QUANTUM-SIZE DEPENDENCE OF FEMTOSECOND ELECTRONIC DEPHASING AND VIBRATIONAL DYNAMICS IN CDSE NANOCRYSTALS

Femtosecond photon-echo techniques are used to probe the dynamics of q uantum-confined excitons in nanocrystals of CdSe. Using three-pulse ph oton 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 measurem ents are reported as a function of both nanocrystal size and temperatu re. The dephasing times vary from 85 fs in nanocrystals of 20-angstrom diameter to 270 fs in 40-angstrom crystals. These rates are determine d by several dynamical processes, all of which depend sensitively on t he size of the nanocrystal. The time scale of the trapping of the elec tronic excitation to surface states increases with increasing size. Th e coupling of the excitation to low-frequency vibrational modes is str ongly size dependent as well, in accordance with a theoretical model. The photon echo also gives information on the polar coupling between t he electronic state and the LO-phonon mode. This coupling is found to peak at an intermediate size. This phenomenon is interpreted as a mani festation of coupling between the interior confined excitons and local ized surface states, which destroys the spherical symmetry of the exci ted state. Using these data, all of the important contributions to the size-dependent homogeneous linewidths can be enumerated.