The Effect of Chemical States of Dopants on the Microstructures and Band Gaps of Metal-Doped ZrO₂ Thin Films at Different Temperatures

The microstructures and band gaps of sol−gel-derived ZrO 2 thin films incorporating seven-transition-metal ions (Cr 3+, Mn 2+, Fe 3+, Co 2+, Ni 2+, Cu 2+, and Zn 2+ ) were investigated at 550 and 950 °C. Results obtained in this study indicate that the chemical states of the dopants play pivotal roles in controlling the physicochemical properties of the metal-doped ZrO 2 thin films at different temperatures. The reduction of Cr 3+ and Co 2+ to zerovalent metals expanded the d spacings and enlarged the crystallite sizes in the doped ZrO 2 thin films at 550 °C, while the incorporation of Mn 2+, Fe 3+, Ni 2+, Cu 2+, and Zn 2+ contracted the d spacings of the ZrO 2 lattices in the solid solutions. The solubility of the dopants in the ZrO 2 lattices decreased at 950 °C, and this phenomenon induced an m-tetragonal-to-monoclinic phase transformation in the doped ZrO 2 . The phase transformation was suppressed in the Cr-, Fe-, and Cu-doped ZrO 2 thin films as a result of the reduction of Cr 3+ and Fe 3+ to species having larger sizes than Zr 4+ . Band gaps of ZrO 2 were reduced in the doped ZrO 2 thin films. The newly generated band gaps of these solid solutions at 550 °C were highly dependent on the d-electronic configurations of the dopants. On the other hand, the segregated metal oxides or zerovalent metals from the ZrO 2 lattice at 950 °C dominated the band gaps of the doped ZrO 2 thin films. Dehydroxylation and deoxygenation processes drove the reduction of metal ions during the thermal treatment. The metal ions having d 5 and d 10 configurations in the ZrO 2 lattice possessed high stability against thermally induced reductions, while the other ions tended to be reduced in the sol−gel-derived ZrO 2 matrix.