Properties of MoO2 and MoO3 Films Prepared from the Chemically Driven Isothermal Close Space Vapor Transport Technique

TL;DR

This study investigates the preparation, phase transformation, and comprehensive characterization of MoO2 and MoO3 films produced via a chemically driven isothermal close space vapor transport technique, highlighting their optical and electrical properties.

Contribution

It introduces a novel method for preparing pure MoO2 films and provides detailed analysis of the phase transition to MoO3, including optical, electrical, and surface properties.

Findings

MoO2 transforms to MoO3 between 225-350°C without other phases.

MoO3 films exhibit high resistivity and semiconducting behavior.

Luminescence spectra of MoO3 are intense and temperature-dependent.

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 temperature_dependent Raman measurements, the MoO2/MoO3 phase transformation was found to occur in the 225 _ 350 oC 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 _ Gruneisen 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.