Carbon – from simple substance to element and back
This object is especially significant for the fact that in the course of its long history it has appeared in different research contexts, allowing us to see that it is not the same object in these various contexts. To be sure, this is true of a great number of objects, most prominently perhaps the gene which began its scientific career as a hypothetical entity introduced for explanatory purposes and which is now a technoscientific design tool. What makes carbon an even more striking example is that it is such an ordinary and familiar object. In the nineteenth century, carbon was identified as a chemical element. In the periodic system set up by Mendeleev in 1869, it appears as a “typical element,” the head of a column which exemplifies the properties of its “family.” Although charcoal was the main pillar of the industrial revolution, Mendeleev was not concerned with the properties of charcoal. Rather he considered the element carbon, the basic substance that exists in all known allotropic forms of carbon, namely diamond, charcoal and graphite. Mendeleev drew a clear distinction between the abstract notion of elements and the concrete stuff of simple substances. Elements cannot be isolated while simple substances come into existence at the end of a process of analysis and purification. As an element carbon is a “separate homogeneous substance, the material but invisible part of compounds” (Mendeleev 1952, p. 439). It is a material entity notably with no essential physical features, as illustrated by its protean role in the chemistry of life. It is characterized by its atomic weight, a property derived from theoretical views about atoms and molecules and with experimental data on the various compounds formed by carbon. It was precisely this abstract distinction between elements and simple substance that provided a clue for Mendeleev’s discovery of the periodicity of the chemical properties of elements. Without this abstract notion, Mendeleev could never have predicted the existence of new elements, before they could be isolated as simple substances – a phenomenological notion of simple substance would not allow predictions of unknown elements.
Mendeleev’s emphasis on the centrality of elements was maintained even reinforced in the early twentieth century with the discovery of isotopes. The periodic system served as the chart of chemistry, the inscription of the basic building blocks that are used by nature and simultaneously the revelation of a unique and general law governing the irreducible diversity of chemical phenomena. Elements such as carbon were significant objects of investigation when chemists were concerned with establishing order in the jungle of individual substances. And thus, although the development of quantum theory shifted the attention of chemists from the macroscopic properties of chemical substances to the inner structure of their atoms, the notion of elements remained fundamental for chemists. When the term ‘isotope’ was forged, this reaffirmed the pertinence of the concept of the element as a distinct chemical entity, albeit now defined in relation to the sub-atomic particles that are the constituents of matter.
From a technoscientific perspective the periodic system is seen as a kind of well-organized toolbox. And over the past decades of the twentieth century, carbon has been the focus of intensive research as a resource for the construction of potential tools for specific applications such as high-modulus reinforcing fibers or semi-conductors. The attention has shifted from the element carbon to the variety of its allotropes and novel manifestations: fullerenes, nanotubes, graphene. The various architectures of carbon molecules have been systematically explored as potential materials for performing specific tasks. What used to be one of the fundamental elements of the material universe became a mine of materials, a vehicle of functional properties that could be useful for technological applications. The old familiar chemical element carbon now exists at the intersection of specific natural dispositions and social, economic, military, environmental concerns.
New instrumental techniques such as electron microscopy and scanning tunneling microscopy reconfigured not only the identity of carbon, but its potential at the level of individual molecules. Significantly, the carbon-60 fullerene structures that were discovered in the 1980s were not scrutinized as exotic laboratory curiosities that would shed light on the structural properties of carbon. In a short time, these buckyballs favored the rediscovery of the long-forgotten nanotubes. These rapidly became the starting point of a race for designing single-wall and multiple-wall carbon nanotubes for electronic or medical technologies. Starting in 2004, graphene – a structural component of graphite – became a major focus of research. This isolated plane of carbon atoms is a thermodynamically unstable sheet of graphite. For this reason, it was not expected to exist in isolation from graphite. Nevertheless, once artificially isolated, it revealed to be perfectly stable. After the Nobel prize awarded in 2010 to Andrei Geim and Konstantin Novoselov “for groundbreaking experiments regarding the two-dimensional material grapheme,” the three novel forms of carbon were brought together in a single narrative – the story of the carbon sheet “unfolding itself” to visibility at increasing dimensionalities: Fullerenes (zero dimension) where first studied with mass spectroscopy and calculations, nanotubes (one dimension) were discovered with electron microscopy, and finally, graphene (two dimensions) can be observed with visible light microscopy.
Graphene is neither an elementary building block nor a bulk material. It is but a pure surface, a surface without bulk, a surface in itself. As a surface, it combines plasticity, sensitivity and reactivity, as a single layer of carbon atoms it promises regularity of structure and control – every atom matters but the atoms are reasonably well behaved. As a pure surface again, it has a unique electronic structure: an anomalous quantum-Hall effect and a zero-gap band structure; its electrons are massless and behave like relativistic particles describable by the Dirac equation (and not by the Schrödinger equation). Graphene thus also furnishes a “pocket playground” to model relativistic quantum mechanics. Moreover, graphene is both a nanoscale and a macroscopic object: It is part of the nanoworld, and part of our world, and can be engineered at both scales. It is therefore considered a promising substitute for silicon in information technology – while there are speculations about silicon-based lifeform.
All this suggests that carbon is an attractive object to scientists and technoscientists alike. What makes it attractive, however, is different at different times and in different contexts. As a pure element it holds a key to the constitution of matter and of life – it is an elusive building block behind the appearances. And as a carrier of functional properties it holds the promise of material proliferation and of technologies to be – its protean character epitomizes the plasticity of nature, that is, its malleability to human ends.
- Mendeleev, D. (1952). The Relation Between the Properties of the Atomic Weight of the Elements. Leicester H.M., Klickstein H.S. (Eds.) Sourcebook in Chemistry 1400-1900, New York: Dover, p. 439.