Interlayer Coupling Affected Structural Stability in Ultrathin MoS2: An

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Interlayer Coupling Affected Structural Stability in Ultrathin MoS: An Investigation by High Pressure Raman Spectroscopy 2

Yalan Yan, Fangfei Li, Yuan-Bo Gong, Mingguang Yao, Xiaoli Huang, Xinpeng Fu, Bo Han, Qiang Zhou, and Tian Cui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06562 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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The Journal of Physical Chemistry

Interlayer Coupling Affected Structural Stability in Ultrathin MoS2: An Investigation by High Pressure Raman Spectroscopy Yalan Yan†, Fangfei Li†*, Yuanbo Gong†, Mingguang Yao†, Xiaoli Huang†, Xinpeng Fu†, Bo Han†, Qiang Zhou†*, Tian Cui† State Key Laboratory of Superhard Materials, College of Physics, Jilin University, No. 2699 Qianjin Street, Changchun 130012, P.R. China.

Corresponding author. Tel: +8643185168881; fax: +8643185168881. E-mail address: [email protected];[email protected]

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ABSTRACT Interlayer coupling play critical roles in determining the lattice vibrations of twodimensional transition-metal dichalcogenides. When compressed, the effects of interlayer coupling remain ambiguous. Pressure-dependent vibrational properties of trilayer and quadlayer MoS2 up to 12.7 GPa were investigated through in situ high pressure Raman spectroscopy measurement. Raman spectrum reveals different responses to pressure in trilayer and quadlayer MoS2 due to their thickness-dependent interlayer coupling interaction. Combining with the First-principles calculations, we demonstrates that the quadlayer MoS2 transforms into an AB’ stacking configuration above 8.6 GPa, where all Mo atoms sit exactly over the Mo atoms in its neighboring layer and all S atoms sit over the center of the hexagons, while the trilayer MoS2 possesses a distorted and wrinkled 2H structure within our studied pressure range. Our study demonstrates that, high pressure Raman spectroscopy measurement is an effective method to explore the structural transformation of ultrathin MoS2 at extreme conditions, as well as to explore their complicated interlayer coupling interaction. It should also be of great benefit for the development of nanotechnology, especially for the design and fabrication of different stacking nanometre devices with tailored properties for specific applications. Keywords: ultrathin MoS2, interlayer coupling, stacking configurations, high pressure, Raman spectroscopy


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INTRODUCTION Since graphene was disclosed, this typical two-dimensional layered material is considered to be applied in electronics with exceptional physical properties such as Hall effect1, high carrier mobility2 and ballistic transport3 at room temperature. Despite of these excellent properties, graphene cannot be used as field effect transistors directly due to the lack of band gap, it is difficult to open a band gap in graphene exceeded 400 meV, and besides, at the cost of reduction of its carrier mobility4. Hence, to find a more desirable semiconductor substitution became an urgent mission. MoS2, a transition metal dichalcogenides (TMD), has been increasingly explored due to its similarities to graphene but with the additional merits of being intrinsic semiconductors with sizable band gaps. MoS2 is composed of repeated molybdenum layers sandwiched between two disulfide layers. Within one S-Mo-S layer, each molybdenum atom strongly bound with six sulphur atoms via covalent bonds in trigonal prismatic coordination. Such a strong intralayer covalent bonding results in a high mechanical strength5. Adjacent layers are joined together by weak van der Waals interactions, allowing MoS2 to be used as superior lubricants6,7, and enabling exfoliation into atomically thin layers. Besides, when intercalated with foreign species such as organic molecules and alkali-metal atoms, MoS2 becomes a super-conductor8,9. Bulk MoS2 is an indirect band gap semiconductor with a band gap of 1.2 eV10, it has been widely applied in photovoltaic11 and photocatalytic12 field due to its strong absorption in the solar spectrum region. However, monolayer MoS2 becomes a direct band gap semiconductor with a band gap of 1.8 eV, exhibiting a dramatic enhancement in luminescence quantum efficiency13, so it becomes ideal candidates for optoelectronic devices such as light emitting diode14, photo diode15, solar cells16, and the field effect transistor17,18. In addition, unusual properties such as valley polarization19 and a strong spin-orbit effect20 have also been demonstrated in monolayer MoS2.



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External perturbations such as high pressure are effective techniques to modify the electric21, optical22, and phonon23 properties of TMDs without inducing impurities. So it is important to understand the pressure effects on TMDs, particularly in applications such as flexible electronics24. Pressure effects on bulk and monolayer MoS2 have been recently reported. For bulk MoS2, combination of Raman spectroscopy, x-ray diffraction analysis and in situ resistivity measurements suggested that high pressure can modify its electronic structure, leading to a transition from semi-conductive to metallic states, accompanied by a phase transition from 2Hc to 2Ha phase at around 20 GPa.21,25 In the case of monolayer MoS2, the energy of direct band gap was suggested to increase with pressure and a transition from direct to indirect band gap occurred at ～ 23 GPa26. Li et al. found that compressing monolayer MoS2 on silicon wafer led to probable distortion of the structural unit with the S atoms sliding within a single layer.23 On the other hand, theoretical calculations predict a metallization of monolayer MoS2 at 68 GPa26. Considering the complex layer number-dependent interlayer interaction in MoS2, the lattice vibrations of ultrathin MoS2 films present distinct features, when compressed, their responses to the pressure may reflect the capability and evolution of interlayer coupling interaction. In this paper, we compared the high pressure Raman spectra of trilayer and quadlayer MoS2 up to 12.7 GPa, combining with the First-principles calculations, we studied their different behaviors under pressure. We demonstrated that the pressure-induced structure transformation from 2H to AB’ stacking occurred in quadlayer MoS2 at 8.6 GPa, while trilayer MoS2 preserves 2H structure with some distortion due to a weaker interlayer coupling interaction. Their vibrational evolutions under pressure give a similar but distinct changes reflect that interlayer interactions play important roles in structural stability. METHODS The bulk 2H-MoS2 materials were bought from SPI supplies (USA), commercial silicon wafers coated with 300 nm of silicon oxide was polished to a thickness of 30 µm to fit the 4 ACS Paragon Plus Environment

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high pressure chamber. Using mechanical exfoliation method, we obtained trilayer and quadlayer MoS2 deposited on the silicon wafer. The optical images of trilayer and quadlayer are presented in Fig. 1a. Their layer number was firstly estimated by optical contrast based on the principle of optical interference through an optical microscope, and then was further confirmed by the characteristic Raman peaks at ambient conditions. The details were described in Results and Discussion section. High Pressure Sample Loading. The high pressure was generated using a gasketed symmetric DAC with a 400 µm culet. A T301 stainless steel gasket was pre-indented to a thickness of 50 µm, and a hole of 160 µm in diameter was drilled in the center of the preindented gasket. The silicon wafers covered with target ultrathin MoS2 sample was cut to 100 µm × 100 µm. The pressure was monitored using the ruby fluorescence method27 and liquid argon was used as the pressure transmission medium (PTM) to produce a quasi-hydrostatic pressure on the sample in the chamber. High Pressure Raman Spectroscopy. The Raman spectra were collected using a HoribaJY T64000 Raman spectrometer in backscattering configuration. Collected signal was dispersed by a 1800 g/mm grating under triple subtractive mode giving a spectra resolution better than 1 cm-1. A solid-state laser centered at 532 nm was used as exciting light source. The 100 × and 50 × objectives were used to collect the Raman spectra in the air condition and under high pressure, respectively. To avoid laser-induced modification or ablation of the samples, the power of incident laser was controlled below 0.65 mW before going through anvil window, the data acquisition time was 500 s. The First-principles calculations. The Plane-wave density functional theory (DFT) calculations were implemented in the CASTEP package28,29. The electron-ion interaction was described by Nom conserving pseudopotentials. A generalized gradient approximation (GGA) in the Perdew–Burke–Ernzerhof (PBZ) scheme was adopted for the exchange and correlation interaction30. The geometric optimization of the unit cell was performed by adopting the 5 ACS Paragon Plus Environment


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Broyden–Fletcher–Goldfarb–Shanno (BFGS) minimization algorithm.29 The kinetic energy cutoff for the plane-wave basis set was 400 eV. A set of (12 × 12 ×12) k-point samplings was used for Brillouin zone integration for all the stacking phases. All the ultrathin MoS2 systems were modeled by a periodic slab geometry using the optimized lattice constants of the bulk materials. For all stacking configurations, a vacuum slab of 20 Å in the (0 0 1) plane was used to avoid interlayer interactions with other replicas. The DFT-D tab with Grimme scheme was used for the interlayer van der Waals dispersion corrections. RESULTS AND DISCUSSION Natural 2H-bulk MoS2 belongs to a space group of P63/mmc31, four Raman-active modes can be detected at ambient conditions, including E2g2 at 32 cm-1, E1g at 286 cm-1, E2g1 at 383 cm-1 and A1g at 408 cm-1. As shown in Fig. 2a, the atomic displacements of Raman-active modes is presented for a direct view. There are the Mo-S covalent bonds (solid line) within one layer, while weak van der Waals interactions (dash line) connect neighbouring layers. The E2g2 mode arises from the vibration of one MoS2 layer against its neighboring layers, i.e. all SMo-S atoms within one layer move towards same direction, called the rigid-layer mode, thus this mode is absent in monolayer MoS2. The E1g mode is associated with the in-plane vibration of two S atoms in opposite directions, and this mode is forbidden in backscattering measurements on a surface perpendicular to the c axis32-34, so that we did not detect this mode in our experiments. The E2g1 mode results from the in-plane vibrations of the Mo and S atoms in opposite directions, while the A1g mode comes from the out-of-plane vibrations of two S atoms in opposite directions. In addition, a “b” band is observed, which is associated with a two-phonon Raman process, i. e., a successive emission of dispersive quasi-acoustic phonon and dispersionless transverse-optical phonon (Fig. 3a-c)35.


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Fig 1. (a) The images of trilayer, and quadlayer MoS2 flakes deposited on silicon wafer. (b) A schematic drawing of gasketed diamond anvil cells (DAC) for compression experiments. Fig 2 present the Raman spectra of monolayer, trilayer, quadlayer and bulk MoS2 at ambient conditions. The monolayer MoS2 is verified according to the absence of the E2g2 mode, as well as the featured peak positions of E2g1 and A1g modes which are located at 384.8 cm-1 and 404.8 cm-1 (Fig 2c), respectively32. As the vibrational frequencies for different Raman modes are layer number dependent32, the E2g2 and A1g modes exhibit an upshift in frequencies with the increase of layer numbers, i.e., a positive growth, while the E2g1 mode is downshifted in frequency with layer number increasing. According to these relations between layer number and vibrational frequency, Raman spectroscopy thus shows obvious advantages in determining the layer number of few layer MoS2 flakes. The E2g2 peak position and the corresponding frequency difference ∆ω between ultrathin flakes and bulk MoS2 are listed in Table 1.



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Fig 2. (a) Atomic displacements of Raman-active modes for a direct view. The yellow solid lines represent the Mo-S covalent bonds, the yellow dashed lines represent the interlayer van der Waals interaction. (b) The low frequency E2g2 mode and (c) the E2g1 and A1g modes of monolayer, trilayer, quadlayer and bulk MoS2 at ambient conditions. The spectra intensity are normalized for E2g2 mode in (b) and A1g mode in (c), the relative intensity between E2g1 and A1g modes is veridical. According to previous references,36-38 in our work, the ∆ω measured from two MoS2 flakes with different thickness are calculated to be 5.5 and 4.6 cm-1, which are identified as trilayer and quadlayer MoS2 respectively. The value of ∆ω (~5.5 cm-1) for trilayer is well consistent with that reported in the references, however, the ∆ω reported for quadlayer is varied from 3.5 to 5.3 cm-1, our measured value of 4.6 cm-1 falls between this ranges which is assigned to quadlayer MoS2 flakes in our experiment. Furthermore, our analysis/assignment is further supported by the interference color of the two samples (see Fig 1a). It should also be noted that the ∆ω ~ 4.6 cm-1 is clearly larger than that reported for pentalayer MoS2 flakes, whose value is no more than 3.5 cm-1.36-38


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Table 1. The peak position of the E2g2 mode for ultrathin MoS2 (2-5 layers) and the frequency difference of E2g2 mode between ultrathin and bulk MoS2 (∆ω). H. Zeng36 Layer number

Y. Zhao37

G. Plechinger38

This work

1 layer

E2g2 (cm-1) -

∆ω (cm-1) -

E2g2 (cm-1) -

∆ω (cm-1) -

E2g2 (cm-1) -

∆ω (cm-1) -

E2g2 (cm-1) -

∆ω (cm-1) -

2 layers

23.3

10.4

24.0

9.0

19.5

10.0

22.7

9.8

3 layers

28.3

5.4

27.0

6.0

24.5

5.0

27.0

5.5

4 layers

30.0

3.7

29.5

3.5

24.2

5.3

27.9

4.6

5 layers

31.7

2.0

30.5

2.5

26.0

3.5

--

--

bulk

33.7

0

33.0

0

29.5

0

32.5

0

Note : “--” represent the absence of experimental results in our work. The pressure-dependent Raman spectra of trilayer, quadlayer and bulk MoS2 up to 12.7 GPa are shown in Fig 3. It can be seen that the trilayer and quadlayer have larger line widths compared to the bulk MoS2 at ambient conditions. This is related to the nanoscale thickness of trilayer and quadlayer, leading to a broadening of Raman peaks. Similar effect has also been observed in MoS2 nanoparticles.34,39 As pressure increases, the peak widths of E2g1 and A1g modes for all MoS2 samples increased, probably due to the structure deformation upon compression.



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Fig 3. Raman spectra of MoS2 under pressure. Multiple peak fitting with Lorentz profile curves is shown with different colors. The inset shows the enlarged image for the emerging shoulder on the left side of A1g mode of trilayer MoS2 at 4.2 GPa. The A1g mode of trilayer, quadlayer and bulk MoS2 can be well fitted by a single peak with Lorentz profile at 0 GPa. As pressure increases, a shoulder peak on the left side of A1g peak for trilayer and quadlayer MoS2 flakes emerges at 4.2 GPa (trilayer) and 5.2 GPa (quadlayer), respectively, and the intensity of this shoulder peak grows gradually with pressure, as shown in Fig 3. In our recent study, a similar shoulder appeared in monolayer MoS2 at 1.74 GPa23. The pressure difference for the shoulder peak emergence indicates that the thinnest monolayer MoS2 is most sensitive to the pressure, and it becomes more difficult to deform the structure of MoS2 flakes with the layer number increasing, so that the pressure for the shoulder peak emergence is higher. Subsequently, the bulk MoS2 exhibits a well symmetric A1g peak (no splitting) in all the spectra recorded in our studied pressure range (Fig 3c). This also suggests that the A1g mode of ultrathin MoS2 is very susceptible to compression. We further calculate the pressure evolutions of A1g and E2g1 vibration modes and the plotted curves are shown in Fig 4. A larger blue-shift of A1g mode compared to E2g1 mode in trilayer and quadlayer MoS2 flakes can be observed. The pressure coefficient of E2g1 mode is 1.86 cm1

GPa-1 for trilayer MoS2 and 2 cm-1GPa-1 for quadlayer MoS2, while that of A1g mode is 2.79

cm-1GPa-1 for trilayer MoS2 and 2.8 cm-1GPa-1 for quadlayer MoS2. The larger pressure coefficients of the A1g modes than those of the E2g1 modes suggest that the compression on caxis is more effective compared to a-axis. This suggests that the out-of-plane compression play a dominant role in the structure modification of ultrathin MoS2. It is noted that the A1g mode is extremely sensitive to the out-of-plane compression due to its out-of-plane S-S vibration.


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Fig 4. The plotted curves for (a) the pressure-dependent frequency of A1g, the shoulder mode (red symbols for trilayer; red symbols with “×” for quadlayer), and the frequency difference between A1g and the shoulder one for trilayer and quadlayer (marked as “∆” as shown in the inset). (b) The pressure-dependent E2g1 mode for trilayer, quadlayer and bulk MoS2, and the frequency difference of E2g1 mode between quadlayer and bulk MoS2 are shown in the inset. (c) The pressure-dependent E2g2 modes for trilayer, quadlayer and bulk MoS2, and left shoulder peak of E2g2 for quadlayer are shown as red hollow symbols with “×”. (d) Side and top views of bilayer MoS2 with 3R, 2H, AB’ and A’B layer stackings. We also compared the pressure evolution of the frequency difference between A1g mode and the emerging shoulder peak (marked as “∆”). It can be seen that the pressure coefficient of “∆” for trilayer is much larger than that of quadlayer, as shown in the inset of Fig 4a. This difference is attributed to the different interlayered interactions, because the interlayered interaction grows as the layer number increases40. Previously, the stronger interlayered interaction in quadlayer can be seen from its higher frequency of E2g2 mode (27.9 cm-1) than that of trilayer (27 cm-1) at ambient condition, since E2g2 mode arises from the vibration of one MoS2 layer against its neighboring layers, the frequency of this mode can directly reflect the strength of the interlayer coupling, i.e. a higher frequency indicates a stronger interlayer 11 ACS Paragon Plus Environment


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coupling.41 As we know, any changes in the interlayered interactions or the layer stacking configurations of ultrathin MoS2 will influence its vibrational behaviors.42,43 For the polymorphous MoS2, depending on interlayer translation (A’B and AB’) or rotation (3R when rotation angle Θ=60°)44, MoS2 has other structures such as those shown in Fig 4d. Liu et al. showed that the A1g mode in bilayer MoS2 exhibits a slight red-shift from 407.5 cm-1 to 405 cm-1 due to the interlayered rotation of the angle Θ from 0° to 15°, indicating that the A1g mode is strongly dependent on the interlayer coupling interaction.42 Thus, the interlayered sliding or structure distortion due to compression may occur in trilayer and quadlayer MoS2, which subsequently leads to different stacking configurations of S atom layers compared with the starting structure, and thereafter A1g mode was affected. We now analyze the pressure evolutions of the in-plane vibration E2g2 and E2g1 modes. 2 At ambient pressure, it is reported that the E2g1 mode is less sensitive to layer number41. As shown in Fig 4b, at pressure below 8.6 GPa, though they all show a blue shift with pressure, we can also see that the E2g1 mode is less sensitive to layer number; nevertheless, the pressure evolutions of the trilayer and the bulk counterpart keep similar behavior above 8.6 GPa, while the pressure slope of E2g1 mode of the quadlayer MoS2 is steeper than those of the trilayer and the bulk. We observed a gradually increasing frequency difference between quadlayer and bulk MoS2 above 8.6 GPa, from 0 to 5 cm-1, as seen in the inset in Fig 4b, this is an unusual phenomenon, which is related to the structure translation of quadlayer MoS2 under high pressure. When the transition from 2Hc to 2Ha structure happens in bulk MoS2, a higher vibration of E2g1 mode can be found after 20 GPa,25 similarly the increase of E2g1 mode in quadlayer MoS2 after 8.6 GPa reflects the transition. Meanwhile, a splitting of the E2g2 mode in quadlayer is found at 8.6 GPa (Fig 5a), which is in contrast to the monotonous increase in frequency and the absence of splitting or change of this mode observed in the trilayer. The pressure evolution of E2g2 mode is also shown in Fig 4c. Since E2g2 mode is directly related to the interlayer interaction, the splitting of E2g2 rigid 12 ACS Paragon Plus Environment

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layer mode in quadlayer may signify the change of the stacking configuration under high pressure. i.e. the structure transition caused by the pressure-induced layer sliding. We preclude the formation of 3R-MoS2 because a 60° rotation is required to transform 2H to 3RMoS2, as shown in Fig 4d. Previous work also suggested that 3R-MoS2 can only be produced by a chemical vapor deposition method.45 In contrast, interlayer sliding is achievable more easily because less external energy is required. The transition from 2Hc to 2Ha phase in bulk MoS2 due to interlayer sliding has been confirmed at about 20 GPa.25,46

Fig 5. The low-frequency E2g2 modes of (a) quadlayer (b) trilayer MoS2 at different pressure. The open circles are experimental data, the curves are fitted by Lorentz profiles, a small shoulder peak emerges in quadlayer MoS2 above 8.6 GPa. In order to uncover the effect of possible layer sliding on the high pressure evolutions of E2g1 and E2g2 modes in quadlayer MoS2, we investigated two high-symmetry AB’ and A’B stacking bilayer MoS2, and their difference can be seen from side view, as shown in Fig 4d. Mo atoms overlap in AB’ and S atoms overlap in A’B stacking. Previous DFT simulations on bilayer MoS2 suggested that, the A’B and AB’ stacking configurations can be distinguished through Raman spectra.43 Compared with the initial 2H structure, the A’B and AB’ configurations respectively show 15.5 and 3.8 cm-1 lower E2g2 mode, while for the E2g1 mode, they all give a 2.5 cm-1 higher frequency than that of 2H structure.43 Compared with our 13 ACS Paragon Plus Environment


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Raman spectra, it is obvious that the AB’ configuration is consistent with the measured result. It is thus believed that the pressure induced layer sliding in quadlayer MoS2, forming the AB’ structure. Moreover, the trilayer MoS2 did not transform into A’B or AB’ stacking configuration, which might be due to the thickness-dependent interlayer coupling interactions. From the Raman spectra of trilayer and quadlayer MoS2 recorded at high pressure, we can find the trilayer MoS2 exhibits an out-of-plane vibration sensitive behavior, showing an obvious shoulder peak on the left side of A1g mode, while the quadlayer MoS2 gives an inplane vibration sensitive behavior, presenting as the E2g1and E2g2 modes changes. These phenomena also indicate that the interlayer coupling interaction plays an important role on the structural stability of ultrathin MoS2 under high pressure. The trilayer possesses weaker interlayer interactions compared with the quadlayer MoS2, so that the pressure-induced distortion or wrinkle effect is prominent, which is evidenced by the variation of the out-ofplane A1g mode. The relatively strong interlayer interaction in quadlayer should suppress the wrinkle effect, and subsequently for the ultimate bulk MoS2, no variation on A1g was found even up to 60 GPa.25 On the contrary, the pressure-induced sliding effect becomes prominent when interlayer coupling interaction enhances, as evidenced by the in-plane E2g1and E2g2 modes. In order to further confirm the pressure induced sliding in quadlayer MoS2, we performed the First-principles calculations for 2H, AB’ and A’B stacking configurations. The pressure dependence of enthalpies for 2H, AB’ and A’B quadlayer MoS2 are shown in Fig 6b. At 0 GPa, 2H is the most stable stacking configuration, whereas, after 8 GPa, AB’ seems to be the more stable stacking configuration when compared with 2H and A’B. This result successfully verifies experimental observations showing pressure-induced interlayer sliding occurred at 8.6 GPa from the 2H phase to the AB’ phase in quadlayer. Similar to quadlayer, at 0 GPa, for trilayer, 2H seems to be the most stable stacking pattern, AB’ and A’B seems to possess very close enthalpy, which are slightly higher than that of 2H, 14 ACS Paragon Plus Environment

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while at high pressure the enthalpy of 2H structure grows higher than that of AB’ and A’B. Considering shear strain exist in the DAC due to the solidification of PTM argon after 4GPa, we also checked what may happen when shear strain is applied on all stacking configuration under pressure, from bilayer to quadlayer MoS2. Then it was found that the distorted 2H structure gives a lowest enthalpy in trilayer MoS2, as shown in Figure 6, while that for quadlayer MoS2 is much higher than the AB’ stacking, indicating the trilayer MoS2 favors a distorted structure under high pressure, but the quadlayer MoS2 favors AB’ stacking configuration through interlayer sliding, due to stronger interlayer coupling in quadlayer MoS2 may inhibit wrinkle or distortion effects of ultrathin MoS2.

Fig 6. The enthalpies of the 2H, AB’ and A’B phases of (a) trilayer (b) quadlayer MoS2 as a function of pressure. The bulk MoS2 transforms from 2Hc to 2Ha structure above 20 GPa, while the quadlayer MoS2 studied here shows a transformation from 2H to AB’ configuration at above 8.6 GPa, suggesting that sliding transformation under high pressure is favored due to the energy minimum priority. In fact, the 2Ha and the AB’ configurations have the same structure but called differently in different literatures for the bulk and bilayer MoS2, respectively. The higher transition pressure in bulk MoS2 suggests more energy is required to overcome the barrier due to strong interactions in it. As Blumberg and Levita demonstrated, the interlayer coupling becomes the dominant factor determining its sliding energy landscape or sliding 15 ACS Paragon Plus Environment


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properties under pressure.47,48 Combining with First-principles calculations, we here confirmed that Raman spectra can directly uncover the interlayer interactions and stacking patterns of 2D TMD materials. CONCLUSIONS Combining the Raman spectra and the First-principles calculations, we have investigated the structure transformation of trilayer and quadlayer MoS2 under high pressure. Our studies suggest that the structure of ultrathin MoS2 can be tuned by pressure. The distinct high pressure Raman responses of trilayer and quadlayer suggest their different structural changes upon compression, which is explained by their thickness-dependent interlayer coupling interaction. Upon compression, the c axis of MoS2 is more susceptible to pressure, indicating the out-of-plane compression play a dominant role in the structure modification of ultrathin MoS2. The structural transition from 2H to AB’ configuration above 8.6 GPa is observed in quadlayer MoS2, as evidenced by the in-plane modes E2g1and E2g2, which is further confirmed by the lowest enthalpy for AB’ stacking after 8 GPa when compared with others. While trilayer MoS2 possesses a distorted and wrinkled structure within our studied pressure range due to the weaker interlayer coupling. The interlayer coupling and stacking configurations were tuned by pressure and detected by Raman spectroscopy measurement, demonstrated that high pressure is another effective tool to enrich the polymorphism of ultrathin MoS2. It is expected to motivate the design and fabrication of different stacking nanometre devices with tailored properties for specific applications, which is especially benefit for the development of nanotechnology. ACKMOWLEDGMENTS This work was supported by the National Basic Research Program of China (No. 2011CB808200), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1132), National Natural Science Foundation of China (Nos. 11574112, 11474127 ， 51032001, 11074090, 10979001, 51025206, 11274137), National Found for 16 ACS Paragon Plus Environment

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Figure captions Fig 1. (a) The images of trilayer, and quadlayer MoS2 flakes deposited on silicon wafer. (b) A schematic drawing of gasketed diamond anvil cells (DAC) for compression experiments. Fig 2. (a) Atomic displacements of Raman-active modes for a direct view. The yellow solid lines represent the Mo-S covalent bonds, the yellow dashed lines represent the interlayer van der Waals interaction. (b) The low frequency E2g2 mode and (c) the E2g1 and A1g modes of monolayer, trilayer, quadlayer and bulk MoS2 at ambient conditions. The spectra intensity are normalized for E2g2 mode in (b) and A1g mode in (c), the relative intensity between E2g1 and A1g modes is veridical. Fig 3. Raman spectra of MoS2 under pressure. Multiple peak fitting with Lorentz profile curves is shown with different colors. The inset shows the enlarged image for the emerging shoulder on the left side of A1g mode of trilayer MoS2 at 4.2 GPa. Fig 4. The plotted curves for (a) the pressure-dependent frequency of A1g, the shoulder mode (red symbols for trilayer; red symbols with “×” for quadlayer), and the frequency difference between A1g and the shoulder one for trilayer and quadlayer (marked as “∆” as shown in the inset). (b) The pressure-dependent E2g1 mode for trilayer, quadlayer and bulk MoS2, and the frequency difference of E2g1 mode between quadlayer and bulk MoS2 are shown in the inset. (c) The pressure-dependent E2g2 modes for trilayer, quadlayer and bulk MoS2, and left shoulder peak of E2g2 for quadlayer are shown as red hollow symbols with “×”. (d) Side and top views of bilayer MoS2 with 3R, 2H, AB’ and A’B layer stackings. Fig 5. The low-frequency E2g2 modes of (a) quadlayer (b) trilayer MoS2 at different pressure. The open circles are experimental data, the curves are fitted by Lorentz profiles, a small shoulder peak emerges in quadlayer MoS2 above 8.6 GPa. Fig 6. The enthalpies of the 2H, AB’ and A’B phases of (a) trilayer (b) quadlayer MoS2 as a function of pressure. 24 ACS Paragon Plus Environment

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Fig 1. (a) The images of trilayer, and quadlayer MoS2 flakes deposited on silicon wafer. (b) A schematic drawing of gasketed diamond anvil cells (DAC) for compression experiments. 47x27mm (300 x 300 DPI)

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Fig 2. (a) Atomic displacements of Raman-active modes for a direct view. The yellow solid lines represent the Mo-S covalent bonds, the yellow dashed lines represent the interlayer van der Waals interaction. (b) The low frequency E2g2 mode and (c) the E2g1 and A1g modes of monolayer, trilayer, quadlayer and bulk MoS2 at ambient conditions. The spectra intensity are normalized for E2g2 mode in (b) and A1g mode in (c), the relative intensity between E2g1 and A1g modes is veridical. 82x84mm (300 x 300 DPI)


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Fig 3. Raman spectra of MoS2 under pressure. Multiple peak fitting with Lorentz profile curves is shown with different colors. The inset shows the enlarged image for the emerging shoulder on the left side of A1g mode of trilayer MoS2 at 4.2 GPa. 46x27mm (300 x 300 DPI)



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Fig 4. The plotted curves for (a) the pressure-dependent frequency of A1g, the shoulder mode (red symbols for trilayer; red symbols with “×” for quadlayer), and the frequency difference between A1g and the shoulder one for trilayer and quadlayer (marked as “∆” as shown in the inset). (b) The pressure-dependent E2g1 mode for trilayer, quadlayer and bulk MoS2, and the frequency difference of E2g1 mode between quadlayer and bulk MoS2 are shown in the inset. (c) The pressure-dependent E2g2 modes for trilayer, quadlayer and bulk MoS2, and left shoulder peak of E2g2 for quadlayer are shown as red hollow symbols with “×”. (d) Side and top views of bilayer MoS2 with 3R, 2H, AB’ and A’B layer stackings. 75x48mm (300 x 300 DPI)


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Fig 5. The low-frequency E2g2 modes of (a) quadlayer (b) trilayer MoS2 at different pressure. The open circles are experimental data, the curves are fitted by Lorentz profiles, a small shoulder peak emerges in quadlayer MoS2 above 8.6 GPa. 66x55mm (300 x 300 DPI)



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Fig 6. The enthalpies of the 2H, AB’ and A’B phases of (a) trilayer (b) quadlayer MoS2 as a function of pressure. 85x60mm (300 x 300 DPI)


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Interlayer Coupling Affected Structural Stability in Ultrathin MoS2: An

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