Structural transformation of ZnO quantum dots to ZnO/ZnS core-shell quantum dots and fluorescence color shift from yellow to blue induced by interfacial sulfurization

In recent years, ZnO quantum dots (QDs) synthesized via solution-based methods have garnered significant attention due to their rich surface defects, which lead to strong yellow-green light emissions. However, these defect emissions often limit their optoelectronic performance. Surface engineering of ZnO QDs offers a promising strategy to enhance their properties and enable the construction of core-shell heterostructures. Among various approaches, ion exchange via sulfurization is an effective method to transform ZnO into ZnS, driven by the higher solubility constant of ZnO. In this study, we explore the transformation of ZnO QDs into ZnO/ZnS core-shell QDs through an interfacial sulfurization reaction using thioacetamide. Transmission electron microscopy and x-ray diffraction analyses confirm the successful formation of ZnO/ZnS core-shell QDs. Ultraviolet-Visible (UV-Vis) absorption spectra reveal a significant change: the characteristic Urbach absorption tail of ZnO QDs disappears in ZnO/ZnS core-shell QDs, replaced by a step-like absorption edge at higher energy. Fluorescence spectroscopy further highlights the impact of this structural modification, showing that the intrinsic emission of ZnO is enhanced, while the yellow-green defect emission is quenched. Additionally, a strong blue emission appears around 450 nm, indicative of higher-energy radiative transitions in core-shell QDs. These findings demonstrate that surface sulfurization effectively alters the optical properties of ZnO QDs, facilitating the formation of ZnO/ZnS core-shell QDs with improved emission characteristics. This study underscores the potential of sulfurization as a powerful tool for tuning the optoelectronic properties of semiconductor nanomaterials, with significant implications for their applications in optoelectronics and photocatalysis.