Surface-passivation-induced optical changes in Ge quantum dots

One of the most interesting properties of quantum dots is the possibility to tune the band gap as a function of their size. Here we explore the possibility of changing the lifetime of the lowest-energy excited state by altering the surface passivation. We show that a moderately electronegative passivation potential can induce long-lived excitons without appreciable changes to the band gap. In addition, for such passivation the symmetry of the valence-band maximum is ${\gamma }_{8_v}\hspace{0.5em}{(t}_1$ derived) instead of the more usual ${\gamma }_{8v}\hspace{0.5em}{(t}_2$ derived). This reverses the effect of the exchange interaction on the bright-dark exciton splitting.