commentary The era of carbon allotropes Andreas Hirsch Twenty-five years on from the discovery of 60C , the outstanding properties and potential applications of the synthetic carbon allotropes — fullerenes, nanotubes and graphene — overwhelmingly illustrate their unique scientific and technological importance. arbon is the element in the periodic consist of extended networks of sp3- and 1985, with the advent of fullerenes (Fig. 1), table that provides the basis for life sp2 -hybridized carbon atoms, respectively. which were observed for the first time by Con Earth. It is also important for Both forms show unique physical properties Kroto et al.3. This serendipitous discovery many technological applications, ranging such as hardness, thermal conductivity, marked the beginning of an era of synthetic from drugs to synthetic materials. This lubrication behaviour or electrical carbon allotropes. Now, as we celebrate role is a consequence of carbon’s ability conductivity. Conceptually, many other buckminsterfullerene’s 25th birthday, it is to bind to itself and to nearly all elements ways to construct carbon allotropes are also the time to reflect on a growing family in almost limitless variety. The resulting possible by altering the periodic binding of synthetic carbon allotropes, which structural diversity of organic compounds motif in networks consisting of sp3-, sp2- includes the synthesis of carbon nanotubes and molecules is accompanied by a broad and sp-hybridized carbon atoms1,2. As a in 19914 and the rediscovery of graphene range of chemical and physical properties. consequence of the expected remarkable in 20045. Keeping in mind the numerous The tools of modern synthetic chemistry physical properties of these elusive carbon possible carbon modifications and the allow the tailored design of these properties allotropes, it has been appealing, for number of scientists investigating this by the controlled combination of structural some time, to develop concepts for their challenge, these revelations have certainly and functional building blocks in new preparation in macroscopic quantities. not come to an end. target systems. However, diamond and graphite represented Elemental carbon exists in two natural the only known allotropes of carbon for Synthesis of allotropes allotropes, diamond and graphite, which a long time. This situation changed in The fullerenes, of which the smallest stable and most prominent is C60, are molecular carbon allotropes. C60 consists of a spherical network of sixty structurally equivalent sp2-hybridized carbon atoms in the shape of a football (or, a soccer ball or, more technically, an Ih-symmetrical truncated icosahedron) composed of 12 pentagons Many undiscovered and 20 hexagons. Following the isolation allotropes for example sp-sp2-graphyne and structural determination of C60 by 20?? Kroto et al., the first successful preparation of 0-dimensional fullerenes in macroscopic Graphene 2004 quantities was reported five years later by Krätschmer and Huffman6. This involved the evaporation and recondensation of graphite using an arc-discharge method to Carbon nanotubes give C60. Now, fullerenes can be prepared on 1991 the ton scale. Many representatives of higher 7 fullerenes such as D5h-C70, chiral D2-C76, Fullerenes D6h-C84 and more recently D5h-C90 (ref. 8) 1985 have been isolated and structurally characterized. Furthermore, there are several Figure 1 | The world of synthetic carbon allotropes. In this family, fullerenes represent the most intensively possible ways of derivatizing fullerene investigated class. Fullerene chemistry is a mature field and many well-defined derivatives with including creating fullerene salts, open-cage outstanding properties have been synthesized. The first fullerene-based products such as organic solar fullerenes and quasi-fullerenes (Fig. 2). cells have already entered the market. The materials properties of the carbon nanotubes and especially The latter are spherical, closed structures those of graphene are considered to be even more promising. However, it is still difficult to control their composed of pentagons and hexagons chemistry and also the bulk production of uniform monodisperse samples, for example, mass production combined with rings of other sizes. Another of tubes with single helicities remains a challenge. Graphene chemistry is in its early infancy. Thinking family of fullerenes, namely endohedral to the future, there are a huge number of elusive carbon allotropes whose predicted properties are fullerenes (Fig. 2f), incorporates guest atoms unprecedented. Synthetic chemists are developing concepts at present for their preparation and have or molecules inside the carbon framework. already synthesized partial structures. Examples of endohedral fullerenes include 868 NATURE MATERIALS | VOL 9 | NOVEMBER 2010 | www.nature.com/naturematerials © 2010 Macmillan Publishers Limited. All rights reserved nmat_Commentary_NOV10.indd 868 12/10/10 14:14:35 commentary La@C and Sc N@C and, most recently, Over the past couple of decades, 82 3 80 a + the first endohedral fullerene containing –– thousands of well-characterized adducts an I -symmetrical C cage, Li@C , was of C and other fullerenes have been h 60 60 a+ + 60 structurally characterized9. Exohederal synthesized and show remarkable + + fullerenes — those that have been modified – – properties. For example, the exohedral externally — are discussed in further R R covalent binding of organic donor f + b detail below. molecules such as porphyrins has attracted Soon after the discovery of the fullerenes, a lot of attention7, to simulate natural the formation of quasi-one-dimensional photosynthesis or to transform light carbon nanotubes out of evaporated graphite into chemical energy. Donor–acceptor 4 was reported . Later, a series of alternative C60 R R dyads or oligoads such as compound 1 production methods for carbon nanotubes, e c (Fig. 3) undergo photoinduced electron such as chemical vapour deposition N N R R transfer from the porphyrin donor methods, were developed10. Depending on to the fullerene acceptor and serve as the preparation conditions, either single- d ideal model compounds for organic walled or multiwalled carbon nanotubes solar cells. Indeed, no other compound are obtained. Carbon nanotubes can be class surpasses fullerenes as electron considered as seamless, rolled-up graphite acceptors in photovoltaic devices11. sheets. These tubular networks of bent Adduct 2, namely phenyl-C61-butyric 2 sp -hybridized atoms are characterized by a Figure 2 | Possible derivatizations of C60 acid methyl ester (PCBM), serves as an pronounced one-dimensionality because of (ref. 6): a, fullerene salts; b, exohedral adducts; n-type conductor in very inexpensive a very high aspect ratio: typical diameters of c, open-cage fullerenes; d, quasi-fullerenes; and comparatively efficient plastic solar 11 single-walled carbon nanotubes are around e, heterofullerenes; f, endohedral fullerenes. cells (bulk heterojunctions) that have one to two nanometres, but their lengths can already entered the market. Water-soluble easily reach millimetres and above. Present fullerenes, such as adduct 3 and 4, exhibit production methods of carbon nanotubes sheer manifold of aesthetically pleasing remarkable biological properties with lead to the formation of mixtures consisting structures but their outstanding, and often regard to anti-HIV activity12 and metal-free of tubes with many different helicities, unprecedented, properties. Common superoxide dismutation, respectively. The characterized by so-called m,n-values of the to fullerenes, carbon nanotubes and latter is a principal mechanism for efficient roll-up vectors10. graphene is the presence of fully conjugated neuroprotection13. Further applications π-electrons confined in either zero-, quasi- of fullerene adducts in materials science Synthetic carbon allotropes one- or two-dimensions. This characteristic include the fields of non-linear optical conjugation leads to pronounced redox properties14 and liquid crystals15. represent a growing activity and outstanding electronic Other types of fullerene derivative show family of fascinating and properties. However, what about the unprecedented properties: for example, chemical properties of the synthetic carbon salts, such as K3C60 or (TDAE)C60 are aesthetically pleasing allotropes and their (and derivatives’) superconductors16 and ferromagnets17, architectures with outstanding application as high-performance respectively. Cluster-opened fullerenes materials? Let us start by taking a look at have been prepared that can be filled with materials properties. C60: immediately after its availability in small molecules, such as hydrogen, and macroscopic quantities, it was realized subsequently reclosed to form H2@C60 The youngest synthetic carbon allotrope that chemical functionalization was the (ref. 18). This opens the door to a whole is two-dimensional graphene, representing key to full exploitation of its vast potential. 5 a single graphite sheet . Graphene, the For example, chemical functionalization ab ultimate example of expanded aromatic enables a dramatic increase in the solubility N Zn N carbon, was considered for a very long time of fullerenes, in any solvent, which is N N O O O to be an exclusively theoretical material. required for their development as new O O Recently, however, single graphene layers materials or as bioactive redox drugs. O O O were prepared successfully by means of a Most importantly, however, chemical O O simple