Since childhood, we are accustomed to the fact that when you add one apple to another, you get two apples. The same thing happens with pencils, typewriters, and balloons. And in physics this is not necessarily the case. If you bring two films of monoatomic thickness, like graphene, close enough to a small distance, you get a new material.
In this case, we will still have two separate objects, which, in principle, can be pulled back. The interaction between them is due to van der Waals forces - a relatively weak interatomic electromagnetic interaction. The result is a new material (heterostructure), the properties of which are determined not so much by its chemical composition as by the arrangement of the layers. A two- (or more) -layer film can be bent and twisted - and this also leads to a change in its physical properties.
Similar experiments have been carried out on graphene for many years, but graphene in this case is not very interesting. Under the conditions we are accustomed to, it does not have a forbidden gap that turns a substance into a semiconductor; special efforts are required to create it. But, there are other materials.
In this case, researchers from the University of Sheffield (Great Britain) used van der Waals heterostructures made of transition metal dichalcogenides. A small digression is appropriate here. Chalcogens are chemical elements of the 16th group of the periodic table: a column starting with oxygen and sulfur from the top and ending with radioactive livermorium. There are many transition metals, in everyday life we are most familiar with copper, molybdenum and zinc.
The researchers put together a "sandwich" of layers of molybdenum disilenide (MoSe2) and tungsten disulfate (WS2). The conductivity of the resulting material changed periodically in the same way as the moiré effect appears on two folded tulle curtains.
As Professor Alexander Tartakovsky of Sheffield University put it, materials influence each other and change each other's properties, and they should be considered as a completely new metamaterial with unique properties, so one plus one does not give two. The scientists also found that the degree of hybridization is highly dependent on the twisting of the "sandwich", during which the distance between the atomic lattices of each layer changes.
“We found that twisting layers in a heterostructure creates a new supra-atomic periodicity called a moire superlattice,” says Tartakovsky. A moire superlattice with a twisting-dependent period determines how the properties of two semiconductors hybridize."
Professor Tartakovsky added: “A more complex picture of the interaction of atomically thin materials in van der Waals heterostructures is emerging. This is interesting because it allows access to a wide range of material properties, such as twist-tunable variable conductivity, optical properties, magnetism, etc. This can and will be used as new degrees of freedom in the development of devices based on two-dimensional materials.”
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You can read the details in an article published in Nature.
Sergey Sysoev