Generations of minds have been inspired by Legos, the small, snap-together plastic blocks. These blocks have become fantastic cars, elaborate castles and many other whole creations that are greater than the sum of their parts. Today a generation of materials scientists is being inspired by a new type of Legos: building blocks on the atomic scale.
These new construction elements are sheets of materials that can be as thin as just one atom and can be stacked, one on top of another, in a designed, precise sequence. This unprecedented fine construction control can produce substances with electrical and optical properties that have been impossible to create before. And they are allowing scientists to imagine devices made of materials that conduct electricity with very little resistance, faster and more powerful computers, and wearable electronic gadgets that could be bendable, foldable and incredibly lightweight.
This breakthrough followed the creation of graphene, a single sheet of carbon atoms that my colleagues and I at the University of Manchester in England isolated from a bulkier block of graphite in 2004. We made this sheet of repeating six-sided crystals—the atomic structure looks something like a chicken-wire fence—by pulling one-atom-deep layers from the top of the block with adhesive tape. In the past 10 years researchers have found several dozen other types of bulk crystals that can be pulled apart in this way, and their number continues growing rapidly. Mica is one example, and so are materials with exotic names such as hexagonal boron nitride and molybdenum disulfide.
These crystal layers are considered to be two-dimensional because a single atom is the smallest possible thickness for any material. (Slightly thicker crystals of three or so atoms can also be used.) Their other dimensions, width and length, can be a lot larger, depending on the maker's desires. In the past couple of years two-dimensional crystals have become a hot topic in materials science and solid-state physics because they exhibit many unique properties.
We can stack these layers in ways that are quite stable. They do not bond together in a conventional way—using covalent bonds that share electrons, for example. But the atoms are attracted to one another when they come into close proximity, through a weak pull known as the van der Waals force. This force is generally not strong enough to hold atoms and molecules together, but because these two-dimensional sheets are so dense with atoms and so close to one another, the cumulative force becomes formidable.
To understand the tantalizing possibilities offered by this kind of materials engineering, think about room-temperature superconductivity. The idea of transmitting electricity with no loss of energy and of doing so without the need to surround devices with almost unimaginable cold has been a goal of scientists for generations. If materials that can do it are found, consequences for our civilization will be far-reaching. There is a consensus that the goal is achievable in principle, but no one knows how. Today the highest temperature at which materials can be made superconducting is less than −100 degrees Celsius. There has been little progress in raising this limit during the past two decades.
We have recently learned that some superconductors made of oxides—compounds with at least one oxygen atom, along with another element—can be disassembled into individual layers in the manner that I have described. What if we reassemble them back in another sequence and insert additional crystal planes in between? We already know that superconductivity in oxides depends on interlayer separation and that adding extra layers between crystal planes can turn some poorly conducting and even insulating materials into superconductors.
This idea has not yet been fully tested, mainly because the technology of making atomic-scale Lego materials is still in its infancy. Indeed, it is difficult to assemble complex multilayer structures. For the moment, these structures rarely contain more than five different layers, and they usually use only two or three different Lego blocks—mostly graphene in combination with two-dimensional crystals of insulating boron nitride and semiconducting material such as molybdenum disulfide and tungsten diselenide. Because the stacks have a variety of materials, they are often referred to as heterostructures. They are currently small—typically only about 10 microns in width and length, which is less than the width of a cross section of a human hair.
Using these stacks, we can run experiments in search of novel electrical or optical properties and new applications. One intriguing aspect: as thin as these sheets are, they are also quite flexible and transparent. This presents opportunities to develop light-emitting devices that can be shaped in various ways, like display screens that can be folded and unfolded as a user needs a bigger size. Computer chips that use energy much more efficiently are also possible.
If researchers find something significant in their investigations of these structures, we believe it will be possible to scale up the technology for industrial use. It has already happened with graphene and some other two-dimensional crystals: Initially those came as tiny crystallites of a few microns across, but they can now be manufactured in sheets of hundreds of square meters.
No “killer app” has been reported yet. Nevertheless, progress in the field is causing a loud buzz of excitement in scientific communities. Human progress has always closely followed the discovery of new materials. Such discoveries were behind the transitions from the Stone to the Bronze to the Iron to the Silicon Ages. Nanoscale Legos represent something that has never been created before. Right now the possibilities seem endless.