Borophene is a single-atom-thick sheet of boron, part of the same family of ultra-thin materials as graphene (which is made of carbon). First synthesized in 2015, it has attracted intense interest because of its unusual combination of metallic conductivity, mechanical flexibility, and potential for energy storage that far exceeds current battery technology. Unlike graphene, borophene cannot be peeled from a bulk crystal. It has to be built atom by atom in a vacuum chamber, which makes it one of the more challenging two-dimensional materials to produce and study.
How Borophene Is Structured
At the atomic level, borophene starts as a flat triangular lattice of boron atoms. What makes it distinctive is that some of those triangles are missing their center atom, leaving behind hollow hexagons scattered across the sheet. The specific pattern and concentration of these hollow hexagons determine which “polymorph,” or structural variety, you end up with. Two of the most studied are called v1/5 and v1/6, named for the fraction of missing atoms. In the v1/5 phase, the hollow hexagons are staggered; in the v1/6 phase, they line up in rows.
This is also what makes borophene unusually polymorphic. Because there are so many possible ways to arrange the hollow hexagons, borophene can take on a vast number of slightly different structures, each with its own electronic and mechanical fingerprint. Growth temperature plays a role too. At higher temperatures, borophene phases shift from being neatly aligned with their substrate to rotating freely, producing yet more structural variety.
How Borophene Is Made
You can’t make borophene the way you can make graphene. Graphene can be exfoliated, essentially peeled off a chunk of graphite with adhesive tape. Boron doesn’t have a layered bulk form that allows this, so borophene has to be grown from scratch using a technique called molecular beam epitaxy. This involves vaporizing boron inside an ultra-high vacuum chamber and letting individual atoms settle onto a metal surface, where they self-assemble into a single-atom-thick sheet.
The choice of metal surface matters. Researchers have successfully grown borophene on silver, copper, gold, aluminum, iridium, and ruthenium surfaces, each cut along specific crystal orientations. Silver has been the most widely used substrate, particularly its (111) face, which was the surface used in the first successful synthesis. More recently, bilayer borophene (two sheets stacked together) has been grown on silver, copper, and ruthenium, producing material with better thermal stability and oxidation resistance than a single layer.
Electrical and Electronic Properties
Metallicity is borophene’s signature electronic trait. While graphene is a semimetal and phosphorene is a semiconductor, borophene is a full metal in every polymorph studied so far. This is unusual: bulk boron is not metallic, so the conductivity emerges specifically from the two-dimensional structure. Electrons in borophene travel with a Fermi velocity of about 6.6 × 10⁵ meters per second, comparable to graphene’s, which makes it a strong candidate for nanoscale electronics.
The conductivity is also highly directional. Along one crystal axis, the electronic bands are deeply dispersed, meaning electrons move freely and quickly. Along the perpendicular axis, a significant gap opens up. This anisotropy could be useful for building devices that conduct in one direction but not another, something that is difficult to achieve with most materials at the atomic scale. Some experimental work has also measured an optical bandgap of about 2.25 eV in certain forms, suggesting that under the right conditions borophene can behave as a direct-bandgap semiconductor with strong photoluminescence.
Borophene also shows superconducting behavior. A bilayer form known as kagome borophene is predicted to superconduct at a critical temperature between roughly 17 and 35 Kelvin, which is high for a phonon-mediated superconductor. That superconductivity is also anisotropic, meaning it is stronger along certain directions in the sheet.
Mechanical Strength
Borophene is remarkably stiff for a material one atom thick, though its strength depends heavily on direction. Along its “armchair” direction, borophene has an in-plane elastic modulus of about 1,372 GPa, well above graphene’s roughly 1,000 GPa. Along the perpendicular “zigzag” direction, it drops to about 586 GPa. Its ultimate tensile strength follows the same pattern: around 78.6 GPa in the armchair direction and 46.5 GPa in the zigzag direction, compared to graphene’s 123.5 GPa.
This directional variation means borophene is stiffer than graphene in one direction but significantly softer in the other. For engineering purposes, that anisotropy could be either a limitation or an advantage, depending on whether a device needs uniform strength or direction-dependent flexibility.
Energy Storage Potential
The application that generates the most excitement is batteries. Borophene’s theoretical capacity for storing lithium ions is 5,268 milliamp-hours per gram. For context, the graphite anodes used in today’s lithium-ion batteries hold about 372 mAh/g, making borophene’s theoretical capacity more than 14 times greater. The numbers for sodium-ion batteries are similarly dramatic: 1,860 mAh/g for borophene versus just 35 mAh/g for graphite, a 53-fold improvement.
These are theoretical maximums calculated from simulations, not figures from commercial batteries. But they illustrate why researchers see borophene as a potentially transformative anode material. Sodium-ion batteries are of particular interest because sodium is far more abundant and cheaper than lithium, and borophene’s enormous storage advantage could help make sodium-ion technology practical for grid-scale energy storage.
Hydrogen Storage
Borophene has also been studied as a platform for storing hydrogen fuel. When decorated with lithium atoms, a form called β12-borophene can theoretically adsorb up to 14 hydrogen molecules per unit cell, reaching a gravimetric storage capacity of 10.85% by weight. The U.S. Department of Energy’s target for practical hydrogen storage systems is 5.5% by weight, so borophene exceeds that benchmark by a wide margin in simulations. The hydrogen binds at energies that allow it to be released at moderate temperatures, which is critical for a usable storage system.
Gas Sensing
Borophene-based sensors have demonstrated exceptional sensitivity to nitrogen dioxide, a toxic gas produced by vehicle exhaust and industrial processes. In experimental tests, a borophene sensor detected NO₂ at concentrations as low as 200 parts per billion, with a usable detection range spanning from 200 ppb up to 100 ppm. The sensor responded within 30 seconds and recovered within about 200 seconds, all at room temperature. These figures outperform sensors based on other two-dimensional materials, and the sensor showed strong selectivity for NO₂ over other common gases.
Stability Challenges
The biggest practical obstacle for borophene is that it oxidizes quickly when exposed to air. A freshly grown sheet can degrade within minutes outside the vacuum chamber, which limits its usefulness for any application that requires long-term ambient exposure. Researchers have made progress on protective strategies, and bilayer borophene grown on certain substrates shows improved oxidation resistance compared to single layers. Still, the gap between borophene’s extraordinary lab-scale properties and a durable, real-world device remains the central challenge for the field.