Researchers at Columbia University have demonstrated the ability to fine-tune the electronic, mechanical and optical properties of 2D heterostructures like graphene on boron nitride by varying the angle between the crystals in real time. Image: Philip Krantz/Krantz NanoArt.
Researchers at Columbia University have demonstrated the ability to fine-tune the electronic, mechanical and optical properties of 2D heterostructures like graphene on boron nitride by varying the angle between the crystals in real time. Image: Philip Krantz/Krantz NanoArt.

Two-dimensional (2D) materials such as graphene have unique electronic, magnetic, optical and mechanical properties that promise to drive innovation in areas from electronics to energy to materials to medicine. Researchers at Columbia University now report a major advance that may revolutionize the field, a ‘twistronic’ device whose characteristics can be varied by simply varying the angle between two different 2D layers placed on top of one another.

In a paper in Science, the team reports a novel device structure that not only gives them unprecedented control over the angular orientation in twisted-layer devices, but also allows them to vary this angle in situ. As a result, they can study the effects of twist angle on the electronic, optical and mechanical properties of a single device.

Led by physicist Cory Dean and mechanical engineer James Hone at Columbia University, the team built upon techniques they previously pioneered to mechanically layer graphene and other 2D materials on top of one another to form new structures.

"This mechanical assembly process allows us to mix and match different crystals to construct entirely new materials, often with properties fundamentally different from the constituent layers," says Hone, leader of Columbia's Materials Research Science and Engineering Center (MRSEC), which investigates the properties of these heterostructures. "With hundreds of 2D materials available, the design possibilities are enormous."

Recent studies have shown that rotational alignment between the 2D layers plays a critically important role in determining the new properties that arise when materials are combined. For example, when conducting graphene is placed on top of insulating boron nitride with the crystal lattices perfectly aligned, graphene develops a band gap. At non-zero angles, however, the band gap disappears and intrinsic graphene properties are recovered. Just this past March, researchers at Massachusetts Institute of Technology reported the ground-breaking discovery that two stacked layers of graphene can exhibit exotic properties, including superconductivity, when the twist angle between them is set to 1.1°, referred to as the ‘magic angle’.

In previous approaches to fabricating structures with rotationally misaligned layers, the angle was set during the assembly process. This meant that once the device was made, its properties were fixed. "We were finding this approach frustrating, since very small errors in alignment could give entirely different results," says Dean. "It would be great to make a device in which we could study its properties while continuously rotating its layers and so the question was, how to do this?"

The answer, the Columbia researchers realized, was to take advantage of the low friction that exists at the interface between the 2D layers, which are held together by van der Waals forces that are much weaker than the atomic bonds within each layer. This low friction – which ensures 2D materials are very good solid lubricants – makes controlled assembly at a desired angle very difficult. But the Columbia group used this low friction characteristic to their advantage, by designing a device structure in which, instead of preventing rotation, they could intentionally and controllably vary the rotation angle.

The team used graphene/boron-nitride heterostructures to demonstrate the reach of their technique. In these structures, when the layers are not crystallographically aligned, the materials preserve their original properties (e.g. graphene will have a semi-metallic character). When the layers are aligned, however, the properties of graphene change, opening an energy gap that causes it to behave like a semiconductor. The researchers showed that this fine-tuning of the heterostructure's properties affects its optical, mechanical and electronic responses.

"Notably, we demonstrated that the energy gap observed in graphene is tunable and can be turned on or off on demand just by changing the orientation between the layers," says Rebeca Ribeiro, who led this work while a post-doctoral researcher at Columbia but is now a CNRS research scientist at the French Center for Nanoscience and Nanotechnology (C2N-CNRS). "The tuning of this energy gap not only represents a major step toward the future use of graphene in varied applications, but also provides a general demonstration in which the device properties of 2D materials are dramatically varied with rotation."

From a technological point of view, the ability to tune the properties of a layered material by varying the twist angle offers a way for a single material platform to perform a variety of functions. For example, electronic circuits are built from a finite number of components including metallic conductors, insulators, semiconductors and magnetic materials. This process requires integration of a variety of different materials and can pose a significant engineering challenge. In contrast, a single material that can be locally ‘twisted’ to realize each of these components could allow significant new engineering opportunities.

In addition, the ability to dynamically tune a system with mechanical twisting offers a new switching capability that could lead to entirely new device applications. For example, traditional switches typically vary between two well-defined states (on or off, magnetic or not, etc). The Columbia platform could allow the possibility of switching between an arbitrary number of complementary states.

Dean and Hone are now using their new technique to study other combinations of 2D materials in which the properties can be tuned by angular alignment. They are particularly focusing on the recent discovery of superconductivity in twisted bilayer graphene and exploring whether it may be a general feature of twisted bilayers made from arbitrary 2D materials.

"Our study demonstrates a new degree of freedom, namely rotational orientation between layers, that just doesn't exist in conventional semiconductor heterostructures," says Dean. "This is a rare occasion in the semiconductor field where we are truly forging a new path, and opens the door to a whole new field of research where material properties can be varied simply by twisting the structure."

This story is adapted from material from Columbia Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.