Twist-assisted intrinsic toughening in two-dimensional transition metal dichalcogenides.

Material fractures are typically irreversible, marking a one-time event leading to failure. Great efforts have been made to enhance both strength and fracture toughness of bulk materials for engineering applications, such as by introducing self-recovery and secondary breaking behaviours. In low-dimensional structures, two-dimensional materials often exhibit exceptional strength but accompanied by extreme brittleness. Here we discover that the toughness of two-dimensional materials can be enhanced without sacrificing strength-by simply twisting the layers. Through in situ scanning transmission electron microscopy, supported by nanoindentation and theoretical analysis, we reveal that twisted bilayer structures enable sequential fracture events: initial cracks heal to form stable grain boundaries, which then shield subsequent fracture tips from stress concentration. This process consumes additional energy compared with conventional fracture, with toughness enhancement tunable through twist angle adjustment. The intrinsic toughening mechanism via twisting, along with the emerging electronic properties of twistronics that are currently attracting substantial attention, presents an exciting opportunity for future devices.