Influence of chromium concentration on the structural, electronic structure, optical and temperature dependent magnetic properties of ZnS nanocrystals

The effect of Cr dopant on the structural, optical, magnetic properties and local electronic structure of aqueous synthesis derived Zn_1−xCr_xS diluted magnetic semiconductor nano crystals have systematically investigated. The nano crystalline structure and crystallite size have been estimated by X-ray diffraction measurements with Rietveld refinement and high-resolution transmission electron microscopy. Effective increase of the lattice parameter has been observed in doped samples. Raman spectroscopy has been employed to study the crystalline quality, structural disorder and defects in the host lattice. The tetrahedral coordination of the sulfur ions surrounding the zinc ions has been studied by FTIR analysis. The decrease of energy band gap for Cr doped samples has observed. Blue emission has been observed by photo luminescence spectroscopy due to defect formation (Cr_i) in Cr-doped samples. The local electronic structures of Zn and Cr sites are thoroughly studied by synchrotron based X-ray absorption spectroscopy comprising of both X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS). EXAFS studies indicate the presence of secondary phase within the sphalerite lattice of diluted Zn_1−xCr_xS nanocrystals while XANES studies exhibit single pre-edge feature. The amplitude of such pre edge is found to be independent of Cr amount on doped ZnS nanocrystals. The results demonstrated that diluted Cr³⁺ ions are substituted on the host ZnS nanocrystal. The Cr doped ZnS sample shows paramagnetism at room (300 K) and at low (5 K) temperature. The Cr–S bonds are the crucial premise for paramagnetic ordering.