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.
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