Direct relationship between lattice volume, bandgap, morphology and magnetization of transition metals (Cr, Mn and Fe)-doped ZnO nanostructures

In this work, we report the detailed investigation of the dependence of structural, optical, morphological and magnetic properties on the type of transition metals (TM; Cr, Mn and Fe) dopants in Zn1−xTMx  O nanostructures. Pure and TM-doped ZnO nanostructures have been fabricated by the microwave assisted solution route. The striking similarities between changes in the lattice volume, bandgap energy, morphology and saturation magnetization indicated a strong correlation among these properties. As the atomic number of the dopant increased (from Cr to Fe), the lattice volume and bandgap energy were found to increase. Morphological studies using field emission scanning electron microscopy and transmission electron microscopy revealed that doping elements affect the shape and size of ZnO nanostructures such as nanorods (Cr–ZnO), nanosheets (Mn–ZnO) and nanorods (Fe–ZnO). Raman study showed that the E2high phonon modes shifted toward a lower frequency with the atomic number of the TM dopant. The optical bandgap of the TM-doped ZnO nanostructures increased with the increase in orbital occupation numbers of 3d electrons of the TM dopant due to the orbital splitting of magnetic ions. Magnetic measurement results revealed that all the TM-doped ZnO samples showed well-defined ferromagnetic features at room temperature and the value of saturation magnetization (Ms) increased with increasing the atomic number of dopants, having a maximum value for Fe-doped ZnO.