Coupled Exciton and Charge-Transfer Resonances in the Raman Enhancement of Phonon Modes of CdSe Quantum Dots (QDs)

We report the observation of the enhancement of a transverse optical (TO) phonon mode of e(1) symmetry and a normally forbidden surface optical (SO) phonon mode of b(1) symmetry in CdSe quantum dots (QDs) due to adsorption of 4-mercaptopyridine molecules. The former is observed in 3, 4, and 5 nm diameter particles, while the latter is observed only in the 2 nm particles. Maximum enhancement of the phonon modes is obtained through a coupling of the charge-transfer transition and the exciton transition which are in resonance with the laser energy. Selection rules using Herzberg Teller (vibronic) coupling are invoked to explain the observed enhancements.