Effect of Variations in Annealing Temperature and Metallic Cations on Nanostructured Molybdate Thin Films
© to the authors 2008
Received: 17 January 2008
Accepted: 26 March 2008
Published: 29 April 2008
Crystalline molybdate thin films were prepared by the complex polymerization method. The AMoO4(A = Ca, Sr, Ba) films were deposited onto Si wafers by the spinning technique. The Mo–O bond in the AMoO4structure was confirmed by FTIR spectra. X-ray diffraction revealed the presence of crystalline scheelite-type phase. The mass, size, and basicity of A2+cations was found to be dependent on the intrinsic characteristics of the materials. The grain size increased in the following order: CaMoO4 < SrMoO4 < BaMoO4. The emission band wavelength was detected at around 576 nm. Our findings suggest that the material’s morphology and photoluminescence were both affected by the variations in cations (Ca, Sr, or Ba) and in the thermal treatment.
KeywordsMolybdates Thin films Complex polymerization method
Molybdates with scheelite-type structures (tetragonal symmetry) are able to produce green luminescence. These compounds have attracted much attention due to their application as phosphors and as scintillating materials in electrooptical applications like solid-state lasers and optical fibers [1, 2].
Molybdates thin films have a wide variety of applications in electronics, optics, ionics, and anticorrosive coatings. A great deal of interest has focused on the preparation of molybdate thin films due to their optical properties and nanometric scale [3–6].
BaMO4 (BMO), SrMO4 (SMO), and CaMO4 (CMO) powders with a scheelite structure have been synthesized by a variety of techniques, including combustion synthesis, the Czochralski technique, co-precipitation by conventional solid-state reaction, and the complex polymerization method (CPM) [1, 5–9]. However, references in the literature about the synthesis of AMoO4 (AMO) thin films are rare and the most commonly cited method is the electrochemical route [3, 10–12]. The electrochemical method is limited because it uses only Mo substrates to prepare AMO films .
Another limitation of the electrochemical method is its lack of homogeneity. BMO thin films produced by an electrochemical method showed a heterogeneous surface and grain sizes varying from 2 to 10 μm . When the CPM is used, the metal complexes become immobilized in a rigid organic polymeric network, reducing the segregation of particular metals and thus ensuring compositional homogeneity on a molecular scale . The additional advantage of CPM is that it does not require expensive equipment or reagents, deposition in high vacuum chambers or special atmospheric control.
The [MoO4 2− ionic group in the AMO scheelite structure, which has strong covalent Mo_O bonds, is coupled to Ca2+, Sr2+, or Ba2+ cations [4–7]. Many authors attribute the photoluminescent property in these materials to the MoO4 2− cluster and its possible intermediary states (MoO4···O·MoO3, MoO4···O··· MoO4) [5–7, 14]. Because the property of photoluminescence (PL) is associated with distortion in tetrahedra, it is reasonable to assume that PL can be affected by metallic cations.
In the present work, we evaluated the effects on molybdate thin films resulting from variations in the metallic cations and thermal treatment. Crystalline AMO thin films were prepared by the CPM, and were characterized by various techniques, such as X-ray diffraction (XRD), optical reflectance spectroscopy (ORS), Fourier transform infrared reflectance (FTIR), atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), and photoluminescence spectroscopy (PLS).
The materials used in this study were molybdenum trioxide MoO3(Synth 85%), BaCO3, SrCO3, and BaCO3(Mallinckrodt 99%), citric acid (H3C6H5O7) (Mallinckrodt 99%) and ethylene glycol (HOCH2CH2OH) (J. T. Baker 99%). All the chemicals were used without further purification.
Preparation of AMO Thin Films (A = Ca2+, Sr2+, Ba2+)
AMO thin films were produced by deposition of the polymeric resin obtained by CPM onto Si substrates. In this synthesis, the molybdenum citrate was formed by dissolving molybdic acid (MoO3—molybdenum trioxide) in an aqueous solution of citric acid under constant stirring at 60–80 °C to homogenize the molybdenum citrate solution (the molar ratio of citric acid to molybdic acid was 6:1). The solution was homogenized, after which the metallic carbonate (Ca2+, Sr2+, and Ba2+) was dissolved and a stoichiometric amount added to the molybdenum citrate solution. The complex was stirred thoroughly for several hours at 60–80 °C to produce a clear and homogeneous solution. Ethylene glycol was then added to polymerize the citrate by polyesterification. The viscosity of the solution increased under continual heating at 80–90 °C, although no phase separation was detected. The molar ratio of metallic cations to molybdenum cations was 1:1. The citric acid/ethylene glycol mass ratio was set at 60:40. The viscosity of the deposition solution was adjusted to 15 mPa/s by controlling the water content.
A total of six thin films were produced using Si(100) wafers as substrates. The substrates were spin-coated by dropping a small amount of the polymeric resin onto them. Rotation speed and spin time were fixed at 700 rpm for 3 s and 7200 rpm for 30 s, using a commercial spinner (Chemat Technology KW-4B spin-coater). After deposition, the films were heat-treated at 80 °C for 30 min, 100 °C for 20 min and 200 °C for 2 h, applying a heating rate of 1 °C/min. Four layers were deposited and the procedure was repeated for each layer. In the last layer, after the heat treatment at 200 °C, two different crystallization procedures were applied, one involving heat treatments at 600 °C for 2 h in a resistance furnace (RF) and the other at 600 °C for 10 min in a microwave (MW) oven.
Characterization of AMO Thin Films
The AMO thin films were characterized by XRD, using CuK α radiation to determine the resulting phases. The optical reflectance was measured in the wavelength range of 200–800 nm, using a UV–vis–NIR Cary 5G spectrophotometer. Fourier transform infrared reflectance spectra for thin films were recorded in the frequency range of 400–2100 cm−1at room temperature, using an Equinox/55 (Bruker) spectrometer equipped with a 30° specular reflectance accessory. A reconstructed 3D image of the surface of the sample was obtained by atomic force microscopy (AFM), using a Digital Instruments Multi-Mode Nanoscope IIIa microscope. This type of image allows for accurate analysis and quantification of highly relevant parameters such as roughness and grain size. The microstructure and surface morphology of the thin films were observed by high resolution scanning electron microscopy (HR-SEM), using a field emission gun (Gemini-Zeiss Supra35). Photoluminescence spectra of AMO thin films were recorded with a Jobin Yvon-Fluorolog spectrofluorometer coupled to a 450 W xenon lamp. All the measurements were taken at room temperature.
Results and Discussion
Lattice constants and cell volume of the tetragonal structure of AMO thin films
Lattice constants (Å) 
Lattice constants (Å)a
Cell volume (Å3) 
Cell volume (Å3)a
F 2(ν 3) vibration in FTIR absorption spectra of AMO thin films
VibrationsF 2(ν 3) (cm−1)
As can be seen in Fig. 4, the thin films heat-treated in the RF and the MW showed distinct morphologies. The thin film heat-treated in the MW tended to be continuous, while the film treated in the RF tended to form nanoislands. This is probably because the temperature in the RF rises more slowly than in the MW. The slow increase in temperature facilitates shrinking of the polymeric resin on the substrate surface, giving rise to nanoisland structures. Because the nanoisland structures were extremely thin, it was difficult to see the thickness of thin films heat-treated in the RF, and we were unable to ascertain the thickness of the CaMoO4thin film heat-treated in the RF.
The micrographs indicate that the thickness, grain size, and type of morphology are dependent on the metallic cations and the heat treatment history. The lattice constants and cell volume of the tetragonal structure of AMO thin films revealed by X-ray diffraction decreased linearly with a diminishing ionic radius. Therefore, the grain size is expected to increase in the following order: CMO < SMO < BMO. As expected, the grain sizes were about 78.38, 83.18, and 133.25 nm, respectively, for CMO, SMO, and BMO thin films. The thickness proved to be dependent on the grain size; therefore, it followed the same order of growth. The morphological structure was devoid of cracks, homogeneous, and distinct in all the films.
The crystalline scheelite-type phase was confirmed in the BMO, SMO, and CMO thin films using the aforementioned characterization techniques. Processing thin films by CPM proved efficient and involved little cost, since this method does not require expensive equipment or reagents, deposition in high vacuum chambers or special atmospheric control. The mass, size, and basicity of the A2+cation was found to be dependent on the intrinsic characteristics of the materials. Our findings suggest that both the morphology and the PL of the material are affected by variations in the cations (Ca, Sr, or Ba) and in the thermal treatment. These results confirmed that AMO thin films are highly promising candidates for PL applications.
This work was supported by CNPq, FAPESP-CEPID, and CAPES.
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