The nano-revolution spawned by carbon

In 1985, scientists reported the discovery of the cage-like carbon molecule C60. The finding paved the way for materials such as graphene and carbon nanotubes, and was a landmark in the emergence of nanotechnology.
Pulickel M. Ajayan is in the Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77006, USA.

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The history of the carbon molecule C60 highlights the fact that discoveries do not happen in a predefined sequence. C60, carbon nanotubes and graphene (single layers of graphite) are essentially members of the same family: all are nanoscale structures that consist of carbon atoms arranged in a periodic crystal lattice. Graphite has been known for a few hundred years, and individual layers of the material could be separated easily. However, the identification of C60 by Kroto et al.1 did not occur until 1985. This, in turn, led to the discovery of graphene nearly two decades later2. Both of these breakthroughs led to Nobel prizes, in chemistry for C60 (1996) and in physics for graphene (2010).

The discovery of C60 occurred on the campus of Rice University in Houston, Texas. Eiji Osawa, a Japanese theoretical chemist, had predicted3 the stable structure of a 60-atom carbon molecule in 1970, but this finding did not come to the attention of the mainstream scientific community. Experimental results from mass spectrometry were also beginning to emerge, showing the stability of 60-atom carbon clusters. However, no one made the connection that these clusters would have the structure that Osawa had predicted. It was against this backdrop that the visit of the British chemist Harry Kroto to the laboratories of Rice scientists Richard Smalley and Robert Curl proved significant.

Kroto was an expert in molecular spectroscopy and had an interest in the molecules that exist in interstellar space. He proposed a simple mechanism for the formation of the small carbon-chain molecules that had been observed in interstellar gas clouds, and suggested that this idea could be tested using Smalley’s experimental apparatus. Smalley, Curl and their students were making many different atomic clusters, such as those of silicon, through ablation — the removal of material from the surface of a target — and were analysing the masses of these clusters in detail. After some delay, Kroto’s proposal was accepted and he journeyed to Houston.

In previous work by other groups4, a peak corresponding to C60 was somewhat prominent in mass spectra. During the experiments at Rice to test the mechanism of carbon-chain formation, it became clear that the C60 peak could be made extremely strong under certain conditions. However, the structure of the C60 molecule was the main puzzle that needed to be solved. The team accomplished this task, and published the first report in 1985.

The structure of C60 turned out to be a beauty (Fig. 1a). It looked exactly like the classic design of a football (soccer ball). More precisely, the structure is about 0.7 nanometres across and is a truncated icosahedron — a polyhedron that has 12 pentagonal and 20 hexagonal faces. This highly symmetric, cage-like shape was first described by Archimedes, and the rules that guide the topology of polyhedra were first developed by Descartes.

Figure 1 | Three major nanoscale carbon structures discovered in the past 35 years. a, In 1985, Kroto et al.1 reported the discovery of the molecule C60. It has a cage-like structure that consists of 12 pentagonal and 20 hexagonal faces. b, Following Kroto and colleagues’ work, carbon nanotubes were first produced10 in 1991. A carbon nanotube can be thought of as a 2D hexagonal lattice of carbon atoms that is rolled up to form a hollow cylinder. c, In 2004, scientists reported the isolation of graphene2 — a single layer of carbon atoms in a 2D hexagonal lattice.

When applied to polyhedra that are made of only pentagons and hexagons, these rules imply that every such closed structure can contain any number of hexagons but must have exactly 12 pentagons. Heptagons can also be introduced, producing negative curvature (saddle-shaped surfaces), but the topological effect of a heptagon is cancelled by that of a pentagon. In the mid-eighteenth century, the Swiss mathematician Leonhard Euler had proposed a formula5 for these geometric rules, which were now profoundly manifested at the nanoscale in C60. Larger closed carbon cages (such as C70 and C82) also exist, and can be formed by simply adding more hexagons to the cage.

The family of C60 and larger molecules have come to be known as the fullerenes, after the US architect Buckminster Fuller. Fuller had become famous for designing stable domes and buildings6 that have shapes similar to that of C60. The correspondence was striking, although the scale differed by a factor of about 10 billion. So it was that the C60 family got its name (its members could well have been called soccerenes).

Kroto and colleagues’ fullerene discovery took other scientists by surprise. Initially, there were quite a few sceptics; many thought that C60 was flat rather than cage-like. However, this perception changed after work by the German chemist Wolfgang Krätschmer, the US chemist Donald Huffman and their students. In 1990, these researchers succeeded7 in isolating C60 molecules from carbon soot in bulk , thereby making the substance available for large-scale experiments.

The fullerene discovery immediately had two major consequences. First, fullerenes were used to synthesize a large variety of unconventional materials. For example, endohedrals8 (fullerenes that enclose metal atoms), fullerene-assembled solids and superconducting fullerene materials9 were produced and characterized with excitement. Fullerenes were seen as a distinctive, stable molecular system and as an ideal building block for making unprecedented materials. They were also touted as a new allotrope (structural form) of carbon that deviated from the familiar graphite and diamond.

Second, the discovery provided the impetus to seek other carbon allotropes — particularly nanoscale materials. The most substantial result from this search was the synthesis and development of carbon nanotubes (Fig. 1b) by the Japanese physicist Sumio Iijima10 and colleagues11 in the early 1990s. Carbon nanotubes showed that the electronic structures of carbon layers could be tuned by structural nanoscale engineering, suggesting possible uses in electronics and other applications.

Over the following two decades, a rush of research activities, publications and patents would make fullerenes and carbon nanotubes the poster children of nanotechnology. It was also during this period, in 2004, that the Russian physicists Andrei Geim and Konstantin Novoselov isolated graphene2 (Fig. 1c). Graphene was the first example of a truly stable 2D material and revealed the physics associated with such 2D systems.

It has been nearly 35 years since Kroto and colleagues’ fullerene paper was published. In spite of all the potential that fullerenes promised, these molecules have not led to any major applications, barring a few encouraging ideas in solar cells and biochemistry. However, the work paved the way for many innovations in nanomaterials that will ultimately find uses in nanotechnology. The fullerene discovery and what followed show the ingenuity of the human mind in solving a nanoscale puzzle. Fullerenes also provide a curious case in which an architect’s name was dragged into a major scientific discovery. Buckminster Fuller would probably not have minded.

Nature 575, 49-50 (2019)

doi: 10.1038/d41586-019-02838-4


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