Valley polarization in stacked MoS2 induced by circularlypolarized light

Juan Xia1, Xingli Wang2, Beng Kang Tay2,3, Shoushun Chen4, Zheng Liu2,3,5, Jiaxu Yan1,6 (), and

Zexiang Shen1,3,7 ()

1 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore

637371, Singapore 2 NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University,

Singapore 639798, Singapore 3 CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, Singapore 637553, Singapore 4 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore 5 Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, Singapore

639798, Singapore 6 Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech

University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China 7 Centre for Disruptive Photonic Technologies, Nanyang Technological University, Singapore 637371, Singapore

Received: 10 August 2016

Revised: 10 October 2016

Accepted: 12 October 2016

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2016

KEYWORDS

circularly polarized

photoluminescence,

first-principles

calculations,

molybdenum disulfide,

ultra-low-frequency

Raman spectroscopy,

valley polarization

ABSTRACT

Manipulation of valley pseudospins is crucial for future valleytronics. The

emerging transition metal dichalcogenides (TMDs) provide new possibilities for

exploring the interplay among the quantum degrees of freedom, including real

spin, valley pseudospin, and layer pseudospin. For example, spin–valley coupling

results in valley-dependent circular dichroism in which electrons with particular

spin (up or down) can be selectively excited by chiral optical pumping in

monolayer TMDs, whereas in few-layer TMDs, the interlayer hopping further

affects the spin–valley coupling. In addition to valley and layer pseudospins, here

we propose a new degree of freedom—stacking pseudospin—and demonstrate

new phenomena correlated to this new stacking freedom that otherwise require

the application of external electrical or magnetic field. We investigated all possible

stacking configurations of chemical-vapor-deposition-grown trilayer MoS2 (AAA,

ABB, AAB, ABA, and 3R). Although the AAA, ABA, 3R stackings possess a sole

peak with lower degree of valley polarization than that in monolayer samples,

the AAB (ABB) stackings exhibit two distinct peaks, one similar to that observed

in monolayer MoS2 and an additional unpolarized peak at lower energy. Our

findings provide a more complete understanding of valley quantum control for

future valleytronics.

Nano Research

DOI 10.1007/s12274-016-1329-x

Address correspondence to Jiaxu Yan, jxyan@ntu.edu.sg; Zexiang Shen, zexiang@ntu.edu.sg

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2 Nano Res.

1 Introduction

In analogy to spintronics [1, 2], valleytronics is proposed

to encode bits of information by manipulation of valleys,

the degenerate extrema of energy dispersion in the

momentum space [3–9]. Recently, transition metal

dichalcogenides (TMDs) have captured centralized

attention because of their intriguing physics associated

with the valley degree of freedom, which implies the

great potential for valley quantum control [10–17]. In

monolayer TMDs, by breaking the crystal inversion

symmetry, the valley pseudospins can be distinguished

by opposite signs at the corners (K and K’) of the

hexagonal Brillouin zone. The giant spin–orbit coupling

(SOC) separates the valence bands at each valley into

spin-up and spin-down states. Such inherent spin–

valley coupling can significantly protect the valley

polarization as the intervalley scattering requires a

simultaneous spin flip. Only carriers with selective

spin can be emitted under the valley optical selection

rule, which has been widely demonstrated by circularly

polarized excitation [10, 11, 13].

Beyond monolayer TMDs, bilayer TMDs introduce

another intrinsic degree of freedom called the layer

pseudospin [17–22]. The layer pseudospin refers to

the location of the carriers, either in the upper or lower

layer. The natural AB-stacked bilayer TMDs, in which

the lower layer is rotated by 180° with respect to

the upper layer, possess crystal inversion symmetry.

Consequently, the layer rotation switches two valleys

but leaves the spin unchanged, which results in a

sign change for the spin–valley coupling from layer

to layer. In theory, the valley-dependent effect should

vanish in the presence of inversion symmetry [14, 23].

Notably, nontrivial circular photoluminescence (PL)

polarization degrees have been observed in bilayer

MoX2 [11, 22] and WX2 [19, 20]. Contrary to early

beliefs, breaking the inversion symmetry by applying

an electric field in AB-bilayer samples is not required.

The possible origin remains in debate. For bilayer WSe2

and WS2, the giant SOC (up to ~0.5 eV) suppresses the

interlayer hopping, thus leading to the localized spin

states within either the upper or lower layer related

to the valley index that results in a nontrivial valley

polarization value [18–21]. This scenario appears

vague for bilayer MoS2 with a moderate SOC value of

approximately 0.1 eV. Zunger et al. [24, 25] claimed

that the circular polarization is intrinsic in centro-

symmetric layer stacks composed of individual

noncentrosymmetric layers.

The interplay among spin, valley pseudospins, and

layer pseudospins opens an unprecedented platform

toward manipulation of quantum states. Both spins

and valleys have magnetic moments and can be

manipulated by magnetic and optical means [26–29].

Layer pseudospin is associated with electric polarization

and can be tuned by an electric field [22]. Numerous

manipulations of degrees of freedom have been reported

to exploit this quantum control in monolayer and

bilayer TMDs. Along with the layer number, the stacking

configurations are also involved [30–32]. Iwasa et al.

[30] reported robust valley polarization in 3R phase

MoS2 using circularly polarized PL spectroscopy. Wu

et al. [31] examined several artificially stacked MoS2

bilayers by folding exfoliated monolayers and observed

different optical nonlinearities. On one hand, the

limited number of stacking patterns studied in the

literature (only natural 2H/3R phases and artificially

stacked MoS2 have been studied) prevents a good

understanding of stacking pseudospins. On the other

hand, the non-oriented folding may yield uncontrolled

interlayer coupling degradations.

In this work, we report on stacking-dependent

valley polarization in high-oriented chemical vapor

deposition (CVD)-grown trilayer samples. Using ultra-

low frequency (ULF) Raman spectroscopy, we observe

two distinct stacking configurations within one special

AAB-stacked MoS2 sample—one half AAB-stacking

and the other half 3R-stacking—with a shear (S) mode

at 16 cm–1 discovered first. Further utilizing circularly

polarized PL measurements, we report two exciton

peaks with distinct valley polarization behaviors in

AAB (ABB) stacking: The high-energy peak exhibits

an anomalously robust valley polarization (~90%),

and the low-energy peak becomes valley polarization

degrading. Combining this investigation with the ab

initio calculations, we uncover a physical picture of the

stacking-modulated spin–valley coupling in few-layer

MoS2. By extending our circularly polarized PL studies

to other stackings, this mechanism was further verified

in all stacked few-layer MoS2 systems. Our work

provides another possible freedom-controlled approach

to desirably manipulate the valley quantum states.

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2 Results and discussion

Figure 1 shows an AAB-stacked trilayer MoS2, where

we can clearly determine the layer number from both

the optical contrast image (Fig. 1(a)) and Raman/PL

intensity mappings (Fig. S1 in the Electronic Sup-

plementary Material (ESM)). The integrated Raman

intensity mapping of the A1g modes reveals the directly

proportional relationship between the Raman intensity

and layer number, and the PL intensity mapping clearly

demonstrates that the direct bandgap monolayer

emits more strongly. The PL intensity weakens with

increasing thickness of the MoS2 sample, in agreement

with the literature. The ULF Raman spectra provide

more information about different atomic arrangements

and interlayer coupling effects within the MoS2

nanoflake shown in Fig. 1(a), which cannot be clearly

observed in the high-frequency Raman and PL

spectrum. In Fig. 1(b), the two peaks labeled S and B

correspond to the shear and layer breathing modes,

involving the rigid motions of adjacent layers that

are parallel and perpendicular to the atomic planes,

respectively. This assignment is confirmed by the

polarized ULF Raman spectra, as clearly indicated in

Fig. S2 (in the ESM). In general, the S modes appear

in both ( ) ( )Z xx Z Π and ( ) ( )Z xy Z configurations,

whereas the B modes exist only in the ( ) ( )Z xx Z Π

configuration and not the ( ) ( )Z xy Z configuration.

Furthermore, the higher intensity ratio of the B mode

to S mode identifies the stacking configuration of the

bilayer region as AA-stacking, which is consistent with

the distinct features of AB-stacked and AA-stacked

bilayers, as shown in Fig. S1 (in the ESM). The features

of the ULF Raman intensity mapping of the bilayer

region using the B mode (Fig. S2 in the ESM) are

also consistent with those of a typical AA-stacking

spectrum.

For trilayer MoS2, two branches of the S modes exist,

i.e., the low and high modes of shear vibration (LS

and HS modes), as shown in Fig. 1(g). The LS mode

represents interlayer vibration in which the first

and third layer move in opposite directions while the

middle layer remains static; the HS mode represents

the motion in which the first and third layer move

together in the same direction while the middle layer

moves in the opposite direction. We identified the

Figure 1 Structure characterizations of AAB-stacked trilayer MoS2. (a) Optical contrast image of AAB-stacked trilayer MoS2. (b) ULFRaman spectra of the bilayer region. (c) and (d) ULF Raman mapping of the LS mode (c) and HS & B mode (d). (e) and (f) PolarizedULF Raman spectra of the left (e) and right (f) part of the trilayer sample. The yellow strips indicate the integrated peaks in (c) and (d),and the blue and orange curves represent the ( ) ( )Z xx Z Π and ( ) ( )Z xy Z polarized configuration, respectively. (g) and (h) Vibrational schematics of LS/HS and B modes in 3R-stacked (g) and AAB-stacked (h) trilayer MoS2. The numerical values were obtained from both experiments and calculations.

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distinct stacking configurations of the trilayer region

using ULF Raman spectroscopy (Figs. 1(c) and 1(d)).

For the left part of the trilayer region, the ULF Raman

spectra show two sharp peaks at 16 and 28 cm–1

(Fig. 1(e)), and both modes (16 and 28 cm–1) exist

under ( ) ( )Z xx Z Π configuration; however, only the low

frequency mode at 16 cm–1 is present in the ( ) ( )Z xy Z

configuration. The simulation identifies the former

as the LS mode and the latter as B modes, which

are assigned as the 3R phase. For the right part of

the trilayer region, the peak at 28 cm–1 represents the

accidental degeneracy of both the S and B modes,

which can also be distinguished using polarized ULF

Raman spectroscopy (Fig. 1(f)). The S mode in the

merged peak of the right region at approximately

28 cm–1 corresponds to the HS mode. The sole peak in

the right region is present under both ( ) ( )Z xx Z Π and

( ) ( )Z xy Z configurations except for a relatively large

intensity decrease up to 70% under the ( ) ( )Z xy Z

configuration. We can thus identify the stacking

configuration in the left region as AAB stacking.

Therefore, combining these results with our simulation

work, we can conclude that the HS (LS) mode exclusively

exists in the AAB (3R) stacking order. This situation

is similar to the lowest S mode observed in ABC-

stacked graphene [33, 34]. Unlike the S modes, only

one B mode centered at approximately 28 cm–1 is

present in all the trilayer samples of different stacking

configurations, indicating that the B mode is not

sensitive to stacking order. The distinct stacking-

dependent behaviors of the Raman S mode between

3R and AAB stackings can be easily understood through

a classical physical picture [33], as discussed in Fig. S7

(in the ESM).

Next, we discuss the stacking-modulated valley

polarization in trilayer MoS2 based on circularly

polarized PL measurements. Figures 2(a)–2(d) show

the valley and spin polarizations for different layer

regions of the trilayer MoS2 under σ– excitation with

left-hand (σ– red) and right-hand (σ+ black) detection

at 77 K. The degree of circular polarization η is defined

as ( ) / ( )I I I I

, where I denotes the

intensity of the detected I emission. The data indicate

high circular valley polarization in the monolayer

(~80%), AA-stacked bilayer (~80%), and 3R trilayer (60%)

MoS2. This observation demonstrates that valley optical

selection rules in 3R-phase MoS2 samples are consistent

with previous reports [30]. With the lack of inversion

symmetry in monolayer MoS2, the extraordinary spin

and valley contrasting physics emerge. For AA-stacked

bilayer MoS2, the lack of inversion symmetry still holds,

in which the spin and valley degrees of freedom are

locked in the AA bilayer similar to two individual

Figure 2 Valley and spin polarizations in 3R-stacked and AAB-stacked trilayer MoS2. (a)–(d) CP-PL spectra of the monolayer (a), bilayer (b), and left (c) and right (d) part of the trilayer regions in the AAB-stacked trilayer MoS2.

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5 Nano Res.

monolayers. A similar situation persists in the trilayer

3R-stacked region, yielding strong circular dichroism.

Notably, there are two peaks under σ– detection for

the right part with ABB stacking. The energies of the

corresponding two excitons are 1.91 eV (high energy,

HE) and 1.86 eV (low energy, LE). Moreover, the HE

peak displays a strong PL polarization (~90%) under

σ– and σ+ detection, whereas the LE peak shows almost

no spin-valley selectivity. We attribute the peaks located

at ~1.76 eV to radiative recombination of bound

excitons [35], i.e., neutral excitons bound to defects,

consistent with the previously reported defect-activated

peaks at ~1.78 eV in monolayer MoS2.

To reveal the different dichroism behaviors, we

calculated the band structures of the 3R-stacked (left)

and AAB-stacked (right) trilayer MoS2 considering

SOC, as shown in Fig. S9 (in the ESM). The spin-

polarized band structures around the K valley are

enlarged. Because of different interlayer coupling and

charge locking effects, the top valence band of the 3R

phase almost exclusively originates from the spin-up

electrons in the upper layer, whereas for the AAB

stacking, the top two valence bands are the mixed

contributions of the upper and middle layer. The

different PL polarization behaviors arise from the

intrinsic stacking-modulated Bloch electrons at K(K’)

valleys. Here, we focus on the spin textures at the

band edge, which are marked with yellow rectangular

blocks. The splittings between the conduction bands

(Δc1,Δc2) and valence bands (Δv1,Δv2) are induced by

interlayer coupling effects. Figure 3(a) shows the energy

level diagram of 3R-stacked trilayer MoS2. The three

layers are coupled with each other by reduced interlayer

coupling in the 3R phase, resulting in nontrivial

energy splittings to form a “triple monolayer”. The

A-exciton transitions can only be excited by σ– polarized

light from the K’ valley, and the Bloch wave functions

are localized in the upper layer. Because of the large

energy splittings (~0.1 eV), the interlayer hopping

is effectively suppressed, and high circular valley

polarization persists similar to the monolayer case. In

the AAB-stacked trilayer, however, the energy levels

can be treated as a mix of coupled AB bilayers and

decoupled AA bilayers, as demonstrated in Fig. 3(b).

Figure 3 (a) and (b) Schematic illustration of circularly polarized-light-induced spin and valley polarizations in 3R-stacked (a) and AAB-stacked (b) trilayer MoS2 under σ − excitation. The spin-up and spin-down bands are marked in red and blue, respectively. The orange (grey) balls/circles denote resonantly excited (after relaxation) electron–hole pairs under a 633-nm laser. The dominant spin relaxation pathway following the intra-valley spin-flip of the photo-excited electron accompanied by dissipation of energy into the environment is depicted by the solid green arrow, and the ω1 emission at –K valley with weak cross section that needs the spin-flip and energy cost is indicated by the dotted green arrow.

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Under σ– excitation, the two detected σ– signals, ω1 and

ω2, correspond to the electron–hole recombinations in

the upper layer at the K valley and the middle layer

at the K’ valley. At the K’ valley, the spin can flip

from the upper layer to the middle layer through the

interlayer coupling by dissipation of Δv1 (~20 meV).

In this process, σ+ PL will be emitted at ω2, which

contributes to the detected σ+ peak in Fig. 2(d). However,

σ– at ω1 must gain an energy cost of ~50 meV (Δv1 for

an electron and Δc1 for a hole) to achieve the spin-flip

from the middle layer to the upper layer at the K

valley, which is greater than the thermal fluctuation

kT. Therefore, the degree of circular polarization η for

ω1 emission is larger than that for ω1 emission.

We note that our observation about stacking-

modulated spin and valley polarization shares some

similarities with gate-induced peak splitting in bilayer

WSe2 [19]. In that case, emissions from the upper and

lower layers at ω1 and ω2 arise from the difference

between the conduction and valence band energy shifts

(Δc and Δv, respectively) induced by an external

electric field, i.e., Δc – Δv. A high voltage is needed to

make this splitting larger than the spectral linewidth

(20-meV splitting under 150 V). Our peak splitting

value almost equals ~50 meV, i.e., Δc + Δv, which has

been certainly observed in experiments. Moreover, the

degrees of polarization for the two peaks behave in

opposite directions: η at ω1 is 90%, whereas η at ω2 is

nearly zero.

Figures 4(a)–4(d) present the circularly polarized

PL spectra of four types of trilayer MoS2 with ABB, AAB,

AAA, and ABA stacking, respectively. Note that ABB

and AAB stacking are equivalent by spatial inversion.

Hence, we observe almost the same PL polarization

features for the ABB stacking as for the ABB stacking,

that is, prominent PL polarization for the HE exciton

and almost no spin–valley selectivity for the LE exciton

(Figs. 4(a) and 4(b)). For the AAA and ABA stackings,

there is only one peak situated at 1.91 eV under both

σ– and σ+ detection, similar to the results for the 3R

stacking. We calculated both charge density distribution

diagrams (Figs. 4(e) and 4(f)) and band structures with

spin electronic states (Figs. S9–S11 in the ESM) for all

four stackings. Because of SOC and the strong interlayer

locking effect, the charges in the top valence band for

the AAA stacking almost exclusively originate from

the middle layer, which is similar to the findings for

3R stacking. However, for ABA stacking, because of

effective interlayer coupling, there is almost no band

splitting among the layers only with a large SOC gap

(Δc1 = Δc2 = 0, Δv1 = Δv2 = 0). Therefore, there is only

one HE peak (1.91 eV) for ABA stacking.

Figure 4 Valley and spin polarizations in ABB-, AAB-, AAA-, and ABA-stacked trilayer MoS2. (a)–(d) Circularly polarized PL spectra from ABB- (a), AAB- (b), AAA- (c), and ABA-stacked (d) trilayer MoS2. (e) and (f) Charge density distribution for AAA- (e) and ABA-stacked (f) trilayers. Because of the weak interlayer coupling, the charges in the top valence band for AAA stacking almostexclusively originate from the middle layer, which shares some similarity with monolayer MoS2. For the ABA stacking case, the charges are contributed from three coupled layers because of the effective interlayer coupling.

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3 Conclusions

In summary, we demonstrated new coupling

phenomena connecting real spin with stacking

pseudospin in trilayer MoS2. Using ULF Raman

spectroscopy, we identified two distinct stacking

configurations within the trilayer region of the MoS2

sample; that is, the left half of the sample exhibits 3R

stacking, and the right half exhibits AAB stacking.

The 3R phase in trilayer CVD-grown samples was

identified for the first time and was characterized by

a unique Raman peak at 16 cm–1 representing the LS

mode among layers. Combining this investigation

with our simulation work, we concluded that the HS

(LS) mode exclusively exists in the AAB (3R) stacking

order. In polarization-resolved PL measurements, we

observed the distinct behaviors of valley polarization

in different structural configurations without any

external field. Similar to monolayer and AA-stacked

bilayer MoS2, the trilayer 3R-stacked region (left)

yields a strong circular dichroism. Notably, the AAB-

stacked trilayer region (right) exhibits two peaks

under σ– detection with exciton energies at 1.91 eV

(HE, ~90%) and 1.86 eV (LE, ~0%), respectively. We

then extended our circularly polarized PL study to four

other types of trilayer MoS2 with ABB, AAB, AAA,

and ABA stacking. All the ABB and AAB stackings

showed a prominent PL polarization for the HE exciton

but almost no spin–valley selectivity for the LE exciton.

Similar to 3R stacking, AAA and ABA stacking result

in only one peak situated at 1.91 eV under both σ–

and σ+ detection. Our calculated band structures with

spin electronic states and charge density distribution

diagrams for all four stackings are in good agreement

with our experimental results. Therefore, such stacking-

modulated coupling between spin and pseudospin

broadens the understanding of valley quantum control

for future valleytronics.

4 Materials and methods

4.1 Sample preparation

A special-stacked AAB trilayer MoS2 nanoflakes with

a size of tens of micrometers was synthesized using

vapor solid deposition technology, in which 1 g MoS2

was placed in a ceramic boat and then placed at the

center of a 1-inch quartz tube with a piece of Si/SiO2

wafer acting as a substrate. The system was pumped

down to 30 Torr before being heated to 900 °C in

35 min; the pressure remained constant at 30 Torr for

another 10 min and was then increased to 760 Torr in

1 min and remained constant for another 20 min. After

the reaction, the growth system was cooled down to

room temperature naturally. Finally, the few-layered

MoS2 films consisting of 1–5 layers were produced on

the Si/SiO2 substrate. For the growth process, 20 sccm

Ar was used as the carrier gas.

4.2 Raman spectra and polarization-resolved PL

measurements

We used a Witec CRM200 backscattering Raman system

equipped with a solid-state yttrium aluminum garnet

laser at 2.33 eV with an appropriate power to avoid

sample heating (below 1 mW). The size of the laser

beam was approximately 250 nm and was focused on

the sample using an objective (Olympus, 100×/NA0.95).

For the ultra-low-frequency Raman spectra, we used

two BragGrate Notch Filters (BNF) centered at 532 nm

with bandwidths as narrow as 5 cm–1 and a large OD

(> 4) to achieve the low-frequency region. In addition,

a linear polarizer with different collection angles was

used to realize the parallel ( ) ( )Z xx Z Π and perpen-

dicular ( ) ( )Z xy Z backscattering configurations. For

the circularly polarized PL studies, an extremely

low laser power of ~60 μW was used before a long-

working-distance objective (Nikon, 50×/NA0.45, WD

13.8 mm) to avoid possible sample heating, and the

sample was maintained at 77 K. The excitation laser

with linear polarization, either horizontal or vertical,

was transformed into a corresponding circularly

polarized wave, either left-hand (σ–) or right-hand

(σ+), using an achromatic quarter-wave plate (Thorlabs,

AQWP05M-600). The emitted PL (σ– or σ+) passed

through the same quarter-wave plate followed by a

linear polarizer, and finally, the circularly polarized PL

was converted into a linearly polarized wave, which

was finally collected by a Si charge-coupled device.

4.3 Ab initio calculations

The first-principles calculations to compute the

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8 Nano Res.

electronic structures were performed with the plane

wave basis set as implemented in the Quantum

ESPRESSO program package [36]. The norm-conserving

pseudopotentials [37], including spin-orbit coupling

effects, were adopted, and the cutoff energy for the

basis set was 140 Ry. The exchange correlation potential

was described using the local-density approximation

in the parameterization of Perdew and Zunger [38].

The Brillouin zone was sampled by a 15 × 15 × 1

k-point mesh. The energy convergence for the relaxa-

tion was selected to be less than 10–5 eV/A. We added

a vacuum layer of 20 A along the z-axis to avoid

mirror interactions between neighboring images.

The phonon spectrum and Raman intensities were

calculated within density-functional perturbation

theory (DFPT) as introduced by Lazzeri and Mauri

[39]. To perform the spin projection calculations,

we computed the Wannier functions using the

maximally localized algorithm [40] implemented in

the package WANNIER90 [41] using Mo d orbitals

and S p orbitals.

Acknowledgements

This research was supported by MOE under AcRF

Tier 2 (No. MOE2012-T2-2-124) and AcRF Tier 3 (No.

MOE2011-T3-1-005) in Singapore. X. L. W. and B. K. T.

would like to acknowledge the funding support from

NTU-A*STAR Silicon Technologies Centre of Excellence

under the program grant No. 112 3510 0003. L. Z.

would like to acknowledge the funding support from

the Singapore National Research Foundation under

NRF RF Award No. NRF-RF2013-08. J. X. Y. and J. X.

acknowledge the technical support from H. L. H. at

WITec. We thank Dr. Jer-Lai Kuo for helpful discussions.

Electronic Supplementary Material: Supplementary

material (systematic Raman and PL characterization

of the sample, Raman selection rules of HS and LS

mode in 3R and AAB trilayer MoS2, circularly

polarized PL set-up details, calculated band structures

and charge density distribution) is available in the

online version of this article at http://dx.doi.org/

10.1007/s12274-016-1329-x.

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The author has requested enhancement of the downloaded file. All in-text references underlined in blue are linked to publications on ResearchGate.The author has requested enhancement of the downloaded file. All in-text references underlined in blue are linked to publications on ResearchGate.