Mar 7 2019
At the University of Sheffield, physicists have discovered that placing a pair of atomically thin graphene-like materials on top of one another changes their properties and yields a material that has unique hybrid properties. This latest discovery paves the way for designing next-generation nano-devices as well as materials.
This phenomenon occurs without manually combining the two atomic layers or using a chemical reaction, but takes place by connecting the layers to one another through a feeble so-called van der Waals interaction – akin to the way a sticky tape fixes to a flat surface.
In the breakthrough study reported in Nature, researchers have also discovered that the characteristics of the novel hybrid material can be accurately regulated by twisting the two atomic layers stacked together, paving the way for using this exceptional degree of freedom for the nano-scale control of nano-devices and composite materials in upcoming technologies.
The concept of stacking layers of various materials to create the so-called heterostructures dates back to the 1960s—the period when semiconductor gallium arsenide was studied for developing tiny lasers, which are extensively being used today.
Heterostructures are very common today and are employed quite widely in the semiconductor sector as a tool for designing and controlling optical and electronic properties in devices.
Of late, in the period of atomically thin two-dimensional (2D) crystals, for example, graphene, novel types of heterostructures have evolved, in which relatively weak van der Waals forces hold together atomically thin layers.
The novel structures dubbed “van der Waals heterostructures” present immense possibilities for producing a variety of innovative devices and “meta”-materials by arranging together any number of atomically thin layers. Many combinations become feasible that are otherwise cannot be accessed in conventional three-dimensional (3D) materials, possibly providing access to novel unexplored functionality of optoelectronic devices or exotic properties of materials.
In the analysis, van der Waals heterostructures created from the so-called transition metal dichalcogenides (TMDs)—a broad class of layered materials)—were used by the researchers. These materials, in their 3D bulk form, are slightly analogous to graphite—which is utilized in pencil leads—and graphene from this material was obtained as a single 2D atomic carbon layer.
The team discovered that when a pair of atomically thin semiconducting TMDs is integrated into a single structure, their properties are hybridized.
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. 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.
Alexander Tartakovskii, Professor, Department of Physics and Astronomy, University of Sheffield.
In other researches, analogous effects have been discovered and examined largely in graphene, which is the “founding” member of the 2D class of materials. The new study demonstrates that other materials, especially semiconductors like TMDs, exhibit powerful hybridization, which can also be regulated by the twist angle.
According to investigators, the study demonstrates considerable possibilities for the development of novel types of devices and materials.
The more complex picture of interaction between atomically thin materials within van der Waals heterostructures emerges. 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.
Alexander Tartakovskii, Professor, Department of Physics and Astronomy, University of Sheffield.
On their part, scientists would like to do more research to investigate more material combinations to find out the exact capabilities of the latest technique.
The study was performed in close association with the University of Manchester, National Institute for Materials Science (Japan), Ulsan National Institute of Science and Technology (Republic of Korea), and the University of Oxford.