Colour centre studies in MoO3 films
TL;DR: The growth of defects in vacuum-evaporated MoO3 films has been studied by optical absorption in the present work as discussed by the authors, where as-grown films were subjected to different kinds of treatment such as thermal annealing, UV and X-ray irradiation, and their optical absorption spectra were recorded between 300 and 1500 nm at room temperature as well as at liquid N2 temperature.
Abstract: The growth of defects in vacuum-evaporated MoO3 films has been studied by optical absorption in the present work. The as-grown films were subjected to different kinds of treatment such as thermal annealing, UV and X-ray irradiation, and their optical absorption spectra were recorded between 300 and 1500 nm at room temperature as well as at liquid N2 temperature The films were found to be amorphous from X-ray and electron-microscopic studies. From these studies it is observed that in addition to point defects, defect aggregates similar to the colloidal centres in silver and alkali halides, are present in these films.
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TL;DR: In this paper, the progress in all of these areas will be reviewed thoroughly, including the early stage of the research, the band structure, photochromic mechanism and the behavior of the oxide.
Abstract: Molybdenum oxide can exhibit pronounced photochromism and thus might act as an excellent photonic material for a number of technical applications. In the early stage of the research, the attention was focused mainly on the (band) structure, photochromic mechanism and the behavior of the oxide. Later, many investigations were carried out on the factors that might influence the photochromic performance. At the same time, the photochromic response has been extended from UV light to visible light. In this review, the progress in all of these areas will be reviewed thoroughly.
258 citations
TL;DR: In this paper, thin film polymorphs of molybdenum trioxide have been synthesized by reactive r.f. sputtering in an atmosphere of argon plus oxygen.
162 citations
TL;DR: In this paper, the properties of rf-sputtered molybdenum oxide films for electrochromic display devices have been investigated, and the dependence of the properties on the oxygen concentration in the sputtering atmosphere was mainly concerned.
Abstract: Properties of rf‐sputtered molybdenum oxide films for electrochromic display devices have been investigated. This report mainly concerns the dependence of the properties on the oxygen concentration in the sputtering atmosphere. The oxide films were prepared by rf sputtering from a compressed powder MoO3 target under an operating pressure of 4×10−2 Torr using a mixture of Ar‐0.5–50% O2. Electrical resistivity of the films formed increases with increasing oxygen concentration in the sputtering atmosphere and ranges from 2.5×1010 to ∼1×1012 Ω cm. Spectral transmittance of the films is about 85% in the visible and near‐infrared region. Optical band gap and refractive index of these films are 2.67–2.76 eV and 2.01–2.26, respectively. Electrochemichromic properties of the films were also studied using asymmetric cells, and it was found that a good electrochromic performance was obtained usually by the cells composed of the films with a resistivity of 2.5×1010 Ω cm.
109 citations
TL;DR: Molybdenum oxide thin films find diverse applications as catalysts, gas sensors, and electrochromic devices as mentioned in this paper, and are produced mainly by reactive sputtering and thermal evaporation but other...
Abstract: Molybdenum oxide thin films find diverse applications as catalysts, gas sensors, and electrochromic devices. Such films are produced mainly by reactive sputtering and thermal evaporation but other ...
91 citations
TL;DR: In this article, the optical properties of slightly blue molybdenum trioxide thin films were investigated with a view to phonon and polaron species and 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.
Abstract: 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, σ, also goes down to 0.67 eV−1 due to annealing treatment. Using the oscillator model, a phonon energy, hνo, 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.
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TL;DR: In this paper, the optical absorption spectra of amorphous and crystalline thin Wo3 films have been measured in the temperature range 110° to 500°K and the temperature coefficient of the band edges was found to be − 5.0 × 10−4 eV/°K.
Abstract: Thin films of Wo3 deposited on quartz substrates at room temperature have been shown to be amorphous in structure. The optical absorption spectra of the amorphous and crystalline films have been measured in the temperature range 110° to 500°K. The fundamental absorption edge of an amorphous film occurs at 3800 A which on crystallization moves to 4500 A. On the high-energy side of the absorption edge several absorption peaks are resolvable in both types of film. The frequency dependence of the absorption coefficient below 104 cm−1 is described by an expression of the form K (v, T) = K 0 exp[− (β/kT) (E 0 − hv)] and above 104 cm−1 it follows a square law dependency. The temperature coefficient of the band edges was found to be − 5.0 × 10−4 eV/°K and the estimated band gaps at 0°K were found to be 3.65 and 3.27 eV for the amorphous and crystalline films, respectively. The electrical conductivity of a thin film has been measured in the temperature range 298–573°K and the activation energy was found t...
933 citations
TL;DR: In this paper, an energy-level diagram has been proposed to account for the optical and electrical properties as well as the color center formation in MoO3, and the growth and decay of color centers has been studied in detail.
Abstract: The optical absorption spectrum of thin films of MoO3 has been measured over the temperature range from 77° to 290°K. The fundamental absorption edge occurs at 3300 A with an absorption coefficient of 105 cm−1. A stoichiometric film of MoO3 shows three absorption peaks at 3350, 2880, and 2700 A, respectively, and these are suggested to be due either to exciton formation or to transitions involving a split valence band.Values for the refractive index have been measured in the wavelength range 0.42–1.2μ. The low‐frequency dielectric constant (18.0±1) has been measured and the comparison with the high‐frequency dielectric constant (5.70) indicates that the bonding is predominantly ionic.The electrical conductivity has been measured on a polycrystalline sample in the temperature range 298° to 900°K and the activation energies of conduction found to be 1.83 eV (intrinsic) and 0.56 eV (impurity).On irradiating a thin film (0.2μ) of MoO3 with light of wavelength shorter than 3300 A, three color‐center bands having maxima around 5000, 6250, and 7700 A are formed. The same type of color centers are also formed on heating the film in oxygen in inert atmosphere. The color centers thus formed cannot be bleached optically. However, they can be bleached thermally in oxygen at 300°C. The growth and decay of color centers has been studied in detail.The formation of color centers is associated with increased dark conductivity of the MoO3. Very weak photoconductivity and photovoltaic effects with threshold energy corresponding to the fundamental absorption edge at 3300 A have been observed.An ESR signal characteristic of Mo5+ state has been observed in a film of MoO3. There is a small increase of ESR signal on color‐center formation which disappears completely on heating in air at 300°C.An energy‐level diagram has been proposed to account for the optical and electrical properties as well as the color‐center formation in MoO3.
260 citations
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