Structural Transformations in Two-Dimensional Transition-Metal Dichalcogenide MoS₂ under an Electron Beam: Insights from First-Principles Calculations

Abstract

The polymorphism of two-dimensional (2D) transition-metal dichalcogenides (TMDs) and different electronic properties of the polymorphs make TMDs particularly promising materials in the context of applications in electronics. Recently, local transformations from the semiconducting trigonal prismatic H phase to the metallic octahedral T phase in 2D MoS have been induced by electron irradiation [ Nat. Nanotech. 2014 , 9 , 391 ], but the mechanism of the transformations remains elusive. Using density functional theory calculations, we study the energetics of the stable and metastable phases of 2D MoS when additional charge, mechanical strain, and vacancies are present. We also investigate the role of finite temperatures, which appear to be critical for the transformations. On the basis of the results of our calculations, we propose an explanation for the beam-induced transformations, which are likely promoted by charge redistribution in the monolayer due to electronic excitations combined with formation of vacancies under electron beam and buildup of the associated mechanical strain in the sample. As this mechanism should be relevant to other 2D TMDs, our results provide hints for further development and optimization of electron-beam-mediated engineering of the atomic structure and electronic properties of 2D TMDs with subnanometer resolution.

Multiscale computational understanding and growth of 2D materials: a review

“…The well-known phase-stability competition between the 2H and 1T′ types in transition metal dichalcogenides (TMDs) has been studied extensively using DFT. Several possible mechanisms for the 1T′phase stabilization in the disulfide, including growth-related factors such as the presence of point defects and residual strain, have been elucidated through DFT calculations 19 . Phase-stability analysis for the entire group -IV metal dichalcogenide family 20 revealed that the 1T′ phases are generally more stable for ditellurides.…”

Strategies on Phase Control in Transition Metal Dichalcogenides

“…Apart from the relatively complex substrate design, physical collision can also render target considerable strain to induce phase transition . Recently, a facile Ar plasma method has been proposed to the trigger phase transition between 2H–1T′ in monolayer MoS 2 .…”

Convergent ion beam alteration of 2D materials and metal-2D interfaces

“…Phase or structural transformation has been demonstrated by electron beam irradiation [42,43], laser beam [44] and electrostatic [45] methods. First principle calculations have also shown that the combination of strain, vacancies, and electronic excitations created by an electron beam can lead to a phase transition [46]. The structure transformation induced by an Ar + ion beam merits further investigation into its mechanism and implications on physical, chemical and electrical properties.…”