  The electronic properties of hydrogenated, spherical, Si/Ge and Ge/Si
core-shell nanocrystals with a diameter ranging from 1.8 to 4.0 nm are studied
within Density Functional Theory. Effects induced by quantum confinement and
strain on the near-band-edge states localization, as well as the band-offset
properties between Si and Ge regions, are investigated in detail. On the one
hand, we prove that Si(core)/Ge(shell) nanocrystals always show a type II
band-offset alignment, with the HOMO mainly localized on the Ge shell region
and the LUMO mainly localized on the Si core region. On the other hand, our
results point out that a type II offset cannot be observed in small (diameter
less than 3 nm) Ge(core)/Si(shell) nanocrystals. In these systems, quantum
confinement and strain drive the near-band-edge states to be mainly localized
on Ge atoms inducing a type I alignment. In larger Ge(core)/Si(shell)
nanocrystals, instead, the formation of a type II offset can be engineered by
playing with both core and shell thickness. The conditions that favor the
transition from a type I to a type II alignment for Ge(core)/Si(shell)
nanocrystals are discussed in detail.
