Effects of pH of the precursor sol on structural and optical properties of Cu-doped ZnO thin films

Cu-doped ZnO thin films deposited on glass substrates by a sol-gel method with sol pH ranging from 6 to 10 were investigated. Characterization of the morphology and microstructures by scanning electron microscopy (SEM) and X-ray diffraction (XRD) revealed that the grain size decreases slightly with an increase of the pH value of the precursor sol. ZnO vary in their orientation along the preferred (0 0 2) plane, while the best c-axis orientation occurs in the thin film fabricated from the precursor sol with pH = 8.5. X-ray photoelectron spectroscope (XPS) and XRD results indicate that Cu ions are indeed substituted at the Zn2+ site into the ZnO lattice. Strong ultraviolet emission, blue double emission and weak green emission peaks are observed in the photoluminescence spectra of all samples at room temperature. Further studies show that adjusting the pH of the precursor sol can well tune the bandgap from 3.19 to 2.42 eV.