Size dependence of confined acoustic phonons in CuCl nanocrystals

Persistent spectral hole burning spectroscopy was used to study the size de pendence of the confined acoustic phonons in CuCl nanocrystals embedded in silicate glass, NaCl, and KCI. It is found that the energies of the confine d acoustic phonons in the nanocrystals in glass and KCI are almost the same , but twice larger than those in NaCl, which were obtained from the Stokes- side acoustic phonon holes with the same excitation energies. The confined acoustic phonons in the nanocrystals in glass and KCl can be well explained in terms of the lowest-frequency vibrational modes calculated on a sphere model with a free boundary condition. However, the energies of the confined acoustic phonons in the nanocrystals in NaCl are lower than the frequencie s of the lowest-frequency vibrational modes predicted by a cube model with a free boundary condition. This observation shows that the energies of the confined acoustic phonons in CuCl nanocrystals depend on the size, the shap e, and the boundary condition of the nanocrystals. [S0163-1829(99)13431-3].