ARTCLES High-yield production of graphene by liquid-phase exfoliation of graphite YENNY HERNANDEZIt, VALERIa NICOLoSIt, MUSTAFA LOTYA FIONA M. BLIGHE1 ZHENYU SUN1, 2 SUKANTA DE1, 2.L.T. MCGOVERN1 BRENDAN HOLLAND MICHELE BYRNE3 YURII K GUN K02,3 JOHN J. BOLAND2,3 PETER NIRAJ2,3 GEORG DUESBERG2,3 SATHEESH KRISHNAMURTHY2,3 ROBBIE GOODHUE4 JOHN HUTCHISON5 VITTORIO SCARDACI6 ANDREA C. FERRARI6 AND JONATHAN N. COLEMAN,2* 'School of Physics, Trinity College Dublin, Dublin 2, Ireland 2Centre for Research on Adaptive Nanostructures and Nanodevices(CRANN), Trinity College Dublin, Dublin 2, Ireland 3School of Chemistry, Trinity College Dublin, Dublin 2, Ireland "Department of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK 6Engineering Department, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 OFA, UK ' These authors contributed equally to this work. *e-mail: colemajetcd ie Published online: 10 August 2008: doi: 10. 1038/nano. 2008.215 Fully exploiting the properties of graphene will require a method for the mass production of this remarkable material. Two main routes are possible: large-scale growth or large-scale exfoliation. Here, we demonstrate graphene dispersions with concentrations up to 0.01 mg ml-, produced by dispersion and exfoliation of graphite in organic solvents such as N-methyl-pyrrolidone. This is possible because the energy required to exfoliate graphene is balanced by the solvent-graphene interaction for solvents whose surface energies match that of graphene. We confirm the presence of individual graphene sheets by Raman spectroscopy transmission electron microscopy and electron diffraction. Our method results in a monolayer yield of l wt%, which could potentially be improved to 7-12 wt% with further processing. The absence of defects or oxides is confirmed by X-ray photoelectron, infrared and Raman spectroscopies. We are able to produce semi-transparent conducting films and conducting composites. Solution processing of graphene opens up a range of potential large-area applications, from device and sensor fabrication to liquid-phase chemistry. The novel electronic properties of graphene have been well in order to make useful devices, either by mechanical transfer or documented, the charge carriers behave as massless Dirac through solution processing fermions, and novel effects such as an ambipolar field effect,a Recently, a large number of papers have described the dispersion room-temperature quantum Hall effect and the breakdown of and exfoliation of graphene oxide(GO) 8-2. This material consists of the Born-Oppenheimer approximation have all been observed. graphene-like sheets, chemically functionalized with compounds A graphene monolayer has also been demonstrated as a such as hydroxyls and epoxides, which stabilize the sheets in ransparent electrode in a liquid crystal device. However, as was water22. However, this functionalization disrupts the electronic the case in the early days of nanotube and nanowire research, structure of graphene. In fact GO is an insulator rather than a graphene suffers from a problem that is common to many novel semi-metal and is conceptually different from graphene. Although materials-the lack of a method for producing it at high yields. the functional groups can be removed by reduction, so far this The standard procedure used to make graphene is leaves a significant number of defects, which continue to disrupt with carrier mobilities up to 200,000 cm2V-Is-1(refs 8-10). solution-phase method to produce significant quantities of deao s micromechanical cleavage,. This gives the best samples to date, the electronic properties, remain. 22. Thus, a non-coval However, the single layers so obtained form a negligible fraction free, unoxidized graphene is urgently required. In this paper w amongst large quantities of thin graphite flakes. Furthermore, it propose one such method. is difficult to envisage how to scale up this process to mass Here we show that high-quality monolayer graphene can be production. Alternatively, growth of graphene is also commonly produced at significant yields by non-chemical, solution-phas achieved by annealing SiC substrates; however, these samples are exfoliation of graphite in certain organic solvents. This work in fact composed of a multitude of domains, most of them builds upon over 50 years of study into chemical exfoliation of submicrometre in scale, and they are not spatially uniform in graphite 4. Previously, intercalated graphite could be partiall number or size over larger length scales -. A number of works exfoliated by reactions involving the intercalant, through thermal have also reported graphene growth on metal substrates but shock or by acid treatment of expandable graphite.However,to this would require transfer of the sample to insulating substrates date, such methods have given thin graphite sheets or graphene naturenanotechnologyivol3septEmbEr2008www.naturecom/naturenanotechnology @2008 Macmillan Publishers Limited. All rights reserved
High-yield production of graphene by liquid-phase exfoliation of graphite YENNY HERNANDEZ1†, VALERIA NICOLOSI1†, MUSTAFA LOTYA1 , FIONA M. BLIGHE1 , ZHENYU SUN1,2, SUKANTA DE1,2, I. T. McGOVERN1 , BRENDAN HOLLAND1 , MICHELE BYRNE3 , YURII K. GUN’KO2,3, JOHN J. BOLAND2,3, PETER NIRAJ2,3, GEORG DUESBERG2,3, SATHEESH KRISHNAMURTHY2,3, ROBBIE GOODHUE4 , JOHN HUTCHISON5 , VITTORIO SCARDACI6 , ANDREA C. FERRARI6 AND JONATHAN N. COLEMAN1,2* 1School of Physics, Trinity College Dublin, Dublin 2, Ireland 2Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland 3School of Chemistry, Trinity College Dublin, Dublin 2, Ireland 4Department of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland 5Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK 6Engineering Department, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK † These authors contributed equally to this work. *e-mail: colemaj@tcd.ie Published online: 10 August 2008; doi:10.1038/nnano.2008.215 Fully exploiting the properties of graphene will require a method for the mass production of this remarkable material. Two main routes are possible: large-scale growth or large-scale exfoliation. Here, we demonstrate graphene dispersions with concentrations up to 0.01 mg ml21 , produced by dispersion and exfoliation of graphite in organic solvents such as N-methyl-pyrrolidone. This is possible because the energy required to exfoliate graphene is balanced by the solvent –graphene interaction for solvents whose surface energies match that of graphene. We confirm the presence of individual graphene sheets by Raman spectroscopy, transmission electron microscopy and electron diffraction. Our method results in a monolayer yield of 1 wt%, which could potentially be improved to 7 –12 wt% with further processing. The absence of defects or oxides is confirmed by X-ray photoelectron, infrared and Raman spectroscopies. We are able to produce semi-transparent conducting films and conducting composites. Solution processing of graphene opens up a range of potential large-area applications, from device and sensor fabrication to liquid-phase chemistry. The novel electronic properties of graphene have been well documented1 ; the charge carriers behave as massless Dirac fermions2 , and novel effects such as an ambipolar field effect3 , a room-temperature quantum Hall effect4 and the breakdown of the Born –Oppenheimer approximation5 have all been observed. A graphene monolayer has also been demonstrated as a transparent electrode in a liquid crystal device6 . However, as was the case in the early days of nanotube and nanowire research, graphene suffers from a problem that is common to many novel materials — the lack of a method for producing it at high yields. The standard procedure used to make graphene is micromechanical cleavage7 . This gives the best samples to date, with carrier mobilities up to 200,000 cm2 V21 s21 (refs 8 –10). However, the single layers so obtained form a negligible fraction amongst large quantities of thin graphite flakes. Furthermore, it is difficult to envisage how to scale up this process to mass production. Alternatively, growth of graphene is also commonly achieved by annealing SiC substrates; however, these samples are in fact composed of a multitude of domains, most of them submicrometre in scale, and they are not spatially uniform in number or size over larger length scales11–13. A number of works have also reported graphene growth on metal substrates14–17, but this would require transfer of the sample to insulating substrates in order to make useful devices, either by mechanical transfer or through solution processing. Recently, a large number of papers have described the dispersion and exfoliation of graphene oxide (GO)18–21. This material consists of graphene-like sheets, chemically functionalized with compounds such as hydroxyls and epoxides, which stabilize the sheets in water22. However, this functionalization disrupts the electronic structure of graphene. In fact GO is an insulator23 rather than a semi-metal and is conceptually different from graphene. Although the functional groups can be removed by reduction, so far this leaves a significant number of defects, which continue to disrupt the electronic properties, remain18,22. Thus, a non-covalent, solution-phase method to produce significant quantities of defectfree, unoxidized graphene is urgently required. In this paper we propose one such method. Here we show that high-quality monolayer graphene can be produced at significant yields by non-chemical, solution-phase exfoliation of graphite in certain organic solvents. This work builds upon over 50 years of study into chemical exfoliation of graphite24. Previously, intercalated graphite could be partially exfoliated by reactions involving the intercalant25, through thermal shock26 or by acid treatment of expandable graphite27. However, to date, such methods have given thin graphite sheets or graphene ARTICLES nature nanotechnology |VOL 3 | SEPTEMBER 2008 | www.nature.com/naturenanotechnology 563 © 2008 Macmillan Publishers Limited. All rights reserved.
ARTICLES fragments- rather than large-scale graphene monolayers. The response to this problem has so far been the exfoliation of chemically modified forms of graphene such as Go or unctionalized graphene2022,28. However, such materials are I graphene, as they are insulators containing numerous structural defects,28 that cannot, so far, be fully removed by chemical treatment22 Our method results h-quality, unoxidized monolayer graphene at yields of l wt%. We show that the process could potentially be improved to give yields of up to 12 wt% of the starting graphite mass with sediment recycling. As a b4000 solution-phase method it is versatile, it may be scaled up, and it can be used to deposit graphene in a variety of environments and substrates not available using cleavage or growth methods. Furthermore, it can be used to produce graphene-based composites or films, a key requirement for many applications, GBL such as thin-film transistors, conductive transparent ele 8001.000 ndium tin oxide replacement or for photovoltaics. Wavelength (nm) DISPERSION OF GRAPHITE a-2460L91m1与 Recently, carbon nanotubes have been successfully exfoliated in a small number of solvents such N-methylpyrrolidone(NMP)29-33 Such exfoliation occurs because the strong interaction between solvent and nanotube sidewall means that the energetic penalty for exfoliation and subsequent solvation becomes small We suggest Concentration向gmr) that similar effects may occur between these solvents and graphene. To test this we prepared rsion or sI d Solvent surface energy, E如mnJm2 powder(Aldrich product 332461, batch number 06106DE)in 405060708090100 NMP (spectrophotometric grade, >99.0%) by bath sonication -10 (see Supplementary Information, Section S1. 2). After sonication we obtained a grey liquid consisting of a homogeneous phase and large numbers of macroscopic aggregates. As with nanotube dispersions,32, these aggregates could be removed by mild ntrifugation, giving a homogeneous dark dispersion. Such dispersions, prepared at different graphite concentrations are hown in Fig. la. Although moderate levels of sedimentation and Solvent surface tension, y(mJ m-2) aggregation occur within three weeks of centrifugation, the dispersions remain of high quality at least five months after preparation(see Supplementary Information, Section S2. 4) Figure 1 Optical characterization of graphite dispersions. a, Dispersions of In order to find the concentration after centrifugation, we graphite flakes in NMP, at a range of concentrations ranging from 6 ug ml-1(A) for residual solvent, gave the concentration of c, Optical absorbance (ex=660 nm) divided by cell length (A/n) as a function dispersed phase after centrifugation. This procedure was repeated of concentration for graphene in the four solvents NMP, GBL, DMA and DMEU for three other solvents known successfully disperse showing Lambert-Beer behaviour with an average absorption coefficient of anotubes 4: N, N-Dimethylacetamide(DMA), y-butyrolactone (a6E0)=2,460Lg-1 m-1. The x-axis error bars come from the uncertainty in (GBL) and 1, 3-dimethyl-2-imidazolidinone (DMEU). These measuring the mass of graphene/graphite in solution. d, Graphite concentration were characterized by UV-vis-IR measured after centrifugation for a range of solvents plotted versus solvent absorption spectroscopy, with the absorption coefficient plotted surface tension. The data were converted from absorbance (660 nm)using versus wavelength(Fig. 1b). The spectra are featureless in the A/l=(ass )C with (aeso)= 2, 460 Lg-1 m-1. The original concentration, before visible-IR region as expected 5. Each of these four dispersions centrifugation, was 0.1 mg ml-1. The y-axis error bars represent the standard as diluted a number of times and the absorption spectra deviation calculated from five measurements Shown on the right axis is the recorded. The absorbance(660 nm)divided by cell length is percentage of material remaining after centrifugation. On the top axis, the surface plotted versus concentration(Fig. Ic), showing Lambert-Beer tension has been transformed into surface energy using a universal value for surface behaviour for all solvents, (a660)=2,460Lg Im-1 ntropy of Sad 0. 1 mJ K-1 m-2. The horizontal arrow shows the approximate Thus, it is clear that graphite can be dispersed in some solvents. range of the reported literature values for the surface energy of graphite As we will show, the graphite is almost completely exfoliated to multilayer structures with <5 layers in NMP, GBL and DMEU, if In addition t quantities pproximately calculate in this case to be ndividual monolayers are also present. The question is what Information, Section S6.0): olvent properties lead to this exfoliation of graphite? Such exfoliation can on if the net energetic cost is ry small. In chemistry, this energy balance is expressed as the (6-6)2b @2008 Macmillan Publishers Limited. All rights reserved
fragments27 rather than large-scale graphene monolayers. The response to this problem has so far been the exfoliation of chemically modified forms of graphene such as GO or functionalized graphene20,22,28. However, such materials are not graphene, as they are insulators containing numerous structural defects22,28 that cannot, so far, be fully removed by chemical treatment22. Our method results in high-quality, unoxidized monolayer graphene at yields of 1 wt%. We show that the process could potentially be improved to give yields of up to 12 wt% of the starting graphite mass with sediment recycling. As a solution-phase method it is versatile, it may be scaled up, and it can be used to deposit graphene in a variety of environments and substrates not available using cleavage or growth methods. Furthermore, it can be used to produce graphene-based composites or films, a key requirement for many applications, such as thin-film transistors, conductive transparent electrodes for indium tin oxide replacement or for photovoltaics. DISPERSION OF GRAPHITE Recently, carbon nanotubes have been successfully exfoliated in a small number of solvents such N-methylpyrrolidone (NMP)29–33. Such exfoliation occurs because the strong interaction between solvent and nanotube sidewall means that the energetic penalty for exfoliation and subsequent solvation becomes small34. We suggest that similar effects may occur between these solvents and graphene. To test this we prepared a dispersion of sieved graphite powder (Aldrich product 332461, batch number 06106DE) in NMP (spectrophotometric grade, .99.0%) by bath sonication (see Supplementary Information, Section S1.2). After sonication we obtained a grey liquid consisting of a homogeneous phase and large numbers of macroscopic aggregates. As with nanotube dispersions30,32, these aggregates could be removed by mild centrifugation, giving a homogeneous dark dispersion. Such dispersions, prepared at different graphite concentrations are shown in Fig. 1a. Although moderate levels of sedimentation and aggregation occur within three weeks of centrifugation, the dispersions remain of high quality at least five months after preparation (see Supplementary Information, Section S2.4). In order to find the concentration after centrifugation, we passed the graphite dispersion through polyvinylidene fluoride (PVDF) filters. Careful measurements of the filtered mass, accounting for residual solvent, gave the concentration of dispersed phase after centrifugation. This procedure was repeated for three other solvents known to successfully disperse nanotubes34: N,N-Dimethylacetamide (DMA), g-butyrolactone (GBL) and 1,3-dimethyl-2-imidazolidinone (DMEU). These dispersions were then characterized by UV–vis – IR absorption spectroscopy, with the absorption coefficient plotted versus wavelength (Fig. 1b). The spectra are featureless in the visible – IR region as expected35. Each of these four dispersions was diluted a number of times and the absorption spectra recorded. The absorbance (660 nm) divided by cell length is plotted versus concentration (Fig. 1c), showing Lambert –Beer behaviour for all solvents, ka660l ¼ 2,460 L g21 m21 . Thus, it is clear that graphite can be dispersed in some solvents. As we will show, the graphite is almost completely exfoliated to multilayer structures with ,5 layers in NMP, GBL and DMEU, if not other solvents. In addition, significant quantities of individual monolayers are also present. The question is what solvent properties lead to this exfoliation of graphite? Such exfoliation can only occur if the net energetic cost is very small. In chemistry, this energy balance is expressed as the enthalpy of mixing (per unit volume), which we can approximately calculate in this case to be (see Supplementary Information, Section S6.0): DHmix Vmix 2 Tflake dG dsol ð Þ2 f ð1Þ AB C D E 4,000 3,000 2,000 1,000 20 15 10 5 0 10 5 0 10 5 0 0 400 600 NMP NMP λ = 660 nm α ~ 2,460 L g–1 m–1 DMA DMA DMEU DMEU GBL GBL 0 10 20 30 40 40 50 60 70 80 90 100 Solvent surface tension, γ (mJ m–2) Concentration after centrifugation (µg ml–1 ) % remaining after centrifugation A /I (m–1) α (L g–1 m–1) 50 60 70 80 2 4 6 8 10 800 Wavelength (nm) Concentration (µg ml–1) 1,000 1,200 1,400 γ = Esur – TSsur sol sol Solvent surface energy, Esur (mJ m–2) sol Figure 1 Optical characterization of graphite dispersions. a, Dispersions of graphite flakes in NMP, at a range of concentrations ranging from 6 mg ml21 (A) to 4 mg ml21 (E) after centrifugation. b, Absorption spectra for graphite flakes dispersed in NMP, GBL, DMA and DMEU at concentrations from 2 to 8 mg ml21 . c, Optical absorbance (lex ¼ 660 nm) divided by cell length (A/l ) as a function of concentration for graphene in the four solvents NMP, GBL, DMA and DMEU showing Lambert–Beer behaviour with an average absorption coefficient of ka660l ¼ 2,460 L g21 m21 . The x-axis error bars come from the uncertainty in measuring the mass of graphene/graphite in solution. d, Graphite concentration measured after centrifugation for a range of solvents plotted versus solvent surface tension. The data were converted from absorbance (660 nm) using A/l ¼ ka660lC with ka660l ¼ 2,460 L g21 m21 . The original concentration, before centrifugation, was 0.1 mg ml21 . The y-axis error bars represent the standard deviation calculated from five measurements. Shown on the right axis is the percentage of material remaining after centrifugation. On the top axis, the surface tension has been transformed into surface energy using a universal value for surface entropy of Ssol sur 0.1 mJ K21 m22 . The horizontal arrow shows the approximate range of the reported literature values for the surface energy of graphite39–42. ARTICLES 564 nature nanotechnology |VOL 3 | SEPTEMBER 2008 | www.nature.com/naturenanotechnology © 2008 Macmillan Publishers Limited. All rights reserved.
ARTCLES b Number of layers per sheet Figure 2 Electron microscopy of graphite and graphene. a, SEM image of sieved, pristine graphite(scale bar: 500 um). b, SEM image of sediment after centrifugation( scale bar: 25 um). C-e, Bright-field TEM images of monolayer graphene flakes deposited from GBL (C), DMEU (d)and NMP(e), respectively(scale ars: 500 nm). f, g, Bright-field TEM images of a folded graphene sheet and multilayer graphene, both deposited from NMP (scale bars: 500 nm). h, Histogram of the number of visual observations of flakes as a function of the number of monolayers per flake for NMP dispersions. where 8=V(Eu)is the square root of the surface energy of phase i, crystallite size in the starting powder was >150 um,the the thickness of a graphene flake and is the graphene preparation procedure must result in tearing of the crystallites. volume fraction. Reminiscent of the Hildebrand-Scratchard This process may be similar to sonication-induced fragmentation quation, this shows the enthalpy of mixing is dependent on the of carbon nanotubes balance of graphene and solvent surface energies. For graphite, the surface energy is defined as the energy per unit area required to EVIDENCE OE EXFOLIATION IO GRAPHENE overcome the van der Waals forces when peeling two sheets apart. From equation(1), we expect a minimal energy cost of It is possible to investigate the state of the material remaining exfoliation for solvents whose surface energy matches that dispersed using transmission electron microscopy (TEM) by of graphene. To test this, we dispersed graphite in a of dropping a small quantity of each dispersion onto holey carbor solvents. By measuring the optical absorbance ld grids. Crucially, this technique is simpler than that previousl centrifugation and using the absorption coefficient( to used to prepare graphene for TEM", which involved under transform absorbance into concentration we can the etching of graphene placed on a silicon substrate. Immediately amount of graphite flakes dispersed as a function of solvent apparent in the present technique is the advantage of having surface energy(calculated from surface tension,38, see Fig. I graphite dispersions. Figure 2c-g shows bright-field TEM images caption)as shown in Fig. ld. As predicted, the dispersed of the objects typically observed, which generally fall into three oncentration shows a strong peak for solvents with a surface classes. The first class, as shown in Fig. 2c-e, comprises energy very close to the literature values of the nanotube/ monolayer graphene. Second, in a number of cases we observe graphite surface energy -2(that is, 70-80 m)m 2). Coupled folded graphene layers(Fig. 2f). Third, we find bilayer and ith equation (1),this strongly suggests that not only is the multilayer graphene( Fig. 2g). In all cases, these objects have nthalpy of mixing for graphite dispersed in good solvents very lateral sizes typically of w micrometres. In some cases the close to zero, but the solvent-graphite interaction is van der sheet edges tend to scroll and fold slightly(see Supplementary 40-50 m] m. Also, we can tell from these data that for the best these samples, graphite has been extensively exfoliated to give solvent(benzyl benzoate),8.3% by mass of the original material monolayer and few-layer graphene. By analysing a large number remained after centrifugation.( For NMP, 7.6% remained of TEM images, paying close attention to the uniformity of the It is crucial to ascertain the exfoliation state of the material that flake edges, we can generate flake thickness statistics remains dispersed after centrifugation. First we examined the state Fig. 2h. From these data we can estimate the number fraction of of the initial graphite powder. Scanning electron microscopy monolayer graphene (number of monolayers/total number of (SEM) studies(Fig. 2a) show the starting powder to consist of flakes observed) in NMP dispersions as 28%. This corresponds to flakes of lateral size <500 um and thickness <100 um. In a solution-phase monolayer mass fraction (mass of all comparison, the sediment separated after centrifugation contains monolayers/ mass of all flakes observed)of 12 wt%, leading to flakes, which are much smaller, with lateral size measured in tens an overall yield(mass of monolayers /starting graphite mass)of of micrometres with thicknesses of a few micrometres(Fig 2b). I wt%(see Supplementary Information, Table S2 and Section Clearly, sonication results in fragmentation of the initial flakes, S2. 3). In fact, we also find that the sediment can be recycled to with the largest removed by centrifugation. We note that, as the produce dispersion naturenanotechnologyivol3septEmbEr2008www.naturecom/naturenanotechnology @2008 Macmillan Publishers Limited. All rights reserved
where di ¼ p (Esur i ) is the square root of the surface energy of phase i, Tflake is the thickness of a graphene flake and f is the graphene volume fraction. Reminiscent of the Hildebrand–Scratchard equation36, this shows the enthalpy of mixing is dependent on the balance of graphene and solvent surface energies. For graphite, the surface energy is defined as the energy per unit area required to overcome the van der Waals forces when peeling two sheets apart. From equation (1), we expect a minimal energy cost of exfoliation for solvents whose surface energy matches that of graphene. To test this, we dispersed graphite in a wide range of solvents. By measuring the optical absorbance after mild centrifugation and using the absorption coefficient (660 nm) to transform absorbance into concentration, we can quantify the amount of graphite flakes dispersed as a function of solvent surface energy (calculated from surface tension37,38, see Fig. 1 caption) as shown in Fig. 1d. As predicted, the dispersed concentration shows a strong peak for solvents with a surface energy very close to the literature values of the nanotube/ graphite surface energy39–42 (that is, 70 –80 mJ m22 ). Coupled with equation (1), this strongly suggests that not only is the enthalpy of mixing for graphite dispersed in good solvents very close to zero, but the solvent –graphite interaction is van der Waals rather than covalent. In addition, it predicts that good solvents are characterized by surface tensions in the region of 40 –50 mJ m22 . Also, we can tell from these data that for the best solvent (benzyl benzoate), 8.3% by mass of the original material remained after centrifugation. (For NMP, 7.6% remained.) It is crucial to ascertain the exfoliation state of the material that remains dispersed after centrifugation. First we examined the state of the initial graphite powder. Scanning electron microscopy (SEM) studies (Fig. 2a) show the starting powder to consist of flakes of lateral size ,500 mm and thickness ,100 mm. In comparison, the sediment separated after centrifugation contains flakes, which are much smaller, with lateral size measured in tens of micrometres with thicknesses of a few micrometres (Fig. 2b). Clearly, sonication results in fragmentation of the initial flakes, with the largest removed by centrifugation. We note that, as the crystallite size in the starting powder was .150 mm, the preparation procedure must result in tearing of the crystallites. This process may be similar to sonication-induced fragmentation of carbon nanotubes43. EVIDENCE OF EXFOLIATION TO GRAPHENE It is possible to investigate the state of the material remaining dispersed using transmission electron microscopy (TEM) by dropping a small quantity of each dispersion onto holey carbon grids. Crucially, this technique is simpler than that previously used to prepare graphene for TEM44, which involved underetching of graphene placed on a silicon substrate. Immediately apparent in the present technique is the advantage of having graphite dispersions. Figure 2c–g shows bright-field TEM images of the objects typically observed, which generally fall into three classes. The first class, as shown in Fig. 2c–e, comprises monolayer graphene. Second, in a number of cases we observe folded graphene layers (Fig. 2f ). Third, we find bilayer and multilayer graphene (Fig. 2g). In all cases, these objects have lateral sizes typically of a few micrometres. In some cases the sheet edges tend to scroll and fold slightly (see Supplementary Information, Fig. S3b). However, we rarely observe large objects with thickness of more than a few layers. Thus we believe that, in these samples, graphite has been extensively exfoliated to give monolayer and few-layer graphene. By analysing a large number of TEM images, paying close attention to the uniformity of the flake edges, we can generate flake thickness statistics as shown in Fig. 2h. From these data we can estimate the number fraction of monolayer graphene (number of monolayers/total number of flakes observed) in NMP dispersions as 28%. This corresponds to a solution-phase monolayer mass fraction (mass of all monolayers/mass of all flakes observed) of 12 wt%, leading to an overall yield (mass of monolayers/starting graphite mass) of 1 wt% (see Supplementary Information, Table S2 and Section S2.3). In fact, we also find that the sediment can be recycled to produce dispersions with number and mass fractions of 0 30 20 10 0 246 Number of layers per sheet Counts 8 N = 100 10 12 14 Figure 2 Electron microscopy of graphite and graphene. a, SEM image of sieved, pristine graphite (scale bar: 500 mm). b, SEM image of sediment after centrifugation (scale bar: 25 mm). c–e, Bright-field TEM images of monolayer graphene flakes deposited from GBL (c), DMEU (d) and NMP (e), respectively (scale bars: 500 nm). f,g, Bright-field TEM images of a folded graphene sheet and multilayer graphene, both deposited from NMP (scale bars: 500 nm). h, Histogram of the number of visual observations of flakes as a function of the number of monolayers per flake for NMP dispersions. ARTICLES nature nanotechnology |VOL 3 | SEPTEMBER 2008 | www.nature.com/naturenanotechnology 565 © 2008 Macmillan Publishers Limited. All rights reserved.
ARTICLES a 14 人人人 Multilayer Distance(A-) 0.005101.5 igure 3 Evidence of monolayer graphene from TEM. a, b, High-resolution TEM images of solution-cast monolayer(a)and bilayer(b) graphene(scale bar 500 nm) C, Electron diffraction pattern of the sheet in a, with the peaks labelled by Miller-Bravais indices. d, e, Electron diffraction pattems taken from the positions of the black(d)and white spots(e), respectively, of the sheet shown in b, using the same labels as in C. The graphene is clearly one layer thick in d and two layers thick in e. f-h, Diffracted intensity taken along the 1-210 to-2110 axis for the patterns shown in c-e, respectively. i, Histogram of the ratios of the intensity of the 11100 and (2110 diffraction peaks for all the diffraction patterns collected. A ratio >1 is a signature of graphene monolayer graphene that we have measured to be m18% and plot a line section through the(1-210)-(0-110)-(-1010) 7 wt%, respectively. This suggests the possibility of full sediment (-2110) axis for the patterns in Fig. 3c-e in Fig. 3f-h. In ecycling and the eventual increase of the yield towards Fig 3f,g we see that the inner peaks,(0-110)and(-1010),are 7-12 wt%(relative to the starting graphite mass) more intense than the outer ones,(1-210) and (-2110) onfirming that that both the flake in Fig 3a and the region IDENTIFICATION OF MONOLAYERS BY ELECTRON DIFFRACTION by the black dot in Fig. 3b are monolayers. Conversely, Fig. 3h shows inner peaks that are less intense than the outer A more definitive identification of graphene can be made by ones, confirming that the area around the white dot in Fig. 3b analysis of electron diffraction patterns5. As an example of this, consists of more than one layer. Further confirmation of the Fig. 3a, b shows what appear to be a graphene monolayer and a presence of monolayer graphene can be found by measuring the graphene bilayer, respectively. Figure 3b is particularly interesting diffraction peak intensity as a function of tilt angle (see as the right side of the flake consists of at least two layers, Supplementary Information, Section S2. 8) whereas on the left side, a single monolayer protrudes. Figure 3c We can use the fact that the ratio of the intensity of the (1100) shows the normal-incidence electron diffraction pattern of the to the (2110) peaks gives an unambiguous local identification of flake in Fig 3a. This pattern shows the typical sixfold symmetry monolayer versus multilayer to provide information on the yield expected for graphite/graphene.5, allowing us to label the of monolayer graphene. We measured the diffraction pattern of peaks with the Miller-Bravais (hkil) indices. Figure 3d, e 45 flakes before measuring the intensity ratio shows normal-incidence selected-area diffraction patterns for the These ratios are plotted as a histogram in Fig flake in Fig. 3b, taken with beam positions close to the black bimodal distribution, with peaks centred at 1(1100)/(2110)=0.35 and white dots, respectively. This means we expect one and I(oo)/4(21101=1.5, representing multilayer and monolaye pattern(Fig. 3d) to reflect monolayer graphene and the other graphene, respectively. These results agree well with reported hexagonal pattern similar to that in Fig. 3c. The main difference graphene and 1(1oo //2110)N 1.4 for monolayer graphene between Fig. 3d and Fig. 3e is that for the multilayers(Fig. 3e), Alhough these data suggest a yield of 51% monolayer graphene, the 21101 spots appear relative to the this is certainly an overestimate, as selected-area electron (1100 spots. This is an important observation, as for diffraction can give monolayer-like patterns for multilayers, such have shown that the intensity ratio is I(1oo /1/211011(ref. 46). Virtually all the of layers per flake, as shown in Fig. 2h. However, we can use objects identified in all the images as multilayers displayed a ratio electron diffraction to check the accuracy of our image analysis, of 411ooy/(21101<1, demonstrating that AB stacking is showing that we can reproducibly use it to identify monolayer graphene, thus confirming the results presented in Fig. 2h. The This identification of AB stacking in these thin multilayers presence of monolayers was also confirmed by measuring TEM allows us to differentiate monolayer from multilayer graphene by identified layers by Raman spectroscopy(see Supplementary pection of the intensity ratio 1(uoo /1( 2110. To do this, we Information, Section $2.9) @2008 Macmillan Publishers Limited. All rights reserved
monolayer graphene that we have measured to be 18% and 7 wt%, respectively. This suggests the possibility of full sediment recycling and the eventual increase of the yield towards 7 –12 wt% (relative to the starting graphite mass). IDENTIFICATION OF MONOLAYERS BY ELECTRON DIFFRACTION A more definitive identification of graphene can be made by analysis of electron diffraction patterns45. As an example of this, Fig. 3a,b shows what appear to be a graphene monolayer and a graphene bilayer, respectively. Figure 3b is particularly interesting as the right side of the flake consists of at least two layers, whereas on the left side, a single monolayer protrudes. Figure 3c shows the normal-incidence electron diffraction pattern of the flake in Fig. 3a. This pattern shows the typical sixfold symmetry expected for graphite/graphene44,45, allowing us to label the peaks with the Miller–Bravais (hkil) indices. Figure 3d,e shows normal-incidence selected-area diffraction patterns for the flake in Fig. 3b, taken with beam positions close to the black and white dots, respectively. This means we expect one pattern (Fig. 3d) to reflect monolayer graphene and the other (Fig. 3e) to reflect multilayer graphene. In both cases we see a hexagonal pattern similar to that in Fig. 3c. The main difference between Fig. 3d and Fig. 3e is that for the multilayers (Fig. 3e), the f2110g spots appear to be more intense relative to the f1100g spots. This is an important observation, as for multilayers with Bernal (AB) stacking, computational studies have shown that the intensity ratio is If1100g/If2110g , 1, whereas for monolayers it is If1100g/If2110g . 1 (ref. 46). Virtually all the objects identified in all the images as multilayers displayed a ratio of If1100g/If2110g , 1, demonstrating that AB stacking is predominant in these samples46. This identification of AB stacking in these thin multilayers allows us to differentiate monolayer from multilayer graphene by inspection of the intensity ratio If1100g/If2110g. To do this, we plot a line section through the (1–210)– (0 –110) – ( –1010) – ( –2110) axis for the patterns in Fig. 3c–e in Fig. 3f –h. In Fig. 3f,g we see that the inner peaks, (0–110) and (–1010), are more intense than the outer ones, (1–210) and ( –2110), confirming that that both the flake in Fig. 3a and the region marked by the black dot in Fig. 3b are monolayers. Conversely, Fig. 3h shows inner peaks that are less intense than the outer ones, confirming that the area around the white dot in Fig. 3b consists of more than one layer. Further confirmation of the presence of monolayer graphene can be found by measuring the diffraction peak intensity as a function of tilt angle (see Supplementary Information, Section S2.8). We can use the fact that the ratio of the intensity of the {1100} to the {2110} peaks gives an unambiguous local identification of monolayer versus multilayer to provide information on the yield of monolayer graphene. We measured the diffraction pattern of 45 flakes before measuring the intensity ratio If1100g/If2110g. These ratios are plotted as a histogram in Fig. 3i. We get a bimodal distribution, with peaks centred at If1100g/If2110g ¼ 0.35 and If1100g/If2110g ¼ 1.5, representing multilayer and monolayer graphene, respectively. These results agree well with reported experimental intensity ratios of If1100g/If2110g 0.4 for bilayer graphene and If1100g/If2110g 1.4 for monolayer graphene45. Alhough these data suggest a yield of 51% monolayer graphene, this is certainly an overestimate, as selected-area electron diffraction can give monolayer-like patterns for multilayers, such as that in Fig. 3b, when the beam is incident on a protruding monolayer. Better statistics can be found by counting the number of layers per flake, as shown in Fig. 2h. However, we can use electron diffraction to check the accuracy of our image analysis, showing that we can reproducibly use it to identify monolayer graphene, thus confirming the results presented in Fig. 2h. The presence of monolayers was also confirmed by measuring TEM identified layers by Raman spectroscopy (see Supplementary Information, Section S2.9). 0–110 1–210 –2110 –1010 –1–120 0.0 0.5 Intensit y (a.u.) 200 14 12 10 8 6 4 2 0 0.0 0.5 1.0 1.5 2.0 N = 45 Graphene I {1100}/I {2110} Multilayer Counts 150 100 50 0 1.0 1.5 Distance (Å–1) Intensity (a.u.) Intensity (a.u.) 200 0 600 100 300 0 0.0 1.0 1.5 0.5 Distance (Å–1) h i Figure 3 Evidence of monolayer graphene from TEM. a,b, High-resolution TEM images of solution-cast monolayer (a) and bilayer (b) graphene (scale bar 500 nm). c, Electron diffraction pattern of the sheet in a, with the peaks labelled by Miller–Bravais indices. d,e, Electron diffraction patterns taken from the positions of the black (d) and white spots (e), respectively, of the sheet shown in b, using the same labels as in c. The graphene is clearly one layer thick in d and two layers thick in e. f–h, Diffracted intensity taken along the 1–210 to –2110 axis for the patterns shown in c–e, respectively. i, Histogram of the ratios of the intensity of the f1100g and f2110g diffraction peaks for all the diffraction patterns collected. A ratio .1 is a signature of graphene. ARTICLES 566 nature nanotechnology |VOL 3 | SEPTEMBER 2008 | www.nature.com/naturenanotechnology © 2008 Macmillan Publishers Limited. All rights reserved.
ARTCLES bilayer, demonstrate does not introduce significant structural bonded to the basal pl 地二 we recorded Rama petra for individual marked TEM grids, 2: NMP cast film, large flake allowing us to identify monolayers, bilayers and multilayers from both the TEM image and the shape of the 2D band, confirming the quality of our exfoliation(see Supplementary Information, 3: NMP cast film. Section S2.9). Furthermore, X-ray photoelectron spectroscopy, as 4: NMP cast individual bilayer and infrared spectroscopy (see Supplementary Information, Section S3. 3)show the absence of oxidization typically associated 1,2501,5001,750250027503000 can produce high-quality, unoxidized graphite and grapher Raman shift (cm-) flakes in solution b EURTHER CHARACTERIZATION OELIOIID-PHASE EXEOLIATION We can briefly illustrate the potential of this method of graphite exfoliation by using it to make thin graphene films. Raman and 12828726085284 SEM analyses show that these films consist predominately of thin graphite flakes with fewer than five layers(see Supplementary Information, Section S1.4). X-ray photoelectron spectroscopy measurements show that these films have ll wt% residual nmP after drying at room temperature at l x 10 mbar. This value remained unchanged after a subsequent vacuum anneal at 400C (see Supplementary Information, Section S3. 2). Combustion roon Binding energy (ev) drying(l x 10-3 mbar), which can be reduced to <7 wt% after annealing(see Supplementary Information, Section S3. 4).These Figure 4 Evidence for defect-free graphene. a, Raman spectra for films have conductivities of 6,500 S m- similar to reduced lk graphite (1), a vacuum filtered film with the laser spot focused on a large graphene oxide films, and optical transparencies of w 42%(see 5 um)flake(2), a vacuum filtered film with the laser spot focused on a small Supplementary Information, Section $4.0) 1 um)flake (3), a large(10 um) bilayer (4). Note that for spectra 2 and 4, volume fraction. We measured the conductivity of such colea. t high We also demonstrate polystyrene-graphene e D line is absent, indicating that virtually no defects are present. For the small flake(spectrum 3), a weak D line is apparent, consistent with edge to be a100 Sm(see Supplementary Information, Section $5.0 effects. b, A carbon 1s core-level XPS spectrum for a thin film(30 nm), for 60-80 vol% films, comparable to the most conductive tube vacuum-deposited trom a graphene dispersion and dred in a vacuum oven at those quoted for graphene-oxide-based composites zo. Finally,we room temperature. The Shirley background has been subtracted for clarity. Main deposited graphene monolayers and multilayers on SiO2 surfaces by fit line represents graphitic carbon(C-C). The remainder, 286 eV, can be very means of spray coating, demonstrating that this processing method the need for The smaller fit lines represent residual NMP; Cong, carbon in the NMP ring an potentially be used to prepare samples for microelectronic bonded to two hydrogen atoms; C-N, carbon in the NMP molecule bonded to a pplications(see Supplementary Information, Section S2.7) nitrogen atom; C=0, carbon in the NMP ring double bonded to an oxygen atom. Left inset: enlarged view of the NMP fit lines(combined and individual) Right inset: structure of NMP. We have demonstrated a scalable method to produce high-quality unoxidized graphite and graphene flakes from powdered graph EVIDENCE FOR DEFEMT-EREF GRAPHENE By using certain solvents, graphene can be dispersed at concentrations of up to 0.01 mg ml. These dispersions can Although Fig. Id suggests a van der Waals type solvent-graphene then be nsed to deposit flakes by spray coating, vacuum filtration drop casting. By adding polymers they can be turned into basal-plane functionalization, which could alter the electronic polymer-composite dispersions. We believe that this work opens structure. Figure 4a shows Raman spectra of three different flakes devices to transparent electrodes and conductive composites with the spectrum of bulk graphite for comparison (see Supplementary Information, Section S1. 4). Spectra 2 and 3 were Received 2 May 2008; accepted 2 July 2008: published 10 August 2008 measured on thin films prepared by vacuum filtration onto alumina filters by focusing the spot on a large diameter(5 pm) 1. Geim, A. K. Nowoselov, K &. The rise of graphene. Nature Mater. 6, 183-191(2007) d a small diameter (1 um) flake, respectively. Spectrum 4 2 Novoselow, K Set al Two-dimensional gas of massless Dirac fermions in graphene Natare 438. was measured on a significantly large bilayer (10 197-200(2005) upplementary Information, Sections Sl I and S2.9). The G line 4. Zhang, Y B. Tan, Y. W Stormer, H L. Kim, P Experimental observation of the quantum hall (1 580 cm")and 2D line(2, 700 cm )are clearly visible in s. Pisana, S. at al. Breakdown of the adiabatic Born-Oppenheimer approximation in graphene. Nature all cases. However, the d peak(1, 350 cm )is only visible in the spectrum of the very small flake, as expected due to edge Nake, le s x giraphemewba-d liqui ar sta ic cre, alk pret xod o sony effects. These data, in particular the spectrum for the individual 10451-10453(2005) naturenanotechnologyvoL3septEmbEr2008www.naturecom/naturenanotechnology @2008 Macmillan Publishers Limited. All rights reserved
EVIDENCE FOR DEFECT-FREE GRAPHENE Although Fig. 1d suggests a van der Waals type solvent–graphene interaction, it is crucial to definitively rule out any inadvertent basal-plane functionalization, which could alter the electronic structure. Figure 4a shows Raman spectra of three different flakes with the spectrum of bulk graphite for comparison (see Supplementary Information, Section S1.4). Spectra 2 and 3 were measured on thin films prepared by vacuum filtration onto alumina filters by focusing the spot on a large diameter (5 mm) and a small diameter (1 mm) flake, respectively. Spectrum 4 was measured on a significantly large bilayer (.10 mm) (see Supplementary Information, Sections S1.1 and S2.9). The G line (1,580 cm21 ) and 2D line (2,700 cm21 ) are clearly visible in all cases. However, the D peak (1,350 cm21 ) is only visible in the spectrum of the very small flake, as expected due to edge effects47. These data, in particular the spectrum for the individual bilayer, demonstrate that our process does not introduce significant structural defects47, such as epoxides covalently bonded to the basal plane22. In addition, we recorded Raman spectra for individual flakes deposited on marked TEM grids, allowing us to identify monolayers, bilayers and multilayers from both the TEM image and the shape of the 2D band, confirming the quality of our exfoliation (see Supplementary Information, Section S2.9). Furthermore, X-ray photoelectron spectroscopy, as shown in Fig. 4b (see Supplementary Information, Section S3.2) and infrared spectroscopy (see Supplementary Information, Section S3.3) show the absence of oxidization typically associated with GO (refs 18,19). These experiments again confirm that we can produce high-quality, unoxidized graphite and graphene flakes in solution. FURTHER CHARACTERIZATION OF LIQUID-PHASE EXFOLIATION We can briefly illustrate the potential of this method of graphite exfoliation by using it to make thin graphene films. Raman and SEM analyses show that these films consist predominately of thin graphite flakes with fewer than five layers (see Supplementary Information, Section S1.4). X-ray photoelectron spectroscopy measurements show that these films have 11 wt% residual NMP after drying at room temperature at 1 1023 mbar. This value remained unchanged after a subsequent vacuum anneal at 400 8C (see Supplementary Information, Section S3.2). Combustion analysis gave an NMP content of 10 wt% after room-temperature drying (1 1023 mbar), which can be reduced to ,7 wt% after annealing (see Supplementary Information, Section S3.4). These films have conductivities of 6,500 S m21 , similar to reduced graphene oxide films19, and optical transparencies of 42% (see Supplementary Information, Section S4.0). We also demonstrate polystyrene–graphene composites at high volume fraction. We measured the conductivity of such composites to be 100 S m21 (see Supplementary Information, Section S5.0) for 60–80 vol% films, comparable to the most conductive polymer–nanotube composites48 and significantly higher than those quoted for graphene-oxide-based composites20. Finally, we deposited graphene monolayers and multilayers on SiO2 surfaces by means of spray coating, demonstrating that this processing method can potentially be used to prepare samples for microelectronic applications (see Supplementary Information, Section S2.7). CONCLUSION We have demonstrated a scalable method to produce high-quality, unoxidized graphite and graphene flakes from powdered graphite. By using certain solvents, graphene can be dispersed at concentrations of up to 0.01 mg ml21 . These dispersions can then be used to deposit flakes by spray coating, vacuum filtration or drop casting. By adding polymers they can be turned into polymer–composite dispersions. We believe that this work opens up a whole new vista of potential applications from sensor or devices to transparent electrodes and conductive composites. Received 2 May 2008; accepted 2 July 2008; published 10 August 2008. References 1. Geim, A. K. & Novoselov, K. S. The rise of graphene. Nature Mater. 6, 183 –191 (2007). 2. Novoselov, K. S. et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 438, 197–200 (2005). 3. Novoselov, K. S. et al. Electric field effect in atomically thin carbon films. Science 306, 666–669 (2004). 4. Zhang, Y. B., Tan, Y. W., Stormer, H. L. & Kim, P. Experimental observation of the quantum Hall effect and Berry’s phase in graphene. Nature 438, 201–204 (2005). 5. Pisana, S. et al. Breakdown of the adiabatic Born–Oppenheimer approximation in graphene. Nature Mater. 6, 198 –201 (2007). 6. Blake, P. et al. Graphene-based liquid crystal device. Nano Lett. 8, 1704–1708 (2008). 7. Novoselov, K. S. et al. Two-dimensional atomic crystals. Proc. Natl Acad. Sci. USA 102, 10451–10453 (2005). Intensity (a.u.) D G 2D 1: Bulk graphite 2: NMP cast film, large flake 3: NMP cast film, small flake 4: NMP cast individual bilayer 1,250 1,500 1,750 Raman shift (cm–1) 2,500 2,750 3,000 290 288 286 284 282 Binding energy (eV) 288 287 286 285 284 CRing C-N C=O CH3 N O Intensity (a.u.) Figure 4 Evidence for defect-free graphene. a, Raman spectra for bulk graphite (1), a vacuum filtered film with the laser spot focused on a large (5 mm) flake (2), a vacuum filtered film with the laser spot focused on a small (1 mm) flake (3), a large (10 mm) bilayer (4). Note that for spectra 2 and 4, the D line is absent, indicating that virtually no defects are present. For the small flake (spectrum 3), a weak D line is apparent, consistent with edge effects. b, A carbon 1s core-level XPS spectrum for a thin film (30 nm), vacuum-deposited from a graphene dispersion and dried in a vacuum oven at room temperature. The Shirley background has been subtracted for clarity. Main fit line represents graphitic carbon (C–C). The remainder, 286 eV, can be very well fitted considering only residual NMP without the need for any oxide lines. The smaller fit lines represent residual NMP; Cring, carbon in the NMP ring bonded to two hydrogen atoms; C–N, carbon in the NMP molecule bonded to a nitrogen atom; C ¼ O, carbon in the NMP ring double bonded to an oxygen atom. Left inset: enlarged view of the NMP fit lines (combined and individual). Right inset: structure of NMP. ARTICLES nature nanotechnology |VOL 3 | SEPTEMBER 2008 | www.nature.com/naturenanotechnology 567 © 2008 Macmillan Publishers Limited. All rights reserved.
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J. & Coleman, J. N. Observation of percolation-like scaling, far from the percolation threshold, in high volume fraction, high conductivity polymer–nanotube composite films. Adv. Mater. 19, 4443 –4447 (2007). Supplementary Information accompanies this paper at www.nature.com/naturenanotechnology. Acknowledgements We acknowledge the Centre for Research on Adaptive Nanostructures and Nanodevices and Science Foundation Ireland for financial support and Nacional de Grafite (Brazil) for supplying flake graphite. V.N. wishes to thank the EU project ESTEEM for facilitating access to the microscopy facilities in Oxford. A.C.F. acknowledges funding from the Leverhulme Trust and the Royal Society. Author contributions J.N.C. conceived and designed the experiments. Y.H., V.N., M.L., F.M.B., Z.S., S.D., B.H., M.B., P.N., S.K., R.G. and V.S. performed the experiments. I.T.McG., R.G., A.C.F. and J.N.C. analysed the data. Y.K.G., J.J.B., G.D., R.G., J.H., A.C.F. and J.N.C. contributed materials/analysis tools. A.C.F. and J.N.C. co-wrote the paper. Y.H. and V.N. contributed equally to this work. All authors discussed the results and commented on the manuscript. Author information Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/. Correspondence and requests for materials should be addressed to J.N.C. ARTICLES 568 nature nanotechnology |VOL 3 | SEPTEMBER 2008 | www.nature.com/naturenanotechnology © 2008 Macmillan Publishers Limited. All rights reserved