Towards controlled synthesis and better understanding of highly luminescent PbS/CdS core/shell quantum dots

A two-step cation exchange procedure has been developed for synthesizing PbS/CdS core/shell quantum dots (QDs) with a much thicker shell than previously reported, which expands the flexibility of the current cation exchange approach. The thick-shell QDs allow relatively easy observation of the core/shell morphology by transmission electron microscopy as well as exhibiting characteristic absorption and emission of CdS when the shell thickness reaches 1.8 nm. X-ray diffraction patterns show gradual transformation from a rock salt PbS pattern to a zinc blend CdS pattern with increasing shell thickness and the overall diffraction pattern is indeed the same as that of the CdS standard when the shell is thicker than 3.6 nm. The thick-shell QDs were further analyzed by energy dispersive X-ray spectrometry. It is found that Pb only exists in the core region and is essentially absent in the shell region. All of these results consistently suggest that the shell is made of CdS, instead of ternary PbxCd1-xS alloy in thick-shell QDs. As direct experimental identification of the shell composition in thin-shell QDs is difficult, experimental data and calculations are combined to indirectly probe this issue. The comparison of band gap versus core size plots of different compositional models indicates that it is highly likely that the thin shell is also made of CdS. Importantly, these core/shell PbS/CdS QDs not only show significantly increased quantum yield up to 67% at the optimal shell thickness of about 0.7 nm, they are also much more photostable and thermally stable than the shell-free PbS QDs.