Reversible Changes in Resistance of Perovskite Nickelate NdNiO₃ Thin Films Induced by Fluorine Substitution

Perovskite nickel oxides are of fundamental as well as technological interest because they show large resistance modulation associated with phase transition as a function of the temperature and chemical composition. Here, the effects of fluorine doping in perovskite nickelate NdNiO₃ epitaxial thin films are investigated through a low-temperature reaction with polyvinylidene fluoride as the fluorine source. The fluorine content in the fluorinated NdNiO_3-xF_x films is controlled with precision by varying the reaction time. The fully fluorinated film (x ≈ 1) is highly insulating and has a bandgap of 2.1 eV, in contrast to NdNiO₃, which exhibits metallic transport properties. Hard X-ray photoelectron and soft X-ray absorption spectroscopies reveal the suppression of the density of states at the Fermi level as well as the reduction of nickel ions (valence state changes from +3 to +2) after fluorination, suggesting that the strong Coulombic repulsion in the Ni 3d orbitals associated with the fluorine substitution drives the metal-to-insulator transition. In addition, the resistivity of the fluorinated films recovers to the original value for NdNiO₃ after annealing in an oxygen atmosphere. By application of the reversible fluorination process to transition-metal oxides, the search for resistance-switching materials could be accelerated.