Two very small forms of carbon
Fullerenes and nanodiamonds are both nanoscale forms of carbon, and both appear in discussions of new carbon materials. They are often mentioned together, but they are built differently. A fullerene is a hollow molecule made of carbon atoms joined in a closed cage. A nanodiamond is a tiny crystal with the same atomic arrangement as ordinary diamond, only extremely small. Placing them side by side shows how bonding type and size shape what carbon does.
Fullerene molecules
The best-known fullerene has 60 carbon atoms arranged in a pattern of pentagons and hexagons, much like the panels of a soccer ball. Larger cages, such as ones with 70 atoms, also exist, and so do elongated and nested variants. Each atom bonds to three neighbors, as in graphite, but the pentagons force the sheet to curve and close up into a ball.
Several points are worth knowing:
- Fullerenes are discrete molecules. They can dissolve in certain organic solvents and can form crystals in which the molecules pack together, unlike a graphite stack or a diamond lattice.
- The closed cage can, in principle, hold other atoms inside, giving so-called endohedral fullerenes studied in research.
- The outside surface can be chemically modified, which allows researchers to attach groups that change solubility and reactivity.
Fullerenes were first identified in the 1980s, and the finding helped lead to interest in related structures such as nanotubes and graphene. A nanotube can be thought of as a fullerene stretched into a cylinder, which links to the discussion of carbon nanotubes.
Where fullerenes are used
Practical use has been narrower than early excitement suggested.
- Organic electronics research. Fullerene derivatives have served as electron-accepting materials in experimental organic solar cells.
- Chemistry and catalysis studies. Their reactivity and cage structure make them useful model systems.
- Lubricant and composite additives. Fullerene-based additives have been investigated, with mixed results.
- Biomedical research. Modified fullerenes have been studied for several roles, but safe, effective medical products are not established, and claims in this area deserve caution.
Fullerenes are produced mainly by vaporizing graphite in an inert atmosphere and separating the fullerenes from the resulting soot. Purification is a significant part of the cost.
Nanodiamond particles
Nanodiamonds are diamond crystals only a few nanometers across in the smallest cases, and up to tens or hundreds of nanometers in others. They retain the strong, three-dimensional bonding of diamond, so the core is hard. At this size, though, the surface is a big fraction of the particle, and the surface chemistry often matters as much as the core. Surfaces commonly carry groups that contain oxygen or hydrogen, or a thin disordered shell that is graphite-like.
Common production routes include:
- Detonation synthesis. Explosives with a negative oxygen balance detonate in a closed chamber, and diamond particles of only a few nanometers form in the soot. They need extensive purification and tend to clump into aggregates.
- Milling of larger diamond. High-pressure, high-temperature diamond grit can be crushed to small particles, though controlling size and shape is difficult.
- Other methods. Laser-based and vapor-based approaches also exist, mostly at research scale.
These particles are related to the synthetic diamond produced by HPHT presses, but they are made for an entirely different size range.
Where nanodiamonds are used
- Polishing and lapping slurries. Fine diamond particles are used to produce very smooth surfaces on hard materials.
- Additives to coatings, lubricants and polymers. Small amounts can alter hardness, wear or friction, though dispersion is a recurring challenge, as it is with nanotubes.
- Seeding for diamond film growth. Nanodiamond particles are sometimes placed on a surface to act as nuclei so a continuous film can grow, which connects to the idea of seed crystals in vapor deposition.
- Biological labeling and sensing. Some nanodiamonds contain defects, such as nitrogen-vacancy centers, that emit light and respond to magnetic fields, making them interesting for sensing and imaging research.
Each of these remains somewhat specialized, and performance depends heavily on the particle quality, which varies by source. The broader subject is treated under diamond nanoparticles.
Comparing the two
| Feature | Fullerene | Nanodiamond |
|---|---|---|
| Bonding | Three neighbors per atom | Mostly four neighbors per atom |
| Form | Hollow molecule | Solid crystal |
| Typical size | About one nanometer for the common cage | A few nanometers to hundreds |
| Behavior | Molecular, soluble in some solvents | Hard particle, dispersed in liquids |
| Typical interest | Chemistry, electronics research | Polishing, additives, sensing research |
Why they belong in the same story
Both show that carbon properties depend on structure and scale. Shrink diamond to nanometers and surface effects start to dominate. Curl a graphite-type sheet into a ball and a new molecule appears. They sit alongside graphite, graphene, tubes and amorphous carbon within the larger overview of carbon allotropes.
A caution applies to both: nanoscale materials can behave differently from their bulk counterparts in the body and the environment, and published findings should be read with attention to the exact material and dose studied.