Morphology Evolution of Gradient-Alloyed Cd_xZn_1-xSe_yS_1-y@ZnS Core-Shell Quantum Dots during Transmission Electron Microscopy Determination: A Route to Illustrate Strain Effects

In this work, we reported the morphology evolution (formation of voids & size reduction) of gradient-alloyed Cd_xZn_1-xSe_yS_1-y@ZnS quantum dots under electron irradiation during transmission electron microscopy observation. By investigating the correlations between shell gradients and morphology evolution, the formation of voids can be explained by the continuous electron irradiation-induced atomic movement under interfacial strain. On the other hand, the size reduction can be attributed to the elastic scattering-enabled sputtering of surface atoms. The as-formed voids of Cd_xZn_1-xSe_yS_1-y@ZnS quantum dots with CdS-rich cores are much larger than those of ZnSe-rich ones, and the sizes of voids decreased with the increasing of shell thickness. The comparison of the morphology evolution of Cd_xZn_1-xSe_yS_1-y@ZnS core-shell quantum dots with different composition gradients demonstrated that the size and shape of as-formed voids illustrate the strain characteristics of shell gradient. This provides a guideline to understand the strain effects in gradient-alloyed core-shell quantum dots through transmission electron microscopy measurement. We believe the deep insights into gradient-alloyed Cd_xZn_1-xSe_yS_1-y@ZnS core-shell quantum dots would push forward their optimization toward commercialized light-emitting technology.