The role of cobalt doping in tuning the band gap, surface morphology and third-order optical nonlinearities of ZnO nanostructures for NLO device applications

Abstract

The work presented here reported the effect of doping cobalt (Co) in ZnO thin films. The thin films were prepared using the spray pyrolysis technique with 0, 1, 5 and 10 wt% cobalt doping concentrations to study the morphological, optical and third-order nonlinear optical (NLO) properties. X-ray diffraction revealed the crystalline nature of the prepared thin films, and the crystallite size was found to increase with the concentration of doped Co. The morphology and surface topography of the films were largely influenced by doping, as indicated by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). With an increase in Co-doping concentration, the direct optical energy band-gap value increased from 3.21 eV to 3.45 eV for pure to 10 at% of Co concentrations respectively. To study the NLO properties of the prepared thin films, the Z-scan technique was adopted; it was observed that with an increase in the doping concentration from 0 to 10 wt%, the nonlinear absorption coefficient (β) was enhanced from 4.68 × 10⁻³ to 9.92 × 10⁻³ (cm W⁻¹), the nonlinear refractive index (n₂) increased from 1.37 × 10⁻⁸ to 2.90 × 10⁻⁸ (cm² W⁻¹), and the third-order NLO susceptibility (χ⁽³⁾) values also increased from 0.79 × 10⁻⁶ to 1.88 × 10⁻⁶ (esu). At the experimental wavelength, the optical limiting (OL) features of the prepared films were explored, and the limiting thresholds were calculated. The encouraging results of the NLO studies suggest that the Co : ZnO thin film is a capable and promising material for nonlinear optical devices and optical power limiting applications.