Defect induced ferromagnetism in luminescent and doped CdS quantum dots

This work reports transition metal (TM = Ni, Mn, Fe, Co and Cr) doping of CdS nanoparticles with the generic formula Cd1-xTMxS; x = 0.04 and 0.08 synthesized by chemical co-precipitation method. Polyvinylpyrrolidone was used as surfactant to prevent agglomeration. The samples were characterized with X-ray diffraction (XRD), UV–Vis absorption spectroscopy, photoluminescence and superconducting quantum interference device (SQUID). XRD confirmed the phase purity of the doped samples. Average crystallite size calculated from Debye–Scherrer formula was 2–9 nm. The UV–Vis spectra of the samples show blue shift, revealing strong confinement effect in the nanoparticles. The band gap calculated from UV–Vis spectroscopy lies in the range of 2.81–2.89 eV. The synthesized samples show luminescent emission ranging from 450 to 600 nm in the visible region of the electromagnetic spectrum. SQUID studies at 300 K reveal that Cd1−xNixS and Cd1−xMnxS show paramagnetic behaviour with small nonlinearity for low magnetic field whereas room temperature ferromagnetism was observed for Cd1−xFexS, Cd1−xCoxS and Cd1−xCrxS nanoparticles.