2D materials are gathering a lot of interest for a range of applications, both on the high-tech end of the application spectrum and on the lower end. There’s been much interest in 2D materials because of the sheer breadth of materials available within the material class. This means that many different properties can be harnessed and tailored for various applications. Within the class of 2D materials, some materials are better known than others, for example, graphene, transition metal dichalcogenides (TMDCs), and boron nitride. Other lesser-known materials are starting to gather some traction—one of which is another boron-based 2D material known as borophene.

What is Borophene?

Unlike boron nitride (the better-known boron-based 2D material), borophene is an all-boron 2D material. So, the chemical structure is entirely made up of boron atoms and is more analogous to the other single element 2D material compositions, such as graphene (all-carbon), phosphorene (all-phosphorus), or stanine (all-tin). Borophene has beneficial properties for electronic applications because of its Dirac electronic properties, and like many 2D materials, it is a highly flexible material even though it is inorganic in nature. Borophene also has high mechanical strength, with a Young’s modulus much higher than many other materials.

One of the key advantages of graphene-based materials is their high stability in many environments and to many stimuli. However, none of the other single element 2D materials share this same level of stability because the atoms can’t form the same level of structural stability and electron structure that the carbons atoms can in graphene. Borophene falls into this instability category—because its atoms form triangular and hexagonal shapes, which is less stable than the all-hexagon arrangement in graphene—and is one of the reasons why applications for borophene have not been developed as much for borophene as other 2D materials—including highly stable boron nitride.

There have been a number of stabilisation routes proffered for borophene. One of the main approaches is to see if borophene can be made into a liquid state at low temperatures. However, borophene’s phase transition is poor, so these fabricated materials only remained stable when attached to a substrate. Borophene oxide has been proffered as another option because the oxygen atoms can help to stabilise the boron backbone. One of the newest approaches being trialled is to create borophene liquid crystals. Given the existing use of liquid crystals in optical displays today, the natural application developments gravitate towards optical devices.

Using Borophene in Liquid Crystal Form

The interest in using borophene as a liquid crystal system is primarily due to the 2D nanolayers being able to retain their coplanar structure (where all the atoms sit on the same atomic plane). So, in a liquid state, the arrangement of atoms remains the same, and for liquid crystals, it means that the 2D planes of boron atoms will be able to move around and over each other, but the structure of the 2D sheets themselves will stay intact. These types of 2D material liquid crystals are gaining attention for different optic and photonic applications. Graphene is already being used to create lyotropic liquid crystal systems, and now the attention has turned to borophene liquid crystals. The inorganic nature of the material could make it more suitable for different applications compared to the applications where organic liquid crystals are already widely used.

Could Borophene Liquid Crystal Phases be Used for Optical Devices?

Researchers in Japan have created a borophene liquid crystal system using a layered structure derived from borophene oxide. The liquid system is a fully inorganic liquid that contains layered 2D materials within it. This inorganic liquid system was shown to have a very high thermal stability, making it suitable for optical devices in harsher application/operating environments where there are no suitable organic counterparts—potentially opening the scope of 2D material and liquid crystal applications.

The borophene liquid crystal structure created by the researchers was found to be a lyotropic liquid crystal system. The liquid crystal system showed a high thermal stability up to 350°C—meaning that the ordered layer structure exists over a very wide temperature range—and an optical switching behaviour driven by a low voltage (1 V). Thin 2D sheets such as these are attracting interest as optical materials because of the surface plasmons they possess. In particular, 2D borophenes with Dirac-type band structure (such as these) are expected to exhibit beneficial electronic and optical properties, including low-loss and highly confined broadband plasmons. Research into these systems has only really just started—so it’s hard to say where they might be used—but they have good properties that are suitable for optical devices, and they have now been integrated into a more stable form—and a form that is already utilised in optical displays.

So, the big question is, could we see borophene liquid crystals in optical devices? It’s definitely possible, and we will likely see some academic developments come out where liquid crystal systems have been integrated into working and functional optical devices. For commercial and real-world uses, that’s a much tougher question to answer at this stage. The whole 2D material industry is still in relative infancy. While we’re seeing a lot of commercial graphene products hitting the market (alongside some TMDC and boron nitride products), the other 2D materials are much less developed at a fundamental level. The industry/commercial adoption is currently non-existent. The other factor is that boron is a lot more expensive and less abundant than carbon, so the likes of borophene may struggle to make the big commercial impact that graphene has from an economic standpoint alone.

It’s possible that we could see such liquid crystals in commercial applications, but not before graphene and the other, more popular, 2D materials mature further and get greater market penetration. If this happens, then the attention may turn to the wider class of 2D materials, including some of the more ‘exotic’ materials, but when this will happen is not a question that can probably be answered for some time. However, from a scientific standpoint, the research and subsequent developments are interesting for seeing how much potential and broad scope the 2D material class has.

References:

Yamamoto K. et al, Liquid crystalline 2D borophene oxide for inorganic optical devices, Nature Communications, 13, (2022), 1037