Thin film of ZnAl2O4:Eu3+ synthesized by a non-alkoxide precursor sol-gel method

SHORT REPORT

Thin film of ZnAl₂O₄:Eu³⁺ synthesized by a non-alkoxide precursor sol-gel method

Renata F. Martins; Osvaldo A. Serra^*

Laboratório de Terras Raras, Departamento de Química, FFCLRP, Universidade de São Paulo, Av. Bandeirantes 3900, 14040-901 Ribeirão Preto-SP, Brazil

ABSTRACT

Europium-doped zinc aluminate (ZnAl₂O₄:Eu³⁺) films were prepared by the dip-coating technique using a non-alkoxide precursor sol-gel method in ethanolic medium. Photoluminescence spectra displayed the main excitation bands at 270 and 393 nm. The emission spectra of the film heated at 500 ºC displayed bands at 578, 590, 612, 650 and 698 nm, assigned to the ⁵D₀→ ⁷F_J (J = 0, 1, 2, 3, 4) transitions. The films are transparent above 320 nm.

Keywords: zinc aluminate, europium, non-alkoxide, sol-gel process, UV-Vis transparent and luminescent films

RESUMO

Filmes de aluminato de zinco dopados com európio (ZnAl₂O₄:Eu³⁺) foram preparados pelo método sol-gel usando precursores não-alcóxidos em meio etanólico através da técnica de dip-coating. Os espectros de fotoluminescência do filme aquecido a 500 ºC apresentaram bandas de excitação em 270 e 393 nm e bandas de emissão em 578, 590, 612, 650 e 698 nm, atribuídas às transições ⁵D₀→ ⁷F_J (J = 0, 1, 2, 3, 4). Os filmes mostraram-se transparentes acima de 320 nm.

Introduction

In recent years, spinel-type zinc aluminate (ZnAl₂O₄) has become technologically important due to its peculiar optical properties. With an optical bandgap of 3.8 eV, which results in effective transparency at wavelengths above 320 nm, it is a useful component of photoelectronic devices operating in the ultraviolet region.^1-3 Also, its high chemical and mechanical stabilities make it suitable for a variety of applications such as flat panel display electrodes, ceramics, and electronic and catalytic materials.^4,5 Bulk non-doped ZnAl₂O₄ has been prepared by the sol-gel process employing alkoxides as precursors, as well as by coprecipitation and wet mixing.^6,7 Phani et al.⁸ have obtained ZnAl₂O₄ thin films by utilizing an alkoxide sol-gel route and deposition by spin-coating. Davolos et al.⁹ have successfully prepared bulk zinc aluminate by a non-alkoxide sol-gel method. Bulk rare earth (RE)-doped zinc aluminate (ZnAl₂O₄:RE³⁺) has been produced by spray drying, combustion and hydrothermal methods.^10-14 The sol-gel process has also been employed in the preparation of luminescent rare earth-based materials incorporated into inorganic hosts in very mild conditions.¹⁵

The non-alkoxide sol-gel method has the advantage of employing stable and less expensive precursors; alkoxides are very reactive and need special care during manipulation and storage (especially aluminum alkoxides). So far, there has been no study devoted to the preparation of rare earth-doped ZnAl₂O₄ using a non-alkoxide sol-gel process. In the present work, thin films of photoluminescent ZnAl₂O₄:Eu³⁺ have been synthesized by a non-alkoxide precursor sol-gel process using the dip-coating technique.

Experimental

Zinc acetate dihydrate (0.329 g, 0.0015 mol) and aluminium nitrate nonahydrate (1.125 g, 0.0030 mol), selected as starting materials, were dissolved in 13.4 mL ethanol. An ethanolic solution of europium chloride (1.6 mL, 0.095 mol L^-1, 0.00015 mol) was then added; the resulting solution, containing Al/Zn 2:1 and Eu/Al 5:100 (molar ratios), was refluxed until formation of a homogeneous and stable sol (4 h).

Borosilicate glass substrates were previously washed with sodium dodecylsulfate, treated ultrasonically with deionized water for 30 min, followed by another 30 min treatment with ethanol and dried in ambient conditions. The film was dip-coated on the glass substrate and treated at 500 ºC for 15 min, and this procedure was repeated ten times. Bulk samples were fabricated by evaporating the initial solution and heating the solid at 500 and 700 ºC for 1 h.

The film and bulk material were characterized at room temperature by photoluminescence (PL), using a SPEX TRIAX 550 Fluorolog III spectrofluorometer. UV-Vis absorbance spectra of the film on the glass substrate were obtained on an HP 8353 diode array spectrophotometer. The X-ray diffraction (XRD) experiment for the bulk materials (500 and 700 ºC) was performed at room temperature with a Siemens/Bruker D.5005 instrument using CuK_α radiation (λ = 1.542 Å). In order to determine the thickness of the film, the substrate was fractured and imaged on a Zeiss EVO 50 scanning electron microscope (SEM).

Results and Discussion

Figure 1 shows the PL excitation spectrum of the 10-layered film with emission wavelength at 612 nm. The spectrum presents a wide band below 260 nm, which corresponds to a charge-transfer transition from O^2- to Eu³⁺ ions, as well as sharp lines of lower intensity, which correspond to f–f transitions ⁷F₀→ ⁵L₆ (393 nm) and ⁷F₀→ ⁵D₂ (462 nm).¹¹ The emission spectrum of the film, obtained with excitation at 270 and 393 nm, displays bands at 578, 590, 612, 650 and 698 nm assigned to the ⁵D₀→ ⁷F_J (J = 0, 1, 2, 3, 4) transitions, with the hypersensitive ⁵D₀→ ⁷F₂ red emission centered at 612 nm being the most prominent (Figure 2). Due to the resolution (spectral bandwidth: 1 nm), the only possible consideration about the emission spectrum is that the environment of the Eu³⁺ ion leads to a low symmetry system, as indicated by the high intensity of the ⁵D₀→ ⁷F₂ electric dipole (ED) transition as compared with the ⁵D₀→ ⁷F₁ magnetic-dipole (MD) transition.¹¹

The chromaticity coordinates (CIE 1931) were calculated using the Spectra Lux Software.¹⁶ The obtained results demonstrated high color purities: x = 0.68 and y = 0.32, which are eligible for optical applications.^11,17,18 The spectra are similar to that obtained by Barros et al.,¹¹ who also considered the europium-doped ZnAl₂O₄ as having properties required for display applications. Figure 3 exhibits the emission spectra of the film after each layer transfer from the first until the tenth, and this same figure shows that the luminescence intensity is approximately linear with respect to the number of layers. For comparison purposes, Figure 4 depicts the emission spectra of the powder (700 ºC) presenting the same features as the film spectrum. The bulk material obtained at 500 ºC remains dark due to carbonaceous impurities, and it is not appropriate for optical measurements, especially with respect to transparency studies. The close similarity of the emission spectra of the film and bulk samples with that presented by Barros et al.¹¹ led us to conclude that Eu³⁺ is embedded predominantly in a Gahnite spinel structure.

The UV-Vis spectrum of the ZnAl₂O₄:Eu³⁺ film displays low absorbance from 320 nm to 800 nm (Figure S1, Supplementary Information), resulting in good transparency in a large spectral window. It was not possible to obtain the absorption spectrum of the bulk material obtained at 500 ºC. On the other hand, the spectrum of the powder obtained at 700 ºC (without carbonaceous material) presented low absorbance above 320 nm, which is very similar to what was observed in the case of the film spectra.

Figure 5 presents the X-ray diffraction patterns of the powder obtained at 500 and 700 ºC. The main peaks are in agreement with the Gahnite spinel crystalline phase (JCPDS 01-082-1043). Analysis of the X-ray pattern depicts the formation of the main crystalline planes (220), (311), (511) and (440) at 2θ = 31, 37, 59 and 65, respectively, and it is evident that (311) is the preferential orientation, as previously reported by Barros et al.¹¹ Some less intense peaks due to ZnO are also present in the sample patterns, as described in Barros’ paper.

In order to estimate the thickness of the 10-layered film, the fractured cross-section of the substrate was imaged by SEM (Figure 6). A 300 nm thickness was observed, which led us to infer that each deposited layer has a thickness of approximately 30 nm. From this value, we can estimate that the size of the particles forming the film is less than 30 nm. Recently, Kapilashrami et al.¹⁹ used the same technique (cross-section) to determine the thickness of ZnO films measuring between ca. 150 and 1000 nm.

Conclusions

We have successfully fabricated ZnAl₂O₄:Eu³⁺ films with nanosized thickness by the dip-coating technique using a non-alkoxide sol-gel method. The films are transparent above 320 nm, and the close similarities between the emission spectra of the film and those of the bulk samples (known to have a Gahnite spinel structure as evidenced by XRD) have led us to conclude that it consists mainly of the Gahnite structure, where Eu³⁺ is embedded. Also, the determined CIE 1931 chromaticity coordinates x ≅ 0.68 and y ≅ 0.32 make this very thin film a possible component for photoelectronic devices transparent in the UV-Vis region.

Supplementary Information

The UV-Vis absorbance spectrum of the ZnAl₂O₄:Eu³⁺ film (Figure S1) is available free of charge at http://jbcs.sbq.org.br, as a PDF file.

Acknowledgments

The authors are grateful to R. F. Silva, J. F. de Lima and C. M. P. Manso for helpful discussions, and to the Brazilian research funding agencies Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for financial support and grants.

Received: November 16, 2009

Web Release Date: May 17, 2010

FAPESP helped in meeting the publication costs of this article.

Supplementary Information

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Publication Dates

• Publication in this collection
  30 July 2010
• Date of issue
  2010

History

• Received
  16 Nov 2009
• Accepted
  17 May 2010