Defect-Induced Phase Transformations in CuO Thin Films by Ag Ion Implantation and Their Gas-Sensing Applications

The present study is aimed to modify the structural, optical, and morphological properties of CuO thin films with Ag ion implantation toward their potential applications. Fabrication of thin films of CuO was carried out using a thermal evaporation technique on silicon (Si) substrate; post implantation was done with 30 keV Ag ions with varying fluences from 1 × 1014 to 1 × 1016 ions/cm2. The pristine and implanted samples were characterized through X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), and diffused reflectance spectroscopy (DRS). SRIM simulation was also carried out to understand the range and other effects. XRD measurements confirmed the formation of the CuO phase for pristine samples, and with the increase in ion fluence, the chemical composition changed from pure CuO to Cu2O + CuO and then to Cu2O + Cu at higher ion fluences. These experimental results from Raman spectroscopy were also consistent with the XRD analysis, which also showed the presence of mixed phases of CuO, Cu2O, and Cu. Results from AFM showed that the value of average roughness increased due to more defects. DRS results revealed the variation in band-gap energy for pristine and implanted samples from 1.96 to 1.71 eV. Density functional theory (DFT) simulations confirmed that Ag ion implantation causes the reduction of CuO surface into Cu2O and Cu. We found that Ag ion implantation-induced defects lead to the mixed phases of CuO, Cu2O, and Cu, which can help in the selective sensing of ethanol and acetone, demonstrating this as a useful technique and having potential applications in energy conversions, and gas and biomolecule sensing.