pubs.acs.org/Langmuir

Layer-by-Layer Self-Assembly of Graphene Nanoplatelets

Jianfeng Shen, Yizhe Hu, Chen Li, Chen Qin, Min Shi, and Mingxin Ye*

Department of Materials Science, Fudan University, The Key Laboratory of Molecular Engineering ofPolymers, Ministry of Education, Shanghai, China

Received January 12, 2009. Revised Manuscript Received February 17, 2009

In this report, graphene nanoplatelets were self-assembled through the layer-by-layer (LBL) method. Thegraphene surface was modified with poly(acrylic acid) and poly(acryl amide) by covalent bonding, which intro-duced negative andpositive charge on the surface of graphene, respectively. Through electrostatic interaction, thepositively and negatively charged graphene nanoplatelets assembled together to form a multilayer structure.Thermogravimetric analysis, Raman spectroscopy, scanning electron microscopy (SEM), and atomic forcemicroscopy were used to demonstrate the modification of graphene nanoplatelets. Fourier transform infraredspectroscopy and SEM proved this method is feasible for preparing graphene-containing films. Ultraviolet-visible spectroscopy confirmed that the adsorption technique resulted in uniform film growth.

1. Introduction

Recently, graphene has attracted a tremendous amount ofattention. Its novel electronic, mechanical, and thermal prop-erties have been well documented.1-7 However, just as withother newly discovered allotropes of carbon, such as carbonnanotubes (CNTs), material availability and processabilityhave been the rate-limiting steps in the evaluation of grapheneapplication.8,9 A number of works about growth and exfolia-tion of graphene have been reported.10-15 Among them,micromechanical cleavage is currently the most effective andreliable method to produce high-quality graphene sheets.10

However, the low productivity of this method makes it un-suitable for large-scale applications. High-yield productionmethods for graphene sheets are desirable for such applica-tions as composite materials and conductive films. Nowa-days, exfoliation of graphite oxide either by rapid thermal

expansion or ultrasonic dispersion has been one of the bestapproaches to obtain graphene oxide (GO) in bulk.16-18

Compared to other production techniques, this process isattractive because of its reliability, amenability to large-scaleproduction, and exceptionally low material costs. Besides,GO consists of graphene-like sheets, chemically functiona-lized with compounds such as hydroxyls and epoxides, whichstabilize the sheets in water. However, since GO is electricallyinsulating, which limits its usefulness for electronic devices,the chemical reduction of exfoliated GO is used to make itelectrically conductive.19,20

Nowadays, one possible route to harnessing excellent prop-erties of graphene sheets for applications is to incorporatethem into composite materials.21-24 The manufacturing ofsuch composites requires not only that graphene sheets beproduced on a sufficient scale but also that they be incorpo-rated, and homogeneously distributed, into various matrices.To achieve this goal, some researchers have focused attentionon the processability of graphene sheets.25 Previous studiesof our group26 have shown that polymer-grafted graphene

*Corresponding author. Tel.: +86-021-55664095. Fax: +86-021-55664094. E-mail address: mxye@fudan.edu.cn.

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Published on Web 3/10/2009

© 2009 American Chemical Society

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nanoplatelets with vey good solubility can be achievedthrough in situ radical polymerization.

On the other hand, interest in layer-by-layer (LBL) self-assembly has increased dramatically in the past decade. It iswidely used as a powerful and versatile method for the pre-paration of ultrathin multilayer films containing CNTs.27-31

Since polyelectrolytes (PEs) allow for interesting surfacepatterning structures, they are widely used in LBL pre-paration of CNT films.32-36 Despite some methods dealingwith the assembly of graphene sheets being proposed, e.g., byfiltration of colloid dispersions of functionalized graphenesheets,37-39 assembly of chemically converted graphenesheets,40,41 and assembly withmolecular templates,42 whethergraphene-based LBL films with PEs can be made remainsunexplored since graphene sheets differ substantially fromCNTs in their geometry,which should change the dynamics ofadsorption.

Here we outline a process for the assembly of graphene-based LBL films. Our basic strategy involved the completeexfoliation of graphite oxide into individual GO sheets fol-lowed by their in situ reduction to produce individual gra-phene sheets. After that, poly(acrylic acid) (PAA) and poly(acryl amide) (PAM) were covalently grafted on the graphenesheets by in situ living free radical polymerization. Then,we show formation of graphene-based multilayer LBL filmsthrough sequential adsorption of PE-functionalized graphenesheets (PEs-g-graphene) and other different charged PEs.There are three advantages of ourmethod: (1) The preventionof aggregation in solution is of particular importance forgraphene sheets because most of their unique properties areonly associatedwith individual sheets. PEs-g-grapheneused inthis method can disperse pretty well in water, which can solvethe aggregation problem. (2) Most of the continuous π-elec-tronic structure of graphene sheets can be sustained throughthis method. (3) The force between anionic and cationic PEs-g-graphene layers of the composite will be relatively strong,which will be very useful for the investigation and applicationof these graphene-containing films.

2. Experimental Section

Materials. Expandable graphite (EG) 8099200 (∼180 μm)was purchased from Qingdao BCSM. CO., LTD., whichwas supposed to expand to nearly 200 times its original sizeafter being treated at about 1000 �C. Dimethylformamide(DMF), 98%H2SO4, 30%H2O2, and potassium permanganate(KMnO4) were purchased from Shanghai Zhenxin ChemicalCompany. Poly(diallyldimethylammonium chloride) (PDDA;Mw: 20 000) and sodium polyacrylate (PAAS;Mw: 30 000) werepurchased fromAldrich and used as received. All other reagentswere at least of analytical reagent grade and used withoutfurther purification. Doubly distilled water was used in all theprocesses of aqueous solution preparations and washings.

Preparation of PEs-g-graphene. EG was first treated at1050 �C in air for 15 s. The size was enlarged up to nearly200 times its original amount. Expandable graphite oxide(EGO) was obtained based on Hummers’ method.43 Generally,5 g of thermal treated EG powders was added to 115mL of 98%H2SO4 in an ice bath. An amount of 15 g of KMnO4 was slowlyadded with stirring, and the rate of addition was controlledcarefully to avoid a sudden increase in temperature. Themixturewas then maintained at 35 �C for 30 min. Deionized water(230 mL) was gradually added, causing an increase in tempera-ture to 98 �C. After 15 min, the mixture was further treated with700 mL of deionized water and 50 mL of 30% H2O2 solution.EGO was washed with deionized water until the pH was 7 andthen dried at 65 �C in vacuum.

EGO (30mg) was loaded in a 250mLdried four-necked flask,and 50 mL of solvent (9:1 DMF/water) was then added. Afterstirring and ultrasonication for 30 min, 0.0248 g (0.0064 mol) ofNaBH4 was added to the mixture. The solution was heated in anoil bath at 80 �C for 4 h, and reduced GO was observed (asconfirmed in the Discussion section, they were shown to begraphene nanoplatelets). Acrylic acid (10 g or 5 g of acryl amide)and 40 mL of H2O were added to the flask. After stirring for30min, the solutionwas purged under dry nitrogen for 30min toremove oxygen, followed by addition of 100 mg of (NH4)2S2O8

(dissolved in 80 mL of water) through a dropping funnel. Theflask was placed in a thermostatted oil bath at 60 �C understirring and sonication. After 48 h the mixture was cooled toroom temperature, diluted with 200 mL of H2O, bath sonicatedfor 1 h, and then centrifuged.28

Preparation of Multilayer Assemblies. The graphene nano-platelet-based LBL films were obtained through a similarprocess based on ref 28. Adsorptions were carried out at roomtemperature in open beakers containing components. Sub-strates for film formation were either silicon or glass slides thatwere first made hydrophilic by treatment with 3:1 v/v % con-centrated H2SO4 and HNO3, followed by extensive rinsingwith doubly distilled water. Multilayer films were grown bycyclic immersion in positively charged water solution of PDDA(1 mg/mL containing 0.5 M NaCl), negatively charged solutionof PAA-g-graphene (0.1 mg/mL), positively charged solution ofPAM-g-graphene (0.1 mg/mL, pH= 3 by adding HCl to makeit protonated), and negatively charged solution of PAAS (1 mg/mL containing 0.5 M NaCl). Exposure times of 20 and 180 minwere used for PEs and PEs-g-graphene, respectively. After everylayer deposition, the sample was repeatedly immersed in puredistilled water 3 times to remove the excess of assembling ma-terials and then dried with bubbling nitrogen. This sequence wasrepeated to obtain the desired number of layers, which wasdesignated as {PDDA/PEs-g-graphene/PAAS}n.

Instruments and Measurement. Water bath sonication wasperformed with a JYD 1200 L sonicator (250 W). The averagemolecular weight and molecular weight distribution were deter-mined by a gel permeation chromatography (GPC) instrument(Waters Breeze, USA) with tetrahydrofuran (THF) as an eluant

(27) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.;Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458–4465.

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8067–8074.(32) Mamedov,A.A.;Kotov,N.A.; Prato,M.;Guldi,D.M.;Wicksted, J. P.;

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Chem. Soc. 2001, 123, 9451–9452.(34) Artyukhin, A. B.; Bakajin, O.; Stroeve, P.; Noy, A. Langmuir 2004,

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2255–2259.(38) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett,

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and narrow polystyrene as the calibration standard. A NetzschTG 209 apparatus was used for the thermogravimetric anal-ysis (TGA). The analysis was fitted to a nitrogen purge gas ata 10 �C/min heating rate. Before the tests, all the samples werecarefully grinded to powders to ensure sufficient diffusion ofheat. Raman spectra were recorded on a Dilor LABRAM-1Bmultichannel confocal microspectrometer with 514 nm laserexcitation. Scanning electronmicroscopy (SEM)was performedwith a Philips XL30 FEG FE-SEM instrument at an accelera-ting voltage of 25 kv. Samples were sputter-coated with 5 nm ofgold to improve contrast. Atomic force micrographs (AFM)were obtained using a Multimode Nano4 in the tapping mode.The samples were prepared by depositing their dispersions onHOPG substrates and allowing them to air dry. Fourier trans-form infrared spectroscopy (FTIR) spectra were recorded on aNEXUS 670 spectrometer. Transmission ultraviolet-visible(UV-vis) spectra were recorded with an APADA UV-1800PCSpectrophotometer. Spectra were acquired from 800 to 200 nmat a scan speed of 200 nm/min and a spectra resolution of 1 nm.

3. Results and Discussion

Just like CNTs and many other nanomaterials, a keychallenge in the synthesis and processing of bulk-quantitygraphene sheets is aggregation. Graphene sheets, due to theirhigh specific surface area, tend to form irreversible agglom-erates or even restack to form graphite through van derWaalsinteractions. The resulting graphene sheet agglomerates ap-pear to be insoluble in water and organic solvents, makingfurther processing difficult.

In the present study, charge-rich PAA and PAM werewrapped along the graphene sheets through in situ radicalpolymerization. Like CNTs, graphene nanoplatelets can beactivated by free radical initiators to open their π-bonds andthen participate in polymerization of monomers. Besides,graphene nanoplatelets will always have some defects onthem, which make them able to be activated much moreeasily. Since PAA and PAM have good water solubility, theyprevented graphene sheets from conglomeration and sedi-mentation. The Mn values of prepared PAA and PAM were1.11� 104 and 1.05� 104, respectively (prepared and tested in

a control experiment). The procedures used in this study wereshown in Figure 1.

TGA is a complementary technique that can reveal thecomposition and changes in thermal stability of the samples.Figure 2 shows TGA curves of EGO, reduced GO, PAMmodified graphene sheets (PAM-g-graphene), and PAAmod-ified graphene sheets (PAA-g-graphene). In agreement withprevious reports in the reference,16 althoughEGO is thermallyunstable and starts to lose mass upon heating even below100 �C, the major mass loss occurs at about 200 �C, presum-ably due to pyrolysis of the labile oxygen-containing func-tional groups, yielding CO, CO2, and steam (Figure 2a-1).On the other hand, the removal of the thermally labile oxy-gen functional groups by chemical reduction increased ther-mal stability for reduced GO (Figure 2a-2), showing 2%weight loss in nitrogen atmosphere at 700 �C. As for PEgrafted graphene nanoplatelets, PAM-g-graphene and PAA-g-graphene gave 65% and 82% weight loss at 700 �C,respectively (Figure 2a-3 and Figure 2a-4). Therefore, thecomposite contains graphene nanoplatelets and PAM (PAA),with weight ratios of 1/2 (2/9), demonstrating high efficiencyof the grafting polymerization. Furthermore, compared withthe net polymers (Figure 2b), the decomposition of polymersduring TGA experiments in the presence of graphene nano-platelets follows a different pathway. Besides, the onset ofdecomposition temperature for PE-g-graphene seems to belower than that for net polymers. This phenomenon is veryinteresting, and it indicates that the presence of graphenemayaffect the decomposition of the attached polymers. Themechanism and reason of this phenomenon are still underinvestigation.

Raman scattering is strongly sensitive to the electronicstructure of samples, and its result is often taken as evidencefor the chemical functionalization of graphite materials.15

Figure 3 shows typical Raman spectra of raw graphite,reduced GO, PAA-g-graphene, and PAM-g-graphene. It isobvious in Figure 3 that after functionalization the character-istic peaks of graphite, namely, the D band at 1330 cm-1 (thedisorder band) and the G band at 1580 cm-1 (the tangential

Figure 1. Procedures used in this study.

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vibration mode), have changed. After reduction, we discern agradual decrease of the D band, indicating the successfulremoval of the oxygen-containing functional groups fromGO. As to the bands of PAA-g-graphene and PAM-g-gra-phene, their G bands are broadened and slightly shifted to1590 cm-1. Besides, comparing with raw graphite, the ratiosof the intensities (ID/IG) for both PAA-g-graphene and PAM-g-grapheneweremarkedly increased, indicating the formationof the sp3 carbon after functionalization. Raman spectralresults described above agree well with those reported byStankovich and Xu,20,39 indicating that the covalent modifi-cation of graphene sheets by PEs was successful. In addition,this result is also consistent with TGA (Figure 2) and FTIR(Figure 6) data.

Figure 4 showsSEMimages of rawgraphite, EGO,PAA-g-graphene, and PAM-g-graphene. Compared with graphite(Figure 4a), the general structure of EGO (Figure 4b) is notgreatly changed, but the imageofEGO is distinguishable frompristine graphite by the appearance of bright regions lackingordered lattice features. They are most likely arising from thepresence of oxygenated functional groups. As for PAA-g-graphene and PAM-g-graphene, the samples also consist ofrandomly aggregated, thin, crumpled sheets closely associatedwith each other and formdisordered solids (Figure 4c and 4d).Besides, their structures seem to be flurry, which may bebecause of the attachment of the polymers.

AFM has been one of the most direct methods of quantify-ing the degree of exfoliation to graphene sheets level after the

dispersion of the powder in a solvent. Representative atomicforce microscopy (AFM) images of reduced GO, PAA-g-graphene, and PAM-g-graphene in tapping mode are shownin Figure 5. Well-dispersed graphene nanoplatelets were ob-served in Figure 5a. It was found that the average thickness ofgraphene nanoplatelets was 1.2 nm, leading to a conclusionthat complete exfoliation of graphene nanoplatelets down to1∼3 layers was achieved under these conditions since theintersheet distance for graphene sheets was about 0.34 nm.As for the PAA-g-graphene (Figure 5b) andPAM-g-graphene(Figure 5c), because of the wrapping of PAA and PAM ongraphene nanoplatelets, the average thicknesses of PAA-g-graphene and PAM-g-graphene become 2.0 and 1.8 nm,respectively. Since PAA-g-graphene and PAM-g-grapheneare expected to be “thicker” due to the presence of the sp3-hybridized carbonatoms slightly above andbelow the originalgraphene plane, we can expect that the heights of them areconsistentwith thedata of reducedGO. Inaddition, it is foundthat the sizes of PEs-g-graphene are smaller than those of thereduced GO. Since PEs-g-graphene can disperse much betterin solution, while reduced GO still tends to agglomeratethough undersonication, the object size of reduced GO de-rived from AFM analysis will be “enlarged”. Besides, areasof low functionalization are also seen at lower heights withsimilar color to that of the background. We attribute thevariability in the height to the local degree of functionaliza-tion, with high points indicating areas of the samples wherethe wrapping of PAA and PAM is especially dense, which isconsistent with the result of TGA.

To prepare LBL films, the uniform distribution of nano-fillers within thematrix is the essential structural requirement.Besides, it is also widely known that graphene sheets tend toform irreversible agglomerates or even restack to form gra-phite through van der Waals interactions. In this work, wetried to use PEs-g-graphene to solve this problem. It is foundthat thePEsneeded tobe present in sufficient concentration tocompete with the hydrophobic interaction between the gra-phene sheets. During the in situ radical polymerization pro-cess, PEs will covalently wrap themselves on the surface ofgraphene sheets, which disrupts the van der Waals interac-tions and leads to isolation of graphene sheets. Once enoughPEs are attached to the surface of graphene sheets, furtheragglomeration of the sheets will be stopped.

Figure 6 shows FTIR spectra of EGO (1), PAA-g-graphene(2), PAM-g-graphene (3), andPDDA/PEs-g-graphene/PAAS(4). In the spectrum of EGO, the peak at 1720 cm-1 is in

Figure 3. Raman spectra of raw graphite, reduced GO, PAA-g-graphene, and PAM-g-graphene.

Figure 2. (a) TGA of EGO (1), reduced GO (2), PAM-g-graphene (3), and PAA-g-graphene(4). (b) TGA of pure PAA and PAM.

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correspondence toCdO stretching vibration. In the spectrumof PAA-g-graphene, the strong peak of the CdO stretchingvibration can also be seen, while peaks at 3400 and 1420 cm-1

are related to the stretching vibration and deformation ofO-H. In the spectrumofPAM-g-graphene, peaks at 3450 and1650 cm-1 are due to the N-H stretching vibrations. As tothe spectrum of PDDA/PEs-g-graphene/PAAS, we can seethat peaks in the spectra of PAA-g-graphene and PAM-g-graphene still exist. Besides, we also observe the asymmetricC-H stretch of the alkyl group at 2960 cm-1 and the stretch-ing peak of C-OH at 1220 cm-1, which also proves the exis-tence of PDDA and PAAS. These results support that thepreparation of PEs-g-graphene is successful, and the LBLprocess is practicable.

SEM images of the films after one cycle (a) and three cycles(b) are shown in Figure 7. It is clearly seen that the first layerhada certain amount of graphene sheets (Figure 7a), revealingthe successful deposition of graphene nanoplatelets on thesubstrate. Random networks of sheets oriented parallel tothe surface were visible in the films, and the surface densityof sheets increased with the number of cycles. After moredeposition cycles (Figure 7b), PEs-g-graphene densely cov-ered the substrate. Besides, it can also be seen that the size ofthe sheets in the films is much larger than that in the solution(as suggested by AFM data). Since the density of graphenenanoplatelets in solution is high, they will connect with eachother and form these “flakes” during the absorption process,which “enlarge” the object size derived from SEM analysis.Given the overlapping morphology of the film, graphene

adsorption occurs as a result of the adsorbing graphenenanoplatelets interacting with multiple layers already on thesurface. As a consequence, a porous film, containing voidsbetween the adsorbed layers of graphene nanoplatelets, wasformed.

While FTIR and SEM were proved to be useful forcharacterizing initial film growth, determining whether PEs-g-graphene adsorption was occurring reproducibly was diffi-cult from them. The prepared filmswere also characterized byUV-vis spectroscopy as shown in Figure 8. Figure 8a showsthe increase in absorbance upon assembly of PEs-g-graphenein each layer on the glass slide, while Figure 8b shows thedependence of the absorption (at wavelength of 600 nm) onthe number of LBL cycles for the films containing graphenenanoplatelets. The clear increase in the absorbance with theassembly step is indicative of the film deposition on thesubstrate. In addition, the linearity between the absorbancewith the layer number (Figure 8b) further suggests that almostthe same amount of PEs-g-graphene was loaded in eachassembling step.

Compared with the system of CNTs in our last study,28

though the same assembly systems are used, the adsorption ofgraphene nanoplatelets is quite different. As found in the laststudy, the dependence of CNTs with the number of depositedlayers is nonlinear and exhibits a decayingmode.While in thesystem of graphene nanoplatelets, the dependence is linear.Themechanisms of the two systemsmaybe as follows: both ofthe systems contain two kinds of PEs-g-graphene (PEs-g-CNTs). One is strongly involved in interaction with PEs to

Figure 4. SEM images of raw graphite (a), EGO (b), PAA-g-graphene (c), and PAM-g-graphene (d) before sonication, with a concentration of0.1 mg/mL.

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Article Shen et al.

constitute the network of the multilayer, and the other kind isweakly bound to the PE surface (mobile PEs-g-graphene andPEs-g-CNTs) to form the film structure. The solution surface

thus acts as a perfect sink for themobile PEs-g-graphene (PEs-g-CNTs) in the film. The weakly bonded PEs-g-graphene(PEs-g-CNTs) constitutes the major “out” driving force. Onthe other hand, when exposed to different charged PEs, manyPEs-g-graphenes (PEs-g-CNTs) will absorb on the film, andthe strongly bonded part will be the major “in” driving force.Inside the film, theremust always be equilibrium between freeand bound PEs-g-graphene (PEs-g-CNTs). The exchangemechanism leads to continuous film restructuring.27 In thePEs-g-CNTs system,with the increase of the number of layers,though more and more MWCNT-PEs are deposited, the vanderWaals force between thembecomes stronger and stronger,leaving fewer and fewer strongly bonded PEs-g-CNTs on thesurface. With higher density (2 mg/mL) in solution, increasedelectrostatic repulsion between like-charged components willbe a compensation effect to reduce the number of adsorbedCNTs as the area of the film expands and the partial deso-rption of previously adsorbed CNTs will occur. As a result,the dependence of the thickness of the film is nonlinear andexhibits a decaying mode. While in the PEs-g-graphenesystem, the linear evolution of thickness increment can bedue to less density (0.1 mg/mL) and longer deposition time

Figure 5. AFM images of reducedGO (a), PAA-g-graphene (b), and PAM-g-graphene (c) solution after sonication,with a concentration of 0.01mg/mL. Image dimensions are 1 μm � 1 μm.

Figure 6. FTIR spectra of EGO (1), PAA-g-graphene (2), PAM-g-graphene (3), and PDDA/PEs-g-graphene/PAAS (4).

DOI: 10.1021/la900126gLangmuir 2009, 25(11), 6122–6128 6127

ArticleShen et al.

(denser structure will be formed with longer deposition time).The compensation effect of electrostatic repulsion and theincrement effect of longer deposition will have a balance.Finally, eachLBLadsorption step results in the formationof acontinuous monolayer, and the repetition of n depositioncycles yields a composite homogeneous film whose thicknessis proportional to n. Besides the different structure of gra-phene nanoplatelets andCNTs, the different densities of PAAand PAM on graphene nanoplatelets and CNTs may also bevery important factors for the different behavior. Furtherexperiments and more efforts are now needed to be able toprove the mechanisms of the two different systems.

4. Conclusions

We present a detailed study on the preparation and char-acterization of graphene nanoplatelet-based LBL films. Thegeneral concept applied in this paper is to arrange a broad

range of macromolecules into highly ordered architectures byself-assembly through electrostatic interactions on flat tem-plates. PAA-g-graphene and PAM-g-graphene, formingstable dispersion in water, were obtained by modifying gra-phene sheets with charge-rich PAA and PAM. Graphene-based multilayer LBL films were formed through sequentialadsorption of PE-functionalized graphene sheets (PEs-g-gra-phene) and other different charged PEs. Taken together,FTIR, SEM, and UV-vis analysis suggest that the increaseof PEs-g-SWCNTs in each assembly step is uniform. By usingthis process, important parameters such as size, geometry, andcomposition of the films can be readily controlled. All in all,this demonstration offers a new route to graphene-basedmultilayer films and composites. It would be reasonable toexpect that this method can use the powerful electrostaticassembly technique tomanipulate graphene nanoplatelets forcreating new and potentially useful nanosystems.

Figure 7. SEM images of the films after one cycle (a) and three cycles (b).

Figure 8. UV-vis spectraof {PDDA/PEs-g-graphene/PSS}nLBLassembled ona glass slide (a) and the absorbance at 385 nmversus the numberof adsorption cycles (b).

DOI: 10.1021/la900126g Langmuir 2009, 25(11),6122–61286128

Article Shen et al.