Improving functional properties of ZnO nanostructures by transition-metal doping: role of aspect ratio

Herein, pure and 3 % transition metals (TM; Cr2+ and Fe2+ ions)-doped ZnO nanostructures with high aspect ratios were prepared by microwave-hydrothermal method. X-ray diffraction, selected area electron diffraction and high resolution transmission electron microscopy analyses revealed that all the TM (Cr2+ and Fe2+ ions)-doped ZnO nanostructures have wurtzite structure and no secondary phase was detected. Field emission scanning electron microscopy and transmission electron microscopy results confirmed a higher aspect ratio and highly crystalline nature of nanostructures. Raman spectra reveled that no defect related mode was observed which indicated that the nanostructures have high quality and negligible defects. The value of bandgap was found to be close to the standard value of ZnO, and increased with the increase in atomic number of TM dopants, which indicated that the Cr2+ and Fe2+ ions were uniformly substituted in ZnO. Room temperature ferromagnetism was observed in all the TM (Cr2+ and Fe2+ ions)-doped ZnO nanostructures and the value of saturation magnetization (Ms) and remanent magnetization (Mr) were increased with TM (Cr2+ and Fe2+ ions) dopants. The modification in the magnetization and H-c by microwave hydrothermal might be due to the high aspect ratio of nanostructures. Hence, these nanostructures pave the way for development of multifunctional spintronics and optoelectronic devices that integrate structural, morphological, optical, and magnetic properties.