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.

Automated summary

Phonon, as a momentum carrier, plays an important role in the photoluminescence of silicon nanocrystals. In this research, inelastic neutron scattering was used to study the effects of particle size, oxidation, and temperature on phonon dynamics in silicon nanocrystals. It was found that the transverse acoustic phonon modes are more sensitive to particle size, oxidation, and temperature compared to optical phonon modes. The changes in phonon dynamics have the largest impact on vibrational entropy and free energy of silicon nanocrystals at intermediate temperatures.