Revealing the Anomalous Tensile Properties of WS2 Nanotubes by in

Letter pubs.acs.org/NanoLett

Revealing the Anomalous Tensile Properties of WS2 Nanotubes by in Situ Transmission Electron Microscopy Dai-Ming Tang,*,†,‡ Xianlong Wei,†,‡ Ming-Sheng Wang,‡ Naoyuki Kawamoto,‡ Yoshio Bando,‡ Chunyi Zhi,‡ Masanori Mitome,‡ Alla Zak,§ Reshef Tenne,∥ and Dmitri Golberg*,‡ †

International Center for Young Scientists (ICYS) and ‡World Premier International (WPI) Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan § Faculty of Science, Holon Institute of Technology, Holon 58102, Israel ∥ Department of Materials and Interfaces, Weizmann Institute, Rehovot 76100, Israel S Supporting Information *

ABSTRACT: Mechanical properties and fracture behaviors of multiwalled WS2 nanotubes produced by large scale fluidized bed method were investigated under uniaxial tension using in situ transmission electron microscopy probing; these were directly correlated to the nanotube atomic structures. The tubes with the average outer diameter ∼40 nm sustained tensile force of ∼2949 nN and revealed fracture strength of ∼11.8 GPa. Surprisingly, these rather thick WS2 nanotubes could bear much higher loadings than the thin WS2 nanotubes with almost “defect-free” structures studied previously. In addition, the fracture strength of the “thick” nanotubes did not show common size dependent degradation when the tube diameters increased from ∼20 to ∼60 nm. HRTEM characterizations and real time observations revealed that the anomalous tensile properties are related to the intershell cross-linking and geometric constraints from the inverted cone-shaped tube cap structures, which resulted in the multishell loading and fracturing. KEYWORDS: WS2 nanotubes, mechanical properties, tensile, defect, cross-linking, in situ microscopy S2 nanotubes were the first synthesized inorganic nanotubes after the carbon ones.1,2 They consist of covalently bonded S−W−S coaxial shells that are weakly joined by the van der Waals forces, similar to the layered structure of carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs). Compared with the C and BN counterparts, WS2 nanotubes have a substantially different atomic structure, that is, each shell consists of three atomic layers rather than has a simple planar sp2 structure. Accordingly, the interlayer distance, ∼0.62 nm, is substantially larger than those of CNTs and BNNTs, ∼0.33 nm. WS2 nanotubes have demonstrated specific mechanical properties, associated with their unique structure.3−10 For example, they have a shock wave resistance superior to that of CNTs and can sustain shock waves up to 21 GPa.3 During the torsion tests, a unique stick−slip behavior was discovered and attributed to the commensurate atomic arrangement.8 Moreover, the mechanical properties under uniaxial tension have been assessed by in situ scanning electron microscopy (SEM); this reveals the unique combination of ultrahigh fracture strength (3.7−16.3 GPa) and large elastic deformations (5−14%).5 In addition, the mechanical behaviors have directly been observed by in situ transmission electron microscopy (TEM), confirming the “sword-in-sheath” fracture mechanism during tensile tests.9 The fact that the mechanical properties were approaching the theoretical limits suggested that some of the WS2 nanotubes were virtually free of defects, while others have discrete atomic defects (vacancies).5 Because

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© 2013 American Chemical Society

of the unique structure and mechanical properties, the mechanical applications of WS2 tubes have logically been expected, such as scanning probes,11 and solid lubricants.12 However, before the practical and large scale applications, there are still important issues remaining to be addressed. For example, it is generally believed that under tension multiwalled WS 2 nanotubes break along with the sword-in-sheath mechanism, that is, only the outermost shell is loaded and fractured, and the inner tube part may be pulled out in a “telescoping manner”.5,9 For such fracture mechanism, the shells are required to be absolutely parallel, atomically smooth, and to have identical cross-sectional shapes over the whole nanotube. Also, before the final fracture, there should be a difference in length, up to 5−14%, for the outermost shell and the inner unloaded nanotube portion, which is difficult to imagine and, indeed, has never been confirmed experimentally. In addition, while the common sword-in-sheath fracture mechanism is reasonable for the ideally structured and defect free “model” nanotubes with open ends, it is necessary to explore the details of the cracking processes under real time observations on “realistic” nanotubes (e.g., synthesized at a large scale) and having specific “imperfections”.13−16 Apparently, the latter tubes are most likely to be practically utilized. Received: November 16, 2012 Revised: February 5, 2013 Published: February 19, 2013 1034

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Figure 1. (a) Experimental configuration during the in situ TEM tensile tests. (b−d) TEM and HRTEM images, and EDX spectrum of the WS2 nanotubes, revealing the high crystallinity and high chemical purity. (e−g) TEM and HRTEM images of the representative layer defect in a WS2 nanotube. An edge dislocation is marked by the white dots in (g).

close to the probe and then focusing the electron beam at the probe/sample interface for decomposing the hydrocarbon vapor, and depositing carbon. During the EBID process, the electron doze was carefully controlled so as to reduce or entirely eliminate the sample damage. Experiments were carried out to assess the potential irradiation damage to the crystalline structure. As demonstrated in Supporting Information Figure S1, under the electron beam illumination with the electron density of 0.06 A/cm2 set by choosing the third condenser lens aperture (CLA-3) and the spot size No. 5 on the TEM (JEOL 3100FEF) used, the lattice fringes of a WS2 nanotube could be clearly identified; they are kept parallel and straight after irradiation over about half an hour. Since the EBID process lasted only about five minutes under the identical conditions, the irradiation damage to the sample could be neglected. Second, the sample was transferred into an atomic force microscopy (AFM)-TEM in situ holder (also Nanofactory Instruments AB) in which the other end of the WS2 nanotube was welded to the AFM cantilever using the same EBID technique. After that, the tungsten nanoprobe attached to a piezo-tube was retracted to apply a tensile force to the tube under the probe movement at a speed of ∼1 nm/s and at a strain rate of ∼10−4/s. To minimize the experimental error, the

Therefore, it is critically important to understand the tolerance and influences of defects in them on the mechanical properties. In the current work, we used an in situ TEM method to investigate the tensile properties of the regarded imperfect multiwalled WS 2 nanotubes. Our work enables direct correlation between the mechanical properties and the fracture mechanisms at the atomic level, because the property measurements and fracture kinetics were analyzed at the same fragment on a tube and at the same time. We found that the specific structures, such as the intershell cross-linking and inverted cone-shaped caps had important influences on the tube mechanics. It is interesting that due to the regarded “imperfections”, most of the present WS2 nanotubes fractured through multiple-shell breakage rather than via a single-shell failure, which resulted in a much higher fracture force and anomalous tensile strength. We used a two-step approach for the in situ TEM tensile tests.17−26 The experimental configuration is demonstrated in Figure 1a. First, a scanning tunneling microscopy (STM)-TEM in situ holder (Nanofactory Instruments AB) was used to choose an individual WS2 nanotube and then clamp it to a tungsten nanoprobe by an electron beam induced carbon deposition (EBID) method by putting a small amount of wax 1035

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Figure 2. Tensile testing of an individual WS2 nanotube with the outer three shells fractured. (a,b) TEM images of the nanotube before and after the test. The nanotube has an outer diameter ∼58.2 nm and a length ∼1.6 μm, which increases to ∼2.1 μm after fracture, clearly demonstrating a swordin-sheath fracture mechanism. (c) HRTEM image of the fractured end, demonstrating that there are three external layers broken. (d) Strain−stress curve recorded. The stress increased with the strain, basically following a nearly linear relationship, until the abrupt fracture at a strain of ∼4.0% and a stress of ∼8.9 GPa.

obviously they are severely bent. Also, as marked by the white spots in Figure 1g, an edge dislocation was formed. Apparently, the atomic stacking will be irregular at the bent area and the core of the dislocation. Another example is presented in Supporting Information Figure S2. The nanotube has a uniform diameter except for the area indicated by the arrow in Supporting Information Figure S2a. Enlarged images (Supporting Information Figure S2b,c) clearly revealed that cross-shell linking had been formed in the outer shells. The different structural features give us an opportunity to investigate the dependence of mechanical properties on the specific tube structures. The tensile testing results for a representative individual WS2 nanotube are demonstrated in Figure 2 and Supporting Information Movie S1. Initially, the nanotube has an outer diameter of ∼58.2 nm and a length of ∼1.6 μm (Figure 2a). After tension, the total length of the sample was increased to ∼2.1 μm (Figure 2b). Obviously, this nanotube fractured in a sword-in-sheath manner, which is in agreement with the previous publications,5,9 with its outer part broken and its inner part pulled out. However, as shown in a high-resolution TEM (HRTEM) image in Figure 2c, three tubular shells fractured simultaneously rather than only the outermost shell (as was reported before). Therefore, in our experiment the loading was applied not only to the outermost shell but also transferred to the inner part of the nanotube. This resulted in the multiple outer shells breakage. After the fracture, both the outer fractured shells and pulled out inner shells were kept intact (except the appeared fracture surfaces). Therefore, no plastic deformation occurred during the elongation, and the interactions between the outer and inner shells were rather weak. The strain−stress curve is presented in Figure 2d; the stress increased with the strain nearly linearly until the abrupt fracture, confirming the brittle failure. The fracture force was measured to be ∼3087 nN and the fracture strength was

nominal spring constant was calibrated before the test, which was 6.9 N/m, and the sample was carefully aligned with the cantilever. After the tests, the loading force was calculated by measuring the displacement of the cantilever. The stress was obtained by using the fractured cross-section area, which was calculated by the following formula: S = π[(do + d)2 − (di − d)2]/4, where do and di are the outer and inner diameters, respectively, and d is the interlayer spacing (0.62 nm). The length and elongation of the tubes were monitored by in situ video recording for the calculation of the tensile strain. The WS2 nanotubes were produced by a large-scale fluidizedbed method (Figure 1b).27 Compared with the “defect-free” WS2 nanotubes with a slender morphology (5−8 walls, 15−25 nm in diameter) that have been obtained during the laboratoryscale synthesis, the nanotubes studied in the current work are much thicker (20−60 nm in diameter, >20 walls), as presented in Figure 1c. They are highly crystalline, as indicated by the clear and straight (002) lattice fringes. In addition, the samples are chemically pure, as revealed by the energy-dispersive X-ray spectroscopy (EDX) analysis (Figure 1d). Only strong tungsten and sulfur signals were detected in the spectrum along with the peaks of carbon and copper coming from the TEM grid; the peak of oxygen was negligibly small. Distinct from the perfect structures of the thin WS2 nanotubes with open ends and parallel and straight shells, the present thick nanotubes have some specific characteristics noticed under more careful structural analysis. For example, diameter variations are common for such nanotubes. The external diameters of the nanotube in Figure 1b are ∼45.8 and ∼43.4 nm on its left- and right-hand sides, respectively. More importantly, layer defects were commonly detected along with the diameter variations, as shown in Figure 1e−g. The outer diameter of the nanotube was measured to be 41.4 and 47.3 nm at the lower and upper parts, respectively. HRTEM images of the transition part revealed that while the lattice fringes could be still clearly resolved, 1036

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calculated to be ∼8.9 GPa, while taking the fractured three layers as the cross-sectional area. The fracture strain was measured to be ∼4.0%. Therefore, the Young’s modulus was calculated to be ∼223 GPa, quite close to that of a bulk WS2 material (238 GPa). In order to investigate the intershell interactions and a load transfer between shells, the fractured nanotube was reconnected again by EBID method, and the tensile test was repeatedly carried out. The results are presented in Supporting Information Figure S3 and Movie S2. In this case, the total length of the sample was not changed after the test (Supporting Information Figure S3a,b). The fractured ends had the same contrast in the TEM image (Supporting Information Figure S3b), therefore all shells were broken. Such fracture mechanism was confirmed by HRTEM imaging of the fractured ends (Supporting Information Figure S3d). The fracture edge was perpendicular to the tube axis and possessed atomically rough steps, as marked with the white dots in Supporting Information Figure S3d. Such fracture morphology suggests that all three shells have been loaded and fractured simultaneously. The ultimate loading force was measured to be ∼3103 nN, and the fracture strength was calculated to be ∼9.2 GPa, almost the same figures as for the whole WS2 nanotube in Figure 2. Therefore, it is assumed that during the tensile tests of multiwalled WS2 nanotubes, the loading has been applied and transferred to the outer three shells. In total, we successfully carried out 20 tensile tests on individual WS2 nanotubes. Most of them fractured following the sword-in-sheath mode displaying a multishell fracture. There was only one case where the sole outmost shell fractured, as shown in the HRTEM images of the fractured shell and the pulled out inner part (Supporting Information Figure S4). The nanotube with an outer diameter ∼30.8 nm was found to be able to sustain a tensile force of ∼1053 nN and the corresponding fracture strength was ∼17.1 GPa. Another exceptional example is demonstrated in Supporting Information Figure S5. A WS2 nanotube with an outer diameter of ∼51.0 nm and a length ∼2.1 μm was tested (Supporting Information Figure S5a,b) with the ultimate force up to ∼3987 nN and an exceptionally high strength ∼19.6 GPa (Supporting Information Figure S5e). HRTEM of the fractured area revealed that the nanotube also fractured in a sword-in-sheath manner with the two outer shells failed (Supporting Information Figure S5c,d). However, in this case the sheath was severely deformed and partially broken, as indicated by a shape change, curving of the lattice fringes, and additional fringes in the middle part. It is noticed that the cap has a larger diameter and forms an inverted cone-shape. Tensile data for all 20 tests are summarized in Table 1; the fracture force and strength are shown in relation to the tube outer diameters and number of fractured shells. The diameters range from ∼20 to ∼60 nm, averaging at ∼40.4 nm. The maximum and minimum forces are ∼6550 and ∼950 nN, respectively, averaging at ∼2949 nN. The range for the tensile strengths is from ∼6.0 to ∼19.6 GPa with the average strength of ∼11.8 GPa. It is noticed that most of the nanotubes were fractured via multiple outer shell breakage. In fact, 18 nanotubes out of 20 were fractured with 2−5 shell failures, and only one case for a single shell breakage and one case for more than 5 shell breakage were documented. To better understand the mechanical properties of the present tubes, the ultimate force and strength were plotted against the tube diameter, as demonstrated in Figure 3, along with the published data from Kaplan-Ashiri et al.,5 where the “defect-free” multiwalled WS2 nanotubes from laboratory-scale

Table 1. Summary of the Tensile Properties of the WS2 Nanotubes sample number

outer diameter (nm)

fractured shells

fracture force (nN)

strength (GPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Average

33.6 33.7 33.8 29.3 30.8 31.2 28.2 25.1 21.0 49.1 43.4 43.0 58.2 57.0 54.5 57.0 50.5 44.5 33.3 51.0 40.4

4 5 3 4 1 2 2 2 2 5 5 10 3 3 3 3 5 3 2 2 3.5

1520 2977 3492 1857 1053 2270 1349 987 956 3622 5135 6551 3087 3103 2827 4043 5076 3526 1560 3987 2949

6.0 9.6 17.9 8.5 17.1 18.6 12.3 10.1 11.9 7.7 12.5 8.8 8.9 9.2 8.8 12.0 10.5 13.5 11.9 19.6 11.8

synthesis were tested using an in situ SEM tensile method. One obvious difference (besides the synthesis route) is a diameter range that was 20−60 nm in the current work and 10−35 nm in the study by Kaplan-Ashiri et al. The ultimate force in the paper by Kaplan-Ashiri et al. was in the range from 114 to 783 nN, averaging at ∼460 nN, obviously much lower than that in our work. More importantly, in their work the ultimate forces were found to decrease when the diameter became larger than 30 nm. This could be understood in terms that the fracture of WS2 nanotubes was governed by the distribution of defects,5 and bigger nanotubes had more chances to have such defects.28 The measured forces in our work were in an agreement with the data by Kaplan-Ashiri et al. in the diameter range of 20−30 nm and then kept on increasing until over 6000 nN for the tube diameter of ∼45 nm. The comparison of strength values for the current work and the published data is presented in Figure 3b. Again, in the range of 20−30 nm the data from the two studies is in a good agreement. However, when the diameter of the nanotubes increased, surprisingly, the strength decreased only slightly, rather than decreased dramatically following the common size dependence. The average fracture strength was calculated to be ∼11.8 GPa, only slightly lower than that in the paper by Kaplan-Ashiri et al. (∼12.9 GPa), even when the average tube diameter increased from ∼20 to ∼40 nm. The unique combination of high loading force and high strength makes the thick WS2 nanotubes promising for the mechanical applications. The fracture mechanism of thin WS2 nanotubes was thoroughly investigated,5 and this could be described as illustrated in Figure 3c. The thin WS2 nanotubes are highly crystalline, and usually they have open ends. Under uniaxial tension, loading force is applied to the outermost shell. Under critical stress, cracks will nucleate from the positions where vacancies exist and propagate transversely through the outermost shell. Also, the inner part will be pulled out like a sword in sheath. The fracture strength is governed by the size of the defects (missing atoms at the vacancy sites). The fact 1037

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Figure 3. Plots of the ultimate tensile force (a) and strength (b) of WS2 nanotubes against their outer diameter. Our in situ TEM measurement results (red dots) are demonstrated along with the published data by Kaplan-Ashiri et al.5 (black triangles) obtained by in situ SEM tensile method. The data boundaries are connected with colored lines as a guide to the eye. (c) Schematics of the sword-in-sheath fracture mechanism for the openended thin WS2 nanotubes. (d,e) Schematics of the intershell cross-linking (marked with red arrow) and geometric constraint effects (marked with black arrows) for the thick WS2 nanotubes.

Figure 4. In situ TEM observation of the crack nucleation and its propagation in a WS2 nanotube under uniaxial tension. (a) TEM image of the WS2 nanotube at the initial stage. The nanotube has a length of ∼2.1 μm, and the outer diameter of ∼39.1 nm. (b−d) HRTEM images of the cracking process, the running time is marked. The corresponding force curve against time is shown in (e). After a linear increase to ∼1200 nN, the force increase was intentionally slowed down so as to observe the fracture process in detail. Schematic images of the fracturing process are demonstrated in (f−i).

that the strength values have a notable scatter with the highest numbers close to the theoretical limit implies that some of the thin nanotubes are defect-free while the others have discrete atomic defects with 1 to 21 atoms missing.5 In our study, it was found that the thick nanotubes had comparable fracture strength with the thin WS2 nanotubes. This could be understood in terms that the thick nanotubes also have very

high crystallinity, as demonstrated by the parallel and straight lattice fringes in Figure 1c, and the loading capacity of each layer is as high as for the layers in thin nanotubes. However, the ultimate loading force was found to be much higher than in case of the thin nanotubes, which means that more than one layer is loaded and the multishell fracture occurs. The structural origin could be confirmed by the intershell cross-linking 1038

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interesting that the imperfections in the nanotubes actually enhance their overall mechanical properties. Similar phenomena have been reported in the well-studied CNTs and BNNTs. For example, electron irradiation was introduced for the multiwalled CNTs to generate cross-linking between the shells, which resulted in a ∼11.6 times increase of loading force although the fracture strength was reduced to ∼60% compared to nonirradiated samples.29 Because of the enhanced interwall interactions and the irregular cross sections of BNNTs, they were able to bear loading force as high as 4500 nN,22 much higher than that of CNTs. In addition, a geometric strengthening effect was discovered in the BN bamboo-shaped nanotubes. Because of the interlocked geometry, the BN nanobamboos, made of short BNNT segments and bonded by van der Waals forces, could withstand a tensile stress up to ∼8.0 GPa.23 The uniqueness of the WS2 nanotubes in the current work can be explained as follows: while intershell cross-linking does exist and enhances the loading force, the atomic structures remain highly crystalline, so that the ultimate strength is not reduced. To summarize, tensile properties of multiwalled WS2 nanotubes were analyzed under direct in situ TEM tests. It was found that the thick WS2 nanotubes synthesized using a large scale production method could sustain a much higher tensile force than “defect-free” thin nanotubes produced at the laboratory scale, also they were not prone to a common degradation of fracture strength as the tube diameter increased from ∼20 to ∼60 nm. High-resolution imaging and video recording demonstrated that due to intershell cross-linking and inverted cone-shaped cap structure the loading on the outermost shell could effectively be transferred to the inner shells and resulted in the multishell fracturing process. The mechanical performance enhancement due to the structural imperfections discovered in our work should provide new inspirations toward mechanical design and applications of WS2 nanotubes.

(Figure 1e−g, Supporting Information Figure S2) and irregular cap structures (Supporting Information Figure S5). As shown by the schematics in Figure 3d, the intershell cross-linking virtually holds several shells together. Even though the loading force is applied only to the outermost shell, the force could be transferred to the inner shells. In the other example, as shown in Figure 3e, due to the inverted cone-shaped tube cap structure, during the elastic deformation, compressive force will be generated at the inclined interface,23 which will also transfer the loading force to the inner shells. Even if the crack is nucleated at the outermost shell, it is obvious that the inner shells have to overcome the geometrical constraint when they are pulled out. Such effect is clearly demonstrated by the severe deformation and damage of the outer shells in Supporting Information Figure S5. The detailed fracture process was observed in real time under high resolution, as demonstrated in Figure 4 and Supporting Information Movie S3. The tested WS2 nanotube was ∼39.1 nm in diameter and ∼2.1 μm in length. To reveal the cracking process, the loading was intentionally slowed down after a linear force increase to ∼1200 nN (Figure 4e). Therefore, the structural evolution was captured instead of the abrupt fracture processes seen in the previous experiments (those proceeded too fast to be recorded). As one can see in Supporting Information Movie S3, although the external loading was only applied to the outermost shell, at the beginning (Figure 4a,b) the inner shells moved together and no slip between the shells took place, indicating that the loading could effectively be transferred to the inner part of the nanotube. During this stage, clear and straight lattice fringes could be resolved and no defects could be observed, indicating purely elastic deformation. As the loading accumulated and came to a ∼1500 nN force (Figure 4c), a crack between the second and third outermost shells was visually captured (indicated by an arrow). It is interesting that the crack propagated along the inner-shell spacing besides its move within the cracked shells. After the initial cracking and crack propagation, the nanotube failed within a few seconds (Figure 4d), leaving the two outer shells fractured and the inner part of the nanotube pulled out, and thus elastically relaxed. The fracture process is demonstrated in the schematics in Figure 4f−i. As proposed above, under uniaxial tension the nanotube could be divided into two parts marked in red and green (Figure 4f). The loading is applied to the red outer parts and transferred within these shells in this part because of the intershell cross-linking, while the interaction between the red and green parts is relatively weak because the intershell cross-linking mainly occurs in the outer several shells, as demonstrated in Supporting Information Figure S2. When the loading reaches a critical value, the red part cracks (Figure 4g). Because of the multiple shells in the loaded red part, the required force must be very high, several times of that for a single shell. Because of the high tube crystallinity (Figure 1c), the ultimate tensile strength should be a considerable fraction of the theoretical limit. After that, the crack will propagate within the fractured red part and along the weakly bonded interface with the green unloaded part (Figure 4h) until finally the loaded part completely ruptures and the inner part is pulled out (Figure 4i). Above discussions have shown that due to the high crystallinity and presence of intershell cross-linking, the thick WS2 nanotubes hold almost equal fracture strength and much higher ultimate loading force compared with the thin and almost defect-free WS2 nanotubes studied previously. It is quite

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ASSOCIATED CONTENT

S Supporting Information *

In situ TEM videos of the tensile tests on a single WS2 nanotube (Movie S1), tube outer shells (Movie S2), and the crack initiation and propagation processes (Movie S3); electron beam damage assessments (Figure S1); TEM images of the intershell cross-linking of a WS2 nanotube (Figure S2); tensile test results of the WS2 nanotubes with three outer shells fractured (Figure S3), with only one shell fractured (Figure S4), and with an inverted cone-shaped cap structure (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (D.-M.T.) [email protected]; (D.G.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the World Premier International (WPI) Center for Materials Nanoarchitectonics (MANA) and the International Center for Young Scientists (ICYS) of the National Institute for Materials Science (NIMS). We thank 1039

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“NanoMaterials Ltd.” for synthesizing and providing the WS2 nanotubes.

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REFERENCES

(1) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444−446. (2) Tenne, R. Nat. Nanotechnol. 2006, 1, 103−111. (3) Zhu, Y. Q.; Sekine, T.; Brigatti, K. S.; Firth, S.; Tenne, R.; Rosentsveig, R.; Kroto, H. W.; Walton, D. R. M. J. Am. Chem. Soc. 2003, 125, 1329−1333. (4) Kaplan-Ashiri, I.; Cohen, S. R.; Gartsman, K.; Rosentsveig, R.; Seifert, G.; Tenne, R. J. Mater. Res. 2004, 19, 454−459. (5) Kaplan-Ashiri, I.; Cohen, S. R.; Gartsman, K.; Ivanovskaya, V.; Heine, T.; Seifert, G.; Wiesel, I.; Wagner, H. D.; Tenne, R. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 523−528. (6) Kaplan-Ashiri, I.; Cohen, S. R.; Apter, N.; Wang, Y.; Seifert, G.; Wagner, H. D.; Tenne, R. J. Phys. Chem. C 2007, 111, 8432−8436. (7) Kaplan-Ashiri, I.; Tenne, R. J. Clust. Sci. 2007, 18, 549−563. (8) Nagapriya, K. S.; Goldbart, O.; Kaplan-Ashiri, I.; Seifert, G.; Tenne, R.; Joselevich, E. Phys. Rev. Lett. 2008, 101, 195501. (9) Wang, M.; Kaplan-Ashiri, I.; Wei, X.; Rosentsveig, R.; Wagner, H.; Tenne, R.; Peng, L. Nano Res. 2008, 1, 22−31. (10) Xu, T.; Chen, Q.; Zhang, C.; Ran, K.; Wang, J.; Rosentsveig, R.; Tenne, R. Nanoscale 2012, 4, 7825−7831. (11) Rothschild, A.; Cohen, S. R.; Tenne, R. Appl. Phys. Lett. 1999, 75, 4025−4027. (12) Rapoport, L.; Fleischer, N.; Tenne, R. J. Mater. Chem. 2005, 15, 1782−1788. (13) Rothschild, A.; Popovitz-Biro, R.; Lourie, O.; Tenne, R. J. Phys. Chem. B 2000, 104, 8976−8981. (14) Zhu, Y. Q.; Hsu, W. K.; Terrones, H.; Grobert, N.; Chang, B. H.; Terrones, M.; Wei, B. Q.; Kroto, H. W.; Walton, D. R. M.; Boothroyd, C. B.; Kinloch, I.; Chen, G. Z.; Windle, A. H.; Fray, D. J. J. Mater. Chem. 2000, 10, 2570−2577. (15) Krause, M.; Mücklich, A.; Zak, A.; Seifert, G.; Gemming, S. Phys. Status Solidi B 2011, 248, 2716−2719. (16) Houben, L.; Enyashin, A. N.; Feldman, Y.; Rosentsveig, R.; Stroppa, D. G.; Bar-Sadan, M. J. Phys. Chem. C 2012, 116, 24350− 24357. (17) Wang, M.-S.; Bando, Y.; Rodríguez-Manzo, J. A.; Banhart, F.; Golberg, D. ACS Nano 2009, 3, 2632−2638. (18) Rodríguez-Manzo, J. A.; Wang, M. S.; Banhart, F.; Bando, Y.; Golberg, D. Adv. Mater. 2009, 21, 4477−4482. (19) Wang, M. S.; Golberg, D.; Bando, Y. Adv. Mater. 2010, 22, 93− 98. (20) Wang, M. S.; Golberg, D.; Bando, Y. Adv. Mater. 2010, 22, 4071−4075. (21) Wang, M. S.; Golberg, D.; Bando, Y. Adv. Mater. 2010, 22, 5350−5355. (22) Wei, X.; Wang, M. S.; Bando, Y.; Golberg, D. Adv. Mater. 2010, 22, 4895−4899. (23) Tang, D. M.; Ren, C. L.; Wei, X. L.; Wang, M. S.; Liu, C.; Bando, Y.; Golberg, D. ACS Nano 2011, 5, 7362−7368. (24) Golberg, D.; Costa, P. M. F. J.; Wang, M.-S.; Wei, X.; Tang, D.M.; Xu, Z.; Huang, Y.; Gautam, U. K.; Liu, B.; Zeng, H.; Kawamoto, N.; Zhi, C.; Mitome, M.; Bando, Y. Adv. Mater. 2012, 24, 177−194. (25) Tang, D.-M.; Ren, C.-L.; Wang, M.-S.; Wei, X.; Kawamoto, N.; Liu, C.; Bando, Y.; Mitome, M.; Fukata, N.; Golberg, D. Nano Lett. 2012, 12, 1898−1904. (26) Yamaguchi, M.; Tang, D.-M.; Zhi, C.; Bando, Y.; Shtansky, D.; Golberg, D. Acta Mater. 2012, 60, 6213−6222. (27) Zak, A.; Sallacan-Ecker, L.; Margolin, A.; Feldman, Y.; PopovitzBiro, R.; Albu-Yaron, A.; Genut, M.; Tenne, R. Fullerenes, Nanotubes, Carbon Nanostruct. 2011, 19, 18−26. (28) Griffith, A. A. Philos. Trans. R. Soc. London, Ser. A 1921, 221, 163−198. (29) Peng, B.; Locascio, M.; Zapol, P.; Li, S.; Mielke, S. L.; Schatz, G. C.; Espinosa, H. D. Nat. Nanotechnol. 2008, 3, 626−631. 1040

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