Phonons in highly-crystalline mesoporous silicon: The absence of phonon-softening upon structuring silicon on sub-10 nanometer length scales

Abstract This article presents inelastic thermal neutron scattering experiments probing the phonon dispersion in mesoporous silicon with pores 8 nm across. Scattering studies reveal the energy–momentum relation for transverse and longitudinal phonons along the high symmetry directions Γ L ¯ , Γ K ¯ and Γ X ¯ in the Brillouin zone. The dispersion up to phonon energies of 35 meV unambiguously proves that the phonon group velocities in highly-crystalline silicon are not modified by nanostructuring down to sub-10 nanometer length scales. On these length scales, there is apparently no effect of structuring on the elastic moduli of mesoprous silicon. No evidence can be found for phonon-softening in topologically complex, geometrically disordered mesoporous silicon putting it in contrast to silicon nanotubes and nanoribbons.