Size-dependent dynamics of coherent acoustic phonons in nanocrystal quantum dots

Confined acoustic phonons in InAs nanocrystals are observed in the time dom ain by femtosecond pump-probe spectroscopy. The size dependence of both fre quency and damping time is investigated for InAs. The frequency of the disc rete modes varies between 18 and 30 cm(-1) for a nanocrystal radius ranging from 28 to 12 Angstrom, and the dependence deviates from the expected 1/R behavior at small sizes. The damping rate is found to vary linearly with II R, suggesting coupling to the matrix through the particle surface as a main damping route. This is corroborated by observing faster damping for the ac oustic mode in solution compared to a polymer environment. The coupling str ength is found to depend on material parameters: in CdSe nanocrystals, weak er coupling is observed. The role of acoustic phonon coupling in the dephas ing dynamics of semiconductor nanocrystals is discussed.