ISSN 2040-3364 www.rsc.org/nanoscale Volume 3 | Number 1 | January 2011 | Pages 1–316 REVIEW ARTICLE Dresselhaus et al. Graphene edges: a review of their fabrication and characterisation
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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.
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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.
This journal is ª The Royal Society of Chemistry 2011
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).
This journal is ª The Royal Society of Chemistry 2011
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).
This journal is ª The Royal Society of Chemistry 2011
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
This journal is ª The Royal Society of Chemistry 2011
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-
This journal is ª The Royal Society of Chemistry 2011
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
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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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