from the parent hydrocarbons is not likely, or at least not a

major process. Previously, Hu et al. stated that carbon radicals

and radical ions would be formed in the strong laser field, and

are likely the primary building blocks in the growth of polyyne

molecules. Lack of strong chain-length correlation between

the product polyynes and solvent molecules is consistent

with this assumption.

Acknowledgements

The authors are grateful to Prof. Walt Duley and Dr. Anming

Hu in University of Waterloo for helpful discussion. This work

is partly supported by a Grant-in-Aid for Scientific Research

(Nos. 20048007 and 21550021) from Japan Society for the Pro-

motion of Science (JSPS). J.S. thanks the support by the JSPS

‘‘Invitation Fellowship Programs for Research in Japan’’.

R E F E R E N C E S

[1] Duley WW, Williams DA. Interstellar polyynes from thedisruption of carbon grains. Mon Not R Astron Soc1984;211:97–103.

[2] Wakabayashi T, Murakami T, Nagayama H, Nishide D,Kataura H, Achiba Y, et al. Raman spectral features of longerpolyynes HC2nH (n = 4–8) in SWNTs. Eur Phys J D2009;52:79–82.

[3] Hu A, Lu QB, Duley WW, Rybachuk M. Spectroscopiccharacterization of carbon chains in nanostructuredtetrahedral carbon films synthesized by femtosecondpulsed laser deposition. J Chem Phys 2007;126:154705-1–5.

[4] Eastmond R, Johnson TR, Walton DRM. Silylation as aprotective method for terminal alkynes in oxidativecouplings: a general synthesis of the parent polyynesH(C@C)nH (n = 4–10, 12). Tetrahedron 1972;28(17):4601–16.

[5] Tsuji M, Tsuji T, Kuboyama S, Yoon S-H, Korai Y, Tsujimoto T,et al. Formation of hydrogen-capped polyynes by laserablation of graphite particles suspended in solution. ChemPhys Lett 2002;355(1–2):101–8.

[6] Cataldo F. Synthesis of polyynes in a submerged electric arcin organic solvents. Carbon 2004;42:129–42.

[7] Hu A, Sanderson J, Zaidi AA, Wang C, Zhang T, Zhou Y, et al.Direct synthesis of polyyne molecules in acetone bydissociation using femtosecond laser irradiation. Carbon2008;46:1823–5.

[8] Kandidov VP, Kosareva OG, Liu W, Becker A, AkozbekN, Bowden CM, et al. Self-transformation of a powerfulfemtosecond laser pulse into a white-light laser pulsein bulk optical media (or supercontinuum generation).Appl Phys B 2003;77(2–3):149–65 [and referencestherein].

[9] Wakabayashi T, Tabata H, Doi T, Nagayama H, Okuda K,Umeda R, et al. Resonance Raman spectra of polyynemolecules C10H2 and C12H2 in solution. Chem Phys Lett2007;433:296–300.

[10] Tabata H, Fujii M, Hayashi S, Doi T, Wakabayashi T. Ramanand surface-enhanced Raman scattering of a series of size-separated polyynes. Carbon 2006;44:3168–76.

Fig. 4 – The chromatograms (analytical HPLC) of the samples

prepared from n-hexane (upper) and n-decane (lower).

1676 C A R B O N 4 8 ( 2 0 1 0 ) 1 6 7 0 – 1 6 9 2

Hybrid Langmuir–Blodgett monolayers of graphite oxidenanosheets

Tamas Szabo a,b,*, Viktoria Hornok a, Robert A. Schoonheydt b, Imre Dekany a,*

a Department of Physical Chemistry and Materials Science, University of Szeged, Aradi v.t. 1, H-6720 Szeged, Hungaryb Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium

0008-6223/$ - see front matter � 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2009.12.050

* Corresponding authors: Fax: +36 62 544042.E-mail addresses: sztamas@chem.u-szeged.hu (T. Szabo), i.dekany@chem.u-szeged.hu (I. Dekany).

A R T I C L E I N F O

Article history:

Received 27 October 2009

Accepted 21 December 2009

Available online 28 December 2009

A B S T R A C T

Highly ordered monolayers of graphite oxide (GO) were assembled on hydrophilic sub-

strates by the Langmuir–Blodgett technique. AFM observations confirm that the film archi-

tecture can be controlled by the pH and GO concentration in the subphase. At pH 10, films

drawn over a 50 mg/L GO suspension consist of well-aligned, closely interlocked single or

bilayer graphene oxide particles without the presence of larger gaps or overlaps between

them.

� 2009 Elsevier Ltd. All rights reserved.

C A R B O N 4 8 ( 2 0 1 0 ) 1 6 7 0 – 1 6 9 2 1677

Deposition of ordered mono- and multilayered films of

amphiphylic molecules by the Langmuir–Blodgett (LB) meth-

od has been established for a long time. Not only molecules,

but also exfoliated particles of layered colloids may be assem-

bled to thin films. Examples are hybrid monolayers of swella-

ble clay minerals [1,2], delaminated nanosheets of layered

titanates [3] or layered double hydroxides [4]. Graphene is

the individual one-atom thick carbon layer of the graphite

structure with emerging potential applications in nanoelec-

tronics. Much of these applications, such as transparent con-

ductors, require large-scale production of graphene in the

form of thin films. As the LB method affords very thin films

(nanofilms) and high degree of structural order, it would con-

stitute an excellent platform for the preparation of graphene-

based conductive nanolayers. Cote et al. reported that LB

mono- and multilayers of graphene could be fabricated by

spreading a methanol/water dispersion of graphite oxide

(GO) over the water surface, followed by chemical reduction

of GO to graphene [5]. However, it is surprising that hydro-

philic GO particles form a stable monolayer at the air–water

interface. Here we present an alternative of the above proce-

dure, which relies on the adsorption of exfoliated GO sheets

in the subphase on a monomolecular layer of a cationic

amphiphylic dye (octadecyl ester of rhodamine B; RhB18).

The resulting hybrid monolayer formed at the gas/liquid

interface is then transferred to hydrophilic substrates.

GO used in this study was synthesized by the Brodie meth-

od. This sample is highly oxidized (C2O0.98H0.40), and it is iden-

tical with that codenamed as GO-4 in an earlier publication

[6]. One gram GO was wet-ground with 15 mL water in an

agate ball mill for 7 h and the slurry obtained was diluted with

ultrapure water to 25 or 50 mg/L. The pH of these dispersions

Fig. 1 – AFM images of hybrid RhB18/GO films assembled over 50

were either left unchanged (5.5 ± 0.2) or set to 7.5 ± 0.05 and

10.0 ± 0.05 by adding 2 M NaOH solution. After stirring at least

for 24 h with intermittent sonication, the pH was reset and

the dispersion was immediately used for monolayer deposi-

tion. LB films were assembled by a NIMA Technology model

611 LB trough on glass microscope cover slips (for AFM) or

quartz slides (for UV–Vis spectroscopy). The deposition proto-

col (Supplementary material Fig. S1) was as follows. The sub-

strates were immersed into the GO dispersion immediately

after cleaning (soaking in chromic acid followed by rinsing

with ultrapure water). Next, 40 lL of 1 mM RhB18 dissolved

in chloroform was spread from a microsyringe over the

subphase containing GO particles. After 20 min, the hybrid

monolayer was compressed with 20 cm2/min barrier speed

until the surface pressure reached 2 mN/m. After 30 min,

the film was transferred by lifting the substrate at a rate of

5 mm/min.

Fig. 1 shows AFM images of hybrid films deposited from

50 mg/L GO dispersions of different pHs. Brighter, irregularly

shaped features of various sizes ranging from several tens

of nanometers to several micrometers are seen on the image

of the film drawn from a pH 5.5 subphase (Fig. 1a). They rep-

resent relatively thick (up to 20–25 nm) aggregates of incom-

pletely exfoliated graphite oxide particles. One can also

observe stacks of thinner sheets, which form by random

adsorption on top of each other, resulting in considerable dis-

order and roughness (13 nm, or 2.7 nm without the five larg-

est aggregates). Remarkable differences in topography are

observed at pH 7.5 (Fig. 1b). First, the lamellar particles cover

very well the underlying substrate (seen as darker regions).

Second, the full height range (16 nm) and roughness

(0.6 nm) are much lower, indicating a more uniform packing

mg/L GO dispersions of (a) pH 5.5, (b) pH 7.5, and (c) pH 10.0.

Fig. 2 – AFM image of the RhB18/GO film prepared from

25 mg/L GO dispersion of pH 10.0 (top). The bottom panel

shows offset section profiles taken over lines (a), (b), and (c)

of the top image.

1678 C A R B O N 4 8 ( 2 0 1 0 ) 1 6 7 0 – 1 6 9 2

of nanosheets. However, dozens of double-layer structures

consisting of one or more lamellae on top of others were ob-

served over the 25 lm2 area. At pH 10, the topography is char-

acterized by densely tiled nanosheets and dark holes of

lower-lying empty areas (Fig. 1c). Since graphene oxide is

not hollow, the latter features indicate wider gaps that are

large enough to be resolved by AFM. These gaps appear due

to shape mismatch of platelets at certain locations, but nano-

sized trenches separating closely interlocking edges of plate-

lets cannot be visualized by the tip. This has two

consequences: (1) the contours of the particles are less readily

visualized, and (2) two or more well-aligned particles may ap-

pear as one. It is remarkable that overlapping particles were

not observed and the roughness of the film at pH 10

(0.35 nm) was very close to that of the bare glass surface

(0.25–0.3 nm). A collection of four images taken on other areas

of the same sample (i.e., at pH 10) are seen in Supplementary

material Fig. S2. They are similar to that displayed in Fig. 1c

confirming that the films are uniform over macroscopic

dimensions. Overlapped, wrinkled or back-folded layers (such

as the two marked with arrows on the top image) are seen

only at very few locations. The observed effect of pH on the

structure is explained by the fact that GO exfoliates only at

slightly alkaline pHs [6]. At the original, acidic pH the particles

do not disaggregate well. They adsorb on the surfactant layer

as stacked sheets, giving rise to rough hybrid films. However,

particles exfoliated at higher pHs are much thinner, and their

larger negative surface charge hinders the vertical stacking of

platelets.

Most of the gaps between GO particles in the close-packed

films are not wide enough to allow the tip to reach the under-

lying substrate at large areas. Probe artifacts may then occur,

making the interpretation of particle height profiles ambigu-

ous. We found that a better separation, i.e., a lower surface

coverage could be obtained by decreasing the GO concentra-

tion to 25 mg/L in the subphase. In Fig. 2, one observes that

the GO platelets are loosely packed with noticeable uncovered

regions between them. Height profiles were taken along the

5 lm long white lines. Height fluctuations of around 0.9–

1.2 nm are detected where these lines cross-over the particle

surfaces,1 with a rare exception as shown by line b, where a 2-

nm high feature is placed on top of 1-nm high one. The 1-nm

jump in the height was reported in many AFM studies of GO

platelets on various substrates (mica [7], HOPG [8], or SiO2/Si

[5]) measured either in contact [9], non-contact [7] or tapping

modes [5,8]. Thus, the 1-nm apparent thickness seems to be

quite general, and it is commonly attributed to fully exfoliated

single layers of GO (ca. 0.6 nm) and water molecules adsorbed

on them. However, the d-spacings of hydrated graphite oxides

range between only 0.7–0.82 nm at 40–50% relative humidi-

ties, under which, e.g., Paredes et al. [8] or Stankovich et al.

[7] determined the layer thicknesses. These XRD distances

are much closer to the AFM-thicknesses of 0.67 nm found

by Pandey et al. for elementary graphene oxide layers [10].

This implies that the 0.9–1.2 nm heights may refer to dou-

ble-layer GO particles.

1 More accurate determination of the platelet thicknesses is not psheets conform to corrugation of the substrate.

Fig. 3 compares the UV–Vis spectrum of the hybrid film

(50 mg/L, pH 10) with that of a RhB18 Langmuir monolayer

transferred from water onto quartz. The spectra of the sup-

ports were substracted to get those characteristic only for

the deposited films. For both films, there is an absorption

band centered at 570 nm with a shoulder around 530 nm.

These bands are, respectively, the 0–0 and the 0–1 vibronic

transitions of monomeric RhB18. A contribution of RhB18 di-

mers to the 530 nm band can however not be excluded [1].

The absorbances at 570 nm are almost the same, which

means that roughly the same amount of RhB18 molecules

are present in the films with or without GO. This allows us

to evaluate the contribution of GO to the total absorbance in

the UV region, simply by substracting the RhB18 spectrum

from that of the RhB18/GO hybrid. The substracted spectrum

fits very well to the spectrum of a GO dispersion with an

ossible as the glass surface is not atomically smooth and the GO

Acknowledgement

T. Szabo acknowledges support from the Magyary Zoltan

postdoctoral fellowship funded by the EEA and Norway

Grants. The authors are also thankful for the financial sup-

port from the Hungarian National Scientific Fund (OTKA)

Project Nr. 73307 and the bilateral agreement Flanders-

Hungary.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.carbon.2009.12.050.

R E F E R E N C E S

[1] Ras RHA, Umemura Y, Johnston CT, Yamagishi A,Schoonheydt RA. Ultrathin hybrid films of clay minerals.Phys Chem Chem Phys 2007;9:918–32.

[2] Szabo T, Wang J, Volodin A, van Haesendonck C, Dekany I,Schoonheydt RA. AFM study of smectites in hybridLangmuir–Blodgett films: Saponite, Wyoming bentonite,hectorite and laponite. Clays Clay Miner 2009;57:513–21.

[3] Akatsuka K, Haga M, Ebina Y, Osada M, Fukuda K, Sasaki T.Construction of highly ordered lamellar nanostructuresthrough Langmuir–Blodgett deposition of molecularly thintitania nanosheets tens of micrometers wide and theirexcellent dielectric properties. ACS Nano 2009;3:1097–106.

[4] Wang J, Ren XM, Feng XS, Liu SY, Sun DJ. Study of assembly ofarachidic acid /LDHs hybrid films containing photoactivedyes. J Colloid Interface Sci 2008;318:337–47.

[5] Cote LJ, Kim F, Huang J. Langmuir–Blodgett assembly ofgraphite oxide single layers. J Am Chem Soc 2009;131:1043–9.

[6] Szabo T, Tombacz E, Illes E, Dekany I. Enhanced acidity andpH-dependent surface charge characterization ofsuccessively oxidized graphite oxides. Carbon 2006;44:537–45.

[7] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA,Kleinhammes A, Jia Y, et al. Synthesis of graphene-basednanosheets via chemical reduction of exfoliated graphiteoxide. Carbon 2007;45:1558–65.

[8] Paredes JI, Villar-Rodil S, Solıs-Fernandez P, Martınez-AlonsoA, Tascon JMD. Atomic force and scanning tunnelingmicroscopy imaging of graphene nanosheets derived fromgraphite oxide. Langmuir 2009;25:5957–68.

[9] Hirata M, Gotou T, Horiuchi S, Fujiwara M, Ohba M. Thin-filmparticles of graphite oxide 1: high-yield synthesis andflexibility of the particles. Carbon 2004;42:2929–37.

Fig. 3 – UV–Vis spectra of GO/RhB18 monolayer deposited at

pH 10, 50 mg/L (blue) and RhB18 monolayer (magenta). The

substracted spectrum (black) is compared to that of a 10 mg/

L GO dispersion at pH 7 (light green); data points of the latter

were divided by 25 to fit in the same range. (For

interpretation of the references to colour in this figure

legend, the reader is referred to the web version of this

article.)

C A R B O N 4 8 ( 2 0 1 0 ) 1 6 7 0 – 1 6 9 2 1679

absorption maximum of 0.024 at 235 nm. This value can be

used to assess the average thickness of GO in the hybrid LB

film as detailed in the supplementary content. The average

thickness was found to be 0.99 nm, which is in good agree-

ment with the AFM height fluctuations. However, the problem

whether the hybrid films involve single or bilayered GO parti-

cles still remains elusive. This is because sheet heights mea-

sured by AFM are influenced not only by the real thickness,

but also by spurious contributions from the different tip-sam-

ple interactions [8] or improperly chosen free amplitude of

the cantilever and the amplitude setpoint [11]. In addition,

RhB18 molecules form hybrid RhB18-GO particles at the air–

water interface and thus, contribute to the measured height

of the particles. However, RhB18 molecules also fill the gaps

between the GO nanosheets in the films. As a consequence,

the measured height, which is the difference between the

top of the hybrid RhB18-GO particles and the top of the

RhB18 film, is still the height of the GO particle. For the pres-

ent LB films, it is thus more likely that bilayers of GO are

deposited because the UV spectroscopic assessment is free

of the above artifacts. Although it yields an average value,

the uniformity of the films are outstanding as shown by

AFM, so the average thickness should characterize well the

height of the individual GO platelets.

Our approach is capable of ordering 2D nanosheets by

lateral compression. Therefore, it is superior to drop-cast-

ing, dip coating, spin coating or self-assembly methods, in

which a random adsorption and alignment of particles oc-

curs. Although Cote et al. reported that ordered LB films

can be assembled by using pristine GO [5], the present hy-

brid monolayers show an important advantage over their

LB films: we have only observed a negligible amount of

imperfections in our films such as GO sheets folded back

at interconnecting edges or face-to-face aggregates. These

films are very promising for advanced electronic applica-

tions because very large areas can be covered by densely

tiled graphene oxide nanosheets, which can provide contin-

uous electrical pathways after reduction to conductive

graphene. LB films of chemically derived graphene may be

especially beneficial for the fabrication of optically transpar-

ent flexible circuits, where the use of indium tin oxide is

limited due to its rigidity and fragility. Further optimization

of this system for various demands may be achieved by

using size fractionated GO particles or functional layered

carbons with ion-exchange properties [12].

[10] Pandey D, Reifenberger R, Piner R. Scanning probemicroscopy study of exfoliated oxidized graphene sheets.Surf Sci 2008;602:1607–13.

[11] Nemes-Incze P, Osvath Z, Kamaras K, Biro L. Anomalies inthickness measurements of graphene and few layergraphene crystals by tapping mode atomic force microscopy.Carbon 2008;46:1435–42.

[12] Bourlinos AB, Georgakilas V, Zboril R, Bakandritsos A,Stassinopoulos A, Anglos D, et al. Pyrolytic formation andphotoluminescence properties of a new layeredcarbonaceous material with graphite oxide-mimickingcharacteristics. Carbon 2009;47:519–26.

Crystallization induced block copolymer decorationon carbon nanotubes

Wei-ru Wang a, Xu-ming Xie a,*, Xiong-ying Ye b

a Advanced Materials Laboratory, Department of Chemical Engineering, Tsinghua University, Beijing 100084, Chinab Department of Precision Instrument and Mechanology, Tsinghua University, Beijing 100084, China

A R T I C L E I N F O

Article history:

Received 17 August 2009

Accepted 31 December 2009

Available online 6 January 2010

A B S T R A C T

A unique noncovalent means to decorate block copolymers on carbon nanotubes (CNTs)

using a controlled polymer crystallization method is presented. Transmission electron

microscope observation and electron diffraction result demonstrated the surface function-

alization of CNTs with a crystalline–noncrystalline triblock copolymer poly(vinylcyclohex-

ane)-b-poly(ethylene)-b-poly(vinylcyclohexane) (PVCH-PE-PVCH), forming a novel nano-

hybrid epitaxial brush structure, which consists of a central CNT and disc-shaped folded-

chain lamellae of PE blocks with random coils of amorphous PVCH blocks surrounding

them.

� 2010 Elsevier Ltd. All rights reserved.

Despite from extraordinary mechanical properties and

thermal conductivity to unique electronic and optical proper-

ties [1] carbon nanotubes (CNTs) offer tremendous opportuni-

ties for the development of fundamentally new material

systems, the intrinsic poor dispersibility and processability

of CNTs have hindered their further use in the practical appli-

cations [2,3]. Accordingly, surface functionalization of CNTs

becomes an essential step [4–6]. Using polymer chains to

‘wrap’ CNTs is a versatile and effective way for CNT function-

alization. In particular, block copolymers (BCPs) may provide

a series of attractive noncovalent wrapping and decoration

methods for the functionalization of CNTs. These approaches

can be driven by distinct interactions between CNTs and poly-

mers including p-stacking, electrostatic interactions, and dec-

oration of CNTs with micelles [7–9]. Compared with

homopolymers, BCPs enhance the dispersibility and stability

of CNTs in a wider range of organic solvents and host polymer

matrices by means of the dual action: one block of the poly-

mer forms a close interaction with CNTs, while the other

block(s) provide the dispersibility and chemical compatibility

to the CNTs [10]. More attractively, the intriguing ability of

BCPs to self-assemble into ordered nanostructures brings us

an ideal nanoscale template for CNT alignment control, and

can be utilized further for fabrication of functional hybrid

materials and functional devices including flexible field emis-

sion display panels. Park et al. [11] reported a two-dimen-

sional alignment of CNT in a lamellar polystyrene (PS)

microdomain of PS-b-polyisoprene (PI) diblock copolymer. In

our previous work [12], a selective quasi one-dimensional

alignment of PS decorated CNTs in the cylindrical PS phase

of a microphase-separated asymmetric styrene–butadiene-

styrene (SBS) triblock copolymer was successfully realized.

The progresses give a new significance to the functionaliza-

tion of CNTs with BCPs and dispersion of CNTs in BCP matri-

ces. However, the challenge for the uniform dispersion of

CNTs in selective microdomains of BCPs and self-assembly

alignment in BCP ordered nanostructure is still huge.

Recently, Li et al. [13,14] achieved a periodical surface dec-

oration of CNTs using controlled crystallization of homopoly-

mers such as poly(ethylene) (PE) and nylon-66. The

0008-6223/$ - see front matter � 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2009.12.059

* Corresponding author: Fax: +86 10 62784550.E-mail address: xxm-dce@mail.tsinghua.edu.cn (X.-m. Xie).

1680 C A R B O N 4 8 ( 2 0 1 0 ) 1 6 7 0 – 1 6 9 2