The moiré superlattice formed by stacked and twisted layers of 2D transition metal dichalcogenides. Image: University of Sheffield.
The moiré superlattice formed by stacked and twisted layers of 2D transition metal dichalcogenides. Image: University of Sheffield.

Physicists at the University of Sheffield in the UK have discovered that placing two atomically thin graphene-like materials on top of each causes the emergence of novel hybrid properties. This finding, reported in a paper in Nature, paves the way for the design of new materials and nano-devices.

The emergence of hybrid properties happens without the two atomic layers becoming physically mixed or undergoing a chemical reaction. Instead, the layers attach via a weak, so-called van der Waals, interaction – similar to how sticky tape attaches to a flat surface.

The physicists found that the properties of the new hybrid material can be precisely controlled by twisting the two stacked atomic layers. This opens the way to using this technique for the nano-scale control of composite materials and nano-devices in future technologies.

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

More recently, in the era of atomically thin two-dimensional (2D) crystals such as graphene, new types of heterostructures have emerged, where atomically thin layers are held together by relatively weak van der Waals forces.

These new structures, nicknamed 'van der Waals heterostructures', offer a huge potential for creating numerous 'meta'-materials and novel devices, by stacking together any number of atomically thin layers. Hundreds of combinations become possible that can’t be achieved with traditional three-dimensional materials, offering access to unexplored optoelectronic device functionality or unusual material properties.

In the study, the researchers produced van der Waals heterostructures made out of so-called transition metal dichalcogenides (TMDs), a broad family of layered materials. In their three-dimensional bulk form, these materials are somewhat similar to graphite – the material used in pencil leads – from which graphene was first extracted as a single 2D atomic layer of carbon.

The researchers found that when two atomically thin semiconducting TMDs are combined in a single structure their properties hybridize. "The materials influence each other and change each other's properties, and have to be considered as a whole new 'meta'-material with unique properties – so one plus one doesn't make two," explained Alexander Tartakovskii of the Department of Physics and Astronomy at the University of Sheffield.

"We also find that the degree of such hybridization 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 hybridize."

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 hybridization, which can be controlled by the twist angle.

"The more complex picture of interaction between atomically thin materials within van der Waals heterostructures emerges," said Tartakovskii. "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."

The researchers would like to conduct further studies to explore more material combinations and investigate the capabilities of the new method.

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