FULLERENES

Name: Fullerenes (named after Buckminster Fuller, due to their structural similarity to his geodesic domes)

Symbol: C (specific forms such as C₆₀, C₇₀, etc.)

Group: 14

Period: 2

Block: p

Category: Nonmetal (molecular allotrope of carbon)

Atomic number: 6

Atomic mass: 12.011 u (per carbon atom; e.g., C₆₀ = 720.66 u)

Electrons per shell: 2.4 (per carbon atom)

Electronegativity: 2.55

Density: ~1.65 g/cm³ (for solid C₆₀)

Melting point: Not defined (C₆₀ sublimes at ~600–700 °C in an atmosphere) (inert)

Boiling point: Not applicable (sublimes at high temperatures)

Thermal conductivity: ~0.4 W/(m K) (low, comparable to polymers)

Electrical conductivity: ~10⁻¹⁴ S/m (insulator, but can be semiconducting when doped)

Magnetic order: Diamagnetic

Ordinary state: Solid (molecular or crystalline in fullerite)

Oxidation states: 0 (mainly), +2, +4 (in derived compounds)

Mohs hardness: Not applicable (molecular material, soft in pure state)

Vickers hardness: Non-standard (low mechanical strength in fullerite)

Brinell hardness: Non-standard (molecular material)

Most stable isotopes: C-12 (98.93%), C-13 (1.07%)

Discoverer: Harold Kroto, Robert Curl, and Richard Smalley (discovered in 1985, Rice University, USA; Nobel Prize in Chemistry 1996)

Fullerenes, a molecular allotrope of carbon (C) characterized by closed structures such as spheres, ellipsoids, or tubes (e.g., C₆₀ or "buckyball"), represent a milestone in materials science due to their unique properties, such as low thermal conductivity (0.4 W/(m K)), electrical insulation (10⁻¹⁴ S/m) in their pure state, and potential for superconductivity when doped. Discovered in 1985, fullerenes ushered in the era of nanotechnology, opening the door to applications in electronics, medicine, and composite materials. Their history combines theoretical and experimental advances and an interdisciplinary context that culminated in the 1996 Nobel Prize in Chemistry, transforming our understanding of carbon and its allotropes.

Although carbon has been known since ancient times in forms such as graphite and diamond, the existence of fullerenes was not suspected until the end of the 20th century. In the 1970s, theoretical chemists speculated about carbon molecular structures based on networks of pentagonal and hexagonal rings, inspired by the geodesic domes of architect Buckminster Fuller, after whom they are named. However, the experimental breakthrough came in 1985, when Harold Kroto (University of Sussex, UK), Robert Curl, and Richard Smalley (Rice University, USA) were investigating the formation of carbon molecules under simulated stellar atmospheres. Using laser mass spectroscopy in a graphite vaporization experiment, they identified a stable molecule of 60 carbon atoms (C₆₀), with an icosahedral structure of 20 hexagons and 12 pentagons, published in Nature (1985). This molecule, nicknamed "buckminsterfullerene," was the first confirmed fullerene. Initially, the discovery was met with skepticism by the scientific community, as closed molecular structures of carbon were unusual. In 1990, Wolfgang Krätschmer and Donald Huffman succeeded in producing macroscopic quantities of C₆₀ by electric arc in inert atmospheres (such as helium, He), enabling detailed studies that confirmed its structure and properties. This breakthrough triggered a boom in fullerene research, including variants such as C₇₀ and tubular structures (precursors to carbon nanotubes, discovered in 1991 by Sumio Iijima). The ability of fullerenes to form crystals (fullerite) and exhibit superconductivity when doped with alkali metals, such as potassium (K₃C₆₀), at temperatures of ~20 K, consolidated their scientific relevance. Kroto, Curl, and Smalley received the Nobel Prize in Chemistry in 1996 for their discovery, which also inspired the study of other nanomaterials.

Although fullerenes are found in nature in small quantities (e.g., in soot or coal deposits), their synthetic production (estimated at approximately 100 tons per year by 2025) is the main source, with a market of approximately USD 500 million projected by 2027. In applications, fullerenes are used in electronics (organic solar cells, semiconductors), medicine (antioxidants, drug delivery systems), and composite materials (polymer reinforcements). Their biocompatibility and ability to encapsulate atoms (endohedral fullerenes) open up possibilities in medical imaging and cancer therapies. The history of fullerenes, from their accidental discovery to their impact on nanotechnology, reflects carbon's ability to surprise and redefine the boundaries of science, cementing them as a key allotrope in modern technology.

They are the third and least abundant (by a wide margin) allotropic form of elemental carbon found naturally. As an anecdotal fact, it's worth noting that the first fullerenes were synthesized in laboratories around the 1970s. They were discovered in small doses in the mineral known as shungite, which comes from Russia and is attributed with unique properties such as healing, radiation protection, chi cleansing... OK, most of its "virtues" are more pseudoscientific than demonstrable, but it's true that they contain fullerenes. What happens is that this is used against scientific knowledge itself, expanding, or rather, exaggerating, the properties of a mineral, such that if it truly had antibacterial properties, as it does (it's been known since the time of Peter I "the Great" of Russia), these characteristics have led us to go from talking about antimicrobial properties to the possibility of curing more serious diseases of all kinds without any demonstrable real basis to date. In any case, since we're talking about fullerenes, let's first describe exactly what they are.

So far, we've seen that both graphite and diamond are elemental carbon, or, in other words, pure carbon. Both forms are called allotropes and differ from each other in many characteristics. However, what makes graphite graphite and diamond diamond are their crystal structures.

As we saw before, crystal structure refers to the way in which the atoms are arranged in bulk in an allotrope.

In graphite, for example, the carbon atoms are hexagonally bonded in layers of graphene stacked together, giving it its characteristics. In diamond, the crystal structure is identical to that of elemental silicon and germanium, the so-called body-centered diamond cubic. This atomic configuration is only present in four of the elements in Group 14 of the Periodic Table, in atomic order from lowest to highest: Carbon (Diamond), Silicon, Germanium, and the β (beta) form of Tin, which forms when the pure metal drops below 13.2°C.

Just as the layers of Graphene intercalated within Graphite give it its properties, both good and bad, the same is true for Diamond, which assumes the structure we see naturally in the aforementioned elements.

Exactly the same thing happens with fullerenes, because their crystalline structure is different from both Graphite and Diamond.

Although there are several modifications, in a typical fullerene, the carbon atoms are linked in a hexagonal shape, forming what is often called a "honeycomb." It is composed of six carbon atoms linked to six others in long chains. Viewed through a microscope, they resemble the cells formed by bees in their honeycombs. It has been shown to be one of the optimal shapes that makes the most use of volume, in the sense that it is capable of forming strong structures in less space than other symmetrical shapes. In other words, the "design" (if we look at it symmetrically) is hexagonal. Now, graphene is also hexagonal, yet its properties are quite different. What changes? In fullerenes, the long chains of carbon atoms linked together to form hexagons take on tubular shapes or even form spheroid bodies. This is the case of the "buckyball," which resembles an atomic-sized soccer ball composed of carbon atoms linked in such a way that they form a perfect sphere where each particle is exactly as far from its neighbor as the one before it and the next: we are talking about a perfectly symmetrical structure.

Fullerenes, as we have already mentioned, come in many forms, but the most famous by far are nanotubes and the aforementioned buckyball.

Nanotubes are fullerenes that have, as their name suggests, a cylindrical, "tube-like" shape. They are the most popular and widely used of the fullerenes, so much so that many people don't even know what they are; that is, it is often mistakenly thought that fullerenes and nanotubes are two different things. The reason is that nanotubes are so popular compared to the rest of their family that they seem to form a category in themselves, which is not the case.

Nanotubes are synthesizable, and unlike graphite and diamond, they have "metallic" properties. Their greatest virtues are their extremely high tensile strength (or rather, tensile strength), which makes them one of the strongest materials, if not the strongest, in this particular aspect. In addition to good chemical resistance, nanotubes are also good thermal and electrical conductors. The allotrope's high tenacity and hardness properties have led to theories regarding the manufacture of the ancient and semi-legendary "Damascus Steel," which was said to be capable of cutting a blade in the air and also breaking a rock (normally, only steel can have a very high edge or high toughness, never both at the same time at such levels). Hence, it has been theorized that carbon nanotubes naturally present in the coke used to produce these weapons may have given them these properties, which are still talked about today. This theory is unconfirmed, but according to it, since graphite (a common form) produces such good results in steel, the presence of an "improved" form of carbon would, by this principle, be responsible for producing far superior steel alloys. This is something I don't agree with for several reasons, such as the fact that the process is possible today, yet it doesn't seem profitable. Or, Damascus steel would have been of very good quality in its day, but like all things of its time, it would have been "sold" in the most exaggerated way, giving it almost mystical or supernatural overtones to give a sense of superior quality. I know this because I myself have written about it in the past. In any case, this is the fullerene that most closely resembles our discipline.

Buckyballs and other fullerenes have applications in the medical industry, but their importance in metallurgy is virtually nonexistent, so I won't go into detail about it in this section.

Fullerenes, molecular allotropes of carbon (C) such as C₆₀ (buckminsterfullerene), are closed structures shaped like spheres, ellipsoids or tubes, characterized by sp² hybridization and unique properties such as low thermal conductivity (0.4 W/(m·K)), electrical insulation (10⁻¹⁴ S/m) in pure state, and potential for superconductivity when doped. With a density of ~1.65 g/cm³ (for solid C₆₀) and an estimated global production of ~100 tons per year (2025), fullerenes are high-value materials in nanotechnology, with a projected market of ~$500 million by 2027. Their versatility, biocompatibility, and ability to encapsulate atoms make them ideal for applications in electronics, medicine, composite materials, and energy, transforming industries despite the challenges of large-scale synthesis using methods such as electric arc or controlled combustion.

In electronics, fullerenes, particularly C₆₀ and C₇₀, are used in organic solar cells (OPVs) as electron acceptors, improving power conversion efficiency (~10–15% in optimized cells) due to their high electron mobility upon doping. They are also used in field-effect transistors (FETs) and sensors, taking advantage of their ability to form functional derivatives, such as PCBM ([6,6]-phenyl-C₆₁-methylbutyrate), which optimizes charge transport. In medicine, the biocompatibility of fullerenes allows their use as antioxidants, capturing free radicals to protect cells in treatments for neurodegenerative diseases or cancer. Endohedral fullerenes, which encapsulate atoms such as nitrogen (N@C₆₀), are explored in medical imaging (MRI) and targeted therapies, while water-soluble derivatives are used in controlled drug-release systems.

In composite materials, fullerenes reinforce polymers and resins, improving mechanical and thermal resistance without adding significant weight, which is valuable in the aerospace and automotive industries. For example, incorporating C₆₀ into polymers increases stiffness by 10–20%, ideal for protective coatings. In energy, alkali metal-doped fullerenes, such as potassium (K₃C₆₀), exhibit superconductivity at temperatures of ~20 K, with potential applications in energy storage devices. Furthermore, they are used in catalysts for chemical reactions, such as hydrogenation, and in nanostructured lubricants due to their friction-reducing spherical structure. Although production costs (USD 100–1,000/g for pure C₆₀) limit their widespread adoption, fullerenes remain fundamental in nanotechnology, consolidating their role as an innovative material with a growing impact on science and industry.