Optical and electrochromic properties of heated and annealed MoO 3 thin films
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Heating and annealing effects on the optical properties of slightly blue substoichiometric molybdenum trioxide thin films are reported partially. During heating and annealing, different levels of coloration seem to be generated by simultaneous reduction and proton injection into thin films, which cause the formation of hydrogen molybdenum bronzes of different concentrations. The optical band gap, Eo, of MoO3 thin film when annealed in between 313 and 473 K is decreased to 2.82 eV, and this reduction in Eo has been explained with a view to phonon and polaron species. Moreover, temperature-dependent change in the optical band gap has also been interpreted in terms of band gap slope, B, and the real part of the refractive index, n. Urbach slope, , also goes down to 0.67 eV−1 due to annealing treatment. Using the oscillator model, a phonon energy, ho, of about 0.08 eV was found for the reported MoO3 thin film which is very close to the characteristic phonon energy of MoO3. Over and above, the intensity for the blue absorption band, over the photon energy range 0.4–3.0 eV, also reaches a maximum as MoO3 thin film is annealed at 473 K for 1 h and then decreases by 32% after annealing at the same temperature for 2 h. This phenomenon is due to polaronic band intensity which rises and falls during annealing. Half-width of the blue band, as found by fitting polaron model, is changed only by 3% on annealing the MoO3 thin film at 473 K as compared to its value at room temperature.
I. INTRODUCTION
Many kinds of electrochromic materials have been discovered, such as inorganic compounds including MoO3,1 V2O5,2 NiO,3 and organic compounds,4 but the study of transition metal trioxides (WO3, MoO3) is of great interest, in relation to the electrochromic effect, especially because of its application in display devices5,6 and optical “smart windows.”6–8 Moreover, a wide variety of transition metal compounds based on either twodimensional van der Waals bonded layer structures or three-dimensional framework tunnel structures are also of great interest as potential electroactive materials for high-energy density secondary lithium batteries,9,10 and in this relation, layerlike compounds of V2O5 and MoO3 have been successfully used as cathodes in microbattery systems.11–14 Another important feature of these oxides is that WO3 and MoO3 thin films act as either a negative or a positive resist15,16 with high-contrast capability for focused ion beam exposure depending on the preparation conditions. This resist work is potentially useful for both nanostructures fabrication17 and nanometer-width refractory metal (W or Mo) writing.18,19 J. Mater. Res., Vol. 16, No. 9, Sep 2001
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Molybdenum oxides are not only important from an academic point of view but are also vital due to polymorphic changes in MoO3 and, therefore, have technological applications such as the optical data storage of display schemes and recording of stable thin holograms20 and also acting as catalysts.8,21 In addition, th
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