CALCULATIONS OF THE ELECTRON-ENERGY-LOSS SPECTRA OF SILICON NANOSTRUCTURES AND POROUS SILICON

The electronic excitations in quantum-size silicon molecules, wires, o r spheres are investigated by the evaluation of the full frequency-dep endent 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 nanostruc tures are dominated by collective excitations corresponding to bulk an d surface modes even for nanostructures containing a small number of a toms. In contrast to the static screening, the dynamical properties ar e not strongly affected by the quantum confinement and are well descri bed by the classical theories. We show that these collective modes are only slightly sensitive to surface defects and that low-energy excita tions below 8 eV are only observable for one-dimensional silicon molec ules, e.g., for polysilanes. These results are used to discuss the rec ent experimental observations made on fresh and oxidized porous silico n.