Size-dependent Raman and infrared studies of PbSe nanoparticles

Micro-probe Raman and far-infrared absorption spectroscopies were used to prove the existence of optical phonon modes of PbSe nanoparticles prepared by colloidal chemistry and preliminarily characterized by transmission electron microscopy. To the best of our knowledge, this is the first time that evidence of the surface phonon (SP) mode by Raman spectroscopy has been experimentally observed. The wavenumber of the SP mode is consistent with its prediction by a dielectric continuum model. While for different PbSe nanoparticle sizes the observed SP mode does not show any obvious change in its position, there is a clear shift by approximately 4 cm^-1 toward higher wavenumber in the appearance of the LO(Γ) in the Raman spectra from the 3 nm to the 7 nm PbSe nanoparticles. Far-infrared measurements demonstrate the presence of the transverse optical TO(Γ) and of the coupled phonon modes.