Why Boron Nitride Is Called “White Graphene”
Graphene is the first known truly two-dimensional crystalline material. It is an allotrope of carbon, and the minimum thickness of a graphene sheet is one atom. Graphene oxide crystals were first received in 1859, the English chemist Benjamin Brodie. The method he proposed is still used today. used in industryhowever, Brody himself did not guess that he had discovered an ultra-thin material. The physical and chemical properties of multilayer graphene films attracted the attention of scientists only in the middle of the 20th century, and Andre Geim and Konstantin Novoselov managed to synthesize single-layer pure graphene in 2004, for which they were awarded the Nobel Prize in Physics in 2010. For context, I will leave a link to Nobel lecture Novoselova.
Prerequisites for the discovery of white graphene
Despite the fact that graphene has unique mechanical, electrically conductive and optical properties propertiesof particular interest is his controlled superconductivityThere is a mechanism that allows you to turn superconductivity on and off in a graphene sheet by adjusting angle of bending of the lattice. Achieve superconductivity in a bilayer graphene sheet succeeded by scientists from the Massachusetts Institute of Technology in 2018. The bending angle was 1.1°, and the temperature was 1.7 K. This is almost absolute zero, so this technology is far from practical application. Also, the technology for the production of graphene transistors has not yet been developed, since the forbidden zone in graphene is zero. In the following figure, the forbidden zone of graphene is indicated by a dashed line and is located between cones, the upper one of which corresponds to the “valence band”, and the lower one to the “conduction band”.
The band gap includes those conductivity values that the electron cannot accept, and it is this zone that is used to switch the semiconductor. Therefore, it is difficult to create a transistor based on pure (even multilayer) graphene – you need to select a substrate on which to then place a layer of graphene. Research of this kind is also being conducted, for example, in 2022, scientists from Tsinghua University designed transistor gate based on 0.34 nm thick graphene on SiO substrate2 – for comparisonthe diameter of a carbon atom is 0.15 nm.
In addition, graphene is chemically active and, when passing current, reacts with the environment, in particular, with the silicon substrate. Because of all these difficulties, a search is underway for a material that would be as similar as possible to graphene in physical properties, but would go into a superconducting state at higher temperatures and would be more convenient for use with silicon or graphene. A flat, inert and flexible material is needed that would have a sufficiently large band gap to be used as a semiconductor. It was this search that led to the discovery of hexagonal boron nitride (h-BN or simply hBN, where h is “hexagonal”).
The topological similarity between the graphene and hBN lattices is ~98.5%. Therefore, it is easy to assemble a graphene lattice on top of hBN with virtually no folds or shifts in the graphene. But it is precisely this minimal mismatch between the lattices that provides a sufficient band gap for graphene to exhibit stable semiconductor properties. To make graphene less reactive with the environment, a three-layer “sandwich” is used, in which graphene is enclosed between two layers of boron nitride.
hBN is not only useful in the production of graphene microelectronics, but also has remarkable properties itself. For example, about ten years ago this compound was studied as anti-radiation coating for deep space missions. Like graphene, boron nitride can be rolled into nanotubesThe first samples of such nanotubes, about 200 nm long and 3 nm thick, were received back in 1995. The elastic modulus of boron nitride nanotubes can theoretically reach 850 GPa, but in real measurements, described, for example, Hereit turns out to be somewhat less – about 770 GPa. Nevertheless, it is one of the strongest and at the same time lightest materials in nature. In addition, layers of boron nitride slide perfectly over each other; this property is called “super slipperiness“The super slipperiness in boron nitride is due to the action of van der Waals forces, also characteristic of graphene lattices.
Cubic modification of boron nitride
While graphene is an exotic allotropic modification of carbon, it is interesting primarily for its two-dimensional structure. Boron nitride, on the other hand, is not found in nature, but, like graphene and diamond, exists in both two-dimensional and three-dimensional (cubic) modifications. The difference in the crystal lattices of boron nitride in these configurations is shown above. At the molecular level, the cubic lattice of boron nitride does resemble diamond.
I would like to draw your attention to this. resumewhich compares the properties of lamellar and cubic boron nitride. Various research groups, in particular from Rice University in Texas led by Pulikel Ajayan, are studying the combined use of these boron nitride variants. What are the properties of these configurations?
hBN is thin and soft, similar to graphite. It does not melt or decompose even at high temperatures. The Rice University team checked these properties by immersing hBN in electrolytes consisting of ionic liquids. Ionic liquids contain only ions, but no regular atoms, so they can be compared to salt, which is liquid at room temperature. Ionic liquids do not ignite or evaporate at all, but only decompose, and such chemical decomposition of ionic liquids occurs at temperatures above 350 °C. Due to such thermal stability, ionic liquids are now considered as a new generation of electrolytes for lithium-ion batteries. Researchers led by Marco Rodriguez have found that hexagonal boron nitride, which is similar in consistency to toothpaste, is very good as a barrier between electrodes if installed in a battery filled with ionic liquid. In traditional lithium-ion batteries, such barriers (membranes) are made of special plastics, but they melt at temperatures of hundreds of degrees, and, unlike boron nitride, can ignite. These experiments showed that boron nitride outperforms plastics not only because it is nonflammable, but also because it can serve as a heat sink if it is (partially) used to line the inside walls of a battery. According to Rodriguez's group, lithium-ion batteries using hBN operate normally in the range from room temperature to 150 °C and show almost no oxidation.
Similar dielectric properties and refractoriness are inherent in cubic boron nitride, but it is also very strong, and can be used to make drills and cutting edges, like diamond.
Thus, both modifications of boron nitride provide heat dissipation and electrical insulation. By combining them, it is possible to develop new strong synthetic coatings in which hard C-BN would provide strong adhesion to the surface, and soft hBN would protect the material from wear due to the aforementioned super-slipperiness (functionally, it would resemble a solid lubricant).
Production of boron nitride and promising areas of its application
Boron nitride is a completely synthetic material, and is currently produced in the laboratory in the form of very thin films by the chemical vapor deposition method. This method is also used for growing synthetic diamonds.
In this case, chemical vapor deposition first requires evaporation of materials containing boron and nitrogen, and then allowing the molecules to condense on a specially cooled surface, where they naturally form a boron nitride lattice.
Potentially, boron nitride may have other applications, especially if it is a mixture of the two modifications described above. For example, when a laser beam hits boron nitride powder, it produces a bright flash, which could be useful in the production of displays or for more precise radiation therapy. Due to its dielectric properties, boron nitride could be used to create completely new memory cellswhich will function at high temperatures, hard radiation and at the same time guarantee the integrity of the recorded information. Also, by mixing hBN and C-BN, it would be possible to simultaneously fine-tune both the width of the forbidden zone and the thermal conductivity (cooling) of the electronics. The low energy consumption of such devices and their operability in a wide temperature range will certainly find application in astronautics and space robotics.
Conclusion
In my opinion, it is remarkable to see how the interest in graphene and, ultimately, the search for new materials to achieve superconductivity, has created conditions for a thorough study of boron nitride, a substance no less interesting than graphene. Ultimately, boron nitride is more promising as a dielectric and semiconductor than as a superconductor. I would not be surprised if boron nitride gradually overtakes graphene as the main raw material for the production of nanotubes, and also finds application in production of memristors and more complex storage devices. We may be so not indifferent to carbon and its allotropic modifications, since we ourselves are a carbon-based life form. However, boron and nitrogen, adjacent to carbon on the left and right in the periodic table (by the way, carbon is located directly above silicon), open up no less interesting paths for the development of electronics and atomic lattices than the more familiar elements of group IV.
