Since graphene was first isolated in 2004, 2D materials have had all kinds of suggested applications. They are ultra-light and immensely robust, yet flexible and stretchable. Often with excellent electronic properties, 2D materials can be found that are highly conductive or with tuneable semiconductor bandgaps.
It is the fact that they are so thin that gives them considerable potential in many applications. As every atom on the surface of a 2D material is directly exposed to its surroundings, the signal from exposure to new material is not diluted.
Using Graphene in Sensor Applications
2D-materials have as large a surface-area to volume ratio as could be hoped for, meaning that those molecules have a significant potential area to interact and bind with. Additionally, graphene, in particular, has high carrier mobility, and density meaning that it is highly sensitive to electrical signals – for example, due to the presence of a particular molecule.
An example of this sensitivity in action was demonstrated when Bosch, in collaboration with Max-Planck, were able to use graphene to create a magnetic sensor over a hundred times more sensitive than its silicon counterpart. The sensor relies on graphene’s high carrier mobility as it operates using the Hall effect, where a voltage is induced by the effects of a magnetic field to deflect moving charge carriers.
In the case of chemical sensors, or some biomedical sensors, you might desire to detect just a single molecule of a potentially dangerous substance. This can be completed with graphene-fabricated sensors (micrometers in size), which can resolve events on the molecular scale.
Graphene has been used to enhance existing sensors as shown in this link - in 2015, graphene was used to improve infrared absorption spectroscopy, allowing for the sensing of individual chemical bonds. There are also clear applications for medicine, defense, and food safety, and crop protection: the earlier potentially hazardous chemicals can be detected, the faster the problem can be addressed.
Other 2D Materials
Graphene is of course not the only 2D material that has been used in sensor applications. Those that do have bandgaps, such as MoS2, have been implicated for use in field-effect transistors - with the bandgap present, the current doesn’t flow “all the time,” and so noise is reduced.
Such sensors were used back in 2014 by a group at UC Santa Barbara to perform specific protein sensing with high sensitivity even at concentrations of 100 femtomoles – the equivalent of detecting a drop of milk dissolved in a hundred tonnes of water.
Common to many of these two-dimensional materials is the flexibility, strength, and small-scale required for sensors – future developments promise sensors in wearable devices that can detect gas and other chemicals, or even in the bloodstream for medical monitoring. They’ve also been implicated for use in the Internet of Things, where for particular device applications space is at a premium.
Early in 2018, a team at the University of Manchester – where graphene was first isolated – announced that they had been able to create humidity sensors combined with RFIDs, and hence the ability to connect to wireless networks.
2D Materials in Electronics
It’s not just in the world of sensors that 2D materials could prove invaluable. Much of the astonishing progress in computing is owed to Moore’s Law: the continued ability to shrink circuits so that the number of transistors on a dense integrated circuit doubles approximately every two years. This has been a robust trend that has fuelled the exponential growth in computing power, but exponential growth in a finite world eventually reaches physical limits.
Increasing levels of ingenuity are required as we approach these physical limits. 2D materials, due to their atomically thin sizes, will be crucial in allowing us to approach the end of this law. A literature review published in Nature in 2014 noted that 2D materials, such as transition metal dichalcogenides as well as graphene, would be useful for thin-channel transistors and potentially new device concepts altogether.
The topological insulator properties of 2D materials, which has been the subject of a great deal of theoretical and experimental investigation, are also likely to find use in some ingenious circuit design. Entirely different architectures can now be investigated with these materials, rather than incremental improvements on structures that already exist.
There are still challenges. Although it has proved possible to construct many different 2D materials such as graphene, silicene, germanene by relatively well-understood techniques like exfoliation or vapor deposition, it has yet to be demonstrated that 2D materials with tuned and desirable electronic properties can be mass-manufactured.
The infrastructure for producing such electronics outside the lab is not yet in place, as it is for silicon. Many structures still need to demonstrate that they will work consistently outside the lab. But, as the researchers and venture capital pouring into this field can attest, most people think that the fundamental properties of 2D materials are too useful for them to remain on the materials science shelf, and many prototypes for sensors and circuits have already been constructed.
Perhaps ironically, moving down to two dimensions has opened up a whole new dimension of parameter space for materials scientists to explore – one with rich and often exciting properties. It seems likely that, with the increased synthesis of Van der Waals heterostructures where layers of 2D materials are stacked together, many of the unique properties of the materials being discovered today will make them valuable building blocks in the sensor and electronic designs of the future.