Photoluminescence study of Eu+3 doped ZnO nanocolumns prepared by electrodeposition method

Photoluminescence study of Eu+3 doped ZnO nanocolumns prepared by electrodeposition method

Accepted Manuscript Title: Photoluminescence Study of Eu+3 doped ZnO Nanocolumns Prepared by Electrodeposition Method. Authors: Abdelhak Nouri, Abdelk...

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Accepted Manuscript Title: Photoluminescence Study of Eu+3 doped ZnO Nanocolumns Prepared by Electrodeposition Method. Authors: Abdelhak Nouri, Abdelkrim Beniaiche, Bernab´e Mar´ı Soucase, Hocine Guessas, Amor Azizi PII: DOI: Reference:

S0030-4026(17)30341-8 IJLEO 58998

To appear in: Received date: Revised date: Accepted date:

11-12-2016 17-3-2017 18-3-2017

Please cite this article as: Abdelhak Nouri, Abdelkrim Beniaiche, Bernab´e Mar´ı Soucase, Hocine Guessas, Amor Azizi, Photoluminescence Study of Eu+3 doped ZnO Nanocolumns Prepared by Electrodeposition Method., Optik - International Journal for Light and Electron Optics This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Photoluminescence Study of Eu+3 doped ZnO Nanocolumns Prepared by Electrodeposition Method.

Abdelhak Nouri1,*, Abdelkrim Beniaiche1, Bernabé Marí Soucase2, Hocine Guessas1, Amor Azizi3

Laboratoire des Systems photonique et d’optique non linéaire, Université Sétif 1, 19000 Algeria

1 2

Departament de Física Aplicada-IDF, Universitat Politècnica de València, Camí de Vera s/n, 46022

València, Spain 3

Laboratoire de Chimie, Ingénierie Moléculaire et Nanostructures,Université Sétif1, 19000 Algeria.


Corresponding autor: [email protected]

Abstract: Europium doped ZnO (ZnO:Eu) nanocolumns were grown on indium tin oxide substrates (ITO) by electrodeposition method in zinc nitrate aqueous solution with different concentration of Eu. According to XRD results electrodeposited ZnO:Eu films exhibit thigh crystalline quality with a preferential orientation parallel to c-axis. Moreover, the (002) peak position shifts toward to lower angles confirming the incorporation of Eu+3 ions into the ZnO lattice. The crystallite size of a pure ZnO decreases when increasing the Eu doping. SEM images reveal a well dense nanocolumns arrays perpendicular to the surface. Furthermore, doping with Eu+3 ions leads to a change in the top of nanocolumns from hexagonal to conic-hexagonal shape. The average diameters of nanocolumns are found to be around 200 nm and 180 nm for undoped ZnO 1

and Eu doped ZnO, respectively. Photoluminescence (PL) emission from ZnO:Eu thin films reveals the typical peaks related to excitons and intrinsic defects in ZnO and four narrow peaks in the red region attributed to 4f-shell transition in Eu+3 ions. As Eu doping increased, PL peaks related to Eu+3 emission showed an increase in intensity and a shift in its wavelength position. A schematic drawing of the excitation mechanism and related emission process is proposed. Keywords: ZnO:Eu; nanostructures; electrodeposition; photoluminescence; shifting.

1. Introduction: Zinc oxide is a wide and direct band-gap n type semiconductor, which has attracted considerable interest for applications in optical and optoelectronic devices such as light-emitting and laser diodes covering the ultraviolet range. Owing to its interesting properties, particularly its wide band-gap of 3.37 eV and its large exciton binding energy of 60 meV [1]. ZnO under thin films or nanostructured morphologies is highly demanded for technological applications, especially for the use in various electronic and optoelectronic devices, such as gas sensing applications, surface acoustic wave devices, transparent coating and solar cell applications [2]. Many growth techniques have been adopted to prepare ZnO thin films and nanostructures, such as chemical vapor deposition (CVD), thermal oxidation, pulsed laser deposition, spray pyrolysis and electrodeposition [3]. Among all methods, the electrodeposition in aqueous solution is a low cost method because it works at ambient pressure and relatively low temperatures. Therefore, it is convenient and economic for large-scale preparation of well-oriented ZnO nanorods, nanowire arrays with relatively higher growth rate. To obtain ZnO nanostructures, temperature and concentration parameters of the solution have to be controlled. Sheng-Nan et al. [4] suggested that the temperature of the bath must be 2

kept between 80°C and 90°C, in the meanwhile, Debabrata et al. [5] propose to use low concentration for Zn precursor. In order to obtain new structural, electrical and optical properties the doping with a transition metal and/or rare earth is highly recommended. For example, doping with aluminum leads to enhance the electrical properties [6], whereas, doping with europium ions has a great impact on optical properties of ZnO nanostructures. In literature, many papers report on europium doped ZnO. Danny E. P. Vanpoucke [7] reveals the impact of Europium doping on the second harmonic generation of ZnO nanowire. Furthermore, Zhong et al. [8] demonstrate in their studies the effect of annealing on the photoluminescence of Eu doped ZnO thin films. In this context, the red and blue shifting appeared in PL spectrum is an important phenomenon that can provide information’s at the atomic scale. A blueshift of PL peak position is observed on a porous Si during thethermal oxidation which is attributed to quantum size [9]. Furthermore, a red-shift is observed on the position of maxima of the UV bands depending on the morphology of ZnO structure grown by the aqueous chemical growth [10]. Thus, P. A. Rodnyi et al [11] report in their studies the properties of ZnO where the maximum of the edge luminescence band is red-shifted from 386 nm to 390 nm depend on the variation of average particles size from 0.9 µm to 0.82 µm. However, the phenomenon of shifting of PL peaks positions can be observed also in PL peaks related to Eu+3 ions. Koen Binnemans [12], suggests that the emission from higher excited states (5D1, 5D2, 5D3 ) can shift the luminescence toward orange and yellow emissions and can be tuned by varying the Eu+3 concentrations in the host matrix. While, Partha P. Pal et al.[13] reported that if Eu+3 ions are either not quantum confined or suffering an insufficient optical confinement by the nanocrystals, there is no shift PL peaks position with the doping concentration.


In the present work, we report a photoluminescence study of europium doped ZnO. Thus, to reveal the reason behind red-shift of 5D0 →7F2 transition in PL spectrum as a function of Eu concentration. In this study, Eu doped ZnO nanostructures are synthesized using the electrodeposition method with the characterization of the emission of ZnO:Eu films at different percentage of Eu doping. The effect of Eu ions on structural, morphological and photoluminescence properties were investigated. Then, the red sharp peaks in the PL spectrum are discussed and the reason behind the observed wavelength shift of PL peaks when varying Eu doping concentration. Moreover, the blue and green emissions in ZnO:Eu nanostructures related to native ZnO defect are also discussed. 2. Experimental: The electrodeposition procedure consists of a classical three electrodes electrochemical cell and an aqueous solution. A solution containing 10-3 M of Zn (NO3)2 as precursor of Zn and 0.5 M KNO3 as supporting electrolyte was used without oxygen bubbling. For doping, another solution containing 10 -3M Eu(NO3)2 is prepared. Three different concentrations of europium have been tested (0.5%, 1.5%, 2.5%).These percentages represent the amount of dissolved Eu ions (obtained from Eu(NO3)2) with respect to the amount of dissolved Zn ions (from Zn(NO3)2). The main parameter controlled during the deposition was the deposited charge. Electrodeposition procedure was stopped for all samples when a charge of 1 coulomb was reached. Finally, samples were rinsed with distilled water without any thermal treatment. The electrodeposition process was performed on indium tin oxide coated glass (ITO) acting as working electrode and Pt and Ag/AgCl as counter and reference electrodes, respectively. The potential of deposition (- 0.9 V) was kept constant during the deposition and the temperature of bath was maintained at 80 °C. The PH of solution was kept about 5 during deposition without further adjustments. Before deposition, the ITO substrates (2cm x1cm) 4

were cleaned sequentially by acetone, ethanol and distilled water during 15 minutes using an ultrasonic bath. For morphological characterization, scanning electron microscopy (SEM) is used. Structural characteristics were measured by X-ray diffraction (XRD) in θ-2θ configuration using a Rigaku diffractometer equipped with a copper anticathode (CuKα=1.54Å). The photoluminescence spectra were obtained at room temperature using a laser of 325 nm wavelength. 3. Results and discussion: We carried out the experiment in aqueous solution of 10-3M of Zn (NO3)2 and 0.5M KNO3 at a voltage of -0.9 V and a temperature of 80 °C. The mechanism of electrodeposition of ZnO nanostructures in nitrate medium is well known and could be summarized as following [6]. Zn ( NO )  Zn 3 2


  2 NO 3

    NO  H O  2e  NO  2 OH 3 2 2



 2OH

 Zn (OH )  ZnO  H O 2 2

(1) (2) (3)

With the presence of Eu3+ ions in the solution, part of Eu3+ ions can be incorporated into the ZnO lattice as a dopant and another part of Eu3+ ions can be reacted with OH- hydroxide to form Eu2O3, according to the following reaction:



 6OH

 Eu O  3H O 2 3 2


3.1 Structural characterization: Fig.1 shows the X-ray diffraction patterns of undoped ZnO and europium doped ZnO with different concentration (0.5%, 1.5% and 2.5%). As shown in Fig.1 5

a wurtzite structure typical of ZnO (JCPDS N° 36-1451) with (002) as preferred orientation, is observed. No other peaks related to europium oxide (Eu2O3) were detected. Moreover, the shift was observed in the peaks position toward lower angles (inset Fig.1) and the extension of the parameters lattice a, c (SeeTable 1) with increasing Eu doping concentration indicates the incorporation of Eu ions in ZnO lattice. It is worthy mentioning that the Eu ions radius (0.095 nm) is larger than that of Zn (0.074nm) and then for Eu atoms substituting Zn atoms the lattice parameters of ZnO should increase and then XRD peaks would shift to low angles, as observed. The intensity of (002) peak shows a significant increase when varying Eu doping from 0.5%Eu to 1.5%Eu, followed by slight decrease when the doping level increases to 2.5%Eu. We believed that the two first doping levels, ZnO lattice receive more Eu ions and the limit of the doping is 2.5%Eu. Doping beyond 2.5%Eu results in a deterioration of the crystal lattice. Similar phenomena have also been observed in the study of O. Lupan et al.[14]. As shown in Table 1, the value of full width at half maximum (FWHM) rises from 0.1277° to 0.1604° when Eu concentration varies from 0% to 2.5%, suggesting that the ZnO crystallites size decreases as Eu doping increases. 3.2 Morphological analysis: Fig.2 (a, b, c, d) display the morphology of undoped ZnO and europium doped ZnO with different concentration (0.5%, 1.5%, 2.5%), respectively. As shown in Figure 2 all samples exhibit the formation of regular arrays of dense nanocolumns with hexagonal (wurtzite) shape perpendicular to the substrate, which confirm the preferred orientation along the c-axis. This result is in a good agreement with XRD results and supported by the literature [15].The nanocolumns of undoped ZnO (shown in Figure 2.a) have hexagonal end shape, whereas for those doped with Eu, their end become hexagonal-conic shape as shown in Fig.2 (b, c, d). This fact reveals the vital role of Eu+3 ions in changing the end shape of nanocolumns. 6

This phenomenon can be explained as follows; the low concentration of Zn(NO3)2 in solution may prevent the amalgamation of nuclei. This last has two directions for growing, along and perpendicular to c-axis. As the top plane (0001) [16] is more active to react with hydroxide than sides planes, the growth rate along the c-axis is higher than the perpendicular growth [17], leading to the formation of nanocolumns as shown in Fig.2a. With addition of Eu(NO3)2 in the solution, the concentration of Zn+2 decreases. Then Eu+3 ions have two roles; first, is the adsorption on the top and side planes where they restraint the lateral (perpendicular to c-axis) and vertical (along c-axis) growth. For this reasons the nanocolumns of Eu doped ZnO look smaller than that of undoped ZnO, second, Eu+3ions enter in competition with Zn+2 on the top planes, leading to the formation and union of three polyhedron on top planes and subsequently resulting in the conical shape observed on the top of the nanocolumns [17]. Hence, the average diameters of the present nanocolumns are found to be around 200 nm for undoped ZnO and 180 nm for those doped with Eu. Our results are in good agreement with literature [5,18]. To confirm the presence of Eu+3 ions in ZnO lattice, EDX (Energy Dispersive X-ray Spectroscopy) analysis were carried out to determine the elemental composition of ZnO:Eu nanocolumns. Fig.3 shows the EDX pattern of undoped ZnO and Eu doped ZnO samples. As shown in Fig.3a, EDX spectrum of undoped ZnO shows that the nanocolumns are composed of Zn and O, whereas the Eu doped ZnO samples as depicted in Fig.3 (b, c, d) are certainly composed of Zn, O and Eu. The concentration of the europium incorporated in ZnO lattice is calculated in the following ratio Eu/(Eu+Zn). The concentrations are found to be 0.5at%, 1.45at% and 2.40at%, respectively (See Table 1). As shown in Table 1, the concentration of Europium incorporated in ZnO host lattice is less than that provided in the solution. This can be due to the difference between ionic radii Eu+3(0.095 nm) and Zn+2(0.074 nm) and the charge imbalance created when Eu+3ions substitute the Zn+2sites. 7

3.3. Photoluminescence properties: Photoluminescence was used to check the presence of Eu+3 ions and the defects in ZnO lattice. Fig.4 depicts the photoluminescence spectra at room temperature of undoped and Eu doped ZnO with different Eu concentrations (0.5%, 1.5% and 2.5%). As shown in Fig.4, all samples display a narrow emission peaks centered at 381, 410, 434, 473 and 519 nm. The peak located about 381nm (3.25 eV) is related to near band-edge emission due to the recombination of free exciton in ZnO lattice [19,20]. The peak centered at 410 nm (3.02 eV) is attributed to the zinc vacancy [13,21,22]. Moreover, the PL peak centered at 473 nm (2.62 eV) is related to the transition from the conduction band to oxygen interstitial sites [13,21,23]. Furthermore, as the samples were not subjected to any heat treatments, there are still trapped hydroxides (reaction Eq. 3) which mean that the PL peak around 434 nm (2.86 eV) is due to the transition from OH- trapped sites to the valence band [23]. The PL peak located at 519 nm can be attributed to the recombination of ionized charge of oxygen vacancy with photo-generated holes [19-21]. Additionally to PL peaks related to undoped ZnO, all Eu-doped ZnO samples exhibit a sharp peaks in the red part were centered at 579, 593, 613, and 659 nm. These peaks are identified as intra-4f shell transition 5D0→7Fj(j=0-4) in Eu+3 ions [13]. This result reveals that the Eu+3 ions act as a luminescent center in ZnO nanocolumns. For further clarification, Table 1 reports the values of asymmetric ratio R. This last is known as the ratio of the intensity of I(5D0→7F2) to I(5D0→7F1) and can provide information about the structural quality of ZnO:Eu films. All values of R are above one. So, it is clear that the dominant transition is the electric dipole 5D0→7F2 transition [24]. From this result it can be inferred that the Eu+3 ions have been incorporated into a lower symmetry site than C3V by substituting Zn sites. Therefore, the most Eu+3 ions are located at the surface of ZnO:Eu nanocolumns. Moreover, the appearance of the peak centered at 579 nm 8

corresponding to the


D0→7F0 transition, is due to the same total angular

momentum and indicates that some Eu+3 ions occupy interstitial sites [25]. Our results are supported by SEM study and are in good agreement with literature [21]. Fig.5 illustrates the variation of the asymmetric ratio as a function of Eu concentration. The empiric graphic was fited to an exponential decay function (see inset Fig.5). As shown, the asymmetric ratio increases with the increase of Eu concentration until reaching the higher value of 2.76 for 2.5% of Eu doping.This result shows an enhancement of the crystal-field around Eu+3 ions with increasing Eu doping. As shown in Fig.4, the position of 5D0→7F2 transition is significantly affected by Eu doping concentration, when Eu doping concentration increases from 0.5% to 2.5% the position of 5D0→7F2 transition shifts toward higher wavelengths, from 611.5 nm to 613 nm. Theoretically, the level 7F2 have three sublevels of crystal-field (A1, E1, E2), where the sublevel A1 and E1 have closer energy level as reported by S. Lôpez-Romero et al.[25] and C. Linarè et al.[26].So, as shown in Fig.4, the positions of 5D0→7F2 transition are located at 611.5 nm (2.02780 eV), 612 nm (2.02614eV) and 613 nm(2.02284 eV) for Eu doping concentrations of 0.5at%, 1.45at% and 2.40at%, respectively (Table 1). As can be seen, two positions have closer energy (611.5 and 612 nm). In our case, we believe that the sublevels (A1, E1, E2) are behind the phenomenon of shifting PL peak positions. We infer that the shift of the peak position of 5D0→7F2 transition is due to the enhancement in the crystal-field around Eu+3 ions, then lead the emission occurred from main emitting level 5D0 to the sublevels A1, E1 and E2 of 7F2 level when varying Eu doping concentration 0.5at%, 1.45at% and 2.40at% respectively. Fig.6 shows the schematic of the energy transfer process from ZnO host to Eu+3 ions (right side) where illustrate the 5D0→7F2 transition from 5D0 level to different sublevels of 7F2 level as a function of Eu+3 ions incorporated in ZnO lattice. Thus the transitions occurred from 5D0 level to 7F0 , 7F1 and 7F3 levels. The emission 9

related to zinc vacancies (VZn), oxygen interstitials (Oi) and surface defects (left side) in Eu doped ZnO nanocolumns are represented by color arrows corresponding to the emission wavelength and their energy levels. 4. Conclusion: Europium doped ZnO nanocolumns were electrodeposited on ITO coated glass was demonstrated and characterized. X-ray diffraction measurement reveals a hexagonal wurtzite structure with high crystalline quality. SEM images confirm the XRD results where the nanocolumn was almost vertical to the surface. Furthermore, the top of nanocolumns changes the shape from hexagonal to conichexagonal when introducing Eu doping; inferring that the growth mode changed and Eu ions are adsorbed on the surface of ZnO. PL spectra contain the peaks related to the intrinsic defect in ZnO and those attributed to the transition in Eu+3 ions. Moreover, the crystal field around Eu+3 ions increases its asymmetric ratio when varying Eu concentration. Increasing Eu concentration results in a shift of the PL peak position corresponding to 5D0→7F2 transitions by the emission from the main 5D0 level to sublevels A1, E1 and E2 of 7F2 level. To better understand the energy transfer process from ZnO host to Eu+3 ions in Eu-doped ZnO nanocolumns a useful schematic diagram has been proposed and investigated.


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Figure Captions: Figure 1: X-ray diffraction patterns of ZnO:Eu deposited on ITO substrate with different Eu concentrations (0.5%, 1.5%, 2.5%). Figure 2: SEM image of undoped ZnO (a) and Eu-doped ZnO; 0.5%Eu (b), 1.5%Eu, (c) and 2.5% Eu (d). Figure 3: EDX spectra for : a) undoped ZnO, b) ZnO:0.5%Eu, c) ZnO:1.5%Eu, d) ZnO:2.5% Eu. Figure 4: Room temperature Photoluminescence (PL) emission spectra of the undoped ZnO and Eu doped ZnO with different Eu concentration (0.5%, 1.5%, 2.5%) Figure 5: Fit of the asymmetric ratio R as a function of Eu concentration where x is the concentration of europium. Figure 6: Schematic drawing of the mechanism of excitation and emission process.



* : ITO Intensity (Arb.Unit)

(002) (101)

0.5% 0%

(112) (201)









34,4 2theta (degree)





Intensity (Arb.Unit)



1.5% 0.5%

0% 30





2theta (degree)

Figure 1



Undoped ZnO

c- Eu doped ZnO (1.5%)


Eu doped ZnO (0.5%)

d- Eu doped ZnO (2.5%)

Figure 2


Figure 3

0% 2.5% 1.5% 0.5%


PL Intensity (Arb.Units)

0,8 5D 7F 0 2


5D 7F 0 1


5D 7F 0 0


5D 7F 0 3

0,0 350



500 550 wavelength (nm)




Figure 4 17



Asymmetric ratio R



y  y 0  a1e


y 0  2.76


a1  2.81


t  0.78 0,0


1,0 1,5 2,0 Eu Concentration (at%)

( x/t )



Figure 5

Figure 6


Tables Captions: Table.1










photoluminescence properties of Eu-doped ZnO nanostructures.

XRD Eu (%)

Eu at%

a (A°)

PL c (A°)


provided in


Asymmetric ratio



position (nm)