Nonuniform Excitonic Charge Distribution Enhances 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 ZnCdSe quantum dots. As x increases and the lowest-energy exciton shifts to the red, the peak initially broadens and then sharpens 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 peak from 251 cm in pure ZnSe to 210 cm in nearly pure CdSe with strong broadening at intermediate compositions. The LO overtone to fundamental intensity ratio, a rough gauge of exciton-phonon coupling strength, increases considerably for intermediate compositions compared with those of either pure ZnSe or pure CdSe. The results indicate that 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.