Substoichiometric Tuning of the Electronic Properties of Titania

Reduced titania phases such as Ti₂O₃, Ti₃O₅, and Ti_xO_2x-1 (the magńeli phase, 4 ≤ x ≤10) are collectively a promising class of materials due to the stoichiometric tunability of their electronic structure. This flexibility, combined with their relatively high electrical conductivity, high corrosion resistance, and photostability makes reduced titania a candidate for many applications. In this work, stable reduced titania films were synthesized on silicon substrates via pulsed laser deposition (PLD) under a forming gas environment at elevated temperatures. The atomic/electronic structure and the electrical properties of the PLD reduced titania films were investigated using four-point probe, Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy (XAS), and ultraviolet photoelectron spectroscopy (UPS). The PLD films demonstrated a notable decrease in resistivity compared to those of stoichiometric TiO₂ films reported in the literature. Raman spectroscopy and XAS revealed that the observed resistivities in these films are correlated to substoichiometric titania phases (Ti₂O₃ between 400-600 °C and the Ti₃O₅-like at 700 °C) present in them. UPS measurements on the sub-stoichiometric titania indicated a decrease in the work functions with a decrease in the titanium oxidation state. The ability to tune such electrical properties with careful control of preparation is key to using reduced titania as a flexible platform for material design.