Transition from band insulator to excitonic insulator via alloying Se into monolayer Ti S3: A computational study

First-principles density functional theory plus Bethe-Salpeter equation calculations are employed to investigate the electronic and excitonic properties of monolayer titanium trichalcogenide alloys TiS3-xSex (x=1 and 2). It is found that the band gap and exciton binding energy display asymmetric dependence on the substitution of Se for S. While the band gap can be significantly decreased as compared to that of pristine TiS3, the exciton binding energy varies slightly, regardless of the position and concentration of the Se substitution. A negative exciton formation energy is found when the central S atoms are replaced by Se atoms, suggesting a many-body ground state with spontaneous exciton condensation. Our work thus offers insight for engineering an excitonic insulator.