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Nano Research
Nano Res
DOI 10.1007/s12274-015-0762-6
Strain-induced direct-indirect band gap transition and
phonon modulation in monolayer WS2
Yanlong Wang1,2#, Chunxiao Cong2#, Weihuang Yang1,2, Jingzhi Shang2, Namphung Peimyoo2, Yu
Chen2, Junyong Kang3, Jianpu Wang1,4, Wei Huang1,4,5 (), and Ting Yu2 ()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0762-6
http://www.thenanoresearch.com on March 5, 2015
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1
TABLE OF CONTENTS (TOC)
Strain-induced direct-indirect band gap transition and
phonon modulation in monolayer WS2
Yanlong Wang,1,2# Chunxiao Cong,2# Weihuang
Yang,1,2 Jingzhi Shang,2 Namphung Peimyoo,2 Yu
Chen,2 Junyong Kang,3 Jianpu Wang,1,4 Wei Huang,1,4,5*
and Ting Yu2*
1Nanyang
Technological University – Nanjing Tech Center
of Research and Development, Nanjing Tech University,
Nanjing 211816, China
2Division
Physical
of Physics and Applied Physics, School of
and
Mathematical
Sciences, Nanyang
Technological University, 637371, Singapore
3Fujian
We plot the fitted light emission intensities of A, A- and I peaks of
CVD monolayer WS2 as a function of the uniaxial tensile strain,
Key Laboratory of Semiconductor Materials and
Applications, Department of Physics, Xiamen University,
Xiamen 361005, China
4Key
Laboratory of Flexible Electronics (KLOFE) and
Synergetic Center for Advanced Materials (SICAM),
Nanjing Tech University (NanjingTech), Nanjing 211816,
China
Laboratory for Organic Electronics & Information
Displays (KLOEID) and Institute of Advanced Materials
(IAM),
Nanjing
University
of
Posts
&
Telecommunications, 9 Wenyuan Road, Nanjing 210046,
China
strain since its appearance, contrary to those of A and A- peaks. It
could be interpreted as the strain-induced direct to indirect band gap
Institute of Advanced Materials (IAM), Jiangsu National
5Key
which show that I peak intensity has a rising trend with increasing
transition and agrees well with the calculated critical strain of around
2.6% which can lead to this transformation.
Nano Research
DOI (automatically inserted by the publisher)
Research Article
Strain-induced direct-indirect band gap transition and
phonon modulation in monolayer WS2
Yanlong Wang1,2#, Chunxiao Cong2#, Weihuang Yang1,2, Jingzhi Shang2, Namphung Peimyoo2, Yu
Chen2, Junyong Kang3, Jianpu Wang1,4, Wei Huang1,4,5 (), and Ting Yu2 ()
Received: day month year
ABSTRACT
Revised: day month year
In-situ strain Photoluminescence (PL) and Raman spectroscopy has been
Accepted: day month year
employed to exploit evolutions of electronic band structure and lattice
(automatically inserted by the
publisher)
© Tsinghua University Press
vibrational responses of chemical vapour deposition (CVD) grown monolayer
tungsten disulphide (WS2) under uniaxial tensile strain. An observable
broadening and appearing of an extra small feature at the longer wavelength
and Springer-Verlag Berlin
side shoulder of the PL peak occur under 2.5% strain, which could be an
Heidelberg 2014
indicator of direct to indirect bandgap transition and further confirmed by our
density functional theory calculations. As strain further increases, the spectral
KEYWORDS
weight of the indirect transition becomes larger gradually. Cross the entire strain
monolayer WS2, strain,
light
emission
tuning,
indirect transition, trion,
crystallographic
orientation
range, with the increase of strain, the light emissions corresponding to each
optical transition such as direct band gap transition (K-K) and indirect band gap
transition (Γ-K, ≥2.5%) show monotonous linear red-shift. In addition, larger
binding energy of the indirect transition compared to the direct one is deduced
and slight lowering of trion dissociation energy with increasing strain is
observed. Not only the electronic band structure, but also the lattice vibrations
could be modulated by the strain. The softening and splitting of in-plane E'
mode is shown under uniaxial tensile strain and the polarization dependent
Raman spectroscopy confirms the observed zigzag-oriented edge of CVD grown
WS2 in previous studies. These findings indeed enrich our understanding of
2
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strained states of monolayer transition metal dichalcogenide (TMD) materials
and further lay a foundation for developing applications such as strain detection
and light emission modulation of such emerging two-dimensional TMDs based
on their strain dependent optical properties.
Address correspondence to Ting Yu, [email protected]; Wei Huang, [email protected]
1 Introduction
breaking strength [10, 19], atomically thin TMDs
provide a good opportunity to further study strain
Transition metal dichalcogenides (TMDs) can be
effects on 2D materials. Phonon softening [20, 21],
generally expressed as MX2 where M symbolizes a
crystal orientation determination [20], bandgap
transition metal element and X represents a
narrowing [22, 23], valley polarization decrease [24],
chalcogen atom. Bulk TMDs have a layered
and giant valley drift [25] under tensile strain have
structure with each layer composed of two
been reported in monolayer MoS2. However,
chalcogenide atom planes and one transition metal
influences of strain on the direct band gap feature,
atom sub layer situated between them. The most
the most interesting and important property of 2D
common stacking polytype is 2H, with a trigonal
TMDs, such as a direct to indirect band gap
prism unit which comprises centered metal atom
transition under strain predicted by the theoretical
and its nearest 6 chalcogen atoms [1, 2]. TMDs have
calculations [26-29] has not been systematically
long been known to possess a remarkable variety of
studied by any experimental approach. Moreover
important physical and chemical properties [3, 4].
the strain effects on trion feature which was
Recently as the fabrication and characterization
recently assigned in several monolayer TMDs [9,
technique becomes mature, two-dimensional (2D)
30-32] remain unstudied experimentally to our best
TMDs have generated widespread research interest
knowledge. Last but not least though the edges of
and consequently many extraordinary results are
CVD grown 2D TMDs are reported to be zigzag
obtained, such as thinning-induced bandgap type
termination by TEM observations, they are not
transition [5-7] , valley confinement due to
atomically sharp and need further investigation to
inversion asymmetry [8, 9] , high eﬀective Young's
accurately identify the edge orientation [33-35],
modulus [10] , nonblinking photon emission [11] ,
which plays an important role in determining
and so on.
properties of TMD materials [36]. 2D tungsten
Strain engineering plays a key role in tuning
disulphide (WS2), one kind of TMD material, has
properties of 2D materials. Strain effects on
been
monolayer graphene have been widely studied. The
optoelectronics, photonics, and nanoelectronics
lattice vibration, the magnetism, and even the
both in independent forms and heterostructures
electronic band structures of graphene could be
[37-40], and CVD method has been employed to
remarkably influenced by applying strains [12-18].
successfully grow monolayer WS2 in large area with
Owing to the as-born bandgap and extremely high
high quality in our recent reports [11, 33]. Study of
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demonstrated
to
hold
potential
for
3
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strain dependent properties could help us gain
and 532 nm laser lines. The laser spot is around 500
further insight into the opportunity for applications
nm in diameter. The PL spectra before and after
of this material especially the flexibility-related ones
transfer process were taken under the same
[41], however, experimental investigation of strain
condition. Raman spectrum of monolayer WS2
effects on light emission and lattice vibration of 2D
transferred to PET substrate was measured with
WS2 is still lacking in stark contrast to the 2D MoS 2
higher integration time and power compared with
counterpart [20-24]. In this paper we report our
that of as-grown one for better comparison. In each
observation of tunable light emission and lattice
strain loading process, at least three different points
vibration of CVD grown monolayer WS2 under
were studied to ensure the observed spectrum
uniaxial tensile strain. Both bandgap energy tuning
change
and transformation from direct to indirect achieved
measurements
by strain can be revealed from photoluminescence
performed under the same condition with a low
(PL) spectra. Furthermore, trion and phonon
laser power to avoid damage to the sample. Under
behaviors subject to strain are discussed. Finally
some specific strain strengths several spectra on the
polarization dependent intensities of split E' and E'
same spot were taken and compared to confirm the
+
modes
are
analyzed
to
determine
-
the
is
typical.
under
Both
Raman
different
and
strain
PL
were
validity of measurement results.
crystallographic orientation of our CVD grown
WS2.
3 Calculation Methods
2 Experimental
The electronic structure calculations of unstrained
and strained WS2 monolayer based on the density
This study involved CVD growth of monolayer WS 2
functional theory (DFT) were performed using the
samples on SiO2/Si substrate and transfer process to
projector
flexible PET substrate for further strained Raman
method with a plane-wave basis set as implemented
and PL measurements.
in the Vienna ab initio simulation package (VASP)
Growth and transfer: CVD method as previously
code [61-63]. The plane-wave cutoﬀ energy was 500
described [33] was employed to grow monolayer
eV and the exchange–correlation functional was
WS2.
with
treated within the local density approximation
corresponding optical and fluorescence images
(LDA) according to the Ceperlay Alder (CA)
taken by an optical microscope (BX 51). Then the
parameterization. The 1H-WS2 monolayer was
grown samples were transferred to PET substrate
modeled by 1×1 unit cell that contains 3 atoms. A
adopting the improved method proposed by Li [60].
vacuum spacing larger than 22 Å was introduced to
Strain application: The samples were mounted on a
hinder the interaction between periodic replicas
strain stage with the desired areas in the middle of
along the c axis, making the monolayer structure
the stage gap and could be controllably elongated
eﬀectively isolated. The convergence condition for
to apply uniaxial tensile strain.
the energy was chosen as 10−6 eV and the structures
Desired
areas
were
located,
augmented
wave
pseudopotentials
were
were relaxed until the forces on each atom were less
collected by a Raman system (Witec CRM200) with
than 0.01 eV/Å . The Monkhorst–Pack scheme was
1800 lines/mm and 2400 lines/mm grating under
used to sample the Brillouin zone. To obtain the
457 and 488 nm excitation laser, respectively. PL
unstrained conﬁguration, the atomic positions and
measurements were conducted in the same Witec
lattice vectors were fully relaxed with a mesh of
system with 150 lines/mm grating under both 457
15×15×1, and the optimized (relaxed) coordinates
Optical
characterization:
Raman
spectra
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4
Nano Res.
were then used for the self-consistent and density of
state (DOS) calculations with the mesh of k space
increased
to
25×25×1.
Band
structures
were
calculated along the high symmetry points using
the path Γ–M–K–Γ. Since the uniaxial strain effects
on band structure (especially the band edge parts)
of monolayer TMD materials have been shown to
be nearly direction independent [23, 24, 26], we
only considered the strain along the [010] direction.
Specifically the lattice vector along this direction
was enlarged according to strain strength while
Figure 1. Optical images of CVD grown monolayer WS2 on (a)
those along the other two directions were kept
SiO2/Si substrate and (b) PET substrate after transfer process. (c)
constant.
Raman and (d) PL spectra of monolayer WS2 before and after
transfer with fitted curves.
4 Results and discussion
symmetric peak usually presents [11, 33]. As can be
To effectively apply uniaxial strain to monolayer
seen from Figure 1(d), an obvious peak splitting
WS2, the as-grown CVD WS2 thin layers on SiO2/Si
occurs after the transfer process. Considering the
substrate are needed to be transferred onto a
truth that the transfer process may cause some
flexible substrate, Polyethylene terephthalate (PET)
non-intentional doping to the monolayer WS2, we
in our case (see the discussion and Figure S1 in the
attribute the doublet split two peaks to neutral
Electronic
for
exciton (higher energy) and charged exciton or trion
details). Optical images of monolayer WS2 before
(lower energy), respectively. The similar excitonic
and after transfer shown in Figure 1(a) and (b)
emission
reveal that the transferred sample maintained its
previously in WS2 and other 2D TMDs [9, 30-32].
original perfect triangle shape. As a unique and
Such excitonic emissions are originated from direct
powerful tool for investigations of layer number
transition at K point in the Brillouin zone [34] and
[42-44], stacking order [45-47], local oxidation [48],
there is switch between neutral exciton (A) and
and doping effects [49, 50], Raman spectroscopy is
trion (A- or A+ depending on extra carrier type) at
used to characterize the samples before and after
different Fermi levels [9, 30-32]. From our previous
transferring with 457 nm excitation laser line. As
studies, our CVD grown WS2 exhibit an n-type
shown in Figure
this excitation
doping property [11]. The existence of excessive
condition one second-order zone-edge phonon
electrons in such n-type WS2 could facilitate the
mode (2LA(M), 347 cm ) plus two zone-center
formation of negative charged exciton (A -). For the
modes (E', 357cm-1 and A′1 , 417cm-1) dominate in the
transferred
range of 300 to 480 cm [51]. No obvious spectral
transformation from A to A in the PL spectra is
change is observed, again indicating the transferred
caused by a decrement of the n-type doping level
sample remains its quality.
resulted from the trapped molecules during transfer
Supplementary
Material
1(c), under
(ESM)
-1
-1
features
have
samples,
also
the
been
spectral
observed
weight
-
Benefitting from a direct bandgap, monolayer
process. The molecular doping effects on the light
TMDs usually possess strong PL emissions. For our
emissions of WS2 monolayer has been discussed in
CVD
our previous study [52].
grown
monolayer
WS2,
a
strong
and
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To better guide our understanding of the
monolayer WS2 from K to Γ point while keep the
observed strain dependent light emission behaviors
CBM unchanged, resulting in the narrowest gap
shown later, we calculated the band structure of
between the transition from Γ to K, as can be seen in
monolayer WS2 under different uniaxial strain by
Figure 2(c). With the continuous increase of strain,
DFT method and presented our findings in Figure 2
the indirect band gap feature becomes more
(See the Calculation Methods for detail). One
obvious (see the band structure with 3.8% strain in
remarkable change of the band structure induced
Figure 2(d)). In addition, the zoom-in view of the
by applying uniaxial tensile strain is the direct to
band structure around the K point (the inset of
indirect band gap transition, as also theoretically
Figure 2(d)) show that the strain can move the local
predicted in other monolayer TMD materials [23, 25,
VBM and CBM slightly away from K point. This
26, 28, 29]. In unstrained state as shown in Figure
so-called valley drift has recently been observed in
2(a), both valence band maximum (VBM) and
uniaxially strained monolayer MoS2 [25] and will be
conduction band minimum (CBM) locate at K point.
detailedly studied in future work.
As the strain increases, the energy difference
To further probe the strain modulated electronic
between the local maximum of K and Γ point in the
band structures revealed by our DFT calculation,
valence band (VB) becomes smaller and they are
the detailed in-situ strain PL spectra measurements
near degenerate under 2.4% strain (Figure 2(b)).
are conducted. Figure 3(a) shows the evolution of
Uniaxial strain of 2.6% can move the VBM of
Figure 2. Calculated band structure of monolayer WS2 under (a) 0%, (b) 2.4%, (c) 2.6% and (d) 3.8% uniaxial strain. Inset:
zoomed-in band structure near CBM and VBM around the K point.
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Figure 3. (a) PL spectra, fitted light emission (b) integrated intensities and (d) energies of A, A- and I peaks of CVD monolayer WS2
as a function of the uniaxial strain under 532 nm laser line. The intensities of all peaks are normalized by the I peak intensity under
3.7% strain, which is the largest one of the sub peaks in measured strain range. (c) and (e) are intensity ratios of direct to indirect
transition peak and position distance trend of A A- peak with increasing strain, respectively. The dashed lines in (b) and (e) serve as
guide for the corresponding trend under strain and the solid lines in (c) are linear fit.
PL spectra in monolayer WS2 as a function of
fittings could be achieved by using this fitting
uniaxial tensile strain. With increasing strain ranged
strategy as demonstrated by some typical spectra
from 0 to 2.2%, there is an obvious redshift of PL
shown in Figure S2 in the ESM. It is noteworthy
emission while the line shape keeps unchanged,
that the newly appearing peak under strain no less
similar to the previous observation for strained
than 2.5% (expressed as I, standing for indirect
monolayer MoS2 [22, 24]. A discernible PL peak
thereinafter) is always broader than the other two,
broadening can be observed under 2.5% strain (see
in line with the feature of the indirect gap emission
Figure S2 in the ESM), after which PL spectrum
peak in few-layer WS2 [7, 34]. This broadening is
continues to be broader and entirely redshifts with
contributed by the phonon which participates in the
increasing strain. Considering our DFT observation
recombination process of the indirect transition to
of the direct to indirect band gap transition occurs
satisfy momentum conservation by providing
under ca. 2.6%, such broadening, especially at the
appropriate momentum [53]. As can be seen from
longer
extra
Figure 3(b), I peak intensity has a rising trend with
contribution of the light emission from the indirect
wavelength
tail
might
be
the
increasing strain contrary to those of A and A - peaks.
band gap (Γ-K). To test this speculation we fitted the
Simultaneous lowering of two intensity ratios (A/I
PL spectra with strain below 2.5% by two Gaussian
and A-/I) (see Figure 3(c)) shows direct to indirect
peaks while the PL spectra under strain since 2.5%
spectral weight conversion with increasing strain,
were fitted by three Gaussian peaks. Excellent
as a result of strain- induced faster energy
7
Nano Res.
narrowing rate of the indirect transition than that of
polarizability [55]. The smaller effective exciton
the direct one. Fitted light emission energies as a
mass and the larger polarizability consequently
function of strain are given in Figure 3(d). In
lead to a reduction of trion dissociation energy [56].
addition to the expected linear redshift of A exciton
To further prove the indirect transition peak does
peak with strain (-0.0113 eV/%) which is also found
contribute to the observed PL spectra, we show the
in monolayer MoS2, [22-24] I peak is linearly
results obtained by fitting all PL spectra under
redshifted at a larger rate of -0.0187 eV/%. It should
strain with two Gaussian peaks in Figure S3 in the
be noted that under 2.5% strain, the energy of I
ESM. In this way the fitted intensity of A- peak
peak is 35 meV smaller than A peak instead of the
increases remarkably in the high strain region
approximate energy degeneracy of these two
where the shortest transition is indirect and the
transitions near the critical strain to transform the
dissociation
band gap type. It can be explained by the larger
increasing strain. Those do not agree with the
binding energy of the indirect transition compared
expected strain-induced light emission change as
to that of the direct one. The measured light
discussed before and provide additional evidence
emission energy by PL spectroscopy is the
for the assignment of I peak in PL spectra under
fundamental band gap minus binding energy of the
high strain.
energy
decreases
slightly
with
corresponding transition. The exciton binding
Not only the electronic band structure, lattice
energy is linearly dependent on effective exciton
vibration of such 2D systems is also very sensitive
mass [54], and the latter is jointly determined by
to strain. Strain dependent Raman spectra of
effective electron and hole mass at the band edge
monolayer WS2 are displayed in Figure 4(a) with
with the expression: μex = memh/(me+mh). Since the
the extreme sensitivity of E' mode to strain
direct and indirect transitions share the same
observed. The strain-induced red shift and splitting
effective electron mass, the much larger hole mass
of this mode under high strain can be clearly seen.
at Γ point than K point [55] lead to the stronger
In contrast the out-of-plane A′1 mode is more inert
binding for the indirect transition. This different
to in-plane strain. As schematically illustrated in
binding energy together with the near-degenerate
Figure 4(b), E' mode involves in-plane opposite
fundamental energy gap lead to lower energy of I
displacements of W and S atoms, while A′1 mode
peak, which makes them distinguishable in the PL
corresponds to vibrations of mere S atoms out of
spectrum.
dissociation
phase perpendicular to the plane [2]. The analysis
energy evolution under strain can be estimated by
of different response of these two phonon modes to
extracting the strain dependent energy distances of
uniaxial strain in monolayer MoS2 based on their
A and A- peaks. A slightly decreasing trend is
distinct
observed as shown in Figure 3(e). The decrease of
breaking has been reported by us [20] and later
trion dissociation energy under applying an
confirmed by Conley [23]. To accurately reveal the
increasing tensile strain has been noticed and
behaviors of phonon modes under strain, multiple
explained by the changes of effective exciton mass
Lorentzian functions were adopted to fit Raman
and polarizability caused by varying strain [55, 56].
spectra under different strain strength. As clearly
In detail, increasing strain could effectively reduce
seen in fitted phonon frequencies versus strain
Furthermore
the
trion
atomic
displacements
and
symmetry
the effective exciton mass meanwhile enlarge the
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8
Nano Res.
Figure 4. (a) Raman spectra of monolayer CVD WS2 with increasing strain under 488 nm laser line. (b) Schematic illustration of
atomic displacements of the two first-order phonon modes. (c) Fitted phonon frequencies of E’ and A′1 modes of monolayer WS2 as
a function of uniaxial strain. (d) and (e) are polar maps of fitted Raman intensities of E’+, E’- and A′1 modes as a function of angle φ
under uniaxial strain of 3.2%.
(Figure 4(c)), double degenerate in-plane E' mode
to be a unique and efficient way to determine
linearly softens under small strain and then splits
crystal
under 1.4% strain, after which both of E' and E'
-
monolayer MoS2 in our previous work [20]. Here
peaks have a linear relationship with strain value.
we adopted this technique to identify the edge and
A′1
+
orientation
of
mechanically
exfoliated
mode remains single Lorentz shape throughout
crystal orientation of our CVD grown monolayer
the applied strain range as expected. It is noticed
WS2. First we show the measured polarization
that a slight red-shift of less than 0.4 cm-1 happened
dependent Raman spectra of the as-transferred
to
A′1
mode under high strains, i.e. 2% - 3.2%. This
CVD grown monolayer WS2 under zero strain in
is a little surprising since this mode should be
Figure S4(a) in the ESM. In our polarization
insensitive to in-plane strain. We attribute this red
configuration the incident light is constantly
shift to laser-induced heating which becomes more
polarized along the horizontal direction and the
remarkable as the laser shining on the sample
scattered light polarization is chosen with an angle
A′1
φ corresponding to the horizontal by a linear
proceeds instead of the applied strain. Since the
mode is reported to be more susceptible to the
polarizer. The fitted results (Figure S4(b) in the ESM)
heating induced by laser [57] and temperature [58,
clearly show that the intensity of E' mode follows a
59] than E' mode for monolayer TMDs, its
cos2φ dependence while that of the A′1 mode is
unexpected red shift under strain is observed in our
insensitive to the polarization angle, which has been
case.
well explained by their different Raman tensors [20].
Polarization
dependent
Raman
spectroscopy
under uniaxial strain has been successfully proven
The
schematic
geometry
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in
diagram
our
of
the
polarization
polar-dependent
Raman
9
Nano Res.
measurements under uniaxial strain is displayed in
uniaxial
Figure S5(a) in the ESM. Zigzag direction is
dissociation energy and soften the in-plane E'
assumed to have the angle θ relative to the x-axis.
phonon mode followed by lifting its two-fold
Strain was applied in the horizontal direction and
degeneracy. The different polarization dependence
the incident light polarization was fixed to this
of the split E'+ and E'- modes could be used to
x-axis direction, which makes ψ equal to 0°. The
efficiently identify the crystal orientation and
polarization of scattered light is selected by an
confirm the zigzag-type edge of CVD grown
analyzer and tuned from -90 to 90 degree relative to
monolayer WS2. These findings extend previous
the strain direction (the angle φ) with a stepzise of
experimental
30 degrees. We fitted the obtained Raman spectra of
corroborating the theoretical prediction of strain
monolayer WS2 as a function of angle φ under
effects on band structural evolution and providing
uniaxial strain of 3.2% (see Figure S5(b) in the ESM)
insights into strain dependent trion behaviors. The
-
and plotted the polar angle dependence of E' , E'
observed sensitivity of light emission to strain in
and A′1 peaks in Figure 4(d) and (e). The intensity
of A′1 mode is proportional to cos2φ similar to the
monolayer WS2 caused by strain-induced band gap
nonstrain case as this out-of-plane mode is seldom
atomically thin TMD material holds promise for
affected by in-plane strain [20]. In contrast the
various applications such as strain detection and
intensities of E'+ and E'- modes follow cos2φ and
optoelectronics.
+
sin2φ
dependence,
respectively.
Since
tensile
strain
strained
can
decrease
study
of
trion
TMDs,
type and energy change further indicates that this
the
polarization dependent intensities of these two split
Acknowledgements
E' and E' modes in our configuration are supposed
+
-
to be proportional to cos2(φ+3θ) and sin2(φ+3θ),
This work is supported by the Singapore National
respectively [20], the θ value is otained to be zero
Research
degree. Thus the zigzag direction of our CVD
NRFRF2010-07, MOE Tier 2 MOE2012-T2-2-049,
grown monolayer WS2 is identified to be along the
A*Star SERC PSF grant no. 1321202101 and MOE
horizontal axis, which happens to overlap with one
Tier
of the triangle edges as evident from Figure 1(b).
acknowledges the support of the ‘‘973’’
This confirms the previous observations that the
(2015CB932200), NSFC (Grants No 21144004, No
CVD-grown WS2 edge is zigzag-terminated [33, 34].
20974046, No 21101095, No 21003076, No 20774043,
1
Foundation
NRF
RF
MOE2013–T1–2–235.
Award
W.
No.
Huang
project
No 51173081, No 50428303, No 61136003, No
50428303), the Ministry of Education of China (No.
5 Conclusions
IRT1148), the NSF of Jiangsu Province (Grants No
SBK201122680, No 11KJB510017, No BK2008053, No
In summary we have studied the strain dependent
light emission and lattice vibration properties of
CVD grown monolayer WS2 and experimentally
demonstrated the possibility of tuning different
optical transition energies and their relative spectral
weight by applying uniaxial strain. This tuneable
optical property is attributed to the strain-induced
direct to indirect band gap transition and confirmed
11KJB510017, No BK2009025, No 10KJB510013 and
No BZ2010043) and NUPT (No NY210030 and
NY211022). J.P. Wang is grateful for the NNSFC (No.
11474164), NSF of Jiangsu province (BK20131413),
and the Jiangsu Specially-Appointed Professor
program. Y.L. Wang thanks Luqing Wang, Dr.
Xiaolong Zou and Dr. Alex Kutana for the
constructive discussion.
by the DFT calculation. We have also shown
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10
Nano Res.
Electronic
Supplementary
Material:
Supplementary
material
(layer
thickness
determination, PL spectra with fitted curve, results
obtained by fitting all PL spectra under strain with
two Gaussian peaks, polarization geometry,
polarization dependent Raman intensity under zero
strain and polar-dependent Raman spectra under
3.2% strain) is available in the online version of this
article at http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher).
[9]
Jones, A. M.; Yu, H.; Ghimire, N. J.; Wu, S.;
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Peimyoo, N.; Shang, J.; Cong, C.; Shen, X.; Wu, X.;
Yeow, E. K. L.; Yu, T. Nonblinking, Intense
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Electronic Supplementary Material
Strain-induced direct-indirect band gap transition and
phonon modulation in monolayer WS2
Yanlong Wang1,2#, Chunxiao Cong2#, Weihuang Yang1,2, Jingzhi Shang2, Namphung Peimyoo2, Yu
Chen2, Junyong Kang3, Jianpu Wang1,4, Wei Huang1,4,5 (), and Ting Yu2 ()
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
Figure S1. (a) Optical and (b) fluorescence images of CVD grown monolayer WS 2 sample on SiO2/Si substrate. The green dot in the
fluorescence image represents the position where spectra shown in Figure 3(a) and Figure 4(a) were taken.
Address correspondence to Ting Yu, [email protected]; Wei Huang, [email protected]
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Nano Res.
Layer thickness determination
In this study, the CVD grown monolayer WS2 sample on SiO2/Si substrate was first identified by its intense
light emission and further confirmed by the frequency difference between E' and A′1 Raman modes. As can
be seen from the fluorescence (FL) image in Figure S1(b), the triangle area with monolayer thickness shows
obviously stronger light emission than the centre part which is thicker than monolayer. This is due to the
indirect to direct band gap transition when thinned to a monolayer limit [1, 2]. In addition, by fitting Raman
peaks of as-grown monolayer WS2 sample on SiO2/Si substrate displayed in Figure 1(c), the frequency
difference of 59.3 cm-1 between E' and A′1 Raman modes was obtained, which verifies the monolayer
identification since this quantity is thickness dependent in thin layers of WS 2 [3, 4].
Overcoming the nonuniformity of the sample
From the FL image in Figure S1(b), the light emission from the sample near edge is enhanced compared
with body counterpart. The quenching of PL in certain areas of CVD grown monolayer WS 2 has been
reported by us and assigned to defect-induced electron doping [5, 6]. In view of this nonuniformity, we
studied the evolution of both PL and Raman spectra under different strain strength by shining the laser on
exactly the same position of sample and compared the PL behaviors at different points to ensure the
observed spectrum change is typical. The green dot in Figure S1(b) is the point where spectra shown in
Figure 3(a) and Figure 4(a) were taken.
Figure S2. PL spectra of CVD monolayer WS2 transferred onto PET substrate with fitted curves under certain strain strengths.
As can be seen from Figure S2, a comparison between PL spectra of monolayer WS 2 obtained at different
strain loading stages clearly shows that the PL shape after strain release is similar to that of zero strain case
and can be fitted by two Gaussian peaks well. The disappearance of I peak when strain is released further
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Nano Res.
confirms it is originated from indirect band gap transition. In addition the sample after strain release
possesses comparable PL emission to that of the as-transfer one. This, together with no emergence of
defect-related bound exciton peak makes us believe that our sample is defect free throughout the whole
process and available for new round of strain measurement.
Figure S3. Results obtained by fitting all PL spectra under strain with two Gaussian peaks. (a) Light emission integrated intensities of A
and A- peaks of CVD monolayer WS2 as a function of the uniaxial tensile strain. The intensities of all peaks are normalized by the Apeak intensity under 3.7% strain. (b) The trend of A and A- peak position distance with increasing strain.
Figure S4. (a) Raman spectra of the as-transferred CVD monolayer WS2 as a function of the angle between the polarizations of the
incident and the scattered lights. (b) Polar plot of the fitted intensities of E’ and A′1 modes as a function of the angle between the
polarizations of the incident and scattered lights.
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Nano Res.
Figure S5. (a) Schematic diagram of the polarization geometry in our polar-dependent Raman measurements. (b) Raman spectra of
monolayer WS2 as a function of angle φ under uniaxial strain of 3.2%.
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