Structural, Electronic, and Optical Properties of 2D Titanium Oxide Layers on MgO (001)*

Two-dimensional transition metal oxides (2DTMOs) are a promising addition to the growing array of functional 2D materials, with potential applications related to their long-lived, strongly bound excitons. In addition, 2DTMOs are expected to be more stable than other 2D materials since they do not react with water or oxygen species. However, unlike some other chalcogenides, 2DTMOs do not naturally occur in stackable van der Waals-bonded layers, so they present challenges for structural prediction and characterization. We report on ab initio density functional theory simulations of 2D titanium oxide layers on the (001) surface of MgO. We consider both monolayer TiO₂, including various surface reconstructions, and few-layer Mg₂TiO₄ based on a bulk inverse spinel structure. We examine the feasibility of various interfaces between the Mg₂TiO₄ overlayer and the MgO(001) substrate. We report on calculations of the optical absorption spectrum of low-energy structures in order to compare to experiment. Finally, we describe the improved treatment of the orbital energies of low-energy structures using a nonlocal hybrid functional.