  The study of charge and spin transport through semiconductor quantum dots is
experiencing a renaissance due to recent advances in nano-fabrication and the
realization of quantum dots as candidates for quantum computing. In this work,
we combine atomistic electronic structure calculations with quantum master
equation methods to study the transport of electrons and holes through strongly
confined quantum dots coupled to two leads with a voltage bias. We find that a
competition between the energy spacing between the two lowest quasiparticle
energy levels and the strength of the exchange interaction determines the spin
states of the lowest two quasiparticle energy levels. Specifically, the low
density of electron states results in a spin singlet being the lowest energy
two-electron state whereas, in contrast, the high density of states and
significant exchange interaction results in a spin triplet being the lowest
energy two-hole state. The exchange interaction is also responsible for spin
blockades in transport properties, which could persist up to temperatures as
high as 77K for strongly confined colloidal quantum dots from our calculations.
Lastly, we relate these findings to the preparation and manipulation of singlet
and triplet spin qubit states in quantum dots using voltage biases.
