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www.rsc.org/nanoscale Volume 3 | Number 1 | January 2011 | Pages 1–316

REVIEW ARTICLEDresselhaus et al.Graphene edges: a review of their fabrication and characterisation

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Graphene edges: a review of their fabrication and characterization

Xiaoting Jia,a Jessica Campos-Delgado,b Mauricio Terrones,cd Vincent Meuniere and Mildred S. Dresselhaus*f

Received 16th August 2010, Accepted 9th October 2010

DOI: 10.1039/c0nr00600a

The current status of graphene edge fabrication and characterization is reviewed in detail. We first

compare different fabrication methods, including the chemical vapor deposition method, various ways of

unzipping carbon nanotubes, and lithographic methods. We then summarize the different edge/ribbon

structures that have been produced experimentally or predicted theoretically. We discuss different

characterization tools, such as transmission electron microscopy and Raman spectroscopy, that are

currently used for evaluating the edge quality as well as the atomic structures. Finally, a detailed

discussion of defective and folded edges is also presented. Considering the short history of graphene edge

research, the progress has been impressive, and many further advances in this field are anticipated.

aDepartment of Materials Science and Engineering, MassachusettsInstitute of Technology, Cambridge, MA, USA. E-mail: [email protected];Tel: +617 253 6860bDivisao de Metrologia de Materiais, Instituto Nacional de Metrologia,Normalizacao e Qualidade Industrial (INMETRO), Duque de Caxias,RJ, Brazil. E-mail: [email protected] Nanocarbon Research Center, Shinshu University, Wakasato 4-17-1, Nagano, Japan. E-mail: [email protected] of Physics and Materials Research Institute, ThePennsylvania State University, 104 Davey Lab., University Park, PA,16802–6300, USA. E-mail: [email protected]; [email protected] of Physics, Applied Physics, and Astronomy, RensselaerPolytechnic Institute, Troy, NY, USA. E-mail: [email protected] of Physics and Department of Electrical Engineering andComputer Science, Massachusetts Institute of Technology, Cambridge,MA, USA. E-mail: [email protected]

Xiaoting Jia

Xiaoting Jia was born in Don-

gyang, Zhejiang province,

China. She received a B.S.

degree in Materials Science

from Fudan University, China in

2004, and a M.S. degree in

Materials Science and Engi-

neering from Stony Brook

University in 2006. She is

currently a PhD candidate in the

Department of Materials

Science and Engineering at

Massachusetts Institute of

Technology. Her research

interests are focused on the

in-situ TEM studies of thermo-

electric nanomaterials, quantum dots, and carbon-based nano-

structures including carbon nanotubes, graphene, and graphitic

nanoribbons, and also on the STM studies of graphene and gra-

phene edges.

86 | Nanoscale, 2011, 3, 86–95

1. Introduction

The study of graphene, and in particular of graphene edges, is

a research topic that has expanded rapidly over the past few

years. The motivation for the popularity of this field largely

stems from the unique electronic, chemical, and magnetic prop-

erties of graphene edges. The graphene community has become

increasingly aware of the importance of clean, atomically sharp

edges, in light of experimental difficulties in designing electronic

logic devices due to the presence of disorder in the edges of

narrow graphene ribbons. As researchers strive to find ways to

produce better quality graphene nanoribbons with improved

edge structures, a review on the present status of graphene edges,

Jessica Campos-Delgado

Jessica Rosaura Campos Del-

gado received her B.S. in

Physics Engineering from the

State University of San Luis

Potosi, Mexico in 2005. She

enrolled in the graduate

program of Applied Sciences at

the Institute of Science and

Technology of San Luis Potosi

in Mexico and obtained her PhD

degree in 2009. She is currently

working as a Post-Doc at the

National Institute of Metrology,

Standardization and Industrial

Quality of Brazil. Her research

interests are towards the

synthesis of carbon nanostructures and their characterization

through Raman spectroscopy and electron microscopy.

This journal is ª The Royal Society of Chemistry 2011







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their fabrication, characterization, and properties seems indeed

timely. Specifically, we here present an overview of the different

methods commonly employed in the production of graphene

nanoribbons and graphene edges. We also discuss the charac-

terization of graphene edges (both open and folded edges) using

transmission electron microscopy (TEM) and Raman spectros-

copy, and we conclude the review with a outlook to the future.

Graphene, a single layer of carbon atoms forming a two-

dimensional (2D) honeycomb lattice, can, in principle, be

considered as an elementary building block for all sp2-hybridized

carbon allotropes. Although graphene was in fact produced

experimentally decades ago,1 it wasn’t until the recent

Mauricio Terrones

Mauricio Terrones obtained his

B.Sc. degree in Engineering

Physics with first class honours

at Universidad Iberoamericana.

In 1997 he obtained his

doctorate degree with Sir Prof.

Harold W. Kroto (Nobel

Laureate, FRS) from University

of Sussex (UK). He has co-

authored more than 270 publi-

cations in leading journals, and

holds >9000 citations to his

work (H-index¼54). He has

numerous awards including: the

Alexander von Humboldt

Fellowship, the Mexican

National Prize for Chemistry, the Javed Husain Prize and the

Albert Einstein medal from UNESCO, the TWAS Prize in Engi-

neering Physics, the Carbon Prize by the Japanese Carbon Society,

the Somiya Award by the International Union of Materials

Research Societies, among others. Aged 41, his research now

concentrates on the theory, synthesis and characterization of novel

layered nanomaterials, including graphene. He is now distinguished

Professor at Shinshu University, and from January 2011 onwards

will also join Penn State University as Professor of Physics.

Vincent Meunier

Vincent Meunier holds the

Kodosky Constellation Chair at

Rensselear Polytechnic Institute

where he is an associate

professor in the department of

physics, applied physics, and

astronomy. He uses computa-

tional physics tools to establish

structure/functionality relation-

ships in a variety of nano-

structured and low-dimensional

materials, with particular

emphasis on their electronic,

structural, and transport prop-

erties. Recent work of Meunier

includes the study of super-

capacitor featuring nanoscale pores, for which he and his

coworkers have established an accurate atomistic model that

accounts for both confinement and quantum mechanical effects.


developments in 2004 by the Manchester group that the field of

graphene research took off rapidly.2–5 These developments in the

‘‘science of graphene’’ prompted an unprecedented surge of

activity and demonstration of new physical phenomena. These

developments also triggered a renewed emphasis on the inter-

disciplinary nature of nanoscience and created new opportunities

in materials science, physics, chemistry, and electrical engi-

neering. Graphene is a unique monolayered two dimensional

crystal, which exhibits a room temperature quantum Hall effect.6

It is the strongest material by weight, so far reported.7 It is

a semimetal with its conduction band and valence band being

degenerate at the K point in the Brillouin zone (a situation that

occurs only for the special unit-cell geometry and orientation

relative to graphene’s high-symmetry axis). Graphene shows

a symmetrical linear dispersion relation about the K point for

electrons and holes, and it has an extremely high room temper-

ature carrier mobility that is about 2 orders of magnitude greater

than that of silicon.5

One of the most widely discussed applications of graphene in

electronics is its potential use in field effect transistors (FETs).

The promise held by graphene for electronic applications is due

to the fact that it is one atomic layer thick and should therefore

yield particularly good performance for high frequency appli-

cations. In order to make FET (field effect transistor) devices for

digital logic applications, a sizable band-gap is required.8 In this

context, graphene nanoribbons with narrow widths (below

20 nm) can generate such a bandgap that is dependent on the

ribbon width and crystallographic orientation of the edges.9

Unfortunately, the as-produced narrow ribbons usually suffer

from disordered edges that make the bandgap poorly defined,10

which in turn results in a dramatically degraded carrier mobility.

This constitutes the main reason why obtaining atomically sharp

edges in narrow graphene nanoribbons of controlled width has

been one of the most important challenges for the applications of

graphene in electronic devices.11

There are two types of achiral edges in graphene nano-

ribbons—zigzag and armchair edges, defined by the

Mildred S: Dresselhaus

Mildred Dresselhaus was born in

1930 in New York and

completed her PhD studies at

the University of Chicago in

1958, and then a postdoc at

Cornell studying the microwave

surface impedance of type 1

superconductors in a magnetic

field. She started working in

carbon science in 1960 when she

joined the MIT Lincoln Labo-

ratory. These studies advanced

and broadened when she joined

the faculty of the Massachusetts

Institute of Technology in 1967

and continues to the present time

with students and collaborators worldwide. The output includes

many research papers, review articles and 4 books, with a new book

on Raman spectroscopy in carbon nanotubes and graphene reach-

ing the bookstands in December 2010.

Nanoscale, 2011, 3, 86–95 | 87


Fig. 1 Zigzag and armchair edges in monolayer graphene nanoribbons.

The edge structure and the number of atomic rows of carbon atoms

normal to the ribbon axis determine the electronic structure and ribbon

properties. (Image courtesy of M. Hofmann, MIT).

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orientation of the hexagons relative to the ribbon length

(see Fig. 1). These achiral edges have been observed to

dominate over chiral edges after Joule heating.12 Theoretical

calculations13 show that zigzag edges are metastable and

a planar reconstruction into pentagon-heptagon configurations

spontaneously takes place at room temperature. In practice,

hydrogenated or oxygenated group passivation during heat

treatment14,15 is commonly used to stabilize the edges in air.

The detailed chemistry and carrier transport associated with

these functionalized edges seem very exciting and further work

is necessary along this direction.

Fig. 2 CVD grown graphitic nanoribbons. (a) SEM micrographs by

Murayama and Maeda of the ribbon-like filaments of graphite prepared

at 700 �C, using a bright catalytic particle that is located on the tip of the

filament; (b) the ‘tail’ of a ribbon-like filament prepared at 700 �C; (c) the

cross-section of a ribbon-like filament prepared at 550 �C ((a), (b) and (c)

are reprinted by permission from Macmillan Publishers Ltd: Nature

(Ref. 20), copyright (1990)); (d) SEM image of a mixture of carbon

nanoribbons and Fe filled MWNTs reported by Mahanandia et al.;21 (e)

Magnified SEM image of carbon nanoribbons ((d) and (e) are reprinted

from Ref. 21, Copyright (2008), with permission from Elsevier). (f) A low

magnification TEM image of many suspended GNRs reported by

Campos-Delgado et al.;19 (g) SEM image showing the wavy structure of

graphitic nanoribbons ((f) and (g) are reprinted with permission from

Ref. 19. Copyright 2008 American Chemical Society).

2. Fabrication

Edged graphene (nanoribbons) can be fabricated in various

ways. Quantum mechanical manifestations are most pronounced

in narrow, high-quality edged graphene ribbons which have

a unique crystallographic orientation. Such graphene nano-

ribbons (GNRs) are therefore expected to be the most important

in graphene electronic device applications. As a result, there has

been significant progress in the large-scale production of GNRs

in recent years with approaches such as the mechanical exfolia-

tion of graphite, chemical vapor deposition, and lithographic,

chemical and sonochemical methods.

It is noteworthy that different types of defects are generally

present in high or low concentrations on the edges of graphene

or graphene nanoribbons.16 From an electronic materials point

of view, the presence of hydroxyl groups on the edges could

affect the nanoribbon carrier transport, and they should be

removed in order to enhance the conductivity of the ribbons.

In this context, Tour’s group demonstrated that hydrazine is

very efficient in removing oxygen from the graphene nano-

ribbons, thus improving their electrical conductivity.17

However, a great majority of the published work in the gra-

phene field does not emphasize (or study in detail) the

importance of edge defects (or edge functionalities), and most

of these papers assume that some of the produced graphene

nanoribbons are defect-free or very close to that. Therefore,

further research involving the detailed chemistry and physics

of sharp- and defective-graphene edges should be carried out,

with the goal of understanding and eventually reaching control

of edge defects.

88 | Nanoscale, 2011, 3, 86–95

2.1 Chemical vapor deposition synthesis of GNRs

Although lithographically produced graphene nanoribbons have

been commonly used in research laboratories, chemically derived

graphene nanoribbons have also been demonstrated.18 Such

narrow GNRs however may not be as appealing for bulk

industrial level applications due to their multi-step processing

and relatively low yields. Therefore, chemical vapor deposition

(CVD) offers an alternative approach for preparing large

quantities of graphene nanoribbons in a relative short amount of

time.19 The first report of CVD graphite nanoribbon samples is

attributed to Murayama and Maeda in 1990.20 Through the

decomposition of a reactant gas containing CO/H2/Fe(CO)5 at

400–700 �C, they were able to produce graphite filaments 10 mm

long, with a rectangular cross-section 100–700 nm wide and 10–

200 nm thick with catalytic Fe3C particles attached to one of the

ends (Fig. 2a–c). Using high-resolution TEM they demonstrated

that the graphene sheets had a uniform orientation perpendicular

to the filament axis and that the graphene edges formed loops at

the edges upon thermal annealing at 2800 �C.20 In 2008, two

different routes were reported for obtaining graphitic nano-

ribbons. One of them involved the decomposition of ferrocene

and tetrahydrofuran at 950 �C, and the material thus produced

contained a mixture of iron filled multiwall carbon nanotubes

(MWNTs) and carbon nanoribbons (Fig. 2d and 2e). Although

the length and width of the nanoribbons are not discussed in the

report, by observation of the published SEM images, the mate-

rial appears to be �200 nm in width and tens of microns in

length. The graphitic structure of the carbon nanoribbons was

confirmed through X-ray diffraction and TEM measurements,

showing that the (002) lattice planes are perpendicular to the axis



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of growth.21 The second report involved the thermal decompo-

sition at 950 �C of ferrocene/ethanol/thiophene solutions,

resulting in the synthesis of a material of �40 graphene layers in

thickness (stacked parallel to the axis of growth), 20–300 nm in

width, and tens of microns in length (Fig. 2f and 2g).19 These

ribbons start oxidizing at 700 �C,19 similar to highly oriented

pyrolytic graphite (HOPG). This method of CVD synthesis

produces large quantities of graphitic nanoribbons, and the as-

synthesized nanoribbons have many open edges that are suitable

for the study of the structural and electronic properties of gra-

phene edges.

2.2 Unzipping of CNTs to yield GNRs

Unzipping carbon nanotubes along their longitudinal direction

constitutes an alternative method for graphene nanoribbon

synthesis. This concept has been adopted by several groups using

different reactants and methods. Kosynkin et al.17 reported that

MWCNTs can be unzipped by using a chemical treatment with

sulfuric acid (H2SO4) and potassium permanganate (KMnO4; an

oxidizing agent) as schematized in Fig. 3a. Multi-layer graphene

nanoribbons and graphene sheets can be produced in this way.

The resulting ribbons are up to 4 mms long, 100–500 nms wide,

and 1–30 graphene layer thick. The as-synthesized ribbon edges

have many oxygen-containing chemical groups that quench the

electrical conductivity of the ribbons, so that reduction or

annealing in hydrogen have to be carried out to remove these

groups from the edges. More recently, Higginbotham et al.22

reported that the nanotube unzipping process can be optimized

by an efficient chemical oxidation process at a somewhat elevated

temperature (60 �C) in the presence of a second acid (C2HF3O2

or H3PO4) in addition to the H2SO4/KMnO4 mixture. The

presence of the second acid (e.g., H3PO4) inhibits the creation of

vacancies in the GNRs due to the protection of the diol groups.22

Interestingly, the degree of oxidation can be adjusted by

Fig. 3 Schematic representation of the method used for unzipping

carbon nanotubes to form graphene nanoribbons: (a) chemical route,

involving acid reactions that start to break carbon-carbon bonds

(e.g., H2SO4 and KMnO4 as oxidizing agents); (b) intercalation-exfolia-

tion of MWNTs, involving treatments in liquid NH3 and Li, and

subsequent exfoliation using HCl and heat treatments; (c) catalytic

approach, in which metal nanoparticles ‘‘cut’’ the nanotube longitudi-

nally like a pair of scissors, (d) physico-chemical method, by embedding

the tubes in a polymer matrix followed by Ar plasma treatment; and (e)

the electrical method, by passing an electric current through a nanotube.

The resulting structures are either (f) GNRs or (g) graphene sheets.

(Images are reprinted with permission from Ref. 16. Copyright 2010

American Chemical Society).


controlling the amount of oxidizing agent (KMnO4) in the

reaction. This process results in much longer (>5 mm) and nar-

rower ribbons (<100 nm), with sharper edges (i.e., more atomi-

cally perfect),22 when compared to previous reports in the

literature (see Fig. 4). Jiao et al. reported a plasma etching

method for unzipping CNTs to form graphene nanoribbons23

(Fig. 3b). In this case, the resulting nanoribbons are 10–20 nms

wide, and typically 1–3 graphene layers thick, and show semi-

conducting properties. Another unzipping method consists of

intercalating nanotubes using alkali-metal atoms (e.g. Li, K, Na)

and washing out the intercalants. This procedure causes the tube

to open along its length. This approach was first reported using

Li intercalation in conjunction with ammonia (NH3)24 (Fig. 3b).

A previous study has shown that transition metals can cut

through graphene sheets under hydrogen flow conditions.25,26

This method can also be used for cutting MWCNTs into GNRs

(Fig. 3c) through a method of selective etching. A method for the

facile synthesis of high-quality graphene nanoribbons has been

reported more recently.27 This method involves two steps in each

of the gas phase and liquid phase steps. In the mild gas-phase

oxidation step, oxygen reacts with pre-existing defects on the

nanotube sidewalls to form etch pits. In the subsequent solution-

phase sonication step, sonochemistry and hot gas bubbles

enlarge the pits and unzip the tubes. The GNRs thus obtained

show smooth edges in the HRTEM image and also show a low

(ID/IG) ratio in the Raman D and G band spectra indicating

a low defect density. These GNRs show the highest electrical

Fig. 4 Graphene nanoribbons produced by unzipping carbon nano-

tubes with an efficient chemical oxidation process. (a,b) Transmission

electron microscopy (TEM) images of graphene nanoribbons obtained by

unzipping CVD-grown multiwalled carbon nanotubes, using an opti-

mized method involving two acids (TFA or H3PO4) in the presence of

KMnO4 and H2SO4 at 65 �C; (c) TEM image of a graphene nanoribbon

showing various bends produced using the method by Higginbotham

et al.;22 (d) atomic force microscopy (AFM) image of a graphene nano-

ribbon segment produced by an optimized oxidation method using

a second acid (H3PO4) at 65 �C, in addition to KMnO4 and H2SO4. (e) An

electron diffraction pattern of a few-layer graphitic nanoribbon obtained

using the same conditions as those shown in panel (d). Note the bright

spots in (e) correspond to the hexagonal lattice. (Images are reprinted

with permission from Ref. 16 and 22. Copyright 2010 American Chemical

Society).

Nanoscale, 2011, 3, 86–95 | 89


Fig. 5 TEM images showing sharp zigzag and armchair edges formed

from irregular edges using an in situ Joule heating method.12 (a) TEM

image shows irregular edges in graphitic nanoribbons before Joule

heating. (b) Sharp zigzag (pink lines) and armchair (green lines) edges are

formed after Joule heating. (c) High resolution TEM image shows well-

defined zigzag and armchair edges (as illustrated by green hexagons) after

Joule heating. (Images are from Ref. 12. Reprinted with permission from

AAAS).

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conductance and mobility reported thus far for 10–20 nm wide

graphene nanoribbons.27 An alternative method for unzipping

carbon nanotubes has been developed by Zettl and co-workers

and it consists of passing a high current through a carbon

nanotube inside an electron microscope.28

Unzipping of nanotubes to yield GNRs appears to be a very

promising technique in terms of mass production. This is mainly

because several companies around the world, are now able to

produce massive quantities of carbon nanotubes per year using

the CVD process. However, the atomic control of the edge

morphologies of the GNRs still needs to be enhanced and scaled

up, in order to take advantage of the special properties of gra-

phene nanoribbons.

2.3 Other methods

In addition to the approaches summarized above, STM lithog-

raphy has been used to cut graphene nanoribbons from graphene

sheets.29 Graphene ribbons with a typical width ranging from

2.5 nm to 15 nm, and a length of �120 nm were produced in this

way, with a measured bandgap of about 0.5 eV for a 2.5 nm-wide

armchair GNR using room temperature STS measurements.29

The advantage of using STM lithography is that it is possible to

pattern bent junctions with nanometer-precision, as well as to

produce a predetermined width and crystallographic orientation

of the nanoribbons; such features are hard to achieve presently

using other cutting techniques. This advantage stems from the

possibility of achieving excellent atomic resolution using STM on

flat graphitic surfaces. However, STM lithography suffers from

low throughput and equipment accessibility, two issues that

make it unlikely for this technique to be used in large-scale

applications. Another chemical method for atomically precise

bottom-up fabrication of GNRs has recently been developed by

Cai et al.,30 which involves a precursor monomer on a gold

substrate that forms GNRs through dehalogenation and cyclo-

dehydrogenation steps. This method produces GNRs with well

controlled widths and various shapes. However, challenges

remain in fabricating these GNRs on a technology relevant

substrate, or in transferring the GNRs onto an arbitrary

substrate.

3. Characterization

Graphene edges are investigated using a variety of character-

ization techniques. Advanced transmission electron microscopes

(TEM) are commonly used in evaluating graphene edge struc-

tures, and Raman spectroscopy is advantageous for dis-

tinguishing zigzag from armchair edges and for studying the

chirality-dependent edge disorder and the quantum confinement

of the electrons in 1D band structures. The use of scanning

tunneling microscopy (STM) and spectroscopy (STS) for

studying the electronic states at graphene edges is also a prom-

ising research field, and STS/STM can be used to study the

magnetic properties of graphene edges, which is reviewed in

detail in Ref. 31.

3.1 TEM characterization and in situ Joule heating

High resolution transmission electron microscopy (HRTEM) is

a highly sensitive tool for studying graphene edges. Graphene

90 | Nanoscale, 2011, 3, 86–95

edges in each layer typically exhibit a dark line in the TEM image

(see Fig. 5b), which enables counting the number of layers in as-

synthesized graphene samples.32,33 The edge roughness can also

be directly estimated using HRTEM imaging.

Significant progress has been reported in the TEM character-

ization of graphene edges.12,34–41 For example, in a previous study

developed by some of the present authors,31 CVD-grown

graphitic nanoribbon edges have been characterized using

a conventional HRTEM (2010F) instrument integrated with an

STM holder. Using this setup, edge modification by in situ Joule

heating was demonstrated for the first time.12 In this experiment,

a piece of graphitic nanoribbon material (Fig. 2f) is suspended in

between two electrodes inside the TEM, and a voltage is applied

across the ribbon length. After a significant amount of time

(typically 20 min) in the HRTEM and exposure to electron beam

irradiation, the graphitic nanoribbon becomes highly defective.

Under these conditions the edges of each graphene layer show

irregular structures (Fig. 5a). As the voltage is increased across

the ribbon, the material gets significant resistive Joule heating,

resulting in in-situ crystallization and edge reconstruction. The

resulting material shows almost entirely achiral edges (either

zigzag or armchair edges (Fig. 5b)). Atomically smooth zigzag,

armchair edges, and edge junctions can be readily resolved here

(Fig. 5c). This postprocessing approach shows that Joule heating

in an electron microscope provides a possible way of modifying

rough edges in graphene nanoribbons, and provides a further

step towards the use of graphene nanoribbons for electronic



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device applications. Various consistent explanations based on

first principles calculations have been provided both to explain

the details of the Joule heating mechanism in cleaning the

edges,12,42,43 and to highlight the role of electron irradiation

induced by the TEM itself in the process.43

Zettl and co-workers40 have also been able to directly observe

5–7 and Thrower-Stone-Wales (TSW)44,45 bulk defects as well as

5–7 edge defects on isolated graphene surfaces using aberration

corrected transmission electron microscopy (see Fig. 6 and

Fig. 7).

In order to resolve the atomic structures at graphene edges,

advanced TEM techniques are required. Using an aberration-

corrected TEM instrument combined with a monochromator

(TEAM 0.5), which has sub Angstrom resolution, the stability of

mechanically exfoliated graphene edges under electron beam

irradiation has been studied in real time. Under these conditions,

edge reconstruction is observed (see Fig. 6). The abundance of

zigzag edges lends evidence for a more stable zigzag configura-

tion under these conditions.40 In Ref. 38 open and closed (folded)

edges in exfoliated HOPG materials have been studied under

conditions when such edges are stable. After furnace annealing at

�2000 �C, open edges are found to become less stable so that

most of the open edges join with edges in the neighboring layer to

form well-stacked loops. Finally, note that Ref. 39 reports on the

in situ observation of graphene sublimation and multi-layer edge

reconstructions at elevated temperatures.

3.2 Raman spectroscopy

Compared to the TEM characterization of graphene edges,

Raman spectroscopy provides a complementary, yet quite

different characterization tool that is also suitable for dis-

tinguishing between zigzag and armchair edges in graphene

Fig. 6 (Left) Molecular model showing the transformation of 4 adjacent

hexagons into a 5-7-7-5 defect or a Thrower-Stone-Wales defect, and

(right) HRTEM image showing two 5-7-7-5 defects located on the edges

(red circles) of a hole in a graphene surface. (Images are taken from

movies from supplementary material of Ref. 40. Reprinted with

permission from AAAS).

Fig. 7 HRTEM images showing a zigzag edge near a hole region of

a graphene layer (left) transformed into a 5–7 defect edge (right). (Images

are adapted from movies in the supplementary material of Ref. 40.

Reprinted with permission from AAAS).


nanoribbons and studying edge disorder. It has been found that

the Raman peak of nanographite ribbons (on a HOPG substrate)

exhibits an intensity dependence on the light polarization direc-

tion relative to the nanographite ribbon axis,46 as shown in

Fig. 8a. The Raman spectrum here is composed of two peaks

centered at 1568 cm�1 (G1 peak) and 1579 cm�1 (G2 peak). The

G2 peak frequency, which remains unchanged when varying the

polarization angle (q) of the incident beam with respect to the

ribbon axis direction, comes from the HOPG substrate. In

contrast, the G1 feature in Fig. 8a comes from the nanographite

ribbon, and its intensity varies with the polarization of the inci-

dent light. The intensity decreases with increasing q and can be

fitted to a cos2 q curve (Fig. 8b). This curve originates from the

relationship between the probability of light absorption W(~k)

and the wave vector of the electron k written as

Wð k!Þf

��P!� k

!��2

k2

where ~P is the polarization vector of the incident light. Since van

Hove singularities occur in the ~T direction (q ¼ 0) in real space

due to the 1D quantum confinement structure in the electronic

density of states, a large Raman signal is obtained when the light

is in resonance with the excitonic optical transition energies

between van Hove singularities. The Raman G-band intensities

are, therefore, different for interior regions of a nanographite

ribbon and at the edges. The G-band intensity of a GNR is also

dependent on the GNR width and crystalline direction. Thus it is

only the electronic ~k vector along the ~KT direction in reciprocal

Fig. 8 Raman spectrum on GNR edges.46,47 (a) Raman spectra obtained

for light incident with different polarization angles (q) with respect to the

ribbon axis direction in nanographite ribbon sheets. The inset shows

a schematic figure of the sample (vertical gray line) denoting the direction

between the ribbon axis and the light polarization vector. (b) Intensity of

the G1 Raman peak versus q. The dotted line is a cos2 q theoretical fit to

the experimental points. (Images (a) and (b) are adapted from Ref. 46).

(c) Raman spectra obtained in three different regions of an HOPG

sample (see (d)). The inset shows an optical image of an edge region of

a sample and the regions where spectra 1, 2 and 3 were taken (open

circles). (d-e) AFM images of the step on the HOPG substrate where the

Raman spectra shown in (c) were taken. (f) The STM measurements and

(g) The FFT (fast Fourier transform) filtered image verifying the zigzag

edge configuration in the marked region 2 in (c, d). (Images (c)-(g) are

reprinted with permission from Ref. 47. Copyright 2004 by the American

Physical Society).

Nanoscale, 2011, 3, 86–95 | 91


Fig. 9 Schematics of graphene nanoribbons with a perfect zigzag edge,

and edges with 5-7 defects and 5-8-5 defects, respectively. (Image courtesy

of L. P. Wang, MIT).

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space that is involved in the optical absorption process. When

q ¼ 0, then we have ~k//~KT, and G1 reaches its maximum value;

however, when q reaches 90�, the G1 peak should vanish and

a cos2 q dependence of the intensity is obtained, as shown in

Fig. 8b. This polarization dependent behavior allows the Raman

technique to distinguish between armchair and zigzag edges.

For example, Figs. 8d–g show the influence of the edge

structure on the Raman spectra of edges in HOPG.47 The edge

configurations shown here are determined by STM measure-

ments. It is found that the disorder-induced D band in the

Raman spectra also provides a powerful way to distinguish

armchair from zigzag edges. Here we see that the D band

intensity is about 3 times less intense for a zigzag edge (spectrum

2) compared to that for an armchair edge (spectrum 1), when

normalized to their G band intensities. The residual intensity of

the D band from the zigzag edge is attributed to disorder in this

edge structure. This shows that the observation of the D-band

could be useful for characterizing the defect structure in nano-

graphite-based devices. Recent calculations of the Raman

spectra of graphene ribbons including the effect of the matrix

elements of the Raman tensor48 represent a first step in a quan-

titative approach to using the Raman G-band scattering inten-

sities to distinguish between armchair and zigzag edges.

Fig. 10 Folded edges formed across graphene layers by furnace heat

treatment. (a) The pristine sample before heat treatment shows open

edges. (b) At 1500 �C, single loops (folded edges) are formed between

adjacent graphene layers. (c) At 2800 �C, multiple loops (folded edges)

are formed. (d) A schematic shows the single, double and multiple loop

(folded edge) configurations in graphitic nanoribbons. (Images are

reprinted with permission from Ref. 57. Copyright 2009, American

Vacuum Society).

3.3 Defective edges

The most common defects one finds in graphene nanoribbons

are: vacancies, heptagon-pentagon pairs (STW transformations),

loops and interstitials. While heptagon-pentagon pairs and loops

preserve the connectivity of the nanoribbon, interstitials and

vacancies do not. Therefore, scientists must now work on defect

edge engineering in order to tailor the reactivity and transport

properties of graphene edges. For example, it could be possible to

achieve specificity for sensing different types of molecules or to

anchor specific polymer groups at the edges of GNRs, in order

to produce stable and well dispersed composites or suspensions.

In addition, electronic and thermal transport properties are

important for understanding the effect of specific and controlled

defects on graphene edges. It is clear that edge chemistry and the

physics of GNRs, a field which is just emerging, could lead to

unexpected catalytic reactions, novel field effect transistors,

efficient electrodes for Li-ion batteries, anchoring centers for

assembling heavy metal, highly conducting and transparent

films, drug delivery devices, and other applications.

Further studies considering edges having pentagon-heptagon

(5-7) or pentagon-octagon-pentagon (5-8-5) rows (as schema-

tized in Fig. 9) need to be carried out in detail from both

experimental and theoretical points of view. For example, very

recently, Batzill and coworkers identified the presence of a 5-8-5

defect line within a graphene sheet using STM.49 These lines

behave as quantum wires. This finding indicates that other defect

topologies should also be considered and studied. In addition,

Botello-M�endez et al. also demonstrated that hybrid ribbons

interconnecting zigzag and armchair nanoribbons behave as spin

polarized conductors.50 Finally, it is also possible to have non-

carbon atoms within either ribbons or graphene using hetero-

atomic doping atoms, such as B and N.51–56 These systems need

to be studied further along with the presence of defects on the

edges caused by the introduction of non-hexagonal rings.13

92 | Nanoscale, 2011, 3, 86–95

3.4 Folded edges

As previously mentioned, folded edges are often observed in

TEM characterizations of multi-layer graphene. Folded edges

are formed when the edges of adjacent graphene layers are

connected with each other to form a curved closed loop. Folded

edges (see Fig. 10) have quite a different structure from other

carbon nanostructures, and therefore should have a different

chemistry and reactivity as compared to the open edges. Thus

they should have different applications. For this reason, it is

important to understand the conditions under which folded

edges form, and how to prevent edge folding (or loop

formation at multi-layer graphene edges) when folded edges are

undesirable.

Ref. 57 describes two methods by which CVD-grown pristine

open edged graphitic nanoribbons (see Fig. 10a) can develop into

stable closed loops. One method consists of furnace heat treat-

ment to temperature in excess of 1500 �C. In this case, the shape



Fig. 11 TEM images of folded edges that are formed across graphene

layers by resistive Joule heating (a) near the electrode, and (b) further

away from the electrode. (Images are reprinted with permission from

Ref. 57. Copyright 2009, American Vacuum Society).

Fig. 12 Molecular models showing the final configurations for graphene

nanoribbons with vacancy (a),(c) and interstitial defects (b),(d). Zigzag

ribbons create loops with both vacancies (a) and interstitials (c), while the

reconstructed zigzag (reczag) edges do not show this behavior and rather

show increased structural order. Interstitials lead to the formation of

monoatomic carbon chains in both zigzag and reczag edges, due to the

low reactivity of a graphene surface. (Images are reprinted from Ref. 43

with permission).

Fig. 13 Carbon K-edge NEXAFS spectra of the pristine graphene NR,

NR1000, NR1500, and NR2000. The pristine sample (NR) and HOPG

are also given for comparison. The count value for HOPG given in the

figure is reduced to half of the original count to make the count scale

comparable with the other traces. The C 1s to p* (285.5 eV) and s*

(291.85 eV) transitions are indicated by dotted vertical lines. (Image is

reprinted from Ref. 60 with permission).

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and size of the loops can be tuned by varying the annealing

temperature. Fig. 10a shows the open edges in a pristine sample

before heat treatment. The open edges of this sample are

passivated by heat treatment, and single loops are formed

between adjacent layers at 1500 �C in order to minimize the free

energy (Fig. 10b). Some double loops are formed by heat treat-

ment to 1500 �C and multi-loops are formed when using higher

heat treatment temperatures. For example, faceted multiple

loops are formed at 2800 �C, as can be readily resolved in TEM

images (Fig. 10c). Fig. 10d shows the schematics of different loop

formations at the edge of multi-layer graphene. A similar

observation was made by the Iijima group in Ref. 38, where an

HOPG sample is heat treated in a furnace for several hours at

2000 �C. In this case, TEM micrographs of the heat treated

sample clearly show loop formation at the edges. Another

method for loop formation, other than furnace heat treatment, is

by direct resistive Joule heating across pristine graphitic nano-

ribbons inside a TEM, as mentioned in section 3.1 above. It is

found that resistive Joule heating without prior electron beam

irradiation damage induces effective loop formation in a way

very similar to that of furnace heat treated samples (see Fig. 11).

The key difference between open edges and loop formation is

the significant role played by electron beam irradiation when

applied before in situ Joule heating. Irradiation indeed enables

edge modification and sharp open edge formation in CVD-

grown graphitic nanoribbons. Recent theoretical work by Cruz-

Silva et al.43 provided an atomistic analysis to highlight the

reasons why loops are formed by Joule heating alone, while

adjacent layers do not coalesce when Joule heating is applied

after high energy electron irradiation. This theoretical work

based on large-scale quantum molecular dynamics calculations

indicates that the presence of both vacancies and interstitials (so-

called ‘‘Frenkel pairs’’) are essential for keeping graphene layers

parallel to one another and for preventing adjacent edges from

coalescing (loop formation). Electron beam irradiation, based on

previous reports,35,58,59 is likely to provide the driving force for

inducing vacancies and interstitials in graphitic nanoribbons. On

one hand, the introduction of vacancies increases the surface

reactivity and interlayer interaction far away from the edges. On

the other hand, the interstitials provide effective feedstock to

ensure interlayer cross-link creation which keeps the layers

parallel and prevents loop formation (see Fig. 12). Quantum

transport calculations further confirm that interlayer cross-


linking increases the backscattering of electrons and promotes

interlayer transport.43 Therefore the cross-linking sites are key

for both Joule heating and defect annealing, and such sites are

susceptible to being healed during the Joule heating process.

Very recently, Joly, et al.60 investigated the edge states in

graphene nanoribbons prepared by the CVD method, using

near-edge X-ray absorption fine structure (NEXAFS) and

Nanoscale, 2011, 3, 86–95 | 93


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electron spin resonance (ESR) spectroscopy. It was noted that

the C 1s to p* transitions of nanoribbon samples, correspond to

the C 1s to edge-state transitions of nanographenes. As the CVD

ribbons were annealed at high temperatures in an Ar atmo-

sphere, the contribution of the edge states decreased due to the

loop formation. These results confirm the presence of magnetic

edge states in the edges of graphene nanoribbons and provide

significant insight into these magnetic edge states (see Fig. 13).

Conclusions and outlook

Since 2004, the research on graphene edges has attracted

increasing attention and has undergone rapid progress. Zigzag

edges can be distinguished from armchair edges because of their

high electron density of states near the Fermi level9,62 relative to

armchair edges. The fabrication of graphene edges and graphene

nanoribbons has made significant advances within the last two

years towards the large scale production of smooth edges and

narrow ribbons. With the stringent demands of electronic device

applications and the desire to continually promote fundamental

physics studies, more efforts in improving the synthesis condi-

tions as well as the post-growth treatment procedures of gra-

phene edges are foreseeable. In the meanwhile, the

characterization of graphene edges has also progressed signifi-

cantly and is now pushing the limit of nanomaterials character-

ization techniques. Integrated characterization techniques, such

as combined STM with TEM, combined Raman spectroscopy

with other electron microscopy techniques, are likely to take

place, which will facilitate our understanding of advanced

nanomaterials and their development towards applications. In

addition, study of the magnetoresistance of graphene nano-

ribbon-based FETs is another emerging research field that is

likely to gain more momentum towards various applications.61

Although extensively investigated in the laboratory, the

fabrication of graphene edges in electronic device applications is

still facing many challenges, including the creation of both

smooth edges and controlled narrow graphene ribbons that can

open a sizable and well-defined bandgap and there are still many

open issues and opportunities for further research effort. Given

the short history of this field, the progress has been impressive

and significant developments in graphene edge applications are

expected to occur in the near future.

Acknowledgements

The authors gratefully acknowledge support from NIRT/NSF/

CTS-05-06830 (XJ) and Navy-ONR-MURI-N00014-09-1-1063

(MSD). We thank JST-Japan for Research funding the Center

for Exotic NanoCarbon Project, the Japanese regional Innova-

tion Strategy Program by the Excellence (MT). We also thank

FAPERJ for research funding in Brazil (JCD).

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graphene edges a review of their fabrication and characterization

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