Van der Waals heterostructures
Ever since the first demonstration of graphene in 2004, the scientific community has discovered more than 2500 other layered, atomically thin (two-dimensional, 2D), materials.
While these materials cover an amazing range of electrical, chemical, optical and mechanical properties, perhaps the most astounding discovery is that these crystals can be combined freely to create altogether new materials.
They discovered that when two or more atomically thin graphene-like materials are placed on top of each other their properties change, and a material with novel hybrid properties emerges, paving the way for design of new materials and nano-devices. The properties of these hybrid material can be precisely controlled by twisting the two stacked atomic layers, opening the way for the use of this unique degree of freedom for the nanoscale control of composite materials and nano-devices in future technologies.
While strong covalent bonds provide in-plane stability of the 2D crystals, these materials are called van der Waals heterostructures because the atomically thin layers are not mixed through a chemical reaction but rather attached to each other via a weak so called van der Waals interaction – similar to how a sticky tape attaches to a flat surface.
Since all atoms and molecules attract each other by the ubiquitous van der Waals (vdW) forces, there are virtually no limitations to how all these new superthin materials can be assembled into stacks – just like Lego blocks.
Due to their unique interlayer coupling and optoelectronic properties, these materials are of considerable interest for the next generation nanoelectronics because they make it possible to create high-performance structures tailored to a specific purpose.
Van der Waals heterostructures open a huge potential to create numerous metamaterials and novel devices by stacking together any number of atomically thin layers. Hundreds of combinations become possible otherwise inaccessible in traditional three-dimensional materials, potentially giving access to new unexplored optoelectronic device functionality or unusual material properties.
Besides the contact between different 2D atomic layers, the passivated, dangling-bond-free surfaces of 2D crystals can bond together with other dimensional materials through vdW force. Consequently, fabrication of mixed-dimensional vdW heterostructures could be carried out through hybridizing 2D crystals, especially graphene, with 0D quantum dots or nanoparticles, 1D nanostructures such as nanowires or carbon nanotubes, or 3D bulk materials.
These heterostructures are completely handcrafted and the fabrication procedures presented several drawbacks such as the difficulty to align the crystal lattices (with atomic accuracy) of the different adjacent materials or to avoid trapping ambient adsorbates between the layers, hampering their performance and reproducibility.
However, in 2017, researchers discovered that franckeite, a mineral belonging to the sulfosalts family, shows a natural crystal structure similar to the man-made van der Waals heterostructures – with the huge advantage of an almost perfect alignment between crystal lattices and the absence of tapped residues between layers.
Conventional 2D heterostructures usually are composed of two layers of opposite charge carrier type using inorganic materials. One of the challenges when creating 2D heterostructures is the painstaking stacking of the individual components on top of each other.
Any industrial application will obviously require a scalable approach to the vdW assembly. To this end, significant efforts have been reported to epitaxially grow graphene, 2D hBN and 2D MoS2 on top of each other. However, it is a daunting task to find the right conditions for so-called vdW epitaxy because the weak interlayer interaction generally favours the island growth rather than continuous monolayers.
Another scalable approach is layer by layer deposition from 2D-crystal suspensions by using Langmuir-Blodgett or similar techniques. One can also mix suspensions of different 2D crystals and then make layer-by-layer laminates, relying on self-organizational assembly (flocculation).
Unfortunately, micrometre-sized crystallites in suspensions cannot provide large continuous layers, and this would limit possible applications for such vdW laminates. Currently, they are considered for use as designer ultra-thin dielectrics8, selectively-permeable membranes and composites materials.
So far, the most feasible approach to industrial scale production of vdW heterostructures seems to grow individual mono- and few- layers on catalytic substrates, then isolate and transfer these 2D sheets on top of each other. This route has already been proven to be scalable. If a particular heterostructure attracts sufficient interest in terms of applications, it seems inevitable that its production can be scaled up by trying a variety of available approaches.
Read a comprehensive review on van der Waals heterostructures by Andre Geim.