Origin of the\nHigh-Frequency Shoulder in the Raman\nSpectra of CdSe Quantum Dots

The origin of the high-frequency shoulder (HFS) observed\nabove\nthe longitudinal optical (LO) peak around 230 cm<sup>–1</sup> in the Raman spectra of CdSe quantum dots (QDs) has been the subject\nof intense debate. We use state-of-the-art <i>ab initio</i> density functional theory applied to small CdSe QDs with various\nrealistic surface passivations and find an intense Raman signal around\n230 cm<sup>–1</sup>, which corresponds to a stretching vibration\nof a defective 2-fold coordinated Se atom. We interpret this signal\nas being the origin of the HFS. Since the signal disappears in fully\npassivated and defect-free (magic size cluster) structures, it can\nbe used as a fingerprint to distinguish defective from nondefective\nstructures.