  It is possible to confine vibrational modes to silicon nanocrystals by
encapsulating them within hydrogenated amorphous silicon. This is not because
of the small impedance mismatch between materials but, rather, is due to higher
order moments in the distribution of density and stiffness in the amorphous
phase--i.e. it is a result of material substructure. The concept is elucidated
using an idealized one-dimensional setting and then demonstrated for a
realistic nanocrystalline geometry. Beyond the immediate focus on amorphous
encapsulation, this offers the prospect of specifically engineering higher
order property distributions as an alternate means of managing phonons. The
approach could be applied, for instance, to design deterministically ordered
materials which exploit this means of control.
