A comparative study of the isoelectronic Cd and Hg substitution in EDTA-capped ZnS nanocrystals

Abstract

In this research, the surface capped pure ZnS nanoparticles, as well as the Cd- and Hg-doped ones were synthesized via a green ultrasonic-assisted co-precipitation route. The products were characterized by X-ray diffraction, scanning and transmission electron microscopies, Fourier transform infrared, UV–Vis and photoluminescence spectroscopies. The results showed that the synthesized spherical-like nanoparticles are single-phased and well-dispersed with diameters of about 3 nm. They were crystallized in a cubic zincblende structure whose lattice constants increase on doping due to the larger ionic radii of the dopants. The Cd/Hg substitution results in slightly less microstrain and so rather smaller particles. The studied nanoparticles are direct band gap materials whose band gap values vary with Cd/Hg doping from 4.31 eV for ZnS to 3.94/4.40 eV as a result of the competition between the quantum size effect and the composition effect. The effect of the isoelectronic Cd and Hg doping is also revealed as the weakening of the blue photoluminescence band around 430 nm originated from the defect states in ZnS matrix, and the appearance of a red excitonic emission at 640 nm. It was found that in these nanoparticles being smaller than Bohr dimension, the particle size is a determinative parameter for governing the efficiency of the radiative emissions.