Distinct Acoustic and Optical Phonon Dependences on Particle Size, Oxidation, and Temperature in Silicon Nanocrystals

Phonon, as a momentum carrier, may play an important role in the photoluminescence of silicon nanocrystals. However, a systematic experimental study on phonon dynamics in spatially confined silicon systems remains limited. We used inelastic neutron scattering to investigate particle size, oxidation, and temperature effects on phonon dynamics of silicon nanocrystals by measuring phonon density of states of 12 and 50 nm silicon nanocrystals with several oxidation levels at different temperatures. We showed that the lattice vibrations of large silicon nanocrystals and bulk silicon are substantially different. We found that transverse acoustic phonon modes have much stronger dependences on particle size, oxidation, and temperature than optical phonon modes. We showed that the changes in phonon dynamics have the largest effect on vibrational entropy and free energy of silicon nanocrystals at intermediate temperatures. Our results shed light on phonon dynamics of silicon-based functional nanomaterials and will facilitate further investigations of electron-phonon interactions in spatially confined silicon systems.