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Register a new userProperties of MoO2 and MoO3 Films Prepared from the Chemically Driven Isothermal Close Space Vapor Transport Technique
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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.
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A novel procedure, based in a closed space vapor transport (CSVT) configuration, has been devised to grow films or flakes of pure MoO2 in a reductive atmosphere, at relatively low temperature and using MoO3 as the source. In contrast with conventional CSVT technique, in the proposed method a temperature gradient is not required for the growth to take place, which occurs through an intermediate volatile transport species that is produced in the complex reduction reaction of MoO3. An added value of this simple method is the possibility of transforming the MoO2 into MoTe2, one of the most interesting members of the transition metal dichalcogenide family. This is achieved in a sequential process that includes the growth of Mo oxide and its (in-situ) tellurization in two consecutive steps.
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