2D materials hold promise in applications ranging from electronic devices to catalysis, and from information storage to medicine. But how close are we to commercialized products?
The arena of 2D materials is becoming densely populated. In the 12 years since the isolation of graphene, many materials — conducting, insulating and semiconducting — have arrived on the stage. Some of these materials are monolayers comprising a single element, for example, phosphorene, borophene, germanene and silicene; others feature different atoms alternating in the same layer, for example, boron nitride, transition metal dichalcogenides (TMDCs) and MXenes. The combination of 2D layers into van der Waals heterostructures, in which different monolayers are freely mixed and matched, further expands the opportunities for exploring new physics and applications1.
For 2D materials, electronic applications are at the forefront of most researchers' minds — after all, one of the properties that makes graphene an exceptional material is its remarkable electron mobility. Indeed, electronics is a prime field of application for 2D materials, which have great potential as field-effect transistor (FET) channels because, alongside their high carrier mobilities, they exhibit large on/off ratios at low input voltages and they enable the miniaturization of FETs. A Review in this Focus issue by Manish Chhowalla and colleagues examines the merits of graphene, hexagonal boron nitride, TMDCs, silicene and phosphorene as FET channels, discussing the benefits and drawbacks of each material.
Beyond electronics, which draws on the charge of electrons to encode information, and spintronics, which leverages the spin of electrons, valleytronics is now emerging as a way to exploit the valley degree of freedom (which specifies the energy extrema, or valleys, that an electron occupies) to store and process information. This is another area in which 2D materials have a leading role. In 2D layers, the valley pseudospin can be addressed both optically and electrically to realize devices. John Schaibley, Xiaodong Xu and colleagues discuss in a Review why 2D materials with a honeycomb structure, in particular graphene and TMDCs, are ideal platforms for the study and implementation of valleytronics.
However, there is more to 2D materials than electronic (or valleytronic) devices. For example, graphene and, more generally, carbon-based materials are being actively pursued as low-cost, efficient catalysts for clean energy production and storage. Their performance rivals that of their metal-based counterparts, which are costly and have relatively poor stability. In a Review by Xien Liu and Liming Dai, research into heteroatom-doped graphene and carbon nanotubes as cost-effective and stable catalysts in fuel cells, batteries, water splitting systems and dye sensitized solar cells is explored.
A new player on the scene — phosphorene — is reported in depth in a Review by Castro Neto and colleagues. Monolayers of black phosphorus, which have an anisotropic lattice that differentiates them from other 2D materials, were first synthesized in 2014 and are attracting considerable interest because of their excellent carrier mobilities and high optical and UV absorption. Some of the properties predicted for phosphorene, such as superconductivity, still await experimental verification, but high-performance devices, such as FETs and photodetectors, are already demonstrated. Some 2D materials have been reported even more recently and their exploration is just beginning; for example, borophene, which was isolated last year, is bendable and stretchable and may be applicable in flexible electronics2, as related in one of our research highlights.
“The stage is set to enable the making and modelling of 2D materials to result in the manufacture of devices”
The research literature is filled with reports on the synthesis and characterization of 2D materials and on proof-of-concept devices. But if 2D layers are such wonderful materials, why are they not invading our everyday lives? In a Comment, Seongjun Park offers an opinion of why this is the case and addresses the challenges posed by graphene commercialization. It is evident that there is a need for stronger collaboration between academics and industrial engineers. The translation of graphene into biomedicine, which is, currently, mainly focused on graphene-based sensors, is discussed in a Comment by Kostas Kostarelos, who warns against inflated expectations and, in a similar vein to Park, identifies collaboration between researchers and medical professionals as crucial for the successful adoption of 2D materials in medicine.
Graphene has led to a plethora of 2D materials and devices. It is possible that a game-changer application will come from one of these materials, but the road to the commercialization of devices based on 2D materials is likely to be very long. However, these materials certainly hold great promise, and the stage is set to enable the making and modelling of 2D materials to result in the manufacture of devices.
Liu, Y. et al. Van der Waals heterostructures and devices. Nat. Rev. Mater. 1, 16042 (2016).
Zhang, Z. et al. Substrate-induced nanoscale undulations of borophene on silver. Nano Lett. 16, 6622–6627 (2016).