  The exciton states of strained CdTe/CdS core-shell tetrapod-shaped
nanocrystals were theoretically investigated by the numerical diagonalization
of a configuration interaction Hamiltonian based on the single-band effective
mass approximation. We found that the inclusion of strain promotes the type-II
nature by confining the electrons and holes in nonadjacent regions. This
carrier separation is more efficient than that with type-II spherical
nanocrystals. The strain leads to a small blue shift of the lowest exciton
energy. Its magnitude is smaller than the red shift induced by increasing the
shell thickness. Moreover, the larger arm diameter reduces the influence of
both shell thickness and strain on the exciton energy. Considering the broken
symmetry, a randomness induced carrier separation was revealed which is unique
to a branched core-shell structure. The calculated shell thickness dependence
of the emission energy agrees well with available experimental data.
