  Using an ab-initio molecular dynamics scheme (the Fireball scheme), we
determined the equilibrium structure of intermediate size Si_nGe_m (n+m=71)
nanoclusters with/without hydrogen passivation on the surface. Due to the
strong surface distortion, defect states are found to permeate the energy gap
of Si_nGe_m clusters. However, the defect states are removed by adding H atoms
on the surface of Si_nGe_m clusters, and the gap opens up to a few eV,
indicating a blueshift for photoluminescence. It is also found that the
radiative transition between the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) states is enhanced by one to two
orders of magnitude for Si_nGe_m nanoclusters with respect to the corresponding
pure Si clusters. This significant increase of the emission probability is
attributed to the strong overlap of HOMO and LUMO wavefunctions that are
centered mostly on the Ge atoms.
