Stacking 2D materials changes their properties

Physicists from the University of Manchester and the University of Sheffield have discovered that when two atomically thin (two-dimensional) materials like graphene are stacked on top of each other, their properties change and a material with novel hybrid properties emerges — a breakthrough that could pave the way for new materials and nano devices.

The idea of stacking layers of different materials to make so-called heterostructures goes back to the 1960s, when semiconductor gallium arsenide was researched for making miniature lasers. Today, heterostructures are common and are used broadly in the semiconductor industry as a tool to design and control electronic and optical properties in devices.

In the era of atomically thin 2D crystals, such as graphene, new types of heterostructures have emerged, where atomically thin layers are held together by nothing more than relatively weak van der Waals forces — similar to how a sticky tape attaches to a flat surface. These so-called ‘van der Waals heterostructures’ offer the potential to create numerous designer-materials and novel devices by stacking together any number of atomically thin layers, with hundreds of possible combinations that would be otherwise inaccessible in traditional 3D materials.

The UK researchers tested van der Waals heterostructures made out of so-called transition metal dichalcogenides (TMDs), a broad family of layered materials that are in their 3D form somewhat similar to graphite. The researchers found that when two atomically thin semiconducting TMDs are combined in a single structure, their properties hybridise.

“The materials influence each other and change each other’s properties, and have to be considered as a whole new metamaterial with unique properties — so one plus one doesn’t make two,” said Professor Alexander Tartakovskii from the University of Sheffield.

“We also find that the degree of such hybridisation is strongly dependent on the twist between the individual atomic lattices of each layer.

“We find that when twisting the layers, the new supra-atomic periodicity arises in the heterostructure — called a moiré superlattice.

“The moiré superlattice with the period dependent on the twist angle governs how the properties of the two semiconductors hybridise.”

The atomic layers being stacked.

In other studies, similar effects have been discovered and studied mostly in graphene, the ‘founding’ member of the 2D materials family. The latest study shows that other materials, in particular semiconductors such as TMDs, show strong hybridisation that can be precisely controlled by twisting the two stacked atomic layers.

“By controlling the hybridisation of electrons’ states in heterostructures and also using moiré superlattice effects, which are generic for heterostructures of atomically thin films, we acquire a new handle for tailoring optical properties of materials,” said Professor Vladimir Falko, Director of the National Graphene Institute at the University of Manchester.

The scientists believe their study, published in the journal Nature, shows huge potential for the creation of new types of materials and devices. They would like to do further studies to explore more material combinations to see what the capabilities of the new method are.

“The more complex picture of interaction between atomically thin materials within van der Waals heterostructures emerges,” Prof Tartakovskii said. “This is exciting, as it gives the opportunity to access an even broader range of material properties such as unusual and twist-tunable electrical conductivity and optical response, magnetism etc. This could and will be employed as new degrees of freedom when designing new 2D-based devices.”

Top image credit: ©stock.adobe.com/au/artegorov3@gmail

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