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

Confined acoustic phonons in InAs nanocrystals are observed in the time domain by femtosecond pump-probe spectroscopy. The size dependence of both frequency and damping time is investigated for InAs. The frequency of the discrete modes varies between 18 and 30 ${\mathrm{cm}}^{\mathrm{-}1}$ for a nanocrystal radius ranging from 28 to 12 Å, and the dependence deviates from the expected $1/R$ behavior at small sizes. The damping rate is found to vary linearly with $1/R,$ suggesting coupling to the matrix through the particle surface as a main damping route. This is corroborated by observing faster damping for the acoustic mode in solution compared to a polymer environment. The coupling strength is found to depend on material parameters: in CdSe nanocrystals, weaker coupling is observed. The role of acoustic phonon coupling in the dephasing dynamics of semiconductor nanocrystals is discussed.