Engineering of Strongly Bound Interlayer Excitons in Mg₂TiO₄ Thin Films

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. 2DTMOs are expected to be stable since they do not react readily with water or oxygen. However, unlike many other 2D materials, 2DTMOs do not natively form stackable van der Waals-bonded layers, so they present challenges for structural prediction and characterization. Recent experimental work on the MgO(001) surface has demonstrated the growth of thin films of Mg₂TiO₄, whose low energy electronic states are dominated by Ti and O orbitals. We review the structure of these thin films, computed at the DFT level, and present many-body calculations of their electronic excitations. Our DFT calculations demonstrate that the film has a band offset favorable for interlayer exciton formation. Motivated by that work, we present GW and GW-BSE calculations of quasiparticle energies, exciton binding energies, and optical absorption spectra. These calculations characterize the suite of intra- and interlayer excitons that exist in Mg₂TiO₄ and shed light on the importance of film thickness in controlling their relative binding energies.