Calculations of the electron-energy-loss spectra of silicon nanostructures and porous silicon

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Abstract
The electronic excitations in quantum-size silicon molecules, wires, or spheres are investigated by the evaluation of the full frequency-dependent dielectric matrix of the system. The calculation is based on a tight-binding framework in the random-phase approximation. The energy-loss spectra derived for fast electrons interacting with the nanostructures are dominated by collective excitations corresponding to bulk and surface modes even for nanostructures containing a small number of atoms. In contrast to the static screening, the dynamical properties are not strongly affected by the quantum confinement and are well described by the classical theories. We show that these collective modes are only slightly sensitive to surface defects and that low-energy excitations below 8 eV are only observable for one-dimensional silicon molecules, e.g., for polysilanes. These results are used to discuss the recent experimental observations made on fresh and oxidized porous silicon.