High Electron Mobility in Epitaxial Trilayer Graphene on Off-axis SiC(0001)

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The van de Waals heterostructure formed by an epitaxial trilayer graphene is of particular interest due to its unique tunable electronic band structure and stacking sequence. However, to date, there has been a lack in the fundamental understanding of the electronic properties of epitaxial trilayer graphene. Here, we investigate the electronic properties of large-area epitaxial trilayer graphene on a 4° off-axis SiC(0001) substrate. Micro-Raman mappings and atomic force microscopy (AFM) confirmed predominantly trilayer on the sample obtained under optimized conditions. We used angle-resolved photoemission spectroscopy (ARPES) and Density Functional Theory (DFT) calculations to study in detail the structure of valence electronic states, in particular the dispersion of π bands in reciprocal space and the exact determination of the number of graphene layers. Using far-infrared magneto-transmission (FIR-MT), we demonstrate, that the electron cyclotron resonance (CR) occurs between Landau levels with a (B)(1/2) dependence. The CR line-width is consistent with a high Dirac fermions mobility of ~3000 cm(2)·V(-1)·s(-1) at 4 K.

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received: 09 July 2015

accepted: 09 November 2015

Published: 07 January 2016

# High Electron Mobility in Epitaxial Trilayer Graphene on Off-axis SiC(0001)

Mahdi Hajlaoui 1,2 , Haikel Sediri 1 , Debora Pierucci 1 , Hugo Henck 1 , Thanyanan Phuphachong 3 ,
Mathieu G. Silly 2 , Louis-Anne de Vaulchier 3 , Fausto Sirotti 2 , Yves Guldner 3 , Rachid Belkhou 2 &
Abdelkarim Ouerghi 1

The van de Waals heterostructure formed by an epitaxial trilayer graphene is of particular interest
due to its unique tunable electronic band structure and stacking sequence. However, to date, there
has been a lack in the fundamental understanding of the electronic properties of epitaxial trilayer
graphene. Here, we investigate the electronic properties of large-area epitaxial trilayer graphene on a
4° off-axis SiC(0001) substrate. Micro-Raman mappings and atomic force microscopy (AFM) confirmed
predominantly trilayer on the sample obtained under optimized conditions. We used angle-resolved
photoemission spectroscopy (ARPES) and Density Functional Theory (DFT) calculations to study in
detail the structure of valence electronic states, in particular the dispersion of π bands in reciprocal
space and the exact determination of the number of graphene layers. Using far-infrared magnetotransmission (FIR-MT), we demonstrate, that the electron cyclotron resonance (CR) occurs between
Landau levels with a (B) 1/2 dependence. The CR line-width is consistent with a high Dirac fermions
mobility of ~3000 cm 2 ·V −1 ·s −1 at 4 K.

Monolayer graphene is a two-dimensional materials with linear dispersions near the _K_ and _K_ ’ points of the Brillouin
zone. Low-energy charge carriers therein obey the Dirac-Weyl equation and behave like massless fermions 1 . This
material is ambipolar and exhibits ballistic transport properties on a micrometer scale up to room temperature.
In multilayer graphene, the interlayer coupling introduces perturbations of the low-energy band dispersions 2 .
Consequently, the linear _π_ and _π*_ bands near the Fermi level in monolayer graphene are modified in multilayer
graphene, showing a strong dependence on the stacking sequence as well as the layer number. Recently multilayer
graphene has attracted considerable interest 3 due to the possibility of inducing a band gap. This band gap can be
generated by breaking the translational symmetry using an external electric field 4,5 or by introducing asymmetrical
doping between the graphene layers resulting in a ‘built in’ electric field 3,6 . Moreover, the number of graphene layers
and their stacking configuration will control the electronic properties of multilayer graphene. When multilayer
graphene are stacked, Bernal stacking represents the lowest energy configuration. The shift of one atom of the
bottom layer along the C-C bond axis (one bond length) allows a transition between the Bernal and the second
stable allotropes of multilayer graphene, rhombohedral stacking. In the case of trilayer graphene, this transition
has to overcome, an energy barrier of about 1.1 meV/atom 7 .

A viable method for obtaining a large-scale graphene production is the epitaxial approach based on SiC graphitization. The epitaxial growth of graphene on semi-insulating SiC has the potential to enable the next generation
of ultra high frequency and low power electronic devices. The main advantage here is the possibility of obtaining a
large graphene area directly on semi-insulating substrates which is one of the main requirements for high frequency
devices, with a huge control of the number of graphene layers. Moreover, the choice of the appropriate SiC polytype
also makes it possible to select for the formation of Bernal or rhombohedral multilayer graphene 8 . Other methods
to obtain large-area graphene include chemical vapor deposition (CVD) on metals 9 . However, this CVD on metal
process is less suitable for device manufacture, as the graphene layers must be transferred onto a semiconducting
substrate. These transferred graphene layers tend to acquire contamination as a result of the transfer process, thus

1CNRS- Laboratoire de Photonique et de Nanostructures, Route de Nozay, 91460 Marcoussis, France. 2SynchrotronSOLEIL, Saint-Aubin, BP48, F91192 Gif sur Yvette Cedex, France. 3 Laboratoire Pierre Aigrain, Ecole Normale
Supérieure-PSL Research University, CNRS, Université Pierre & Marie Curie-Sorbonne Universités, 24 rue Lhomond,
[75005 Paris, France. Correspondence and requests for materials should be addressed to A.O. (email: abdelkarim.](mailto:abdelkarim.ouerghi@lpn.cnrs.fr)
[ouerghi@lpn.cnrs.fr)](mailto:abdelkarim.ouerghi@lpn.cnrs.fr)

Figure 1. Structural properties of epitaxial trilayer graphene. ( a ) Schematic structure of our trilayer
graphene on SiC(0001), ( b ) Morphological AFM image of trilayer graphene on an off-axis SiC(0001) substrate,
( c ) The AFM phase image of trilayer graphene.

resulting in graphene which is not suitable for device oriented applications. Coletti _et al._ 10, studied epitaxial trilayer
graphene obtained using hydrogen intercalated bilayer graphene on 3C-SiC(111) and on 6H-SiC(0001) substrates
using ARPES. Their ARPES results suggest that the resulting trilayer graphene showed a tendency towards the
development of presumably large-area ABC and ABA type stacking respectively. Recently, Lalmi _et al._ 11 studied
trilayer graphene on 4H-SiC(0001) using STM/STS and ARPES. They showed the presence of a different types of
structural defect 11 in which a transition from ABA to ABC stacking was observed.

The Si-face 4 H-SiC substrate is preferred to the C-face for the growth of graphene. This is mainly due to the
fact that graphene growth is easier to control on the Si-face, and under optimized conditions the thickness of the
graphene can be limited to just a mono, bi and few layers over large areas. The off-axis 4H-SiC(0001) substrate
could therefore, be a good candidate to obtain trilayer graphene. Although further, experiments are required to
obtain large-area and high quality sample with high electron mobility.

We demonstrate here the successful growth of ordered trilayer graphene layers on off-axis SiC(0001) substrates
up to an unprecedentedly large area of hundreds of micrometers. The high quality of the graphene layers was further
confirmed by AFM, micro-Raman mappings and X-ray photoelectron spectroscopy (XPS) measurements. Using
FIR-MT measurements, we show that trilayer graphene exhibits high electron mobility (~3000 cm 2 ·V −1 ·s −1 at 4 K)
for the observation of the electron cyclotron resonance, despite their high doping level. By comparing ARPES
measurements and DFT calculations, we show that the ordered trilayer graphene presents an ABA stacking. Using
the near-edge X-ray absorption fine structure (NEXAFS) technique, we also measured the unoccupied electronic
states (π * and σ *). These observations could be of prime importance, particularly regarding the development of
multilayer graphene-based electronic devices.

The trilayer graphene used in this study was obtained by annealing 4° off-axis 4H-SiC(0001) at 1550 °C in 800
mbar argon for 10 min (Fig. 1(a)). For Si terminated SiC substrates the first carbon layer which is formed, the
so-called interface or buffer layer, is insulating because one third of its carbon atoms are covalently bound to the SiC
substrate. The epitaxial graphene refers so to the carbon layer formed on top of this interfacial layer, and features
linear dispersion typical of isolated single layer graphene. The thickness of our sample was confirmed by XPS and
ARPES measurements. The typical morphology of the graphene sample can be seen in the AFM image displayed
in Fig. 1(b). Step direction and terrace width are directly determined by the initial misorientation of the substrate
with respect to the crystallographic (0001) plane. The (0001) terraces are characterized by a width of ~ 2–3 μ m
separated by steps heights of 10 nm, while the (11–20) nanofacets are characterized by a width of ~ 300–600 nm.

During the graphitization process, graphene growth starts in the vicinity of the nanofacets regions consisting
of step edges 12 and gradually propagates at the center of the terraces. This growth process generates a large area of
carpet-like graphene, which covers the steps without interruption as clearly shown in the AFM image in Fig. 1(c).

In addition to AFM imaging, we carried out micro-Raman spectroscopy mapping. Figure 2(a,b) show the maps
of the 2D peak position and _fwhm_ on a 12 × 12 μ m 2 scan. The different color scale highlights a blue shift of the 2D
peak from the terraces to the nanofacets, combined with an increase in the _fwhm_ . This effect was not surprising,
as the reduced bonding coordination of the nanofacets led to an easier Si sublimation from the substrate, which as

Figure 2. Micro-Raman spectroscopy of epitaxial trilayer graphene. ( a ) and ( b ) show the Raman shift and
_fwhm_ of the 2D vibrational mode respectively. Image dimensions are 12 _μ_ m × 12 _μ_ m, ( c ) Micro-Raman spectra
of the D, G and 2D vibrational mode collected on the terrace (black spectrum) and on the step edge (blue
spectrum), with a 532 nm wavelength excitation and a spatial resolution better than 1 μ m.

explained before, facilitated graphene growth. This different growth rate between the terraces and the nanofacets
may induce the formation of up to a trilayer along the nanofacets 6 . Taking two representative spectra from the
terraces and nanofacets (Fig. 2(c)) it was easier to evaluate the shift and the _fwhm_ of the 2D peak. As usual for
epitaxial graphene on SiC, several intense peaks were visible in the wavenumber range between 1300–800 cm −1,
corresponding to second-order Raman bands originating in the SiC substrate 13 . In addition to the 2D peak, other
main structures originating from graphene were present: the D and the G bands. The low intensity of the D peak
shows that there were only a small number of defects or disorders in the graphene structure 13,14 .

This is an indication of the high quality of epitaxial trilayer graphene on off-axis SiC under argon flux. As
shown by the Raman maps, the 2D peak from the (11–20) nanofacets had blue-shifted with respect to those of the
spectrum obtained for the (0001) terrace of about 50 cm −1 (from 2700 to 2750 cm −1 ), with an increased _fwhm_ of
about 60 cm −1 (from 50 to 110 cm −1 ). At the same time, we also saw a shift in the G peak of about 20 cm −1 (from
1590 to 1610 cm −1 ). This shift shows a strong correlation between the strain and the charge density on epitaxial
graphene. Thus, the observed blue-shift indicates that there is a difference in strain (associated with the shift of
the G and the 2D lines) and carrier density (associated with the shift of the G line) between the (0001) terraces
and (11–20) nanofacets.

We probed the electronic properties of the sample further using XPS and NEXAFS experiments. The C1s spectra
(Fig. 3(a)) were acquired using two different photon energies (hν = 340 eV, and hν = 510 eV). The variation in photon energy reflects a variation of surface and bulk sensitivity, allowing the depth positions of the different species to
be identified across the surface. The different components contributing to the spectra were decomposed by a curve
fitting procedure. The experimental data points are displayed with dots, meanwhile the solid line is the envelope
of the fitted components. The C1 _s_ spectra show the conventional deconvolution expected for epitaxial graphene
on SiC(0001) 12,15,16, characterized by three contributions. The peak G, at a binding energy of 284.6 eV, indicates
the presence of sp 2 hybridized C–C bonds and it this is the signature of the graphene layers. This component was
fitted using a Doniach-Sunjic line shape with an asymmetry factor α of 0.1 and a _fwhm_ of 0.5 eV. The two other
peaks at binding energies of 283.8 eV (SiC) and 285.3 eV (IL) are attributed to the SiC bulk and to the interfacial
layer respectively. Due to the non-metallic nature of these components, the peaks are symmetric (asymmetric
factor α = 0 eV). Assuming that the graphene-SiC sample can be modeled as a semi-infinite SiC substrate with a

Figure 3. Electronic properties of epitaxial trilayer graphene. ( a ) High resolution XPS spectra of C 1s core
level hν = 510 eV (bulk sensitive, top panel) and hν = 340 eV (surface sensitive, bottom panel), ( b ) Carbon
K-edge NEXAFS spectra, measured for different incidence angles of linearly polarized synchrotron light. The
insert shows a schematic representation of the NEXAFS geometry: θ represents the angle between the incident
photons and the sample surface.

uniform graphene overlayer, the graphene thickness can be calculated from the ratio between the intensity of the
G and SiC components 17 extracted from XPS data using the following relation:

( _α_ ) = _λ_ ⋅ cos _α_ ⋅ _ln_  1 + _IG_ ⋅

_d_ ( _α_ ) = _λ_ ⋅ cos _α_ ⋅ _ln_  1 + _IISiCG_ ⋅ _NNSiCG_  (1)

_SiC_

_G_





_G_

_SiC_

_N_



cos _α_ ⋅ _ln_  1 + _IISiCG_ ⋅ _NNSiCG_  (1

For a photon energy of 510 eV, we have an escape depth λ ~ 7.2 Å. In the geometry of our XPS experiment the
take-off angle α was 0°, and _IG_ and _I SiC_ represented the area of the G and SiC components of the C 1s peak extracted
from the deconvolution of Fig. 3(a – top). For epitaxial graphene on SiC, the areal density of the C atoms in
graphene _NG_ = 3.8 × 10 15 cm −2 is about three times of that of the C atoms in the SiC bilayer _N SiC_ = 1.22 × 10 15 cm −2 .
We obtained a thickness _d_ = 7.9 Å, corresponding to about three layers of graphene.

Unoccupied electronic states of the trilayer graphene were also examined by NEXAFS spectroscopy. The C
k-edge spectra were measured for different incidence angles of linear polarized synchrotron light. As shown in the
insert of Fig. 3(b), θ is the angle between the sample surface and the incident photons. The spectra present the
characteristic features of epitaxial graphene on SiC 12,18 : two sharp resonances at 285.3 eV and 291.7 eV and a broad
structure at 292.7 eV, assigned to the _π_ ⁎, _σ_ 1 ⁎ and _σ_ 2 ⁎ resonances, respectively. Changing the incidence angle of the

light from grazing incidence (θ = 10°) to normal (θ = 85°) provided information on the orientation of the trilayer
graphene with respect to the SiC substrate. As expected for a well-ordered planar π conjugated graphene system,
the intensity of �� _π_ ⁎ resonance was enhanced for grazing light incidence, when the direction of polarization vector
_E_ was close to the surface normal, and decreased with increases in the incident angle such that it wasalmost suppressed for normal incidence. Meanwhile the _σ_ ⁎ resonances revealed the opposite angular dependence behaviour.
As no other significant features, apart from the σ * and π * transitions, were observed on these NEXAFS spectra,
this meant the graphene layers presented a high degree of crystallinity and were uniformly parallel to the substrate
surface. Moreover, the sharpness of the NEXAFS features and of the graphene C 1s core level peak a well-defined
bonding environment and a long-range periodic order in the electronic structure of trilayer graphene.

The electronic structure was also probed using ARPES, which is a powerful method for this purpose, as it gives
direct access to the spectral function containing the information on electron energy band dispersion. Figure 4(a)
shows the valence band structure around the K point, along the MΓ K direction, in the first graphene Brillouin
zone. ARPES data are measured for hν = 60 eV. The momentum distribution curve (MDC) is shown in Fig. 4(b).
The MDC was extracted for E – EF = –1.32 eV. The three peaks observed in the MDC result from the presence of

Figure 4. Band structure of epitaxial trilayer graphene. ( a ) ARPES map of the trilayer graphene acquired
along the MΓ K direction at hν = 60 eV, ( b ) MDC extracted from the ARPES map at E–EF = – 1.32 eV, ( c ) Second
derivative of the ARPES map in ( a, d ) DFT band structure calculation for Bernal stacking trilayer graphene.

three bands in the ARPES spectrum in Fig. 4(a). As is already well-known, multilayer graphene presents a band
structure different from that of a single layer 19 . Furthermore, the band structure of few-layer graphene films depends
on the number of layers, as well as on the stacking sequence. Clearer evidence of the band structure of our epitaxial graphene is given by the second derivative of the ARPES intensity map shown in Fig. 4(c). We observed the
dispersion of three bands, as indicated by the MCD; two of these touching each other at the K point. These band
dispersions showed evidence of a trilayer graphene on the off-axis 4H SiC (0001). They are plotted on Fig. 4(d) to
give a comparison with theoretical data. This band dispersion was also consistent with our DFT calculations of ABA
stacking. The ARPES measurements show the growth of Bernal stacked trilayer graphene. In particular, the sharp
and intense structure of the three π -bands underlines the high structural quality of these graphene layers. However,
we do not currently exclude the formation of the ABC stacking at low scale. As explained before, Bernal-stacking
is energetically more favorable than rhombohedral stacking 7,20, a fact that is confirmed by our results, where no
contribution of ABC stacking was observed with respect to ABA stacking. As it is usually observed in the case
of trilayer graphene grown on SiC, the π bands form a cone, for which the π branches cross at the Dirac point
(ED) at –320 meV below the Fermi level (EF). This energy shift Δ E (Δ E = ED - EF) is known to be related to the
charge transfer from the substrate to the graphene layers 21 . From the position of the Dirac point,with respect to
the Fermi energy, we estimate a carrier concentration _n_ ~ 9 × 10 12 cm −2 . Considering that the x-ray spot size in the
ARPES measurements was about 180 × 80 (H × V) μ m 2, we can assume that we are able to obtain large areas of

Figure 5. Magneto-transmission measurements of epitaxial trilayer graphene. ( a ) FIR transmission
spectra of an ABA trilayer graphene for various magnetic fields B. The arrow shows the calculated energy of
the cyclotron resonance 5 → 6 at 15 T. ( b ) Calculated LL spectrum of an ABA trilayer graphene. Monolayer
graphene-like and bilayer graphene-like LLs are represented by the black and red lines, respectively. The blue
line represents the Fermi energy and the arrows indicate the measured cyclotron resonances between LLs of
index n.

mono-domain trilayer with Bernal stacking, in contrast to other works, where the epitaxial graphene showed different domains with different numbers of layers or different stacking 10,22 . These results make the off-axis SiC(0001)
substrate, a good candidate for producing large areas of trilayer graphene with the favorable Bernal stacking.

Finally, to study the electronic quality of our graphene layers, we determined the carrier mobility of the sample
using FIR-MT. In order to measure the FIR-MT in the range 0–80 meV, the trilayer graphene sample (typically
5 × 5 mm 2 ) was placed in a 15 T superconducting coil at 4 K and exposed to the radiation from a mercury lamp
(Faraday geometry). The transmitted light was analyzed by a Fourier transform spectrometer using a Si composite
bolometer directly below the sample for detection. The transmission at a given magnetic field T(B) was normalized
by the zero field transmission T(0). Typical transmission spectra are shown in Fig. 5(a). For B ≥ 3T, a broad
absorption line was mainly observed and attributed to the electron cyclotron resonance of the conduction bands
which touch at the K point. Note that the energy position of this minimum did not vary linearly with B, which is
the feature of Dirac fermions 23 . The Landau levels (LLs) of an ABA trilayer graphene have been calculated analytically 24,25 and are shown in Fig. 5(b). The trilayer graphene LL consists of both a bilayer graphene-like and a
monolayer graphene-like LL spectrum. This is expected since the trilayer graphene band structure consists of one
massless monolayer graphene-like band and two massive bilayer graphene-like bands, as discussed above. The
energy separation of the bilayer graphene-like LL is too narrow to be experimentally observed (i.e. of the order of
the LL broadening so that the different LLs are not resolved) and we essentially measured the monolayer
graphene-like spectrum given by _En_ = _vF_ 2 _e_ _ħn B_, where n is the LL index. The Fermi velocity _v_ _F_ ≈ 106 _m_ . _s_ −1

is deduced from the ARPES measurements. The cyclotron resonance corresponds to the transition n→ n+ 1, where
the n th level is the upper populated LL ( _En_ < _E_ _F_ < _En_ +1 ), as shown by the arrows in Fig. 5(b). Taking Fermi energy
_E_ _F_ = 320 _meV_ above the Dirac point in agreement with the ARPES measurements, the cyclotron resonance energy
can be calculated for each magnetic field. For example, the absorption at B = 15 T associated with the transition
5→ 6 is indicated by the arrow in Fig. 5(a). By considering that a clear absorption minimum is only defined for
B ≥ 3T, one can deduce a mobility of the Dirac fermions higher than 3000 _cm_ 2 _V_ −1 _s_ −1 at 4K. The carrier mobility
obtained was particularly high and comparable to that which we usually obtained in graphene monolayers 26 . Even
if the Bernal stacked trilayer graphene uniformly covers the terraces of the sample creating a very homogenous
sample, the presence of the steps edges can effectively increase short-range scattering and affect the mobility 27 . A
reduction in step edge density using a gentle hydrogen intercalation between the SiC substrate and the graphene 26,
could probably improve the electron mobility further.

In summary, we have investigated the electronic properties of Bernal stacking trilayer graphene. When
large-area of trilayer graphene is measured at low temperature, the CR observation condition μ B ≥ 1 allows to
deduce an electron mobility of ~3000 cm 2 V −1 s −1 . We used NEXAFS to study the lowest unoccupied electronic
states, mainly π * and σ *, providing information on the two-dimensional nature of graphene layers on a SiC substrate. By comparing ARPES measurements with band structure calculations, we show that the trilayer graphene
presents an ABA stacking. Our approach represents a significant step towards the scalable synthesis of high quality

6 _m_ . _s_ −1

_v_ _F_ ≈ 106 _m_ . _s_

Bernal stacking trilayer on a large scale. It is compatible with the high structural qualities and fine thickness control
needed to develop graphene-based electronic devices.

Methods
The trilayer graphene used in this study was obtained by annealing an off-axis 4H-SiC(0001) substrate. Before
graphitization, the substrate was hydrogen etched (100% H2) at 1550 °C to produce well-ordered atomic terraces of
SiC. The sample was heated to 820 °C and deoxidized at 1100 °C in order to remove the native oxide and a possible
surface contamination. The substrate was then heated to 1550 °C in an Ar atmosphere (800 mbar) for 10 min. The
samples were cooled down to room temperature and transferred ex-situ to perform the different measurements.

The AFM measurements were carried out under ambient conditions and the images recorded in tapping mode.
The Micro-Raman mapping was performed at room temperature with a Renishaw spectrometer using 532 nm laser
light auto-focusing on the sample by a DMLM Leica microscope with a 100× objective and a power of 5 mW with
a spot size of about 1 μ m.

The XPS, NEXAFS and ARPES experiments were carried out on the TEMPO beamline 28 (SOLEIL French
synchrotron facility) at room temperature 29 . The photon source is a HU80 Apple II undulator set to deliver linear
polarized light. During the XPS measurements, the photoelectrons were detected at 0° from the sample surface normal  _n_ and at 42° from the polarization vector _E_ � . The zero binding energy (BE) (i.e. the FERMI level) was taken

polarized light. During the XPS measurements, the photoelectrons were detected at 0° from the sample surface normal  _n_ and at 42° from the polarization vector _E_ � . The zero binding energy (BE) (i.e. the FERMI level) was taken

at the leading edge of a clean molybdenum foil in electriclal contact with the sample. The C 1s NEXAFS spectra
were recorded in the Auger yield mode using an energy window of 13 eV centered at 260 eV. For ARPES measurements, the photon energy (h _ν_ = 60 eV) and sample orientation were set in order to explore the k-space region
around the K point in the Γ K direction of the Brillouin zone.

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Acknowledgements
The authors received funding from the French National Research Agency (ANR) SUPERTRAMP project, LABEX
NanoSaclay (Grant No. ANR-10-LABX-0035) and GANEX (Grant No. ANR-11-LABX-0014).

Author Contributions
M.H., D.P. and H.S. grows the graphene sample, D.P., M.S., F.S, R.B. and A.O. conducted the measurements by
XPS/ARPES. H.S., M.H. and H.H. characterized it by AFM and Raman spectroscopy. T.P., L.V. and Y.G., carried
FIR-MT experiment. All the authors participated in analyzed the data and writing the paper.

Additional Information
Competing financial interests: The authors declare no competing financial interests.

How to cite this article : Hajlaoui, M. _et al._ High Electron Mobility in Epitaxial Trilayer Graphene on Off-axis
SiC(0001). _Sci. Rep._ 6, 18791; doi: 10.1038/srep18791 (2016).

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