What Makes 2D Materials Unique?
The unique characteristics of 2D materials derive from the fact that specific properties of materials (such as their electrical conductivity and interaction with light) change significantly at the nanoscale (nm). (Consider that 1 nm equals 1/1,000,000 mm—or, put another way, it’s around 100,000 times thinner than a strand of human hair.)
2D materials are incredibly thin and lightweight, have excellent electrical conductivity, and are stable at sizes below 10 nm (the point at which traditional semiconductor materials begin to fail). Graphene, silicene, germanene, and phosphorene are some of the most studied 2D materials. These are outlined below.
A graphic, atomic diagram that reflects the lattice structure of graphene: a key material in 2D materials. Image Credit: Pixabay.
The first 2D material to be synthesised was graphene in 2004. Graphene is a carbon allotrope that consists of a single layer of carbon atoms that are 0.14 nm in thickness. These form a 2D hexagonal matrix that allows electrons to move through the graphene at very high speeds, giving it superior thermal conductivity. Graphene is also highly transparent: it absorbs only about 2% of incident light in the visible spectrum.
Another 2D material, silicene, consists of a monolayer of silicon atoms and is thinner than graphene (although it’s much more difficult to synthesise than the latter). Unlike graphene, silicene is not a strictly planar material. Rather, it has a ‘buckled honeycomb’ lattice structure at the atomic level. This structure gives silicene exceptional magnetic properties, thermal insulation, a tunable bandgap, and high mechanical strength.
Germanene is a heavier analogue of graphene that was first synthesised in 2014. Like silicene, it also features a buckled lattice structure with a buckling angle that gives it a tunable bandgap. Germanene is a 2D topological insulator and active optical material.
Phosphorene is a 2D material derived from phosphorus. It comprises a monolayer of black phosphorus, an allotrope of phosphorus with atoms having weak Van der Waals forces. Phosphorene has electrical, optical, and ultraviolet absorption properties (comparable to those of graphene).
2D materials can be used to design powerful transistors for computing applications. Pictured: a closeup of a central processing unit. Image Credit: Pexels.
Optoelectronic Applications of 2D Materials
2D materials, such as silicene, germanene, and phosphorene, have interesting potentials in optoelectronic applications: these include optical signal processors, light-emitting diodes, and photovoltaics. Due to their layer-dependent and optimally-sized bandgaps, these 2D materials have exceptional optical and electronic properties. The next three sections look at some of their other applications.
Graphene has also been used to coat the touchscreens of consumer electronics, such as smartphones, to improve their touch sensitivity. Indium-tin oxide—the conventional material used—has similar conductivity, but it is much more brittle than graphene. Owing to their lightweight, 2D materials are also suitable for the designing of flexible, wearable technology.
Graphene is one of the most studied 2D materials in the microelectronics industry. It is celebrated for its unique combination of thinness, excellent thermal conductivity, and stability at the nanoscale. In 2008, researchers at the University of Manchester created the world’s smallest transistor by carving tiny electronic circuits into the material with individual transistors (roughly the size of a molecule).
In 2015, a team of scientists led by Deji Akinwande, professor of Electrical and Computer Engineering at the University of Texas at Austin, found a way to harness the electrical characteristics of silicene to produce very fast switching transistors that are also smaller and more efficient than conventional types.
The team developed a new method for developing silicene layers while minimising the material’s exposure to air. To achieve this, the team condensed a hot vapour of silicon atoms on a crystalline silver block in an airless chamber. Then, they formed one-atom-thick silicene on a thin layer of silver and deposited a 1-nanometre thick layer of alumina on top. The result was silicene sandwiched between two protective layers that could easily be transferred to an oxidised silicon substrate to design a silicene field-effect transistor.
A further promising application of 2D materials relates to the energy industry. Graphene-coated solar cells designed by researchers at MIT allows the technology to be installed on virtually any surface, including windows, walls, the surface of consumer electronics, and even paper.
Moreover, using graphene as the electrodes of rechargeable batteries (see graphene batteries) significantly enhances the technology’s charging speed, storage capacity, and lifespan, even when compared to its lithium-ion counterparts. Graphene sheets can also create supercapacitors with significantly higher energy densities when compared to that of standard electric double-layer capacitor (or EDLC) materials while drastically reducing the size and weight.
Another interesting application of graphene involves fuel cells. Graphene has a significantly high surface area, which makes it an outstanding low-cost alternative material. Moreover, protons can pass through graphene, which increases fuel cell efficiency by lowering the fuel crossover.
Weighing up the competition: steel sheets (pictured) are heavier and weaker when compared to the lightweight, robust graphene sheets. The latter also have exceptional electrical conductivity. Image Credit: Public Domain Pictures.
Considering Both the Present and Future Potential of 2D Materials
Although the outstanding properties of 2D materials like silicene and germanene are being extensively studied, scientific knowledge beyond graphene is currently limited. Nonetheless, these materials show great promise for improving the efficiency of existing electronic applications.
On top of this, many more 2D materials are coming to light. According to research by leading scientists, there are as many as 6,000 2D materials analogous to graphene. Nanoelectronics is a rapidly developing field set to gain rapid popularity and ultimately disrupt several conventional technologies—as electronics continue to become smaller and more efficient.