Lattice dynamics of β−FeSi2 nanorods

Abstract

Here we present a combined experimental and ab initio lattice dynamics study of the semiconducting $\ensuremath{\beta}$ phase of $\mathrm{Fe}{\mathrm{Si}}_{2}$. A polycrystalline $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}$ film was prepared on Si(111) and single-crystalline, self-assembled $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}$ nanorods were grown on Si(110) by molecular beam epitaxy. Both types of nanostructures were obtained by annealing of precursor structures, an epitaxial Fe film in the case of the film and high-aspect-ratio $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}$ nanowires in the case of the nanorods. The morphology and crystalline structure of the samples were investigated by reflection high-energy electron diffraction, atomic force microscopy, as well as x-ray diffraction and x-ray absorption spectroscopy. The Fe-partial phonon density of states (PDOS) was obtained from nuclear inelastic scattering. The PDOS of the film was investigated in the temperature range of 296 K down to 11 K and shows an excellent agreement with the ab initio calculations. In the PDOS of the nanorods, a shift in the number of states in the main features and an additional vibrational mode at 20 meV are observed. While the first effect can fully be explained by the specific orientation of the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}$ unit cell on the Si(110) surface, the second effect is attributed to the formation of an $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}$ interlayer at the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}/\mathrm{Si}$ interface. Furthermore, the thermoelastic properties of the film show a harmonic behavior in the investigated temperature range. For the nanorods, no significant deviation from the film is observed, except for a small decrease of the sound velocity.