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Graphene

Graphene-Based Materials: Synthesis, Characterization,Properties, and Applications

Xiao Huang, Zongyou Yin, Shixin Wu, Xiaoying Qi, Qiyuan He, Qichun Zhang,Qingyu Yan, Freddy Boey, and Hua Zhang*

Graphene, a two-dimensional, single-layer sheet of sp2hybridized carbon atoms, has attracted tremendousattention and research interest, owing to its exceptional

physical properties, such as high electronic conductivity,good thermal stability, and excellent mechanical strength.Other forms of graphene-related materials, including

graphene oxide, reduced graphene oxide, and exfoliated

graphite, have been reliably produced in large scale.The promising properties together with the ease ofprocessibility and functionalization make graphene-based materials ideal candidates for incorporation intoa variety of functional materials. Importantly, grapheneand its derivatives have been explored in a widerange of applications, such as electronic and photonicdevices, clean energy, and sensors. In this review, after a

general introduction to graphene and its derivatives, thesynthesis, characterization, properties, and applications ofgraphene-based materials are discussed.

1. Introduction . . . . . . . . . . . . . . . . . . . . . . .1877

2. Graphene-Based Materials. . . . . . . . . . . 1878

3. Applications of Graphene andGraphene-Based Materials . . . . . . . . . . 1885

4. Conclusion and Outlook . . . . . .. . . . . . . 1893

From the Contents


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Graphene-Based Materials

1877 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.comsmall2011,7,No. 14, 18761902

X. Huang, Dr. Z. Y. Yin, S. X. Wu, X. Y. Qi, Q. Y. He, Prof. Q. C. Zhang,

Prof. Q. Y. Yan, Prof. F. Boey, Prof. H. Zhang

School of Materials Science and Engineering

Nanyang Technological University50 Nanyang Avenue, Singapore 639798, Singapore

Fax: +656790-9081

Website: http://www.ntu.edu.sg/home/hzhang/

E-mail: [email protected]

Prof. F. Boey, Prof. H. Zhang

Centre for Biomimetic Sensor Science

Nanyang Technological University

50 Nanyang Drive, Singapore 637553, Singapore

X. Y. Qi

Singapore Institute of Manufacturing Technology

71 Nanyang Drive, Singapore 638075, Singapore

DOI: 10.1002/smll.201002009

1. Introduction

Graphene is a 2D single layer of carbon atoms with

the hexagonal packed structure.[1,2] The carbon bonds are

sp2 hybridized, where the in-plane C-C bond is one of the

strongest bonds in materials and the out-of-plane bond,

which contributes to a delocalized network of electrons, is

responsible for the electron conduction of graphene and pro-

vides the weak interaction among graphene layers or between

graphene and substrate. With these unique structural charac-

teristics, graphene has shown exceptional physical properties,

which have attracted enormous research interest in both sci-

entific and engineering communities.[317]

One of the most remarkable properties of graphene is

that its charge carriers behave as massless relativistic parti-

cles or Dirac fermions, and under ambient conditions they

can move with little scattering. This unique behavior has

led to a number of exceptional phenomena in graphene. [18]

First, graphene is a zero-bandgap 2D semiconductor with

a tiny overlap between valence and conduction bands. [18]

Second, it exhibits a strong ambipolar electric field effect so

that the charge carrier concentrations of up to 1013 cm2 and

room-temperature mobilities of 10 000 cm2 s1 are meas-

ured.[18] Third, an unusual half-integer quantum Hall effect

(QHE) for both electron and hole carriers in graphene has

been observed by adjusting the chemical potential using the

electric field effect.[19,20] In addition, graphene is highly trans-

parent, with an absorption of2.3% towards visible light.[6]

Its thermal conductivity, k, is measured with a value of

5000 W mK1 for a single-layer sheet at room temperature.[21]

Graphene also possesses the excellent mechanical strength.

The intrinsic mechanical properties of free-standing monolayer

graphene membranes were measured by nano-indentation

in an atomic force microscope.[3] The breaking strength

is 42 N m1 and the Young's modulus is 1.0 TPa, indicating it

is one of the strongest materials ever measured.

Up until now, versatile methods have been developed for

fabrication, growth, or synthesis of graphene and its deriva-

tives. In graphite, the adjacent graphene layers are bound

by weak van der Waals forces.[22] Therefore, the pristine

graphene can be obtained from the mechanical exfoliation

of graphite using adhesive tapes.[14,18] The bottom-up growth

of graphene sheets is an alternative to the mechanical exfo-

liation of bulk graphite. Chemical vapor deposition (CVD)

has been used to grow single- and few-layer graphene

sheets on metal surfaces, such as Ni and Cu.[8,2356] Large-

area epitaxial graphene films up to a few micrometers in

size can be subsequently transferred to other substrates.

Carbon segregation can also become graphene layers on

carbon-containing substrates, such as SiC,[5772] through high-

temperature annealing. However, these methods are imprac-

tical for large-scale solution-based processes. Therefore, the

oxidation and exfoliation of graphite oxide, followed by the

chemical reduction,[73] has been used to prepare reduced

graphene oxide (rGO) sheets[74116] or chemically function-

alized graphene (CFG). This is one of the most developed

methods in the literature, and several types of reduction

agents have been reported, such as hydrazine,[7486,9093]

strong alkaline media,[117] vitamin C,[116] and bovine serum

albumin (BSA).[118] Besides the reduction with chemical

agents, electrochemical,[109,119121] photochemical[87,122] and

thermal[94,111,123131] reduction methods have been devel-

oped as well. As the precursor of rGO, graphene oxide

(GO) sheets,[5972,132145] exfoliated from graphite oxide,

are obtained by the Hummers method, via the reaction of

graphite with a mixture of potassium permanganate (KMnO4)

and concentrated sulfuric acid (H2SO4).

[

7478

,

8082

,

146

]

GO sheets are thus highly oxidized and characterized by the

dominant presence of epoxides, alcohols, and carboxylic acid

groups.[147,148] These functional groups render GO and rGO

advantageous compared to the pristine graphene, in terms of

the tunability in electrical and optical properties via chemical

reactions. [149,150] Unfortunately, rGO shows electric conduc-

tivity several orders lower than the pristine graphene because

of the incomplete reduction and the presence of numerous

defects, which disrupt the sp2 network.[75,77,80,150152] In order

to overcome the poor conductivity of rGO, several methods

have been developed. For example, the non-oxidation liquid-

phase exfoliation of graphite has been demonstrated, which

can produce large-area single-layer graphene by ultrasoni-cation of graphite in both aqueous[153] and non-aqueous[154]

solutions. Re-intercalation and ultrasonication of thermally

exfoliated expandable graphite (EG) can produce single-

layer graphene sheets as well.[155] Even without ultrasonica-

tion, the readily dispersible graphene sheets from intercalated

compounds of graphite have been reported by using ternary

potassium salt as the intercalation agent.[156] One-step elec-

trochemical exfoliation of graphite into graphene layers has

also been realized in ionic liquids.[157] Moreover, the arc

discharge method,[158] and the direct chemical synthesis of

graphene in solution have been developed from nongraphitic

substances, such as the reaction of ethanol and sodium fol-

lowed by pyrolysis,[

159

]

and the organic synthesis of graphene-like polyaromatic hydrocarbons.[160]

Besides the graphene-based 2D sheets, other graphene-

related materials have been fabricated/synthesized as well,

such as zero dimensional (0D) graphene quantum dots,[161163]

1D graphene nanoribbons (GNRs),[5,163181] and graphene

nanomeshes (GNMs).[182187] These materials are expected to

possess different electrical and optical properties, due to the

variation in size and geometry, and the presence of a large


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of GO or rGO.[148] For example, rGO dispersions in THF,

CCl4, and 1,2-dichloroethane (EDC) were obtained by con-

verting the edge carboxylic acid groups of rGO into octade-

cylamines, which sterically stabilized the graphene sheets.[272]

The treatment of GO with organic isocyanates could lead

to the derivatization of both the edge carboxyl and surface

hydroxyl functional groups via the formation of amides or

carbamate esters (RnCO) (Figure 1A).[273] Esterification of

the carboxylic groups in GO with the hydroxyl groups in

poly(vinyl alcohol) (PVA) was also realized in the synthesis

of GOpolymer composite sheets.[274] On the other hand, the

basal surface of GO can be functionalized by the nucleophilic

ring-opening reaction between the epoxy groups of GO and

the amine groups of an amine-terminated organic molecular,

such as 1-(3-aminopropyl)-3-methylimidazolium bromide (anionic liquid) (Figure 1B) and 3-amino-propyltriethoxysilane

(APTS).[275,276] Also, rGO platelets were covalently function-

alized with diazonium salts (e.g., sodium dodecylbenzenesul-

fonate, SDBS), and the resulting rGO was readily dispersed

in several polar organic solvents (Figure 1C).[277] Addition-

ally, by the [2+1] cycloaddition of nitrenes onto the C=C

double bonds, a number of different organic compounds, such

as azido-phenylalanine (Figure 1D) and azidotrimethylsilane,

have been attached to the graphene surfaces.[278280] Recently,

the surface of graphene was covalently functionalized by

grafting polystyrene-polyacrylamide (PS-PAM) through in-

situ free-radical polymerization.[281]

Compared to covalent functionalization, noncovalentfunctionalization based on the van der Waals force or the

interaction between rGO and stabilizers not only gives less

negative impact on the structure of graphene and its deriva-

tives, but also provides the feasibility to tune their solubility

and electronic properties.

The first noncovalent functionalized stable graphene

dispersion was produced by reducing an aqueous GO

dispersion with hydrazine in the presence of poly(sodium

4-styrenesulfonate) (PSS).[152] In this experiment, the rGO

sheets were stabilized via the association with the hydro-

phobic backbone of PSS, while the hydrophilic sulfonate side

groups sustained the whole graphenePSS complex in water.

amount of edge defects. For example, the GNRs and GNMs

have exhibited band opening with much enhanced onoff

ratios in field-effect transistors (FETs) compared to 2D

graphene sheets.[5,182]

Featuring unique physical and chemical properties, and

having reliable synthetic methods for both solid and solu-

tion-phase processes, graphene and its derivatives have been

incorporated into a number of functional materials to formcomposites, and have been used as building blocks for var-

ious kinds of applications, including FETs,[18,75,76,110,188216]

memories,[85,86,217233] photovoltaic devices,[23,80,84,161,234252]

photocatalysis,[251,253256] sensors,[75,119,257266] cell cultures,[83]

intracellular imaging,[267] and matrices for matrix-assisted

laser desorption/ionization time-of-flight mass spectroscopy

(MALDI-TOF-MS).[82,268,269] In the following context, we

will focus on the synthesis and characterization of graphene-

based materials, the exploration of their properties, and

studies of their applications.

2. Graphene-Based MaterialsPristine graphene is a hydrophobic material, and has

no appreciable solubility in most solvents. Nevertheless,

the processing of graphene composites concerns itself

foremost with the solubilization of graphene. To improve

the solubility of graphene, different functional groups

have been attached to the carbon backbone by chemical

modification,[73,101,270,271] covalent,[272296] or noncovalent

functionalization.[74,77,152,287,289,290,293,297306]

It is impossible to directly disperse hydrophobic graphite

flakes or graphene sheets in water without the assistance

of dispersing agents. Because of the presence of oxygen-

containing groups, the chemically reduced graphene oxide(rGO), or so called chemically modified graphene, can form

the homogeneous aqueous suspension by controlled reduc-

tion of graphene oxide (GO) with hydrazine and dialysis

while maintaining the pH of the solution at about 10 by the

addition of amonia.[73] However, the resulting solubility of

rGO in water is very limited, with a value of


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Noncovalent functionalization of graphene sheets through

the interaction was reported by using the water-soluble

aromatic organic molecule 1-pyrenebyturate (PB) as a sta-

bilizer; and through subsequent vacuum filtration, large-area

flexible PBgraphene films with layered structures could

be prepared.[297] This approach provides a general route for

the preparation of functionalized graphene through the

interaction. Other reports based on this strategy have shownsignificantly improved solubility and conductivity of the func-

tionalized graphene sheets.[289,298,300,301]

2.1. GraphenePolymer Composites

Because of their good conductivity, thermal stability, and

excellent mechanical strength, graphene and its derivatives

are important filler materials for polymer composites. The

properties and performances of graphenepolymer com-

posites not only depend on the quality of graphene filler

and polymer matrix, but also depend on the dispersity of

the filler, the bonding between the filler and matrix, and the

ratio of filler to matrix. These factors are mainly determined

by the fabrication processes. Similar to the conventional

polymer processing, the methods applied for the fabrication

of graphenepolymer composites are solution mixing,[307312]

melt blending,[313315] and in-situ polymerization.[316327]

Solution mixing is one of the most commonly used

methods for preparation of polymer composites, since it is

straightforward, requires no special instruments, and allows

for large-scale production. One of the major concerns in

solution mixing is the solubility or dispersity of graphene

sheets in the polymer solution. For example, the water sol-

uble polymer, such as poly(vinyl alcohol) (PVA), is easy to

be mixed with aqueous GO dispersions at various concen-trations.[307,328] However, the low solubility of GO and rGO

in organic solvents poses great challenge in synthesis of

graphenepolymer composites in organic solutions. One way

to solve this problem is to use ultrasonication to produce

short-time metastable dispersions of GO or other graphene

derivatives, which are then mixed with polymer solutions,

such as poly(methyl methacrylate) (PMMA),[329] polycapro-

lactone (PCL),[308] polyurethane (PU),[309,330] and polyaniline

(PANI).[310] High-speed shearing has also been utilized to

mix graphene-based fillers and the polymer matrices, while

ice baths have been used to prevent extra heating from the

shearing process.[311] However, re-aggregation of the rGO

sheets might occur during the slow process of solvent evap-oration. Therefore, it is important to modify the graphene

sheets or graphene derivatives with functional molecules, in

order to increase the solubility in various kinds of solvents.

For example, after GO sheets were functionalized with

phenyl isocyanate and mixed with polystyrene (PS) solution

in DMF,[331] they were further reduced to rGO. During the

experiment, the polymer matrix prevented the re-aggregation

of rGO sheets and the dispersed suspension maintained very

well. However, these studies only rely on the stabilizers with

weak electronic delocalization systems (such as small aro-

matic molecules), which limit the solubility of the obtained

rGO based materials.

In addition, biomolecules have been involved in the func-

tionalization process of graphene based on the van der Waals

force, such as using single-strand DNA (ssDNA) to stabilize

aqueous suspensions of single-layer graphene sheets.[299]

Figure 1. A) Isocyanate treatment of GO where organic isocyanatesreact with the hydroxyl (left oval) and carboxyl groups (right oval) ofGO sheets to form carbamate and amide functionalities, respectively.Reproduced with permission.[273] Copyright 2006, Elsevier, Inc.B) Covalent functionalization of epoxy groups of GO by an ionic liquid.

Reproduced with permission.[276] Copyright 2009, Royal Society ofChemistry. C) Covalent functionalization of rGO with diazonium salts.Reproduced with permission.[277] Copyright 2008, American ChemicalSociety. D) Functionalization of exfoliated microcrystalline graphene(G) with azido-phenylalanine (Phe-N-G) in o-dichloro-benzene (ODCB)by nitrene chemistry. Reproduced with permission.[278] Copyright 2009,Royal Society of Chemistry.


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Recently, in our group, a specially designed conjugated

polyelectrolyte (CPE), PFVSO3 with a planar backbone and

charged sulfonate and oligo(ethylene glycol) (OEG) side

chains, was used to modify rGO sheets (Figure2A,B).[77] Due

to the strong interaction between PFVSO3 and rGO, the

resulting CPE-functionalized rGO (PFVSO3-rGO) shows

excellent solubility and stability in a variety of polar solvents,

including water, ethanol, methanol, DMSO, and DMF. Inorder to further improve the solubility of graphenepolymer

composites in both high polarity (such as water and ethanol)

and low polarity (such as toluene and chloroform) solvents,

an amphiphilic coil-rod-coil conjugated triblock copolymer

(PEG-OPE, chemical structure shown in Figure 2C),[74] which

is composed of one lipophilic -conjugated oligomer and two

hydrophilic PEG coils, has been designed and synthesized

to modify rGO sheets. The conjugated rigid-rod backbone

of PEG-OPE can attach to the basal plane of rGO by the

strong interaction (Figure 2D), whereas the lipophilic

side chains and two hydrophilic coils of the backbone fly

away from the rGO surface to form an amphiphilic outer-

layer. As a result, the obtained graphenepolymer compositeare soluble in both organic low-polarity and water-miscible

high-polarity solvents (Figure 2E).

Melt compounding is another popular fabrication method

in industry for high-yield fabrication of polymer composites,

which involves the blending of filler materials and polymer

matrices using high-shear forces at elevated temperatures.

Polylactide (PLA)-exfoliated graphite (EG) composites have

been successfully prepared by mixing PLA chips and EG in

a mechanical mixer at 175200 C.[313] In another example,

graphene platelets were obtained by thermal reduction of

GO, and then melt-blended with polyethylene terephthalate

(PET) at 285 C to give the PETgraphene composite.[314]

However, melt compounding is not as effective as the solu-

tion mixing method, in terms of the ability to achieve good

dispersity and distribution of the filler materials in the matrix.

In addition, the use of high shear forces can sometimes break

the filler materials, such as carbon nanotubes and graphenesheets.[315]

In-situ polymerization involves the mixing of monomer

solution and the suspensions of graphene- based materials,

in the presence of catalysts at proper reaction conditions.

Epoxy is a typical polymer suitable for in-situ polymeriza-

tion.[316318,332] For example, a graphene nanoplatelet suspen-

sion was mixed with epoxy resins and subjected to high-shear

mixing. While stirring and heating the mixture to remove

the solvent, epoxy curing agent was added to finalize the

polymerization process.[316] Polyaniline (PANI) is another

type of polymer, which is usually synthesized by in-situ

polymerization.[319323] Since the polymerization of PANI is

an oxidative process, an oxidative agent, such as ammoniumpersulfate, is added to the reaction solution to facilitate the

polymerization.[321] In addition to solution-based oxidation,

graphenePANI composite can be prepared by the in-situ

anodic electropolymerization,[324] where graphene paper

was obtained using H2 reduction of thermally expanded GO,

followed by filtration and vacuum drying. PANI was then

electrochemically deposited with the graphene paper as the

working electrode in a three-electrode cell containing the

electrolyte with aniline monomers. Besides epoxy and PANI,

Figure 2. A) Chemical structure of the designed PFVSO3. B) Schematic illustration of PFVSO3-stabilized rGO sheets. Reproduced with permission.[77]

C) Chemical structure of PEG-OPE. D) Schematic illustration of PEG-OPE stabilized rGO sheets. E) Photograph of PEG-OPE-rGO dispersed in differentsolvents. [PEG-OPE-rGO] = 1.22 mgmL1. Reproduced with permission.[74]


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flexible and exhibit good mechanical properties, which can

be used in capacitive pressure sensors, an alternative to the

stiff silicon in micro-electromechanical systems (MEMS).[333]

In another report, strong and ductile poly(vinyl alcohol)

(PVA)graphene composites were made by vacuum filtra-

tion. At 3 wt% of GO loading, a Young's modulus of 4.8 GPa

and tensile yield strength of110 MPa were achieved. To fur-

ther improve the interaction of the graphene filler and thepolymer matrix, thus facilitating the effective load transfer,

polyurethane (PU) was covalently bonded with GO plate-

lets, and the Young's modulus and hardness were increased

by 900% and 327%, respectively. The resulting improved

hardness indicates a high resistance to scratching, which is

important in surface coating applications.[309] The effective

enhancement of mechanical properties of these polymer

graphene composites arises from not only the intrinsic prop-

erties of graphene, but also the good dispersion of filler, as

well as the effective interfacial interaction between the filler

and host polymer, which prevents the movement of polymer

chains under the external force.

The thermal stability of polymers is generally character-ized by the glass transition temperature (Tg), above which

the molecular chains begin to slide pass each other when

an external force is applied. By adding stiff filler materials,

which interfere with the flowing process of the chains, the Tg

of polymer composites can be increased. For example, after

poly(amido amine) (PAMAM) was mixed with graphitic

nanoplatelets (FNPs) in THF by ultrasonication and high-

speed shearing, the obtained composite exhibited up to a

30 C increase in Tg with 15 wt% loading of the filler.[329]

The performance of the composites was further enhanced

by replacing the FNPs with functionalized graphene sheets

(FGSs) as the nanofiller, which was obtained by the thermal

exfoliation of highly oxidized graphite oxide. Compared toFNPs, FGSs possess a larger amount of single-layer graphene

sheets with wrinkled morphology and functionalized surfaces,

which in turn give stronger fillermatrix interactions. With a

loading of 0.05 wt% FGS, there is an increase of 30 C in Tg

for the formed PAMAMFGS composite, while with a loading

of 1 wt% FGS, the improvement of Tg in poly(acrylonitrile)

(PAN)FGS composite reaches more than 40 C.[311] Impor-

tantly, at a


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matrix is expected to significantly enhance the thermal con-

duction of the composite. Thermally exfoliated graphite nano-

platelets were incorporated in an epoxy matrix. It was found

that the introduction of few-layer graphene (layer number:

4) resulted in a thermal conductivity enhancement of more

than 30 times at k= 6.44 W/mK with a loading of25 vol%

of graphene.[316] This result outperforms conventional fillers,

such as silver, alumina, and silica, which require 5070%loading to achieve a similar result. In addition, when FGSs

were added as the filler in silicon foam matrix, 0.25 wt% of

loading had led to a 6% increase in the ultimate thermal con-

ductivity.[325] The large aspect ratio, 2D geometry, high stiff-

ness, and low thermal interface resistance of graphene have

caused the enhanced thermal conductivity in these reports.

Such composite foams with excellent heat dissipation prop-

erty are advantageous for coatings on electrical wires to elim-

inate conductive heat.

2.2. GrapheneMetal Nanoparticle Composites

The attractive properties of graphene and its deriva-

tives have made them ideal templates for the synthesis

of metal nanoparticles (NPs), such as Au,[81,87,118,334359]

Ag,[81,360362] Pd,[165,334,341,349,363368] Pt,[118,334,369] Ni,[370] and

Cu.[341] Dependent on the type of the anchored NPs, the

graphenemetal NP composites have been applied in a

broad range of areas, such as surface-enhanced Raman scat-

tering (SERS),[81,337] catalysis,[349,363] and electrochemical

sensing.[119,348,371373] The graphenemetal NP composites can

be prepared by chemical reduction,[334,336,339,373,374] photo-

chemical synthesis,[87,98] microwave assisted synthesis,[341]

electroless metallization,[81] and thermal evaporation.[375]

One of the most straightforward approaches to preparegraphenemetal NP composites is the direct chemical reduc-

tion of the metal precursors in the presence of GO or rGO

suspensions. The GO and rGO sheets possess defects and

residual oxygenated functional groups, which can act as the

nucleation sites for the growth of metallic nanostructures. The

first grapheneAu NP composite was prepared by the reduc-

tion of AuCl4with NaBH4 in a rGO octadecylamine (ODA)

solution.[339] The ODA functionalization of rGO offers good

solubility to the composites in low-polarity solvents, e.g.,

THF. In addition to Au NPs, Pd NPs supported on GO were

prepared by bubbling hydrogen through a suspension of

Pd2+GO in ethanol, and the obtained composites were used

as catalysts in the SuzukiMiyaura coupling reaction.[

349

]

Thepd/GO and pd/rGO composites exhibited higher catalytic

activities compared to conventional Pd/carbon catalysts, with

turnover frequencies exceeding 39 000 h1, accompanied by

very low Pd leaching (


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the identification method based on atomic force microscopy

(AFM) measurements.

2.3. GrapheneSemiconductor Nanomaterial Composites

There is great demand for the synthesis of graphene

semiconductor nanomaterial composites because of their

promise in electronics, optics, and energy-based applications

such as solar cells, Li-ion batteries, and supercapacitors. To

date, various kinds of semiconductor nanomaterials have been

synthesized and supported on graphene-based templates,which include TiO2,[88,122,251,253,254,256,376386] ZnO,[78,80,387391]

SnO2,[392397] MnO2,

[398401] Co3O4,[402406] Fe3O4,

[407417]

Fe2O3,[418,419] NiO,[221] Cu2O,

[78,79,420] RuO2,[421,422]

CdS,[238,245,423428] and CdSe.[100,429432] The synthetic methods

for preparation of these graphenesemiconductor nano-

material composites include in-situ crystallization,[378,380,424]

solution mixing,[256,376,404] microwave-assisted growth,[400,402]

electrochemical deposition,[78,80,429] and vapor deposition.[221]

The in-situ crystallization approach has been considered

as one of the most commonly used methods to synthesize

composites of GO or rGO and semiconductor nanomate-

rials. For example, grapheneCdS composites were prepared

groups present in GO compared to rGO. Similar phenom-

enon is also reported in the GO/rGO templated growth of

Ni nanocrystals, where the higher degree of oxidation of

graphene sheets has led to larger nucleation density of Ni

NPs.[370]

As discussed above, the surface chemistry of chemi-

cally modified graphene can affect the nucleation density of

anchored metal NPs,[81] whereas the lattice atomic structure

of rGO can direct the assembly and pattern formation of

Au NDs.[87] In addition to these observations, the number of

pristine graphene layers also showed a direct impact on the

particle size and density of thermally evaporated Au NPs,[

375

]

i.e., the particle size decreased and density increased with

increasing layer numbers of the graphene film (Figure 3C,D).

Two factors have been considered to cause this interesting

phenomenon. First, the surface free energy of graphene is

dependent on the layer number, which controls the inter-

action between graphene and the evaporated Au atoms.

Second, the diffusion coefficient of Au atoms varies on dif-

ferent surfaces, which determines the outcome of the compe-

tition between nucleation and growth of the Au islands. [375]

It is also suggested that, the different densities and sizes of

NPs observed in SEM could be used to identify the layer

numbers of graphene, which might give an alternative to

Figure 3. A) Transmission electron microscopy (TEM) image of ODT-capped Au NDs synthesized in situ and assembled on a rGO surface. Inset of(A) is a high-resolution TEM image of an Au ND. Reproduced with permission.[87] B) Scanning electron microscopy (SEM) image of Ag NPs denselygrown on GO sheets. Reproduced with permission.[81] Copyright 2009, American Chemical Society. C) Au NPs on monolayer, bilayer, and trilayergraphene, respectively. D) Statistics of the size and density of Au NPs on n-layer graphenes. Reproduced with permission.[375] Copyright 2010,American Chemical Society.


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network. Consequently, the anode made from the graphene

Co3O4 NP composites showed a very high reversible capacity

of 1100 mA h g1 in the first 10 cycles, and over 1000 mA h g1

after 130 cycles (Figure 4C).Thin-film-based applications require postsynthetic depo-

sition of the composite materials on substrates by techniques

such as spin-coating, drop casting, and transfer printing.

Therefore, the direct electrochemical deposition of NPs on

graphene-based substrates is an attractive approach to pre-

pare certain types of graphenesemiconductor nanomaterial

hybrid films, for example, ZnO, Cu2O, and CdSe.[78,80,429] For

instance, ZnO nanorods were deposited on spin-coated rGO

thin films on quartz, using an oxygen-saturated aqueous solu-

tion of ZnCl2 and KCl as the electrolyte.[80] It is found that

the crystal quality of the deposited ZnO nanorods depends

on the thickness of the rGO thin films. For example, the thick

rGO film electrode with low resistivity gives better qualityof the deposited ZnO nanorods. This method has also been

applied to deposit Cu2O nanostructures on rGO films, spin-

coated on flexible polyethylene terephthalate (PET) sub-

strates.[78] Different from the random deposition of ZnO

and Cu2O on rGO films, the ordered CdSe NP patterns on

graphene films were realized through a templated electro-

chemical process.[429] In this work, after graphene was epitax-

ially grown on Ni film, a layer of mesoporous silica film was

formed on the graphene sheets via a solgel process. Then

CdSe NPs were electrochemically deposited on the graphene

surface through the precoated porous silica film. This work

demonstrates a ligand-free process for the pattern formation

by mixing GO and Cd(CH3COO)2 in DMSO, which was

then heated in an autoclave at 180 C for 12 h.[424] During

the synthetic process, the hydrothermal process results in the

simultaneous formation of CdS nanoparticles (NPs) and the

reduction of GO to rGO in DMSO, which acts as both

the solvent and the sulfur source. Time-resolved fluorescence

spectroscopy data showed a picosecond ultrafast elec-

tron transfer process from CdS NPs to the graphene sheet,which demonstrates the potential optoelectronic applica-

tion of this grapheneCdS hybrid material. In another work,

grapheneCo3O4 hybrid material was synthesized by reacting

Co(NO3)26H2O and ammonia solution in the presence of

GO sheets, followed by drying and heating at 450C to result

in the Co3O4graphene composite used for the Li ion bat-

tery application.[403] The in-situ crystallization approach is

also applicable to synthesis of many other types of semicon-

ductor nanostructures on graphene-based templates such

as MnO2 nanoneedles,[398] TiO2 rods,

[380] and SnO2 NPs.[433]

Alternatively, the in-situ microwave irradiation offers a

fast and easy way to synthesize graphenesemiconductor

NP composites, which has been applied to prepare grapheneMnO2 NPs[400] and grapheneCo3O4 NPs.

[402] For example,

after a water suspension of rGO was mixed with potas-

sium permanganate powders by ultrasonication, the mix-

ture was heated in a household microwave oven for only

5 min to obtain the grapheneMnO2 NP composites.[400]

Another efficient and direct means to prepare graphene

semiconductor NP composites is the solution mixing

approach.[256,376,379,404] For example, commercialized TiO2

NPs (P25) were mixed with Nafion-coated graphene to fab-

ricate dye-sensitized solar cells, where the Nafion served as a

glue to tightly bind graphene sheets and NPs.[379] In another

example, after the pre-synthesized CdS NPs were functional-

ized with benzyl mercaptan molecules and then mixed withrGO sheets, the benzyl mercaptan-capped CdS NPs absorbed

on rGO surfaces through interactions.[423] However, in

some cases the pre-synthesized NPs from an organic synthetic

route are not compatible with the aqueous dispersion of GO

sheets. In order to solve this problem, a two-phase approach

was developed to prepare grapheneTiO2 composites. For

example, after a solution of oleic acid-capped TiO2 nanorods

in toluene was mixed with GO water suspension and stirred

for 24 h, the TiO2 nanorods were able to assemble on the GO

surface at the water/toluene interface.[256]

Very recently, a novel strategy used to prepare graphene-

encapsulated metal oxides hybrids has been developed. In

this work, after the negatively charged GO sheets wrappedaround the positively charged Co3O4 NPs (modified by ami-

nopropyltrimethoxysilane, APS) through the electrostatic

interaction, GO was chemically reduced to rGO. The obtained

composites were successfully used for the Li ion battery

application (Figure 4AC).[404] This approach is essentially

important for the preparation of graphenemetal oxide NP

composites as anode materials for lithium ion storage due to

the following reasons. First, the graphene encapsulation can

suppress the aggregation of oxide NPs during the charge

discharge process. Second, it gives rise to high oxide content

in the composites (91.5 wt%). Third, an overall high electro-

conductivity of the composite is maintained by the graphene

Figure 4. A) Schematic illustration of fabrication of graphene-encapsulated metal oxide NPs. B) Typical SEM image of graphene-encapsulated Co3O4. C) Cycle performance of graphene-encapsulatedCo3O4. Reproduced with permission.

[404]


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3. Applications of Graphene andGraphene-Based Materials

3.1. Field-Effect Transistors

One of the most extensively explored applications

of graphene is the field-effect transistor (FET). The first

graphene-based FET was reported by Novoselov et al. in

2004,[18] where single- and few-layer graphene (FLG) with

sizes up to 10 m was obtained by mechanical exfoliation of

highly oriented pyrolytic graphite (HOPG). 2D FLG behaves

with metallic characteristics and exhibits a strong ambi-

polar electric field effect. The pristine graphene could not

be used for the fabrication of effective FET devices because

the onoff current ratio (Ion/Ioff


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graphene oxide (GO),[223,441] graphene

Au hybrid systems,[104] graphenepolymer

composites,[218] and rGO-electrodes.[219,220]

In the graphene-based nonvola-

tile memory device with ferroelectric

gating,[217] the binary information, i.e., high

and low resistance states of the graphene

working channel, can be switched by con-trolling the polarization of the ferroelec-

tric thin film induced by the electrostatic

doping of graphene. In the case of GO-

based memory devices, the properties of

desorption/absorption of oxygenated func-

tional groups on the GO sheets[441] and

the unique charge trapping properties of

GO[223] have enabled the switching effect

with the threshold voltage of 1 and 1.4 V,

respectively. [223,441]

Thanks to the feasibility of large-area

production of rGO films, the fabrication

of both conductivity- and type-switching memory deviceshas been successfully demonstrated in a rGO/Au NP hybrid

system (Figure6).[104] The real-time type-switching of the car-

rier types in the rGO-FETs can be controlled by applying a

gate bias to adjust the charges in the Au NPs, thus in turn

transconductance. Therefore, a dense array of ordered nanori-bbons is desirable, which, however, remains a significant

challenge. Alternatively, another type of graphene nanostruc-

tures, called graphene nanomesh (GNM)[182,183] or nanoper-

forated graphene,[184] was developed, which not only opens

the bandgap of graphene but also provides

high driving current. With block copolymer

lithography, the fabricated GNMs could

have variable periodicities with a neck

width as small as 5 nm. The FETs fabri-

cated with GNM as the channel material,

referred to as GNM-FETs, can support

currents 100 times greater than individual

GNR devices, while the onoff ratio, whichcan be tuned by varying the neck width,

is comparable with that achieved in indi-

vidual GNR devices (Figure5). Addition-

ally, the block copolymer lithography is

scalable, and allows for the rational design

and fabrication of GNM-FET devices with

standard semiconductor processing.

3.2. Memory Devices

Graphene-related materials have

also been widely explored in memorydevices.[222] It was demonstrated theo-

retically by Gunlycke et al. that when

the graphene nanostrips are under a

non-equilibrium state, e.g., in the pres-

ence of a ballistic current, both spin-

polarized and spin-unpolarized nanostrip

states exist, which could be switched

through the applied bias in a binary

memory device. After that, extensive

experimental investigations on graphene-

related memory devices were carried

out, based on pristine graphene,[217]

Figure 5. A) Schematic of a GNM-FET. B) Drain current (Id) versus drainsource voltage (Vd),recorded at different gate voltages for a GNM device with a channel width of2 mm andchannel length of1 mm. The on-state conductance at Vg= 10 V is comparable to an arrayof 100 parallel GNR devices. C) Transfer characteristics for the device in B at Vd= 210, 2100,and 2500 mV. The ratio between Ion and Iofffor this device is 14 at Vd= 2100 mV. D) Transfercharacteristics at Vd= 2100 mV for GNMs with different estimated neck widths of15 nm(device channel width 6.5 m and length 3.6 m), 10 nm (channel width 2 mm and length1 mm), and7 nm (channel width 3 mm and length 2.3 mm). Reproduced with permission.[182]Copyright 2010, Nature Publishing Group.

Table 1. Comparison of methods used for generation of GNRs and the resulting onoffcurrent ratio obtained from the GNR-based FETs.

Methods for

preparation

of graphene

Methods for

generation of

graphene nanoribbons

Width of graphene

nanoribbons

[nm]

Maximum

bandgap opening

[meV]

Current

onoff

ratio

References

Mechanically

peeled from

graphitecrystals

Oxygen plasma with

e-beam lithography

generated resistpattern as etch mask

15 200 N.A. [170]

Mechanically

peeled from

graphite crystals

Oxygen plasma with

dielectric nanowire as

etch mask

15 N.A. 12 [169]

Exfoliation plus

ultrasonication

based on

expandable graphite

Chemical

derivation

2 400 107 [5]

Unzipping multiwall

carbon nanotubes

Plasma etching 6 N.A. >100 [177]


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memory device exhibits a high onoff ratio

(104105) and low switching threshold

voltage (0.51.2 V), which are related to the

sheet resistance of the rGO electrode. The

rGO films in this work are prepared from

the thermal reduction of GO at 1000 C.

We have also developed a method to fab-

ricate highly reduced GO films by chem-ical reduction with smeared hydrazine at

low temperature (e.g., 100 C) combined

with a multilayer stacking technique.[85]

The resulting rGO films show a low sheet

resistance of 160500 sq1. By using

them as the electrodes in memory devices

with a configuration of rGO/P3HT:PCBM/

Al, a high onoff current ratio of up to106

is achieved.

3.3. Photovoltaic Devices

Owing to its good electronic con-

ductivity, optical transparency, and large

specific surface area, graphene-based

materials have been extensively explored

in the field of photovoltaics. Until now,

lots of studies on graphene-based photo-

voltaic applications have been reported, in

which graphene-based materials are used

as the transparent electrodes,[23,80,105,234,244,248,435,442] electron

acceptor,[161,237238,246,376,378379] and light absorber.[161]

3.3.1. Transparent Electrodes

Graphene-based transparent electrodes are mainly employed

in three types of photovoltaic devices: organic,[23,105,244,435,442]

inorganicorganic hybrid,[80] and dye-sensitized solar cells.[234]

In the organic photovoltaic (OPV) solar cells, the highest

power conversion efficiency (PCE, ) of 1.71% has been

reported based on the use of large-area, continuous, trans-

parent, and highly conducting few-layer graphene films

deposited by the CVD process.[244] The good performance is

attributed to the noncovalent modification of the graphene

films with pyrene buanoic acid succidymidyl ester. In another

report, CVD-deposited-graphene is successfully transferred

onto a flexible PET substrate for fabrication of OPV devices.

It is worth noting that the graphene/PET-based and ITO/PET-based OPV devices showed comparable performances with an

of 1.18 and 1.27%, respectively.[23] Importantly, the CVD-

grown graphene-based solar cells show outstanding capability

to be operated under bending conditions with bending angles

up to 138, while the ITO-based devices display cracks and

irreversible failure under bending of only 60. Therefore, the

epitaxially grown graphene used as a highly flexible, transparent,

and continuous electrodes for OPVs presents itself as a poten-

tial replacement for ITO in flexible photovoltaic applications.

As an alternative to CVD-grown graphene, rGO film has

been used as transparent electrode as well.[80] In this study,

ZnO film is electrochemically deposited on the rGO electrode

affecting the charges in the FET channel. Importantly, such

type-switching behavior of the rGO-based devices is success-

fully applied to build the reconfigurable inverter logic gates

by controlling the charges in the Au NPs.[104]

Graphenepolymer composites have been investigatedin memory devices as well, through a reliable and low-cost

synthetic route.[218] As reported, GO was modified by a

conjugated-polymer of triphenylamine-based polyazome-

thine (TPAPAM), and then incorporated into a nonvola-

tile memory device. This device exhibits a typically bistable

electrical switching and nonvolatile rewritable memory

effect. An onoff current ratio of more than 103 is achieved.

Under application of a constant voltage stress, both

the on and off states from such TPAPAMGO-based

memory device are stable up to 108 read cycles at a read

voltage of 1.0 V.

The use of indium tin oxide (ITO) and some metal

materials as electrodes in memory devices has been limitedbecause of their brittle nature, high cost, and insufficient ele-

ment resources. Consequently, the memory device based on

polymer composites of poly(3-hexyl thiophene) (P3HT) and

methanofullerene [6,6]-phenyl C61-butyric acid methyl ester

(PCBM) bulk-heterojunction (BHJ) on rGO film electrode

has been developed in our group.[86] The currentvoltage

(IV) curves from this rGO/P3HT:PCBM/Al memory show

the electrical bistable behavior, a write-once-read-many-

times (WORM) memory effect. The carrier transport mecha-

nism related to the conductivity-switching is affected by the

polarization of PCBM domains and the formation of a local-

ized internal electrical field among the adjacent domains. The

Figure 6. A) Operation principle of type-switching memory device. B) Typical hysteresiscurve of type-switching memory device based on GO and NPs. It exhibited both n-type andp-type characteristics near zero gate bias voltage. C) Real-time switching of the reduced GOchannel type. After applying a large positive (or negative) top-gate bias, a small back-gatebias was used to confirm the p-type (or n-type) behavior of the channel. D) Switching theinverter functionality by using a transistor that consisted of graphene and NPs. Reproducedwith permission.[104]


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2D structure and high electron mobility. To date, to the best

of our knowledge, graphene-based materials that have been

used as an electron acceptor in photovoltaic devices include

functionalized graphene,[237] a grapheneTiO2 composite,[376]

and a layered graphenequantum dot hybrid.[238]

In order to fabricate functionalized graphene-based

photovoltaic devices, GO sheets were first reacted with

phenyl isocyanate to change their surfaces from hydrophilicto hydrophobic. The resulting solution-processed functional-

ized graphene (SPFGraphene) was then mixed with poly(3-

octylthiophene) (P3OT) to form the P3OT/SPFGraphene

composite, which was used as the active layer in the bulk

heterojunction (BHJ) OPV device.[237] After optimizing the

annealing process (160 C for 20 min) for the fabricated OPV

device, the best PCE of 1.4% was obtained. This work sug-

gests the promises of SPFGraphene materials to serve as a

competitive alternative to PCBM as the electron acceptor for

high-performance OPV devices.

The grapheneTiO2 composite, as another type of effec-

tive electron acceptor, has been used as the photoanode in

DSSCs.[

376

]

In fabricated DSSCs, the graphene works as abridge which could enhance the charge transport rate to pre-

vent the charge recombination and increase the light collec-

tion efficiency, thus improving the photoelectrical conversion

efficiency of DSSCs. Compared with pure TiO2 photoanode-

based DSSCs, the short-circuit current density for the

grapheneTiO2 photoanode based device is increased by

45%, without sacrificing the open-circuit voltage. As a result,

the total conversion efficiency is measured as 6.97%, which

shows a 39% increase compared to that of pure TiO2-based

DSSCs.

A simple bottom-up approach, i.e., the electrophoretic

deposition of graphene layers followed by the chemical

bath deposition of a CdSe quantum dot (QD) layer, isused to create a novel graphene/CdS-QD bilayer structure,

which works as the electron transfer system in photovoltaic

devices.[238] The best performance with a PCE of 16% is

obtained from the photovoltaic device

fabricated with eight repeated graphene/

CdS-QD bilayers. In this work, the signifi-

cantly improved photoresponse, especially

the photocurrent, suggests that graphene

is a good candidate for the collection and

transport of photogenerated charges.

3.3.3. Light Absorbers

The tunable bandgap and large optical

absorptivity of graphene are appealing

characteristics for the efficient light har-

vesting. By using small organic compounds,

graphene QDs that contain 168 conju-

gated carbon atoms have been synthesized

via a stepwise solution-based chemical

route.[161] The uniform-sized graphene

QDs are made highly soluble after their

edges are covalently reacted with 1,3,5-

trialkyl phenyl moieties. This novel solu-

bilization strategy enables the QDs to act

and subsequently incorporated to a hybrid solar cell based on a

ZnOP3HT system. The PCE of the inorganicorganic hybrid

solar cell with a layered structure of quartz/rGO/ZnO NR/

P3HT/PEDOT:PSS/Au (PEDOT:PSS = poly(3,4-ethylenediox

ythiophene):poly(styrenesulfonate)) is around 0.31% (Device

II in Figure 7A). The rGO film with a higher conductivity

after thermal annealing gives a higher work function, which is

changed from 5.0 to 4.7 eV. It results in a better matchingbetween the conduction band of ZnO and Fermi level of rGO,

thus improving the performance of the fabricated hybrid solar

cells. In addition, the chemically derived rGO films have been

transferred onto PET substrates, used as transparent and flex-

ible electrodes in OPV devices,[84] with a configuration of rGO/

PEDOT:PSS/P3HT:PCBM/TiO2/Al. Importantly, we found that

when the optical transmittance of rGO is above 65%, the per-

formance of the OPV devices mainly depends on the charge

transport efficiency through the rGO electrodes, whereas if the

transmittance is less than 65%, the performance of the devices

is dominated by the light transmission efficiency.

In addition to OPV devices, graphene has been used

as the transparent electrode for dye-sensitized solar cells(DSSCs).[234] A graphene thin film, prepared by dip coating in

a hot, aqueous GO dispersion followed by thermal reduction,

exhibits a high conductivity of 550 S/cm and a transparency

of more than 70% over the 10003000 nm range. It was used

to fabricate a solid-state graphene/TiO2/dye/spiro-OMeTAD/

Au device (Spiro-OMeTAD = 2,2,7,7-tetrakis(N,N-di-p-

methoxyphenilamine)-9,9-spirobifluorene) (Figure 7B). This

is the first demonstration of a solid-state DSSC based on a

graphene electrode; however the PCE (0.26%) of this device

is lower than that (0.84%) of the corresponding fluorine tin

oxide (FTO)-based solid-state DSSC.

3.3.2. Electron Acceptors

Graphene has emerged as a new electron-accepting mate-

rial in photovoltaic device applications because of its unique

Figure 7. A) Under simulated globe sun illumination, the obtained IVcurves for ZnO/P3HThybrid solar cells by using I) one-step and II) two step reduced GO films as electrodes. Inset:schematic illustration of the fabricated solar cell. Reproduced with permission. [80] B) Top:illustration of dye-sensitized solar cell using graphene film as electrode. The four layersfrom bottom to top are Au, dye-sensitized heterojunction, compact TiO2, and graphene film.Bottom: IVcurve of graphene-based cell (black) and the FTO-based cell (grey), illuminatedunder AM solar light (1 sun). Reproduced with permission. [234] Copyright 2008, AmericanChemical Society.


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Such excellent performance is attributed to the high carrier

mobility of graphene[1819] and the resulting extremely low

noise.[1920]

By electrochemical gating through the top gate, GFET

is readily configured as a biosensor in physiological buffer

solution. It has been shown that GFET is sensitive to the pH

value of an electrolyte,[448] and is able to detect protein in the

nanomolar scale in bovine serum albumin (PBS) buffer.

[

445

]

Recently, the Lieber group reported that compared to a

nanowire-based sensor, GFET gave competitive perform-

ance for recording bioelectrical signals from living cells. [447]

Besides the mechanically exfoliated graphene, the epitaxially

grown graphene is also used as the channel material, which is

able to detect DNA with single-base-mismatch sensitivity.[444]

Reduced graphene oxide (rGO), prepared from chemical

or thermal reduction of chemical derived GO, is a prom-

ising substitute for pristine graphene as a channel material

due to its low cost and large-scale production. For example,

a GFET sensor is used for the detection of chemical warfare

agents and explosives at a sensitivity in the part-per-billion

(ppb) level based on rGO thin films, which are fabricated byspin-coating GO sheets on a Si/SiOx substrate, followed by

subsequent chemical reduction.[108] In another example, a

flexible GEFT sensor, fabricated by ink-jet-printed rGO films

on PET), is used to detect NO2 and Cl2 vapors with an air

sample at the ppb level.[116] Recently, our group reported a

flexible GFET sensor using a micopatterned rGO thin film as

the channel material to detect chemicals, metal ions and pro-

teins.[75] Moreover, such a sensor is able to detect hormonal

catecholamine molecules and their dynamic secretion from

living cells in physiological buffer solution.[75]a] As shown

in Figure 8, neuron cells (PC12) are directly cultured on a

relatively long channel (1 cm) of rGO thin film. The source

drain current is continuously monitored to detect the secre-tion of catecholamine molecules by PC12 cells in the high K+

solution.[75a]

as light absorbers (sensitizers) and to replace the tradition-

ally used ruthenium complexes in nanocrystalline TiO2 solar

cells. However, the resulting energy conversion perform-

ance is not high, which requires improvement by covalently

binding functional groups to the QDs for higher charge injec-

tion, or optimizing the preparation procedure for the elec-

trode. Nevertheless, this solubilization strategy opens exciting

opportunities to tune the optical and electronic properties ofgraphene for photovoltaics.

3.4. Sensing Platforms

3.4.1. FET Sensors

Chemical and biological sensors based on graphene

(or graphene derivatives) field-effect transistors (GFETs)

are receiving continuous interest due to their high sensi-

tivity,[258] low noise,[19,20,258] facile fabrication, and biocompat-

ible nature.[83,443] Gas sensors based on GFET have shown

promising results, even being able to detect the absorptionof a single gas molecule.[258] Recently, several groups have

reported GFET-based biosensors, which include the detec-

tion of biomolecules such as DNA[444] and protein,[94,445] and

biosignals arose from the interaction between graphene and

bacteria[446] or cells.[75,447]

In a typical GFET-based sensor, graphene is used as con-

ducting channel between drain and source electrodes. Gate

potential is applied through back-gate (typical SiO2 thin

layer) or top-gate (electric double layer in electrolyte) for gas

and biosensing, respectively. The absorption of analyte mol-

ecules or change of local environment leads to the change of

its electrical conductance.

The first gas sensor based on GFET was prepared by usingmechanically exfoliated graphene as the channel material. [258]

An outstanding single-molecule-detection limit is achieved.

Figure 8. A) Schematic illustration of the experimental setup of front-gate GFET for sensing application. B) Schematic illustration of the interfacebetween a PC12 cell and GFET. C) Real-time response of rGO/PET FET to the vesicular secretion of catecholamines from PC12 cells stimulated byhigh K+ solution. Reproduced with permission.[75a] Copyright 2010, American Chemical Society.


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nitrogen plasma. The obtained N-doped rGO results in high

electrocatalytic activity for H2O2 reduction, fast electron

transfer for glucose oxidase, and a low glucose detection limit

(0.01 mm). In another approach, pyrene-grafted poly(acrylic

acid) (PAA) modified-graphene sheets are deposited alter-

natively with poly(ethyleneimine) (PEI) on a glassy carbon

electrode via an LbL assembly approach.[452] The obtained

multilayer graphenepolymer films display enhanced elec-tron transfer and excellent electrocatalytic activity for H2O2.

The surface modification of Au electrode is realized

with thiol-terminated molecules, e.g., n-octadecylmercaptan

(C18H37SH), which are used to adsorb graphene sheets via the

noncovalent interaction.[259] In this report, the heterogeneous

electron transfer blocked by the thiol molecules have been

restored by graphene sheets, and the graphene-modified Au

electrode gives a smaller interfacial capacitance compared

with the pure Au electrode.

In order to take advantages of the good catalytic activity

of noble metal NPs, grapheneNP composites have been

applied in electrochemical sensing.[343,372,471,473] In one of the

attempts, a grapheneAu NP composite is mixed with chi-tosan solution and used to modify a Au electrode for sensing

H2O2 and O2, which exhibits good electrocatalytical activity.

By further hybridizing this composite with glucose oxidase

(GOD), the graphene/Au NPs/GOD/chitosan electrode

shows excellent response to glucose with a linear range from

2 to 10 mm at 0.2 V.[473] In addition, graphenePt NP is used

to modify glassy carbon electrode, which exhibits superior

detection performance compared to the electrode only modi-

fied by graphene, with a wide linear range and low detec-

tion limit for H2O2 and trinitrotoluene (TNT).[372] In these

reports, the enhanced performance of graphenemetal NP

composites arise from the excellent electrical conductivity

and high specific surface area of graphene, as well as thesynergistic effect from the graphene and the anchored metal

NPs.[372,473] In addition to using grapheneNP composites to

modify the electrode, nanogold enwrapped graphene nano-

composites (NGGNs) are employed as trace labels in

clinical immunoassays, where horseradish peroxidase (HRP)-

conjugated anti-carcinoembryonic antigen (CEA) is attached

to the surface of NGGNs, used as the secondary antibodies

for the detection of CEA.[471] Compared to the HRP-anti-

CEA-nanogold used as the antibody, the graphene-containing

sandwich immunoassay exhibits higher sensitivity and lower

detection limit, which is attributed to the improved electron

transfer between the analyte and the electrode via graphene.

3.4.3. Fluorescence Sensors

Fluorescence-based detection method is sensitive, selec-

tive, rapid, and cost-effective in the analysis of biomolecules.

One of the strategies to use graphene in fluorescence-based

bio-detection takes the advantage of graphene's capability in

fluorescence quenching,[261,262,264,474476] induced by the fluo-

rescence resonance energy transfer (FRET).[477] Recently, a

graphene-based DNA and protein sensing platform has been

demonstrated by binding the dye-labeled ssDNA to GO,

which completely quenches the fluorescence of dyes (e.g.,

fluorescein amidite (FAM), a fluorescein-based dye). During

A hybrid material of rGO and Au NPantibody conju-

gates has also been employed as the channel material for

protein detection.[94] The target protein immunoglobulin G

(IgG) interacts specifically with the anti-IgG-conjugated Au

NPs, resulting in the change of electrical characteristics of the

device. A detection limit of 2 ng/mL IgG is achieved, which

is among the lowest detection limit for carbon-nanomaterial-

based sensors.

3.4.2. Electrochemical Sensors

Graphene-based materials possess large specific sur-

face area, excellent conductivity, and availability for sur-

face functionalization, which are important characteristics

in the electrochemical applications.[449] Until now, many

graphene-based electrochemical sensors have been reported

to detect glucose,[119,265,344,348,371,450460] ascorbic acid,[461463]

dopamine,[462470] H2O2,[343,372] DNA,[257] DNA bases,[259] and

antigen.[471] It has been shown that graphene and graphene-

based materials are promising supplements or replacements

for conventional carbon materials, such as carbon nanotubes

and graphite.[472] Even superior performance of graphene-based electrochemical sensors compared to carbon nano-

tubes has been reported, which is attributed to the presence

of more sp2-like planes and edge defects in graphene.[462,464]

In a typical set up of electrochemical sensing, a graphene

thin film is used as the electrode.[462,466] For example, the

epitaxially grown graphene is prepared on a silicon carbide

substrate by the CVD process, and then incorporated as the

working electrode in a three-electrode electrochemical cell.

Anodization is used to generate oxygenated groups on the

surface of graphene. It is shown that the resulting sensing

platform exhibits good detection ability towards nucleic acids,

uric acids, dopamine, and ascorbic acid, as well as to differen-

tiate single-stranded DNA from double-stranded DNA.[466]

In another report, by using a microwave plasma-enhanced

CVD method, graphene nanoflakes are grown on silicon sub-

strates, which are used to simultaneously detect dopamine,

ascorbic acide, and uric acide. The graphene flakes process

abundant graphitic edge planes and defects, which are essen-

tially responsible for the fast electron transfer and good elec-

trocatalytic activity observed in this work.[462]

In addition to the direct use of graphene as the working

electrode, graphene or graphene derivatives have been

used to modify the conventional glassy carbon elec-

trode[119,257,265,451,465] and Au electrode.[259] The glassy carbon

electrode is usually functionalized with APTES, which con-

tains the positively charged amine groups to absorb GO

sheets. The adsorbed GO sheets, electrochemically reduced to

rGO, are further modified with, for example, glucose oxidase

(GOx) used for glucose detection.[119] Recently, we have used

rGO-modified glassy carbon electrode to detect methicillin-

resistant Staphylococcus aureus (MRSA) DNA by using the

electrochemical impedance spectroscopy. The detection limit

of 100 fm for MRSA DNA is achieved.[257] The modification

of glass carbon electrode can also be carried out by directly

drop-casting a rGO suspension[451] or a mixed slurry of rGO

solution and polymer (e.g., chitosan) on the electrode sur-

face.[265] In the latter case, the rGO is further treated with


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event takes place, fluorescence resonance energy transfer

between the Au NP and GO occurs, and the reduction in GO

fluorescence is detected.[478] This selective and sensitive plat-

form can be expanded further to a GO microarray format for

the multiple pathogen analysis.

3.4.4. Matrices for Mass Spectrometry

Laser desorption/ionization mass spectrometry (LDI-MS)

is an important tool for accurate, fast, and sensitive analysis

of biomolecules,[481] organic molecules,[269] and metal clus-

ters.[482] MALDI-TOF MS is one of the most popular types

of LDI-MS, which provides an accurate and easy approach

for high-molecular-mass species.[268,483] However, for low-

mass analytes (with a molecular mass of


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derived graphene suspensions. Typically, a layer of catalyst

(e.g., Ni) is lithographically patterned on a Si/SiO2 substrate,

which is used to grow graphene sheets by CVD. The pat-

terned graphene layer can be moved to other substrates, by

either transfer with a polydimethylsiloxane (PDMS) stamp,

or etching SiO2 to leave the graphene layer on the surface

of aqueous solution followed by scooping it with the target

substrate.

[

24

]

Other types of top-down lithographic techniques have also

been used to pattern graphene thin films or large area pris-

tine graphene sheets, such as focused ion beam lithography

(FIB)[166] and e-beam lithography (EBL),[494] which can pro-

vide much smaller sized graphene patterns compared to con-

ventional photolithography. GNR arrays that are 2030 nm

wide are fabricated by using EBL to deposit Al lines on

large-area pristine graphene and then etching the unpro-

tected areas of graphene with O2 plasma.[494] However, GNR

of only a few nanometers in width is required for fabrication

of room-temperature FETs with a large bandgap and high

onoff current ratios, since the energy gap of a GNR scales

inversely with the ribbon width.[

170,171

]

A chemical etchingmethod is thus developed to further reducing the widths of

the EBL-prepared GNRs, in order to improve the onoff

ratio in FET applications.[494] In gas-phase chemical etching

process, the etching gas O2 is mixed with Ar, H2, and NH3,

which are used as the dilution gas or provide a reducing envi-

ronment. The etching rate depends on the ratio of mixed gas,

the reaction temperature, and layer numbers of graphene.

As a result, GNRs with widths of 5 nm are fabricated

(Figure10A), which are used as FET channel materials with

an onoff ratio of104.

Nanoscale featured structures, such as block co-polymer

films,[182] nanowires,[166] nanospheres,[492,493] can also be used

as effective etch masks in lithographic processes. Graphenenanomeshes (GNMs) were generated for the first time using

poly(styrene-block-methyl methacrylate) (P(S-b-MMA))

block copolymer thin films with cylindrical domains as the

etch mask, where CHF3-based reactive-ion etching (RIE)

and O2 plasma etching are combined to generate holes in the

graphene layers (Figure 10B).[182] In nanosphere lithography,

uniformly sized spherical particles are able to assemble into

monolayer, forming closely packed hexagonal lattice arrange-

ment with tunable periodicity controlled by the particle size.

3.5. Patterning of Graphene-Based Materials

There is increasing demand on the fabrication of pat-

terned graphene thin films with well-controlled feature size

and periodicity, and tunable electronic properties, such as

patterned graphene electrodes, arrays of graphene nanori-

bons (GNRs), and nanomeshes (GNMs). Various methods

have been developed to generate graphene-based patterns,which include the conventional photolithography,[200,489491]

nanosphere lithography,[492,493] e-beam lithography,[494,495]

direct laser writing,[224] scanning tunneling microscopy

(STM) lithography,[167,496] local anodic oxidation,[497,498] tip-

based thermal chemical reduction,[499] dip-pen nanolitho-

graphy (DPN),[500] microcontact printing,[501,502] plasma etc

hing,[164,166,182,200,490,494] transfer printing,[200,490,503] scratching

method,[76,504] as well as the direct growth of graphene on

prepatterned catalyst substrates.[24,505506]

Conventional photolithography uses light to transfer a

geometric feature from a photo mask to a photoresist film,

which allows for high throughput and good quality con-

trol. In a typical experiment, patterned graphene films withvarious feature sizes and shapes are fabricated as follows. A

substrate is first modified with a photoresist, e.g., functional-

ized perfluorophenylazide (PFPA) such as PFPA-silane for

a silicon or glass substrate or PFPAdisulfide for gold films.

Then a suspension of graphene flakes is spin-coated onto the

modified substrate, which is, after drying, subjected to UV-

irradiation through a photomask. Upon light irradiation, the

azido group on PFPA is converted to the highly reactive sin-

glet perfluorophenylnitrene that can undergo C=C addition

reactions with the adjacent graphene. The subsequent ultra-

sonication and rinsing processes result in the removal of non-

reacted graphene, and the graphene film with desired patterns

is obtained.[

489

]

The photolithography process, based on otherphotoresists and lift-off strategies, can be applied to different

graphene- based substrates, such as HOPG[490] and epitaxi-

ally grown graphene films.[491] For example, a novel method

is developed to fabricate line patterns of pristine graphene

from HOPG.[490] In this work, after graphene patterns are

made on HOPG by photolithography and O2 plasma etching,

a gold film is deposited on the patterns. Peeling off the gold

film results in the exfoliation of patterned graphene layers

from the bulk HOPG, which can be transferred to other sub-

strates followed by etching away the gold film.

Another conventional means to fabricate patterns for

graphene-based materials is based on the electrostatic

adsorption of the target graphene materials on a patternedmolecular layer. For example, microcontact printing is used

to generate positively charged 11-amino-1-undecanethiol

(AUT) patterns on Au surface[501] or APTES on Si/SiOx[502]

to absorb negatively charged GO sheets, resulting in line pat-

terns[502] or dot arrays[501] of GO sheets. Recently, by using

dip-pen nanolithography (DPN), an AFM-based lithography

method,[507516] AUT patterns have been generated, which

are used to precisely position GO sheets on Au surfaces.[500]

Direct growth of epitaxial graphene film on prepatterned

catalyst substrate presents another route to achieve pat-

terned graphene films,[24,505,506] which offers better electrical

properties compared to those films made from chemically

Figure 10. A) AFM images of a GNR array after chemical narrowing.Reproduced with permission.[494] Copyright 2010, Nature PublishingGroup. B) SEM image of a GNM structure after removing the top SiOxmesh mask. Scale bar = 100 nm. Reproduced with permission.[182]Copyright 2010, Nature Publishing Group.


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a GO sheet via temperature control (Figure 12A).[499] The

rGO nanoribbons with widths down to 12 nm are generated.

The reduced region shows increase of four orders of magni-

tude in conductivity compared to that of GO.

By using STM lithography, smaller features in graphene,

as well as atomic structures, can be imaged and precisely

tailored.[167,496] This technique is therefore extremely

attractive, because it allows for the engineering of the elec-

tronic property of the graphene nanostructures. In addition

to lithographical patterning of GNRs with predetermined

energy gaps, STM can generate more complicated archi-

tectures as well, such as a GNR bent junction which con-

nects an armchair and a zigzag ribbon (Figure 12B).[

167

]

Recently, STM has been used to remove hydrogen in

hydrogen-saturated graphene, and nanoscaled graphene

structures are obtained. Importantly, it is observed that the

electronic property of graphene patterns with feature size

smaller than 20 nm does not change compared to hydrogen-

saturated graphene. But if the graphene patterns are larger

than 20 nm, the intrinsic property of pristine graphene is

restored.[496]

4. Conclusion and Outlook

Graphene exhibits a unique chemicalstructure, and outstanding electronic, optical,

thermal, and mechanical properties. Its

derivatives, e.g., epitaxially grown graphene,

graphene oxide (GO), reduced GO (rGO),

and exfoliated graphite, have been produced

with various synthetic methods in order to

meet the increasing requirements for thin

film processing, composite incorporation,

and device integration.

A great number of materials have been

composited with graphene derivatives

namely, polymers, metal NPs, semiconductor

Arrays of graphene nanodisks are thus

prepared by using a polystyrene (PS)-

assembled monolayer as the etch mask.[492]

Alternatively, an assembled monolayer of

silica nanospheres is used as the template

for the deposition of a porous metal film,

which in turn serves as the etch mask for

the fabrication of GNMs instead of nano-disk arrays.[493] Both the block copolymer

and nanosphere lithography are powerful

tools to fabricate GNMs with tunable

periodicity and neck width. The former

method is able to generate GNMs with

periodicity of 2739 nm and a neck width

down to 5 nm,[182] whereas the latter one

provides possibilities to fabricate GNMs

with periodicity ranging from 100 nm to

several micrometers and a neck width of

less than 20 nm.[493]

Since the aforementioned litho-

graphic methods require multiple stepsand special experimental setups, a facile approach to gen-

erate graphene patterns has recently been developed in our

group. A sharp nonmetal object is used to directly scratch

the graphene films. The generated rectangular rGO patterns

(Figure11A) are successfully used as electrodes in electronic

devices.[76] The scratching process is later developed and con-

ducted by an AFM tip, which is controlled programmablly

with a NSCRIPTOR DPN system.[517] By using this method,

various single-layer GO patterns such as gaps, ribbons,

squares, triangles, and zigzags are easily fabricated on Si/SiO2

(Figure 11B).[504]

Another type of patterning methods is the local anodic

oxidation lithography by AFM.[

497,498

]

Usually, a bias voltageis applied between the graphene substrate and the conducting

AFM tip, which creates an electric field to induce electro-

chemical oxidation of the graphene at room temperature with

controlled humility. Although the feature width obtained in

this technique could reach as low as


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and rapid device integration, and to prepare molecular pat-

terns on large-area graphene films in order to partially func-

tionalize the graphene film and thus modify its properties.

AcknowledgementsThis work was supported by AcRF Tier 1 (RG 20/07) and AcRF

Tier 2 (ARC 10/10, No. MOE2010-T21-060) from MOE, CRP

(NRF-CRP22007-01) from NRF, ASTAR SERC Grants (No. 092

101 0064) from ASTAR, CREATE program (Nanomaterials for

Energy and Water Management) from NRF, and New Initiative Fund

FY 2010 (M58120031) from NTU in Singapore. Q.Y. appreciates the

support from A*STAR SERC grant 1021700144 and Singapore MPA

23/04.15.03 grant.

This Review is part of the Special Issue dedicated to Chad Mirkin

in celebration of 20 years of influential research at Northwestern

University.

NPs, CNTs, organic materials, and so on. The properties of

these functional hybrid materials not only depend on the

individual components, but also on the interactions between

them. Therefore, it is important to control the distribution,

density, kind of chemical bonding, as well as 3D arrange-

ment of the components in graphene-based composites for

enhancing their performances. This has been demonstrated in

some reports, such as the graphene-wrapped Co3O4 NPs ina Li ion battery,[404] and the 3D grapheneCNT sandwiched

structure for a supercapacitor.[438] Future efforts are still in

need to improve the fabrication techniques and provide new

possibilities in achieving improved properties. In addition,

the residual functional groups and defects in GO and rGO,[81]

and the layer number of pristine graphene, [375] have shown

some effects on the nucleation density of deposited metal

NPs; whereas the lattice of rGO is able to direct the assembly

of anchored Au nanodots.[87] It can be anticipated that, given

its rich surface chemistry and 2D configuration, graphene-

based materials may serve as templates for the growth of

both inorganic and organic crystals with unusual morpholo-

gies and properties.Graphene-based applications in FETs, memory, photo-

voltaic devices, and sensing platforms have been summarized.

The onoff ratio of FETs has been improved greatly via the

fabrication of GNRs and GNMs. On the other hand, GO/

rGO thin films have shown advantages in memory devices,

due to the ease of processing, and functional group-induced

charge controlling properties. In photovoltaic applications,

graphene-based materials are able to function as the trans-

parent electrodes, electron acceptors and light absorbers. In

particular, thin films made of graphene-based materials have

shown promises as cheap, flexible, and durable supplements

to conventional transparent ITO electrodes in optoelec-

tronic devices. Although the electrical and optical propertiesof GO/rGO can be controlled by varying their size (related

to edge defects),[161] surface functionality,[77] and degree

of reduction,[149] the effective tuning of their bandgap via

chemical routes remains great challenge. In sensing applica-

tions, graphene-based materials, featured with good conduc-

tivity, large specific surface area and functionalizable surfaces,

have demonstrated accurate, rapid, selective, sensitive, and

even single-molecular sensing abilities. Future investigations

on graphene-based sensing platforms, combined with versa-

tile sensing strategies, are expected to continuingly lower the

detection threshold.

One of the major contributions of graphene patterns lies in

the generation of GNRs and GNMs, with controllable featuresize and density. However, the current fabrication of ordered

GNRs arrays from pristine graphene with high onoff current

ratio only relies on e-beam lithography, and has not been real-

ized in large scale with a facile procedure. Nanoscale features

can be made in graphene-based substrates by using AFM based

anodic oxidation, thermal reduction, and STM litho graphy,

with tunable local electronic properties. GO sheets have

been deterministically positioned on substrates by interaction

with molecular patterns generated by microcontact printing

or dip-pen nanolithography. As a future research direction, it

would be useful to directly write small graphene entities, such

as graphene quantum dots or GNRs on substrates for precise

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