Quantitative modeling of the absorption and resonance Raman spectra, including absolute cross-sections and depolarization ratios, is used to determine the strength of exciton-phonon coupling for the longitudinal optical (LO) phonon in CdSe and ZnSe quantum dots (QDs). The Huang-Rhys factor for the LO phonon in the lowest excitonic transition is about 0.2 for CdSe QDs in the 3-5 nm diameter range [1], and 0.3 to 0.5 for ZnSe QDs in the 4-5 nm range. Partial cation exchange of ZnSe QDs is then used to create randomly alloyed Cd<sub>x</sub>Zn<sub>1-x</sub>Se QDs where x ranges from 0 to nearly 1. As the Zn to Cd exchange proceeds, the absorption spectrum shifts to the red and first broadens and then narrows while the overtone to fundamental intensity ratio dramatically increases and then decreases, suggesting that the exciton-phonon coupling is maximized for compositions near a 1:1 Cd:Zn ratio. Simple model calculations suggest that this arises from partial localization of the hole wavefunction in the alloyed structures, maximizing the electron-hole separation and thus the local electric fields to which the polar phonons couple.

Resonance Raman study of exciton-phonon coupling in CdSe and ZnSe quantum dots and their alloys

253rd National Meeting (2017)

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