Optical Transmittance of Thin Films of SrFeO 2.5+x at Elevated Temperatures and Applications to Gas Sensing
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EXPERIMENTAL Preparation of Perovskites and Films Polycrystalline samples of SrFeO2.5+x were prepared from stoichiometric mixtures of SrCO3 and Fe203 in a high temperature sintering process under oxygen or argon as described earlier [3]. Pellets of this material for use as targets for film deposition were then formed by isostatic pressing and thermal annealing. The thin-films, (200-300nm), of either SrFeO2.5 or SrFeO~3 were grown onto optically transparent (1102) sapphire substrates by pulsed laser ablation-deposition, (PLD), using an excimer laser operating with Kr/F mixtures giving 248nm radiation. A stainless steel chamber, heated substrate mount and low p(O2) atmospheres were used during PLD; details of the system and other conditions used are reported elsewhere [5]. Spectrophotometric Batch and In-Situ Measurements Optical data were obtained with a Cary 1E spectrophotometer. The transmittance, (Y), spectra were measured in experiments of two types; a) where the films had the oxygen composition, (x), adjusted in a batch method, and b) where the films were exposed to flowing 02/N 2 mixtures in-situ at elevated temperature. In the batch method, a series of films was exposed to 02 at T = 670K in a low volume, (V-25cm 3),gas manifold which facilitated the controlled addition of oxygen at partial pressures over the range 10-4 < p (02) < 105 Pa. The films were then cooled to ambient temperature under as close to equilibrium conditions as possible, depending on p(02), and the transmittance spectra recorded over the wavelength range 190 < X• 600K), the in-situ transmittance data have been collected for films over a smaller wavelength range, (400 _ X 5 650 nm). Typically, during the collection of preliminary spectrophotometric data, the film is initially under a flow of nitrogen and then the gas phase composition is changed to a N2/02 mixture. Several spectra are then collected at various time intervals; finally a return to flowing N2 is done to confirm reversibility. RESULTS AND DISCUSSION The films at each end of the oxygen composition range showed distinctly different colouration. Those with compositions close to SrFeO2.5 were light yellow and transparent, while those with compositions toward the oxygen rich end of the range, i.e. SrFeO~ 3 , were dark silverblack, mirror like and almost opaque. At elevated temperatures, the change between these two forms has been shown to be reversible when exposed to cyclic changes in p (02) between low and high values [4,6]. Transmittance spectra for four different films of thickness d-300nm, for which x was adjusted in the batch technique, are shown in Fig. 2. For SrFeO2.5, (Fig. 2a), optical transmission is moderate to high for X > 400nm. Below this wavelength, transmittance decreases to a minimum of T = 0.05 in the near uv at X = 300nm. For SrFeO~3, (Fig. 2b), transmittance is low over the whole wavelength range, approaching its largest value, T - 0.05, at X - 900nm. The highest5 degree of opacity occurs for the wavelength range 390 < X < 400nm, where a transmittance 9x10- < T
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