Elsevier

Materials Letters

Volume 58, Issue 5, February 2004, Pages 727-732
Materials Letters

Microstructural and optical characterization of CaWO4 and SrWO4 thin films prepared by a chemical solution method

https://doi.org/10.1016/j.matlet.2003.07.002Get rights and content

Abstract

Stoichiometric CaWO4 and SrWO4 thin films were synthesized using a chemical solution processing, the so-called polymeric precursor method. In this soft chemical method, soluble precursors such as strontium carbonate, calcium carbonate and tungstic acid, as starting materials, were mixed in an aqueous solution. The thin films were deposited on glass substrates by means of the spinning technique. The surface morphology and crystal structure of the thin films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Nucleation stages and surface morphology evolution of the thin films on glass substrates were studied by atomic force microscopy. The films nucleate at 300 °C, after the coalescence of small nuclei into larger grains yielding a homogeneous dense surface. XRD characterization of these films showed that the CaWO4 and SrWO4 phases crystallize at 400 °C from an inorganic amorphous phase. No intermediate crystalline phase was identified. The optical properties were also studied. It was found that CaWO4 and SrWO4 thin films have an optical band gap, Egap=5.27 and 5.78 eV, respectively, of a direct transition nature. The excellent microstructural quality and chemical homogeneity confirmed that this soft solution processing provides an inexpensive and environmentally friendly route for the preparation of CaWO4 and SrWO4 thin films.

Introduction

In recent years, crystal structures based on the scheelite-type have attracted much interest because of their approved use as scintillating medium and in electro-optic applications [1], [2], [3]. In addition, scheelite-type structure materials have been a subject of numerous investigations on the luminescent properties [4], [5]. Typical oxides of scheelite-type structure are SrWO4, CaWO4, PbWO4 and BaWO4. In the scheelite structure, W ions are within tetrahedral O-ion cages and are isolated from each other, while Ca, Ba, Sr and Pb ions are surrounded by eight oxygen ions [6].

A variety of preparation techniques have been proposed to produce these materials, including solid-state reaction, hydrothermal, sputtering and the sol–gel processes [7], [8], [9], [10]. However, new routes for obtaining solutions for thin film deposition and crystal growth remain an interesting subject. The use of the chemical process for thin film production allows the accurate control of the chemical composition. Nowadays, new solution deposition methods, based on wet chemistry, have been used for the preparation of oxide thin films. In this regard, chemical processing using solutions, including soft solution processing, has been attracting increased interest. The soft solution processing can be defined as environmentally friendly, using aqueous solutions [11]. Together with a few techniques of soft solution processing, the polymeric precursor method [12], [13], [14], [15] can be included, because of the liquid nature of the constituents and the relatively low processing temperatures used. This technique is a very promising alternative for better three-dimensional, molecular-scale control of nanostructured materials and for being environmentally friendly. The basic idea behind the polymeric precursor methods is to reduce individualities of different metal ions, which can be achieved by encircling stable metal complexes with growing polymer networks. Immobilization of metal-complexes in such rigid organic polymeric networks can reduce the segregation of particular metals, thus ensuring the compositional homogeneity at a molecular scale. This is of vital importance for the synthesis of multicomponent oxides with complicated compositions, since the chemical homogeneity with respect to distribution of cations throughout the entire gel system often determines the compositional homogeneity of the final complex mixed oxides.

Considering that this method has been successful in preparing some oxide thin films, this work reports on the preparation and characterization of CaWO4 and SrWO4 thin films based on a soft solution processing, the so-called polymeric precursor method.

Section snippets

Experimental procedure

The flow chart for the SrWO4 and CaWO4 thin films synthesis used in this study is outlined in Fig. 1. Tungsten citrate was formed by dissolution of tungstic acid (H2WO4) (Vetec—purity 99.98%) in an aqueous solution of citric acid under constant agitation to homogenize the tungsten citrate solution. After homogenization of the solution, SrCO3 (Aldrich—purity 99.99%) or CaCO3 (Aldrich—purity 99.99%) salt was slowly added. The complex was well stirred for several hours at 70–80 °C to produce a

Results and discussion

Fig. 2, Fig. 3 show the X-ray patterns of the SrWO4 and CaWO4 thin films deposited onto glass substrates after calcination at several temperatures. A diffuse X-ray diffraction (XRD) pattern is observed at 200, 300 and 350 °C, indicating the formation of an inorganic amorphous phase after the pyrolysis process for the SrWO4 thin films. As for temperatures above 350 °C, the crystallization of SrWO4 phase is observed at 400 °C. No intermediate phase was noticed, what suggests a direct

Conclusions

We have successfully demonstrated the growth of CaWO4 and SrWO4 thin films with scheelite-type structure by a soft chemical route, called polymeric precursor method. Polycrystalline, homogeneous, dense, and crack-free thin films were successfully prepared on glass substrates using a spin-coating technique. The XRD results showed that no intermediate phase was detected and a single CaWO4 or SrWO4 phase was crystallized from an amorphous matrix. XRD results showed a tetragonal structure for the

Acknowledgements

The authors gratefully acknowledge the financial support of the Brazilian financing agencies FAPESP/CEPID, CNPq/PRONEX and CAPES.

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