Stokes shift in quantum dots: Origin of dark exciton

A very interesting feature of semiconductor quantum dots(QDs) is the redshift of emission peaks with respect to absorption spectra and its size dependence. The red shift of the emission spectra with respect to absorption spectra is known as Stokes shift, which is commonly observed in semiconductor quantum dots (QDs) and is one of the most important quantities that determine the optical properties of QDs. As the radius increases the redshift decreases and disappears beyond a certain radius. The mechanism of Stokes shift in semiconductor quantum dots is investigated by calculating the energy of the excitonic states. We have taken into account all possible contributions to the total electronic energy in the dot, i.e, dielectric mismatch between dot and surrounding medium, the effects of finite barrier height and electron-hole exchange interaction. The Stokes shift is calculated as a function of radius of dot and compared with experimental data on two different semiconductor based quantum dots. These results provide evidence for exchange splitting of excitonic states, as the mechanism of Stokes shift in QD.