Eu³⁺ singly-doped and Eu³⁺/Sm³⁺ co-doped ZnS quantum dots: structure, optical properties and energy transfer

In this study, Eu³⁺ ion singly doped and Eu³⁺/Sm³⁺ ions co-doped ZnS semiconductor quantum dots (QDs) were successfully synthesized using a wet chemical method in 1-octadecene (ODE) solvent. The successful doping of Sm³⁺ and Eu³⁺ ions into the ZnS host lattice and the composition and valence of the elements present in the sample were confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Structural and morphological studies revealed the presence of Eu³⁺-doped ZnS and Eu³⁺/Sm³⁺ co-doped ZnS QDs of about 3-4 nm size with a zinc blende structure. The Judd-Ofelt (J-O) intensity parameters of Eu³⁺-doped ZnS QDs were determined from their fluorescence spectra. The optical properties of Eu³⁺/Sm³⁺ co-doped ZnS QDs were studied by changing the Eu³⁺ ion concentration and fixing the Sm³⁺ ion concentration. The photoluminescence (PL) spectra of Eu³⁺/Sm³⁺ co-doped ZnS QDs were obtained at an excitation wavelength of 403 nm (⁶H_5/2 → ⁶P_3/2 transition of Sm³⁺ ions) in the 525-725 nm range. The effective energy transfer (ET) process from Sm³⁺ to Eu³⁺ ions was confirmed and explained in detail using the Reisfeld approximation and the Inokuti-Hirayama model. The CIE color coordinates of the Eu³⁺/Sm³⁺ co-doped ZnS QDs were obtained using PL spectral data. Eu³⁺/Sm³⁺ co-doped ZnS QDs exhibited a long lifetime and emitted warm red light. These interesting properties give Eu³⁺/Sm³⁺ co-doped ZnS QDs great potential for applications in photovoltaics, photocatalysis, and biomarkers.