Exciton-phonon coupling in ZnSe/CdSe alloyed quantum dots

Zinc to cadmium cation exchange of ZnSe quantum dots has been used to produce a series of alloyed Zn$_{\mathrm{1-x}}$Cd$_{\mathrm{x}}$Se quantum dots. As x increases, the lowest-energy exciton shifts to the red, initially broadening and then sharpening as x approaches 1. Resonance Raman spectra obtained with excitation near the lowest excitonic absorption peak show a gradual shift of the longitudinal optical phonon from 251 cm$^{\mathrm{-1}}$ in pure ZnSe to 210 cm$^{\mathrm{-1}}$ in nearly pure CdSe with strong broadening at intermediate compositions. The overtone to fundamental intensity ratio, a rough gauge of exciton-phonon coupling strength, is much larger for intermediate compositions than for either pure ZnSe or pure CdSe. Partial localization of the hole in locally Cd-rich regions of the alloyed particles increases the strengths of local internal electric fields, increasing the coupling between the exciton and polar optical phonons.