The era of carbon allotropes

commentaryabFigure 4 | Some examples of elusive synthetic carbon allotropes: a, one-dimensional sp-carbyne;b, two-dimensional sp-sp 2 -graphyne; c, three-dimensional sp-sp 3 -yne-diamond.world of new endohedral derivatives.The substitution of carbon atoms withheteroatoms such as nitrogen, realized inthe heterofullerene (C 59 N) 2 (Fig. 2e) allowsfor further variation of the electronicproperties of the fullerene core 19 .nanotubes and grapheneThe electronic and materials propertiesof carbon nanotubes and graphene are atleast as remarkable as those of fullerenes.Depending on their helicity, carbonnanotubes are either semiconducting ormetallic. Both properties are appealingfor applications in the field of molecularelectronics or for the refinement ofmaterials, such as antistatic paints andshieldings. Indeed, metallic carbonnanotubes and graphene are the firstrepresentatives of stable organic metalswhere no further activation, dopingor charge transfer is required for theestablishment of extremely high chargecarriermobilities. A fascinating recentexample for the performance of carbonnanotubes is their ability to act as singleelectronpumps 20 . This could be a key forthe development single-electron transistors;devices that can confine charges downto single-electron level and hence, areapplicable for quantized current generation.Carbon nanotubes hold the record forthe mechanically strongest material,making them useful for the reinforcementof polymers in composite materials. Forexample, coating cotton thread withconductive carbon nanotubes leads to theformation of a lightweight thread that canbe easily woven into fabrics 21 .Compared with fullerenes, chemicalfunctionalization of carbon nanotubes iscconsiderably less mature and graphenechemistry is still in its earliest infancy.Nevertheless, graphene-based technologyis at present considered to have thegreatest potential for applications. In thisregard, Nobel Laureate Frank Wilczekfrom the Nobel Symposium on Grapheneheld in May 2010 said: “Graphene isprobably the only system where ideasfrom quantum field theory can leadto potential applications.” The idea ofusing graphene for digital electronicswas born in 2004 with the realization ofa graphene-based field effect transistor 5driven mainly by three extraordinaryproperties: extremely high charge-carriermobilities at room temperature; anenormous current-carrying capability; andthe fact that graphene is the ultimate thinmaterial. Already it has been shown thatgraphene meets the electrical and opticalrequirements for transparent conductiveelectrodes used in touch screens anddisplays 22 . However, in contrast to thematerial used at the moment, indiumtin oxide, the graphene electrodes areflexible and mechanically more robust. Toproceed with these developments and togain access to mass production, chemicalfunctionalization of graphite/graphenecould be a powerful tool to solve a numberof problems, such as increased solubilityand processability, stabilization in solution,separation and purification, as well asdevice fabrication.elusive allotropesDuring the investigation of sp 2 -hybridizedallotropes, attention has also beenpaid towards the development offurther synthetic carbon allotropes 23 .The extension of the constructionprinciple by incorporating both sp- andsp 3 -hybridized atoms into carbon networksoffers almost limitless structuralpossibilities. The simplest example forsuch synthetic carbon allotropes isone-dimensional carbyne consisting ofinfinite chains of sp-hybridized carbonatoms (Fig. 4a). Whereas the parentallotrope itself is still elusive, a number ofmodel compounds, namely end-cappedpolyynes, have been synthesizedand characterized.The combination of sp-, sp 2 - andsp 3 -hybridized carbon atoms leads tomany possible two- and three-dimensionalcarbon allotropes, such as graphyne 23(Fig. 4b). Significantly large molecularsegments of graphyne (pictured in redin Fig. 4b) have been synthesized andtheir study suggests the material hasattractive optoelectronic and self-assemblyproperties. Allotropes with a skeletonof sp- and sp 3 -hybridized carbon are theleast-studied class of hybrid allotropes, andthe only notable example reported thus faris expanded cubane 23 . The study of thesematerials is motivated by the prediction ofimpressive mechanical properties, such ashardness for allotropes like yne-diamond,which is composed of the hypotheticalbuilding block adamantyne (picturedin red in Fig. 4c). Although a varietyof low-molecular partial structures ofthese networks have been synthesized,preparative access to these designerallotropes is still elusive.ConclusionsSynthetic carbon allotropes representa growing family of fascinating andaesthetically pleasing architectures withoutstanding materials properties. Theexisting representatives, namely fullerenes,carbon nanotubes and graphene, willbe complemented in the future bynew carbon forms whose constructionprinciples are based on the numerouspossibilities to assemble carbon atomsin one, two and three dimensions. Ata time when silicon-based technologyis approaching its fundamental limits,the possible incorporation of syntheticcarbon allotropes in the production ofelectronic devices is welcome. As a resultof their spectacular electronic, thermaland mechanical properties they areexpected to offer an exceptional choice.However, fully realizing their application innanoelectronics, sensors, nanocomposites,batteries and supercapacitors still requiresmuch fundamental research, keepingchemists, physicists, materials scientistsand engineers busy for years to come. ❐870 nature materials | VOL 9 | NOVEMBER 2010 | www.nature.com/naturematerials© 2010Macmillan Publishers Limited. All rights reserved

commentaryAndreas Hirsch is in the Department of Chemistryand Pharmacy & Interdisciplinary Center ofMolecular Materials (ICMM), Friedrich-Alexander-University Erlangen-Nuremberg, Henkestrasse 42,91054 Erlangen, Germany.e-mail: andreas.hirsch@chemie.uni-erlangen.deReferences1. Karfunkel, H. R. & Dressler, T. J. Am. Chem. Soc.114, 2285–2288 (1992).2. Diederich, F. & Rubin, Y. Angew. Chem. Int. Ed.31, 1101–1123 (1992).3. Kroto, H. W. et al. Nature 318, 162–163 (1985).4. Iijima, S. Nature 354, 56–58 (1991).5. Novoselov, K. & Geim, A. et al. Science 306, 666–669 (2004).6. Krätschmer, W. et al. Nature 347, 354–358 (1990).7. Hirsch, A. & Brettreich, M. Fullerenes – Chemistry and Reactions(Wiley, 2004).8. Yang, H. et al. Angew. Chem. Int. Ed. 49, 886 (2010).9. Aoyagi, S. et al. Nature Chem. 2, 678–683 (2010).10. Hirsch, A. & Vostrowsky, O. Top. Curr. Chem.245, 193–237 (2005).11. Dennler, G. et al. Adv. Mater. 21, 1323–1338 (2009).12. Friedman, S. H. et al. J. Am. Chem. Soc. 115 6506–6509 (1993).13. Liu, G-F. et al. Angew. Chem. Int. Ed. 47, 3991–3994 (2008).14. Koudoumas, E. et al. Chem. Eur. J. 9, 1529 (2003).15. Campidelli, S. et al. Chem. Commun. 4282–4284 (2006).16. Holczer, K. et al. Science 252, 1154–1157 (1991).17. Allemand, P. M. et al. Science 253, 301–302 (1991).18. Komatsu, K. et al. Science 307, 238–240 (2005).19. Vostrowsky, O. & Hirsch, A. Chem. Rev. 106, 5191–5207 (2006).20. Siegle, V. et al. Nano Lett. doi:10.1021/nl101023u (2010).21. Shim, B. S. et al. Nano Lett. 8, 4151–4157 (2008).22. Bae, S. et al. Nature Nanotech. 5, 574–578 (2010).23. Diederich, F. & Kivala, M. Adv. Mat. 22, 803–812 (2010).AcknowledgementsA.H. thanks GRAPENOCHEM and the Cluster ofExcellence EAM for financial support.Green carbon as a bridge torenewable energyJames M. Tour, Carter Kittrell and Vicki L. ColvinA green use of carbon-based resources that minimizes the environmental impact of carbon fuels couldallow a smooth transition from fossil fuels to a sustainable energy economy.Carbon-based resources (coal, naturalgas and oil) give us most of the world’senergy today, but the energy economyof the future must necessarily be far morediverse. Energy generation through solar,wind and geothermal means is developingnow, but not fast enough to meet ourexpanding global energy needs. We advocatethat ‘green carbon’, which enables us to usecarbon-based sources with high efficiencyand in an environmentally friendly manner,will provide our society time to developalternative energy technologies and marketswithout sacrificing environmental oreconomic quality. Green carbon will helpto reduce the loss of our precious carbonresources, which are better reserved forhigh-value chemicals, and it will ensurethat those hydrocarbons used for fuelswill minimize carbon emissions. Throughintensive research and development ingreen carbon, our society can guarantee anenergy future that uses carbon strategically,without smokestacks, greenhouse gases andextensive environmental damage.Building a solid bridgeThere is a chasm between the diminutiveproportions of renewable energy currentlyavailable and our overwhelming dependenceon fossil fuels that currently propel society.The energy policy review of the Obamaadministration makes this soberinglyclear: “The use of renewable energy todayand even in the next 5 to 10 years is stillextremely limited when put into thecontext of total world use of fossil fuels.For example, the world used the equivalentof 113,900 terawatts hours [TWh] of fossilenergy to fuel economic activity, humanmobility, and global telecommunications,among other modern day activities in2007. Replacing those terawatts hours withnon-fossil energy would be the equivalent ofconstructing an extra 6,020 nuclear plantsacross the globe or 14 times the number ofnuclear power plants in the world today.In renewable energy terms, it is 133 timesthe amount of solar, wind and geothermalenergy currently in use on the planet.” 1Barring a huge reduction in our globalstandard of living, we will need to relyon carbon-based energy for some time.Whether this will last for several decades orinto the next century is unclear, but whatis apparent is that renewable approachesto energy generation are increasing at anannual rate of 7.2% compared with 1.6% fornon-renewable growth 2 , and the continuedgrowth of renewables will demand sustainedgovernment support. During this transitionwe propose a green carbon bridge thatminimizes the environmental impact ofcarbon fuels and lowers our reliance on theseresources for primary energy generation.Ultimately, green carbon will use hydrogenfrom renewable sources, while at the sametime producing basic chemical feedstocks.The concept of a bridge to a renewableenergy future is not new, and many strategieshave been offered as near-term alternativesto conventional carbon resources 3 . Althoughsome of these concepts appear at first glanceto be ‘green’ solutions, further scrutinythat takes into account the energy lifecycleand environmental consequences suggestsa more murky set of trade-offs. Biodiesel,for example, has led to destruction of vastareas of tropical rainforest to make palmoil 4 . A shift of transportation to lithiumbattery-powered cars requires enormousamounts of fossil fuels to mine vast stretchesof the Earth in search of lithium, andthe vehicle would need to be driven tensof thousands of kilometres entirely onrenewable energy just to accrue back the~1.7 GJ energy per kilowatt hour capacityused for the lithium mining and batterymanufacture 5 . The electric grid promisedto serve the purportedly clean electric-cartechnology may just become a long tailpipeconnecting the plug-in batteries to coalfiredpower plants 6 . And even if renewableenergies could supply the electricity, thecurrent electrical grids could not sustainthe vast increase in demand. We need toquantify the full environmental or lifecycleconsequences of various options for ourenergy bridge if we are to build it from theproper technologies.We must also consider energytechnology options in a broader context. Innature materials | VOL 9 | NOVEMBER 2010 | www.nature.com/naturematerials 871© 2010Macmillan Publishers Limited. All rights reserved