Microstructural and optical characterization of CaWO 4 and SrWO 4 thin films prepared by a chemical solution method M.A.M.A. Maurera a,b , A.G. Souza b, * , L.E.B. Soledade a , F.M. Pontes a , E. Longo a , E.R. Leite a , J.A. Varela c a LIEC, CMDMC, Department of Chemistry, UFSCar, Via Washington Luiz, km 235, CP-676, CEP-13565-905 Sa ˜o Carlos, S.P., Brazil b CCEN, Departmento de Quimica, Universidade Federal da Paraiba, Campus I, Joa ˜o Pessoa, Paraiba 58059 900, Brazil c Institute of Chemistry, UNESP, Araraquara, S.P., Brazil Received 6 January 2003; received in revised form 1 July 2003; accepted 5 July 2003 Abstract Stoichiometric CaWO 4 and SrWO 4 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 jC, after the coalescence of small nuclei into larger grains yielding a homogeneous dense surface. XRD characterization of these films showed that the CaWO 4 and SrWO 4 phases crystallize at 400 jC from an inorganic amorphous phase. No intermediate crystalline phase was identified. The optical properties were also studied. It was found that CaWO 4 and SrWO 4 thin films have an optical band gap, E gap =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 CaWO 4 and SrWO 4 thin films. D 2003 Elsevier B.V. All rights reserved. Keywords: CaWO 4 ; SrWO 4 ; Scheelite-type structure; Chemical solution method; Thin films 1. 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 [1–3]. In addition, scheelite-type structure materials have been a subject of numerous investigations on the lumines- cent properties [4,5]. Typical oxides of scheelite-type struc- ture are SrWO 4 , CaWO 4 , PbWO 4 and BaWO 4 . 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, hy- drothermal, sputtering and the sol – gel processes [7–10]. 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 in- creased 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 [12–15] 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 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.07.002 * Corresponding author. Fax: +55-83-216-7441. E-mail address: gouveia@quimica.ufpb.br (A.G. Souza). www.elsevier.com/locate/matlet Materials Letters 58 (2004) 727 – 732