Origin of the High-Frequency Shoulder in the Raman Spectra of CdSe Quantum Dots

The origin of the high-frequency shoulder (HFS) observed above the longitudinal optical (LO) peak around 230 cm^-1 in the Raman spectra of CdSe quantum dots (QDs) has been the subject of intense debate. We use state-of-the-art ab initio density functional theory applied to small CdSe QDs with various realistic surface passivations and find an intense Raman signal around 230 cm^-1, which corresponds to a stretching vibration of a defective 2-fold coordinated Se atom. We interpret this signal as being the origin of the HFS. Since the signal disappears in fully passivated and defect-free (magic size cluster) structures, it can be used as a fingerprint to distinguish defective from nondefective structures.