Observation of Novel Superparamagnetism in ZnS:Co Quantum Dots
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ORIGINAL PAPER
Observation of Novel Superparamagnetism in ZnS:Co Quantum Dots S. Elsi 1,2 & S. Mohanapriya 2 & K. Pushpanathan 2 Received: 6 April 2020 / Accepted: 9 June 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract 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 cobaltdoped 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. Keywords ZnS nanoparticles . Cobalt dopant . Crystal structure . Single domain . Superparamagnetism
1 Introduction Zinc sulfide is a compound semiconductor whose chemical formula is ZnS. It exists in two main crystalline forms such as in cubic (zinc blende or sphalerite) and hexagonal form. This dualism is the outstanding property of polymorphism of ZnS [1]. Also, ZnS is a perfect II–VI semiconductor, and they assume structures related to other semiconductors, such as GaAs. Of the two crystalline forms, the zinc blende is the more stable cubic form which has a bandgap of about 3.54 eV and large exciton binding energy of 40 meV at room temperature while the hexagonal form has a wide bandgap of about 3.91 eV. In both cases, the coordination geometry at Zn and S is tetrahedral. Both cubic and wurtzite structure materials are intrinsic, wide-bandgap polar semiconducting * K. Pushpanathan [email protected] 1
Department of Physics, Arignar Anna Government Arts College, Attur, Tamil Nadu 636 121, India
2
Department of Physics Nanomaterials Research Laboratory, Government Arts College, Karur, Tamil Nadu 639 005, India
materials. Besides these cubic and hexagonal structures, recently, it is reported that ZnS crystallizes into a rhombohedral structure [2]. Upon heating around 1020 °C, ZnS undergoes a structural transformation that takes place from cubic to wurtzite
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