1 s2.0 S2452262717300508 main
FlatChem 4 (2017) 20–32
Contents lists available at ScienceDirect
FlatChem
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl a t c
Graphene: Fundamental research and potential applications
Yujia Zhong, Zhen Zhen, Hongwei Zhu ⇑
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, and Center for Nano and Micro Mechanics, Tsinghua University,
Beijing 100084, China
a r t i c l e
i n f o
Article history:
Received 11 May 2017
Revised 20 June 2017
Accepted 20 June 2017
Available online 22 June 2017
a b s t r a c t
Graphene is a representative two-dimensional (2D) material and has been receiving considerable interest
from both academia and industry. In this review, we recollect the latest development in the synthesis, structural analysis, characteristics, and possible applications of graphene materials. The discussion helps
researchers to better understand the properties of graphene and design novel graphene-based applications.
Ó 2017 Elsevier B.V. All rights reserved.
Keywords:
Graphene
Two-dimensional materials
Contents
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Synthesis and transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solid phase method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solution phase method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preparation of graphene nanoribbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Structure and morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mechanical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemical modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mechanical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mass transport. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction
Graphene is the first synthetic two-dimensional (2D) atomic
crystal. It has attracted immense attention because of its excellent
⇑ Corresponding author.
E-mail address: hongweizhu@tsinghua.edu.cn (H. Zhu).
http://dx.doi.org/10.1016/j.flatc.2017.06.008
2452-2627/Ó 2017 Elsevier B.V. All rights reserved.
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properties, including stiffness, strength, elasticity, high thermal
conductivity, extremely high electron mobility, and tunable band
gap. Being such a fascinating all-in-one material, graphene can
replace other materials in many applications and bring about
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Y. Zhong et al. / FlatChem 4 (2017) 20–32
Alternatively, graphene can also be epitaxially grown on single
crystal SiC by vacuum graphitization [4] (Fig. 1B). The number of
epitaxial graphene layers can be controlled, and the quality of such
graphene is good. However, the carrier scattering at epitaxial graphene on SiC is induced by geometry. Thus the size of electrically
uniform domains is limited [6]. This method allows preparing graphene of larger sample size, but it is costly because it requires high
reaction temperature and the very expensive SiC wafers. In addition, the prepared graphene is inferior to the mechanical exfoliated
graphene in quality and crystallite size.
technological breakthroughs. Graphene has been investigated theoretically since the 1940s [1]. However, its studies mostly focused
on theoretical calculations. It was only until 2004 [2], through
exfoliating highly oriented pyrolytic graphite [3], that graphene
was first prepared in the laboratory. This experimental success
stimulated research in 2D materials. From 2004 till now, more than
one hundred research articles on graphene have been published in
Science and Nature. In this review, we recollect the latest development in the synthesis, structural analysis, characteristics, and
potential applications of graphene materials, to address the continued developments and challenges with a wide scope of interest,
highlighting fundamental understanding of the synthesis and
characterization procedures, future outlook, as well as an indepth discussion of high-end application areas. The discussion
helps researchers to better understand the properties of graphene
and design novel graphene-based applications.
Solution phase method
The oxidation–reduction reaction is a widely used solution
phase method to synthesize graphene-derived materials, such as
graphene oxide (GO). This method has low cost and gives high yield,
but the product has inferior quality. The GO fabricated via solvent
casting can completely exfoliate in water (Fig. 1D) to produce suspensions containing almost entirely individual GO sheets [7,8].
The suspension can be deposited as a thin film on many
surfaces, and the film can then be reduced [5]. Subjecting GO to
reducing agent under certain condition gives reduced GO (rGO).
Alternatively, GO can be reduced by heating in an inert atmosphere. Catalysts induce GO reduction upon illumination or
annealing at high temperatures. And it is environmentally friendly
to reduce GO by applying voltage [9]. The use of heated atomic
force microscope (AFM) tip [10], laser beam [11] and pulses of
microwaves [12] allows reducing GO with precision (Fig. 1E). Thermochemical nanolithography with heated AFM tip can pattern the
nanoscale rGO without tip wear or sample tearing. The width of
the rGO patterns can be controlled between 12 nm and 20 lm
[10]. Similarly, reduction through laser irradiation can also pattern
rGO [11]. Both methods are reliable, flexible, clean, and rapid.
Synthesis and transfer
The synthesis of graphene is currently still a key concern in graphene research. Although several methods, with varying cost and
yield, are available, they still pose restraints on the studies and
applications of graphene.
Solid phase method
The synthetic methods can be classified by the phase of carbon
source and the synthetic environment (Fig. 1). Solid phase methods
include mechanical exfoliation [3] and synthesis on SiC [4]. Graphene can be acquired by mechanically exfoliating highly oriented
pyrolytic graphite with tape (Fig. 1A). The resulting graphene has
excellent quality, but the yield is low and the cost is high [3,5].
Solid phase methods
Solution phase methods
C
A
Liquid-phase exfoliation
Mechanical exfoliation
B
F
D
Hot AFM tip
E Laser
Graphene
GO
Graphene
SiC
Synthesis on SiC
CNT unzipping
Reduction of GO
Monomer assembly
Chemical vapor deposition (CVD)
CH4
G
Graphene
H
H
CH4
Graphene
H
C
C
Ni
Cu
Fig. 1. Making graphene. (A–C) Schematic illustration of solid phase methods: (A) mechanical exfoliation [5], (B) synthesis graphene on SiC [4,5], (C) CNTs unzipping by
plasma etching to make GNRs [19]. (D-F) Solution phase methods: (D) liquid-phase exfoliation to produce individual GO sheets [5], (E) reduction of GO induced by hot AFM
tip [10] or laser [11], (F) assembling different monomers to make GNRs with different morphologies [18,20,21]. (G) Device of CVD. (H) Growth mechanisms in CVD. (Step 1)
Methane decomposition to produce C. (Step 2) Left panel: surface segregation and precipitation in Ni substrate. Right panel: surface adsorption in Cu substrate. (Step 3)
Graphene nucleation and continued growth.
22
Y. Zhong et al. / FlatChem 4 (2017) 20–32
Beside the oxidation–reduction method, a non-chemical
solution-phase exfoliation of graphite in organic solvents is developed [13,14]. Few-layer graphene can be obtained by dispersion
and exfoliation of graphite in N-methyl-pyrrolidone (NMP),
N,N-Dimethylacetamide (DMA), c-butyrolactone (GBL) and 1,3dimethyl-2-imidazolidinone (DMEU).
Preparation of graphene nanoribbons
Graphene is a 2D zero band gap semimetal. Therefore, its band
gap must be opened up before it can replace Si and be used in electronics. Because quantum confinement and edge effects can introduce band gaps (Eg) (1/wa, where w is the width and a is near
unity) in graphene nanoribbons (GNRs), the GNRs whose width is
less than 10 nm exhibit semiconductor nature and have substantial
band gap (Eg > 0.3 eV) [15]. Narrow GNRs can be produced through
chemical sonication. For example, sonicating the solution of poly
(m-phenylenevinylene-co-2,5-dioctoxy-pphenylenevinylene)
(PmPV) in 1,2-dichloroethane (DCE) gives a suspension of exfoliated graphite, which is further centrifuged to obtain the GNRs [15].
Lithographic patterning [16] of graphene film is a conventional
technique to acquire GNRs. GNRs prepared by lithographic patterning are wide, but limited in width and smoothness. Carbon nanotube (CNT) can be regarded as a GNR rolled up into a seamless
tube. As a result, it is logical to produce GNRs by unzipping CNTs.
CNTs partly embedded in a polymer film can produce GNRs
through plasma etching (Fig. 1C). If the starting CNTs have small
diameter and certain chirality, the obtained GNRs may have welldefined widths and edges. Selecting the CNTs and modifying the
plasma etching time will manipulate the number of graphene layers and affect the yield. This method allows large scale production
of narrow (8 nm) zigzag GNR is ferromagnetic with no bandgap
[65].
The electrical properties of graphene are also affected by the
symmetry of its hexagonal layer structure. Topological currents
can be observed in graphene superlattices that have broken inversion symmetry [66]. Because of its parabolic dispersion, the bilayer
graphene is susceptible to the symmetry breaking induced by the
interactions among charge carriers even at zero magnetic fields
[67]. When the BLG has Bernal stacking, the band structure
consists of p, p⁄ states and two lower energy bands through the
splitting valley from interlayer coupling. The band gap of the pristine Bernal-stacked BLG is zero. The band gap of BLG can also be
opened or closed by rendering the individual graphene layer, doping through adsorption [47], or strong electrical gating in dual-gate
BLG FETs (Fig. 3D) [68]. The electromechanical properties [69,70]
are used to explore the electrical properties. Because graphene
has massless Dirac fermions-like band structure and lattice symmetry, the pseudo-magnetic fields induced by strain can approach
300 T, which offers a method to control the electronic structure of
graphene [69].
The sheet resistance5 of highly doped graphene reaches
30 X/sq. The electron mobility (l) can rise up to 2 105 cm2/
(Vs) at electron densities (n) of 2 1011 cm 2 in suspended SLG
(Fig. 3E) [71]. The CVD SLG transferred to SiO2 substrate [26] shows
electron mobility l = 3700 cm2/ (Vs) at n = 5 1012 cm 2. The
conductivity r = nel is controlled by both electron mobility and
charge density. Compared with Cu, graphene shows higher electron mobility but has much lower charge density. Zhao et al. found
that the electronic structure of the individual nitrogen dopant in
SLG changed only within a few lattice spacing, and this allows
increasing carrier density while conserving graphene quality [72].
Under magnetic field, supercurrent could be observed in encapsulated graphene in contact with superconducting electrodes [73] or
in a graphene layer contacted by two closely spaced superconducting electrodes [74]. In graphene junction which is contacted by two
closely space superconducting electrodes, a finite supercurrent is
observed even at zero charge density [74]. In some situations,
graphene can show ballistic transport property [75–77]. Ballistic
propagation means that scattering only occurs at the quantum billiards boundaries, as is observed in the low-temperature transport
spectroscopy of SLG or BLG. Epitaxial GNR (40 nm wide) features
single-channel room-temperature ballistic conductors with a ballistic length more than 10 lm [78]. Ju et al. observed ballistic
valley-polarized conducting channel with a ballistic length of
400 nm at 4 K [79]. Rutter et al. found that intravalley and intervalley backscattering was dominated by the in-plane atomic defect,
thus the defect influenced the transport properties of graphene
[80]. Bandurin et al. demonstrated that electrons in graphene
behave like a viscous liquid (Fig. 3F) [81], because in graphene
electron-phonon scattering is weak but electron-electron collisions
abound. The vicinity resistance is negative over a large range of
carrier density and temperature, and the viscosity of electron
Fermi liquid in graphene is 0.1 m2/s.
Optical properties
The optical transmittance (T) and reflectance (R) of graphene
are given by T (1 + 2pG/c) 2 and R 0.25p2a2T, where G = e2/4⁄
(⁄ = h/2p, h is the Planck constant, e is the electron charge) is the
high-frequency conductivity for Dirac fermions in graphene, c is
speed of light, and a = e2/⁄c 1/137 is the fine structure constant
that describes the coupling between light and relativistic electrons.
Reed et al. reported that the value of effective fine-structure constant is a = 0.14, which is smaller than the nominal a = 2.2 and
suggests that the strength of interaction between quasiparticles
is weaker than previously believed [82]. The reflectance of
25
graphene is negligible (
Contents lists available at ScienceDirect
FlatChem
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl a t c
Graphene: Fundamental research and potential applications
Yujia Zhong, Zhen Zhen, Hongwei Zhu ⇑
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, and Center for Nano and Micro Mechanics, Tsinghua University,
Beijing 100084, China
a r t i c l e
i n f o
Article history:
Received 11 May 2017
Revised 20 June 2017
Accepted 20 June 2017
Available online 22 June 2017
a b s t r a c t
Graphene is a representative two-dimensional (2D) material and has been receiving considerable interest
from both academia and industry. In this review, we recollect the latest development in the synthesis, structural analysis, characteristics, and possible applications of graphene materials. The discussion helps
researchers to better understand the properties of graphene and design novel graphene-based applications.
Ó 2017 Elsevier B.V. All rights reserved.
Keywords:
Graphene
Two-dimensional materials
Contents
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Synthesis and transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solid phase method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solution phase method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preparation of graphene nanoribbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Structure and morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mechanical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemical modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mechanical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mass transport. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction
Graphene is the first synthetic two-dimensional (2D) atomic
crystal. It has attracted immense attention because of its excellent
⇑ Corresponding author.
E-mail address: hongweizhu@tsinghua.edu.cn (H. Zhu).
http://dx.doi.org/10.1016/j.flatc.2017.06.008
2452-2627/Ó 2017 Elsevier B.V. All rights reserved.
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properties, including stiffness, strength, elasticity, high thermal
conductivity, extremely high electron mobility, and tunable band
gap. Being such a fascinating all-in-one material, graphene can
replace other materials in many applications and bring about
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Y. Zhong et al. / FlatChem 4 (2017) 20–32
Alternatively, graphene can also be epitaxially grown on single
crystal SiC by vacuum graphitization [4] (Fig. 1B). The number of
epitaxial graphene layers can be controlled, and the quality of such
graphene is good. However, the carrier scattering at epitaxial graphene on SiC is induced by geometry. Thus the size of electrically
uniform domains is limited [6]. This method allows preparing graphene of larger sample size, but it is costly because it requires high
reaction temperature and the very expensive SiC wafers. In addition, the prepared graphene is inferior to the mechanical exfoliated
graphene in quality and crystallite size.
technological breakthroughs. Graphene has been investigated theoretically since the 1940s [1]. However, its studies mostly focused
on theoretical calculations. It was only until 2004 [2], through
exfoliating highly oriented pyrolytic graphite [3], that graphene
was first prepared in the laboratory. This experimental success
stimulated research in 2D materials. From 2004 till now, more than
one hundred research articles on graphene have been published in
Science and Nature. In this review, we recollect the latest development in the synthesis, structural analysis, characteristics, and
potential applications of graphene materials, to address the continued developments and challenges with a wide scope of interest,
highlighting fundamental understanding of the synthesis and
characterization procedures, future outlook, as well as an indepth discussion of high-end application areas. The discussion
helps researchers to better understand the properties of graphene
and design novel graphene-based applications.
Solution phase method
The oxidation–reduction reaction is a widely used solution
phase method to synthesize graphene-derived materials, such as
graphene oxide (GO). This method has low cost and gives high yield,
but the product has inferior quality. The GO fabricated via solvent
casting can completely exfoliate in water (Fig. 1D) to produce suspensions containing almost entirely individual GO sheets [7,8].
The suspension can be deposited as a thin film on many
surfaces, and the film can then be reduced [5]. Subjecting GO to
reducing agent under certain condition gives reduced GO (rGO).
Alternatively, GO can be reduced by heating in an inert atmosphere. Catalysts induce GO reduction upon illumination or
annealing at high temperatures. And it is environmentally friendly
to reduce GO by applying voltage [9]. The use of heated atomic
force microscope (AFM) tip [10], laser beam [11] and pulses of
microwaves [12] allows reducing GO with precision (Fig. 1E). Thermochemical nanolithography with heated AFM tip can pattern the
nanoscale rGO without tip wear or sample tearing. The width of
the rGO patterns can be controlled between 12 nm and 20 lm
[10]. Similarly, reduction through laser irradiation can also pattern
rGO [11]. Both methods are reliable, flexible, clean, and rapid.
Synthesis and transfer
The synthesis of graphene is currently still a key concern in graphene research. Although several methods, with varying cost and
yield, are available, they still pose restraints on the studies and
applications of graphene.
Solid phase method
The synthetic methods can be classified by the phase of carbon
source and the synthetic environment (Fig. 1). Solid phase methods
include mechanical exfoliation [3] and synthesis on SiC [4]. Graphene can be acquired by mechanically exfoliating highly oriented
pyrolytic graphite with tape (Fig. 1A). The resulting graphene has
excellent quality, but the yield is low and the cost is high [3,5].
Solid phase methods
Solution phase methods
C
A
Liquid-phase exfoliation
Mechanical exfoliation
B
F
D
Hot AFM tip
E Laser
Graphene
GO
Graphene
SiC
Synthesis on SiC
CNT unzipping
Reduction of GO
Monomer assembly
Chemical vapor deposition (CVD)
CH4
G
Graphene
H
H
CH4
Graphene
H
C
C
Ni
Cu
Fig. 1. Making graphene. (A–C) Schematic illustration of solid phase methods: (A) mechanical exfoliation [5], (B) synthesis graphene on SiC [4,5], (C) CNTs unzipping by
plasma etching to make GNRs [19]. (D-F) Solution phase methods: (D) liquid-phase exfoliation to produce individual GO sheets [5], (E) reduction of GO induced by hot AFM
tip [10] or laser [11], (F) assembling different monomers to make GNRs with different morphologies [18,20,21]. (G) Device of CVD. (H) Growth mechanisms in CVD. (Step 1)
Methane decomposition to produce C. (Step 2) Left panel: surface segregation and precipitation in Ni substrate. Right panel: surface adsorption in Cu substrate. (Step 3)
Graphene nucleation and continued growth.
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Y. Zhong et al. / FlatChem 4 (2017) 20–32
Beside the oxidation–reduction method, a non-chemical
solution-phase exfoliation of graphite in organic solvents is developed [13,14]. Few-layer graphene can be obtained by dispersion
and exfoliation of graphite in N-methyl-pyrrolidone (NMP),
N,N-Dimethylacetamide (DMA), c-butyrolactone (GBL) and 1,3dimethyl-2-imidazolidinone (DMEU).
Preparation of graphene nanoribbons
Graphene is a 2D zero band gap semimetal. Therefore, its band
gap must be opened up before it can replace Si and be used in electronics. Because quantum confinement and edge effects can introduce band gaps (Eg) (1/wa, where w is the width and a is near
unity) in graphene nanoribbons (GNRs), the GNRs whose width is
less than 10 nm exhibit semiconductor nature and have substantial
band gap (Eg > 0.3 eV) [15]. Narrow GNRs can be produced through
chemical sonication. For example, sonicating the solution of poly
(m-phenylenevinylene-co-2,5-dioctoxy-pphenylenevinylene)
(PmPV) in 1,2-dichloroethane (DCE) gives a suspension of exfoliated graphite, which is further centrifuged to obtain the GNRs [15].
Lithographic patterning [16] of graphene film is a conventional
technique to acquire GNRs. GNRs prepared by lithographic patterning are wide, but limited in width and smoothness. Carbon nanotube (CNT) can be regarded as a GNR rolled up into a seamless
tube. As a result, it is logical to produce GNRs by unzipping CNTs.
CNTs partly embedded in a polymer film can produce GNRs
through plasma etching (Fig. 1C). If the starting CNTs have small
diameter and certain chirality, the obtained GNRs may have welldefined widths and edges. Selecting the CNTs and modifying the
plasma etching time will manipulate the number of graphene layers and affect the yield. This method allows large scale production
of narrow (8 nm) zigzag GNR is ferromagnetic with no bandgap
[65].
The electrical properties of graphene are also affected by the
symmetry of its hexagonal layer structure. Topological currents
can be observed in graphene superlattices that have broken inversion symmetry [66]. Because of its parabolic dispersion, the bilayer
graphene is susceptible to the symmetry breaking induced by the
interactions among charge carriers even at zero magnetic fields
[67]. When the BLG has Bernal stacking, the band structure
consists of p, p⁄ states and two lower energy bands through the
splitting valley from interlayer coupling. The band gap of the pristine Bernal-stacked BLG is zero. The band gap of BLG can also be
opened or closed by rendering the individual graphene layer, doping through adsorption [47], or strong electrical gating in dual-gate
BLG FETs (Fig. 3D) [68]. The electromechanical properties [69,70]
are used to explore the electrical properties. Because graphene
has massless Dirac fermions-like band structure and lattice symmetry, the pseudo-magnetic fields induced by strain can approach
300 T, which offers a method to control the electronic structure of
graphene [69].
The sheet resistance5 of highly doped graphene reaches
30 X/sq. The electron mobility (l) can rise up to 2 105 cm2/
(Vs) at electron densities (n) of 2 1011 cm 2 in suspended SLG
(Fig. 3E) [71]. The CVD SLG transferred to SiO2 substrate [26] shows
electron mobility l = 3700 cm2/ (Vs) at n = 5 1012 cm 2. The
conductivity r = nel is controlled by both electron mobility and
charge density. Compared with Cu, graphene shows higher electron mobility but has much lower charge density. Zhao et al. found
that the electronic structure of the individual nitrogen dopant in
SLG changed only within a few lattice spacing, and this allows
increasing carrier density while conserving graphene quality [72].
Under magnetic field, supercurrent could be observed in encapsulated graphene in contact with superconducting electrodes [73] or
in a graphene layer contacted by two closely spaced superconducting electrodes [74]. In graphene junction which is contacted by two
closely space superconducting electrodes, a finite supercurrent is
observed even at zero charge density [74]. In some situations,
graphene can show ballistic transport property [75–77]. Ballistic
propagation means that scattering only occurs at the quantum billiards boundaries, as is observed in the low-temperature transport
spectroscopy of SLG or BLG. Epitaxial GNR (40 nm wide) features
single-channel room-temperature ballistic conductors with a ballistic length more than 10 lm [78]. Ju et al. observed ballistic
valley-polarized conducting channel with a ballistic length of
400 nm at 4 K [79]. Rutter et al. found that intravalley and intervalley backscattering was dominated by the in-plane atomic defect,
thus the defect influenced the transport properties of graphene
[80]. Bandurin et al. demonstrated that electrons in graphene
behave like a viscous liquid (Fig. 3F) [81], because in graphene
electron-phonon scattering is weak but electron-electron collisions
abound. The vicinity resistance is negative over a large range of
carrier density and temperature, and the viscosity of electron
Fermi liquid in graphene is 0.1 m2/s.
Optical properties
The optical transmittance (T) and reflectance (R) of graphene
are given by T (1 + 2pG/c) 2 and R 0.25p2a2T, where G = e2/4⁄
(⁄ = h/2p, h is the Planck constant, e is the electron charge) is the
high-frequency conductivity for Dirac fermions in graphene, c is
speed of light, and a = e2/⁄c 1/137 is the fine structure constant
that describes the coupling between light and relativistic electrons.
Reed et al. reported that the value of effective fine-structure constant is a = 0.14, which is smaller than the nominal a = 2.2 and
suggests that the strength of interaction between quasiparticles
is weaker than previously believed [82]. The reflectance of
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graphene is negligible (