Modulation of the NiO_x bandgap by controlling oxygen stoichiometry

Transition metal oxides are a class of functional materials widely used in optoelectronics, spintronics, and memory technology. The oxygen stoichiometry of these oxides plays a vital role in determining their electronic, optical, and thermal properties. Post-growth annealing in ozone has shown to be effective in modifying these properties. Here, we choose NiO, an antiferromagnetic Mott insulator in perfect stoichiometry, as an example to show that its stoichiometry can be tuned continuously in a broad range by the control of the oxidation power during growth or a post-growth topotactic reduction process. The bandgap of the as-processed NiO x films was modulated in accordance with their resistivity, lattice constant, and Ni chemical valence. This method can be readily applied to other transition metal oxides for the optimization of their properties.