  Self-organized silicide nanowires are considered as main building blocks of
future nanoelectronics and have been intensively investigated. In
nanostructures, the lattice vibrational waves (phonons) deviate drastically
from those in bulk crystals, which gives rise to anomalies in thermodynamic,
elastic, electronic, and magnetic properties. Hence, a thorough understanding
of the physical properties of these materials requires a comprehensive
investigation of the lattice dynamics as a function of the nanowire size. We
performed a systematic lattice dynamics study of endotaxial FeSi$_2$ nanowires,
forming the metastable, surface-stabilized $\alpha$-phase, which are in-plane
embedded into the Si(110) surface. The average widths of the nanowires ranged
from 24 to 3 nm, their lengths ranged from several $\mu$m to about 100 nm. The
Fe-partial phonon density of states, obtained by nuclear inelastic scattering,
exhibits a broadening of the spectral features with decreasing nanowire width.
The experimental data obtained along and across the nanowires unveiled a
pronounced vibrational anisotropy that originates from the specific orientation
of the tetragonal $\alpha$-FeSi$_2$ unit cell on the Si(110) surface. The
results from first-principles calculations are fully consistent with the
experimental data and allow for a comprehensive understanding of the lattice
dynamics of endotaxial silicide nanowires.
