Composition and strain effects in Type I and Type II heterostructure ZnSe/Cd(Zn)S and ZnSe/Cd1-xZnxS core/shell quantum dots

Abstract

Abstract We investigated the effect of ternary shell alloy composition on the bandgap and diameter of core of Z n S e / C d 1 − x Z n x S heterostructure core/shell quantum dots, which were synthesized by using a simple colloidal technique. Characterization by using the x-ray diffraction (XRD), transmission electron microscopy (TEM), UV–Vis absorption and fluorescence emission spectroscopic techniques indicate that (i) there is a transition of Z n S e / C d 0.6 Z n 0.4 S Type-I heterostructure (electrons and holes tend to localize in core) to Z n S e / C d 0.75 Z n 0.25 S quasi-Type-II heterostructures (holes tend to localized in the core and electrons are delocalized) and (ii) then after large red shift and Stock-shift in PL emission spectra but not a distinct absorption peak in UV spectra become noticeable in ZnSe/ Cd 0.75 Zn 0.25 S quasi-Type II and ZnSe/CdS Type II heterostructures (electrons are localized in core and holes are localized in shell). Furthermore, the increase of Cd:S ratio in shell alloy composition shifts the XRD peaks to lower 2θ degrees which corresponds to tensile strain in the ZnSe core. Finally, the hydrostatic interfacial strain has effect on the squeezing or stretching the capped core: A decrease of compressive force on core from ZnSe/ZnS to tensile force in ZnSe/CdS with increase in Cd:S ratio indicates that transition of compressive strain to tensile strain takes place with the transition from Type-I to II heterostructure.