A discovery in November 1985 meant that all chemistry textbooks became outdated overnight. Until then, scientists believed that carbon only occurred in one of two allotropes: as graphite or as diamond. Allotropy refers to the ability of a chemical element to exist in two or more different forms in the same physical state. These different forms can vary in the arrangement of their atoms, leading to distinct physical, mechanical and chemical properties. Diamond and graphite are classic examples of allotropy: In diamond, the carbon atoms are arranged in a tetrahedral structure, whereas in graphite the atoms form layers of hexagonal lattices. 

From Soccer to Science

In November 1985, Sir Harold W. Kroto, Robert Curl, and Richard Smalley at Rice University discovered a third carbon allotrope – C60 fullerene. This cage-like molecule consists of 60 carbon atoms joined together by single and double bonds to form a hollow sphere with 12 pentagonal and 20 hexagonal faces, giving it a shape that resembles a soccer ball. The called it “Buckminsterfullerene” because of its resemblance to geodesic domes designed by the American architect Buckminster Fuller.

A fullerene is an allotrope of carbon. Its molecules consist of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to six atoms. The molecules may have hollow sphere- and ellipsoid-like forms, tubes, or other shapes.

A Serendipitous Discovery: The Magic of C60

As so often in science, the discovery was accidental: Originally, the scientists were investigating the absorption spectra of interstellar dust. To achieve their research goal, they used a powerful laser to vaporize graphite rods in an atmosphere of helium gas. This process was designed to simulate the extreme conditions found in the atmospheres of carbon-rich red giant stars. After producing carbon clusters in this way, they used a mass spectrometer to analyze the resulting molecules. But in addition to the expected long carbon chains, they also discovered a previously unknown, pure-carbon molecule in the sooty residue, which is known today as fullerene.

The C60 molecule forms very readily and exhibits extraordinary stability. This has a number of reasons:

• Fullerenes have a closed-cage structure where carbon atoms form a continuous surface with no dangling bonds. This contributes significantly to their stability and resilience.
• Each carbon atom is bonded covalently with three others, creating a network of interconnected hexagons and pentagons in icosahedral symmetry. This bonding arrangement distributes stress evenly across the molecule.
• The arrangement of carbon atoms in a sphere-like structure allows for even distribution of forces, making fullerenes resistant to external pressures.
• In C60, there are 20 hexagons and 12 pentagons arranged in a pattern similar to a soccer ball. This structure minimizes strain in the molecule.
• Isolated pentagon rule: In C60, no two pentagons share an edge, which contributes to its stability by further reducing strain.
• The carbon atoms in fullerenes are connected by both single and double bonds. In C60, the bonds between two hexagons are shorter than those between a hexagon and a pentagon, creating a balanced and stable structure.
• While not as extensive as in some other carbon structures, there is some delocalization of π-electrons across the fullerene molecule, contributing to its stability.
• Fullerenes, especially C60, exhibit a high degree of symmetry, which contributes to their stability and unique properties.

In 1996, Curl, Kroto and Smalley received the Nobel Prize in Chemistry for their roles in the discovery of buckminsterfullerene and the related class of molecules, the fullerenes. The presenter of the Nobel noted its implications for all fields of natural science. Indeed, research on fullerenes has resulted in the synthesis of more than a thousand new compounds.

From Fullerenes to Carbon Nanotubes: Shaping the Future of Nanotechnology

Fullerenes, particularly C60, were among the first well-defined nanoscale structures to be discovered and studied. This achievement laid the groundwork for the emerging field of nanotechnology, opening up new avenues for research and applications at the molecular level.

Carbon nanotubes (CNTs), discovered and synthesized in 1991, are a type of cylindrical fullerene. These carbon tubes typically have diameters in the nanometer range. Despite their small width, they can vary significantly in length, extending from less than a micrometer to several millimeters. The maximum length of carbon nanotubes (CNTs) can vary significantly depending on the synthesis method used.

The unique C–C bonding and cylindrical structure of these one-dimensional materials give rise to some remarkable macroscopic characteristics:

• Mechanical strength: Along the longitude directions, carbon nanotubes show superior mechanical strength, with the highest known tensile strength and elastic modulus among known materials.
• Electrical properties: Carbon nanotubes are either metallic or semiconducting along the tubular axis. In theory, metallic nanotubes can carry an electric current density of 4 × 109 A/cm2, which is more than 1,000 times greater than those of metals like copper.
• Superior heat conductivity: outperforming diamond as the best thermal conductor.
• Chemical inertness: Their cylindrical and planar structure, lacking exposed atoms that can easily be displaced, contributes to their relative chemical stability.
• High ductility: CNTs can undergo large elastic deformations, meaning they can stretch and return to their original shape. Under certain conditions, they can also exhibit plastic deformation, where they permanently change shape but still maintain their overall structural coherence.

Based on the number of overlapping cylinders, carbon nanotubes can be categorized into the following groups:

1. Single-walled carbon nanotubes (SWCNTs) have diameters ranging from approximately 0.5 to 2.0 nanometres. SWCNTs are composed of carbon atoms arranged in a hexagonal lattice, forming a seamless cylindrical tube. This unique arrangement gives them their remarkable properties.
2. Multi-walled carbon nanotubes (MWCNTs) consist of two or more single-wall carbon nanotubes in a nested, tube-in-tube structure. The distance between adjacent shells is about 0.34 nanometer. Double- (DWCNTs) and triple-walled carbon nanotubes are specific types of MWCNTs.

Tailoring Properties for Advanced Applications

Functionalized fullerenes have shown promise in various biomedical fields, such as MRI and X-ray imaging, photodynamic therapy, and drug/gene delivery.

A key advantage of DWCNTs this is the ability to modify the outer tube without affecting the inner one, allowing for functionalization (= grafting chemical functions to the surface of the nanotubes to add new properties) while maintaining the overall characteristics of the material. These characteristics make DWCNTs valuable for various applications including composite materials (reinforcing polymer matrices), hydrogen storage, catalysts, solar cells, biosensors, and nanoelectronics.

The telescopic motion ability of inner shells, allowing them to act as low-friction, low-wear nanobearings and nanosprings, has potential in nanoelectromechanical systems (NEMS).

Recent studies have explored using carbon nanotubes to create three-dimensional macroscopic all-carbon devices. Researchers have developed methods to fabricate free-standing, porous, all-carbon scaffolds using single- and multi-walled carbon nanotubes. These scaffolds feature pores at multiple scales (macro, micro, and nano) and can be customized for various applications.

Conclusion 

The discovery of fullerenes not only expanded our understanding of carbon chemistry but also provided researchers with new building blocks for creating advanced materials, devices, and technologies. Today, they are at the heart of nanotechnology, driving innovation across multiple sectors, from electronics and materials to medicine and energy. 

The term “fullerenes” derives from buckminsterfullerene (C60), the first fullerene molecule to be discovered. Due to their resemblance to soccer balls, closed fullerenes, particularly C60, are also known as “buckyballs”. The suffix “ene” indicates that the carbons are unsaturated (connected to only three other atoms, rather than the typical four).

Sources:  

• Kroto, H.W.; Heath, J. R.; Obrien, S. C.; et al. (1985). "C60: Buckminsterfullerene". Nature. 318 (6042): 162–163 
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• https://www.acs.org/molecule-of-the-week/archive/b/buckminsterfullerene.html 
• Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors, Shivani Dhall, 2023 Elsevier Inc.