Tunable optoelectronic properties in multilayer 1T-TiS2: the effects of strain and an external electric field

In this work, we study the electronic properties of mono- and multilayer titanium disulfide (TiS2) with the aid of first-principles calculations based on density functional theory. We find that the band gap can slightly be tuned as a function of the number (N) of stacked layers, ranging from 0.49 eV in the monolayer down to 0.40 eV in the bulk form—as a result of quantum confinement and the formation of sub-bands. However, the introduction of external agents such as biaxial strain and electric fields can significantly change the electronic properties of the system and induce strong gap modifications. Compressive strains and electrical fields are found to reduce the indirect band gap and induce a semiconductor to semimetal transition beyond a critical value, which is a decreasing function of N. In contrast, under tensile strains, the gap increases up to a maximum value and can reach about 0.90 eV under a 5% strain. Furthermore, we also report the optical properties of these systems, which display strong absorption peaks in both visible and UV regions of the spectrum, thus making the most of incident solar light. These properties also display a good tunability, as the peak intensities increase with N and the peak positions show a strong dispersion with strain. However, the spectra are less sensitive to electrical fields, despite their response being very similar to that found under compressive strains. Finally, k-resolved band structure calculations suggest the existence of both intralayer and interlayer excitons in optical transitions in the visible range. In light of these results, we believe that TiS2 can efficiently be explored in the design of novel vdW heterostructures in combination with other 2D materials, thus opening the way to novel applications in future nano- and optoelectronic devices.