Diamond Nanothreads without the Carbon?

Boron Nitride Could Bring Polar Lattices to the Nanothread Geometry

Nanotechnology has had a significant impact on materials engineering in recent years. The ability to manipulate matter at the nanoscale level has enabled the development of new materials and the modification of existing materials to improve their properties and performance. One of the key advantages of nanotechnology in materials engineering is the ability to control and manipulate the structure and properties of materials at the atomic and molecular level. This has allowed researchers to create materials with unique and desirable properties, such as increased strength, improved electrical conductivity, and enhanced chemical reactivity. Nanotechnology has also enabled the development of new and advanced manufacturing techniques, such as additive manufacturing or 3D printing. By using nanomaterials, it is possible to create parts and components with intricate geometries and high precision that would be difficult or impossible to achieve with traditional manufacturing techniques. A new example of nanomaterial is the nanothread – a one-dimensional covalently bonded material formed mainly by pressured-induced polymerization of unsaturated molecules like benzene that looks like the thinnest possible diamond wire.

Unlike carbon nanotubes, which are hollow, carbon nanothreads are solid structures, composed of densely packed carbon atoms arranged in a unique crystal structure. They are also thinner than carbon nanotubes, with a diameter of just a few angstroms (10^-10 meters). Carbon nanothreads are believed to have a number of potential applications due to their unique physical and chemical properties. They are extremely strong under tension and remarkably stiff for their thickness, with a tensile strength that is estimated to be 100 times greater than that of steel. They are also expected to be excellent conductors of heat. Theoretically, carbon nanothreads could be used in a wide range of applications, including as ultra-strong and lightweight materials for construction and engineering, nanometer-scale actuators or sensors, and potentially in electronics, energy storage, or catalysis.

Carbon nanothreads have been successfully synthesized from molecules such as pyridine, benzene, furan, aniline and thiophene, with evidence of fully saturated and partially saturated threads. Extensive research has been conducted to enable the synthesis of nanothreads from diverse organic precursors with great success. This success has revealed a great variety of structures, properties and functionalities, most of which can be accessed via proper selection of organic precursor molecules – a direction that is yet to be fully explored. Whereas most solid-state materials form along thermodynamic pathways, nanothreads marry the synthetic capabilities of organic chemistry with the rigidity and crystalline organization of solid-state materials, opening up a vast new design space for materials.

Nevertheless, the fact that nearly all reported nanothreads have carbon-based backbones is not an indication that inorganic nanothreads are not possible. Recently, replacing all carbon dimers with BN pairs has been identified as one approach of producing an isoelectronic BN-based nanothread. Borazine, also known as “inorganic benzene”, is isoelectronic to benzene but with considerable bond polarity. Thus, since organic nanothreads can be formed from benzene under pressure, there is a possibility of forming inorganic BN nanothreads from borazine. Additionally, there are speculations on borazine’s ability to trigger polymerization at a lower pressure than benzene due to its reduced aromaticity, thus improving production quantities by allowing the use of lower-pressure synthesis apparatus. However, these assertions and speculations are yet to be verified experimentally.

Research interests in nanothreads have been increased by its anticipated properties and applications. Besides possessing similar mechanical strength and thermal conductivity as carbon nanothreads, borazine nanothreads could also possess fascinating electromechanical properties. To spark further research on borazine nanothreads, Dr. Tao Wang and Dr. En-Shi Xu from Pennsylvania State University, Dr. Bo Chen from Donostia International Physics Center in Spain, Professor Roald Hoffmann from Cornell University and Professor Vincent Crespi from Pennsylvania State University carried out a theoretical examination of borazine-derived nanothreads with varying degrees of saturation. To design the atomic frameworks of the nanothreads the authors utilized the methods similar to those previously used for benzene-derived nanothreads, except disallowing homopolar bonds due to the large energy penalty of a B–B or N–N bond compared to a B–N bond. Their work is recently published in the journal ACS Nano.

The researchers showed an increase in energy when transitioning from molecular borazine to degree-2 borazine-derived threads (i.e. those with two atoms in each borazine bonded to a neighbouring molecule). The energy then decreased upon continuing to degree-4 and degree-6 nanothreads with the formation of more σ bonds. Limiting structures to those with at most two borazine formula units within the crystalline repeat unit, only 13 fully saturated (degree-6) borazine-derived nanothreads were obtained that avoid forming energy-intensive homopolar bonds. Of the 13 fully saturated BN nanothreads, only two were more stable than borazine molecules. This limited number of low-energy outcomes could help in kinetic control of reaction products. Compared to partially saturated benzene nanothreads, where only even-degree threads avoid the formation of high-energy radicals, degree-3 and degree-5 threads are also possible for boron nitride threads containing three-coordinate borons and nitrogen. In addition to high mechanical strength, borazine-derived nanothreads may also exhibit remarkable piezoelectricity and flexoelectricity.

In summary, the study enumerated the possible structures of borazine-derived nanothreads with a focus on their reaction pathways and piezoelectric properties. The resulting borazine nanothreads exhibited remarkable properties, some of which are absent in their benzene nanothreads. In a statement to Advances in Engineering, first author Dr. Tao Wang stated that their study provided a systematic approach for exploring the potential structures and plausible reaction pathways for borazine-derived nanothread, and highlighted the intriguing electromechanical properties of borazine-derived nanothreads arising from the unusual combination of extremely thin yet rigid backbone and a polar lattice, which makes them promising candidates for various applications such as nanoelectronics, energy storage, and sensing. Overall, nanomaterials such as borazine-derived nanothreads have a transformative effect on materials engineering, opening up new avenues for research and development and enabling the creation of materials with unique and desirable properties. With ongoing advances in nanotechnology, it is expected that materials engineering will continue to be an important field for the development of new and innovative materials with applications in a wide range of industries.