Paper SS+AS-TuA4
Vibrational Spectroscopy of Iron Oxide Nanostructures and Thin Films Supported on Graphite

Tuesday, November 8, 2016, 3:20 pm, Room 104E

Iron oxide nanostructures supported on highly oriented pyrolytic graphite have been investigated with high resolution electron energy loss spectroscopy (HREELS) and Auger electron spectroscopy (AES). The average O:Fe ratio, as measured by AES, can be increased or decreased by annealing in an oxygen background of 1x10^-7 Torr or in vacuo, respectively. Depending on annealing temperature, and oxygen exposure, the O:Fe ratio can range from near metallic to hematite (Fe₂O₃) stoichiometry. Regardless of stoichiometry, no iron oxide vibrational modes were observed in the specular HREELS spectra. Only the collective free charge carrier excitation of the graphite substrate was observed. The absence of iron oxide modes in specular HREELS is due to an electrostatic screening from the surface dipole generated by the collective graphite excitation. This screening effect is supported by calculations of the electron energy loss function for a thin iron oxide film supported on graphite. Off specular HREELS shows that the graphite phonon dispersion is unperturbed by the presence of iron oxide nanostructures. Thus, there is minimal interaction between the graphite substrate and the supported iron oxide nanostructures. HREELS spectra of water and carbon monoxide adsorbed on iron oxide nanostructures show hindered vibrational modes. The intensity of the hindered mode is high when compared to the intramolecular modes and the elastic peak. The intensity enhancement is due to a resonance effect between the hindered mode and a longitudinal phonon mode of the nanoparticle i.e. a substrate Fermi resonance. A more descriptive interpretation of this resonant enhancement and finite relaxation lifetime based off perturbation theory will be discussed.