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.