Semiconductor-metal transition in vulcanized NiCo2O4 film

The effects of vulcanization on the electrical transport properties and magnetic structure of NiCo2O4 were studied in this work. NiCo2O4 films were epitaxially grown on Al2O3 substrate, and then vulcanized. The NiCo2O4 structures with O replaced by S atoms were calculated based on the density functional theory. As O were replaced by S atoms, the orbital overlap between S and cations increases, the localized electrons around cations are transformed into collective electrons, and the conductive behavior of the film changes from semiconductor to metal. The formation of covalent bonds caused by S substitution leads to the weakening of the interaction between tetrahedral Co and octahedral Ni ions. Meanwhile, the d-orbital energy level of cations shifts. The spin direction of Ni ions and half tetrahedral Co ions is flipped in the vulcanized NiCo2O4 structure, and the electrons at Ni3+ eg level enter the conduction band, which makes the carrier type of the vulcanized NiCo2O4 film change from p to n-type. These results indicate that S atoms play an active role in improving the conductivity of NiCo2O4.