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 sp
2
-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
pi-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

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 sp
2
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. 1133