Er3+ and Sm3+ co-doped ZnS quantum dots: Judd-Ofelt, luminescent properties and energy transfer

ZnS semiconductor quantum dots (QDs) co-doped with Er3+ and Sm3+ ions were successfully fabricated using a wet chemical method. The presence of Er3+ and Sm3+ in the ZnS host was confirmed using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD and transmission electron microscopy (TEM) analyses showed that the QDs had a zinc blende (ZB) structure with spherical shape and a particle size of approximately 4 nm. The optical properties of ZnS QDs were investigated by changing the Er3+ ion concentration and fixing the Sm3+ ion concentration. Judd-Ofelt (J-O) intensity parameters were determined from optical absorption spectra. The obtained J-O parameters illustrate the covalency related to the ligands and rare-earth ions. The radiative transition probabilities (Ar), branching ratios (βr), and radiative lifetimes (τr) of various excited states of the Er3+ ions were calculated. For the first time, the energy transfer (ET) processes from the ZnS host to Er3+ ions and from Er3+ to Sm3+ ions were confirmed and explained in detail. Er3+ and Sm3+ co-doped ZnS QDs showed a very long lifetime (up to milliseconds) compared to undoped ZnS. This exciting property has potential applications in photocatalysis, biosensing, and photovoltaics.