Excited states and size-dependent electro-optical properties of ${\mathrm{CdS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{-}\mathit{x}}$ quantum dots

The electric-field response of ${\mathrm{CdS}}_{0.44}$ ${\mathrm{Se}}_{0.56}$ 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 sizes 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 ∝${\mathit{R}}^4$ and the decrease in oscillator strength is ∝${\mathit{R}}^6$, 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 field-induced redshift of the second excited state is found to increase as a function of decreasing particle size with a maximum occurring for particles with radius nearly equal to bulk Bohr exciton radius. © 1996 The American Physical Society.