OBSERVATION OF OPTICAL PHONONS AND TQ-LO SPLITTING IN CUBR AND CUCL NANOCRYSTALS

Optical transverse (TO) and longitudinal (LO) phonons and surface (SO) phonons have been studied for CuBr and CuCl nanocrystals with radii o f a few nanometers by using a resonant Raman spectroscopy. It is found that the observed eigen-frequencies of the TO and LO phonons of long wavelength do not appreciably differ from those in the corresponding b ulk crystals, which indicates that the continuum theory still holds ev en in such small nanocrystals, leading to the same TO-LO splitting wit h that in the bulk. The observed SO phonon frequency in CuBr nanocryst als is also consistent with that estimated from the continuum theory b y considering the boundary conditions.