22-04-2021 | By Liam Critchey
The field of 2D materials is one of the most rapidly advancing areas of nanomaterials, both on their own and in layered materials known as van der Waals (vdW) heterostructures. Inspired by the discovery of graphene in 2004, there are now over 100 known 2D materials in existence. These can be layered into so many different structures, that the application scope for the field of 2D materials is huge.
One of the areas where 2D materials are being touted for a lot of potential is electronic devices. Again, this has come about because of the efficient charge carrier and electrical conductivity properties of graphene. Still, there is a range of other 2D materials with similar properties, and materials such as hexagonal boron nitride can be used as an insulating material in electronic devices.
Despite their own properties, you can also sandwich them together to make heterostructures, and these materials have synergistic effects that are not often seen from the sum of their parts. One of the most recent areas being investigated is the use of heterostructures to build reconfigurable electronics.
2D materials are ultra-thin nanomaterials. In a lot of cases, they are one atomic layer thick. Many people think this is where the ‘2D’ name comes from, but you can have 2D materials that are more than 1 atomic layer thick. For example, transition metal dichalcogenides (TMDCs), such as molybdenum disulphate (MoS2) and tungsten diselenide (WSe2), are composed of three atomic layers and are classed as 2D materials. Graphene has multi-layered forms that can be 8 or 9 atomic layers thick, so why the name ‘2D materials’?
The name actually arises from the quantum properties of the material, namely, the quantum confinement of the charge carriers in the materials. So, in 2D materials, the charge carriers (electrons and holes) can only travel in 2 spatial dimensions and are confined in the third. This is why it’s possible to have multi-layered materials and still classify them as 2D materials.
You can stack these 2D materials on top of each other to created multi-layered structures. If it is all the same type of material, then it’s just a multi-layered material structure. Still, if the layers are composed of different materials, then it becomes a heterostructure. The layers in a heterostructure are not chemically linked. Rather, they are held together by van der Waals intermolecular forces (hence the name), so they are physically bound together. The properties of the heterostructures are often different from their constituents' individual properties, which is one of their key selling points.
The inherent thinness of 2D materials means that (for the most part) they are much more flexible than bulkier structures. However, when you start adding the layers together, the flexibility sharply reduces (like it does for most materials). The same can be said for rotation and twisting. In recent years, researchers have been twisting 2D materials. Many media coverage has centred around the fact that you can twist graphene layers (including bilayers) into a ‘magic angle’ where insulating and superconducting properties can be induced across the material. TMDC monolayers have also been twisted in this way.
However, the layered network of a heterostructure makes it hard to rotate, even when it’s held together by forces rather than bonds. Moreover, the rotation angle of vdW heterostructures can’t be changed after they have been fabricated and to deduce the angle of any fabricated heterostructure is a complex process in itself.
Because the layers are held together with intermolecular forces rather than chemical bonds, it’s possible to disassemble the individual 2D building blocks, and subsequently stack them again in different combinations, angles, and sequences. This could open different device functions by reconfiguring the building blocks of the device. Until recently, this has been challenging because of a lack of ability to disassemble the heterostructure layers, but new research has emerged that could unlock this reconfigurable potential.
A new approach has materialised where the layers of a heterostructure can be rearranged into different architectures, presenting the opportunity to create reconfigurable electronic devices. The individual building blocks were created by fabricating each of the 2D material layers onto nanometre-sized polymer substrates. These building blocks could then be assembled, disassembled, rotated, twisted, and changed into several different orientations and structural conformations—including completely new vdW heterostructures.
This process enabled the researchers to change a molybdenum disulphide flake into both a p-type and an n-type transistor. From a back-gate to a front-gate device, by restacking different metal contacts and insulating layers. The approach extended out to heterostructures containing up to 4 layered heterostructures (using 3 different 2D materials). In this instance, the first heterostructure was used as a non-volatile floating memory device, and after reconfiguration, it performed the function of a Schottky diode.
These reconfigurable electronic devices work slightly differently from conventional electronics. In these reconfigurable electronics, the essential building blocks, i.e., the electrical contacts, dielectrics and barrier layers are chemically bound to the device (in the form of ionic and covalent bonds) and the 2D materials are subsequently fabricated to perform the intended function (and rearranged/rotated to perform a different function).
Heterostructures in electronics are nothing new, but the ability to reconfigure them to change the device's core functions is a novel approach. It could unlock multi-functional electronic (and optoelectronic) devices that only require a limited number of building blocks. While we’re starting to see 2D materials used commercially, it’s likely to be a while before seeing the likes of reconfigurable heterostructure devices implemented on a commercial level. Nevertheless, recent developments are starting to show the potential for both 2D materials and heterostructures and how we could use them to build the next generation of electronic devices.