Types of Fullerenes

Fullerenes are a distinct class of carbon allotropes composed entirely of carbon atoms arranged in the form of a closed hollow structure, such as spheres, ellipsoids, or tubes. They are also known as buckyballs or buckytubes, named after the architect Buckminster Fuller, whose geodesic domes resemble their molecular geometry. Fullerenes are made up of sp²-hybridised carbon atoms linked by single and double bonds, forming networks of pentagons and hexagons. Since their discovery in 1985, fullerenes have become an important subject of study in materials science, chemistry, and nanotechnology due to their unique electrical, mechanical, and chemical properties.

Discovery and Background

Fullerenes were first identified in 1985 by Harold Kroto, Robert Curl, and Richard Smalley during experiments involving the vaporisation of graphite using a laser. The team discovered a new molecular form of carbon with 60 atoms (C₆₀) arranged in a spherical structure resembling a soccer ball. This molecule was named buckminsterfullerene, and the discovery earned the scientists the 1996 Nobel Prize in Chemistry.
Since then, many other fullerene structures have been synthesised, varying in size, shape, and carbon atom count. Fullerenes can be categorised into several types based on their structure and configuration.

Classification of Fullerenes

Fullerenes are classified into three main structural types:

1. Spherical Fullerenes
2. Cylindrical Fullerenes (Carbon Nanotubes)
3. Ellipsoidal and Other Variants

Additionally, derivatives and modified forms of fullerenes exist, including endohedral, exohydrogenated, and heterofullerenes, which display modified properties for specialised applications.

1. Spherical Fullerenes

Spherical fullerenes are the most well-known type and are composed of carbon atoms arranged in a pattern of 12 pentagons and varying numbers of hexagons forming a closed cage. Each carbon atom is bonded to three others, maintaining sp² hybridisation.
Examples of Spherical Fullerenes:

• C₆₀ (Buckminsterfullerene):
  • The most common and stable fullerene.
  • Composed of 60 carbon atoms forming a truncated icosahedron with 12 pentagons and 20 hexagons.
  • Resembles a football (soccer ball) structure.
  • Highly symmetrical and stable, used extensively in nanotechnology and materials research.
• C₇₀ Fullerene:
  • Comprises 70 carbon atoms forming a prolate spheroid (rugby-ball shape).
  • Contains additional hexagons compared to C₆₀, leading to elongated geometry.
  • Exhibits similar properties to C₆₀ but with altered electronic characteristics.
• Higher Fullerenes (C₇₆, C₈₄, C₉₀, etc.):
  • Larger molecules with more carbon atoms.
  • Increased size leads to structural diversity and variation in electronic behaviour.
  • Found in soot or synthesized through arc discharge methods.

Properties of Spherical Fullerenes:

• Highly symmetrical structures with delocalised π-electrons.
• Excellent electron acceptors, useful in photovoltaic and superconducting materials.
• Chemically modifiable to form derivatives with specific functions.

2. Cylindrical Fullerenes (Carbon Nanotubes)

Carbon Nanotubes (CNTs) are cylindrical structures formed by rolling a single or multiple layers of graphene into tubes. They are considered a tubular form of fullerenes and exhibit exceptional mechanical, thermal, and electrical properties.
Types of Carbon Nanotubes:

• Single-Walled Carbon Nanotubes (SWCNTs):
  • Consist of a single graphene sheet rolled into a cylindrical shape.
  • Diameter typically ranges from 0.7 to 2 nanometres.
  • Exhibit either metallic or semiconducting behaviour depending on chirality (the angle at which the graphene sheet is rolled).
• Multi-Walled Carbon Nanotubes (MWCNTs):
  • Composed of multiple concentric graphene cylinders with interlayer spacing of about 0.34 nm.
  • Larger diameters (up to several tens of nanometres).
  • Mechanically stronger but less flexible than SWCNTs.

Applications of Carbon Nanotubes:

• Used in nanocomposites, batteries, transistors, sensors, and drug delivery systems.
• Their exceptional tensile strength and electrical conductivity make them ideal for aerospace and electronics industries.

3. Ellipsoidal and Other Fullerenes

Ellipsoidal fullerenes differ from the spherical types by having elongated or distorted geometries. These variations arise due to differences in the arrangement of pentagonal and hexagonal rings.
Examples include:

• C₇₀: Prolate (rugby-ball shaped).
• C₇₆ and C₇₈: Exhibit slightly distorted structures with lower symmetry.
• C₈₄: Possesses multiple isomeric forms, some nearly spherical, others ellipsoidal.

Such geometrical variations influence their electronic and optical properties, affecting their use in photonics and nanotechnology.

Special and Derived Types of Fullerenes

Apart from the basic structural types, several special categories of fullerenes exist, defined by the inclusion of other elements or chemical modifications.
1. Endohedral Fullerenes

• Contain foreign atoms, ions, or clusters trapped inside the carbon cage.
• The encapsulated species can be metals, noble gases, or small molecules (e.g., helium, scandium, or lanthanum).
• Example: La@C₆₀ (lanthanum inside a C₆₀ cage).
• Exhibits unique magnetic and electronic properties, used in molecular electronics and quantum computing.

2. Exohedral Fullerenes

• Contain atoms or groups chemically bonded to the outer surface of the fullerene cage.
• Functionalisation alters solubility, stability, and reactivity.
• Used in developing fullerene-based polymers and pharmaceuticals.

3. Heterofullerenes

• Some carbon atoms in the fullerene structure are replaced with heteroatoms like nitrogen, boron, or phosphorus.
• Example: Azafullerenes (C₅₉N) and Boron-substituted fullerenes (C₅₉B).
• Exhibit tunable electronic properties, useful in semiconductor and catalyst design.

4. Fullerene Polymers and Dimers

• Formed by linking multiple fullerene molecules through covalent bonds.
• Example: C₆₀–C₆₀ dimers or polymeric C₆₀ created under high-pressure conditions.
• Exhibit enhanced mechanical strength and conductivity.

Electronic and Structural Diversity

Each fullerene structure exhibits distinct electronic configurations depending on the number of carbon atoms and the symmetry of the molecule.

• Smaller fullerenes (e.g., C₂₀) tend to be less stable due to high strain.
• Larger fullerenes (C₆₀ and above) are more stable and exhibit delocalised π-electron systems, leading to unique optical and conductive properties.

The bonding pattern (12 pentagons and varying hexagons) ensures a curvature that differentiates fullerenes from planar carbon forms such as graphene and graphite.

Applications of Fullerenes

The versatility of fullerenes has led to their use in numerous scientific and technological fields:

• Medicine: Drug delivery systems, antiviral agents, and photodynamic therapy.
• Electronics: Semiconductors, photovoltaic cells, and superconductors.
• Materials Science: Reinforcement of composites and lubricants.
• Energy Storage: Battery electrodes and hydrogen storage materials.
• Environmental Technology: Catalysts for pollutant degradation.

Their ability to accept and donate electrons also makes them effective in molecular electronics and organic solar cells.