2D structures are becoming immensely interesting in materials science because of the enormous range of potential applications. Among these is graphene, a 2-D allotrope of carbon which is outstandingly strong, flexible and has excellent heat conductance. It is in a honeycomb lattice form which is one atom thick, with a hexagonal format and has semi-metallic properties. It was discovered in 2004 by Geim and Novoselov, both professors at the University of Manchester, and won them the Nobel Prize for Physics in 2010.
Graphene aroused great interest in pioneering research laboratories in all parts of the world because of its potential for use in all kinds of electronics applications, from RAM (random-access memory) to displays, printed graphene for paper electronics, and car batteries. It allows electrons to pass through it at enormous speed, and is thus regarded with a kind of awe because it can serve as the material for the design of all kinds of strong and fast electronics which can be transparent.
However, graphene has its limitations too. Not the least of these is the high cost of production, with a single flake of graphene just 1 micron in size costing over a thousand dollars, thus placing it among the most expensive materials on the planet.
Active research is going on to find new ways to produce graphene more plentifully and cheaply, and success seems to be near. For instance, nanowires made of silver treated with graphene have been visualized, and if this process is realized it could mean a more affordable alternative for manufacturing displays based on nanowires. This in turn could displace the indium-tin oxide combination that is now in use for making flexible touchscreen displays at cost-effective rates.
Other difficulties with the use of graphene have to do with its mechanical stiffness, making it unsuitable for devices which require tolerance of significant compression, stretching or torsion. It has a band gap that is not suitable for simple switch on/off operation. It cannot be used as a catalyst in oxidizing environments because of its vulnerability to oxidation. Moreover, it may have sharp edges which could potentially tear the cell membranes and disrupt their functions.
Other 2-D materials following graphene in close succession include hexagonal boron-nitride, graphitic carbon-nitride, silicene and germanene, as well as dichalcogenides such as molybdenum disulfide. Among the latest developments is borophene.
Borophene is based upon boron just as graphene is based upon carbon. This is significant because at nano level, small atomic clusters of carbon and boron are quite astonishingly similar, even though their macroscopic-level allotropes are quite different. Thus the presence of a 2D boron substance like graphene was predicted theoretically before it was discovered.
Image Credits: Materialscientist [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
The actual substance was developed by the process of molecular beam epitaxial growth. That is, elemental boron was deposited under very high vacuum on to a silver surface. The first reported resulting 2D sheet was metallic and bonded weakly to the silver substrate, with a characteristic buckled or crinkled surface. Three different types of 2-D boron films have been successfully created since then, using boron atoms on a silver substrate,
As predicted theoretically, these are metallic sheets even at nanoscale despite the fact that its parent atom, boron, is an undoubted non-metallic semiconductor. Moreover, the new film is strong and flexible and has high electronic conductance, like graphene, but it is much stronger mechanically, and weighs markedly less because of its low mass density, making it a prize indeed.
The flexibility is far above that of graphene, and has been reported as being at a “record high” level. Its ideal strength surpasses the best of all known polymer materials so far. It has a higher ratio of stiffness to weight as well. On the application of strain, in fact, borophene refuses to fracture and instead undergoes strain-specific phase transitions in its structure that make it still stronger.
A unique wavy or corrugated surface has been observed on scanning microscopy when it grows on a silver substrate, which has roused the attention of researchers in flexible electronics, such as flexible electrodes and nanoelectronic contacts. Its strength and lightness make it useful as a potential reinforced component in designing composites. It is thus one of a kind in the category of 2D nanoelectronic materials.
The mechanical properties have been studied independently in China and in the USA. It has two unique features which may be responsible for its novel characteristics. One is the boron atom network which is composed of very variable hollow hexagons (HHs) forming a reference triangular lattice. It is possible to tune this lattice for desired mechanical properties by customizing the HH content. In other words, if more HHs are inserted, the lattice becomes a stronger material.
Another feature is the delocalized multi-center bonding which makes it highly metallic in character, a development whose significance is yet to be explored fully. Its superconductivity is also critical. Some research indicates a higher electron density than graphene, which might mean that if cooled, it could carry electricity with zero losses.
Image Credits: Bo Peng, Hao Zhang, Hezhu Shao, Zeyu Ning, Yuanfeng Xu, Gang Ni, Hongliang Lu, David Wei Zhang & Heyuan Zhu [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons
Work is proceeding apace to synthesize it as well as characterize it fully. Even now it is known that both smooth and striped sheets of borophene have been synthesized, as seen under the scanning tunnelling microscope, depending on the temperature at which it is formed and the way in which the boron atoms relate to the silver surface beneath them. The corrugated type has superior conductance to electrons along the ridges, and possibly higher stiffness as well, compared to graphene itself. This directional component could make borophene the first choice in filtering polarized light experimentally, for instance.
Boron is extremely reactive, and a free-standing sheet of borophene has so far proved elusive as a result. This would be necessary to measure its conductivity more accurately, which has not been possible so far. However, this reactivity is far from being a disadvantage, as it may make borophene easy to modify using other chemical groups, or by sandwiching it between sheets of other material, to refine its properties as required. The hardness of boron may also mean borophene holds the promise of being a more sturdy substance for various applications compared to silicene and germanene, which are easily torn.
The lattice heat conductivity of borophene also increases at first as the temperature rises, but at 150 K and above it starts to fall, in common with other crystalline materials. Thus at room temperature the lattice thermal conductivity is about 14.34 W/mK which is very much lower than that of graphene in suspension, about 3500 W/mK. This points to its great use in managing heat energy in various applications. Another way to manage the thermal conductivity is by introducing the required nanostructure design, such as the grain boundaries and the type and location of nanoinclusions.
With such remarkable characteristics, it is little wonder that this wonder material promises many more uses from electronic to other photovoltaic devices.