- For graphene-like materials, 1+1 doesn’t make 2.
- When such atomically thin materials are stacked on top of each other their properties change drastically.
- This yields a new material with novel hybrid characteristics.
- The study provides an opportunity to explore unusual material properties.
The concept of stacking layer of distinct materials to build heterostructures was introduced in 1962, when gallium arsenide (direct band semiconductor) was studied for producing laser diodes, which are now extensively used to convert electrical energy into light.
These types of heterostructures have become common nowadays: they are widely used in the semiconductor industry to control optical characteristics in electronic devices.
A few recent discoveries of atomically thin two-dimensional materials like graphene have inspired scientists to build new kinds of heterostructures. Recently, physicists at the University of Sheffield found that when two incredibly thin graphene-like materials are stacked on top of each other their properties change drastically, and they yield a completely new material with novel hybrid characteristics.
How Does It Happen?
The atomically thin layers of these two materials, when placed on top of each other, are held together by van der Waals forces, a relatively weak force ranging from 0.5 to 1 kcal/mol – the same forces that help geckos walk effortlessly along walls.
The new structures named van der Waals heterostructures are made of TMDs (short for transition metal dichalcogenides). They are quite similar to graphite in their 3D form. Graphene is extracted from graphite as a single layer of carbon atoms arranged in a hexagonal lattice.
According to the researchers, when two atomically thin materials are combined in a single structure, they influence each other’s properties and form a completely new metamaterial with unique properties. Thus, for graphene, you can say that 1+1 doesn’t make 2.
The extent of hybridization depends on the twist between every single atomic lattice of each layer. By twisting the layer, an overarching periodicity — known as moiré superlattice — emerges in the structure. The moiré superlattice determines how the properties of two atomically thin materials hybridize.
Similar results have been observed in previous studies, especially in the case of graphene, which can be considered as an indefinitely large polycyclic aromatic hydrocarbon. The latest research demonstrates that other semiconductors, such as TMDs, exhibit strong hybridization, and their properties can be manipulated by the twist angle.
The findings could enable scientists to access various material properties including twist-configurable optical response, electrical conductivity, and magnetism. It can also pave the way for designs of new materials and nano-scaled instruments.
The research team plan to investigate more combinations of materials to figure out the actual potential of the new method. Hundreds of combinations are possible in 2D materials that are otherwise inaccessible in conventional 3D materials, which provide opportunities to explore unusual optoelectronic device functionality.