Ultrafast carrier dynamics in core and core/shell CdSe quantum rods: Role of the surface and interface defects

Abstract

Femtosecond pump-probe spectroscopy, in the visible spectral range, is used to study fast processes after photoexcitation in quantum confined wurtzite CdSe core and $\mathrm{CdSe}∕\mathrm{CdS}∕\mathrm{ZnS}$ core/shell nanorods. Effective-mass theory is applied to assign the energy levels in linear absorption spectra. Sequential bleaching of the excitonic states and photoinduced absorption are observed in the transmission difference spectra. A strong difference was found between the core and core/shell samples. In fact, in the transient transmission difference spectra, the decay time of the higher energy states is faster in the core than in the core/shell samples (200 fs versus 500 fs), due to the trapping in the unpassivated surface states of the bare core. Stimulated emission (SE) has been achieved in CdSe core and core/shell quantum rods at room temperature. As a striking feature, the SE is sustained for a shorter time in the core/shell sample (25 ps) compared with the core sample (50 ps). We demonstrate that this result is due to the competition between SE and photoinduced absorption from defect states in the midgap of the core/shell sample. Such midgap states are related to the formation of extended defects at the core/shell interface, due to lattice strain relaxation.