Raman and Infrared Phonon Spectra of Ultrasmall Colloidal CdS Nanoparticles

Raman and infrared phonon spectra of ultrasmall (1.8 nm) colloidal CdS nanoparticles (usNPs) are presented. Multiphonon scattering by optical phonons up to the third order is observed in the Raman spectra at low temperature and resonant (325 nm) excitation. The first-order optical phonon peak is a superposition of several components, two of which can be assigned to surface optical (SO) and longitudinal optical (LO) modes, respectively. The LO mode, being markedly broadened compared to that of spectra of regular (>2 nm) NPs, is related to phonon confinement and bond distortion induced by a significant structural relaxation in usNPs. A shoulder observed above the LO frequency is either due to the density of phonon states induced by distorted surface bonds or due to higher-order scattering processes involving optical and acoustic phonons. The Raman peaks of usNPs do not reveal the upward shift and narrowing upon decreasing temperature from 300 K down to 85 K typical for crystalline semiconductors, even though their intensity increases as expected. The abnormal thermal behavior of phonon peaks is likely related to the significant structural reorganization of the usNP lattice. A broad feature in the range of 200-300 cm(-1) observed in the infrared phonon spectrum of usNPs correlates with the Raman data and is distinct from the SO mode previously reported for NPs of larger size.