Giant low-temperature anharmonicity in silicon nanocrystals

The phonon density of states of silicon nanocrystals with size between 4 and 7.5 nm was measured by inelastic neutron scattering in the 5∼600K temperature range. The narrow particle size distributions enable the study of size effects on phonon dynamics. Giant softening of phonon features below 30 meV, universal broadening of phonon features, and the disappearance of intermediate-energy phonons were observed with decreasing nanocrystals size. Such size effects are mostly attributed to the structure variations within the nanocrystals. The phonons below 30 meV in silicon nanocrystals show temperature dependence opposite to the bulk silicon, explained by the large anharmonicity of the under-constrained near-surface phonons. This is supported by the abnormal atomic mean-square-displacement, and low energy phonon population in small silicon nanocrystals. This work provides crucial information on the phonon dynamics in spatially confined materials.