Optical Properties and Color-Center Formation in Thin Films of Molybdenum Trioxide

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 Å with an absorption coefficient of 105 cm−1. A stoichiometric film of MoO3 shows three absorption peaks at 3350, 2880, and 2700 Å, 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 Å, three color-center bands having maxima around 5000, 6250, and 7700 Å 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 Å 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.

Production of Molybdenum Trioxide Nanosheets by Liquid Exfoliation and Their Application in High-Performance Supercapacitors

“…80,88,89 It has been previously demonstrated that oxygen vacancies, impurities and defects create additional states between the valence and conduction band resulting in a reduction of the optical band gap yielding typical fluorescence features centered around 400-450 nm (2.75-3.1 eV). 80,88,[90][91][92][93][94] Accordingly, it has been suggested that photoluminescence on MoOx (where x3) films provides a valuable handle to probe crystallinity, stochiometry and even surface roughness of the samples. 91 However, the broad near-band edge emission at < 400 nm usually obscures the lower energy features which are characteristic of sample quality.…”

Effect of a ZnSe Layer on the Thermochromic Properties of MoO3 Thin Films

“…The foregoing result is explained in terms of charge carrier injection provided from ZnSe particles into the MoO 3 film through the barrier at the interface of both materials. Moreover, the decrease of the thermochromic response above certain annealing temperatures, e.g., 200 • C for the MoO 3 films, has been associated with the bleaching of MoO 3 due to the replenishment of oxygen vacancies when thermal annealings take place in air or O 2 atmospheres [1]. A further increase in the annealing temperature leads to crystallization of the MoO 3 films [2]; this process is connected with other changes in the material properties, such as the bandgap energy, as observed in Fig.…”

“…This material is being used in an increasingly wide variety of applications, ranging from photocatalytic processes and electrochemistry to dielectric coatings. In addition, considerable attention has been given to the chromogenic properties of MoO 3 , that is, the ability to change its optical properties due to the formation of color centers when exposed to specific external agents [1,2]. In particular, when coloration is induced by heat, it is named a thermochromic material.…”

Thin films for solar control applications

“…The amorphous film has a perovskite-like type structure which consists of corner-sharing MoO 6 octahedra (Carcia & McCarron III 1987;Ramans et al 1987;Granqvist 1994) and has been produced by a number of techniques. The most common way of producing these films is by thermal evaporation from a resistive source (Deb & Chopoorian 1966;Deb 1968;Rabalais et al 1974;Yao et al 1990Yao et al , 1992Chudnovskii et al 1992). Vacuum evaporation is the standard technique; however, evaporation under slightly reactive conditions (Ramans et al 1987) and flash evaporation (Julien et al 1995) have also been used.…”