Frictional Characteristics of Suspended MoS2

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Frictional Characteristics of Suspended MoS

Peng Huang, Andres Castellanos-Gomez, Dan Guo, Guoxin Xie, and Jian Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07735 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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The Journal of Physical Chemistry

Frictional Characteristics of Suspended MoS2 Peng Huang1,2, Andres Castellanos-Gomez3, Dan Guo1,*, Guoxin Xie1,*, Jian Li4 1State

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

2Science

and Technology on Surface Physics and Chemistry Laboratory, Mianyang 621908, Sichuan,

China 3Materials

Science Factory, Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain 4Wuhan

Research Institute of Materials Protection, Wuhan 430030, Hubei, China

ABSTRACT Molybdenum disulfide (MoS2), a booming layered two-dimensional (2D) nanomaterial, has gain intensive interests for its remarkable physical properties. In this work, the friction characteristics of suspended MoS2 are systematically investigated with atomic force microscopy (AFM). The friction on the suspended MoS2 is much larger than that on the supported MoS2 because of the softening bending rigidity and easier formation of puckering at the AFM tip-MoS2 contact interface, and the difference would increases with the applied load. Similar to the supported MoS2, the friction on the suspended MoS2 also decreases with the increasing layers because of the enhanced bending rigidity. The friction on the suspended MoS2 is relatively insensitive to the shapes of holes below but sensitive to the dimensions. This work can provide beneficial guidance for the diverse design requirements of MoS2-based nanoelectromechanical devices. INTRODUCTION Atomically thin molybdenum disulfide (MoS2) has attracted wide attentions for electronic and optoelectronic applications because of its excellent semiconductor properties, that monolayer MoS2 owns a large direct band gap of 1.9 eV while the band gap will decrease with the increasing layers.1,2 Then, the exceptional mechanical properties of MoS2 have been investigated finding that atomically thin MoS2 has an ultra-strong breaking strength and a Young’s modulus similar to steel.3,4 MoS2 also emerges as an effective lubricant and protective layer because of the excellent mechanical properties and low frictional behaviors.5,6 This superior frictional performance of MoS2 is attributed to the weak layered interactions.5,7-9 The investigation on the friction properties of MoS2 is essential for its nanomechanical applications in nanoelectromechanical systems, such as nano-lubricant and surface coating, etc. AFM is a versatile method to investigate the frictional properties of 2D nanomaterials.7-10 Atomically thin MoS2 behaves an increasing friction with the decreasing number of layers, which is similarly to graphene.8 This phenomenon is attributed to the puckering effect (the tip-sample adhesion and low bending rigidity yields to a local deformation of the flake under the tip when the AFM tip is scanned over the flake).8 Furtherly, atomic simulations reveal that the true contact area between the tip and the nanomaterial does not change obviously with the layer numbers while the evolving contact quality has a significant effect on the thickness dependent

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friction.11 Pre-compression and contact quality are effective ways to tune the friction on the suspended graphene.12 First principle calculations also proved that the sliding friction of a graphene-graphene system would decrease with increasing applied normal force and collapse to nearly zero at a critical point, and this abnormal phenomenon is attributed to the transition of potential energy surface from corrugated to countercorrugated state under the critical pressure.13 Besides, no thickness dependent friction was observed on supported graphene strongly bonded to a flat surface because of the much reduced freedom and increased bending stiffness.11 Similarly, substrate effect was also proved to have a significant influence on the frictional characteristics of MoS2. Hexagonal boron nitride (h-BN) substrate with tens of nanometers in thickness can better preserve the atomic flatness of MoS2 and bring the smaller sliding friction and roughness than the MoS2 on SiO2 and mica substrates.9 Relative humidity (RH) is another important factor that can affect the friction of 2D nanomaterials. The friction of monolayer MoS2 was proved to increase slightly with the increasing humidity.14,15 For the MoS2-MoS2 interface, the friction shows a sudden increase at ~50% RH and it is accompanied with appearance of wear.15 More interestingly, the intercalated water between the graphene and mica substrate also apparently increases the friction by a factor of ~3 because of the enhanced spectral range of graphene vibrations and facilitated coupling and energy transfer.16 Monolayer MoS2 can be obtained by many possible ways, such as mechanical exfoliation, liquid exfoliation, chemical vapor deposition (CVD), etc.7,9,17,18 The friction of CVD grown monolayer MoS2 was higher than that obtained by the mechanical exfoliation because of the imperfect crystalline structures. 7 Besides, the friction at the grain boundary was also much larger than that on the grain due to the defects. 7 The friction coefficient between the incommensurate monolayer MoS2 by in situ scanning electron microscope(SEM) technique with a Si nanowire force sensor came as low as ~10-4, reaching the superlubricity region. 5 AFM-based scratch tests on MoS2 nanoflakes proved that the increasing interfacial strength and the decreasing friction with layers significantly improve the tribological performances.10 These researches could provide a convincing evidence for the lubricating and coating applications of MoS2. Although some frictional properties of supported MoS2 have been conducted, the research on the frictional characteristics of suspended MoS2 is still lacking. To satisfy the diverse design and usage requirements of MoS2-based nanodevices (and very special nanomechanical devices), suspended MoS2 is also of great concern. For example, suspended monolayer MoS2 resonators have a fundamental resonance frequencies (𝑓0) up to 10 MHz to 30 MHz (affected by the geometry) and a quality factor (𝑄) about ~55 in vacuum environment at room temperature, and the value of 𝑓0 × 𝑄 can reach 2 × 1010 in other experiments.19,20 Thus, in this research, the frictional properties of suspended MoS2 nanoflakes on holes obtained by mechanical exfoliation with different thickness are investigated using AFM. The MoS2 nanoflakes obtained by mechanical exfoliation from a molybdenite single-crystal can minimize the adverse effect of grain boundaries and defects induced by CVD grown method.7 In our work, the friction on the suspended MoS2 is noticeably larger than that on the supported MoS2,


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and the friction on the suspended monolayer MoS2 is almost 12% larger than that on the supported MoS2 when the load is ~50 nN, and the difference increases with the applied load. The friction on the suspended MoS2 also behaves an increasing variation trend as the number of layers decrease to monolayer, just like the friction behaviors on the supported MoS2.8 The interface effect (mainly the underlying interaction strength between the MoS2 nanoflake and the substrate) is proved to have a significant influence on the frictional characteristics of MoS2,9,16 that the puckerings are more apt to generate on the suspended MoS2 nanoflake and lead to the increased friction. METHODS Fabrication of MoS2 nanoflakes. Monolayer MoS2 was obtained by mechanical exfoliation method with Nitto tape from a bulk molybdenite rock. SiO2/Si substrate with arrays of holes of different dimensions was designed and patterned through the lithographic etching method. Deterministic transfer by all dry viscoelastic stamping method was adopted to obtain the suspended MoS2 on the SiO2/Si substrate nanoflakes with different thickness.21 AFM characterization. A commercial AFM instrument (Dimension Icon, Bruker) was utilized in the experiment. Lateral force mode (LFM) was utilized to conduct the frictional tests between the probe and the MoS2 nanoflakes. Topographic images could also be obtained at the same time. A standard cantilever holder for operation in ambient conditions was used. The normal deflection sensitivities of a probe were calibrated on a specialized clean sapphire wafer three times at different locations, and then the average value was obtained to reduce the accidental errors. The resonant frequency and the normal spring constant of the cantilever were obtained by using the thermal tune method. In order to derive the frictional force from the voltage signal, the lateral spring constant of the probe was calibrated using the wedge calibration method,22 which was estimated to be about 0.52nN/mV. The scanning area was chosen in the middle of the suspended part with 200 × 200 nm2 and the scanning rate was 2 Hz. AC 240-TM probe was adopted in the LFM experiments. The spring constant is ~3 N/m and the Platinum coating on the probe side can reduce the tip wear as much as possible during the friction tests. The applied load during the friction tests varied from 50nN to 250nN. The temperature and relative humidity in the lab room were ~24±2 ℃ and ~26±5%, respectively. Raman spectra analysis. Raman spectra were obtained using the Raman spectroscopy (Labram HR Evolution) with Ar+ laser (532 nm). The laser spot size was ~0.5 μm and far less than the diameters of the holes that the laser beam can be exerted right on the suspended part of MoS2 nanoflake. The laser power was ~2.5 mW and this could eliminate the local heating effect of laser beam on the MoS2 nanoflake. The accumulation time was 10 seconds and 3 cycles were adopted one time to ensure the sufficient signal intensity. The temperature and relative humidity in the lab room were ~21±1℃and ~27±5%, respectively. RESULTS AND DISCUSSION MoS2 nanoflakes with different thickness are obtained by the mechanical exfoliation



method from a bulk molybdenite rock, see more details about the fabrication in the Methods section. The MoS2 nanoflakes on SiO2/Si substrate behave a clear thickness dependent optical contrast and the contrast decreases with the decreasing layers. This can help to identify the location and the layer numbers of the MoS2 nanoflake. Both AFM and Raman instruments can be utilized to identify the exact layers of the MoS2 nanoflake, while Raman spectra analysis is only suitable for the identification of few layers MoS2.23 Lateral force microscopy (LFM) mode was utilized to conduct the frictional tests between the AFM probe and the MoS2 flakes. The SEM images of a suspended MoS2 nanoflake are shown in Figure S1. Ripples can not be completely avoided during the mechanical exfoliation process because of the mechanical stress. However, ripples only appear on part of the suspended nanoflakes, thus the suspended MoS2 nanoflakes without ripples are chosen for the friction tests and this can reduce the adverse effect on the friction from the surface ripples. Besides, the friction on the suspended MoS2 nanoflakes can exclude the effect of the substrate surface roughness. Figure 1 shows the AFM images of a representative monolayer MoS2 nanoflake suspended on the circular and square holes. Figure 1b and 1d show the friction images of the MoS2 nanoflake. Apparently, the friction on the suspended MoS2 is much larger than that on the supported MoS2 because of the brighter area. This can be attributed to the smaller out-of-plane bending stiffness and easier puckering tendency on the suspended MoS2.9,24 Figure S2 shows the 3D AFM height and friction images of a suspended and supported MoS2 within the 200 × 200 nm2 area. Larger surface fluctuations in the vertical direction can be observed on the suspended MoS2 nanoflake. This can also explain for the larger friction on the suspended MoS2 nanoflake. Strong increase can be observed on the friction loop curves at the edge of the hole as shown in Figure S3, and these are caused by the sudden height change.25 Other friction images of a suspended MoS2 nanoflake are shown in Figure S3.


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Figure 1. a,c. AFM topographic images of a suspended monolayer MoS2 on the circular and square holes. b,d. Friction images of the corresponding suspended MoS2. Figure 2a shows the optical image of a representative MoS2 nanoflake on the SiO2/Si substrate with continuous thickness varying from 1 to 4 layers. The friction on the suspended and supported MoS2 nanoflake with different thickness is shown in Figure 2b. The Raman spectra frequency differences (𝐴1𝑔-E12𝑔) of the MoS2 nanoflake with different layers are shown in Figure S4.For the MoS2 supported on the SiO2/Si substrate, the friction force decreases with the increasing layer numbers. This is in good agreement with previous works and it is attributed to the puckering effect.8 For the suspended MoS2 nanoflake, the friction force shows a similar thickness dependence. However, the friction force on the suspended monolayer MoS2 is almost 12% larger than that on the supported MoS2 when the load is ~50 nN, and this difference increases with the applied load. This can be attributed to the interface effect, mainly the underlying interaction strength between the MoS2 nanoflake and the substrate that affects the friction. The probe on the suspended MoS2 is more apt to generate puckerings and surface fluctuations because of the reduced bending rigidity without the substrate supporting.24 The friction force on the suspended MoS2 decreases with the increasing layer numbers and tends to be close to that on the suspended bulk MoS2. As shown in Figure 2b, the friction force on the 4L suspended MoS2 is quite close to that on the 10L suspended MoS2 when the applied load is small. The bending rigidity of suspended MoS2 increases with the thickness and the puckering effect also reduces with the increasing thickness. This can be proved by the variation of sinking depth of



suspended MoS2 under the fixed applied load using lateral force mode (see Figure S5). In previous research, the friction on the suspended graphene membrane decreased with increasing load in the positive load regime and behaved a switch during the near zero load region, and this can be attributed to the effect of adhesion force.12 This phenomenon is to the contrary of our result that the friction on the suspended MoS2 increases with the applied load. The explanation for this is that the applied load in our experiments is much larger than that in the literature and it can overcome the adverse influence of the adhesion force between the tip and the MoS2 nanoflake. The bending rigidity of monolayer graphene is 1.44 eV while the bending rigidity of monolayer MoS2 is 9.61 eV.26,27 What’s more, the bending rigidity is proved to increase with the thickness.28 The bending rigidity of monolayer MoS2 is significantly higher than that of monolayer graphene because of the larger height and additional pairwise and angular interactions between the out-of-plane Mo and S atoms in the sandwich structures.27 However, the Young’s modulus of MoS2 is much smaller than the graphene.3,29 Then, the vertical deformation of suspended MoS2 membrane under the same applied load is much larger than that of graphene. Thus, the puckering effect between the probe and the suspended MoS2 nanoflake would dominate the load dependent friction.

Figure 2. a. A representative MoS2 nanoflake suspended on the SiO2/Si substrate and the continuous varying thickness are shown in the inset. b. Thickness and load dependent friction forces for supported and suspended MoS2 nanoflakes. Raman spectroscopy has been a universal method to identify the layers and investigate the structural properties of 2D layered materials.30,31 Both suspended and supported MoS2 nanoflakes are chosen to obtain the Raman spectra. The Raman spectra curves are smoothly fitted with the Lorentz function. Obvious in-plane E12𝑔 and out-ofplane 𝐴1𝑔 vibration modes are observed. The frequency shifts between the E12𝑔 and the 𝐴1𝑔 vibration modes are shown in the inset of Figure 3b and this can verify the monolayer of the MoS2 nanoflake.23 The red shift of E12𝑔 and 𝐴1𝑔 vibration modes on suspended MoS2 is attributed to the release from the residual substrate strain or doping.32,33 The frequency differences of the suspended MoS2 are a little larger than those of the supported MoS2 because of the tensile strain over the hole and the increased out-of-plane vibration mode intensity is due to the absent substrate inhibitation.33


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Figure 3. a. Optical image of a suspended monolayer MoS2. b. Raman spectra of the monolayer MoS2. Both suspended and supported locations on the MoS2 nanflake are chosen. The effects of the hole shapes and dimensions on the friction characteristics of suspended MoS2 are also discussed. Figure 4a shows a monolayer MoS2 nanoflake suspended on the SiO2/Si substrate with different shapes and dimensions of holes. The parameters of the holes beneath the suspended MoS2 nanoflake are shown in Table S1. The dimensions of the circular and square holes for the large and small sizes are almost equivalent, respectively. The Raman spectra of monolayer and bilayer MoS2 nanoflakes suspended on circular and square holes are shown in Figure S6 and S7. Both the wavelengths of in-plane E12𝑔 and out-of-plane 𝐴1𝑔 vibration modes of suspended MoS2 display a red shift. However, the wavelengths of in-plane E12𝑔 and out-of-plane 𝐴1𝑔 vibration modes on circular and square holes are almost the same. This can indicate that there is no obvious difference of the mechanical properties of MoS2 nanoflakes suspended on the circular and square holes. The friction force on the suspended MoS2 is shown in Figure 4b and it shows an about ~30% increase of when the dimension of the hole varies from 800 nm to 1500 nm. This can be attributed to the smaller bending rigidity and worse structural stability of the suspended MoS2 on the bigger holes. The friction on the circular hole is similar to that on the square hole when the dimensions of the holes are almost equivalent. This indicates that the influence of shape on the friction is far less obvious than the dimensions (which dominates the bending rigidity). Another friction image of a suspended MoS2 nanoflake on different circular holes are shown in Figure S8. Another thickness effect on the friction of suspended MoS2 is shown in Figure S9.



Figure 4. a. Topographic AFM image of a suspended monolayer MoS2 nanoflake. b. Friction on the suspended MoS2 nanoflake with varying dimensions. CONCLUSION In summary, the frictional characteristics of mechanically exfoliated suspended and supported MoS2 are systematically investigated with AFM. The friction force on the suspended MoS2 is much larger than that on the supported MoS2, and it also shows a decreasing variation trend with the increase of layer numbers. For the very thin MoS2 nanoflakes, the easier formation of puckerings caused by the small bending rigidity and large out-of-plane deformation is mainly responsible for the large friction force on the suspended MoS2. The friction on the suspended bulk MoS2 is also much larger than that on the supported bulk MoS2 and this proves the interface effect on the frictional characteristics of MoS2. The dimensions of the holes are proved to have a larger influence on the friction of suspended MoS2 nanoflake than their shape. This work is expected to provide important guidance for the applications of MoS2-based nanomechanical systems. ASSOCIATED CONTENT Supporting Information SEM images of a suspended MoS2 nanoflake (Figure S1); 3D height and friction images of suspended and supported MoS2 nanoflakes (Figure S2); Height and friction images of a suspended monolayer MoS2 nanoflake (Figure S3); Thickness dependent Raman spectra differences of MoS2 nanoflakes (Figure S4); Sinking depth of suspended MoS2 nanoflakes on the holes with increasing thickness (Figure S5); Parameters of the suspended monolayer MoS2 nanoflakes (Table S1); Raman spectra of suspended and supported monolayer MoS2 (Figure S6); Raman spectra of suspended and supported bilayer MoS2 (Figure S7); Effect of hole dimensions on the friction of suspended MoS2 (Figure S8); Thickness dependent frictional properties of suspended MoS2 (Figure S9). AUTHOR INFORMATION Corresponding Authors *E-mail:[email protected] *E-mail:[email protected]


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ORCID Peng Huang: 0000-0003-0530-7338

Andres Castellanos-Gomez: 0000-0002-3384-3405 Dan Guo: 0000-0002-7681-2377 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (Grant Nos. 51527901, 51375255). The authors thank Prof. M.L. Li and Dr. H.S. Pang for providing the SiO2/Si substrates with holes. The authors also give thanks to Miss W.Q. Wang for the guidance in the AFM measurements. REFERENCES (1) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F., Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 13680513. (2) Yoon, Y.; Ganapathi, K.; Salahuddin, S., How Good Can Monolayer MoS2 Transistors Be? Nano lett. 2011, 11 (9), 3768-3773. (3) Bertolazzi, S.; Brivio, J.; Kis, A., Stretching and Breaking of Ultrathin MoS2. A. ACS Nano 2011, 5 (12), 9703-9709. (4) Castellanos-Gomez, A.; Poot, M.; Steele, G. A.; van der Zant, H. S. J.; Agrait, N.; Rubio-Bollinger, G. Elastic Properties of Freely Suspended MoS2 Nanosheets. Adv. Mater. 2012, 24 (6), 772-775. (5) Li, H.; Wang, J.; Gao, S.; Chen, Q.; Peng, L.; Liu, K.; Wei, X. Superlubricity Between MoS2 Monolayers. Adv. Mater. 2017, 29, 170147427. (6) Huang, Y.; Liu, L.; Lv, J.; Yang, J.; Sha, J.; Chen, Y., MoS2 Solid-lubricating Film Fabricated by Atomic Layer Deposition on Si Substrate. AIP Adv. 2018, 8, 0452164. (7) Ky, D. L. C.; Khac, B. T.; Le, C. T.; Kim, Y. S.; Chung, K., Friction Characteristics of Mechanically Exfoliated and CVD-grown Single-layer MoS2. Friction 2017, 1-12. (8) Lee, C.; Li, Q.; Kalb, W.; Liu, X.; Berger, H.; Carpick, R. W.; Hone, J., Frictional Characteristics of Atomically Thin Sheets. Science 2010, 328 (5974), 76-80. (9) Quereda, J.; Castellanos-Gomez, A.; Agrait, N.; Rubio-Bollinger, G., Single-layer MoS2 Roughness and Sliding Friction Quenching by Interaction with Atomically Flat Substrates. Appl. Phys. Lett. 2014, 105, 0531115. (10) Bien-Cuong, T. K.; DelRio, F. W.; Chung, K., Interfacial Strength and Surface Damage Characteristics of Atomically Thin h-BN, MoS2, and Graphene. ACS Appl. Mater. Inter. 2018, 10 (10), 9164-9177. (11) Li, S.; Li, Q.; Carpick, R. W.; Gumbsch, P.; Liu, X. Z.; Ding, X.; Sun, J.; Li, J., The Evolving Quality of Frictional Contact with Graphene. Nature 2016, 539 (7630), 541-546. (12) Deng, Z.; Klimov, N. N.; Solares, S. D.; Li, T.; Xu, H.; Cannara, R. J., Nanoscale Interfacial Friction and Adhesion on Supported versus Suspended Monolayer and Multilayer Graphene. Langmuir 2013, 29, (1), 235-243. (13) Sun, J.; Zhang, Y.; Lu, Z.; Li, Q.; Xue, Q.; Du, S.; Pu, J.; Wang, L., Superlubricity Enabled by Pressure-Induced Friction Collapse. J. Phys. Chem. Lett. 2018, 9 (10), 2554-2559. (14) Schumacher, A.; Kruse, N.; Prins, R.; Meyer, E.; Luthi, R.; Howald, L.; Guntherodt, H. J.; Scandella,



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Frictional Characteristics of Suspended MoS2

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