Thin Film of ZnAl 2 O 4 : Eu 3 + Synthesized by a non-Alkoxide Precursor Sol-Gel Method

Filmes de aluminato de zinco dopados com európio (ZnAl 2 O 4 :Eu) 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 D 0 → F J (J = 0, 1, 2, 3, 4). Os filmes mostraram-se transparentes acima de 320 nm.


Introduction
In recent years, spinel-type zinc aluminate (ZnAl 2 O 4 ) has become technologically important due to its peculiar optical properties.2][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. 4,5Bulk non-doped ZnAl 2 O 4 has been prepared by the sol-gel process employing alkoxides as precursors, as well as by coprecipitation and wet mixing. 6,7Phani et al. 8 have obtained ZnAl 2 O 4 thin films by utilizing an alkoxide solgel route and deposition by spin-coating.Davolos et al. 9 have successfully prepared bulk zinc aluminate by a nonalkoxide sol-gel method.1][12][13][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. 15e 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 earthdoped ZnAl 2 O 4 using a non-alkoxide sol-gel process.In the present work, thin films of photoluminescent ZnAl 2 O 4 :Eu 3+ have been synthesized by a non-alkoxide precursor sol-gel process using the dip-coating technique.
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 (l = 1.542Å).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 Eu 3+ ions, as well as sharp lines of lower intensity, which correspond to f-f transitions 7 F 0 → 5 L 6 (393 nm) and 7 F 0 → 5 D 2 (462 nm). 11The 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 5 D 0 → 7 F J (J = 0, 1, 2, 3, 4) transitions, with the hypersensitive 5 D 0 → 7 F 2 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 Eu 3+ ion leads to a low symmetry system, as indicated by the high intensity of the 5 D 0 → 7 F 2 electric dipole (ED) transition as compared with the 5 D 0 → 7 F 1 magnetic-dipole (MD) transition. 11he chromaticity coordinates (CIE 1931) were calculated using the Spectra Lux Software. 16The obtained results demonstrated high color purities: x = 0.68 and y = 0.32, which are eligible for optical applications. 11,17,18he spectra are similar to that obtained by Barros et al., 11 who also considered the europium-doped ZnAl 2 O 4 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 Eu 3+ is embedded predominantly in a Gahnite spinel structure.
The UV-Vis spectrum of the ZnAl 2 O 4 :Eu 3+ 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 ZnAl 2 O 4 :Eu 3+ 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 Eu 3+ 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.