The impact of sulfur deficiency on the structural, optical and photoluminescence properties of Zn_0.75Cd_0.25S quantum dots

Zn_0.75Cd_0.25S_x (x = 1, 0.9, 0.8, 0.7, 0.6, 0.5) quantum dots were produced via thermolysis method. The structure and microstructures behaviors of Zn_0.75Cd_0.25S_x were examined using Rietveld refinement for the X-ray diffraction (XRD) data. A portion of the integrated O binds with Zn to create a segregated ZnO phase beside ZnS phase. The threshold sulfur deficiency with some O content incorporation into the Zn_0.75Cd_0.25S_x lattice without generating a new phase (ZnO) is x = 0.6. Zinc sulfide is the predominant phase, while the percentage of ZnO rises with increasing S deficiency (x). Zn_0.75Cd_0.25S_0.8 has homogeneous spherical particle morphology with a very small size 2–4 nm, consistent with the XRD value. Zn_0.75Cd_0.25S_0.8 has a homogeneous spherical particle morphology with very small size of 2–4 nm. The disorder’s characteristics and defects were examined through Urbach energy analysis. The optical dielectric constant, optical conductivity and nonlinear optical parameters were calculated for all samples. The optical band gap lowers as the sulfur content increases up to x = 0.6, eventually reaching 2.71 eV. With a reduction in sulfur content to x = 0.5, two optical band gaps (2.8, 3.16) eV were obtained. The refractive index (n) values obtained using different models for all samples. As the molar ratio (x) of Zn/Cd to S increased, the PL emission peak of Zn_0.75Cd_0.25S_x nanoparticles exhibited a Stokes shift caused by S vacancies. The emitted colors from each samples were obtained by using Gaussian fitting for PL spectra. The fluorescence intensity greatly increased by increasing the S deficiency reaching its maximum value at x = 0.6.