RESUMEN
Metal oxides are suitable for detecting, through conductive measurements, a variety of reducing and oxidizing gases in environmental and sensing applications. Metal-oxide gas sensors can be developed with the goal of sensing gases under specific conditions and, as a whole, are heavily dependent on the manufacturing process. Tungsten oxide (WO3) is a promising metal-oxide material for gas-sensing applications. The purpose of this paper is to determine the existence of a correlation between thermal effects and the changes in the Raman spectra for multiple WO3 structures. We have obtained results utilizing Raman spectroscopy for three different structures of WO3 (monoclinic WO3 on Si substrate, nanopowder, and nanowires) that have been subjected to temperatures in the range of 30-160 °C. The major vibrational modes of the WO3:Si and the nanopowder samples, located at ~807, ~716, and ~271 cm(-1), correspond to the stretching of O-W-O bonds, the stretching of W-O, and the bending of O-W-O, respectively; these are consistent with a monoclinic WO3 structure. However in the nanowires sample only asymmetric stretching of the W-O bonds occurs, resulting in a 750 cm(-1) band, and the bending of the O-W-O mode (271 cm(-1)) is a stretching mode (239 cm(-1)) instead, suggesting the nanowires are not strictly monoclinic. The most notable effect of increasing the temperature of the samples is the appearance of the bending mode of W-OH bonds in the approximate range of 1550-1150 cm(-1), which is related to O-H bonding caused by humidity effects. In addition, features such as those at 750 cm(-1) for nanowires and at 492 and 670 cm(-1) for WO3:Si disappear as the temperature increases. A deeper understanding of the effect that temperature has on the Raman spectral characteristics of a metal oxide such as WO3 has helped to extend our knowledge regarding the behavior of metal oxide-gas interactions for sensing applications. This, in turn, will help to develop theoretical models for the identification of specific metal oxide-gas relationships.
RESUMEN
Ultrashort laser-induced breakdown spectroscopy was used to detect the emission radiation from the breakdown of surface contaminants by a femtosecond laser pulse. This study focused on the detection of visible to near-infrared radiation signatures from molecular fragments of the nitro (NO(x)) group present in the breakdown plasma, where target chemicals of potassium nitrate (KNO(3)) and sodium nitrate (NaNO(3)) were used. Spectral signatures at a wavelength region around 410 nm were observed for both KNO(3) and NaNO(3), and were identified as the fluorescence transitions of the NO(x)-molecular structures. The signatures obtained were systematically analyzed and studied as functions of laser parameters. It is shown that for laser parameters used in this study, laser pulse durations ≥1 ps were not as effective as shorter pulses in generating these signatures. A visible wavelength NO(x) signature and the extended high-intensity propagation of a femtosecond laser could be advantageous to detecting nitro-group energetic materials at standoff distances.
