Nonpolar 1T-to-1T' order-disorder transition in a MoS2 monolayer

Finding two-dimensional (2D) materials with ferroelectricity is of great interests towards polarization-related applications and nanosized devices. Despite much theoretical efforts that predict the existence of novel 2D ferroelectrics, only a small portion have been realized in experiments. The well-known 2D transition-metal dichalcogenide molybdenum disulfide (MoS2) monolayer was predicted to have a ferroelectric d1T polymorph resulting from condensation of soft phonons from the centrosymmetric 1T phase. However, experiments mostly obtain a nonpolar monoclinic 1T ' phase, whereas the d1T phase has not been observed. In this study, we use various first-principles techniques, including density-functional theory total-energy calculations, ab initio molecular dynamics, and the temperature-dependent effective potential method to show that the nonpolar 1T' phase is thermodynamically more stable than the d1T phase. Furthermore, the 1T phase at high temperature is averaged among degenerate 1T' structures, and the 1T-to-1T' transition upon cooling is predicted to be of first order with a strong order-disorder character. Effect of strain on the stable structure and the likely ground state of other MoX2 (X = Se, Te) monolayers are also discussed.

Automated summary

Finding ferroelectricity in two-dimensional (2D) materials is important for polarization-related applications and nanosized devices. This study investigates the existence of ferroelectricity in 2D materials and discovers that the nonpolar 1T' phase of 2D transition-metal dichalcogenide molybdenum disulfide is thermodynamically more stable. The study also discusses the effect of strain on other MoX2 monolayers.