Ti-fraction-induced electronic and magnetic transformations in titanium oxide films

Titanium dioxide has been widely used in modern industrial applications, especially as an effective photocatalyst. Recently, freestanding TiO₂ films with a markedly reduced bandgap of ∼1.8 eV have been synthesized, indicating that the dimension has a considerable influence on the bulk band gap (>∼3 eV) and enhances the adsorption range of visible light. Titanium oxide compounds have various stoichiometries and versatile properties. Therefore, it is very necessary to explore the electronic properties and functionalities of other titanium oxide films with different stoichiometries. Here, we combined structure searches with first-principle calculations to explore candidate Ti-O films with different stoichiometries. In addition to the experimentally synthesized TiO₂ film, the structure searches identified three new energetically and dynamically stable Ti-O films with stoichiometries of Ti₃O₅, Ti₃O₂, and Ti₂O. Calculations show that the Ti-O films undergo several interesting electronic transformations as the Ti fraction increases, namely, from a wide-gap semiconductor (TiO₂, 3.2 eV) to a narrow-gap semiconductor (Ti₃O₅, 1.80 eV) and then to metals (Ti₃O₂ and Ti₂O) due to the abundance of unpaired Ti_d electrons. In addition to the electronic transformations, we observed nonmagnetic (TiO₂) to ferromagnetic (Ti₃O₅, Ti₃O₂, and Ti₂O) transformations. Notably, the Ti₃O₅ film possesses both narrow-gap semiconductive and ferromagnetic properties, with a large magnetic moment of 2.0 µ_B per unit cell; therefore, this film has high potential for use in applications such as spintronic devices. The results highlight metal fraction-induced electronic and magnetic transformations in transition metal oxide films and provide an alternative route for the design of new, functional thin-film materials.