Pressure-Induced Phase Transition in Weyl Semimetallic WTe2

Article abstract of DOI:10.1002/smll.201701887

Tungsten ditelluride (WTe₂) is a semimetal with orthorhombic T_d phase that possesses some unique properties such as Weyl semimetal states, pressure-induced superconductivity, and giant magnetoresistance. Here, the high-pressure properties of WTe₂ single crystals are investigated by Raman microspectroscopy and ab initio calculations. WTe₂ shows strong plane-parallel/plane-vertical vibrational anisotropy, stemming from its intrinsic Raman tensor. Under pressure, the Raman peaks at ≈120 cm^?1 exhibit redshift, indicating structural instability of the orthorhombic T_d phase. WTe₂ undergoes a phase transition to a monoclinic T′ phase at 8 GPa, where the Weyl states vanish in the new T′ phase due to the presence of inversion symmetry. Such T_d to T′ phase transition provides a feasible method to achieve Weyl state switching in a single material without doping. The new T′ phase also coincides with the appearance of superconductivity reported in the literature.

Full text of DOI:10.1002/smll.201701887

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Phase Transitions
Pressure-Induced Phase Transition in Weyl
Semimetallic WTe2
Juan Xia, Dong-Fei Li, Jia-Dong Zhou, Peng Yu, Jun-Hao Lin, Jer-Lai Kuo, Hai-Bo Li,
Zheng Liu, Jia-Xu Yan,* and Ze-Xiang Shen*
Tungsten ditelluride (WTe2) is a semimetal with orthorhombic Td phase that
possesses some unique properties such as Weyl semimetal states, pressure-induced
superconductivity, and giant magnetoresistance. Here, the high-pressure properties
of WTe2 single crystals are investigated by Raman microspectroscopy and ab initio
calculations. WTe2 shows strong plane-parallel/plane-vertical vibrational anisotropy,
stemming from its intrinsic Raman tensor. Under pressure, the Raman peaks at
≈120 cm−1 exhibit redshift, indicating structural instability of the orthorhombic Td
phase. WTe2 undergoes a phase transition to a monoclinic T′ phase at 8 GPa, where
the Weyl states vanish in the new T′ phase due to the presence of inversion symmetry.
Such Td to T′ phase transition provides a feasible method to achieve Weyl state
switching in a single material without doping. The new T′ phase also coincides with
the appearance of superconductivity reported in the literature.
J. Xia, Dr. J.-X. Yan, Prof. Z.-X. Shen
Prof. Z. Liu
Division of Physics and Applied Physics
School of Physical and Mathematical Sciences
Nanyang Technological University
NOVITAS, Nanoelectronics Centre of Excellence,
School of Electrical and Electronic Engineering
Nanyang Technological University
Singapore 639798, Singapore
Singapore 637371, Singapore
E-mail: jxyan@ntu.edu.sg; zexiang@ntu.edu.sg
Prof. Z. Liu, Prof. Ze-X. Shen
Dr. D.-F. Li, Prof. H.-B. Li
Centre for Disruptive Photonic Technologies
Nanyang Technological University
Singapore 637371, Singapore
Key Laboratory of Functional Materials Physics
and Chemistry of the Ministry of Education
Jilin Normal University
Dr. J.-X. Yan
Changchun 130103, China
Institute of Advanced Materials (IAM), Jiangsu National
Synergistic Innovation Center for Advanced Materials (SICAM)
Nanjing Tech University (NanjingTech)
J.-D. Zhou, Dr. P. Yu, Prof. Z. Liu
Center for Programmable Materials
School of Materials Science and Engineering
Nanyang Technological University
Singapore 639798, Singapore
30 South Puzhu Road, Nanjing 211816, P. R. China
Prof. Z.-X. Shen
CINTRA CNRS/NTU/THALES, UMI 3288
Research Techno Plaza
Dr. J.-H. Lin
Materials Science and Technology Division
Oak Ridge National Lab
Oak Ridge, TN 37831, USA
Singapore 637553, Singapore
Prof. J.-L. Kuo
Institute of Atomic and Molecular Sciences
Academia Sinica
Taipei 10617, Taiwan
DOI: 10.1002/smll.201701887
(1 of 7) 1701887
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Pressure is a clean and convenient tool to modify the physical
Therefore, A1 and A2 modes can be detected under plane-
properties of diverse transition metal dichalcogenide (TMDs) parallel configuration with incident laser along c-axis, while
materials,[1–6] via strengthening the van der Waals interactions B1 and B2 modes can be observed with incident laser along
along c-axis and shortening the covalent bonds within the a- or b-axis. In polarized Raman spectroscopy, A(BC)D are
atomic plane. For instance, the typical first-order structural used to describe the excitation/collection configurations for
transition in MoS2 (2Hc to 2Ha),[4,5] metallization in MoSe2,[6] sample measurements. In our case, A and D stand for the
[7]
and superconductivity in T′-MoTe2 have been reported incident laser and output scattering direction, with respect to
under high pressure. Among the layered TMDs family, semi- WTe2 atomic plane. B and C stand for laser polarization and
metallic WTe2 have recently attracted considerable attention, collection polarization direction, respectively. For perfectly
with reports of the nonsaturating large magnetoresistance plane-parallel aligned samples, A1 symmetry is obtained in
Z(YY)Z
(LMR),[8–10] pressure-induced superconductivity,[11,12] and as the Z(XX)Z and
configurations, while A2 symmetry
the most promising candidate of a type-II Weyl semimetal.[13] is obtained in the Z(XY)Z configurations.[24] In the case of
Weyl Fermions were observed following the discovery of top- plane-vertical configuration, B2 modes can only be observed
ological superconductors and semimetals, where they show in X(ZY)X configuration, while for B1 symmetry modes,
low-energy excitation properties and have two types of Weyl they are forbidden under both X(ZZ)X and X(ZY)X
points. A point-like Fermi surface are referred to as type-I, configurations. As far as we know, the latter Raman geo-
while a protected crossing appearing at the contact of electron metries with plane-vertical orientation in WTe2 have not
and hole pockets are called type-II Weyl point. The type-II been reported in the literature. First, we address a full mode
Weyl points in WTe2 endows many novel physical properties assignment in WTe2 crystal for both plane-parallel and plane-
that are different to those of type-I Weyl semimetals.[13] So vertical orientations via the polarized Raman spectroscopy.
far, the pressure-driven superconductivity in WTe2 has been For the scanning transmission electron microscope (STEM)
attributed to a possible structural instability[11] or Lifshitz experiments and plane-parallel property study, large-area
phase transition without structural transition.[12] Therefore, monolayer WTe2 samples with high quality are used to com-
the structural evolution of WTe2 under high pressure is still pletely exclude the nonuniformity impact on Raman signals.
unclear and needs to be further clarified.
As for the plane-vertical orientation, we vertically align the
In this paper, we investigate the effects of hydrostatic samples to achieve the X(ZZ)X and X(ZY)X geometries,
pressure on both plane-parallel and plane-vertical orientation so that the laser propagation direction is within the ab-plane
of vibrational properties in WTe2 as well as the corresponding of bulk WTe2 sample, as shown in Figure 1c.
electronic properties. A first-order structural transition from
Figure 1a shows the Z-contrast STEM images of 1L
the Td phase to a monoclinic T′ phase occurs above 8 GPa, and 2L WTe2 in the Td phase which vividly reveal that the
marked by the vanishing of the Raman band at 120 cm−1 where Td phase 1L WTe2 consists of zigzag W chains along a-axis
the Raman active B2 mode in the Td phase becomes Raman as skeleton with Te chains connected therein, in excellent
inactive (B2u symmetry) in the T′ phase. This B2 Raman mode agreement with the atomic model of 1L WTe2. The STEM
can only be observed in the Td phase with incident laser beam image of 2L WTe2 indicates this stacking order is similar
perpendicular to the c-axis of WTe2 crystal, and it softens con- to that of 2H (AB) stacking for MoS2 yet the atoms of top
siderably with pressure, while all other Raman modes stiffen. and bottom layers cannot perfectly overlap due to its dis-
Beyond the plane-parallel Raman studies generally conducted torted lattice structure. The optical images of 1L (grown
in 2D materials, our plane-vertical measurements can detect by chemival vapor deposition (CVD) method) and single
the inactive Raman modes, which may become the indicator crystal bulk WTe2 samples are shown in Figure 1b,c, where
of defects, stacking, or phase transition as the characteristic the atomic planes (010) are parallel and perpendicular to the
peaks.To the best of our knowledge, it is the first report on the substrate, respectively. Figure 1d shows the Raman spectra
anomalous redshift phenomenon of the Td-WTe2 and the new of the monolayer and bulk WTe2 samples with incident
T′ phase under high pressure. We further show that these two laser along c- (marked as P) and a-axes (marked as V) for
phases have distinct electronic properties, especially in Weyl the study of plane-parallel and plane-vertical anisotropies,
ꢀ
ꢀ
semimetallic states. Our findings offer the possibility to switch respectively. All the Raman signals with both ei ꢁ es and


on/off the Weyl states in sole material via high pressure.
ei ⊥ es configurations can be detected. For plane-parallel
Unlike most other semiconducting TMDs with the hexa- configuration (P), the peaks located at 89, 110, 117, 160, and
gonal (2H) phase,[14–16] WTe2 exhibits a unique distorted 210 cm−1 (marked as P1, P2, P3, P4, and P5 in Figure 1d) can
octahedral coordination of the W atom, referred to as be assigned to A2, A2, A1, A1, and A1 symmetry, respectively.
Td-polytype.[17–23] Its space group (Pmn21) endows the corre- Compared with plane-parallel case, the plane-vertical con-
sponding Raman tensors as:
figuration exhibits a different set of Raman peaks, where
the four peaks V1, V2, V3, and V4 located at 93, 123, 141,
and 170 cm−1 are assigned as B2, B2, A1, and A1 symmetry,
respectively. The polarized Raman features of plane-parallel
and plane-vertical configurations are further discussed both
experimentally and theoretically in Supporting Information
by using various setups, which reaffirms the precise attribu-
tions of each mode. Our experimental frequencies of the
observed peaks are excellent agreed with the calculated








a
0
0
0
b
0
0
0
c
0
d
0
d
0
0
0
0
0
R(A1) =
R(B1) =
R(A2 ) =












(1)








0
0
0
0
0
f
0
0
e
0
0
0
e
0
0


R(B ) =
2










0
f
0
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Figure 1. a) Scanning transmission electron microscope (STEM) images of 1L and 2L WTe2. Optical image of 1L CVD grown WTe2 b) vertically aligned
bulk WTe2 c). The white rectangle in (b) highlights the corresponding Raman intensity mapping at 210 cm−1, indicating its homogenous quality.
The inset shows the atomic structures from the top view and the green arrows represent the reference sample orientation. d) Raman spectra of
the monolayer (blue) and bulk (red) WTe2 samples with incident laser along c- (marked as P) and a-axes (marked as V). The solid curves are from
experiments, and the gray curves (doted and solid) are corresponding Lorentzian fitting results.
frequencies and the corresponding symmetry assignments is a particularly strong peak envelope at around 120 cm−1
match perfectly.[25–27]
showing up under V-type configuration. Figure 2b,c shows the
Next, we utilize Raman spectroscopy to extend the high- evolution of lattice vibrational properties of bulk Td-WTe2
pressure study on plane-parallel and plane-vertical aniso- with the increasing of pressure for P- type (b) and V-type
tropies of the vibrational properties, in which the ab-plane (c) sample orientation, respectively. In the case of P-type, all
of bulk Td-WTe2 are parallel (P type) or vertical (V type) the observed Raman peaks gradually blueshift with no dis-
to the diamond anvil surface. Figure 2b shows the Raman continuities as the increasing of hydrostatic pressure from
spectra of bulk Td-WTe2 sample with the incident laser prop- 0 GPa to final 17.0 GPa, indicating no structural phase transi-
agation along c- (P) and a- (V) axes at ambient pressure, tion within the whole studied pressure range. The stiffening
respectively. The difference of Raman behaviors between and broadening phenomenon of Raman modes under pres-
P-type and V-type can be evidently noticed, that is, there sure has been well known, similar to other layered materials.
Figure 2. a) Illustration of the diamond anvil cell loaded with parallelly (P) and vertically (V) aligned WTe2 crystals. b,c) The evolution of Raman
vibrations with pressure for different sample orientations.
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Figure 3. a,b) Evolution of Raman peaks with pressure for perpendicular a) and parallel b) to the laser polarizations. The transparent lines indicate
the linear fit of Raman peak center as a function of pressure. c) Calculated pressure dependence of the enthalpy difference between the Td and T′
unit cells. d) Calculated phonon dispersion curve of T′-WTe2 at 12 GPa.
However, in the case of V-type, upon increasing pressure, Information) at 0 and 15 ps, respectively, which certified that
the new Raman modes (≈120 cm−1) of bulk Td-WTe2 red- the Td phase of WTe2 cannot stably exist under 20 GPa and
shift while all other modes blueshift. More interestingly, the a phase transformation from Td to T′ phase actually happens.
peaks at around 120 cm−1 nearly disappear with the pressure Besides, we have also calculated the enthalpy difference as
above 10.9 GPa. Both redshift and vanishing of Raman peaks a function of pressure between the Td and T′ structures of
cannot be observed under the P-type configuration, which WTe2, as shown in Figure 3c. Before 8 GPa, the value of
confidently declares that there is a phase transition point at
is small and positive, and then it rapidly changes
HT′ − HT
d
≈10 GPa. Figure 3a,b is the statistic diagrams of peak center to negative after the pressure increasing above 8 GPa, which
evolution of WTe2 as a function of pressure for V-type (a) further elucidates that the phase transition of WTe2 is from
and P-type (b) configurations. Our experimental observa- Td phase to T′ phase with the turning point pressure at
tion on pressure-dependent Raman frequencies shows good around 8 GPa. Figure 3d shows the phonon dispersion curves
qualitative agreement with the calculated Raman frequen- for T′-WTe2 at 12 GPa, where all the phonon frequencies
cies as a function of pressure. The pressure-dependent lattice are positive, indicating the T′ phase can stably exist under
vibrational properties of the V-type WTe2 are sketched in high pressure. The instability of Td phase at 12 GPa is also
Figure 3a, where the peaks, V2 and V3, show an anomalous discussed in Figure S8 in the Supporting Information, with
redshift as the pressure increases while other peaks show a imaginary frequencies based on our calculations. Therefore,
blueshift. We can also find the consistent blueshift of all the the MD simulation, enthalpy with increasing pressure, and
Raman peaks in Figure 3b, and yet the change slop for peak phonon calculation for WTe2 at 12 GPa are also consistent
P1 is smaller than that of other peaks. This insensitive prop- with that the application of pressure tends to stabilize the T′
erty of Raman peak P1 to pressure vibration indicates this phase for MoTe2.[7]
kind of peak represents the Raman vibration within atomic
Furthermore, in order to explain the anomalous Raman
plane, where the pressure has a weaker impact on this plane- behavior with V-type configuration, we have performed
parallel vibration compared with plane-vertical vibration. the first-principles calculations of the phonon modes at the
This phase transition under high pressure (8 GPa) should Brillouin zone (BZ) center within the framework of the den-
strongly account for the dramatic change in magnetic and sity functional perturbation theory (DFPT).[28] Figure S8c
electronic properties of WTe2 beyond a certain pressure as in the Supporting Information depicts the vibrational sche-
previously reported.[11,12]
matics of two Raman active B2 modes in V-type bulk Td-WTe2
To further identify the possible phase transition under and T′-WTe2 corresponding to the two redshifted peaks in
high pressure, we have performed the molecular dynamics pressure-dependent Raman spectra (Figures 2c and 3a).
(MD) simulation to examine the evolution of the atomic Compared with Td phase, T′-WTe2 undergoes an opposite
structure of WTe2 under 20 GPa (Figure S8, Supporting sliding within adjacent layers, leading to the unit cell change
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Figure 4. a) Calculated band structures of Td (left) and T′ (right) WTe2 with SOC at 0 and 10 GPa. b) Artificially modification of the band structure
of Td (left) and T′ (right) WTe2 along momentum space cut along the line K–K′, where the Weyl points W1(−), (0.12314, 0.03455, 0) and W2(+))
(0.12185, 0.05280, 0) are given for Td.
from orthorhombic to monoclinic symmetry. This phonon may provide a potential way to switch on/off the Weyl states
softening of the peaks at 120 cm−1 can be explained by the in one material through high pressure.
intermediate states engagement before the phase transition
point. These states originate from the electron–phonon inter-
actions, where intervalence charge transfer between the WTe2
layers plays an important role in the structural distortion of
Experimental Section
the crystal symmetry. Several materials have been previously
Sample Preparation: (1) The few-layer WTe2 were synthesized
reported to exhibit an intermediate state with soften phonon by CVD method using WO3 and WCl6 (Sigma) as the W sources.
behavior while undergoing a transition from a semiconductor The Te powder was used as the Te sources. The crystals were syn-
to a metallic state or when stress or strain is applied.[19] To thesized in quartz tube (1 inch diameter) with temperature from
the best of our knowledge, it is the first report on the anoma- 700 to 850 °C. The mixed gas of H2/Ar with 15 and 150 sccm
lous redshift of the Td-WTe2 Raman modes. Our calcula- were used as the carrier gas, the silicon boat contained 30 mg
tions indicate that the two modes become inactive from Td mixed powders with WO3:WCl6: Te = 1:1:1 (weight ratio) was
to T′ phase, in consist with our experimental findings that the put in the center of the tube. The SiO2/Si substrate was placed
peaks at around 120 cm−1 finally vanish at 10 GPa.
downstream. Another silicon boat containing 0.5 g Te powder
We have also calculated their band structures with spin- was put on the upstream. The temperature ramps up to the
orbit coupling (SOC) for Td and 1T′ phase of WTe2 obtained 820 °C in 17 min, and keeping at the reaction temperature for
from different pressures, as shown in Figure 4. At 0 GPa, about 5–15 min. Then the furnace cooled down to room temper-
four pairs of Weyl Fermions exist in Td phase, while these ature gradually. (2) Bulk WTe2 single crystals were prepared by
Weyl states vanish in the monoclinic T′ phase at 10 GPa, chemical vapor transport method with iodide as the transporting
due to the presence of inversion symmetry. Such differ- agency. The stoichiometric amounts of W powder and Te lumps
ence in band structure may pave a potential way for Weyl (300 mg) with iodide (30 mg) were sealed in a quartz tube under
state tuning in diverse 2D materials by phase engineering. vacuum at 10−6 Torr, which was placed in a three-zone furnace.
The type-II Weyl semimetal property in Td-WTe2 was first The reaction zone was prereacted at 750 °C for 24 h with the
reported by Soluyanov et al.[13] and then Tamai et al. also grown zone at 850 °C, preventing the transport of the samples.
demonstrated that the MoTe2 with 1T′ phase also held the The reaction zone was then programed at a higher temperature
similar property as Td-WTe2.[29] Moreover, one arc-tunable of 950 °C with the growth zone at a lower temperature of 850 °C
Weyl Fermion metallic state has been found in alloy mate- for 7 d to give the temperature gradient in order to achieve the
rials, MoxW1−xTe2, in which they demonstrated that the growth of single crystal. Finally, the furnace was naturally cooled
Fermi arc length and topological strength can be modified by down to room temperature and WTe2 single crystals were col-
Mo concentration.[30] In our work, we propose a possibility lected in the growth zone.
of switching on/off the Weyl states in one material without
doping.
TEM Characterization: The STEM samples were prepared with
a poly(methyl methacrylate) (PMMA) assisted method. A layer of
In summary, we have investigated the high-pressure prop- PMMA of about 1 µm thick was spin coated on the wafer with WTe2
erties of WTe2 single crystals by Raman microspectroscopy samples deposited, and then baked at 180 °C for 3 min. After-
and ab initio calculations. WTe2 shows strong plane-parallel/ ward, the wafer was immersed in NaOH solution (1 m) to etch the
plane-vertical vibrational anisotropy, stemming from its SiO2 layer over night. After lift-off, the PMMA/WTe2 film was trans-
intrinsic Raman tensor. Under pressure, the Raman peaks at ferred into (deionized water) DI water for several cycles to wash
≈120 cm−1 gradually soften upon increasing pressure and are away the residual contaminants, and then it was fished by a TEM
totally suppressed above 10 GPa, indicating the phase transi- grid (Quantifoil Mo grid). The transferred specimen was dried natu-
tion from Td to T′ phase. Reminiscent of the reported super- rally in ambient environment, and then dropped into acetone over-
conductivity emerging in WTe2 at 10 GPa, one can expect night to wash away the PMMA coating layers. The STEM imaging
that the phase transition from Td to T′ phase at 10 GPa sepa- was performed on an aberration-corrected Nion UltraSTEM-100
rates the LMR state and superconducting state. Our findings operating at 100 kV.
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Angle-Resolved Raman Spectra Measurements:
A
Witec maximally localized algorithm[39] and were implemented in the
CRM200 backscattering Raman system was used equipped with a package WANNIER90[40] using W s and d orbitals and Te p orbitals.
solid-state (yttrium aluminum garnet) YAG laser at 2.33 eV (532 nm) The Weyl chirality was calculated by the software package Wanni-
with a proper power avoiding sample heating (below 1 mW). The erTools, which was based on the iterative Green’s function.[41,42]
size of the laser beam is around 250 nm which was focused on
the sample using an objective (Olympus, 100×/NA0.95). To obtain
stronger Raman signal with high (signal to noise) SN ratio, for the
excitation path, the laser polarization was fixed and the sample
was rotated from 0o to 360o with respected to the laser polariza-
tion direction. For the collection path, both the Raman scattering
Supporting Information
signals with z(xx)z and z(xy)z configurations can be detected simul-
taneously without any analyzer before output fiber. As the few-
layer WTe2 samples are easily degraded, all the related Raman
experiments for each sample were completed within 2 h.[31]
High Pressure Measurements: The high-pressure cell used
in this experiment is based on a symmetric diamond anvil cell
(DAC) having two diamonds with 400 µm culet size. The sample
and pressure transmitting medium (methanol/ethanol, 4:l) with a
small ruby chip (10 µm) are loaded in a 150 µm hole drilled in a
250 µm thick T301 gasket preindented to 60 µm thickness. The
high-pressure Raman spectra were collected using a Renishaw
InVia micro Raman system under backscattering configuration,
with excitation laser wavelength of 532 nm and 20× objective
lens. The laser power is controlled below 0.4 mW to avoid possible
laser-induced heating. The signal was dispersed by a 2400 g mm−1
grating under the triple subtractive mode with a spectra resolu-
tion of 1 cm−1. The acquisition time of each spectrum was 200 s.
Frequency calibration of the Raman spectrum is realized using the
characteristic 520 cm−1 line of silicon. For WTe2 sample loading in
DAC, a very fine tip was used to cut the WTe2 crystal and adjust its
orientation within the punched gasket to obtain parallel and ver-
tical alignments.
Supporting Information is available from the Wiley Online Library
or from the author.
Acknowledgements
This research was supported by Ministry of Education (MOE) under
Academic Research Fund (AcRF) Tier 1 (Reference No. RG103/16),
AcRF Tier 2 (MOE2015-T2-1-148), and AcRF Tier 3 (MOE2011-
T3-1-005) in Singapore. Z.L. would like to acknowledge the
funding support from the Singapore National Research Foundation
(NRF) under NRF RF Award No. NRF-RF2013-08. J.-H.L. acknowl-
edge (Japan Society for the Promotion of Science) JSPS KAKENHI
(P16823) for financial support. J.-X.Y. and J.X. acknowledge the
technical support from H.-L.H at WITec. J.X. and D.-F.L. performed
the experiments. J.-X.Y. performed simulation studies. J.-D.Z, P.Y.,
and Z.L. helped to prepare the samples. J.-H.L provided the STEM
measurements. All authors discussed the results. J.X., J.-X.Y., and
Z.-X.S. conceived the study. J.X. and J.-X.Y. wrote the paper.
Conflict of Interest
Ab Initio Calculations: The first-principles calculations
including geometry optimization, vibrational properties, and mole-
cule dynamics were all carried out using the projector-augmented
wave[32] method as implemented in the Vienna ab initio simula-
tion package (VASP) code[33] including the spin–orbit coupling
effects. The exchange correlation potential was described using
the Perdew, Burke and Ernzerhof functional within the general-
ized gradient approximation.[34] The kinetic energy cutoff was set
to 500 eV. The BZ was sampled by a 16 × 8 × 1 k-point mesh using
Monkhorst–Pack (MP) method. The energy convergence for the
relaxation was chosen to be less than 10−5 eV Å−1. To account for
the interlayer van der Waals interactions, all the simulations were
performed using the nonlocal vdW-DF-optB86b functional.[35] The
phonon frequencies at the Γ point and Raman intensities were
calculated within DFPT as introduced in Phonopy.[36] For the MD
simulation study, a 96-atoms 2 × 2 × 2 supercell and 2 × 2 × 2 MP
k-point sampling grid. To adopt Normal Pressure and Temperature
(NPT) ensemble, the simulation system was coupled to an Par-
rinello–Rahman barostat[37] running with Langevin stochastic ther-
mostat, as recently implemented in VASP code. The time step in
the molecular-dynamics simulations was 1 fs and the external pres-
sure maintains at 20 GPa. The Raman peak intensity of a particular
The authors declare no conflict of interest.
[1] X. Dou, K. Ding, D. Jiang, B. Sun, ACS Nano 2014, 8, 7458.
[2] A. P. Nayak, T. Pandey, D. Voiry, J. Liu, S. T. Moran, A. Sharma,
C. Tan, C.-H. Chen, L.-J. Li, M. Chhowalla, J.-F. Lin, A. K. Singh,
D. Akinwande, Nano Lett. 2015, 15, 346.
[3] M. Peña-Álvarez, E. del Corro, Á. Morales-García, L. Kavan,
M. Kalbac, O. Frank, Nano Lett. 2015, 15, 3139.
[4] A. P. Nayak, S. Bhattacharyya, J. Zhu, J. Liu, X. Wu, T. Pandey,
C. Jin, A. K. Singh, D. Akinwande, J.-F. Lin, Nat. Commun. 2014, 5,
3731.
[5] Z.-H. Chi, X.-M. Zhao, H. Zhang, A. F. Goncharov, S. S. Lobanov,
T. Kagayama, M. Sakata, X.-J. Chen, Phys. Rev. Lett. 2014, 113,
036802.
[6] Z. Zhao, H. Zhang, H. Yuan, S. Wang, Y. Lin, Q. Zeng, G. Xu, Z. Liu,
G. K. Solanki, K. D. Patel, Y. Cui, H. Y. Hwang, W. L. Mao, Nat.
Commun. 2015, 6, 7312.
[7] Y. Qi, P. G. Naumov, M. N. Ali, C. R. Rajamathi, W. Schnelle,
O. Barkalov, M. Hanfland, S.-C. Wu, C. Shekhar, Y. Sun, V. Susz,
M. Schmidt, U. Schwarz, E. Pippel, P. Werner, R. Hillebrand,
T. Forster, E. Kampert, S. Parkin, R. J. Cava, C. Felser, B. Yan,
S. A. Medvedev, Nat. Commun. 2016, 7, 11038.
[8] M. N. Ali, J. Xiong, S. Flynn, J. Tao, Q. D. Gibson, L. M. Schoop,
T. Liang, N. Haldolaarachchige, M. Hirschberger, N. P. Ong,
R. J. Cava, Nature 2014, 514, 205.
[9] I. Pletikosic´, M. N. Ali, A. V. Fedorov, R. J. Cava, T. Valla, Phys. Rev.
Lett. 2014, 113, 216601.
normal mode v was calculated as[38] ν ∝|ei ⋅ Aν ⋅es |(1/ωv )(nv +1) ,
I
where ei(es ) is the incident (scattered) light polarization, nv is
the Bose–Einstein distribution, and Aν is the Raman suscep-
tibility associated with the vth normal mode. To do the spin pro-
jection calculations, the Wannier functions were computed by the
1701887 (6 of 7)
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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small 2017, 13, 1701887

www.advancedsciencenews.com
[10] L. R. Thoutam, Y. L. Wang, Z. L. Xiao, S. Das, A. Luican-Mayer, [27] J. Yan, J. Xia, X. Wang, L. Liu, J.-L. Kuo, B. K. Tay, S. Chen, W. Zhou,
R. Divan, G. W. Crabtree, W. K. Kwok, Phys. Rev. Lett. 2015, 115,
Z. Liu, Z. X. Shen, Nano Lett. 2015, 15, 8155.
046602.
[28] M. Lazzeri, F. Mauri, Phys. Rev. Lett. 2003, 90, 036401.
[11] X.-C. Pan, X. Chen, H. Liu, Y. Feng, Z. Wei, Y. Zhou, Z. Chi, L. Pi, [29] A. Tamai, Q. S. Wu, I. Cucchi, F. Y. Bruno, S. Ricco, T. K. Kim,
F. Yen, F. Song, X. Wan, Z. Yang, B. Wang, G. Wang, Y. Zhang, Nat.
Commun. 2015, 6, 7312.
M. Hoesch, C. Barreteau, E. Giannini, C. Besnard, A. Soluyanov,
F. Baumberger, Phys. Rev. X 2016, 6, 031021.
[12] D. Kang, Y. Zhou, W. Yi, C. Yang, J. Guo, Y. Shi, S. Zhang, Z. Wang, [30] T. R. Chang, S. Y. Xu, G. Chang, C. C. Lee, S. M. Huang, B. Wang,
C. Zhang, S. Jiang, A. Li, K. Yang, Q. Wu, G. Zhang, L. Sun, Z. Zhao,
Nat. Commun. 2015, 6, 7804.
[13] A. A. Soluyanov, D. Gresch, Z. Wang, Q. Wu, M. Troyer, X. Dai,
B. A. Bernevig, Nature 2015, 527, 495.
[14] X. Zhang, X.-F. Qiao, W. Shi, J.-B. Wu, D.-S. Jiang, P.-H. Tan, Chem.
Soc. Rev. 2015, 44, 2757.
[15] C. Tan, H. Zhang, Chem. Soc. Rev. 2015, 44, 2713.
G. Bian, H. Zheng, D. S. Sanchez, I. Belopolski, N. Alidoust,
M. Neupane, A. Bansil, H. T. Jeng, H. Lin, M. Zahid Hasan, Nat.
Commun. 2016, 7, 10639.
[31] Y. Kim, Y. I. Jhon, J. Park, J. H. Kim, S. Lee, Y. M. Jhon, Nanoscale
2016, 8, 2309.
[32] P. E. Blöchl, Phys. Rev. B 1994, 50, 17953.
[33] G. Kresse, J. Furthmüller, Phys. Rev. B 1996, 54, 11169.
[16] J. Ribeiro-Soares, R. M. Almeida, E. B. Barros, P. T. Araujo, [34] J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson,
M. S. Dresselhaus, L. G. Cançado, A. Jorio, Phys. Rev. B 2014, 90,
115438.
M. R. Pederson, D. J. Singh, C. Fiolhais, Phys. Rev. B 1992, 46,
6671.
[17] A. Mar, S. Jobic, J. A. Ibers, J. Am. Chem. Soc. 1992, 114, 8963.
[18] B. Brown, Acta Crystallogr. 1966, 20, 268.
[35] J. Klimeš, D. R. Bowler, A. Michaelides, Phys. Rev. B 2011, 83,
195131.
[19] E. Lorchat, G. Froehlicher, S. Berciaud, ACS Nano 2016, 10, 2752.
[36] A. Togo, I. Tanaka, Scr. Mater. 2015, 108, 1.
[20] D. Wolverson, S. Crampin, A. S. Kazemi, A. Ilie, S. J. Bending, ACS [37] M. Parrinello, A. Rahman, Phys. Rev. Lett. 1980, 45, 1196.
Nano 2014, 8, 11154. [38] D. Porezag, M. R. Pederson, Phys. Rev. B 1996, 54, 7830.
[21] D. A. Chenet, O. B. Aslan, P. Y. Huang, C. Fan, A. M. van der Zande, [39] N. Marzari, D. Vanderbilt, Phys. Rev. B 1997, 56, 12847.
T. F. Heinz, J. C. Hone, Nano Lett. 2015, 15, 5667.
[22] R. He, J.-A. Yan, Z. Yin, Z. Ye, G. Ye, J. Cheng, J. Li, C. H. Lui, Nano
Lett. 2016, 16, 1404.
[23] S. Yang, C. Wang, H. Sahin, H. Chen, Y. Li, S.-S. Li, A. Suslu,
F. M. Peeters, Q. Liu, J. Li, S. Tongay, Nano Lett. 2015, 15, 1660.
[40] A. A. Mostofi, J. R. Yates, Y.-S. Lee, I. Souza, D. Vanderbilt,
N. Marzari, Comput. Phys. Commun. 2008, 178, 685.
[41] WannierTools: An Open-Source Software Package for Novel Topo-
logical Materials, arXiv:1703.07789 [physics.comp-ph], https://
github.com/quanshengwu/wannier_tools.
[24] W.-D. Kong, S.-F. Wu, P. Richard, C.-S. Lian, J.-T. Wang, C.-L. Yang, [42] M. P. Lopez Sancho, J. M. Lopez Sancho, J. M. L. Sancho, J. Rubio,
Y.-G. Shi, H. Ding, Appl. Phys. Lett. 2015, 106, 081906.
[25] Y. C. Jiang, J. Gao, L. Wang, Sci. Rep. 2016, 6, 19624.
[26] Y. Zhao, X. Luo, H. Li, J. Zhang, P. T. Araujo, C. K. Gan, J. Wu,
H. Zhang, S. Y. Quek, M. S. Dresselhaus, Q. Xiong, Nano Lett.
2013, 13, 1007.
J. Phys. F: Met. Phys. 1985, 15, 851.
Received: June 5, 2017
Revised: July 14, 2017
Published online: August 28, 2017
(7 of 7) 1701887
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