Excited states and size-dependent electro-optical properties of CdSxSe1-x quantum dots

The electric-field response of CdS0.44Sc0.56 quantum dots in glass is studied as a function of particle size using electroabsorption spectroscopy. Up to six quantum-size levels can be observed in the data. The evolution of the transitions through many particle size provides evidence for mixing of the valence bands due to quantum confinement. The electromodulated absorption data were fit with a first-derivative line-shape function to separate the effects of the electric field on the energy level, width, and oscillator strength associated with each electron-hole state. For the lowest excited state in particles with radius R less than the bulk exciton Bohr radius, the size dependence of the field-induced redshift is proportional to R(4) and the decrease in oscillator strength is proportional to P-6, consistent with perturbation theory. Although perturbation theory predicts the proper power-law dependence, the magnitude of the response is many times smaller than predicted. The redshift of the second excited state is found to increase as a a function of decreasing particle size with a maximum occurring for particles with radius nearly equal to bulk Bohr excition radius.