Growth of Co-doped ZnO nanoparticles by porous alumina assisted sol-gel route: Structural optical and magnetic properties

We report a simple sol-gel method with spin coating to grow Co-doped ZnO nanoparticles (NPs) using porous alumina template. X-ray diffraction analysis confirms the wurtzite hexagonal structure for the Zn1−xCoxO NPs. Transmission electron microscopy (TEM) micrograph reveals clearly the formation of NPs with average diameter in the range 40-50 nm. High resolution TEM lattice images and the electron diffraction (SAED) pattern show that the NPs are single crystalline with lattice plane spacing of 2.47 Å for the (101) plane. UV-Vis absorption spectra show a slight blue shift in the absorption edge due to doping related modification in the band structure. PL spectra show a band edge related UV emission around 378 nm at low temperature, while defect (Zni, OV) related visible emission dominates at room temperature. Room temperature ferromagnetism (RTFM) is observed in Zn1−xCoxO NPs with the moment of 450-750 memu/g. The temperature dependent magnetization (M-T) curve shows high transition temperature (Tc) as 750 K. The observed FM is explained using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Co ions and Zni, OV related defects. This study provides an effective way to obtain HTFM with lower doping concentration as well as to have control over NP size by alumina pores in the Zn1−xCoxO system.