The isolation of graphene in 2004 was a major event, leading to the Nobel Prize in 2010. This “two-dimensional” material, made up of a single carbon layer, continues to attract the attention of researchers. But it's not alone anymore. In the 2010s, other 2D materials were isolated or synthetized Their exceptional properties open up an increasing number of applications. Some of them are now leaving the labs.
Evoking "two-dimensional" or 2D materials is actually wrong, since their finesse may be extreme but they have a thickness. Most of these materials do not exist in nature and must therefore be isolated or synthesized. Some, such as graphene, borophene, germanene, silicene, are made of a single element. Others are composed of two or three. What is of particular interest to research and industry today is their electronic and optoelectronic properties. Some are very good conductors, others have superconducting properties; among them, some have the ability to conduct electricity in the form of Dirac and Majorana fermions, particles without mass that move 300 times faster than the electron, almost at the speed of light. In the field of superconductivity, two-dimensional materials open up several breakthroughs, which could have major consequences in the field of advanced computing. Other 2D materials are semiconductors and could offer alternatives to silicon. 2D semiconductors open above all new perspectives in spintronics, an emerging specialty that differs from conventional electronics in that it exploits not only the electric charge of the electron, but also its quantum spin property. The last fifteen years have been that of pioneers and fundamental science. The current sequence is marked by the emergence of industrial uses: printing of 2D materials by ink jet and disposable sensors open up possibilities for, in particular, connected objects. Until now, the promises of 2D materials seemed a bit overstated, following the well-known Gantner model, which sees a phase of disappointment succeeding the "hype" phase when a major innovation occurs. In the case of 2D materials, it would appear that a third phase has started: delivering.
Let us be precise. Evoking “two-dimensional” or 2D materials is actually wrong, since even if their finesse is extreme, they have a thickness - of only one atom.
Most of these materials do not exist in nature and must therefore be isolated or synthesized. It can be done by exfoliation (this is how graphene was isolated in 2004, by detaching extrafine graphite sheets) or by deposition. Professor Guy Le Lay, from the University of Aix-Marseille, and his colleagues obtained germanene in 2014 by depositing germanium atoms on a gold substrate at high temperatures and in a very high vacuum. One of the challenges of today's researchers is to use less expensive substrates and develop processes that are both faster and allow for larger sample surfaces.
Some of these materials are made of a single element: graphene is an allotropic form of carbon, borophene (of which a first nanostructure of 36 atoms was synthesized in 2014) a boron allotrope, germanene an allotropic form of germanium, silicene (synthesized in 2012) a silicon allotrope.
Several of these materials belong to the Xenes group, which are related in the periodic table: germanene, phosphorene, stanene (tin-based).
Other 2D materials are made of two or three elements. This is the case of graphane, which results from the complete hydrogenation of a graphene sheet, or of the hexagonal form of boron nitride.
Many of these materials have a honeycomb structure, which gives them amazing mechanical properties. A graphene sheet is thus more resistant to breakage than steel (its tensile strength exceeds 130 gigapascals), totally flexible... and impermeable to all gases.
These mechanical performances go hand in hand with great fragility (the edges of the sheet are extremely brittle). This has led to the exploration of nanotube structures, which partly alleviate this disadvantage. Researchers are currently working on the development of heterostructures of Van der Waals, made up of different layers of 2D materials, which a much more resistant.
But what is of particular interest to research and industry today is their electronic and optoelectronic properties.
Graphene has an exceptional electrical conductivity, superior to that of copper for example, which can open up to high-value applications. Samsung is now working on graphene batteries, which will recharge very quickly. The technology used is graphene “balls,” which are easier to produce than sheets.
But it goes further. Since 2017 we know that when added calcium or aluminium, graphene acquires superconducting properties. These are found in other materials of the 2D family, such as chromium germanium telluride, which could make them key elements of the next generation of high-speed computers.
There are two breakthroughs here, one quantitative and the other qualitative.
The quantitative breakthrough was carried out by physicist Jing Xia (University of California) and his colleagues, who were able to verify that a two-atom thick CGT flake retained the magnetic properties of the material on a macro scale. This opens up the possibility of building computer storage devices of a few atoms in thickness.
Some of these materials conduct electricity in the form of Dirac and Majorana fermions, particles without mass that move 300 times faster than the electron, almost at the speed of light.
Above all, and this is where the qualitative rupture occurs, these materials make it possible to exceed the physical limits of the electron, since they have the ability to conduct electricity in the form of Dirac and Majorana fermions, particles without mass that move 300 times faster than the electron, almost at the speed of light.
One of the challenges for all superconductors today is to get out of laboratory conditions and the near-absolute cold that is needed to carry out these experiments. The work of Jing Xia and his colleagues on chromium germanium telluride suggests the possibility of breaking the -100°C limit to reach much more manageable temperatures of -30°C.
In the field of superconductivity, two-dimensional materials thus open up several radical breakthroughs, which could have major consequences in the industry of advanced computing.
Other 2D materials are semiconductors: in some states they are insulating, but when they receive a certain amount of energy (in the form of electricity, light or heat), they conduct electricity. This property makes them good candidates for use in electronics, and again, in the race for miniaturisation, their advantages are decisive compared to silicon.
This is particularly the case for the Xenes. Unlike graphene, which is perfectly flat, they are characterized by an inherent “buckling": their structure shows a slight deformation, a slightly undulating relief that has an effect on their electrochemical properties and makes them remarkable semiconductors.
But in this field, however, it is materials made of several elements that are now attracting the attention of researchers, and in particular transition metal dichalcogenides, where a transition metal atom (molybdenum or tungsten) is sandwiched between two chalcogen atoms (most often tellurium; chalcogenes are the 16th column of the periodic table).
The phototransistors and field effect transistors developed with these materials have aroused a great deal of enthusiasm because of their excellent performance.
2D semiconductors open above all new perspectives in spintronics, an emerging specialty that differs from conventional electronics in that it exploits not only the electric charge of the electron, but also its quantum spin property.
As Maximilien Cazayous, Yann Gallais and Alain Sacuto explain, the electron looks like a tiny spinning top. “The axis and direction of rotation determine the spin orientation. However, the laws of quantum physics show that measuring a spin can only yield certain values according to well-defined rules. (...) When an appropriate magnetic field is applied to it, the electron spin tilts from one orientation to another. The use of spin in spintronic devices is based on this property, which makes it possible to store information: the material support is divided into tiny zones corresponding to as many information bits, and a magnetic field is applied bit by bit to orient the spins of the electrons of the atoms present.”
Until the early 2000s, spintronics mainly used ferromagnetic materials and therefore depended on their properties. However, researchers have recently shown that Xenes may have topological isolator states, which should allow observation of the quantum Hall spin effect. This observation suggests that it is possible to sort carriers according to their spin state in all semiconductors and non-magnetic metals, which will allow manufacturers to free themselves from the constraints associated with ferromagnetic materials.
Spintronics opens up both high-performance sensors and new methods of information storage, the most famous being the quantum computer, which performs operations using the quantum properties of matter, such as superposition and intrication.
The last fifteen years have been that of pioneers and fundamental science. Contrary to the buzz that followed the 2010 Nobel prize, one should not expect a rapid and all-out revolution. But though timid, the transition to industrial uses is definitely on its way. Samsung batteries are the best known example. Other applications are emerging, which allow us to get an idea of the directions for innovation.
The isolation and synthesis of 2D materials is the main obstacle to their development, for reasons of cost, physical limitations and process complexity. Molecular jet epitaxy methods have been developed that allow crystals to grow at the rate of one cm2 per second. Still far from any industrial scale.
More innovative and promising, researchers at the University of Manchester (the same university where graphene was isolated) have developed a method of printing 2D materials by inkjet, which could allow the transition to industrial production of 2D crystals.
Cambridge researchers are currently developing graphene-based inks for high-speed, low-cost manufacturing of printed electronics using rotary printers. This could open up a wide range of practical applications, including printed electronics, smart packaging and disposable sensors.
Currently, printed conductive patterns use a combination of low-conductive carbon with other materials, usually silver, which is expensive. The new graphene-based formulation would cost a few dozen euros per kg of ink, 25 times cheaper. Graphene-based inks have been printed at a speed of over 100 metres per minute, which is consistent with commercial production rates for graphic printing and much faster than previous prototypes.
In the short and medium term, printing will make it possible to manufacture printed disposable biosensors, energy sensors, heat sensors... and cameras.
Felice Torrisi, a university professor of graphic technology, received a 2014 International Young Investigator Award from the National Science Foundation of China to study how graphene and two-dimensional materials could be used to develop flexible cameras printed on paper or plastic.
This opens the way for cameras to be inserted into clothing, packaging, wallpapers, posters, touch screens or even buildings. The project consists of developing flexible, crystals-based, ink-jet 2D photodetectors and studying their integration with commercial electronics.
The current generation of flexible, organic polymer-based photoactive materials has a slow response time: a few milliseconds, too slow for photodetection. This limits their applications. In addition, organic polymers suffer from chemical instability under ambient conditions, which requires additional protective coatings or special processing of printed products, resulting in increased financial and environmental costs.
Some two-dimensional crystals, such as molybdenum sulphide, have remarkable optoelectronic properties (with a rapid response time) and good environmental stability. Combined with highly graphene in printed multi-layer structures, they have excellent performance.
Kostya Novoselov (one of two graphene Nobel Prizes), Zhirun Hu, and their colleauges at the University of Manchester have designed graphene sensors embedded in RFID chips, which have the potential to revolutionize the Internet of Things (IoT).
Again, the key is to create a heterostructure and work on multilayer. Novoselov and his colleagues superimposed graphene oxide on a layer of graphene, creating moisture sensors that can connect to any wireless network - and without the need for a battery!
Once again, using a printing process paves the way for low cost mass production.
Sensors equipped with an RFID module are one of the major technologies in the Internet of Things, and low-cost, battery-free processes could be adapted to monitor, for example, food products or medicines.
These applications suggest, in addition to products with a limited diffusion such as quantum computers, very large-scale uses.
Until now, the promises of 2D materials seemed a bit overstated, following the well-known Gantner model, which always sees a phase of disappointment succeeding the “hype” phase for a major innovation. 2D materials are now over this phase.