Structural and optical investigation of Co-doped ZnO nanoparticles for nanooptoelectronic devices

Cobalt-doped ZnO nanoparticles (NPs), with concentration of Co2+ varying between 1 and 5%, have been synthesized by a sol–gel procedure. The structural, morphological and optical properties of these nanoparticles were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and ultraviolet–visible (UV–Vis) measurements. The X-ray diffraction shows that these semiconductors crystallize in a wurtzite single crystalline phase with P63mc space group. In this structure, the cobalt ion Co2+ substitutes for a Zinc ion Zn2+ in a ZnO semiconductor and occupies a tetrahedral Td site symmetry surrounded by four oxygen atoms with a slight distortion. The TEM images characterize the morphology and crystalline structure of these semiconductors. From the UV spectra, a direct bandgap semiconductor was assumed for ZnO:Co2+ NPs. The bandgap energy of the ZnO lattice gradually decreases following the addition of Co2+ ions. The red absorption spectra are ascribed to the electronic transitions of Co2+ ion in ZnO. To determine the electronic structure of the transition ion Co2+, the crystal field theory is applied for the visible spectrum associated with the d–d transitions of this ion located at a Td site symmetry in ZnO NPs. The experimental and theoretical results of energy levels are in agreement. The results were compared to that published for the Co2+ ion in NP ZnO.