Fabrication and properties of Co doped ZnO spherical assemblies

• Fabrication of Co doped ZnO spherical assemblies composed of numerous nanocrystals.
• Incorporation of Co2+ ions into ZnO at Zn2+ site and their uniform distribution.
• Oriented attachment of nanocrystals in Co doped ZnO assembly.
• Appearance of additional Raman mode at 527 cm−1 due to defects in host lattice.
• Defect induced room temperature ferromagnetic like behavior in Co doped ZnO.

Spherical assemblies of Co doped ZnO comprised of numerous nanocrystals of size 10–15 nm were prepared by refluxing their acetate precursors in diethylene glycol medium. The detailed structural analysis by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) reveal that Co ions are successfully incorporated into the hexagonal wurtzite ZnO nanostructure and do not appear as precipitates or secondary phases. These Co ions are uniformly distributed in the ZnO host lattice and exist in +2 oxidation state. Further, HRTEM analysis also confirmed the oriented attachment of nanocrystals as well as their defects structure. Raman spectra clearly indicate the incorporation of Co2+ ions into ZnO nanostructures at Zn2+ site (appearance of an additional Raman mode at 527 cm−1) and the resulted defects and lattice disorder in host lattice (shifting, broadening and weakening of 2E2 (low) and E2 (high) phonon modes). These Co doped ZnO samples exhibit ferromagnetic like behavior at room temperature, presumably due to the presence of defects in host ZnO lattice.