Optical and electrical properties of MoO₂ and MoO₃ thin films prepared from the chemically driven isothermal close space vapor transport technique

Abstract

Chemically -driven isothermal close space vapour transport was used to prepare pure MoO2 films which were eventually converted to MoO3 by annealing in air. According to temperaturedependent Raman measurements, the MoO2/MoO3 phase transformation was found to occur in the 225 -350 o C range; no other phases were detected during the transition. A clear change in composition and Raman spectra, as well as noticeable modifications of the band gap and the absorption coefficient confirmed the conversion from MoO2 to MoO3. An extensive characterization of films of both pure phases was carried out. In particular, a procedure was developed to determine the dispersion relation of the refractive index of MoO2 from the shift of the interference fringes the used SiO2/Si substrate. The obtained refractive index was corrected taking into account the porosity calculated from elastic backscattering spectrometry. The Debye temperature and the residual resistivity were extracted from the electrical resistivity temperature dependence using the Bloch -Grüneisen equation. MoO3 converted samples presented very high resistivity and a typical semiconducting behaviour. They also showed intense and broad luminescence spectra, which were deconvoluted considering several contributions; and its behaviour with temperature was examined. Furthermore, surface photovoltage spectra were taken and the relation of these spectra with the photoluminescence is discussed.

Part I: NiMoO4 Nanostructures Synthesized by the Solution Combustion Method: A Parametric Study on the Influence of Synthesis Parameters on the Materials’ Physicochemical, Structural, and Morphological Properties

“…It is also evident that, at this calcination temperature, the intensity of the peak corresponding to NiO attenuated, while that of MoO 3 increased. This is reasonable since MoO 2 is stable in the temperature range of 225–350 °C, and in other studies, nanostructured MoO 3 has been found to exhibit stability between 230 °C and 490 °C [ 31 ]. The comparative analysis of the XRD patterns suggest that between 300 °C and 400 °C, a portion of MoO 2 changed to MoO 3 and reacted with NiO to produce NiMoO 4 .…”

“…The relatively small band gap of the material prepared at 300 °C is due to the significant presence of MoO 2 . Conflicting reports about the band gap of MoO 2 can be found in the literature, where some authors suggested that MoO 2 exhibits a metallic behavior, while other findings purported that this material conveys a mixed semi-conductive/metallic behavior [ 31 , 47 ]. However, reduced MoO 3 materials have been reported to attain a band gap between 2.4 and 2.7 eV, and recently Melo et al reported a value of 1.83 eV for pure MoO 2 [ 31 ].…”

“…Conflicting reports about the band gap of MoO 2 can be found in the literature, where some authors suggested that MoO 2 exhibits a metallic behavior, while other findings purported that this material conveys a mixed semi-conductive/metallic behavior [ 31 , 47 ]. However, reduced MoO 3 materials have been reported to attain a band gap between 2.4 and 2.7 eV, and recently Melo et al reported a value of 1.83 eV for pure MoO 2 [ 31 ]. Therefore, it is the large presence of MoO 2 that explains the comparatively small band gap of the sample prepared at 300 °C.…”

Structural, Optical and Electrical Properties of Al+MoO3 and Au+MoO3 Thin Films Prepared by Magnetron Codeposition

“…The reason could be the formation of Al 2 O 3 in the reaction of Al with O. The refractive index of alumina is lower than that of MoO 3 in the measured range of wavelengths [37][38][39], so the increase in the amount of Al 2 O 3 content in the film may reduce the total absorbance of the films, as observed experimentally. This is in agreement with the GIWAXS results that indicate formation of Al 2 O 3 in the Al-based films.…”

Extinction Coefficient Modulation of MoO3 Films Doped with Plasmonic Nanoparticles: From an Effective Medium Theory Description

“…To enhance the optical properties, materials with a greatest potential to be applied as an absorbing layer are the transition metal oxides (TMO’s) and among the most studied are molybdenum oxides (MoO x ) [ 12 ]. Depending on the desired applications, MoO x can be deposited or synthesized by physical or chemical techniques, respectively [ 13 ]. The deposition or synthesis techniques allow us to modify the characteristics of the oxide in order to enhance the desired properties according to the application [ 14 ].…”