Honeycomb Carbon: A Review of Graphene
Matthew J. Allen,† Vincent C. Tung,‡ and Richard B. Kaner*,†,‡
Department of Chemistry and Biochemistry and California NanoSystems Institute, and Department of Materials Science and Engineering, University
of California, Los Angeles, Los Angeles, California 90095
Received February 20, 2009
Contents
1. Introduction 132
2. Brief History of Graphene 133
2.1. Chemistry of Graphite 134
3. Down to Single Layers 134
3.1. Characterizing Graphene Flakes 136
3.1.1. Scanning Probe Microscopy 136
3.1.2. Raman Spectroscopy 136
4. Extraordinary Devices with Peeled Graphene 136
4.1. High-Speed Electronics 137
4.2. Single Molecule Detection 138
5. Alternatives to Mechanical Exfoliation 138
5.1. Chemically Derived Graphene from Graphite
Oxide
139
5.1.1. Depositions 139
5.1.2. Defect Density in Chemically Derived
Graphene
139
5.1.3. Field-Effect Devices 139
5.1.4. Practical Sensors 140
5.1.5. Transparent Electrodes 141
5.2. Total Organic Synthesis 141
5.3. Epitaxial Graphene and Chemical Vapor
Deposition
142
6. Graphene Nanoribbons 143
7. Future Work 143
8. Conclusions 144
9. Acknowledgments 144
10. References 144
1. Introduction
Graphene is the name given to a two-dimensional sheet
of sp2-hybridized carbon. Its extended honeycomb network
is the basic building block of other important allotropes; it
can be stacked to form 3D graphite, rolled to form 1D
nanotubes, and wrapped to form 0D fullerenes. Long-range
π-conjugation in graphene yields extraordinary thermal,
mechanical, and electrical properties, which have long been
the interest of many theoretical studies and more recently
became an exciting area for experimentalists.
While studies of graphite have included those utilizing
fewer and fewer layers for some time,1 the field was delivered
a jolt in 2004, when Geim and co-workers at Manchester
University first isolated single-layer samples from graphite
(see Figure 1).2 This led to an explosion of interest, in part
because two-dimensional crystals were thought to be ther-
modynamically unstable at finite temperatures.3,4 Quasi-two-
dimensional films grown by molecular beam epitaxy (MBE)
are stabilized by a supporting substrate, which often plays a
significant role in growth and has an appreciable influence
on electrical properties.5 In contrast, the mechanical exfo-
liation technique used by the Manchester group isolated the
two-dimensional crystals from three-dimensional graphite.
Resulting single- and few-layer flakes were pinned to the
substrate by only van der Waals forces and could be made
free-standing by etching away the substrate.6-9 This mini-
mized any induced effects and allowed scientists to probe
graphene’s intrinsic properties.
The experimental isolation of single-layer graphene first
and foremost yielded access to a large amount of interesting
physics.10,11 Initial studies included observations of graphene’s
ambipolar field effect,2 the quantum Hall effect at room
temperature,12-17 measurements of extremely high carrier
mobility,7,18-20 and even the first ever detection of single
molecule adsorption events.21,22 These properties generated
huge interest in the possible implementation of graphene in
a myriad of devices. These include future generations of
high-speed and radio frequency logic devices, thermally and
electrically conductive reinforced composites, sensors, and
transparent electrodes for displays and solar cells.
Despite intense interest and continuing experimental
success by device physicists, widespread implementation of
graphene has yet to occur. This is primarily due to the
difficulty of reliably producing high quality samples, espe-
cially in any scalable fashion.23 The challenge is really 2-fold
because performance depends on both the number of layers
present and the overall quality of the crystal lattice.19,24-26
So far, the original top-down approach of mechanical
exfoliation has produced the highest quality samples, but the
method is neither high throughput nor high-yield. In order
to exfoliate a single sheet, van der Waals attraction between
exactly the first and second layers must be overcome without
disturbing any subsequent sheets. Therefore, a number of
alternative approaches to obtaining single layers have been
explored, a few of which have led to promising proof-of-
concept devices.
Alternatives to mechanical exfoliation include primarily
three general approaches: chemical efforts to exfoliate and
stabilize individual sheets in solution,27-32 bottom-up meth-
ods to grow graphene directly from organic precursors,33-36
and attempts to catalyze growth in situ on a substrate.37-43
Each of these approaches has its drawbacks. For chemically
derived graphene, complete exfoliation in solution so far
requires extensive modification of the 2D crystal lattice,
which degrades device performance.31,44 Alternatively, bot-
tom-up techniques have yet to produce large and uniform
† Department of Chemistry and Biochemistry and California NanoSystems
Institute.
‡ Department of Materials Science and Engineering and California Nano-
Systems Institute.
Chem. Rev. 2010, 110, 132–145132
10.1021/cr900070d 2010 American Chemical Society
Published on Web 07/17/2009
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single layers. Total organic syntheses have been size limited
because macromolecules become insoluble and the occur-
rence of side reactions increases with molecular weight.36
Substrate-based growth of single layers by chemical vapor
deposition (CVD) or the reduction of silicon carbide relies
on the ability to walk a narrow thermodynamic tightrope.40
After nucleating a sheet, conditions must be carefully
controlled to promote crystal growth without seeding ad-
ditional second layers or forming grain boundaries.
Despite tremendous progress with alternatives, mechanical
exfoliation with cellophane tape still produces the highest
quality graphene flakes available. This fact should not,
however, dampen any interest from chemists. On the
contrary, the recent transition from the consideration of
graphene as a “physics toy” to its treatment as a large carbon
macromolecule offers new promise. Years of carbon nano-
tube, fullerene, and graphite research have produced a myriad
of chemical pathways for modifying sp2 carbon structures,45-50
which will undoubtedly be adapted to functionalize both the
basal plane of graphene and its reactive edges. This not only
promises to deliver handles for exploiting graphene’s intrinsic
properties but also should to lead to new properties altogether.
This review will discuss the field of graphene from a
materials chemistry standpoint. After a brief history of the
topic, the exciting progress made since 2004, in both the
production of graphene and its implementation in devices,
will be discussed. For a thorough discussion focused on the
physics of graphene, see refs 10, 11, 51, and 52.
2. Brief History of Graphene
To understand the trajectory of graphene research, it is
useful to consider graphene as simply the fewest layer limit
of graphite. In this light, the extraordinary properties of
honeycomb carbon are not really new. Abundant and
naturally occurring, graphite has been known as a mineral
for nearly 500 years. Even in the middle ages, the layered
morphology and weak dispersion forces between adjacent
sheets were utilized to make marking instruments, much in
the same way that we use graphite in pencils today. More
recently, these same properties have made graphite an ideal
material for use as a dry lubricant, along with the similarly
structured but more expensive compounds hexagonal boron
Matthew J. Allen is a graduate student in the Kaner laboratory at the
University of California, Los Angeles (UCLA). He received his B.S. in
physics at Rice University, where he researched carbon nanostructures
in the laboratories of Richard Smalley and Robert Curl.
Vincent C. Tung is a graduate student in the Yang laboratory coadvised
by Prof. Kaner at the University of California, Los Angeles (UCLA). He
received his M.S. in chemistry from the National Tsing-Hua University in
Hsinchu, Taiwan. His previous work was on the photochemistry of organic
light emitting diodes (OLEDs).
Richard B. Kaner received a Ph.D. in inorganic chemistry from the
University of Pennsylvania in 1984. After carrying out postdoctoral research
at the University of California, Berkeley, he joined the University of
California, Los Angeles (UCLA), in 1987 as an Assistant Professor. He
was promoted to Associate Professor with tenure in 1991 and became a
Full Professor in 1993. Professor Kaner has received awards from the
Dreyfus, Fulbright, Guggenheim, and Sloan Foundations, as well as the
Exxon Fellowship in Solid State Chemistry and the Buck-Whitney
Research Award from the American Chemical Society for his work on
refractory materials, including new synthetic routes to ceramics, intercala-
tion compounds, superhard materials, graphene, and conducting polymers.
Figure 1. Single layer graphene was first observed by Geim and
others at Manchester University. Here a few layer flake is shown,
with optical contrast enhanced by an interference effect at a carefully
chosen thickness of oxide. (Reprinted with permission from Science
(http://www.aaas.org), ref 2. Copyright 2006 American Association
for the Advancement of Science.)
Honeycomb Carbon: A Review of Graphene Chemical Reviews, 2010, Vol. 110, No. 1 133
nitride and molybdenum disulfide. High, in-plane electrical
(∼104 Ω-1 cm-1) and thermal conductivity (∼3000 W/mK)
enable graphite to be used in electrodes and as heating
elements for industrial blast furnaces.53,54 High mechanical
stiffness of the hexagonal network (1060 GPa) is also utilized
in carbon fiber reinforced composites. These uses and others
generate an annual demand of more than 1 million tons of
graphite worldwide.55
The anisotropy of graphite’s material properties continues
to fascinate both scientists and technologists. The s, px, and
py atomic orbitals on each carbon hybridize to form strong
covalent sp2 bonds, giving rise to 120 ° C-C-C bond angles
and the familiar chicken-wire-like layers. The remaining pz
orbital on each carbon overlaps with its three neighboring
carbons to form a band of filled π orbitals, known as the
valence band, and a band of empty π* orbitals, called the
conduction band. While three of the four valence electrons
on each carbon form the σ (single) bonds, the fourth electron
forms one-third of a π bond with each of its neighbors
producing a carbon-carbon bond order in graphite of one
and one-third. With no chemical bonding in the c-direction,
out-of-plane interactions are extremely weak. This includes
the propagation of charge and thermal carriers, which leads
to out-of-plane electrical and thermal conductivities that are
both more than 103 times lower than those of their in-plane
analogues.56
2.1. Chemistry of Graphite
Graphite has a rich chemistry in which it can participate
in reactions as either a reducing agent (electron donor) or
an oxidizer (electron acceptor). This is a direct consequence
of its electronic structure, which results in both an electron
affinity and an ionization potential of 4.6 eV.53
A large number of experiments for graphite focus on the
insertion of additional chemical species between the basal
planes, or intercalation. Shaffault is credited with the first
intercalation compound using potassium, dating back to
1841.57 Graphite intercalation compounds (GICs) appear to
be the only layered compounds sufficiently ordered to exhibit
“staging” in which the number of graphitic layers in between
adjacent intercalants can be varied in a controlled fashion.
The stage of a compound refers to the number of graphitic
layers in between adjacent planes of intercalant. The inter-
layer spacing can increase from 0.34 nm (3.4 Å) in native
graphite to more than 1 nm in some GICs, which further
exaggerates the anisotropy of many properties.56,58
The increased interlayer spacing in GICs also means a
significant reduction in the van der Waals forces between
adjacent sheets, which leads one to consider their exfoliation
as a possible route to single layers of graphene. Our group
tried just that in 2003 by violently reacting a stage-1
potassium intercalation compound (KC8) with various sol-
vents such as alcohols, but exfoliation produced only
metastable slabs around 30 layers thick that had a tendency
to scroll under high-powered sonication (see Figure 2).53,59,60
The interlayer spacing in GICs can be further increased by
thermal shock to produce “expanded” graphite, which has
now served as a starting material for recent techniques,
including a nanoribbon synthesis developed by Dai (see
Figure 3).53,61
A second focus of experiments on graphite has been
substitutional doping by the replacement of carbon with other
elements. This includes work by Bartlett and co-workers at
Berkeley in which substitution of carbon with boron and
nitrogen resulted in p- and n-type graphite, respectively.62,63
In light of recent progress with CVD of single layer graphene,
such work will almost certainly be revisited as an alternative
to external gating for controlling electronic behavior in
graphene-based devices, or perhaps to form graphene-only
p-n junctions.
It is also important to mention a few points about progress
in the chemistry of carbon nanotubes. Among the most
important observations have been of the differences in
reactivity between the different crystallographic directions
(zigzag or armchair).64-66 This knowledge should transfer
directly to the “unrolled” or “flattened” case of planar
graphene. A myriad of techniques have also been developed
to selectively modify either the sidewalls of carbon nanotubes
or their end-caps. Such reactions are important looking
forward because they correspond to modification of the basal
plane of graphene and its edges. In fact, in situ TEM was
recently used to study reactions on graphene’s zigzag edge
by Zettl and others at Berkeley.67
3. Down to Single Layers
Researchers have used mechanical exfoliation of layered
compounds to produce thin samples for some time. In 1999,
Ruoff’s group presented one such approach for graphite by
using an atomic force microscope (AFM) tip to manipulate
small pillars patterned into highly oriented pyrolytic graphite
(HOPG) by plasma etching (see Figure 4).1 The thinnest slabs
observed at that time were more than 200 nm thick or the
equivalent of ∼600 layers. Kim’s group at Columbia later
improved the method by transferring the pillars to a tipless
cantilever, which successively stamped down slabs as thin
as 10 nm, or ∼30 layers, on SiO2.68 Electrical measurements
made on the thin crystallites foreshadowed a wealth of work
to come. Other early groups working toward graphene
included Enoki’s in Tokyo, who used temperatures around
1600 °C to convert nanodiamonds into nanometer-sized
regions of graphene atop HOPG in 2001.69
While these elegant methods produced thin samples, it was
ultimately a much simpler approach that led to the first
isolation of single layer graphene in 2004 by a Manchester
group led by Geim (see Figures 5).2 In its most basic form,
the “peeling” method utilizes common celluphene tape to
successively remove layers from a graphite flake. The tape
is ultimately pressed down against a substrate to deposit a
Figure 2. Schematic diagram showing the intercalation and
exfoliation process to produce thin slabs of graphite. Potassium is
inserted between the layers and reacted violently with alcohols.
The exfoliated slabs are ∼30 layers thick. (Reprinted with permis-
sion by The Royal Society of Chemistry from ref 60.)
134 Chemical Reviews, 2010, Vol. 110, No. 1 Allen et al.
sample (see Figure 1). Although the flakes present on the
tape are much thicker than one layer, van der Waals attraction
to the substrate can delaminate a single sheet when the tape
is then lifted away. The method requires a great deal of
patience, as depositions put down by inexperienced scientists
are often a mess of thick slabs in which locating a single
layer can be extremely difficult. With practice, the technique
results in high-quality crystallites, which can be more than
100 µm2 in size.
Perhaps the most important part of isolating single layer
graphene for the first time was the ability to spot an
atomically thin specimen in some readily identifiable fashion.
Optical absorbance of graphene has since been measured at
just 2.3%, ruling out direct visual observation (see Figure
6).70,71 In order to visualize single flakes, the Manchester
group took advantage of an interference effect at a specially
chosen thickness (300 nm) of SiO2 on Si to enhance the
optical contrast under white-light illumination.72 Although
seemingly a simple idea, this was a major step forward and
has contributed a great deal toward progress in this field.
Figure 3. Scanning electron micrographs of natural graphite before (a) and after (b) expansion by acid intercalation and thermal shock.
(Reprinted with permission by The Royal Society of Chemistry from ref 60.)
Figure 4. Scanning electron microscope images of early attempts at mechanical exfoliation using graphite pillars. (a and b) Ruoff’s group
peeled away layers with an AFM tip. (Reprinted with permission from ref 1. Copyright 1999 Institute of Physics.) (c and d) Kim’s group
transferred the pillars to a tipless cantilever and deposited thin slabs onto other substrates in tapping mode. A series of scanning electron
microscope images show thin samples cleaved onto the Si/SiO2 substrate and a typical mesoscopic device. (Reprinted with permission from
ref 68. Copyright 2005 American Institute of Physics.)
Figure 5. Mechanical exfoliation produced the very first single
layer graphene flakes. (a) An atomic force microscopy image shows
the substrate-graphene step height of <1 nm and a folded step
height of 0.4 nm. (Reprinted with permission from ref 9. Copyright
2005 PNAS.) (b) TEM image of a free-standing graphene film after
etching of the underlying substrate. [Reprinted with permission from
Nature (http://www.nature.com), ref 6. Copyright 2007 Nature
Publishing Group.]
Honeycomb Carbon: A Review of Graphene Chemical Reviews, 2010, Vol. 110, No. 1 135
Groups have since adapted the same effect to image graphene
on a variety of substrates and under nonwhite-light condi-
tions.72-75
3.1. Characterizing Graphene Flakes
With new access to 2D crystallites, experimentalists
scrambled to confirm results long predicted by theory. Before
they could do so, techniques needed to be developed for the
characterization of deposited flakes. While optical micros-
copy using the interference effect was a good method for
identifying thin candidates, it could not provide conclusive
evidence that a given flake was single, double, or multilay-
ered. This is an important issue because some of the more
interesting properties of graphene are dependent on crystallite
thickness. The most obvious example is electronic band
structure. Single-layer graphene is a zero band gap semi-
conductor or semimetal in which the highest occupied
molecular orbital (HOMO) touches the lowest unoccupied
molecular orbital (LUMO) at a single Dirac point. For thicker
flakes, stacking of multiple layers leads to some overlap of
their carrier wave functions and the overall behavior becomes
metallic. To match observations with theory, reliable iden-
tification of the number of layers present in a given sample
became imperative.
3.1.1. Scanning Probe Microscopy
Scanning probe microscopy was perhaps the most obvious
choice for verification of crystallite thickness. The method
is relatively slow, but the 0.34 nm (3.4 Å) step height for
each successive layer is well within the detection limits for
modern atomic force microscopes (AFMs). Resolving the
substrate-graphene height profile proved difficult, however,
due to th