a cubic structure (JCPDS cardNo. 43-1036). Lattice constant was calculated from the diffractionpeak of (222) according to the Bragg equation and is plotted as afunction of R, as shown in Fig. 2 (b).

2d sin θ = nλ ð2Þ

As R decreases, the diffraction pattern gradually shifts to the left, i.e.,diffraction angle θ decreases. According to Eq. (2), d increases withdecreasing θ. Therefore the calculated lattice constant increases as Rdecreases. The reason for this may be that as R decreases, more Eu3+

ions would be incorporated into the Y2O3 lattice. Compared with Y3+

ions (0.89 Å), Eu3+ has a larger ionic radius of 0.947 Å. Therefore, thelattice constant would increase with increasing atomic content of Eu3+

ions in the film, i.e., with decreasing R values.All the Y2O3:Eu3+ thin films have similar surface morphology. A

typical SEM image of the Y2O3:Eu3+ thin film is shown in Fig. 3 and theinserted image is a magnified view. The Y2O3:Eu3+ thin film has alamellar surface morphology. EDS was performed to characterize theelement distribution and the atomic ratio of Y3+ to Eu3+ in the films.Results are shown in Figs. 4 and 5, respectively. Apparently, O, Y and Eudistribute uniformly in thefilmand there is no sign of aggregation of anyelement. Besides thedistributionof the elements, the composition of thefilmwasalso characterized. Atomic ratios of Y3+ to Eu3+(Rc) and atomic

content of Eu3+ (CEu3+) for films deposited from solutionswith differentR values are calculated from the EDS spectra and were plotted as afunction of R in Fig. 5. Rc has a simple linear relationwithR, which can befit by the following equation:

Rc = 0:981R + 0:052: ð3Þ

Fig. 4. EDS mapping results of the sample deposited from the solution with R=5(atomic content of Eu3+ equal to 5.07 at.%).

Fig. 5. Calculated atomic ratio of Y3+ to Eu3+ and atomic content of Eu3+as a function of R.

Fig. 6. Photoluminescence emission spectra of samples deposited from solutions withdifferent R values.

4819L. Wang et al. / Thin Solid Films 518 (2010) 4817–4820

Apparently, the fitting result shows that Rc is about the same valueas R, which indicates that the atomic ratio of Y3+ to Eu3+ in Y2O3 filmcan be controlled by simply adjusting the value of R.

CEu3+ decreases exponentially with R values and the data can befit by Eq. (4). The atomic content of Eu3+ can be roughly controlledby R values.

CEu3+ = 3:85 expð−R= 13:9Þ + 17:3 expð−R= 0:99Þ + 0:05 ð4Þ

It is also found that all the films are oxygen rich. For example, filmdeposited from solution with R equal to 5 contains: 15.63 at.% Y3+,3.08 at.% Eu3+ and 81.29 at.% O2−. The spare O2− ions may exist asinterstitial ions in the Y2O3 lattice.

Samples deposited from solution with different R values were usedfor photoluminescence spectrummeasurement. As shown in Fig. 6, thephotoluminescence emission spectra consist of several peakscorresponding to different energy transitions of Y2O3:Eu3+ phosphor.The strongest emission peak for all these samples is from the 5D0–

7F2transition of Eu3+. The fact that thedominant emission is from theparityforbiddenelectric dipole transition rather than fromthemagneticdipoletransition (5D0–

7F1) indicates that Eu3+ is located at the site with noinversion symmetry in the Y2O3 lattice (C2 site) [25–27]. Comparedwiththe 5D0–

7F2 transition, intensity of the 5D0–7F1 transition corresponding

to the orange color is much lower, which makes the electro-depositedY2O3:Eu3+ a purer red phosphor.

As shown in Fig. 6, the intensity of the 5D0–7F2 transition varies

with R. Fig. 7 shows the dependence of the emission intensity of the5D0–

7F2 transition on the atomic content of Eu3+ in the films. Clearly,the intensity of the 5D0–

7F2 transition reaches the maximum value as

Fig. 7. Dependence of the emission intensity of the 5D0–7F2 transition on the atomic

content of Eu3+.

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the atomic content of Eu3+ is around 1.85 at.%. It could be attributedto the competition between two opposing mechanisms: opticalactivation of Eu3+ ions leads to the increasing photoluminescenceintensity, and the pairing or clustering of Eu3+ ions leads to theconcentration quenching. At lower atomic content of Eu3+, the firstmechanism prevails in the photoluminescence process and thephotoluminescence intensity increases with the atomic content ofEu3+. The second mechanism becomes progressively dominant as theatomic content of Eu3+ increases over a critical value and thephotoluminescence intensity decreases dramatically with the atomiccontent of Eu3+. Concentration quenching of Eu3+ in different hostmaterials has been widely studied and the common mechanism isknown that concentration quenching is mainly caused by pairing ofEu3+ ions and the interaction through an intermediate oxygen ion,which is called super-exchange effect [28–32].

4. Conclusions

Y2O3:Eu3+ TFP was prepared by electro-deposition from a solutioncontaining Y(NO3)3 and Eu(NO3)3. The atomic ratio of Y3+ to Eu3+ canbe well controlled by the volume ratio of Y(NO3)3 solution to Eu(NO3)3 solution. As evidenced by XRD and SEM results, the electro-deposited Y2O3:Eu3+ thin film phosphor has excellent crystallinityand a lamellar surface morphology. Photoluminescence characteriza-tion shows that the emission intensity of the 5D0–

7F2 transition hasstrong dependence on the concentration of Eu3+ and the optimumatomic content of Eu3+ is around 1.85 at.%. As the atomic content of

Eu3+ is over 1.85 at.%, concentration quenching occurs and emissionintensity of the 5D0–

7F2 transition decreases dramatically.

Acknowledgements

This work is supported by the National Science Foundation of Chinaunder Grant 60906031, Zhejiang Provincial Natural Science Foundationof China under Grant Y1090742, the Qianjiang Talent Programme ofZhejiang Province under Grant 2009R10018, and the Scientific ResearchFoundation for the Returned Overseas Chinese Scholars (State Educa-tion Ministry) under Grant No. 111383A4C08690.

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