Defects engineer fracture resistance in 2D materials

(a) High-resolution transmission electron microscopy (HRTEM) characterization of a suspended MoSe2 monolayer at a dose rate of 5.8 × 105 e-nm-2-s-1. The isolated Se (iSe) vacancies, isolated Se divacancies (iSe2) and isolated Mo (iMo) vacancies are marked by green, yellow, and blue circles, respectively; (b) The corresponding vacancy densities in (a), measured from 23 regions of 2 × 2 nm in an HRTEM image, and (c) their atomic representation. (d-e) HRTEM images depicting the emergence of Se vacancy lines (lSe) at a dose rate > 1.5 × 107 e-nm-2-s-1, and (f) atomic representation of a vacancy line sharing the same characteristic length and orientation as in (e). (g) The evolution of iSe vacancies and vacancy lines as a function of electron dose rate. The total dose was kept constant for the four conditions. Scale bar: (a) 1 nm, (d) 2 nm, (e) 1 nm. Reproduced with permission from: Nguyen et al., Materials Today (2023), https://doi.org/10.1016/j.mattod.2023.10.002.

Two-dimensional transition metal dichalcogenides (TMDs) are highly attractive for applications because of their electronic and optoelectronic properties, which can be fine-tuned by manipulating defects known as vacancies. Now researchers from Northwestern University, Argonne National Laboratory, and Rice University have shown how these vacancies affect the mechanical strength of one such TMD, hexagonal MoSe₂, too [Nguyen et al., Materials Today (2023), https://doi.org/10.1016/j.mattod.2023.10.002].

Vacancies play a complex role in TMDs: depending on the type and density, vacancies can reduce crack resistance rendering devices less reliable or they can trigger the formation of phases that improve toughness and fracture behavior.

“[We wanted to know] would defects… alter the mechanical behavior of TMDs? And, if so, what are the mechanisms and can these defects be designed to engineer the mechanical properties of TMDs,” explains first author of the study, Hoang Nguyen.

The team, led by Horacio D. Espinosa, used controlled electron irradiation doses in a high-resolution transmission electron microscope to create different types and densities of vacancies. At low dose rates, zero dimensional (0D) isolated Se (iSe), Se₂ (iSe₂), and Mo (iMo) vacancies are formed, while at higher dose rates 1D Se line (lSe) vacancies form. The effect of these vacancy types on the mechanical behavior of MoSe₂was modelled using molecular dynamics (MD) simulations.

“We used the types and densities [of] observed defects to make predictions about their interaction with cracks to see how they affect one another,” says Nguyen. “These predictions were analyzed and explained using a series of mechanical-chemical tools.”

By comparing experimental results and first principle calculations, the researchers reveal that three vacancy types affect the interaction of cracks and defects while one type does not. Isolated metal monovacancies (iMo) and chalcogenide divacancies (iSe₂) arrest the motion and development of crack tips, increasing toughness and resistance to fracture. High-dose chalcogenide line vacancies (lSe) also affect cracks but in a different way. This type of 1D defect attracts propagating cracks, altering the propagation direction and leading to kinking. Isolated chalcogenide monovacancies (iSe), meanwhile, have no significant effect on crack behavior.

“This study shows that defect engineering using controlled electron dose rate or other methods could be feasible… [to] bring about promising enhancement of TMDs’ ductility, a material that is notorious for its brittleness,” points out Nguyen.

Designing patterns of defects could offer a means of controlling fracture behavior in TMDs, at the same time as optimizing optical and electronic properties, say the researchers.