PNAS 2005

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Two-dimensional atomic crystals
K. S. Novoselov*, D. Jiang*, F. Schedin*, T. J. Booth*, V. V. Khotkevich*, S. V. Morozov
†
, and A. K. Geim*
‡
*Centre for Mesoscience and Nanotechnology and School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, United Kingdom;
and
†
Institute for Microelectronics Technology, Chernogolovka 142432, Russia
Edited by T. Maurice Rice, Swiss Federal Institute of Technology, Zurich, Switzerland, and approved June 7, 2005 (received for review April 6, 2005)
We report free-standing atomic crystals that are strictly 2D and can
be viewed as individual atomic planes pulled out of bulk crystals or
as unrolled single-wall nanotubes. By usingmicromechanical cleav-
age, we have prepared and studied a variety of 2D crystals
including single layers of boron nitride, graphite, several dichal-
cogenides, and complex oxides. These atomically thin sheets (es-
sentially gigantic 2D molecules unprotected from the immediate
environment) are stable under ambient conditions, exhibit high
crystal quality, and are continuous on a macroscopic scale.
wafer (Fig. 1
d
), because even a monolayer adds up sufficiently
to the optical path of ref lected light so that the interference color
changes with respect to the one of an empty substrate (phase
contrast). The whole procedure takes literally half an hour to
implement and identify probable 2D crystallites. Their further
analysis was done by atomic force microscopy (AFM), for which
single-layer crystals were selected as those exhibiting an appar-
ent (12) thickness of approximately the interlayer distance in the
corresponding 3D crystals.
Despite its simplicity, the described cleavage technique has
several nonobvious features that are instructive to analyze,
because it also allows one to understand why 2D crystals were
not discovered earlier (e.g., see refs 9–11, 13, and 14, in which
mechanically cleaved graphitic f lakes 10–100 layers thick were
reported). First, monolayers are in a great minority among
accompanying thicker f lakes. Second, unlike nanotubes, 2D
crystals have no clear signatures in transmission electron mi-
croscopy (6–8). Third, monolayers are completely transparent to
visible light and cannot be seen in an optical microscope on most
substrates (e.g., on glass or metals). Fourth, AFM is currently the
only method that allows definitive identification of single-layer
crystals, but it has a very low throughput (especially for the case
of the high-resolution imaging required), and in practice it would
be impossible to find cleaved 2D crystallites by scanning surfaces
at random. Finally, as mentioned earlier, it was not obvious that
isolated atomic planes could survive without their parent crystals
[for example, mechanically cleaved quasi-1D NbSe
3
crystallites
100 nm in diameter were found to deteriorate rapidly (16)].
With the benefit of hindsight, the critical step that allowed us to
find 2D crystallites is the discovered possibility of their tentative
identification in an optical microscope when they are placed on
top of an oxidized Si wafer.
Representative samples of several 2D materials (namely, of
BN, MoS
2
, NbSe
2
,Bi
2
Sr
2
CaCu
2
O
x
, and graphite) obtained and
identified by the procedures described above were investigated
further by scanning tunneling, scanning electron, and high-
resolution transmission electron microscopy (HRTEM). Fig. 2
shows examples of the obtained atomic-resolution images. These
studies
§
confirmed that the prepared 2D crystallites remained
monocrystalline under ambient conditions and no degradation
was noticed over periods of many weeks. Within experimental
resolution, the crystal structure of isolated layers remained the
same as for stacked layers within 3D crystals. Note that 2D
Bi
2
Sr
2
CaCu
2
O
x
showed a superstructure with a unidirectional
modulation period of
28 Å, which is similar to the superstruc-
graphene
layered material
D
imensionality is one of the most defining material param-
eters; the same chemical compound can exhibit dramatically
different properties depending on whether it is arranged in a 0D,
1D, 2D, or 3D crystal structure. Although quasi-0D [e.g., cage
molecules (1)], quasi-1D [e.g., nanotubes (2–4)], and, of course,
3D crystalline objects are well documented, dimensionality two
is conspicuously absent among experimentally known crystals.
On the other hand, there are many layered materials with strong
in-plane bonds and weak, van der Waals-like coupling between
layers. Because of this layered structure, it has long been
tempting to try splitting such materials into individual atomic
layers, although it remained unclear whether free-standing
atomic layers could exist in principle [thin films become ther-
modynamically unstable (decompose or segregate) below a
certain thickness, typically, of many dozens layers]. Thus far,
most efforts have focused on chemical exfoliation of strongly
layered materials and, in particular, of stage I intercalated
graphite (5). During exfoliation, monolayers at some moment
must separate from each other. However, no 2D crystals have
ever been isolated from the resulting slurries, possibly because
single layers appear only as a transient state and involve detach-
ments over microscopic regions. Indeed, the latest studies of
chemically exfoliated graphite have shown that its sediments
consist of restacked and scrolled multilayer sheets rather than
individual monolayers (6–8). An alternative approach has been
the use of mechanical cleavage (9–14). The earlier reports
described mechanically cleaved f lakes consisting of tens and
hundreds of layers, but the recently renewed interest in thin
graphitic films led to f laky materials with a thickness of just a few
graphene layers (12–15). Now we have extended the approach to
its ultimate limit: We have isolated individual crystal planes from
a large variety of strongly layered materials and shown that the
resulting 2D crystals exhibit high crystal quality and macroscopic
continuity.
Materials and Methods
Fig. 1 shows several examples of cleaved samples and illustrates
that they are only one atomic layer thick but nearly macroscopic
laterally. To extract such 2D crystallites, we used a simple but
effective procedure. A fresh surface of a layered crystal was
rubbed against another surface (virtually any solid surface is
suitable), which left a variety of f lakes attached to it (the rubbing
process can be described as similar to ‘‘drawing by chalk on a
blackboard’’). Unexpectedly, among the resulting f lakes we
always found single layers. Their preliminary identification amid
thicker f lakes and other residue was done in an optical micro-
scope. 2D crystallites become visible on top of an oxidized Si
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: AFM, atomic force microscopy; HRTEM, high-resolution transmission elec-
tron microscopy.
‡
To whom correspondence may be addressed. E-mail: geim@man.ac.uk.
§
In the case of HRTEM studies [we used an FEI (Eindhoven, The Netherlands) Tecnai F30], the
cleaved material was deposited directly on holey carbon films, which made the described
route of preliminary identification of 2D crystallites in an optical microscope impossible.
To find them on top of holey carbon among thicker flakes, a different procedure was
developed. First, we used scanning electron microscopy imaging at low acceleration
voltages (FEI Sirion at 500 V). Then, the flakes that were found most transparent in
scanning electron microscopy were studied by AFM (i.e., directly on top of holey carbon)
to define their thickness and select single-layer crystals.
© 2005 by The National Academy of Sciences of the USA
www.pnas.org
cgi
doi
10.1073
pnas.0502848102
PNAS
July 26, 2005
vol. 102
no. 30
10451–10453
Fig. 1.
2D crystal matter. Single-layer crystallites of NbSe
2
(
a
), graphite (
b
),
Bi
2
Sr
2
CaCu
2
O
x
(
c
), and MoS
2
(
d
) visualized by AFM (
a
and
b
), by scanning
electron microscopy (
c
), and in an optical microscope (
d
). (All scale bars: 1
Fig. 2.
Atomic-resolution images of 2D materials. (
a
) Unfiltered scanning
tunneling microscopy image of the crystal lattice in the NbSe
2
monolayer on top
of an oxidized Si wafer. Note that for the scanning tunneling microscopy mea-
surements, an Au film was deposited around 2D crystallites to provide an elec-
trical contact. (
b
) HRTEM images of the 2D Bi
2
Sr
2
CaCu
2
O
x
crystal shown in Fig. 1
c
.
(
c
) HRTEM image of a double-layer MoS
2
. This image is shown to make a con-
nection between our approach based on AFM identification of 2D crystals and the
traditional HRTEM approach used for quasi-1D crystals (all nanotubes were first
found by using HRTEM, where dark lines indicating the nanotube’s walls parallel
to the electron beam are easily visible). No similar signature exists for 2D crystals
(see refs. 6 – 8), and we also found it difficult to align 2D samples exactly parallel
to the electron beam. However, for two-layer crystals, their thickness is easily
identifiable not only in AFM but also in HRTEM because of folded regions seen as
two dark lines (in the case of
c
, the separation is
m.)
The 2D crystallites are on top of an oxidized Si wafer (300 nm of thermal SiO
2
)
(
a
,
b
, and
d
) and on top of a holey carbon film (
c
). Note that 2D crystallites were
often raised by an extra few angstroms above the supporting surface, prob-
ably because of a layer of absorbed water. In such cases, the pleated and
folded regions seen on many AFM images and having the differential height
matching the interlayer distance in the corresponding 3D crystals help to
distinguish between double-layer crystals and true single sheets such as those
shown here.
ture observed in thinned samples of bulk Bi
2
Sr
2
CaCu
2
O
x
pre-
pared for HRTEM (17).
6.5 Å, in agreement with the
interlayer distance in bulk MoS
2
). We occasionally observed short dark lines
(compare with ref. 8) that might be folded monolayers, but no independent
proof for this (e.g., by simultaneous AFM studies) has been obtained yet.
Results and Discussion
We also investigated electrical conductivity of the selected five
2Dmaterials to assess their microscopic quality and macroscopic
continuity. This was done by using field-effect-transistor-like
devices such as the one shown in Fig. 3
Inset
(devices were
prepared by electron-beam lithography). 2D Bi
2
Sr
2
CaCu
2
O
x
and
BN were found to be highly insulating, and no induced conduc-
tivity was detected even at gate electric fields as high as 0.3 V
nm
(i.e., close to the electrical breakdown of SiO
2
), which probably
indicates that band gaps in these 2D materials are larger than in
SiO
2
. We also tried annealing single-layer Bi
2
Sr
2
CaCu
2
O
x
in
oxygen, but the crystals always remained insulating.
On the contrary, 2D graphite (graphene) and both 2D dichal-
cogenides were found to be metallic and exhibited a pronounced
electric field effect (Fig. 3). Their carrier mobilities were deter-
mined as
between 0.5 and 3 cm
2
Vs for different samples, in agreement
with mobilities for the corresponding 3D crystals at room
temperature. Both 2D dichalcogenides were found to be electron
(
V
g
)
en
(
V
g
), where
e
is the electron charge and
n
V
g
is the carrier concentration induced by gate voltage
V
g
(
n
7.2
10
10
cm
2
V for 300-nm SiO
2
). As seen in Fig. 3,
was proportional to
V
g
over large intervals of
n
, showing that
is independent of carrier concentration. Furthermore, by ex-
trapolating the experimental dependences
(
V
g
) to zero
,we
could determine initial (
V
g
0) concentrations of charge
carriers and their type. Graphene behaved rather similarly to
few-layer graphitic samples reported in ref. 12 and is either a
shallow-gap semiconductor or a small-overlap semimetal, in
which positive and negative gate voltages induce 2D electrons
and holes, respectively, in concentrations up to
Fig. 3.
Electric field effect in single-atomic-sheet crystals. Changes in electrical
conductivity
of 2D NbSe
2
,2DMoS
2
, and graphene as a function of gate voltage
are shown (300 K). (
Inset
) Our typical devices used for such measurements: It is an
optical image (in white light) of 2D NbSe
2
on top of an oxidized Si wafer (used as
a gate electrode) with a set of Au contacts. The crystal is seen as a bluer region in
the center. (Scale bar: 5
cm
2
.
10
13
Graphene exhibited typical values of
between 2,000 and 5,000
cm
2
Vs. For 2D NbSe
2
and MoS
2
, we measured mobilities
m.)
10452
www.pnas.org
cgi
doi
10.1073
pnas.0502848102
Novoselov
et al
.
conductors with
n
10
12
to 10
13
cm
2
. Detailed studies of their
conductivities as a function of temperature and
V
g
revealed that
2D MoS
2
was a heavily doped semiconductor with an activation
gap of
0.6 eV, whereas NbSe
2
was a semimetal. The found
electron concentration in 2D NbSe
2
is two orders of magnitude
smaller than carrier concentrations per monolayer in 3D NbSe
2
,
which indicates significant changes in the energy spectrum of
NbSe
2
from a normal metal in 3D to a semimetal in 2D.
In conclusion, we have demonstrated the existence of 2D
atomic crystals that can be prepared by cleavage from most
strongly layered materials. It is most unexpected, if not coun-
terintuitive, that isolated 2D crystals can be stable at room
temperature and in air, leaving aside the fact that they maintain
macroscopic continuity and such high quality that their carrier
mobilities remain almost unaffected. The found class of 2D
crystals offers a wide choice of new materials parameters for
possible applications and promises a wealth of new phenomena
usually abundant in 2D systems. We believe that, once investi-
gated and understood, 2D crystals can also be grown in large
sizes required for industrial applications, matching the progress
achieved recently for the case of single-wall nanotubes (18).
We thank P. B. Kenway for help with transmission electron microscope
studies. This work was supported by the Engineering and Physical
Sciences Research Council (United Kingdom). K.S.N. acknowledges The
Leverhulme Trust for financial support.
1. Kroto, H. W., Fischer, J. E. &Cox, D. E., eds. (1993)
The Fullerenes
(Pergamon,
Oxford).
2. Iijima, S. (1991)
Nature
354,
56–58.
3. Chopra, N. G., Luyken, R. J., Cherrey, K., Crespi, V. H., Cohen, M. L., Louie,
S. G. & Zettl, A. (1995)
Science
269,
966–967.
4. Tenne, R., Margulis, L., Genut, M. & Hodes, G. (1992)
Nature
360,
444–446.
5. Dresselhaus, M. S. & Dresselhaus, G. (2002)
Adv
.
Phys
.
51,
1–186.
6. Shioyama, H. (2001)
J
.
Mater
.
Sci
.
Lett
.
20,
499–500.
7. Viculis, L. M., Mack, J. J. & Kaner, R. B. (2003)
Science
299,
1361.
8. Horiuchi, S., Gotou, T., Fujiwara, M., Asaka, T., Yokosawa, T. & Matsui, Y.
(2004)
Appl
.
Phys
.
Lett
.
84,
2403–2405.
9. Ebbesen, T. W. & Hiura, H. (1995)
Adv
.
Mater
.
7,
582–586.
10. Ohashi, Y., Hironaka, T., Kubo, T. & Shiiki, K. (1997)
Tanso
180,
235–238.
11. Lu, X., Huang, H., Nemchuk, N. & Ruoff, R. S. (1999)
Appl
.
Phys
.
Lett
.
75,
193–195.
12. Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos,
S. V., Grigorieva, I. V. & Firsov, A. A. (2004)
Science
306,
666–669.
13. Zhang, Y., Small, J. P., Amori, M. E. S. & Kim, P. (2005)
Appl
.
Phys
.
Lett
.
86,
073104.
14. Bunch, J. S., Yaish, Y., Brink, M., Bolotin, K. & McEuen, P. L. (2005)
Nano
Lett
.
5,
287–290.
15. Berger, C., Song, Z., Li, T., Li, X., Ogbazghi, A. Y., Feng, R., Dai, Z.,
Marchenkov, A. N., Conrad, E. H., First P. N. & de Meer, W. A. (2004)
J
.
Phys
.
Chem
.
B
108,
19912–19916.
16. Slot, E., Holst, M. A., van der Zant, H. S. J. & Zaitzev-Zotov, S. V. (2004)
Phys
.
Rev
.
Lett
.
93,
176602.
17. Shaw, T. M., Shivashankar, S. A., La Placa, S. J., Cuomo, J. J., McGuire, T. R.,
Roy, R. A., Kelleher, K. H. & Yee, D. S. (1988)
Phys
.
Rev
.
B Condens
.
Matter
.
37,
9856–9859.
18. Zheng, L. X., O’Connell, M. J., Doorn, S. K., Liao, X. Z., Zhao, Y. H., Akhadov,
E. A., Hoffbauer, M. A., Roop, B. J., Jia, Q. X., Dye, R. C.,
et al
. (2004)
Nat
.
Mater
.
3,
673–676.
Novoselov
et al
.
PNAS
July 26, 2005
vol. 102
no. 30
10453
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