The Illinois Materials Resarch Science and Engineering Center (MRSEC) team has found that when it comes to deforming a two-dimensional membrane, the rule is: the curvier, the better. 

We caught up with Yingjie Zhang, Beckman Postdoctoral Fellow in the MRSEC, and he let us in on the team’s new result: 

“The adhesion of a thin film on a curved substrate is oftentimes required in everyday life, such as placing bandages on joints, covering a watermelon with a plastic food wrap, and anti-reflective coatings of camera lenses,” Zhang said. “Due to the geometrical frustrations, the deformed film or membrane typically experiences some tensional and shear stress. The magnitude of this stress depends on both the adhesion between the film and substrate, and the curvature of the substrate. Now if we thin down this film to one atomic layer of carbon atoms, like graphene, it will be strongly deformed on top of corrugated substrates.”

This mechanical deformation can serve as a knob to tune the electrical properties of graphene, transforming graphene from a semi-metal to a semiconductor, which can potentially enable high speed nano-transistors. In this system, a key "knob" researchers can adjust is the substrate curvature. 

“So here comes the question we sought to solve,” Zhang explained. “What kind of substrate corrugation feature do we need, in order to achieve a strong strain in graphene, or any other two-dimensional material in general?”

The Illinois MRSEC team has solved this problem. Yingjie Zhang (Beckman Postdoctoral Fellow), Nadya Mason (I-MRSEC director), Pinshane Huang (I-MRSEC PI), Narayana Aluru (I-MRSEC PI) and additional researchers discovered that a higher strain can be induced in graphene when the substrate radius of curvature is smaller. 

“We used a close-packed array of SiO₂dielectric nanospheres as a model system to induce deformation of graphene, and systematically varied the diameter of the nanospheres (in the range of 20 nm – 200 nm) to examine the evolution of strain in graphene,” Zhang said. "Due to a spatially inhomogeneous van der Waals interaction between graphene and the periodically spherically curved substrate, smaller spheres induce stronger interaction force, which leads to larger strain in graphene." 

“This molecular-level understanding is universal for not only graphene, but also a variety of other atomically thin membrane materials, such as transition metal dichalcogenides and black phosphorus,” Zhang noted. “This mechanistic discovery is important for strain engineering and nanoelectronic device design based on two dimensional materials.”

Keep in touch with the Illinois MRSEC. 

Read the full paper, published in NanoLetters.