  The effect of quantum confinement on the direct bandgap of spherical Si
nanocrystals has been modelled theoretically. We conclude that the energy of
the direct bandgap at the $\Gamma$-point decreases with size reduction: quantum
confinement enhances radiative recombination across the direct bandgap and
introduces its "red" shift for smaller grains. We postulate to identify the
frequently reported efficient blue emission (F-band) from Si nanocrystals with
this zero-phonon recombination. In a dedicated experiment, we confirm the "red"
shift of the F-band, supporting the proposed identification.
