Optical and acoustic phonon modes confined in gallium phosphide nanoparticles

The main aim of this paper is to discuss the confinement effects on the optical and acoustic phonon vibrational modes in gallium phosphide (GAP) nanoparticles (cylindric grain). The Raman scattering from the GaP nanoparticles was investigated. It was found that the redshifts of the longitudinal optical (LO) mode and transverse optical (TO) mode were 15 cm-1 and 13.8 cm-1, respectively. It is generally accepted that the red-shifts of the optical phonon modes are due to the presence of smaller nanosized particles (～1.2 nm) acting as the nanoclustered building blocks of the Gap nanoparticles. In the low frequency Raman spectrum, a set of Stokes lines with almost the same spacing was clearly observed. The scattering feature originates from the discrete phonon density of states of the nanoelustered building blocks. According to Lamb's vibrational theory, the Raman shift wavenumbers of the spheroidal mode and torsional mode of the lowest energy surface modes for the nanoclustered building blocks were calculated. Good agreement can be achieved between the calculated results and the observed scattering peaks. These results indicate that the corresponding Raman peaks are due to scattering from the localized acoustic phonons in the nano-clustered building blocks in the Gap nanoparticles.