Corel shell quantum dots are heterogeneous nanoparticles composed of an inorganic core enveloped by at least one inorganic shell of a second material. These structures have attracted increasing attention over the past decade, especially because of their enhanced photoluminescence properties and the possibility to create spatially separated excitons by means of a staggered corelshell band alignment. Synthesis of corelshell structures involves epitaxial growth of a second semiconductor onto seed particles, in either a single-step or multistep reaction. An alternative mechanism to form this type of heterogeneous nanocrystals exploits cation exchange, which has been demonstrated in various materials such as CdS-PbS, CdS-Ag2S, and CdSe-ZnSe. More recently, the formation of PbSICdS and PbSeCdSe coreshell QDs by cation exchange has been demonstrated, which avoids the problems these materials have with luminescence stability and oxidation. Although attractive because of its simplicity, the cation exchange method has its limitations. For some systems, there is a size limit below which the geometry of the particles is not maintained, or diffusion may alter the composition of the core.
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