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to letters nature Acknowledgements We would like to thank O. Avenel, N. Bruckner, D. Goodstein, W. Holmes, K. Schwab, E. Varoquaux and P. Welander for discussions and support. This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Y.M. was funded by CEA-Saclay, France. Correspondence and requests for materials should be addressed to Y.M. (e-mail: muh@drecam.saclay.cea.fr). ................................................................. Conversion of silicon carbide to crystalline diamond-structured carbon at ambient pressure Yury Gogotsi* , Sascha Welz , Daniel A. Ersoy & Michael J. McNallan * Drexel University, Department of Materials Engineering, Philadelphia, Pennsylvania 19104, USA University of Illinois at Chicago, Department of Civil and Material Engineering, 842 West Taylor Street, Chicago, Illinois 60607, USA .............................................................................................................................................. Synthetic diamond is formed commercially using high-pressure1, chemical-vapour-deposition2 and shock-wave3 processes, but these approaches have serious limitations owing to low production volumes and high costs. Recently suggested alternative methods of diamond growth include plasma activation4, high pressures5, exotic precursors6,7 or explosive mixtures8, but they suffer from very low yield and are intrinsically limited to small volumes or thin lms. Here we report the synthesis of nano- and micro-crystalline diamond-structured carbon, with cubic and hexagonal structure, by extracting silicon from silicon carbide in chlorine-containing gases at ambient pressure and temperatures not exceeding 1,000 8C. The presence of hydrogen in the gas mixture leads to a stable conversion of silicon carbide to diamondstructured carbon with an average crystallite size ranging from 5 to 10 nanometres. The linear reaction kinetics allows transformation to any depth, so that the whole silicon carbide sample can be converted to carbon. Nanocrystalline coatings of diamondstructured carbon produced by this route show promising mechanical properties, with hardness values in excess of 50 GPa and Young's moduli up to 800 GPa. Our approach should be applicable to large-scale production of crystalline diamond-structured carbon. We have previously shown that selective etching of carbides is an attractive technique for synthesis of carbon coatings9. Supercritical water10 or halogens11 can be used to remove silicon from silicon carbide (SiC), producing carbide-derived carbon (CDC) lms and powders that may have a variety of structures depending on the experimental conditions12. Hydrothermal etching of SiC produced sp3-bonded carbon and some diamond10,13, but the process was plagued by a low yield and poor reproducibility, because of the dif culty in controlling the concentrations/activities of species in high-pressure autoclaves. Our synthesis of CDC was conducted at ambient pressure in a quartz tube furnace at or below 1,000 8C, as described in the Methods section. The SiC samples were exposed to owing gas mixtures of 1 3.5% Cl2, 0 2% H2 and balance Ar, which was used as a carrier gas. Because SiCl4 is much more thermodynamically stable than CCl4, chlorine reacts selectively with the silicon at SiC surfaces by the reaction produced by treatment of sintered SiC in Ar/3.5% Cl2 were black and graphitic, according to X-ray diffraction (XRD) and Raman spectroscopy. They showed a low hardness and Young's modulus (Fig. 1). However, cross-sectional hardness measurements using a nano-indenter showed the existence of an intermediate layer, several micrometres thick, between SiC and graphitic carbon. The material in this layer had an average hardness of about 20 30 GPa and Young's moduli of 200 300 GPa. Transmission electron microscopy (TEM) studies of this layer (Fig. 2) showed that it consisted of a mixture of graphitic carbon (as `onions', ribbons and disordered carbon) and nanocrystalline diamond-structured carbon (Fig. 2a). In some regions, large nanocrystalline areas producing characteristic fringing (Fig. 2b) or containing microcrystals of diamond-structured carbon (Fig. 3) were found. Lattice fringing, selected-area electron diffraction (SAED), convergent-beam electron diffraction (CBED) and electron energy-loss spectroscopy (EELS) techniques con rmed the formation of diamond-structured carbon in this layer. However, lattice fringes and diffraction spots at ,0.193 nm and ,0.218 nm (Table 1), as well as characteristic CBED images (see Supplementary Information), suggest formation of 2H hexagonal diamond (lonsdaleite) along with cubic diamond-structured carbon. Lonsdaleite has often been observed in nanocrystalline diamond lms14,15 and accompanies SiC in some natural samples16. Addition of hydrogen to the gas to achieve a chlorine/hydrogen ratio of 2:0.575 to 2:1 resulted in changes in the appearance and structure of the carbon coatings. The layers produced at high hydrogen contents were grey, translucent in thin sections, and a 10.0 8.0 Load (mN) 6.0 SiC 4.0 2.0 0.0 0 50 100 150 200 Displacement (nm) SiC H = 51 GPa E = 544 GPa H = 1.8 GPa E = 18 GPa 25 m Treated in Cl2/H2 H = 50 GPa E = 605 GPa Carbon Treated in Cl2 250 b 100 Nanocrystalline CVD diamond SiC DLC (high sp3 content) Single crystal Si Hardness (GPa) 10 Glassy carbon CDC Cl2 treatment only CDC Cl2/H2 treatment DLC (hydrogenated, as-deposited) DLC (hydrogenated, annealed) 100 Modulus (GPa) 1,000 1 Pyrolytic carbon 0.1 1 10 SiC 2Cl2 SiCl4 C 11 1 thereby leaving carbon on the SiC substrate . CDC coatings NATURE | VOL 411 | 17 MAY 2001 | www.nature.com Figure 1 Results of nano-indentation tests. a, Typical indentation load displacement curves obtained on polished surfaces of sintered SiC (blue curve), as well as carbon coatings produced in Ar/3.5% Cl2 (black curve) and in Ar/2.77% Cl2/1.04% H2 (red curve). Inset, SEM micrograph of the hard coating produced by treatment in Ar/2.77% Cl2/1.04% H2 for 30 h at 1,000 8C. H, hardness; E, Young's modulus measured on the polished surface using depth-sensing indentation. b, Hardness versus Young's modulus for various carbon materials, silicon and SiC. Bars show standard deviations. DLC, diamond-like carbon. 283 2001 Macmillan Magazines Ltd letters to nature fracture surfaces showed a continuous fracture pattern from the coating into SiC (Fig. 4a), suggesting that the mechanical properties of these coatings are close to that of SiC. These coatings were grown to a thickness of 50 mm (Fig. 1). CDC coatings produced with a chlorine to hydrogen ratio of about 2:1 had hardness in excess of 50 GPa and a Young's modulus of ,600 800 GPa (Fig. 1). These values exceed that of diamond-like carbon, but are below that of single-crystal or chemical vapour deposition (CVD) diamond, when measured using the same instrument (Fig. 1b). TEM showed that these coatings were built of nanocrystals, with an average size of 5 10 nm (Fig. 4b). In the SAED pattern from this lm (see Supplementary Information), sharp Bragg re ections are visible up to the order of (800), indicating good crystallinity. No scattering intensity from either graphite or amorphous carbon can be seen. However, kinematically forbidden diamond re ections (200), (222) and (420) were consistently observed during this study (Table 1, Fig. 3d). These are common for Si and diamond crystals, and may appear because the allowed {111} beam acts like a new incident beam and can be rediffracted by (111) planes exciting weak (200) and (222) re ections. However, the intensity of these re ections is expected to be very low for the Fd3m cubic diamond structure. Incorporation of impurity atoms in diamond and the formation of an ordered superstructure could also cause the above-mentioned re ections. Traces of oxygen, silicon and chlorine were observed in energy-dispersive spectroscopy (EDS), but it is dif cult to say if any of these elements were dissolved in the diamond lattice. Even relatively large microcrystals of diamond-structured carbon (Fig. 3a), which produced EELS spectra typical of diamond (Fig. 3c), showed unusually strong forbidden re ections (200) and (222) in SAED patterns (Fig. 3d). They may suggest formation of diamond polytypes14 similar to that of SiC. For example, if the symmetry is lowered from Fd3m to F43m (the same as in b-SiC), a high intensity of the (200) and (222) re ections is expected17. Detailed analysis of the structure of CDC will be published elsewhere. Thus, treatment of SiC in chlorine or chlorine-hydrogen mixtures at Cl2/H2 ratios equal or larger than 2:1 at 1,000 8C results in the conversion of the silicon carbide to crystalline diamond-structured carbon. An increase in the hydrogen content of the gas, resulting in a 1:1 ratio and leading to the formation of HCl, resulted in very thin lms or no coating at all. This may be due to a lower thermodynamic probability of the reaction SiC 4HCl SiCl4 C 2H2 DG8 2 25 kJ mol 2 1 2 Table 1 Comparison of d spacings Experimental* d spacing* (nm) Cubic diamond Fd3m (JCPDS 6-0675) hkl d spacing (nm) 0.206 0.178 0.126 0.1075 Lonsdaleite P63/mmc (JCPDS 19-0268) hkl 100 002 101 d spacing (nm) 0.218 0.206 0.193 ............................................................................................................................................................................. 0.218-0.222 0.206 0.207 0.192 0.194 0.178 0.182 0.150 0.126 0.117 0.110 0.106 111 200 220 311 ............................................................................................................................................................................. * Typical values from SAED, CBED and lattice fringe measurements on about 30 diamond single crystals (5 500 nm in size) from three different samples. Forbidden cubic diamond re ection, typical for n-diamond32. 102 110 103 020 112 0.150 0.126 0.116 0.1092 0.1075 compared to reaction (1), which has DG8 = -434.1 kJ mol-1 (ref. 11) where DG8 is Gibbs free energy change. In Ar-Cl2 and Ar-Cl2/H2 environments, the thickness of the carbon layer increases linearly with time, following the equation d = klt, where d is the layer thickness, t is time, and kl is the linear rate constant. For the Ar/2.77% Cl2/1.04% H2 environment, kl = 1.6 mm h-1, which is only about 20% lower than for Ar/3.5% Cl2. Because the kinetics are linear, the controlling factor of the reaction is not the diffusion of reactant species through the growing carbon layer. If this were the case, we would expect a parabolic rate equation18. In order for the chlorination reaction to proceed, two molecules of Cl2(g) or four molecules of HCl(g) must be transported to the SiC/C interface, and one molecule of SiCl4(g) as well as two molecules of H2(g) must be transported away from the interface for each atom of carbon produced. Linear kinetics implies that the carbon lm is nanoporous and allows easy permeation of Cl2, HCl, H2 and SiCl4 molecules, in spite of its dense appearance in scanning electron microscopy (SEM; Fig. 4a) at magni cations up to 500,000. This nanoporosity may be responsible for the hardness values being lower than that of CVD diamond (Fig. 1b). An important implication of the linear layer-growth kinetics is the possibility of growing very thick coatings on SiC, or the complete conversion of SiC powders or components into diamondstructured carbon. Recent studies have shown that it is not dif cult to form nanometre-sized diamonds19. Moreover, several groups have reported the nucleation of sp3-bonded carbon and nanocrystalline diamond after surface treatment of SiC by uorocarbon plasma20 a Graphite (3.34 ) b 1 nm Figure 2 High-resolution TEM micrographs showing the structure of the carbon coating within a micrometre of the SiC/carbon interface. a, Nanocrystals of diamond-structured carbon (indicated by black arrows) surrounded by graphitic carbon (shown by white 284 1 nm arrows), including onion-like structures. b, Region of nanocrystalline diamond-structured carbon. The sample was treated for 24 h at 1,000 8C in Ar/3.5% Cl2. 2001 Macmillan Magazines Ltd NATURE | VOL 411 | 17 MAY 2001 | www.nature.com letters to nature a Amorphous carbon Crystalline diamondstructured carbon b 100 nm 200 nm c 10,000 8,000 Counts 6,000 4,000 a-C 2,000 260 280 300 320 340 360 * * Crystalline diamondstructured carbon d 222 311 400 311 200 111 022 0 111 200 111 022 111 Energy loss (eV) Figure 3 Characterization of crystalline diamond-structured carbon embedded in amorphous carbon. a, TEM image. b, SEM image. c, EELS spectra from the particle and surrounding carbon material in a. d, [011] SAED pattern from the particle imaged in a. SAED and the EELS spectra show that the rounded particles are diamond-structured carbon. The light material in a was identi ed as carbon with a high content of sp3 bonding and some sp2 bonding using EELS (c). Etching in hydrogen plasma selectively removes amorphous and disordered carbon, revealing crystals of diamond-structured carbon in the carbon layer (b). The rounded shape of the microcrystals can be explained by their solid-state growth via coalescence of nanocrystals when constrained by the carbon matrix. and bombardment with hydrogen21 or carbon ions22. Thus, there is consistent evidence of conversion of carbides into diamond, after removal of metal atoms from the carbide lattice under various experimental conditions. The growth of diamond-structured carbon from SiC in pure chlorine with no hydrogen added (Figs 2 and 3) is also in agreement with the synthesis of diamond from fullerene6,19, which showed that hydrogen is not essential for diamond growth outside its range of thermodynamic stability. According to the original concept for the metastable growth of diamond suggested by Spitsyn23, it is necessary to conserve the orientational effect of the surface carbon atoms and to use carboncontaining molecules with sp3 bonding that can be attached to the diamond surface in a complementary manner. Both conditions can be satis ed when Si is extracted from SiC, forming carbon atoms in sp3 hybridization. We note that the rst diamond growth experiments of Derjagin and Spitsyn23 were conducted in the carbon halogen system, using CBr4 and CI4. It can be assumed that the tetrahedrally coordinated SiC lattice, which is preserved during chlorination, acts as a template for the growth of diamond10,11 and that diamond-structured carbon grows by direct transformation of the SiC lattice because of the sp3 bonding of carbon in SiC. The work we report here was performed using a-SiC; but we note that b-SiC has a similar structure, namely a diamond lattice where 50% of carbon atoms are replaced with Si, and that this polytype could also be converted to crystalline diamond-structured carbon (results available, but not shown here). We have therefore shown that both SiC polytypes can be converted to crystalline diamondstructured carbon, although the question remains as to whether the carbide lattice structure (cubic or hexagonal) affects the structure of the resulting CDC. Molecular-dynamics simulations using empirical interatomic Tersoff potentials have shown that for a Siterminated (1000) 6H-SiC surface, very high lattice strains do not allow direct growth of diamond on SiC, and fragmentation leading to nanocrystalline material must occur24. Small diamond clusters on SiC are predicted to have a good adhesion to the substrate and maintain sp3 coordination of carbon atoms in the cluster24. Our TEM study of the chlorine-treated SiC suggested that SiC was converted to amorphous sp3 carbon, and formation of crystalline diamond-structured carbon occurred from disordered, Si-depleted SiC within nanometres of the SiC/carbon interface. Random orientation of nanocrystals of diamond-structured carbon (Fig. 2) in CDC supports non-epitaxial growth of diamond-structured carbon. Thus, growth of nanocrystalline diamond-structured carbon occurred from highly disordered sp3 carbon produced by selective etching of SiC. Growth of larger crystals of diamondstructured carbon (Fig. 3a, b) was probably the result of coalescence of continuous nanocrystalline regions (Fig. 2b). However, if no hydrogen was added to the gas, nanocrystalline diamond-structured carbon was slowly transformed to the thermodynamically stable (under ambient pressure) graphitic carbon during the long-term treatment at 1,000 8C, and we observed only amorphous and graphitic carbon at a distance of more than 3 mm from the SiC/ carbon interface. Thus, the role of hydrogen is primarily in stabilization of dangling bonds of carbon. This helps to maintain sp3 hybridization of carbon and prevent formation of sp2-bonded carbon. Therefore, addition of hydrogen stabilized the diamondstructured phase carbon and allowed the continuous growth of a lm of this phase on the surface. Theoretical aspects of hydrogenassisted diamond growth at low pressures23 and the thermodynamic a b SiC Carbon 1 m 5 nm Figure 4 Microstructure of a carbon lm produced in Cl2/H2. a, SEM micrograph of a fracture surface showing a carbon layer over the SiC substrate. b, Typical TEM micrograph of the nanocrystalline layer of diamond-structured carbon produced with NATURE | VOL 411 | 17 MAY 2001 | www.nature.com hydrogen present in the reaction gas. Sample was sintered a-SiC treated in Ar/2.77% Cl2/ 1.04% H2 for 30 h at 1,000 8C. Arrows in a show a continuous fracture pattern from the coating into SiC. 285 2001 Macmillan Magazines Ltd letters to nature coupling model that can be used to explain the formation of diamond in reactions (1) and (2) have been well developed25, and will not be discussed here. The transformation of SiC to crystalline diamond-structured carbon that we observed can explain diamond-SiC paragenesis26, and probably the formation of carbonado diamonds in nature27. It also shows that the occurrence of diamond/lonsdaleite in association with SiC can be caused by reasons other than hypervelocity impact on Earth16. The manufacture of CDC has been developed and scaled up during the past few years12,28, and a variety of useful products ranging from nanoporous carbons29 for supercapacitors and batteries, to nanotubes, onion-like carbon30, and tribological coatings31 have been reported. Some of these have been commercialized or are on their way to commercialization. Facilities for halogenation of carbides, which are able to produce tons of carbon per day, have been created (for example, see http://www.skeletontechnologies.com). Only minor modi cation of the process (hydrogen addition) is required for synthesis of crystalline diamondstructured carbon using the same equipment. The process that we have developed is versatile because it allows synthesis of powders or coatings of virtually any thickness. As the transformation is conformal and does not change the shape of the particle, powders with any grain size could be produced by using raw SiC powders of different particle sizes. However, the product will be grains built of nanocrystals of diamond-structured carbon. The presence of micrometre-size crystals in our samples (Fig. 3a) shows that coalescence of nanocrystals of diamond-structured carbon may occur under appropriate conditions, and lead to growth of microcrystalline diamond-structured carbon. The mechanical, electrical and optical properties of nanocrystalline diamond-structured carbon are altered from those of single diamond crystals, because the grain-boundary carbon is p-bonded, as shown by the Raman spectra and EELS. The fraction of atoms residing at grain boundaries can be up to 10% when the average crystallite size is 3 5 nm (ref. 19). Nanocrystalline diamond lms have found applications in tribology: for example, to coat SiC dynamic seals for water pumps19. Graphitic carbon in CDC lms can act as a solid lubricant31. Remarkable electron emission with turn-on elds of ,1 V mm-1 and a high current density has been achieved using thin lms of nanocrystalline diamond19. Nanocrystalline diamond has a higher conductivity than boron-doped microcrystalline diamond, and can be used for electrodes in chemically aggressive environments19. Conformal coatings produced by selective etching11 would be useful in microelectromechanical system (MEMS) applications where very thin and uniform coatings are required. In addition, permeability of the lms produced by chlorination of SiC and an extremely narrow pore-size distribution in CDC29 may enable the production of molecular sieves, high-surface-area electrodes and other applications, where vapour-deposited diamond lms cannot be applied. The large-scale, solid-state synthesis that we report here, being relatively low-cost, and operating at ambient pressure and moderate temperatures with no plasma activation, may open the way for diamond-structured materials to be used in a variety of highvolume applications such as brake pads. M obtained in experiments conducted at 1,000 8C. At the end of each experimental run, an argon purge was initiated through the reaction chamber during the cool-down period. Analysis The reacted specimens were analysed using optical microscopy and SEM, EDS, EELS, XRD and Raman spectroscopy (Ar-ion laser, 514.5-nm excitation wavelength). JEOL JEM-3010 (300 kV) and JEOL JEM-2010F (200 kV) TEMs, as well as a JEOL 6320 eldemission SEM, were used for this work. EDS was used to identify the carbon areas which were free from impurities and showed only traces of silicon and chlorine, and EELS was used to identify areas of diamond-structured carbon from the carbon edge shape. Subsequently, SAED was performed on ,1-mm nanocrystalline areas and microcrystals, and CBED was performed with a 5-nm and 10-nm electron probe. A Nano Indenter XP (MTS) equipped with a Berkovich indenter (diamond pyramid) was used to measure the hardness and Young's modulus of the coating under a load of 10 mN and a loading rate of 100 mN s-1. Received 5 December 2000; accepted 9 February 2001. 1. Bundy, F. P., Hall, H. T., Strong, H. M. & Wentorf, R. H. Man-made diamonds. Nature 176, 51 (1955). 2. Angus, J. C. & Hayman, C. C. Low-pressure, metastable growth of diamond and ``diamondlike'' phases. Science 241, 913 921 (1988). 3. Burkhard, G., Dan, K., Tanabe, Y., Sawaoka, A. B. & Yamada, K. Carbon phase transition by dynamic shock compression of a copper/graphite powder mixture. Jpn J. Appl. Phys. 33, L876 L879 (1994). 4. Roy, R. et al. New process for 1 atm diamond synthesis: from metallic solutions. Innov. Mater. Res. 1, 65 87 (1996). 5. Zhao, X.-Z., Roy, R., Cherian, K. A. & Badzian, A. Hydrothermal growth of diamond in metal-C-H2O systems. Nature 385, 513 515 (1997). 6. Gruen, D. M., Liu, S., Krauss, A. R. & Pan, X. Buckyball microwave plasmas: fragmentation and diamond- lm growth. J. Appl. Phys. 75, 1758 1763 (1994). 7. Regueiro, M. N., Monceau, P. & Hodeau, J.-L. Crushing C60 to diamond at room temperature. Nature 355, 237 239 (1992). 8. Li, Y. et al. A reduction-pyrolysis-catalysis synthesis of diamond. Science 281, 246 247 (1998). 9. Gogotsi, Y. G. & Yoshimura, M. Formation of carbon lms on carbides under hydrothermal conditions. Nature 367, 628 630 (1994). 10. Gogotsi, Y. G., Kofstad, P., Nickel, K. G. & Yoshimura, M. Formation of sp3-bonded carbon upon hydrothermal treatment of SiC. Diamond Relat. Mater. 5, 151 162 (1996). 11. Gogotsi, Y. G., Jeon, J. D. & McNallan, M. J. Carbon coatings on silicon carbide by reaction with chlorine-containing gases. J. Mater. Chem. 7, 1841 1848 (1997). 12. Gogotsi, Y. in Proc. NATO ARW on Nanostructured Films and Coatings (eds Chow, G.-M., Ovid'ko, I. A. & Tsakalakos, T.) 25 40 (Kluwer, Dordrecht, 1999). 13. Gogotsi, Y. G., Nickel, K. G. & Kofstad, P. Hydrothermal synthesis of diamond from diamond-seeded b-SiC powder. J. Mater. Chem. 5, 2313 2314 (1995). 14. Silva, S. R. P., Amaratunga, G. A. J., Salje, E. K. H. & Knowles, K. H. Evidence of hexagonal diamond in plasma-deposited carbon lms. J. Mater. Sci. 29, 4962 4963 (1994). 15. Zarrabian, M., Fourches-Coulon, N., Turban, G., Marhic, C. & Lancin, M. Observation of nanocrystalline diamond in diamondlike carbon lms deposited at room temperature in electron cyclotron resonance plasma. Appl. Phys. Lett. 70, 253 255 (1997). 16. Hough, R. M. et al. Diamond and silicon carbide in impact melt rock from the Ries impact crater. Nature 378, 41 44 (1995). 17. Rossi, M., Vitali, G., Terranova, M. L. & Sessa, V. Experimental evidence of different crystalline forms in chemical vapour deposited diamond lms. Appl. Phys. Lett. 63, 2765 2767 (1993). 18. Kofstad, P. High Temperature Corrosion (Elsevier, London, 1988). 19. Gruen, D. M. Nanocrystalline diamond lms. Annu. Rev. Mater. Sci. 29, 211 259 (1999). 20. Grannen, K. J. & Chang, R. P. H. Diamond growth on carbide surfaces using a selective etching technique. J. Mater. Res. 9, 2154 2163 (1994). 21. Lannon, J. M. J., Gold, J. S. & Stinespring, C. D. Hydrogen ion interactions with silicon carbide and the nucleation of diamond thin lms. J. Appl. Phys. 77, 3823 3830 (1995). 22. Heera, V., Skorupa, W., Pecz, B. & Dobos, L. Ion beam synthesis of graphite and diamond in silicon carbide. Appl. Phys. Lett. 76, 2847 2849 (2000). 23. Spitsyn, B. V. in Handbook of Crystal Growth (ed. Hurle, D. T. J.) 401 456 (Elsevier, London, 1994). 24. Gogotsi, Y. et al. in Proc. NATO ASI on Functional Gradient Materials and Surface Layers Prepared by Fine Particles Technology (eds Baraton, M.-I. & Uvarova, I. V.) 239 255 (Kluwer, Dordrecht, 2001). 25. Wang, J.-T., Cao, C.-B. & Zheng, P.-J. Theoretical aspects of low pressure diamond synthesis. J. Electrochem. Soc. 141, 278 281 (1994). 26. Leung, I., Guo, W., Friedman, I. & Gleason, J. Natural occurrence of silicon carbide in a diamondiferous kimberlite from Fuxian. Nature 346, 352 354 (1990). 27. Daulton, T. L. & Ozima, M. Radiation-induced diamond formation in uranium-rich carbonaceous materials. Science 271, 1260 1262 (1996). 28. Gogotsi, Y. et al. Formation of carbon coatings on SiC bres by selective etching in halogens and supercritical water. Ceram. Eng. Sci. Proc. 19, 87 94 (1998). 29. Kyutt, R. N., Smorgonskaya, E. A., Danishevskii, A. M., Gordeev, S. K. & Grechinskaya, A. V. Structural study of nanoporous carbon produced from polycrystalline carbide materials: small angle X-ray scattering. Phys. Solid State 41, 1359 1363 (1999). 30. Zheng, J., Ekstrom, T. C., Gordeev, S. K. & Jacob, M. Carbon with an onion-like structure obtained by chlorinating titanium carbide. J. Mater. Chem. 10, 1039 1041 (2000). 31. Ersoy, D. A., McNallan, M. J., Gogotsi, Y. & Erdemir, A. Tribological properties of carbon coatings produced by high temperature chlorination of silicon carbide. STLE Tribol. Trans. 43, 809 815 (2000). 32. Hirai, H. & Kondo, K.-I. Modi ed phases of diamond formed under shock compression and rapid quenching. Science 253, 772 774 (1991). Methods Synthesis The apparatus used to investigate the interactions of SiC with halogens at atmospheric pressure has been described elsewhere11. Experiments were performed using several commercially available b-SiC powders, sintered a-SiC and CVD b-SiC materials; however, the experiments described here are limited to sintered a-SiC. These samples were sectioned into disks 16 mm in diameter and 1 mm thick. The disks were cleaned ultrasonically, rinsed in acetone, and placed in a quartz sample holder. This was in turn suspended via a silica wire connected to a fused-silica rod in the centre of a fused-silica reaction tube in the hot zone of a furnace. Experiments were continued for between 30 min and 30 h over a broad temperature range (600 1,100 8C). The results presented here were Supplementary information is available on Nature's World-Wide Web site (http://www.nature.com) or as paper copy from the London editorial of ce of Nature. NATURE | VOL 411 | 17 MAY 2001 | www.nature.com 286 2001 Macmillan Magazines Ltd letters to nature Acknowledgements We thank A. Nicholls for discussions and help with TEM analysis. The work done at UIC was supported by the US NSF, and the work done at Drexel University was supported by DARPA via ONR contract. The electron microscopes used in this work are operated by the Research Resources Center at UIC. The JEM-2010F purchase was supported by the NSF. Correspondence and requests for materials should be addressed to Y.G. (e-mail: gogotsi@drexel.edu). ................................................................. A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles Gregory J. Retallack Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA .............................................................................................................................................. To understand better the link between atmospheric CO2 concentrations and climate over geological time, records of past CO2 are reconstructed from geochemical proxies1 4. Although these records have provided us with a broad picture of CO2 variation throughout the Phanerozoic eon (the past 544 Myr), inconsistencies and gaps remain that still need to be resolved. Here I present a continuous 300-Myr record of stomatal abundance from fossil leaves of four genera of plants that are closely related to the present-day Ginkgo tree. Using the known relationship between leaf stomatal abundance and growing season CO2 concentrations5,6, I reconstruct past atmospheric CO2 concentrations. For the past 300 Myr, only two intervals of low CO2 (,1,000 p.p.m.v.) are inferred, both of which coincide with known ice ages in Neogene (1 8 Myr) and early Permian (275 290 Myr) times. But for most of the Mesozoic era (65 250 Myr), CO2 levels were high (1,000 2,000 p.p.m.v.), with transient excursions to even higher CO2 (.2,000 p.p.m.v.) concentrations. These results are consistent with some reconstructions of past CO2 (refs 1, 2) and palaeotemperature records7, but suggest that CO2 reconstructions based on carbon isotope proxies3,4 may be com- promised by episodic outbursts of isotopically light methane8,9. These results support the role of water vapour, methane and CO2 in greenhouse climate warming over the past 300 Myr. As atmospheric CO2 levels have risen over the past 200 years of industrial fossil fuel consumption, plants have responded by decreasing the density of stomates on their leaves5,6. Here I use the well-known inverse relationship between atmospheric CO2 concentration and stomatal density as a palaeobarometer of atmospheric CO2 during growth of fossil plant leaves. Stomatal density of living plants has, however, been shown to be related to differences in insolation, water stress and stomatal position within leaves, which also affect cell size, and thus stomatal density. The effects of these competing variables are minimized by using stomatal index (percentage stomates over stomates plus epidermal cells) rather than stomatal density (stomates per unit area6). Further limits to such competing variables are suggested by the habitats of the fossil leaves studied: these were mostly humid, lowland, uvial and swamp environments suitable for preservation of fossil plant cuticles10. The plants had nutrient-poor peaty and siliceous substrates, suffered periodic ood disturbance and were in open vegetation early in the ecological succession after disturbance, as is indicated by sedimentological and taphonomic studies of several of the studied fossil sites10,11. Another potentially misleading effect is the differences between trees of different sex in Ginkgo: pollen-producing trees Permian Triasssic 12 10 Stomatal index 8 6 4 2 300 Jurassic Cretaceous Cenozoic 250 200 150 100 50 0 Millions of years ago 12 10 Stomatal index Ginkgo biloba female 8 6 Ginkgo biloba male Figure 2 Stomatal index has varied considerably over the past 300 million years, but was as high during the early Permian ice age as it has been during the Neogene. Data are from fossil leaves of Rhachiphyllum (early Permian, 295 265 Myr ago), Lepidopteris (mid-Permian to late Triassic, 258 200 Myr ago), Tatarina (late Permian, 252 250 Myr ago) and Ginkgo (late Triassic to recent, 229 0 Myr ago). Filled symbols and the solid line are reliable data as established by rarefaction analysis (Fig. 1); un lled symbols are statistically inadequate samples, and are plotted to show proven data density of the cuticular record of these taxa. All data are available as Supplementary Information. 2,000 Lepidopteris stormbergensis 2 0 0 2,000 4,000 6,000 Number of epidermal cells counted Carbon dioxide (p.p.m.v.) 4 1,500 1,000 500 0 6 7 8 9 10 11 Stomatal index y = 155.77x2 2897.8x + 13773 R2 = 0.96 Figure 1 Rarefaction analysis indicates how many cells need to be counted for reliable determination of stomatal index (SI). Shown here are leaves of a female (circles) and male (diamonds) tree of Ginkgo biloba collected from Eugene, Oregon, 5 September 2000, and of Lepidopteris stormbergensis from the late Triassic (Carnian) Molteno formation at Little Switzerland, South Africa12. The Ginkgo samples were ideal: images of the same size from different leaves of two trees. The Lepidopteris samples were numerous but far from ideal: images of irregular shape and varied size, with great natural variation in SI, and from an unknown number of leaves or plants. NATURE | VOL 411 | 17 MAY 2001 | www.nature.com Figure 3 Palaeoatmospheric CO2 can be inferred from fossil leaf SI by using this transfer function derived from Ginkgo leaves grown in greenhouse experiments5 and taken from herbarium sheets dating back to 1888 (see Supplementary Information). 287 2001 Macmillan Magazines Ltd

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