Investigation on microstructure, energy gap, photoluminescence and magnetic studies of Co and Cu in situ doped ZnO nanostructures

Abstract

Co (3%)-doped ZnO and Co, Cu (Co = 3% and Cu = 2 to 4%) dual-doped ZnO nanostructures were prepared using chemical co-precipitation route. The structural analysis indicated no alteration in the structure of hexagonal ZnO and the absence of secondary/impurity phases induced by Co/Cu addition into ZnO. The reduction of crystallite size (≈ 25 nm) at Cu = 2% is due to the suppression of growth rate and the dissimilarities between Co2+/Cu2+ and Zn2+ ions, and the enhanced crystallite size (≈ 29 nm) at Cu = 4% is responsible for the more defect sites associated with interstitials and vacancies of Co2+ and Cu2+ in Zn–O lattice. The persistent c/a ratio (~ 1.602) signified the absence of structural modification by Co/Cu substitution for Zn. The decrease in optical absorption, increase in transmittance and the enhanced energy gap of ZnO by Co/Cu addition were discussed by consideration of dopants and the stimulated defect states. The continuous widening of energy gap (ΔEg ≈ 0.08 eV) with Cu substitution is clarified using Burstein–Moss (BM) band filling effect through energy-level diagram. The existence of Zn–O and Zn–Co/Cu–O bondings around ≈ 442–468 cm−1 was verified by Fourier transform infra-red analysis. The elevated intensity ratio between green and ultra-violet photoluminescence (IG/IUV) at higher Cu concentration, Cu = 4% (≈ 0.74), revealed the occurrence of higher number of defects, particularly oxygen-related defect states, in (Zn, Co, Cu)O lattice. The observed room temperature ferromagnetism (RTFM) in Co, Cu-doped ZnO nanostructures is discussed based on the oxygen vacancy-mediated bound magnetic polarons (BMP) and the exchange interaction between the free electrons and local spin-polarized electrons.