Haque, Nam publish groundbreaking discovery in atomically thin semiconductors

MechSE PhD student Md Farhadul Haque and Associate Professor SungWoo Nam recently published findings from their ongoing studies on flexoelectricity in Materials Today. Titled “Strongly enhanced electromechanical coupling in atomically thin transition metal dichalcogenides,” their study investigates the viability of flexoelectricity in ultrathin semiconductor materials.

Piezoelectricity, meaning electricity resulting from pressure, describes a commonly employed type of electromechanical response in materials that demonstrate asymmetry in their crystalline structure. A mechanical force applied to these materials can generate an electrical response; similarly, a charge within these materials can produce uniform strain (deformation due to stress). Flexoelectricity is a similar phenomenon, but it is not limited to materials that have asymmetric crystalline orientations—in a flexoelectric response, strain is experienced from point to point instead of uniformly along the material’s length.

In thick materials, a flexoelectric approach has historically been unable to sustainably match the same output that a piezoelectric approach can achieve, making the latter a more popular and highly studied method for manipulating electromechanical responses. However, in atomically-thin materials, also referred to as two-dimensional materials, flexoelectricity may have an advantage.

“In the past two decades of research, 2D materials have gained popularity because they have unique mechanical and electrical properties,” Haque said. “There was the potential that we could extract this electromechanical response from a 2D material, and that’s why I was interested in studying flexoelectricity.”

The research team, which is comprised of authors collaborating from Illinois and the University of South Florida, experimentally investigated the semiconductors molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂). “This work shows for the first time that flexoelectricity can surpass conventional piezoelectricity in atomically thin materials,” Nam wrote.

While the studied materials can still have asymmetry in their single atomic layer, qualifying them for piezoelectric responses, the asymmetry is only demonstrated along the materials’ plane. Perpendicular to the plane, these materials show symmetrical structure, which is where a flexoelectric response becomes valuable.

“You can extract the electromechanical response in this out-of-the-plane direction if you can harness the flexoelectricity,” Haque said, explaining that these ultrathin materials require support from a substrate in order to properly receive input. “What we’ve found is that if you can choose proper substrates, you can hit a point where you can extract an electromechanical response in the out-of-the-plane direction so high that flexoelectricity can surpass piezoelectricity in terms of the output.”

The research team’s discovery is significant for developing the next generation of energy harvesters and actuators. These actuators have a variety of applications; for example, small-scale actuators are a common working component in rocket ships. Rockets designed to probe deep space will need to operate in extremely cold conditions; however, piezoelectricity has temperature constraints and can cease to operate in low temperatures, whereas flexoelectricity does not demonstrate this limitation. “It’s possible that we can make actuators that can be used in rockets in extreme conditions,” Haque said of applying flexoelectric materials.

Other team members and Haque are currently working to develop actual devices that use flexoelectricity. “This particular work was mostly a fundamental study—I did experimental measurements of just the material itself,” Haque explained. “We are now exploring devices based on this phenomenon and comparing them with piezoelectricity-driven devices to show differences in operation.”