![Richard Feynman](https://www.nanowerk.com/nanotechnology-news2/id50580_1.jpg)
This can be likened to spray painting with atoms. You start by vaporising ultra-pure source materials like gallium, aluminium or indium, and combine them with the likes of arsenic or phosphorus. The vaporised atoms fly through a vacuum chamber towards a base layer made of similar materials. The atoms stick to it and slowly build up a crystal one atomic layer at a time. The ultra-high vacuum ensures impurities are minimal.
Atomic architects
While the process is relatively slow – typically only a few atomic layers per minute – the precision is remarkable. It allows technicians to stack different semiconductor materials on top of each other to create crystals known as heterostructures, which can have extremely useful properties. By alternately stacking layers of aluminium arsenide and gallium arsenide, for example, you could produce a material that is extremely good at storing electricity. Once this technique had been perfected in 1990s and 2000s, scientists were able to control the number of electrons and their energies in a particular crystal. And since light then interacts with these electrons, having more control over electron behaviour means you also gain more control of how they are stimulated by light. Heterostructures have led to many new discoveries, particularly regarding the quantum behaviour of particles such as electrons within them. Nobel Prizes in Physics have been awarded five separate times (1973, 1985, 1998, 2000, and 2014), and the resulting materials have revolutionised civilisation. Semiconductor heterostructures enable solar cells, LEDs, lasers and ultra-fast transistors. Even the internet would otherwise be impossible: the lasers which send the light pulses that encode the bits of information online are made from heterostructures, as are the photodetectors that measure these light pulses and decode the information. There are restrictions, however. The atomic size, spacing and arrangement of these heterostructures cannot be too dissimilar between layers without defects arising. This limits the possible material combinations and the potential to freely engineer the electronic and optical properties. Also, crystals naturally consist of atoms which form bonds in all three directions. This means there are always unsatisfied atoms with “dangling” bonds at the edges. Foreign impurities seek these bonds and create defects that can destroy other properties. This becomes especially important with smaller crystals, preventing them being integrated to their full extent into modern transistors, lasers and so forth.Enter 2D crystals
The ultimate in ultra-thin sheets of materials is a single layer of atoms. Fortunately, nature devised such “two-dimensional crystals”. The most famous is graphene, which is just carbon atoms arranged in a hexagonal pattern.![Graphene](https://www.nanowerk.com/nanotechnology-news2/id50580_2.jpg)
In a perfect graphene crystal, all the atoms are completely bonded to one another and there are no dangling bonds. It is famously possible to produce graphene by peeling apart layers of graphite using scotch tape: graphite is actually many layers of graphene all held together by Van der Waals forces, which are far weaker than the bonds in each constituent sheet of graphene.
Besides graphene, there are many other 2D crystals, each with unique properties. Several occur naturally as gems in the ground, such as molybdnimum disulphide, an important industrial lubricant. Others can be made by molecular beam epitaxy, such as the insulator boron nitride, and crystals in the same family of transition metal dichalcogenides as molybdnimum disulphide.
Like graphene is to graphite, scientists “peel off” (or exfoliate) single 2D sheets from larger quantities of these compounds. The inherent thinness of these sheets means they can behave quite differently from the heterostructures described earlier. Different atomically thin materials can be insulating, semiconducting, metallic, magnetic or even superconducting. Scientists are also able to pick, place and combine these materials at will to form new heterostructures, known as Van der Waals heterostructures, with different properties to the 2D sheets. Crucially, these don’t have the same limitations as their cousins made by molecular beam epitaxy. They can comprise layers of very different atomic crystals, enabling unprecedented and unlimited possibilities for combining different materials. For example, you can combine magnetic layers with semiconductors and insulators without attracting contaminants like moisture or oxides between layers – impossible with epitaxial heterostructures. This can be used to create devices that control magnetism using electricity, which is the basis for magnetic memory in hard drives. You can also stack together two identical atomic layers with one turned at an angle. This creates a lattice called a moiré pattern, which provides a new degree of freedom to engineer the electronic and optical properties. Our poster on this subject for the University of Heriot-Watt’s demonstration at the current Royal Society Summer Exhibition in London gives a flavour of how this works:![moiré pattern](https://www.nanowerk.com/nanotechnology-news2/id50580.jpg)