Anti-wear performance of monolayer MoS2 modulated by residual

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Anti-wear performance of monolayer MoS2 modulated by residual straining Yuehua Huang, Quanzhou Yao, Zhixing Lu, Liying Jiao, Shuai Zhang, Qunyang Li, and Yonggang Meng ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01904 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 17, 2018

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ACS Applied Nano Materials

Anti-wear performance of monolayer MoS2 modulated by residual straining Yuehua Huang1,3*, Quanzhou Yao2,*, Zhixing Lu4, Liying Jiao4, Shuai Zhang2, Qunyang Li1,2,&, Yonggang Meng1,&&

1

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

2

Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China

3

College of Engineering and Technology, Southwest University, Chongqing 400715,

China 4

Key Laboratory of Organic Optoelectronics and Molecular Engineering of the

Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China

Keywords: molybdenum disulfide, solid lubricant, residual strain, wear, friction

*

These authors equally share the first authorship.

*

These authors equally share the first authorship.

&

Corresponding author: Tel:86-10-62772933 .E-mail: [email protected] (Qunyang

Li) &&

Corresponding author: Tel:86-10-62773867 . E-mail: [email protected] (Yonggang

Meng)

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Reducing friction and wear is a long-standing concern for machinery. Bulk solid lubricants have been widely adopted to address the tribological challenges at the macroscale, yet their implementation is relatively limited for small scale applications. Recently, the emergence of two-dimensional (2D) materials brings a new horizon of a range of ultra-thin solid lubricants for micro- and nanoscale sliding interfaces. Using ball-on-disk friction tests, we found that monolayer MoS2 possessed outstanding lubrication performance when the normal load was below a threshold, beyond which wear set in. Further scratch tests on monolayer MoS2 showed that this critical normal load was significantly affected by the residual strain in MoS2. As revealed by finite element simulations, MoS2 failed primarily due to in-plane stretching during tribological sliding and the tensile residual strain would add to the in-plane strain inside MoS2 resulting in noticeably weaker anti-wear performance. This work provides guideline for optimizing the anti-wear performance of 2D materials that are subjected to residual strain.


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Introduction Friction and wear are important phenomena that exist in sliding interfaces and they often cause energy loss and mechanical failures in traditional machinery 1. With rapid developments of sensing and actuating technologies, various new machinery with visibly reduced sizes such as micro-electro-mechanical systems (MEMS) have emerged. Given the considerable shrinkage in sizes, the surface-to-volume ratio of moving components increases rapidly and surface-related issues, such as friction and wear, become more prominent and severe for these micro-and nanoscale systems

2-3

.

Reducing friction and wear for small-scale sliding interfaces is scientifically and technically challenging. Typically liquid lubricants cannot be employed for micro-and nanoscale devices due to the high surface tension and viscosity of the liquid and the poor lubricating performance under relative low sliding velocity of moving surfaces 4. The traditional solid lubricants are not suitable either, because of their relatively large sizes and thicknesses. Therefore, there is a pressing need in the society for ultra-thin solid coatings or lubricants. During the past decade, graphene as a typical two-dimensional (2D) material has been extensively investigated and demonstrated to have unique mechanical properties. Its unprecedentedly-high mechanical strength

5-6

and the intrinsically lubricating

property at the nanoscale 7-8 make graphene an ideal candidate for atomically-thin solid lubricant. Despite its strong wear resistance at the nanoscale becomes worn in micro-scale friction tests

11-15

9-10

, graphene often

. Macroscopically, other 2D materials

such as MoS2 exhibit fundamental properties and practical applications complementary



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to graphene; however, their 2D counterparts are much less explored. Both molecular dynamics simulation 16 and atomic force microscope (AFM) experiments 17 show that monolayer MoS2 also demonstrates a high mechanical strength. Nano-scale friction experiments have showed that monolayer MoS2 has obvious lubrication effect8, 18-19. Compared with the poor lubrication performance of graphite in dry conditions

20-21

,

MoS2 has excellent lubrication performance even in vacuum22-23. As a molecular unit of the bulk material, monolayer MoS2 may be a complementary molecularly-thin lubricant to graphene in terms of environmental susceptibility. Previous tribological studies of MoS2 mainly focus on MoS2 coatings deposited by sputtering 24-26 or MoS2 nano-sheets as additives for lubricating oil

27-29

. An in-depth understanding of the

tribological properties of monolayer MoS2 is essential for its practical applications. To study the tribological behavior of monolayer MoS2 at the micro-scale, we performed friction tests on chemical vapor deposition (CVD) grown MoS2 using a 10mm-diameter glass lens. Our experiments confirmed that monolayer MoS2 indeed exhibited an outstanding lubricating property without wear when normal load was below a critical value. By adjusting the CVD growth parameters for MoS 2, we further demonstrated that the critical normal load could be significantly altered by the residual strain of MoS2. Assisted by finite element simulations, we showed that the tensile residual strain in MoS2 could greatly reduce its load carrying capacity due to enhanced in-plane stretching.

Results and discussion


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Tube furnace

Heating belt Quartz tube Mo oxide SiO2/Si substrate

Sulfur powder

Ar

(a)

(b)

1.86

Lens

Lens Residual strain

Residual strain

20μm

20μm

Photon energy MoS2 strain

2.0

1.84

1.5

1.82

1.0

1.80

0.5

1.78

Exfoliated

(c)

Continuous

Island-type

MoS2 strain (%)

Ar

Photon energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

(d)

Fig.1. (a) Schematic of the MoS2 synthesis setup by chemical vapor deposition method; (b) Schematic of monolayer MoS2; (c) Schematic of the sliding test and the optical microscope images of continuous and island-type monolayer MoS2; (d) The photoluminescence (PL) peak positions and the residual strain of exfoliated monolayer MoS2, continuous monolayer MoS2 and island-type monolayer MoS2. The structure of monolayer MoS2 and the synthesis setup by chemical vapor deposition are shown in Fig.1. Island-type MoS2 and continuous MoS2 films were synthesized by changing growth parameters, as mentioned in the Methods section. The CVD-grown MoS2 samples were first identified by optical microscopy, as shown in Fig.1(c). The island-type MoS2 was uniform in color, suggesting a uniform thickness. The large-area continuous MoS2 film was mostly homogeneous in color, although some dark purple sites (corresponding to the initial nucleation sites) and stripes (corresponding to the overlapping regions at the grain boundaries) could be observed. To further inspect the layer thicknesses, the height of the two MoS2 samples was measured by AFM (Fig.S1), which confirmed their monolayer nature 17.



Given the strong interaction between SiO2/Si substrate and MoS2, residual strain is often found in CVD-grown MoS2 on SiO2/Si, which is likely due to the thermal mismatch during the cooling process

30-31

. It is well known that when the MoS2 is

thinned down to a monolayer, a strong photoluminescence (PL) emerges 32 and the PL peak position is proven to be an effective indicator to detect the intrinsic strain in monolayer MoS2

31, 33

. The average PL peak position of continuous monolayer MoS2

was 1.825eV and that of island-type monolayer MoS2 was 1.787eV, and that of exfoliated monolayer (Fig.S1) MoS2 without residual strain was 1.848eV. The representative PL spectra of these MoS2 samples are shown in Fig.S2. In accordance with the red shift of the PL peak position

31, 33-34

, the residual tensile strains of

continuous monolayer MoS2 and island-type monolayer MoS2 were estimated to be around 0.5% and 1.5%, respectively. The in-plane lattice vibration E2g1 mode was sensitive to strain, and it usually red-shifted with the increasing strain 35, which can also indicate the presence of residue stress. The Raman characterization of different MoS2 samples has been showed in Fig. S3. The E2g1 mode of continuous monolayer MoS2 and island-type monolayer MoS2 were 383.6 cm-1 and 381.7 cm-1, respectively, both lower than that of exfoliated monolayer MoS2 (384.7 cm-1), which demonstrated the residue stress in these two types of MoS2 samples was different.


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0.20

0.10

20μm

0.00

200

0

0.25

0.6 0.20 0.15

0.4

20μm

0.10

0.2 0.05 0.00

0

20

80 60 40 Cycle number

100

0.0 120

0.2mN

0.8

Linear position (μm)

Friction coefficient Normal load

Normal load (mN)

0.30

0.1mN

(a)

600 400 Cycle number (b)

800

0.6mN

0.05

0.5mN

SiO2/Si

0.15

0.4mN

MoS2

Lens

0.1mN

0.3mN

Friction coefficient

Lens


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

COF 0.3 0.25

40

0.2

30

0.15

20

0.1

10

0.05

20

40 60 80 100 120 Cycle number

(d)

(c)

Fig.2. (a) A schematic diagram of the friction experiment setup; (b) The friction coefficient of continuous monolayer MoS2 under 0.1mN as a function of cycle number. The inset picture is the optical microscope image of the sliding track after test; (c) The friction coefficient on continuous monolayer MoS2 as a function of normal load and cycle number. The load test was conducted at the same location and the normal load was increased every 20 cycles. The inset picture is the optical microscope image of the sliding track after test; (d) The spatial variation of the friction coefficient along the sliding track as a function of cycle number, linear position denotes the true position of the indenter after subtracting the cantilever deformation. Fig.2(a) shows a schematic of the tribological test on the continuous MoS2. Compared to the high friction coefficient (around 0.7) on bare substrate, the friction coefficient of the continuous MoS2 remained steadily low (0.025 around) under 0.1mN normal load throughout 1000 cycles, as shown in Fig.2(b). No any sign of wear of the continuous MoS2 was detected after the friction test. Similar wearless behavior was reproduced for two other tests performed at different locations under 0.05mN and 0.1mN load, as shown in Fig.S4. To identify the maximum load carry capacity before



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wear sets in, we performed a series of friction tests while gradually increasing the normal load every 20 cycles along a same sliding track. As shown in Fig.2(c), the friction coefficient initially decreased and stayed low up to 0.4mN normal load. This initial decrease in friction was likely due to the cleaning effect of the sliding lens by pushing away the surface contamination on MoS2

36

. When the load was further

increased to 0.5mN, the friction coefficient increased abruptly from 0.02 to 0.05; and then the value quickly went up to 0.25 when the normal load ended at 0.6mN. To reveal the detailed wear process, we plotted a two-dimensional (2D) evolution map in Fig.2(d), where friction coefficient was represented as a function of slide position and cycle number. As indicated by the 2D map, when the load was lower than 0.4mN, the friction coefficient remained low and uniform within the entire sliding track. However, starting from 0.5mN, the friction coefficient increased locally, which suggested the local failure of MoS2. The high friction region quickly expanded to the entire sliding track when the load was increased to 0.6mN. The 2D map revealed that the onset of wear occurred at 0.5mN. Below the critical load, wear of monolayer MoS2 was absent and the wearless regime had a low friction coefficient of 0.02, which was even lower than that of 2.5μm thick MoS2 coatings

24

. The excellent lubrication effect of monolayer MoS2 was also

comparable to monolayer graphene. The friction coefficient of CVD-grown graphene on Cu substrate was 0.02 at the wearless regime 37 and it increased to 0.22, which led to immediate wear when graphene was transferred on SiO2 substrate

14

. Our friction

tests demonstrate that monolayer MoS2 has a high potential to be an ultra-thin solid lubricant at the micro-scale.


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0.8

0.05mN

20μm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1mN 20μm

0.6 0.4 0.2 0.0

0

5

10 15 Cycle number

20

Fig.3. The friction coefficient of island-type monolayer MoS2 under 0.05mN and 0.1mN as a function of cycle number. The inset pictures are the optical microscope images of the corresponding sliding track after tests. We also performed friction test on the island-type MoS2. As shown in Fig.3, the friction coefficient on the island-type MoS2 under 0.05mN was already 0.17 during the first cycle and it slightly increased to 0.2 after 17 cycles. After friction test, an obvious wear track was observed. Under a higher normal load of 0.1mN, the friction coefficient exhibited a similar high value and ended at around 0.4 after 20 cycles, leaving a wider wear track. Compared to the outstanding lubrication property of the continuous MoS 2, the performance of the island-type MoS2 was less satisfactory. The detailed characterization of wear track of both continuous and island-type MoS2 by AFM is shown in Fig.S5. The friction map demonstrated that both the continuous and islandtype MoS2 in the sliding track were completely worn out after sliding tests. The friction force on the exposed SiO2/Si substrate was obviously larger than that on MoS2 sample. The topography image and height profile revealed that the exposed SiO2/Si substrate showed no obvious wear under the normal load(≤0.6mN) in our sliding tests on the



tribometer. The PL peak position of the island-type monolayer MoS2 shifted from 1.78 (collected at the sliding track before test) to 1.82 (collected near the wear track of MoS2 after test), as shown in Fig.S6. This demonstrates the residual strain of island-type monolayer MoS2 was released near the wear track. More friction tests are shown in Fig.S7, which confirmed that the anti-wear property of the island-type monolayer MoS2 was consistently weaker than that of the continuous monolayer MoS2.

Continuous MoS2

Island-type MoS2

(a)

6

(b)

6

Forward

4

Lateral force (nN)

Lateral force (nN)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2 0 -2 -4

Forward

4 2 0 -2 -4

Backward

Backward -6

0

0.5

1.5 1 Distance (μm) (c)

2

-6

0

0.5

1.5 1 Distance (μm) (d)

2

Fig.4. The AFM friction force maps (2μm0.5μm) of (a) continuous monolayer MoS2 and (b) island-type monolayer MoS2. The corresponding friction traces extracted from dashed lines in friction force maps are shown in (c)continuous MoS2 and (d)island-type MoS2. These tests use the same AFM tip and the same normal load 9nN. To investigate the mechanism underling the different lubrication performance, we evaluated the intrinsic friction property of the two MoS2 samples before wear using AFM. The influence of normal load, relative humidity38, tip radius and tip material39 are excluded in our test by using the same AFM tip, normal load and testing environment. The nanoscale friction force maps collected by AFM are shown in Fig.4(a)


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and Fig.4(b) and the corresponding lateral force traces are given in Fig. 4(c) and Fig.4(d). The nanoscale friction tests in Fig.4 was carried out by AFM under a very light load (9nN) in the AFM measurement. Based on the friction images, no observable damage was found in MoS2 samples after the friction tests. As shown in Fig. 4, the nanoscale friction measured on continuous MoS2 is almost the same as that of the island-type. Previous results show that nanoscale friction on 2D materials is extremely sensitive to chemical changes or defects on sample surfaces40. Therefore, we speculated that the intrinsic quality of these two MoS2 samples were similar. The distinct frictional behavior between the island-type and continuous MoS2 exhibited at the micro-scale friction tests by tribometer (Fig.2 and Fig.3) is likely to be caused by the wear of MoS2. Instead, we speculate that the different levels of residual strain within MoS2 films might be a viable mechanism. The subtle difference in friction between the island-type and the continuous MoS2 samples may be due to different levels of puckering effect8. The higher residual tension in the island-type MoS2 could suppress the puckering effect and result in smaller friction. To validate this hypothesis, we conducted finite element (FE) simulations to analyze the effect of residual strain on load carrying capacity of the MoS2 films.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Load P

MoS2 In-plane stress

R Lens Residual strain

MoS2

Si

Maximum in-plane stress (GPa)


25 20

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0.5% residual strain 1.5% residual strain MoS2

15

23GPa

Lens

Si Continuous MoS2 MoS2 Lens

10 5

Si Island-type MoS2

0 0.0

0.2 0.4 Normal force (mN)

(a)

0.6

(b)

Fig.5. (a) A schematic showing the simulation model;(b) Maximum in-plane 2D stress in MoS2 with 0.5% and 1.5% residual strain as a function of normal force. The inset picture is schematic showing the wear mechanism of MoS2 with 0.5% and 1.5% residual strain.

The schematic of the FE model is shown in Fig.5(a), where a rigid hemispherical indenter representing the lens is pressed against a thin elastic sheet representing MoS 2 adhered to an elastic substrate. To incorporate the effect of residual strain, an additional calculation step with thermal loading was performed to introduce 0.5% and 1.5% tensile strain before the indentation step. According to a previous study on wear mechanisms of graphene under tribological load, failure of the 2D materials when slid in the interior region was primarily caused by in-plane stretching 37. Therefore, we focused on the inplane stress inside the MoS2 film when it was loaded by the indenter. The maximum inplane stress inside MoS2 as a function of normal load is shown in Fig.5(b) for two systems where MoS2 has 1.5% and 0.5% initial strain respectively. As shown in Fig 5(b), the maximum in-plane stress of monolayer MoS2 with 1.5% residual strain is always higher than that of monolayer MoS2 with 0.5% residual strain under the same normal load. To reach the strength of MoS2 (about 23GPa 17), the critical normal load


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needed for the MoS2 film with 1.5% residual strain (0.35mN) is significantly less than that for the MoS2 film with 0.5% residual strain (0.50mN). In our experiments, the reduction in critical load was somewhat larger than the prediction from the FE simulations. This difference could be due to the effects of the substrate roughness and the extra friction shearing, which are not included in the model. Nevertheless, our simulations clearly suggest that the residual tensile strain in the MoS2 film would lead to additional in-plane stress inside MoS2, which helps break the film when an external load is applied.

Conclusion Friction and wear characteristics of monolayer MoS2 grown on SiO2 substrate were investigated by scratch tests with a glass lens. Our experimental results demonstrated that MoS2, even with its thickness down to one molecular layer, was lubricious at the microscale. Notably, we found that the maximum normal load that the monolayer MoS2 could sustain before wear depended critically on the growth induced residual strain inside MoS2. Our experiments suggested that higher tensile residual strain would reduce the load carrying capacity of MoS2 film because of enhanced in-plane stretching, as confirmed by finite element simulations. This work may offer guidelines for optimizing the tribological performance of monolayer MoS2 or other two-dimensional materials that are subjected to residual straining.



Methods MoS2 Synthesis: The MoS2 was synthesized via CVD, and its setup is schematically shown in Fig.1(a). A quartz boat containing a piece of SiO2/Si covered by Mo oxide was inserted into the middle of the quartz tube furnace. Another corundum boat containing the sulfur powder was placed at the upstream location inside the tube furnace. A heating belt was wrapped outside the tube where the sulfur powder was located. After purging the tube with argon gas for 20min, the tube furnace was heated to 800℃ (for the island-type sample) or 900℃ (for the continuous sample) with 20℃ /min heating rate. During the heating process, the heating belt was heated to 180℃ and maintained at 180℃ when the temperature of the tube furnace reached 650℃. After maintaining the temperature at 800 °C for another 10 min (for the island-type sample) or 900℃ for 15 min (for the continuous sample), the tube furnace was cooled naturally. When the furnace was cooled to 400℃ the heating belt was removed. The MoS2 sample on SiO2/Si was obtained after the furnace cooled to the room temperature. Sample Characterization: The fluorescence spectra of MoS2 samples were acquired by a Raman spectrometer (LabRAM HR800, Horiba). The Raman laser wavelength was 532nm and the spot size was about 1μm. The thickness of MoS2 samples were confirmed by atomic force microscope (Ntegra, NT-MDT) with a Si tip (CSC37B, Mikro Masch). Experimental Measurements: For friction tests at the micro-scale, a reciprocating sphere-on-flat Nano Tribometer (NTR2, CSM Instruments) and a double-leaf cantilever were used. The counterparts were BK7 planoconvex lens (Sigma) and the profile is


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shown in Fig.S8. Using a white-light profilometry (Nexview, Zygo), the curvature radius of the lens was 5mm and the root-mean-square roughness was around 6nm measured in a 167μm×167μm area after removing the curvature. Before each friction test, the lens was cleaned by air-laid paper dipped in ethanol, and its cleanliness was confirmed by optical microscopy (2700M, Leica). We described the friction test using the “micro-scale” because the contact radius was several micrometers judging from the wear track. We also believe that the “macro-scale” can be reasonably used to describe the friction test when considering the counterpart we used was a 10mm-diameter glass lens. The friction tests were performed in ambient environment, with temperature around 25℃ and relative humidity around 35%. The sliding length was 50μm. For nano-scale friction measurements, an atomic force microscope (Ntegra, NT-MDT) with a Si tip (CSC37B, Mikro Masch) was used using the lateral force microscopy mode. Finite Element Simulation: The finite element simulation model consisted of a rigid hemispherical indenter pressing against a thin elastic sheet adhered to an elastic substrate (Fig.5(a)). Given that a rigid hemispherical indenter was used in the simulation model, the effective Young’s Modulus and Poisson’s ratio of the substrate were 45 GPa and 0.34 by considering the elastic properties of Si and BK7 glass 41. The effective Young’s modulus, thickness, and Poisson’s ratio of MoS2 were taken as 270 GPa, 0.65 nm, and 0.27, respectively 17. To incorporate the residual strain inside MoS2 sheet, an additional calculation step with thermal loading was performed to introduce a certain level of thermal expansion of the substrate before the indentation step. For the results of the present work, two different levels of tensile strain about 0.5% and 1.5%



were considered in the simulations. The maximum in-plane stress of the MoS2 sheet during the indentation step was monitored.

Supplementary Materials See supplementary material for more details.

Author Contributions Y.H. performed all experiments and acquired the experimental data and analyzed the data from other authors. Q. Y performed the simulations. Z.L. prepared the MoS2 samples. L.J. supervised the MoS2 samples synthesis. S.Z. help Y.H. to calibrate the AFM tip’s lateral stiffness. Q.L and Y.M. supervised the simulation work and the experimental work. The manuscript was accomplished through contributions of all authors. All authors have given approval to the manuscript.

Acknowledgements We gratefully acknowledge the support from the National Natural Science Foundation of China (11772169 and 11432008), the National Basic Research Program of China (2015CB351903) and the Fundamental Research Funds for the Central Universities of China (SWU118027).

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Table of Contents Graphic 0.6 0.5

Critical normal load (mN)

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0.5% residual strain Lens

Si

Lens

0.4

Lens MoS2

0.3

Si 1.5% residual strain

0.2

Lens

0.1 0.0

Si

Continous MoS2

Island-type MoS2


Anti-wear performance of monolayer MoS2 modulated by residual

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