Quantum size dependence of femtosecond electronic dephasing and vibrational dynamics in CdSe nanocrystals

Femtosecond photon-echo techniques are used to probe the dynamics of quantum-confined excitons in nanocrystals of CdSe. Using three-pulse photon echoes, the modulation of the echo signal from the LO-phonon mode is effectively suppressed, and both the electronic dephasing and the couplings to lattice vibrations are probed directly. Detailed measurements are reported as a function of both nanocrystal size and temperature. The dephasing times vary from 85 fs in nanocrystals of 20-\AA{} diameter to 270 fs in 40-\AA{} crystals. These rates are determined by several dynamical processes, all of which depend sensitively on the size of the nanocrystal. The time scale of the trapping of the electronic excitation to surface states increases with increasing size. The coupling of the excitation to low-frequency vibrational modes is strongly size dependent as well, in accordance with a theoretical model. The photon echo also gives information on the polar coupling between the electronic state and the LO-phonon mode. This coupling is found to peak at an intermediate size. This phenomenon is interpreted as a manifestation of coupling between the interior confined excitons and localized surface states, which destroys the spherical symmetry of the excited state. Using these data, all of the important contributions to the size-dependent homogeneous linewidths can be enumerated.