The influence of pyridine ligand onto the structure and phonon spectra of CdSe nanocrystals

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Abstract
The influence of ligand exchange for pyridine onto the structure and phonon spectra of oleic acid-stabilized CdSe nanocrystals (NCs) is studied by resonant Raman and optical absorption spectroscopy, nuclear magnetic resonance and transmission electron microscopy. The removal of oleic acid ligand by pyridine treatment results in change of intensity ratio of the longitudinal optical (LO) phonon peak to its overtones. The latter effect is attributed to a changed electron-phonon coupling in NCs upon introduction of the hole-capturing ligand (pyridine). The upward shift and broadening of the LO phonon peak are also observed and supposed to be the result of interplay between partial oxidation of the NC and strain induced by surface reconstruction. The relative contribution of these two effects is found to be dependent on the NC size. The activation of two additional Raman features, in the low-frequency range and above the LO band, for pyridine-treated NCs is supposed to be related with induced disorder or reconstruction on the NC surface. No noticeable effect of the surface treatment and concomitant NC aggregation onto the surface optical phonon mode was observed.