Contribution of core/shell and core/shell/shell lattice interfaces on the optical properties of quantum dots?

by Omogo, Bernard O.; Benamara, Mourad; Heyes, Colin D.

Quantum Dots (QDs) are promising components in a range of optical applications because of their high photo-stability, broad absorption spectrum and easily tunable emission. In particular, they have become widely used as fluorescence probes in ultrasensitive biomedical imaging due to their advantages over other probes. In order to use QDs in such applications, it is preferable to isolate the QD core from the biol. environment with a shell, comprising of less-toxic material with a higher band gap. Thus, the biol. system is protected from the more toxic core while the fluorescent core is protected from the effects of the biol. environment. Since the shell has different lattice parameters to the core, a lattice mismatch is introduced which can affect the lattice parameters by introducing a charge-carrier trap states. One soln. to reduce the lattice strain is to use core/shell/shell approach, where the internal shell has a smaller mismatch with the core than the outer shell, but this introduces an addnl. interface that can also affect the optical properties. In order to investigate the effects of adding this addnl. interface, we have synthesized CdSe/ZnS Core/Shell and CdSe/CdS Core/Shell as well as CdSe/CdS/ZnS Core/Shell/Shell QDs. We have characterized the structural properties of these QDs with high resoln. electron microscopy and one particle elemental anal., and the optical properties by ensemble and single QDs Spectroscopy. We have studied the effect of the shell material and thickness on the quantum yield, fluorescence lifetime and blinking- a phenomenon in which the QDs switches between the bright and dark states under const. illumination. These results will contribute to the understanding of the origin of the trap states that are known to cause non-radiative decay pathways and hence blinking in core/shell and core/shell/shell systems and consequently improve their biomedical imaging as well as other optoelectronic applications.