Influence of defects on the structural, optical, photoluminescence and magnetic properties of Cr/Mn dual doped ZnO nanostructures

Abstract

Cr (2%) doped ZnO and Cr, Mn (Cr = 2% and Mn = 2–4%) dual doped ZnO nanostructures were synthesized by chemical co-precipitation route. The hexagonal structure of Cr/Mn doped ZnO was not distorted by Mn doping and no secondary/impurity phases of Cr/Mn were detected in the XRD pattern. The reduced crystallite size at Mn = 2% is due to depression of growth rate and the dissimilarities between dopant and host ions and elevated crystallite size at 4% is accountable for the more defect sites in Zn-Cr-O lattice. The stable c/a ratio (~ 1.603) indicates that the hexagonal ZnO wurtzite structure of the synthesized composite is not affected by Mn substitution. The reduced crystallite size and the elevated UV–visible absorption at the higher wavelength side i.e., visible range at Mn = 2% recommended it for improved photocatalytic and efficient for solar cell applications. The broadening of energy gap at Mn = 2% is explained by Burstein-Moss (BM) band filling effect through energy level diagram. The presence of Zn-O-Cr/Mn bonding is confirmed by Fourier transform infra-red (FTIR) studies. The elevated PL emission and the intensity ratio between the green and blue emissions revealed the existence of more oxygen vacancies and oxygen related defect sites in Zn-Mn-Cr-O lattice. The exhibited room temperature ferromagnetism (RTFM) with strong magnetization in Cr, Mn doped ZnO is induced by both the oxygen vacancy mediated bound magnetic polarons (BMP) and the enhanced exchange coupling among the free electrons and local spin polarized electrons.