journal of materials chemistry c - cnr€¦ · materials for opto-electronics applications. their...

Journal ofMaterials Chemistry C

FEATURE ARTICLE

Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article OnlineView Journal

Graphene–organ

AddRhDEeiaYtt

currently pursuing a joint PhD projUniversite de Strasbourg, France wHer current research interests archaracterization of graphene organ

aIstituto per la Sintesi Organica e la Foto

Ricerche, via Gobetti 101, 40129 Bologna, I

Cite this: DOI: 10.1039/c3tc32153c

Received 31st October 2013Accepted 2nd December 2013

DOI: 10.1039/c3tc32153c

www.rsc.org/MaterialsC

This journal is © The Royal Society of

ic composites for electronics:optical and electronic interactions in vacuum,liquids and thin solid films

A. Schlierf,ab P. Samorıb and V. Palermo*a

Graphene exhibits exceptionalmechanical, optical and electrical properties that are unfortunately accompanied

by poor processability and tunability of its properties. The controlled interaction of graphene with tailor-made

organic semiconductors (OSs) can offer a solution to solve these two problems simultaneously. The use ofwell-

chosen organic semiconducting molecules interacting with graphene enables optimal control over the

molecular self-assembly process forming low-dimensional graphene–organic architectures. Moreover, OSs

allow modulation of numerous physical and chemical properties of graphene, including controlled electrical

doping, ultimately making it possible to boost the performance of conventional organic electronic devices.

Significantly, the interaction of organic molecules with graphene is strong not only at short distances but it is

relevant also at longer distances, up to 30 nm. This feature article reviews some of the most enlightening

results in the field, giving an overview of the interaction between graphene and organic molecules, starting

from the simplest systems at the molecular scale, single molecules on single layer graphene in UHV, up to

mesoscopic, more complex systems i.e., thick interpenetrated layers of graphene–organic composites

embedded in working electronic or photovoltaic devices.

ndrea Schlierf received heriploma in Chemistry (master'segree) from the University ofegensburg, Germany. Duringer studies, she was awardedAAD (German Academicxchange Service) foreignxchange scholarship for a R&Dnternship on organic electronicst the Ricoh R&D center inokohama, Japan. Aer researchraining at Paul Scherrer Insti-ute, Switzerland, she isect with CNR Bologna, Italy andithin a Marie Curie Fellowship.e formulation, processing andic composite materials.

Paolo Samorı is DistinguishedProfessor (PRCE) and director ofthe Institut de Science etd'Ingenierie Supramoleculaires(ISIS) of the Universite deStrasbourg (UdS) & CNRS. He isalso a Fellow of the RoyalSociety of Chemistry (FRSC) anda junior member of the InstitutUniversitaire de France (IUF).He received his Laurea (master'sdegree) in Industrial Chemistryfrom the University of Bologna

in 1995 and his Ph.D in Chemistry from the Humboldt UniversityBerlin in 2000. He was previously a research scientist at Istitutoper la Sintesi Organica e la Fotoreattivita of the Consiglio Nazio-nale delle Ricerche of Bologna. His work has been awarded variousprizes, including the young scientist awards at EMRS (1998) andMRS (2000) as well as the IUPAC Prize for Young Chemists 2001,the Vincenzo Caglioti award 2006 granted by the AccademiaNazionale dei Lincei, the “Nicolo Copernico” award 2009 (Italy)for his discoveries in the eld of nanoscience and nanotechnology,the prix “Guy Ourisson” 2010 du Cercle Gutenberg, the ERCstarting grant 2010 and the Silver Medal of CNRS 2012.

reattivita – Consiglio Nazionale delle

taly. E-mail: [email protected]

bISIS & icFRC, Universite de Strasbourg & CNRS, 8 allee Gaspard Monge, 67000

Strasbourg, France

Chemistry 2014 J. Mater. Chem. C

http://dx.doi.org/10.1039/c3tc32153c

http://pubs.rsc.org/en/journals/journal/TC

Fig. 1 Schematic representation and scanning probe microscopyimages of different classes of carbon-based materials for (opto)elec-tronics: (a and b) small molecules,83 (c and d) polymers,84 (e and f)graphenes.42,85 Adapted with permission from ref. 42, CopyrightAmerican Chemical Society. Adapted from ref. 83 and 84 withpermission from Wiley VCH.

Journal of Materials Chemistry C Feature Article

Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

1. Introduction

Carbon-based systems such as small molecules, polymers,nanotubes and graphene are active components for manymaterials for opto-electronics applications. Their physical andchemical properties are tunable and strictly depend on theirprecise chemical structure and size. All these carbon-basedsystems possess an extended p-conjugated scaffold that can bedecorated with functional groups. Their functionalization canbe optimized in order to programme their self-assembly intohighly ordered supramolecular architectures, as driven bynon-covalent interactions such as p–p stacking, hydrogen-bonding, etc.

Small p-conjugated molecules (pyrenes, pentacenes, coro-nenes, polythiophenes, etc.) possess a well-dened electronicbandgap and a mono-dispersed nanometric size, the latterenabling molecular self-assembly into highly ordered low-dimensional architectures such as bers, nano-crystals oruniform monolayers (Fig. 1a and b). Polymers, conversely,feature a larger linear backbone of sp2 carbons, rangingbetween a few to hundreds of nm in length; unlike smallaromatic molecules, they tend to form more disorderedassemblies (Fig. 1c and d), but can have an elongated and highlyexible shape that allows high solubility and efficient transportof electric charge.

Carbon nanotubes (CNTs) and graphene are based on anaromatic honeycomb network reaching sizes up to a few tens ofmicrons (Fig. 1e and f). This exceptional molecular structureleads to extraordinary optical, electronic and mechanicalproperties that have triggered research activities in variouselds and resulted in the Nobel Prize in physics. However, twoof the greatest challenges that remain to be addressed in theresearch of carbon nanotubes and graphene are: (1) processthem using up-scalable approaches and (2) obtaining an ad-hocchemical functionalization to control their physical and

Vincenzo Palermo is the leader ofthe research unit “FunctionalOrganic Materials” of theNational Research Council ofItaly (CNR). He works on theproduction and nanoscale char-acterization of new materialsusing self-assembling moleculesand graphene. He is a member ofthe scientic committee of Euro-graphene and WP leader of theGraphene Flagship, the largeststrategic research initiative

launched by the European Union in 2013. He is a coordinator of theproject of the European Science Foundation, GOSPEL and of theInternational Training Network GENIUS. In 2012 he was awardedthe Lecturer Award for Excellence of the Federation of EuropeanMaterials Societies. In 2013 he was awarded the Research Award ofthe Italian Society of Chemistry (SCI).

J. Mater. Chem. C

chemical properties. CNT samples are usually a mixture ofsemiconducting and metallic tubes; graphene, while havingvery high charge mobility, is a semi-metal, a zero-bandgapsemiconductor, thus giving electrical devices with very poorIon/Ioff ratio.

While many OSs, thanks to their small size and exible sidechains, are easily sublimed in vacuum or processed from solu-tion, pristine graphene sheets have a strong tendency toundergo aggregation, in solvents or in polymer matrices, to givegraphitic clusters. Composites merging together the excellentproperties of graphene with the high tunability and process-ability of organic molecules are in great demand in differentresearch elds. For example, graphene could improve themechanical and electrical properties of commercial polymers,but the interaction between graphene and the polymer matrix isoen poor, and the graphene sheets tend to aggregate whenprocessed with polymers. Graphene is also an interestingsubstrate for biomedical applications, but its interaction withbiomolecules, as well as its biological activity, is stillcontroversial.1

In particular, many of the possible applications of grapheneare in electronics, but its zero-bandgap and poor processabilityon insulating substrates are major issues hindering thispotential technological development. Many groups are trying toopen a bandgap in graphene, for example by using graphenenanoribbons2,3 or stacked bilayers,4 to combine high chargecarrier mobility with high Ion/Ioff ratios in transistors.

Organic semiconductors (OSs) already have a well-denedand tunable bandgap. By merging together their properties withthose of graphene one may obtain a “dream material” for thesemiconductor industry, optimal for exible electronics appli-cations. However, even if small polyaromatics, conjugatedpolymers and graphene or nanotubes exhibit an sp2 carbonbased backbone, these different classes of materials cannot

This journal is © The Royal Society of Chemistry 2014


Fig. 2 (a) STM images of 1-pyrene butyric acid deposited on gra-phene/Pt (111) with superimposed molecular structure.7 (b and c) STMimages of a single domain of CoPc molecules on (b) G/SiO2 and (c)G/h-BN.13 (d) Schematic representation of a TbPc2 single moleculemagnet (SMM) on graphene.57 (a) Reprinted from ref. 7 with permissionfrom Elsevier, others adapted with permission from ref. 13 and 57,Copyright American Chemical Society.

Feature Article Journal of Materials Chemistry C

Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

always be blended together effectively enough to yield benecialeffects on the materials' properties.

The co-processing of graphene and organic semiconductormolecules remains a challenge and a very active eld ofresearch, with a particular aim to control and optimize theprocess of intermolecular charge transport, and exploit this tofabricate high-performance devices. This requires structuraland energetic tuning of the interfaces between the twocomponents to ensure efficient fundamental processes such ascharge exchange or exciton splitting in the nal material. A highnumber of published results are available, giving in some casesconicting outcomes. In light of this, an overview of this eld istimely and needed.

In the following sections, we will provide an extensive over-view of possible ways that graphene and organic semi-conductors can interact. To obtain a clear and useful picture ofthe state-of-the-art, we will use a “bottom up” approachdescribing the interaction of graphene with organic moleculesstarting from the simplest systems at the molecular scale, i.e.single molecules on single layer graphene in UHV, to themesoscopic, most complex materials, i.e. thick layers of gra-phene–organic composites embedded in working electronicdevices.

For each section, we will rst briey describe the structureand the production techniques of the graphene–organic system,and then provide details on how the interaction changes theproperties of graphene and/or of the interacting organic mole-cules. Given the wide extent of the eld considered and the highnumber of papers published on graphene every single week,this feature article is not intended to be exhaustive; we thusapologize to the authors whose work is not cited herein.

2. Graphene–organic interactions invacuum: single molecules on singlesheets

The simplest graphene–organic interaction one can imagine isthat of a single molecule adsorbed on a graphene sheet, invacuum, i.e. not surrounded by neither gas nor solvent mole-cules. Such systems can be routinely obtained in ultra-high-vacuum (UHV) chambers, by thermal evaporation of differentsmall molecules on graphene produced by chemical vapourdeposition (CVD)5 or by epitaxial growth on silicon carbide.6 Amain advantage of molecules deposited on graphene is that thespatial arrangement and electronic properties of the molecule–graphene interaction can be studied simultaneously with highresolution by means of Scanning Tunneling Microscopy(STM, Fig. 2a).

It is well known that small conjugated molecules can adsorbon solid and at sp2-carbon surfaces forming highly orderednanostructures, similar to 2-dimensional crystals. Thisapproach is thus very useful to create a periodic electronicmodulation of the graphene surface through self-assembly ofaromatic molecules. Many different classes of molecules canform robust, stable physisorbed self-assembled monolayers ongraphite or graphene, from simple alkanes to phthalocyanine,


benzenes, anthracene, pyrenes and perylenes, to larger poly-cyclic aromatic hydrocarbons such as coronene derivatives.7

Generally, themorphology of the self-assembled architectureis driven by a complex interplay between non-covalent inter-molecular and molecule–substrate interactions.8 In the case ofpolyaromatic molecules such as most organic semiconductors,p–p interactions play of course a signicant role in governingthe molecular orientation and packing. Polyaromatic moleculescan be also considered as “nanographenes”,9 given that theiraromatic core is like a small 2D fragment of graphene, and thushave a strong tendency to stack over each other and over gra-phene with a stacking distance of ca. 3.3 A, i.e. similar to theinterlayer distance in graphite.

Overall, several results indicate that nucleation, orientation,and packing of OSs on graphene are different compared tothose grown on conventional substrates such as silicon or evengraphite.10–14

A detailed theoretical analysis of the adsorption of neutralmolecules on graphene was performed by Mat Persson's groupapplying density functional theory.15 It revealed that for neutral(poly)-aromatic, antiaromatic, and more generally for p-conju-gated systems, purely dispersive forces drive the docking ofadsorbates on graphene, whereas short-range electrostaticinteractions ultimately stabilize the complex.

However, if the aromatic backbone of a molecule is deco-rated by side groups exposing well dened functionalities, thelatter can work in synergy or in opposition to the p–p interac-tions between the adsorbed molecules and graphene, leading tomore complex self-assembly pathways. Functionalization of the

J. Mater. Chem. C



Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

aromatic core gives rise to strong medium-range interactionsinvolving the substituents: e.g., ab initio simulations revealedthat derivatives of benzoic acids serve as sub-units for thecontrolled formation of ordered nanopatterns on graphiticsurfaces. Three types of molecule–molecule or molecule–substrate interactions control the nature of the ultimateassembly: (1) long-range repulsive p–p interactions (0.10–0.30 eV), (2) medium-range attractive p–p* interactions (0.30–0.60 eV), and (3) short-range hydrogen bonding (0.60–0.80 eV),the latter giving strong directional intermolecular interactionsthat position adjacent molecules in predictable ways.16

These strong intermolecular interactions on graphene havebeen observed experimentally in continuous uniform layers ofperylenes bearing carboxyl groups (perylene-3,4,9,10-tetra-carboxylic dianhydride).14,17,18 An extensive study by STM, X-rayreectivity and modelling indicates that such molecules form ahighly uniform and coherent layer of molecules, parallel to thegraphene surface but weakly interacting with it, held togetherby hydrogen bonds. Perylene layers adsorb on the graphenesurface with characteristic p–p stacking bond lengths, thusindicating weak interaction with the graphene surface.

One can expect molecules with a greater electron acceptor ordonor character to interact more effectively with graphene; infact, another example of directed, self-assembled layers ongraphene describes the epitaxial growth of peruoropentaceneyielding ap-stacked arrangement of coplanar molecules with anexceedingly low p-stacking distance of 3.07 A, among the lowestp-stacking distance ever reported for organic semiconductorcrystal lattices, likely due to the effect of uorine side-groups.This low stacking distance gives rise to signicant electronicband dispersion along the p-stacking direction.19

Interfacial dipole interactions induced by charge transferbetween copper phthalocyanine (CuPc) molecules and gra-phene are shown to epitaxially align the CuPc molecules in aface-on orientation in a series of ordered superstructures onCVD graphene.10 Furthermore, the assembled structure can beused as a molecular probe to visualize graphene's domainboundaries as Ogawa et al. demonstrated for chloro-aluminumphthalocyanine on CVD graphene.11 In another example, Gao'sgroup demonstrated that monolayer graphene can act as atemplate for fabrication of unique nanoarchitectures. Amolecular layer of magnetic iron(II)-phthalocyanine wasdeposited on a single crystal, monolayer graphene grown viachemical vapour deposition. The iron(II)phthalocyaninearranged in regular Kagome lattices on the surface, followingthe lattice of the graphene moire pattern. A monolayer of gra-phene is thus sufficient to direct the self-assembly of moleculesand to decouple the adsorption process of the phthalocyaninederivative from the underlying metal surface.12

Slightly different results have been obtained on grapheneand graphite14 indicating that, while graphene can templateeffectively molecule adsorption, it does not shield completelythe molecules from the underlying substrate, which shallanyhow inuence the nal structure.

The inuence of the substrate underlying graphene was infact demonstrated for cobalt phthalocyanine (CoPc) on gra-phene13 (Fig. 2b and c) transferred onto silicon dioxide (SiO2) or

J. Mater. Chem. C

hexagonal boron nitride (BN). With both underlying substrates,CoPc molecules formed a square lattice on the graphene layer.However, with graphene on SiO2, the domain size was limitedby the surface corrugation due to SiO2 atomic roughness andchemical inhomogeneity, and there was a degree of disorder inthe molecular ordering within the domains which was notobserved for graphene on BN. Additionally, Scanning TunnelingSpectroscopy (STS) measurements revealed that the energy ofthe orbitals shied from one molecule to another, and that thiseffect was stronger on graphene/SiO2 than on graphene/BNhybrids.

In graphene/SiO2 the substrate underlying the graphenelayer does not only govern disorder in the self-assembled cobaltcentred phthalocyanine layer, but also inuences the system'selectronic properties.

Interestingly, the molecule–graphene interaction could bedramatically enhanced by introducing metal atoms obtainingmolecule/metal/graphene sandwich structures.20

3. Graphene–organic interactions inliquids: supramolecular self-assembly,stabilization and processing

Graphene processability is key for large-scale exploitation ofgraphene's unique properties. To this end, soluble graphene–organic hybrid systems have a clear advantage over CVD orepitaxial graphene because of the tunability of their propertiesand low cost.21

Graphene–organic suspensions can be processed with largearea deposition techniques in coatings, screen or ink-jet printedelectronic devices22,23 allowing embedment of graphene intofunctional composite materials.24

Graphene suspensions are typically obtained by liquid-phaseexfoliation (LPE) in a suitable organic solvent, frequentlyassisted by ultrasonication treatment.25–27 Aromatic cores basedon anthracene,28 pyrene,29–31 perylene32,33 and coronene34 withside functionalization to tailor solubility and electronic prop-erties have also been employed as agents to promote grapheneliquid phase exfoliation and stabilize the exfoliated sheets.Table 1 shows some examples of how the strong interaction ofgraphene with organic molecules has been used to exfoliate andstabilize graphene in solution. The surfactant effect ofaromatics was rst demonstrated for aqueous dispersions ofreduced graphene oxide (RGO); by means of non-covalentinteractions water soluble, sulphonated derivatives of pyreneand perylenediimide can efficiently stabilize the RGO sheets inaqueous suspension, while aqueous suspensions of RGOundergo heavy agglomeration and precipitation.32

More interestingly, the approach of non-covalent function-alization works also for pristine, non-oxidized graphene;graphite is directly transferred into a solution of a smallaromatic exfoliation agent in a process assisted by ultra-soni-cation (Fig. 3). Dong et al. showed that the aromatic moleculetetrasodium 1,3,6,8-pyrenetetrasulfonic acid (TPA) can effec-tively exfoliate graphite into graphenemonolayers by sonicationin aqueous solutions, yielding up to 90% of graphene single



Table 1 Examples of published studies reporting graphene–organic interactions to exfoliate or stabilize graphene in solution

Use of graphene–organic semiconductors interactions for graphene production and solubilisation

Precursor Molecule Solvent Ref.

Graphite Sulphonated and hydroxylatedpyrene

Water 30,31,37 and 55

1,3,6,8-Pyrene-tetrasulfonic acidtetrasodium salt (Py-(SO3)4)aminomethylpyrene (Py-Me-NH2)

Water 20

1-Pyrenecarboxylic acid MeOH/water 29Perylene-based bolaamphiphiles Water 86 and 877,7,8,8-Tetracyanoquinodimethane(TCNQ)

Water 88

9-Anthracene carboxylic acid (ACA) Water 28Sodium cholate (SC) Water 89 and 90Sodium dodecylbenzene sulfonate(SDBS)

Water 91

Ethyl cellulose ETOH 23Ionic and non-ionic surfactants Water 17N,N-Dimethyl-2,9-diaza-peropyrenium dication (MP2+)

Water 36

N,N-Dimethyl-2,7-diazapyrene (DAP$2Cl)

Water 36

Hexadecyltrimethylammoniumbromide (CTAB)

DMF 92

5,10,15,20-Tetraphenyl-(4,11-acetylthiounde-cyl-oxyphenyl)-21H,23H-porphine (TATPP)

NMP (with org. Ammonium ions) 93

Expandable graphite Tetrabutylammonium hydroxide +oleum + 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-(methoxy(poly-ethyleneglycol)-5000)(DSPE-mPEG)

DMF 94

Thermally exfoliated graphite oxide Coronene tetracarboxylic acidtetrapotassium salt

Water 34

Arc evaporation of graphite Coronene tetracarboxylic acidtetrapotassium salt

Water 34

Reduced graphene oxide 3,4,9,10-Perylenetetra-carboxylicdiimide-bis-benzenesulfonic acid(PDI)

Water 32

Pyrene-1-sulfonic acid sodium salt(PyS)

Water 32

Pyrene-block copolymer polyPA-b-polyPEG-A

Water or DMF 38

Carbon nanotubes Bolaamphiphilic or amphiphilicperylene bisimides

Water 33


Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

layers.35 Besides sulphonated pyrene and perylenediimidederivatives, stabilization of graphene in aqueous suspensionwas successfully demonstrated with carboxyl functionalizedcoronene34 and a cationic aza-pyrene and aza-perylenederivative.36

Direct exfoliation of graphite to graphene in aqueous mediawith organic dyes involves a complex process that requires notonly a strong graphene–molecule interaction, but also well-dened kinetics of the adsorption process and solvent choice.In particular, the adsorption of molecules on grapheneproceeds by a “sliding” mechanism through the solvent media(Fig. 3c); a key step to increase interaction is to displace the lastlayer of solvent molecules from the graphene surface, as wedemonstrated combining experimental exfoliation data withmodelling (see our studies31,37 and references therein).


The choice of suitable aromatic systems that non-covalentlyinteract with graphene has a triple bonus: (i) improves the yieldof exfoliation in liquid-phase exfoliated graphene, (ii) stabilizesthe graphene suspension, and (iii) confers new physical andchemical properties to graphene.

Specic interactions between graphene and an aromatic coredo come into play not only in the case of small polyaromaticmolecules, but also if the aromatic system is incorporated into apolymer chain. In this regard, pyrene-functionalized amphi-philic block copolymers can drive exfoliation of graphene ineither aqueous or organic media to give composite lms withimproved tensile strength and tunable conductivity.38

The non-covalent interaction with graphene and organicdyes involving a conjugated p-system can be controlled byvarying the aromatic core and functionalization; this allows

J. Mater. Chem. C


Fig. 3 (a) Picture of different pyrene–graphene solutions in water after sonication with graphite. (b) UV/Vis absorption spectra of the suspensions(samples diluted 1 : 20 for measurement). (c) Snapshots of molecular dynamic simulations of pyrene sulphonate molecules adsorbing on gra-phene in water. Reproduced from ref. 85 with permission from The Royal Society of Chemistry.


Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

systematic ne-tuning of the electronic properties of graphene.As an additional advantage, the exceptional optical properties ofthese organic molecules (strong and well-dened absorptionspectra in the visible spectrum, efficient light-emission, etc.)make it possible to use optical spectroscopy as an easy methodto investigate the interactions between aromatic molecules andgraphene (Fig. 3b).

4. Effect of graphene–organicinteractions on electronic, optical andmagnetic properties

Before describing the most complex systems used in devices, wedetail a bit more the different possible effects of OSs on gra-phene. As mentioned in the introduction, organic moleculescan be used to modify, in certain cases signicantly, the prop-erties of graphene, and vice versa. Table 2 reports a list ofdifferent molecules that have been employed to interact with/modify graphene along with the underlying mechanism and theeffects obtained.

Doping

The adsorption of molecules such as anthracene, naphthaleneand pyrene on graphene has a noteworthy effect on grapheneelectronic properties, inducing p- or n-type doping as demon-strated indirectly by Raman spectroscopy.39 G-band splittingwas observed in these systems, and explained by liing of thetwo-fold degeneracy of the optical phonons at the G point.35

Larger aromatic rings such as anthracene and pyreneproduce wider G-band splitting (z23 cm�1) than smaller ringssuch as naphthalene (z20 cm�1). This kind of doping wasobserved also monitoring charge transport in transistors,7,39 aswill be described in the following section.

J. Mater. Chem. C

From a theoretical point of view, Yong Peng's groupaddressed the binding of organic donor, acceptor and metalatoms on graphene sheets, and revealed the effects of differentnon-covalent functionalizations on the electronic structure andtransport properties of graphene.20 Their simulations suggestthat strong hybridization between the molecular levels and thegraphene valence bands can be achieved through adsorption ofparticular OSs such as 2,3-dichloro-5,6-dicyano-1,4-benzoqui-none (DDQ) and tetrathiafulvalene (TTF).

p-Doping and n-doping were monitored by combinedRaman and X-ray photoelectron spectroscopy (XPS) on anoxidized few layer thick graphene combined with tetracyano-ethylene (TCNE) and tetrathiafulvalene (TTF) molecules,observing shis in S 2p and N 1s XPS peaks for the two mole-cules.40 This kind of dopant layer can be very stable, and bepreserved in air up to 200 �C.41

Fluorescence quenching by charge or energy transfer

The interaction between a light-emitting dye and graphene isinteresting from a fundamental point of view (because it can beconsidered as the interaction between a 0D small emitter and a2D semi-innite quencher), and from a technological point ofview (for applications in LEDs, photovoltaics, sensors, etc.).

Fluorescence quenching and excited energy transfer fromdyes to graphene have been demonstrated with reduced gra-phene oxide, carbon nanotubes, and pristine graphene33,42–44

(Fig. 4).The interaction between uorescent emitters and graphene

is associated with local dipole-induced electromagnetic eldsthat are strongly enhanced due to the unique properties ofgraphene: graphene is an extraordinary energy sink with greatpotential for prospective application in photo-detection, nano-photonic and photovoltaic devices.



Table 2 Examples of published studies reporting different mechanisms of graphene interaction with organic dyes

Different types of graphene–organic semiconductors interactions

Effect Mechanism Carbon allotrope Molecule Solvent Ref.

Emissionquenching

Energy transfer Mechanically cleavedgraphene

Rhodamine Solid 47

Charge or energy transfer Graphene oxide Quaterthiophene Solid 42Energy transfer Reduced graphene oxide P3HT and MEH-PPV Solid 95Charge transfer Amino-functionalized

graphenePyrenebutanoic acid succinimidylester (PyBS)

DMF 49

Charge transfer Amino-functionalizedgraphene

Oligo(p-phenylenevinylene)-methylester (OPV-ester)

CHCl3 49

Charge or energy transfer Few layer graphene (LPE) Dendronized perylenebisimide NMP 44Charge or energy transfer Graphene (in situ LPE) MP$2Cl, DAP$2Cl Water 36Phonon transfer Reduced graphene oxide

(RGO)Sulphonated pyrene derivatives,HPTS and DHPDS

Water 55

Energy transfer and partial chargetransfer

Carbon nanotubes MWNT Poly(phenylene-vinylene) (PPV) Solid 96

Charge and energy transfer Graphene oxide Covalently bound amine-functionalized porphyrin

DMF 48

Optical powerlimiting

Charge transfer complex Epitaxial graphene Tetracyanoquinodimethane (TCNQ) Solid 56

Photo-catalysis Broad-band absorption, chargetransfer

RGO and GO P3HT, covalently graed on (r)GO CHCl3 68

Self-assembly Supramolecular interactions CVD graphene Phthalocyanine derivative Solid 10–13Supramolecular interactions CVD graphene Terephthalic acid (TPA) Solid 97Supramolecular interactions CVD graphene Peruoropentacene Solid 19Supramolecular interactions CVD graphene Oligothiophene Solid 75

Fig. 4 (a) Images of GO–oligothiophene covalent composites inEtOH at neutral pH, upon acidification with HCl and re-neutralizationby triethanolamine (TEA) (from left to right) under normal light (top)and UV irradiation (bottom) showing the reversible emission switch.54

(b and c) Optical fluorescence image of GO monolayers deposited (b)over and (c) below a fluorescent OS layer, showing strong fluores-cence quenching.42,64 (d) Decay rate enhancement in the graphene–rhodamine system, obtained from the lifetime measurements as afunction of graphene–rhodamine distance.47 Reproduced from ref. 54and 64 with permission from The Royal Society of Chemistry. Adaptedwith permission from ref. 42 and 47, copyright 2009 and 2013,American Chemical Society.



Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

Quenching of pyrene emission through energy transfer tographene is proportional to graphene–pyrene distance d. Therate of energy transfer proportional to d�4 has been calculated,suggesting that quenching should be observable for distancesup to 30 nm, much larger than what observed for any othersurface.45,46 The d�4 distance dependence on the quenching ofemitters with monolayer graphene was conrmed experimen-tally with rhodamine molecules by Koppens et al.47 (Fig. 4d). Inthis work the emitter lifetimes were also measured as a functionof emitter–graphene distance d, revealing agreement with auniversal scaling law governed by the ne-structure constant.On graphene, the emitter decay rate exhibited a 90-foldenhancement (corresponding to an energy transfer efficiency of99%) with respect to the decay in vacuum at distance d ¼ 5 nm.The high energy transfer rate was associated mainly with thetwo-dimensionality and gapless character of monolayer gra-phene.47 Thin layers of reduced graphene oxide and porphyrinshowed uorescence quenching and superior optical limitingeffects, better than the benchmark optical limiting material C60

and the control sample.48

Hirsch and co-workers demonstrated the binding and electronicinteractions of pristine single- and few-layer thick graphene with anorganic dyemolecule in homogeneous solutions.44 Electronic cross-talk with dendronized perylenebisimides and graphene suspendedin NMP was conrmed spectroscopically by photoluminescenceand Raman measurements, conrming experimentally an efficientuorescence quenching with graphene. The emission quenchingby energy or electron-transfer mechanisms provides clear evidencefor the non-covalent binding of the two p-systems.

J. Mater. Chem. C



Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

Quenching with graphene in liquid phase has beenexplained with the occurrence of both energy and electrontransfer, according to the system under study. Investigations onthe uorescence quenching of graphene with two organic donormolecules, pyrene butanoic acid succinimidyl ester (PyBS) andoligo(p-phenylenevinylene)methylester (OPV-ester) have beencarried out by Matte and co-workers.49 They did not use pristinegraphene, but amide-functionalized graphene soluble in chlo-roform (CHCl3) and dimethylformamide (DMF). For the twomolecules selected, the absorption and photoluminescencespectra were recorded in mixtures with graphene at increasingconcentration of the latter component. The emission spectra ofthe molecules in a solution with graphene did not show anyadditional bands that could be ascribed to a charge-transfercomplex. Unlike the absorption spectra, the emission markedlychanges with the increasing graphene concentration, resultingin a dramatic loss in emission intensity. Given that no changewas observed in absorption, Matte et al. attributed the strongemission quenching to photo-induced electron transfer, anexcited state phenomenon. The ability of graphene to quenchthe uorescence of these aromatic molecules was attributed tophoto-induced electron transfer on the basis of uorescencedecay and time-resolved transient absorption spectroscopicmeasurements. The occurrence of energy transfer could not beentirely excluded, but the main effect is attributed to photo-generated, charge-separated species in which graphene acts asan acceptor. These charge-separated species are long-lived andthus interesting for the design of photovoltaics.

A more stable, irreversible graphene–molecule interactionis obtained by direct covalent graing of organic semi-conductors such as porphyrins,48 dendrons50 fullerenes51 oroligothiophenes to graphene.52–54 In this case, however, thequality of the resulting material is lower, because the graingcreates a defect on the graphene sheet, or because the graingis directly performed on already defective graphene oxide. Thecovalent “tethering” of a molecule to graphene anyhow givesinteresting systems, because the optoelectronic properties arecontrolled not only by p–p or electrostatic interactions, butalso by the length and exibility of the covalent linker. Forexample, signicant uorescence quenching (>60%) can beobserved when oligothiophene dyes are tethered to GO usingshort linkers (ca. 0.7 nm).53 Conversely, by using a long andmore exible linker, smaller quenching (<16%) is monitored,along with an absence of the perturbation of the most delicateoptical properties of the dye, like pH-dependent light-emission.54

Photo-induced interactions of graphene–organic hybridscan proceed as well through the transfer of phonon energy:since a phonon is a quantum mechanical description of aspecic type of vibrational motion, the enhancement of thephonon energy transfer will increase the vibrational energy of aphonon acceptor. Pan and co-workers demonstrated phononenergy transfer to reduced graphene oxide with water-soluble,sulphonated pyrene derivatives in solution. Phonon transferwith water soluble systems is of particular interest as it allowsfor prospective medical application in photo-thermaltherapy.55

J. Mater. Chem. C

Magnetization

The adsorption of the OS can also confer a magnetic function tographene, complementing its outstanding electronic, mechan-ical and optical properties. A charge-transfer complex characterhas been observed with graphene–organic structures based onmagnetic functional layers adsorbed on graphene.56 Withadsorption of TCNQ on rippled graphene/Ru(0001), each iso-lated molecule acquires charge from graphene, developing alocalized magnetic moment and showing a prominent Kondoresonance. On completion of the TCNQ monolayer, the delo-calization on a spatially extended band preserves a commonorientation of the electron spins, as revealed by spin-polarizedscanning tunneling microscopy. Thus, a magnetically ordered,2D organic layer can be created by electron transfer from gra-phene/Ru(0001) to the strong acceptor TCNQ, similar to what isobserved in intermolecular charge-transfer complexes. Asorganic metals andmagnets rely on charge-transfer processes tocreate either the metallic character or the magnetic properties,these ndings open a pathway towards applications of gra-phene–organic hybrid materials in spintronics: the TCNQmonolayer could act as a spin lter or 2D spin polarizer, addingmagnetic functionalities to graphene by altering the spinpolarization of a current owing in graphene.

In another nice study, a magnetoconductivity signal as highas 20% was obtained in graphene nano-constrictions decoratedwith pyrene-substituted terbium(III)bis(phthalocyanine)quantum magnets (TbPc, Fig. 2d). In this approach, pyrenemoieties are used to control the adsorption of the TbPc ongraphene that becomes sensitive to the magnetization of theterbium-phthalocyanine molecules.57

In addition, the nding that the adsorption of benzene turnsthe magnetic Fe/graphene into nonmagnetic Fe/graphene mayhelp to design novel magnetic sensing or switching devices.20

5. Graphene–organic interactions insolid: thin films and electronic devices

Besides fundamental studies in vacuum or solutions, grapheneinteraction with organic semiconductors can be exploited inbulk systems consisting of OS/graphene blends. These bulkcomposite systems are particularly relevant for applications inorganic eld-effect transistors (FETs), photovoltaics andsensing devices (Fig. 5). Table 3 shows some examples of howgraphene–OS interactions have been used to produce workingelectronic devices.

To better evaluate the possibilities offered by graphene–organic FETs, we should also mention briey the numerousinteresting results obtained with carbon nanotubes.

Nanotube bundles can be effectively employed to templatethe growth of organic crystals under certain experimentalconditions, resulting in the formation of organic nucleates withpreferred orientations. Incorporation of an appropriate amountof random bundles of single-walled nanotubes in pentacene orsexithiophene lms was observed to give 20-fold enhancementin eld-effect mobility without reduction of the Ion/Ioff ratio.58



Fig. 5 (a) Work function of nickel, RGO and P3HT in an RGO-P3HT transistor. Arrows schematize the charge transport across the device, fromsource to drain, passing across different RGO sheets and P3HT domains. The image shows the experimental WF values as measured by KPFM.The inset shows a schematic representation of the device.64 (b) Schematic structure of a transistor using a modified perylene (PTCDI-13) as anactive layer and graphene as the source/drain.74 (c–e) Examples of energy diagrams of graphene interacting with metal, conductive oxides andOSs, from ref. 72–74 respectively. Reproduced from ref. 64 with permission from The Royal Society of Chemistry. Adapted with permission fromref. 73 and 74, Copyright American Chemical Society.


Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

High mobility, air-stable organic transistors have been real-ized from sequential deposition of high conductivity carbonnanotubes beneath an organic semiconductor (HBC); in thisway the transconductance and charge carrier mobility of thesemiconductor have been signicantly improved. The effectivemobility was increased by a factor of 6 while preserving theIon/Ioff ratio above 104.59

The use of carbon nanotubes as all-carbon electrodes inte-grated in both nanoscale and thin-lm pentacene transistorsmade it possible to improve the charge injection at the elec-trode–semiconductor interfaces, leading to increased deviceperformances when compared to devices based on traditionalnoble metals as electrodes.60

However, studies on CNTs showed that they do not act just aspreferential, highly conductive additives to enhance themobility ofOSs; the electronic processes within such devices are morecomplex, with a key effect on charge transport characteristics of theinterfaces present in the devices. Charge trapping at the dielectric/polymer (e.g. polythiophene) interfaces has been used in transis-tors based on carbon nanotubes for sensing applications.61 In adifferent study, optically gated FETs based on CNTs coated withpoly(3-octylthiophene-2,5-diyl) (P3OT) have been realized in whichthe switching mechanism is due to trapping of photo-excitedelectrons at the polymer/dielectric interface. Sufficient power andselection of an eligible wavelength induces a change in conduc-tance upon illumination, up to four orders of magnitude.62 Allthese approaches anyhow have to deal with the well-known nega-tive aspects of CNTs such as the mixture of properties (semicon-ducting & metallic) typical in CNT batches. The same approachescan be exploited using graphene, having better processability anddifferent dimensionality as compared to 1D nanotubes.


FETs based on pristine graphene typically show very highcharge mobility (>105 cm2 V�1 s�1), but a very poor Ion/Ioff ratio.Achieving high mobility while maintaining a high Ion/Ioff ratio isthe greatest challenge to allow development of graphene-based(exible) electronics; a viable approach intensively exploredrelies on the incorporation of graphene into solution processedorganic eld-effect transistors (OFET). The addition of anorganic semiconductor can bring into play new physicalprocesses like the controlled doping of graphene.

The effect of adsorbed molecules on graphene doping andrelated charge transport was explored by drop casting a solutionof either pyrene, naphthalene or anthracene derivatives on a“scotch tape” graphene layer (as mentioned above). Shis inthreshold voltage from �108 V to +62 V could be observed inthis way, together with signicant changes in Raman spectraindicating signicant doping.39

Transistors based on CVD graphene were realized in a recentstudy with self-assembly of solution processed triethylsilyle-thynyl-anthradithiophene (TES-ADT). TES-ADT displayed astanding-up molecular assembly, which facilitates lateralcharge transport on graphene.63

Blends of reduced graphene oxide microsheets and OSpolymers such as poly-3-hexyl thiophene (P3HT) and poly-(p-phenylene vinylene) (PPV) have shown signicant uores-cence quenching as compared to the pure OS, providingevidence for a good interaction between the twomaterials in thesolid state.17,64 In particular, efficient charge transport acrossgraphene and polymers has been demonstrated for simplebilayer systems composed of reduced graphene oxide (RGO)and P3HT, with a 20-fold increase of the effective mobility andIon/Ioff ratios >103.64 The use of bi-component systems as active

J. Mater. Chem. C


Table 3 Examples of published studies reporting different mechanisms of graphene–organic interactions in working devices

Graphene–organic semiconductors – applications in electronic devices

Device Effect Mechanism Carbon allotrope

ComponentcontainingG or CNTs Molecule Ref.

Field effecttransistor

Increase in conductance Photoexcitation Carbon nanotube Bulk composite P3OT (polythiophene) 62Hole injection Better injection barrier Carbon nanotubes Electrode Pentacene 60Increase in mobility,high on/off ratio

Electric eldenhancement atCNT tips

Carbon nanotubes Bulk composite Pentacene, sexithiophene 58

Increase in mobility,stable on/off ratio

“Fast lanes” for chargecarriers in conductionchannels

Graphene (LPE) Bulk composite P3HT or PQT-12poly(3,3-didodecyl-quaterthiophene

67

Work function tuning Doping Graphene S/D electrodes N,N’-Ditridecyl-3,4,9,10-perylenetetracarboxylicdiimide (PTCDI-C13)

98

Increase in conductivityand ambipolar mobility

Charge percolation GO-isocyanatefunctionalized

Bulk composite Polystyrene (PS) 76

Increase in conductivity Doping Graphene Electrode 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)

20

Increase in conductivity Doping Graphene Electrode Tetrathiafulvalene (TTF) 20n- or p-doping ofgraphene

Doping Mechanically cleavedgraphene

Active layer 1,5-Naphthalenedi-amine, dimethyl-anthracene,dibromoanthracene,pyrene-tetrasulfonic acid

39

Increase in mobility “Fast lanes” for chargecarriers in conductionchannels

RGO Active layer P3HT 66

Tunable memory effect Photoexcitation CNTs Bulk composite Polythiophene 61Tunable tunnel barrier Electric eld Mechanically cleaved

grapheneTunneling electrode MoS2/BN

heterostructures79

Charge transportmodulation

Photoexcitation RGO Bulk composite 1-Pyrene butyric acid 7

Increase in conductivityand mobility

‘‘Fast lanes’’ for chargecarriers in conductionchannels

Carbon nanotubes Bulk composite Alkyl-substitutedhexabenzocoronene

59

Enhancement inperformance

Photoexcitation Functionalized CVDgraphene

Electrode SAM of pyrene butanoicacid succidymidyl ester(PBASE)

73

Photovoltaic cells Increase in quantumefficiency

Photoexcitation Carbon nanotubesMWNT

Hole-collectingelectrode

Poly(p-phenylenevinylene) (PPV)

96

Increase inphotoconversionefficiencies

Photoexcitation Carbon nanotubes Single andsandwiched layers

Polythiophene 99

Increased short-circuitphotocurrent densitiesand ll factors

High specicsurface area

Graphene Electrode PEDOT–PSS 100

Nonvolatileelectronicmemory cell

Bistable switching Electric eld GO-g-PtBA polymergraed GO

Sandwiched P3HT 69

Bistable switching,charge transfer

Electric eld Hexadecylamine-graphene oxide(HDAGO)

Bulk composite P3HT 70

Non-covalentfunctionalization

n/a Exfoliated graphene Electrode 1-Pyrenecarboxylic acid 29

Sensors Increase in conductivity Interaction of theOS with gas

Exfoliated graphene Electrode 1-Pyrenecarboxylic acid 29

Molecular magneticgate

Magnetic coupling Mechanically cleavedgraphene, patterned

Nano-constriction Pyrene-substitutedterbium-bis(phthalocyanine)TbPc2

57

Super-capacitors Non-covalentfunctionlization

n/a Exfoliated graphene Electrode 1-Pyrenecarboxylic acid 29,101

J. Mater. Chem. C This journal is © The Royal Society of Chemistry 2014


Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online



Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

layers of FET has amajor disadvantage: charges will have to faceseveral injection barriers, hopping continuously from P3HT toRGO and vice versa. However, the WF values measured by KelvinProbe Force Microscopy65,66 (KPFM) on RGO monolayers andP3HT amounted to 4.75 � 0.02 and 4.80 � 0.03 eV, respectively.The low WF difference favors charge transport from one mate-rial to the other.

It should be noted that the WF of organic semiconductorslike P3HT depends signicantly onmolecular packing, which inturn depends on other parameters such as deposition condi-tions and molecular weight. The value obtained by KPFMmeasurements for the P3HT is in fairly good agreement with theHOMO of P3HT estimated by cyclic voltammetry measure-ments, i.e. 4.9 eV. The WF of P3HT is close to the RGO valuethus allowing efficient charge transport across the twomaterialsas shown in Fig. 5a, which is a schematic representation ofcharge transport across the RGO-P3HT blend reporting theexperimental WF values as measured by KPFM.

Huang et al. showed that, when compared to pure organicsemiconductor layers, graphene–organic hybrid FETs exhibit upto 20-fold increase in effective eld-effect mobilities maintain-ing an Ion/Ioff ratio comparable to or better than what observedwithout graphene. The mobilities determined in P3HT/gra-phene PQT-12/graphene hybrid FETs resulted in values as highas 0.17 cm2 V�1 s�1 and 0.6 cm2 V�1 s�1, respectively.67 Inaddition, non-covalent interactions in bulk GO-P3HT hybridassemblies were found to enhance optical absorption, chargetransfer, and photocatalytic properties. With their broadabsorption, excited states can be generated with sunlight,rendering GO-P3HT hybrids highly promising for potentialapplication as efficient green photocatalysts in organicsynthesis.68

Organo- and water-dispersible GO-polymer nanosheets wereused also for organic electronic memory devices. Thin lms ofpoly(tert-butyl acrylate), covalently bound to GO sheets, showedbistable electrical conductivity switching behavior and a non-volatile, rewritable memory effect in an Al/GO-g-PtBA + P3HT/ITO sandwich device.69 The non-volatile rewritable memoryeffect has also been demonstrated with a nanocomposite ofhexadecylamine-functionalized graphene oxide (HDAGO) andpoly(3-hexylthiophene) (P3HT). The device had an ITO/P3HT-HDAGO/Al sandwich structure, in which the composite lm ofP3HT-HDAGO was prepared by simple solution phase mixingand spin-coating. This memory device exhibited typical bistableelectrical switching behavior and a non-volatile rewritablememory effect, with a turn-on voltage of about 1.5 V and anIon/Ioff ratio of 105. The electrical switching behavior wasattributed to the electric-eld-induced charge transfer betweenP3HT and HDAGO nanosheets.70

The full potential of graphene in integrated circuits can beaccessed only with a reliable ultrathin high-k top-gate dielectric.In top-gated graphene devices an oxide layer, typically Al2O3 andHfO2, is grown on graphene by atomic layer deposition. Even inthis case, OSs can have a benecial effect; a molecularly thinorganic seeding layer of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) has been found to beneciallydirect oxide deposition, with signicant improvements in


dielectric performance limits for ALD-grown oxides ongraphene.71

A main potential application of graphene, intensivelystudied worldwide, is to use it as a transparent conductor, andthis application has been studied also for organic electronicdevices.

Although GO is known to be insulating, when deposited as athin coating layer it can have a benecial effect on chargetransport in organic photovoltaic devices. The incorporation ofGO deposited from neutral solutions between the photoactivepoly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methylester (PCBM) layer and the transparent and conducting indiumtin oxide (ITO) results in a decrease in recombination of elec-trons and holes and leakage currents, and a dramatic increasein the photovoltaic efficiencies to values that are comparable tothose of devices fabricated with PEDOT:PSS as the hole trans-port layer (Fig. 5d).72 Large-area, continuous anodes composedof pristine few-layered graphene were also realized by CVD andtested in organic photovoltaic devices: non-covalent function-alization of graphene with pyrene butanoic acid succidymidylester (PBASE) allows for alignment of the Fermi level of gra-phene very close to the highest occupied molecular orbital ofPEDOT:PSS for efficient hole collection. Graphene anodesmodied by self-assembled PBASE exhibit excellent perfor-mance characteristics73 (Fig. 5e).

Also the work function of graphene electrodes in OFETs canbe engineered by functionalizing the surface of SiO2 substrateswith self-assembled monolayers (SAMs). In general, themeasured WF of graphene can vary signicantly, as shown aswell in the band diagrams of Fig. 5; graphene WF depends onthe production method and on the underlying substrate thatcan induce relevant doping. The electron-donating NH2-termi-nated SAMs with aminotriethoxy-silane induce strong n-dopingin graphene, whereas the CH3-terminated SAMs neutralize thep-doping induced by SiO2 substrates. Graphene doped by SAMsdisplays a Dirac voltage shi over 150 V in transistor applicationand enables graphene to have a tunable work function. Thework function of graphene shis down to 3.9 eV for NH2-SAM-modied SiO2, facilitating electron injection at this low workfunction graphene electrode74 (Fig. 5b and c).

It should be noted that the quality of the underlying CVDgraphene is critical for interaction with OSs in devices; Sun et al.studied the assembly of oligothiophene derivatives on grapheneand found that defect-like ripples and wrinkles signicantlyweaken the molecule–graphene interaction. By using twodifferent quaterthiophene derivatives they showed that suchstructural irregularities in graphene hinder stable molecularadsorption.75

An original approach was also reported by Eda and Chho-walla, who managed to obtain working transistors even viablending RGO with an insulating polymer. By preparing a welldispersed mixture of RGO with polystyrene (PS), they obtainedworking ambipolar transistors with electron and hole mobilitiesof 0.2 cm2 V�1 s�1 and 0.7 cm2 V�1 s�1, respectively, althoughaccompanied with very low Ion/Ioff ratios ranging between 3 and6. In the latter case, of course, the polymer was not activelycontributing to the charge transport process, the latter taking

J. Mater. Chem. C



Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

place via percolation among RGO sheets dispersed in theinsulating PS matrix.76 Blending an insulating polymer in theactive layer of microelectronic devices is not meant to have highperformance, but this result is interesting as it lies at the edgebetween organic electronics and research on bulk composites:nowadays, many groups are trying to use graphene incommercial polymers to increase electrical conductivity andgive “functional” properties to structural materials already usedin automotive and aerospace elds.

6. Beyond graphene – emerging 2Dmaterials and inorganicheterostructures with graphene

With recent developments in large-scale production techniquessuch as liquid-phase exfoliation and CVD growth, new layeredmaterials beyond graphene such as BN, NbSe2, TaS2, and MoS2based nanosheets holding great potential in both fundamentaland applied research can be made available (see ref. 77 and 78).If the electronic interaction of organic molecules with graphene(a semi-metal) is rather complex, their interaction with semi-conducting 2D nanosheets will give systems even more chal-lenging to be understood, but more versatile to be used as 2Dcomposites79,80 as alternative gate insulators, photo-responsivecomponents, active materials for eld-effect transistors orelectrode materials. A polymer nanocomposite of liquid phaseexfoliated MoS2 and polyethylene oxide (PEO) has been recentlydemonstrated as an anode material for lithium ion batteries.MoS2/PEO electrodes exhibit high charge storage capacities andlong-term reversibility.81

Composites from organic semiconductors and other non-carbon based 2D materials, such as tin sulphite or tin selenide,are particularly interesting. For instance, it has been found thatthe presence of 1,10-phenanthroline can govern themorphology of SnSe giving 2D nanosheets, instead of 3D SnSenanoowers obtained in the absence of the molecule. Single-crystalline nanosheets are obtained from the coalescence of theSnSe nucleus in an oriented attachment mechanism. Bandgapdetermination and optoelectronic test based on hybrid lms ofSnSe and poly(3-hexylthiophene) indicate the great potential ofthe ultrathin SnSe nanosheets in photodetection andphotovoltaics.82

7. Conclusions

Overall, we have attempted to give an overview and some amongthe most enlightening examples of how the interaction of gra-phene with molecules, and in particular with organic or poly-meric semiconductors, can give nanocomposites featuringinteresting novel chemical and physical properties that arebeyond the simple sum of the properties of the isolatedcomponents. These hybrid systems are not only interestingmaterials to study, but can also be used as key components fortechnologically relevant applications. Some general consider-ations can be drawn:

J. Mater. Chem. C

1. Graphene can interact in a controlled way with a very wideportfolio of organic semiconductors, driving their self-assemblytowards ordered 2D crystals. Many molecules can physicallyadsorb on graphene. However, only some of them (for examplestrong acceptors and uorinated OSs) show an interactionstrong enough to perturb effectively graphene's electronicproperties.

2. The interaction of OSs with graphene is strong not only atshort distances, by charge transfer or p–p interactions, butenergy transfer can also be relevant at longer distances, up to30 nm.

3. The interaction of organic molecules with graphene canalso be used in solution, to foster graphene exfoliation. Ascompared to other exfoliation techniques based on organicsolvents or non-aromatic surfactants, the use of OSs allows us tomonitor effectively graphene–organic interactions by opticalspectroscopy, and to obtain graphene–organic hybrids suitablefor electronic applications.

4. Graphene can be used as a nano-additive or functionalcoating either in the active layer or contacting electrodes ofmany organic electronic devices, allowing the boost of theperformance of conventional OSs.

Thanks to their processability and broad arsenal of deriva-tives that are available, organic molecules can interact withgraphene in innite ways, leading to the formation of varioustypes of assemblies spanning from simple monolayers on singlesheets to complex, bulk graphene–organic composites. Thephysical interaction has a dramatic effect on the optical andelectronic characteristics of the hybrid system, inuencingnumerous properties such as processability and charge trans-port which can be tuned e.g., via controlled doping. Graphenecan strongly interact with and tune the morphology of mostorganic molecules, in a more controlled way than other tech-nologically relevant materials such as silicon or metals. Thepossibility to combine together carbon-based materials havingvery different properties shall allow a seamless integrationbetween high-speed electronics, organic electronics andcomposite science.

Acknowledgements

This work was supported by the European Science Foundation(ESF) under the EUROCORES Program EuroGRAPHENE(GOSPEL), the Operative Program FESR 2007–2013 of RegioneEmilia-Romagna – Attivita I.1.1., the EC Marie-Curie ITN-GENIUS (PITN-GA-2010-264694), the European Union SeventhFramework Programme under grant agreement no. 604391Graphene Flagship, FET project UPGRADE (project no. 309056),the Agence Nationale de la Recherche through the LabEx projectNIE, and the International Center for Frontier Research inChemistry (icFRC). We thank A. Liscio, E. Treossi, X. Xia and K.Agalou for the joint research activities that were an essentialsource of inspiration for this review.

Notes and references

1 A. Bianco, Angew. Chem., Int. Ed., 2013, 52, 4986–4997.




Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

2 J. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun,S. Blankenburg, M. Muoth, A. P. Seitsonen, M. Saleh,X. Feng, K. Mullen and R. Fasel, Nature, 2010, 466, 470–473.

3 C. A. Palma and P. Samorı, Nat. Chem., 2011, 3, 431–436.4 T. Ohta, A. Bostwick, T. Seyller, K. Horn and E. Rotenberg,Science, 2006, 313, 951–954.

5 X. S. Li, W. W. Cai, L. Colombo and R. S. Ruoff, Nano Lett.,2009, 9, 4268–4272.

6 C. Berger, Z. M. Song, X. B. Li, X. S. Wu, N. Brown, C. Naud,D. Mayou, T. B. Li, J. Hass, A. N. Marchenkov, E. H. Conrad,P. N. First and W. A. de Heer, Science, 2006, 312, 1191–1196.

7 S. R. Pathipati, E. Pavlica, E. Treossi, R. Rizzoli,G. P. Veronese, V. Palermo, L. Chen, D. Beljonne, J. Cai,R. Fasel, P. Ruffieux and G. Bratina, Org. Electron., 2013,14, 1787–1792.

8 V. Palermo and P. Samorı, Angew. Chem., Int. Ed., 2007, 46,4428–4432.

9 P. Samorı, N. Severin, C. D. Simpson, K. Mullen andJ. P. Rabe, J. Am. Chem. Soc., 2002, 124, 9454–9457.

10 K. Xiao, W. Deng, J. K. Keum, M. Yoon, I. V. Vlassiouk,K. W. Clark, A.-P. Li, I. I. Kravchenko, G. Gu, E. A. Payzant,B. G. Sumpter, S. C. Smith, J. F. Browning andD. B. Geohegan, J. Am. Chem. Soc., 2013, 135, 3680–3687.

11 Y. Ogawa, T. Niu, S. L. Wong, M. Tsuji, A. T. S. Wee, W. Chenand H. Ago, J. Phys. Chem. C, 2013, 117, 21849–21855.

12 J. H. Mao, H. G. Zhang, Y. H. Jiang, Y. Pan, M. Gao,W. D. Xiao and H. J. Gao, J. Am. Chem. Soc., 2009, 131,14136–14137.

13 P. Jarvinen, S. K. Hamalainen, K. Banerjee, P. Hakkinen,M. Ijas, A. Harju and P. Liljeroth, Nano Lett., 2013, 13,3199–3204.

14 Q. H.Wang andM. C. Hersam,Nat. Chem., 2009, 1, 206–211.15 J. Bjork, F. Hanke, C. A. Palma, P. Samori, M. Cecchini and

M. Persson, J. Phys. Chem. Lett., 2010, 1, 3407–3412.16 A. Rochefort and J. D. Wuest, Langmuir, 2009, 25, 210–215.17 R. J. Smith, M. Lotya and J. N. Coleman, New J. Phys., 2010,

12, 125008.18 J. D. Emery, Q. H. Wang, M. Zarrouati, P. Fenter,

M. C. Hersam and M. J. Bedzyk, Surf. Sci., 2011, 605,1685–1693.

19 I. Salzmann, A. Moser, M. Oehzelt, T. Breuer, X. Feng,Z.-Y. Juang, D. Nabok, R. G. Della Valle, S. Duhm,G. Heimel, A. Brillante, E. Venuti, I. Bilotti,C. Christodoulou, J. Frisch, P. Puschnig, C. Draxl,G. Witte, K. Mullen and N. Koch, ACS Nano, 2012, 6,10874–10883.

20 Y. H. Zhang, K. G. Zhou, K. F. Xie, J. Zeng, H. L. Zhang andY. Peng, Nanotechnology, 2010, 21, 065201.

21 K. S. Novoselov, V. I. Fal'ko, L. Colombo, P. R. Gellert,M. G. Schwab and K. Kim, Nature, 2012, 490, 192–200.

22 F. Torrisi, T. Hasan, W. Wu, Z. Sun, A. Lombardo,T. S. Kulmala, G.-W. Hsieh, S. Jung, F. Bonaccorso,P. J. Paul, D. Chu and A. C. Ferrari, ACS Nano, 2012, 6,2992–3006.

23 E. B. Secor, P. L. Prabhumirashi, K. Puntambekar,M. L. Geier and M. C. Hersam, J. Phys. Chem. Lett., 2013,4, 1347–1351.


24 Y. Xu, M. G. Schwab, A. J. Strudwick, I. Hennig, X. Feng,Z. Wu and K. Mullen, Adv. Energy Mater., 2013, 3, 1035–1040.

25 Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Y. Sun,S. De, I. T. McGovern, B. Holland, M. Byrne, Y. K. Gun'ko,J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy,R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari andJ. N. Coleman, Nat. Nanotechnol., 2008, 3, 563–568.

26 M. Lotya, Y. Hernandez, P. J. King, R. J. Smith, V. Nicolosi,L. S. Karlsson, F. M. Blighe, S. De, Z. M. Wang,I. T. McGovern, G. S. Duesberg and J. N. Coleman, J. Am.Chem. Soc., 2009, 131, 3611–3620.

27 A. Ciesielski and P. Samori, Chem. Soc. Rev., 2014, 43, 381–398.

28 B. Saswata, K. Tapas, M. Ananta Kumar, K. Nam Hoon andL. Joong Hee, Nanotechnology, 2011, 22, 405603.

29 X. H. An, T. J. Simmons, R. Shah, C. Wolfe, K. M. Lewis,M. Washington, S. K. Nayak, S. Talapatra and S. Kar,Nano Lett., 2010, 10, 4295–4301.

30 D. Parviz, S. Das, H. S. T. Ahmed, F. Irin, S. Bhattachariaand M. J. Green, ACS Nano, 2012, 6, 8857–8867.

31 A. Schlierf, H. Yang, E. Gebremedhn, E. Treossi, L. Ortolani,L. Chen, A. Minoia, V. Morandi, P. Samori, C. Casiraghi,D. Beljonne and V. Palermo, Nanoscale, 2013, 5, 4205.

32 Q. Su, S. Pang, V. Alijani, C. Li, X. Feng and K. Mullen, Adv.Mater., 2009, 21, 3191–3195.

33 C. Backes, C. D. Schmidt, K. Rosenlehner, F. Hauke,J. N. Coleman and A. Hirsch, Adv. Mater., 2010, 22, 788–802.

34 A. Ghosh, K. V. Rao, S. J. George and C. N. R. Rao, Chem.–Eur. J., 2010, 16, 2700–2704.

35 X. C. Dong, Y. M. Shi, Y. Zhao, D. M. Chen, J. Ye, Y. G. Yao,F. Gao, Z. H. Ni, T. Yu, Z. X. Shen, Y. X. Huang, P. Chen andL. J. Li, Phys. Rev. Lett., 2009, 102, 135501.

36 S. Sampath, A. N. Basuray, K. J. Hartlieb, T. Aytun,S. I. Stupp and J. F. Stoddart, Adv. Mater., 2013, 25, 2740–2745.

37 H. Yang, Y. Hernandez, A. Schlierf, A. Felten, A. Eckmann,S. Johal, P. Louette, J. J. Pireaux, X. Feng, K. Mullen,V. Palermo and C. Casiraghi, Carbon, 2013, 53, 357–365.

38 Z. Liu, J. Liu, L. Cui, R. Wang, X. Luo, C. J. Barrow andW. Yang, Carbon, 2013, 51, 148–155.

39 X. C. Dong, D. L. Fu, W. J. Fang, Y. M. Shi, P. Chen andL. J. Li, Small, 2009, 5, 1422–1426.

40 D. Choudhury, B. Das, D. D. Sarma and C. N. R. Rao, Chem.Phys. Lett., 2010, 497, 66–69.

41 C. Coletti, C. Riedl, D. S. Lee, B. Krauss, L. Patthey, K. vonKlitzing, J. H. Smet and U. Starke, Phys. Rev. B: Solid State,2010, 81, 235401.

42 E. Treossi, M. Melucci, A. Liscio, M. Gazzano, P. Samorı andV. Palermo, J. Am. Chem. Soc., 2009, 131, 15576–15577.

43 J. Kim, L. J. Cote, F. Kim and J. X. Huang, J. Am. Chem. Soc.,2010, 132, 260–267.

44 N. V. Kozhemyakina, J. M. Englert, G. A. Yang, E. Spiecker,C. D. Schmidt, F. Hauke and A. Hirsch, Adv. Mater., 2010,22, 5483–5487.

45 R. S. Swathi and K. L. Sebastian, J. Chem. Phys., 2009, 130,086101.

J. Mater. Chem. C



Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

46 R. S. Swathi and K. L. Sebastian, J. Chem. Phys., 2008, 129,054703.

47 L. Gaudreau, K. J. Tielrooij, G. E. D. K. Prawiroatmodjo,J. Osmond, F. J. G. de Abajo and F. H. L. Koppens, NanoLett., 2013, 13, 2030–2035.

48 Y. F. Xu, Z. B. Liu, X. L. Zhang, Y. Wang, J. G. Tian,Y. Huang, Y. F. Ma, X. Y. Zhang and Y. S. Chen, Adv.Mater., 2009, 21, 1275.

49 H. Matte, K. S. Subrahmanyam, K. V. Rao, S. J. George andC. N. R. Rao, Chem. Phys. Lett., 2011, 506, 260–264.

50 M. Quintana, A. Montellano, A. E. D. Castillo, G. VanTendeloo, C. Bittencourt and M. Prato, Chem. Commun.,2011, 47, 9330–9332.

51 X. Y. Zhang, Y. Huang, Y. Wang, Y. F. Ma, Z. F. Liu andY. S. Chen, Carbon, 2009, 47, 334–337.

52 Y. Liu, J. Zhou, X. Zhang, Z. Liu, X. Wan, J. Tian, T. Wangand Y. Chen, Carbon, 2009, 47, 3121.

53 M. Melucci, E. Treossi, L. Ortolani, G. Giambastiani,V. Morandi, P. Klar, C. Casiraghi, P. Samori andV. Palermo, J. Mater. Chem., 2010, 20, 9052–9060.

54 M. Melucci, M. Durso, M. Zambianchi, E. Treossi, Z. Y. Xia,I. Manet, G. Giambastiani, L. Ortolani, V. Morandi, F. DeAngelis and V. Palermo, J. Mater. Chem., 2012, 22, 18237–18243.

55 X. Y. Pan, H. Li, K. T. Nguyen, G. Gruner and Y. L. Zhao,J. Phys. Chem. C, 2012, 116, 4175–4181.

56 M. Garnica, D. Stradi, S. Barja, F. Calleja, C. Diaz,M. Alcami, N. Martin, A. L. V. de Parga, F. Martin andR. Miranda, Nat. Phys., 2013, 9, 368–374.

57 A. Candini, S. Klyatskaya, M. Ruben, W. Wernsdorfer andM. Affronte, Nano Lett., 2011, 11, 2634–2639.

58 S. Liu, S. C. B. Mannsfeld, M. C. LeMieux, H. W. Lee andZ. Bao, Appl. Phys. Lett., 2008, 92, 053306.

59 K. D. Harris, S. Xiao, C. Y. Lee, M. S. Strano, C. Nuckolls andG. B. Blanchet, J. Phys. Chem. C, 2007, 111, 17947–17951.

60 C. M. Aguirre, C. Ternon, M. Paillet, P. Desjardins andR. Martel, Nano Lett., 2009, 9, 1457–1461.

61 C. Anghel, V. Derycke, A. Filoramo, S. Lenfant, B. Giffard,D. Vuillaume and J. P. Bourgoin, Nano Lett., 2008, 8,3619–3625.

62 J. Borghetti, V. Derycke, S. Lenfant, P. Chenevier,A. Filoramo, M. Goffman, D. Vuillaume andJ. P. Bourgoin, Adv. Mater., 2006, 18, 2535.

63 J. Jang, J. Park, S. Nam, J. E. Anthony, Y. Kim, K. S. Kim,K. S. Kim, B. H. Hong and C. E. Park, Nanoscale, 2013, 5,11094–11101.

64 A. Liscio, G. P. Veronese, E. Treossi, F. Suriano, F. Rossella,V. Bellani, R. Rizzoli, P. Samori and V. Palermo, J. Mater.Chem., 2011, 21, 2924–2931.

65 V. Palermo, M. Palma and P. Samorı, Adv. Mater., 2006, 18,145–164.

66 A. Liscio, V. Palermo and P. Samori, Acc. Chem. Res., 2010,43, 541–550.

67 J. Huang, D. R. Hines, B. J. Jung, M. S. Bronsgeest,A. Tunnell, V. Ballarotto, H. E. Katz, M. S. Fuhrer,E. D. Williams and J. Cumings, Org. Electron., 2011, 12,1471–1476.

J. Mater. Chem. C

68 S. Wang, C. T. Nai, X.-F. Jiang, Y. Pan, C.-H. Tan,M. Nesladek, Q.-H. Xu and K. P. Loh, J. Phys. Chem. Lett.,2012, 3, 2332–2336.

69 G. L. Li, G. Liu, M. Li, D. Wan, K. G. Neoh and E. T. Kang,J. Phys. Chem. C, 2010, 114, 12742–12748.

70 L. Zhang, Y. Li, J. Shi, G. Shi and S. Cao,Mater. Chem. Phys.,2013, 142, 626–632.

71 V. K. Sangwan, D. Jariwala, S. A. Filippone, H. J. Karmel,J. E. Johns, J. M. P. Alaboson, T. J. Marks, L. J. Lauhonand M. C. Hersam, Nano Lett., 2013, 13, 1162–1167.

72 S. S. Li, K. H. Tu, C. C. Lin, C. W. Chen and M. Chhowalla,ACS Nano, 2010, 4, 3169–3174.

73 Y. Wang, X. H. Chen, Y. L. Zhong, F. R. Zhu and K. P. Loh,Appl. Phys. Lett., 2009, 95, 063302.

74 J. Park, W. H. Lee, S. Huh, S. H. Sim, S. B. Kim, K. Cho,B. H. Hong and K. S. Kim, J. Phys. Chem. Lett., 2011, 2,841–845.

75 X. Sun, J. Zhang, X. Wang, C. Zhang, P. Hu, Y. Mu, X. Wan,Z. Guo and S. Lei, Chem. Commun., 2013, 49, 10317–10319.

76 G. Eda and M. Chhowalla, Nano Lett., 2009, 9, 814–818.77 V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano

and J. N. Coleman, Science, 2013, 340, 1420.78 A. K. Geim and I. V. Grigorieva, Nature, 2013, 499, 419–425.79 L. Britnell, R. V. Gorbachev, R. Jalil, B. D. Belle, F. Schedin,

A. Mishchenko, T. Georgiou, M. I. Katsnelson, L. Eaves,S. V. Morozov, N. M. R. Peres, J. Leist, A. K. Geim,K. S. Novoselov and L. A. Ponomarenko, Science, 2012,335, 947–950.

80 L. Britnell, R. M. Ribeiro, A. Eckmann, R. Jalil, B. D. Belle,A. Mishchenko, Y. J. Kim, R. V. Gorbachev, T. Georgiou,S. V. Morozov, A. N. Grigorenko, A. K. Geim, C. Casiraghi,A. H. C. Neto and K. S. Novoselov, Science, 2013, 340,1311–1314.

81 J. Xiao, D. Choi, L. Cosimbescu, P. Koech, J. Liu andJ. P. Lemmon, Chem. Mater., 2010, 22, 4522–4524.

82 L. Li, Z. Chen, Y. Hu, X. Wang, T. Zhang, W. Chen andQ. Wang, J. Am. Chem. Soc., 2013, 135, 1213–1216.

83 A. Liscio, M. Bonini, E. Treossi, E. Orgiu, M. Kastler, F. Dotz,V. Palermo and P. Samori, J. Polym. Sci., Part B: Polym. Phys.,2012, 50, 642–649.

84 V. Palermo, E. Schwartz, A. Liscio, M. B. J. Otten, K. Mullen,R. J. M. Nolte, A. E. Rowan and P. Samori, SoMatter, 2009,5, 4680–4686.

85 A. Schlierf, H. Yang, E. Gebremedhn, E. Treossi, L. Ortolani,L. Chen, A. Minoia, V. Morandi, P. Samori, C. Casiraghi,D. Beljonne and V. Palermo, Nanoscale, 2013, 5, 4205–4216.

86 J. M. Englert, J. Rohrl, C. D. Schmidt, R. Graupner,M. Hundhausen, F. Hauke and A. Hirsch, Adv. Mater.,2009, 21, 4265–4269.

87 C. D. Schmidt, C. Bottcher and A. Hirsch, Eur. J. Org. Chem.,2007, 2007, 5497–5505.

88 R. Hao, W. Qian, L. Zhang and Y. Hou, Chem. Commun.,2008, 6576–6578.

89 M. Lotya, P. J. King, U. Khan, S. De and J. N. Coleman, ACSNano, 2010, 4, 3155–3162.




Publ

ishe

d on

16

Dec

embe

r 20

13. D

ownl

oade

d by

CN

R B

olog

na o

n 20

/03/

2014

14:

20:0

5.

View Article Online

90 S. De, P. J. King, M. Lotya, A. O'Neill, E. M. Doherty,Y. Hernandez, G. S. Duesberg and J. N. Coleman, Small,2010, 6, 458–464.

91 M. Lotya, Y. Hernandez, P. J. King, R. J. Smith, V. Nicolosi,L. S. Karlsson, F. M. Blighe, S. De, Z. Wang, I. T. McGovern,G. S. Duesberg and J. N. Coleman, J. Am. Chem. Soc., 2009,131, 3611–3620.

92 S. Vadukumpully, J. Paul and S. Valiyaveettil, Carbon, 2009,47, 3288–3294.

93 J. Geng, B.-S. Kong, S. B. Yang and H.-T. Jung, Chem.Commun., 2010, 46, 5091–5093.

94 X. L. Li, G. Y. Zhang, X. D. Bai, X. M. Sun, X. R. Wang,E. Wang and H. J. Dai, Nat. Nanotechnol., 2008, 3, 538–542.

95 Y. B. Wang, D. Kurunthu, G. W. Scott and C. J. Bardeen,J. Phys. Chem. C, 2010, 114, 4153–4159.


96 H. Ago, K. Petritsch, M. S. P. Shaffer, A. H. Windle andR. H. Friend, Adv. Mater., 1999, 11, 1281–1285.

97 R. Addou and M. Batzill, Langmuir, 2013, 29, 6354–6360.

98 J. Park, W. H. Lee, S. Huh, S. H. Sim, S. B. Kim, K. Cho,B. H. Hong and K. S. Kim, J. Phys. Chem. Lett., 2011, 2,841–845.

99 G. M. A. Rahman, D. M. Guldi, R. Cagnoli, A. Mucci,L. Schenetti, L. Vaccari and M. Prato, J. Am. Chem. Soc.,2005, 127, 10051–10057.

100 W. J. Hong, Y. X. Xu, G. W. Lu, C. Li and G. Q. Shi,Electrochem. Commun., 2008, 10, 1555–1558.

101 S. Ghosh, X. An, R. Shah, D. Rawat, B. Dave, S. Kar andS. Talapatra, J. Phys. Chem. C, 2012, 116, 20688–20693.

J. Mater. Chem. C


journal of materials chemistry c - cnr€¦ · materials for opto-electronics applications. their...

Documents