Observation of Novel Superparamagnetism in ZnS:Co Quantum Dots

Cobalt-doped zinc sulfide quantum dots with different cobalt concentrations were prepared by the solution route. Structural, optical, morphological, and magnetic responses of the prepared quantum dots were analyzed. X-ray diffraction study confirmed that cubic (zinc blende) structure is the dominant structure of synthesized samples. Crystallite size and lattice constant were found to decrease with cobalt dopant. Transmission electron microscope analysis shows that the mean size of the particles lies in the range of 7–10 nm. The absorption edge of cobalt-doped zinc sulfide nanoparticles is shifted to lower wavelength as compared to that of bulk zinc sulfide which confirmed the quantum confinement effect. The bandgap variation was observed with doping, and it varied from 4.3 to 5.6 eV. The emission spectrum reveals that the cobalt dopant suppresses the ultraviolet emission and enhances the visible emission. Fourier-transform infrared spectrum confirmed the formation of zinc sulfide and the substitution of cobalt ion. Magnetization versus magnetic field result demonstrates that the zinc sulfide and cobalt-doped zinc sulfides are superparamagnetic. Electron spin resonance spectra also confirmed the superparamagnetic nature of zinc sulfide and cobalt-doped zinc sulfide samples. Micro-Raman study also confirmed the incorporation of cobalt in the lattice and the size of the order of the exciton Bohr radius for ZnS.