Material strength and toughness simultaneously achieved through layer twisting

The mechanical strength and toughness of engineering materials are often mutually exclusive, posing challenges for material design and selection. To address this, a research team from The Hong Kong Polytechnic University (PolyU) has uncovered an innovative strategy: by simply twisting the layers of 2D materials, they can enhance toughness without compromising material’s strength.

This breakthrough facilitates the design of strong and tough new 2D materials, promoting their broader applications in photonic and electronic devices. The findings have been published in Nature Materials.

While 2D materials often exhibit exceptional strength, they are extremely brittle. Fractures in materials are also typically irreversible. These attributes limit the use of 2D materials in devices that require repeated deformation, such as high-power devices, flexible electronics and wearables.

Efforts to improve toughness by introducing defects, such as vacancies and grain boundaries, often degrade intrinsic electrical properties, leading to a trade-off between mechanical durability and electronic performance. Therefore, enhancing both the strength and toughness of bulk materials for engineering applications has remained a significant challenge.

To overcome these limitations, a research team led by Prof. Jiong Zhao, Professor of the PolyU Department of Applied Physics, has pioneered a novel twisting engineering approach whereby twisted bilayer structures enable sequential fracture events, addressing the conflict between strength and toughness in 2D materials. The finding was supported by nanoindentation and theoretical analysis.

Typical transition metal dichalcogenides (TMDs) are a class of 2D materials known for their unique electronic, optical and mechanical properties. These characteristics enable their diverse applications in electronics and optoelectronics, energy storage and conversion, sensors and biomedical devices, quantum technologies, mechanics and tribology. By focusing on TMDs, such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), the team discovered a new fracture mechanism in twisted bilayers.

Using in situ transmission electron microscopy, the team found that when cracks propagate in twisted bilayer structures, the lattice orientation mismatch between the upper and lower layers leads to the formation of interlocking crack paths.

Following the initial fracture, the crack edges in both layers spontaneously form stable grain boundary structures through interlayer self-assembly. This distinctive “crack self-healing” mechanism protects subsequent fracture tips from stress concentration, effectively preventing further crack propagation. Notably, this process consumes more energy than conventional fracture, and the degree of toughness enhancement can be tuned by adjusting the twist and twist angle.

Prof. Jiong Zhao said, “By breaking through the framework of conventional fracture mechanics theory, this study presents the first demonstration of autonomous damage suppression in 2D materials, establishing a groundbreaking strategy for designing integrated novel strong-and-tough 2D materials. This research also extends the application of twistronics to mechanical performance design, such as with regard to material strength, opening exciting possibilities for the development of future electronic and photonic devices.

“As fabrication techniques for twisted 2D materials continue to mature, a new generation of smart materials combining superior mechanical properties with exotic electrical characteristics, holds great promise for technological innovation in the fields of flexible electronics, energy conversion, quantum technology and sensing.”