Highly conducting graphene sheets and Langmuir-Blodgett films

Highly conducting graphene sheets and Langmuir–Blodgett?lms

XIAOLIN LI1,GUANGYU ZHANG1,XUEDONG BAI2,XIAOMING SUN1,XINRAN WANG1,ENGE WANG2 AND HONGJIE DAI1*

1Department of Chemistry and Laboratory for Advanced Materials,Stanford University,Stanford,California94305,USA

2Institute of Physics,Chinese Academy of Sciences,Beijing100080,China

*e-mail:hdai@https://www.sodocs.net/doc/e717021536.html,

Published online:1August2008;doi:10.1038/nnano.2008.210

Graphene is an intriguing material with properties that are distinct from those of other graphitic systems1–5.The?rst samples of pristine graphene were obtained by‘peeling off’2,6 and epitaxial growth5,7.Recently,the chemical reduction of graphite oxide was used to produce covalently functionalized single-layer graphene oxide8–15.However,chemical approaches for the large-scale production of highly conducting graphene sheets remain elusive.Here,we report that the exfoliation–reintercalation–expansion of graphite can produce high-quality single-layer graphene sheets stably suspended in organic solvents.The graphene sheets exhibit high electrical conductance at room and cryogenic https://www.sodocs.net/doc/e717021536.html,rge amounts of graphene sheets in organic solvents are made into large transparent conducting?lms by Langmuir–Blodgett assembly in a layer-by-layer manner.The chemically derived, high-quality graphene sheets could lead to future scalable graphene devices.

Several methods have been explored to date to obtain graphene in solution phase by means of chemical routes.Graphite oxide (GO)has been prepared by harsh oxidation using Hummer’s method16.The as-made GO obtained in this way is electrically insulating,but chemical reduction9,10,15partially recovers the conductivity,albeit at values orders of magnitude below that of pristine graphene.Irreversible defects and disorder exist in the GO sheets9,10.The reduced GO exhibits non-metallic behaviour,with the conductance decreasing by about three orders of magnitude upon cooling to low temperatures13,whereas pristine graphene is nearly metallic2,17.Recently,we obtained high quality graphene nanoribbons(GNR)by sonicating thermally exfoliated graphite in a1,2-dichloroethane(DCE)solution of poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene)(PmPV)18.However,the yield was low and most of the ribbons had two or more layers. Despite these and other efforts8–15,19–22,solution phase derivation of single-layer graphene with high electrical conductivity from widely available parent graphite materials has not been achieved on a large scale.The production of stable suspensions of graphene in organic solvents is also an important goal in chemical processing and other areas.

In the current work,to make high-quality graphene sheets(GS), we started by?rst exfoliating commercial expandable graphite (160–50N,Grafguard)by brief(60s)heating to1,0008C in forming gas.We then ground the exfoliated graphite, reintercalated it with oleum(fuming sulphuric acid with20%free SO3),and inserted tetrabutylammonium hydroxide(TBA, 40%solution in water)into the oleum-intercalated graphite (Fig.1a)in N,N-dimethylformamide(DMF,see Methods).We then sonicated the TBA-inserted oleum-intercalated graphite (Fig.1b)in a DMF solution of1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)-5000] (DSPE-mPEG)for60min to form a homogeneous suspension. Centrifugation was used to remove large pieces of material from the supernatant(Fig.1c;see also Methods).This method resulted in large amounts of GS suspended in DMF and can be transferrable to other solvents including water and organic solvents.

We used atomic force microscopy(AFM)to characterize the materials deposited on the substrates from the supernatant and observed that$90%of the graphene sheets were single-layer GS of various shapes and sizes(Fig.1d).For the hundreds of GS measured,we found that the average size of single-layer GS was $250nm(see Supplementary Information,Fig.S2a)and the average topographic height was$1nm(see Supplementary Information Figs.S2b and S3).Transmission electron microscopy (TEM,Fig.1e)and electron diffraction(ED,Fig.1f)were used to characterize the single-layer GS.The ED pattern of our GS was similar to that of peeled-off graphene23,suggesting a well-crystallized,single-layer graphene structure.

Our starting expandable graphite was prepared by chemical intercalation of sulphuric acid and nitric acid24.Upon heating, they exfoliated violently due to volatile gaseous species released from the intercalant.Most of the exfoliated graphite was still in a multilayer graphene form25.In order to obtain single-layer GS, we used a process of reintercalation with oleum,a chemical known to strongly debundle carbon nanotubes through intercalation26.TBA is a molecule capable of inserting into and expanding the distance between heavily oxidized graphite layers27. We proposed that TBA could insert into oleum-intercalated graphite to increase the distance between adjacent graphitic layers (Fig.1a),facilitating the separation of GS upon sonication in a surfactant solution27.The success of this step was made evident by showing that without this TBA treatment step,with the rest of the method being otherwise identical,the yield of single-layer GS was extremely low(see Supplementary Information,Fig.S1,for control experiments).We also found that DMF was a better solvent than water for our method.DSPE-mPEG is a surfactant capable of suspending nanotubes28,another important factor in obtaining a homogeneous suspension of GS.

Our weak oleum treatment condition (soaking in oleum at room temperature for one day)is important to obtain high-quality GS without excessive chemical functionalization and thus property degradation.The conjugate graphene plane is largely free of irreversible modi?cations throughout the treatment steps.Room-temperature oleum treatment is much less oxidative than Hummer’s method,as is clear from the infrared (IR)spectra,which show that as-made GS has signi?cantly fewer functional groups (Fig.2a,b)than as-made Hummer’s GO (Fig.2d,e).The IR spectrum of as-made GS (Fig.2a)showed weaker signals for the carboxylic groups than the Hummer’s GO (shaded areas in Fig.2a,d)29.X-ray photoelectron spectroscopy (XPS)(Fig.2b)of our as-made GS showed small but noticeable signals at higher

binding energy corresponding to small amounts of C–O species 9,29.These species were removed by 8008C annealing in H 2,indicating the formation of high-quality graphene (Fig.2b).The annealed GS exhibited the same XPS spectrum as a pristine highly oriented pyrolytic graphite (HOPG)crystal (Fig.2b),con?rming the lack of signi?cant defects or covalent modi?cations of sp 2carbon in the ?nal GS product.

We propose the schematic structures of the intermediate and ?nal products of our GS and Hummer’s GO as shown in Fig.2c,f.Oxidization of our intermediate,as-made GS was relatively mild,and the few covalently attached functional groups such as the carboxylic (seen in the IR spectrum,Fig.2a)and hydroxyl groups were most likely at the edges of the as-made GS (Fig.2c).

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Figure 1Chemically derived single-layer GS from the solution phase.a ,Schematic of the exfoliated graphite reintercalated with sulphuric acid molecules (teal spheres)between the layers.b ,Schematic of TBA (blue spheres)insertion into the intercalated graphite.c ,Schematic of GS coated with DSPE-mPEG molecules and a photograph of a DSPE-mPEG /DMF solution of GS.d ,An AFM image of a typical GS several hundred nanometres in size and with a topographic height of $1nm (see Supplementary Information for height details).The scale bar is 300nm.e ,Low-magni?cation TEM images of a typical GS several hundred nanometres in size.The scale bar is 100nm.f ,Electron diffraction (ED)pattern of an as-made GS as in e ,showing excellent crystallization of the GS.

This was supported by the fact that the electrical conductivity of our as-made GS was similar to that of 8008C vacuum-annealed GS (Fig.3b,c),an unlikely result if the graphene plane was heavily modi?ed covalently.The Hummer’s GO was heavily oxidized,with disrupted conjugation in the plane,missing carbon atoms in the plane 30,and abundant functional groups such as epoxide,hydroxyl,carbonyl and carboxyl at both the edges and in the plane (Fig.2f)9,10.Importantly,these abundant functional groups weaken the van der Waals interactions between the layers of GO and make them hydrophilic,which is the reason for the occurrence of single-layer GO exfoliation in aqueous media to form stable suspensions without the need for an insertion agent such as TBA or the assistance of a surfactant for suspension.Thermal annealing removed some of the functional groups but was unable to completely repair the holes and other irreversible defects formed within the plane of the Hummer’s GO sheets (Fig.2f)9,10.

We fabricated single-GS electrical devices with as-made and annealed GS and Hummer’s GO.We used palladium or titanium /gold as source /drain (S /D)metal contacts (channel length L %100nm),a P tt-Si backgate and 500-nm SiO 2for the

gate dielectrics (see Methods and Supplementary Information).A typical resistance for $100-nm-wide GS (Fig.3a)at room temperature is 10–30k V (Fig.3b,c).The average resistance histogram (error bar is the standard deviation)for large numbers of devices showed that the room-temperature resistance of as-made GS was similar to that of annealed GS devices (for both palladium and titanium /gold contacted devices),and about 100times lower than annealed GO (Fig.3b).As-made GO devices without annealing were all electrically insulating.This result strongly supports the proposed atomic structures of GS and GO (Fig.2c,f)and the assertion that our GS comprise nearly pristine graphene.Our thermally annealed GS retained high electrical conductivity,with only a slight increase in resistance at low temperatures (for both palladium and titanium /gold contacted devices),in strong contrast to annealed GO,which was insulating at low temperatures (Fig.3c).Devices comprised of as-made GS showed reduced metallic characteristics over annealed GS devices (but were still .1,000times more conducting than GO devices)with a larger increase in resistance at low temperatures (Fig.3c).This suggested the as-made GS contained a small amount of disorder in the

structures.

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Figure 2Comparison of GS and GO sheets.a ,IR spectrum (400–4,000cm 21)of as-made GS.The shaded region from $1,400to 1,900cm 21includes the signal of the carboxylic groups.b ,XPS spectra of as-made,annealed GS and a HOPG crystal.Note the similarity between the spectra of the annealed GS and HOPG.c ,Schematic of the atomic structure of as-made (top)and annealed GS.d ,IR spectrum (400–4,000cm 21)of as-made GO.The shaded region from $1,400to 1,900cm 21includes the signal of the carboxylic groups.e ,XPS spectra of as-made,annealed GO and a HOPG crystal.f ,Schematic of the atomic structure of as-made (top)and annealed GO.

T o explore the utility of our high-quality GS,we transferred large quantities of GS from DMF to organic solvent DCE,with excellent stability against agglomeration.The fact that our as-made GS was stably suspended in DCE without the use of additional surfactant indicates high hydrophobicity of the graphene,consistent with a low degree of graphene oxidation and covalent functionalization.In contrast,Hummer’s GO was highly hydrophilic and completely insoluble in organic solvents.The organic stability of our GS enabled Langmuir–Blodgett (LB)?lms to be made on various transparent substrates,including glass and quartz (see Methods and Supplementary Information)to produce transparent and conducting ?lms.This was done by adding GS suspensions onto the water subphase,vaporizing the DCE solvent from the water surface,compressing the ?oating GS,and transferring the GS LB ?lm onto a substrate by dip-coating.The GS ?oated on water due to the hydrophobicity within the sheet.The edges of the GS contain functional groups,giving rise to planar amphiphilic species.We were able to transfer the GS repeatedly to achieve multilayer ?lms.The one-,two-and

three-layer LB ?lms on quartz (Fig.4a)afforded sheet resistances of $150,20and 8k V at room temperature (Fig.4c)and transparencies (de?ned as transmittance at a wavelength of 1,000nm)of $93,88and 83%,respectively (Fig.4b,c).With a three-layer LB ?lm,a sheet resistance of 8k V can be achieved with a transparency greater than 80%,which compares favourably with reduced GO ?lms 11,12.The conductance and transparency of our ?lms are comparable to those made of GS formed by sonication of natural graphite in DMF 31.This is the ?rst time that high-quality GS have been assembled by the LB technique in a layer-by-layer manner on large substrates.Note that with the same method we also succeeded in making GS using pristine graphite ?akes as the starting material,and the structural,electrical and spectroscopic properties of the GS made from pristine ?akes were similar to those made from expandable graphite.Thus,our large-scale synthesis of GS and the ability to process it in various solvents for assembly enables its potential use in high-performance,scalable applications such as solar cells using transparent conducting ?lms.

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100Figure 4Large-scale Langmuir-Blodgett (LB)?lms of GS.a ,A photograph of a two-layer GS LB ?lm on quartz with part of it left clear.The scale bar is 10mm.b ,Transparency spectra of one-(black curve),two-(red curve)and three-layer (green curve)GS LB ?lms.The transparency

was de?ned as the transmittance at a wavelength of 1,000nm.c ,Resistances (red)and transparencies (blue)of one-,two-and three-layer LB ?lms.The small percentage of bilayer and few-layer GS in our sample and GS overlapping in the LB ?lm over the substrate contributed to the transparency loss.

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Figure 3Electrical characterization of a single GS.a ,AFM image of a typical device with a single GS (thickness $1nm,single layer)bridging the channel (channel length L %100nm GS with titanium /gold contacts and silicon backgate)between the source (S)and drain (D)electrodes.The scale bar is 200nm.b ,Mean resistance histograms for 10devices each of as-made GS,annealed GS and annealed GO.The resistances of as-made GS and annealed GS were similar (within the error bars representing statistical variations between the GS devices)indicating the high quality of our as-made GS.c ,Resistances of as-made GS (green curve),GS annealed at 800oC with a titanium /gold contact (black);GS annealed at 800oC with a palladium contact (blue),and GO annealed at 800oC (red).The resistance of GS,particularly the annealed GS,showed only a very small drop in conductance (similar to some of the peel-off pristine graphene samples reported in the literature)at low temperature.

METHODS

PREPARATION OF GRAPHENE SHEET SUSPENSION

Our single-layer GS preparation started by exfoliating expandable graphite (160–50N of Grafguard)at1,0008C in forming gas for60s.Exfoliated graphite ($10mg)was then ground with NaCl crystallites for3min,forming a uniform greyish mixture.Small pieces of exfoliated graphite were separated and collected by dissolving the NaCl with water and then?ltration.The resulting sample was then treated with oleum at room temperature for a day.After complete removal of acid by?ltration and repeated washing,the resulting sample was ultrasonicated using a cup-horn sonicator in a DMF(10ml)solution of TBA (130m l)for5min.The suspension was held at room temperature for3days to allow the TBA to fully insert into the graphene layers.A5ml suspension was then taken out and bath-sonicated with DSPE-mPEG(Laysan Bio.)(15mg)for1h, forming a homogeneous suspension.After centrifuging the suspension at 24,000g for3min,we obtained a black suspension with predominantly single-layer GS retained in the supernatant.

CHARACTERIZATION OF GRAPHENE SHEETS

AFM images were taken of the GS with a Nanoscope IIIa multimode instrument. The samples were prepared by soaking a SiO2substrate(pretreated with a4mM 3-aminopropyltriethoxysilane(APTES)water solution for20min)in the graphene suspension for20min,rinsing with water and blow-drying with argon. The substrate was calcined to3508C and annealed at8008C in H2before AFM. The IR spectrum(400–4,000cm21)was measured using a Nicolet IR100FT-IR spectrometer with pure KBr as the background.After removal of the surfactant by?ltration and repeated washing,the graphene sample was collected and ground with KBr.The mixture was dried and compressed into a transparent tablet for measurement.We characterized our GS using a JEOL2010F FEG transmission electron microscope(TEM)at an accelerating voltage of120kV. The TEM samples were prepared by drying a droplet of the graphene suspension on a lacey carbon grid.High-resolution XPS measurement was carried out using an SSI S-Probe monochromatized XPS spectrometer,which used Al(K a) radiation as a probe.The analysis spot size was150?800m m.Sample preparation involved removal of the surfactant by?ltration and repeated washing,and depositing the materials onto a silicon substrate by repeated drop-drying.The GO sample was prepared by depositing materials onto a silicon substrate by repeated drop-drying.The HOPG sample was used for XPS measurement without any treatment.

GRAPHENE SHEET AND GRAPHITE OXIDE DEVICE FABRICATION

GS and GO were deposited onto a500-nm SiO2/PttSi substrate(pretreated with 4mM APTES solution).After removal of the surfactant by3508C calcination and8008C H2annealing,we used electron-beam lithographic patterning followed by electron-beam evaporation of palladium(20nm)or titanium (1.5nm)/gold(20nm)to randomly form the source and drain electrodes (channel length$100nm,width$2m m)on the substrate.The sample was then annealed in argon at3008C for15min to improve the contacts between the source and drain metal and the GS/GO in the channel region.

LANGMUIR–BLODGETT FILM FABRICATION

The DMF suspension of GS was centrifuged at24,000g for1h to remove the surfactants.The aggregates were then resuspended in fresh DMF by a brief sonication step.This centrifugation and resuspending process was repeated three times.The GS samples were then resuspended in fresh DCE and the centrifugation and resuspending process repeated a further three times to ensure complete removal of the DSPE-mPEG.The resulting GS were suspended in DCE by a5-min sonication step.GS LB?lms were made using a commercial KSV-Minimicro2000LB trough.Approximately1.2ml of GS/DCE suspension was added to a water subphase in the LB trough.A platinum plate was used to monitor the surface tension during compression of the GS on the water subphase by moving the two opposing barriers towards each other.At a target surface pressure of$27mN m21,GS were compressed to form a dense LB?lm transferrable onto a solid substrate(up to1?1inch2)by slowly pulling the substrate out of the aqueous subphase.The transferred GS LB?lm was typically calcined at3508C to remove DSPE-mPEG and TBA residues before transparency and resistance measurement.After calcination of the quartz substrate with a one-layer LB?lm we then transferred another layer GS?lm onto it by repeating the LB procedure.We were able to obtain multilayer LB?lms by this layer-by-layer transfer method.The transparency of the GS?lms was measured with a Cary6000i spectrophotometer using pure quartz as the background.The transparency was de?ned as the transmittance at a wavelength of1,000nm.

Received31March2008;accepted27June2008;published1August2008.

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Author contributions

H.D.and X.L.conceived and designed the experiments.X.L.and G.Z.performed the experiments and analysed the data.H.D.and X.L.co-wrote the manuscript.All authors discussed the results and commented on the manuscript.

Author information

Reprints and permission information is available online at https://www.sodocs.net/doc/e717021536.html,/reprintsandpermissions/. Correspondence and requests for materials should be addressed to H.D.