Exceptional Mechanical Properties of Phase-Separation-Free Mo3Se3

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Exceptional Mechanical Properties of Phase-Separation-Free MoSe Chain-Reinforced Hydrogel Prepared by Polymer Wrapping Process Si Hyun Kim, Seungbae Oh, Sudong Chae, Jin Woong Lee, Kyung Hwan Choi, Kyung Eun Lee, Jongwha Chang, Liyi Shi, Jae-Young Choi, and Jung Heon Lee Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02343 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 1, 2019

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Exceptional Mechanical Properties of PhaseSeparation-Free Mo3Se3--Chain-Reinforced Hydrogel Prepared by Polymer Wrapping Process Si Hyun Kim1,, Seungbae Oh2,, Sudong Chae2, Jin Woong Lee2, Kyung Hwan Choi1, Kyung Eun Lee3, Jongwha Chang4, Liyi Shi5, Jae-Young Choi1,2,*, Jung Heon Lee1,2,6,* 1SKKU

Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU),

Suwon, Gyeonggi 16419, Republic of Korea 2School

of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU),

Suwon, Gyeonggi 16419, Republic of Korea Biomedical Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792,

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Republic of Korea 4School

of Pharmacy, University of Texas, El Paso, TX 79968, USA

5Research

Center of Nanoscience and Nanotechnology, Shanghai University, Shanghai 200444,

China 6Biomedical

Institute for Convergence at SKKU (BICS), Sungkyunkwan University (SKKU),

Suwon, Gyeonggi 16419, Republic of Korea

ACS Paragon Plus Environment

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KEYWORDS Mo3Se3- chain nanowire, mechanical properties, natural hydrogel, gelatin, composite

ABSTRACT

As Mo3Se3- chain nanowires have dimensions comparable to those of natural hydrogel chains (molecular-level diameters of ~0.6 nm and lengths of several micrometers) and excellent mechanical strength and flexibility, they have large potential to reinforce hydrogels and improve their mechanical properties. When a Mo3Se3--chain-nanowire-gelatin composite hydrogel is prepared simply by mixing Mo3Se3- nanowires with gelatin, phase separation of the Mo3Se3nanowires from the gelatin matrix occurs in the micro-network, providing only small improvements in their mechanical properties. In contrast, when the surface of the Mo3Se3nanowire is wrapped with the gelatin polymer, the chemical compatibility of the Mo3Se3- nanowire with the gelatin matrix is significantly improved, which enables the fabrication of a phaseseparation-free Mo3Se3--reinforced gelatin hydrogel. The composite gelatin hydrogel exhibits significantly improved mechanical properties, including a tensile strength of 27.6 kPa, fracture toughness of 26.9 kJ/m3, and elastic modulus of 54.8 kPa, which are 367 %, 868 %, and 378 % higher than those of the pure gelatin hydrogel, respectively. Furthermore, the amount of Mo3Se3nanowires added in the composite hydrogel is as low as 0.01 wt%. The improvements in the mechanical properties are significantly larger than those for other reported composite hydrogels reinforced with one-dimensional materials.


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Introduction Hydrogel is a porous three-dimensional (3D) material composed of hydrophilic polymer networks and large amount of water.1 As many properties of hydrogels are similar to those of human tissues,1 they have been widely used for biomedical applications including biosensing,2-4 drug delivery,5-6 tissue engineering,7-9 and regenerative medicine.1 Particularly, natural hydrogels have been extensively used in biomedical fields owing to their simple preparations and excellent biocompatibilities.10-11 Despite these advantages, the natural hydrogels are limited by their low mechanical strengths and fragility.12 Although synthetic hydrogels can be used as alternatives,13 they lack bioactive functional groups and lead to toxicity issues with crosslinking agents.14 Therefore, it is required to develop a new strategy that can enhance the mechanical properties of natural hydrogels. Due to their unique thermal, electrical, optical, and mechanical of properties,15-17 onedimensional (1D) nanomaterials, such as Ag nanowires, Au nanowires, Au nanorods, carbon nanotubes, can be used with other materials as composite materials to develop interesting opportunities in new directions.18-42 Especially, 1D nanomaterials have been considered as potential reinforcement materials to enhance the mechanical properties of hydrogels owing to their nanoscale dimensions, large surface areas, and excellent mechanical properties. However, in most cases, the hydrogel composites exhibit limited improvements in their mechanical properties compared with those of the original hydrogels.43-44 Regarding the composite fabrication, intimate mixing and homogeneous distribution of the reinforcement materials in the matrix materials are required to obtain excellent mechanical properties. Therefore, it is very important to choose a 1D nanomaterial that satisfies these conditions in hydrogels.


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Recently, studies on 1D single chain nanowires such as LiMo3Se3,45-49 Mo6S9-xIx,50 NaNb2PS10,51 Te,52 Nb2Se9,53 and V2Se9,54 revealed their unique properties, such as high thermal stabilities, small band gaps, and large surface areas. In addition, the electronic structure of the carbon nanotube is determined by its chirality, while single chain nanowires exhibit identical electronic structures. Moreover, bulk 1D crystals can be produced with earth-abundant materials through several methods, such as solid-state reaction and flux method.53-54 It is worth noting that well-dispersed nanowires have diameters of ~1 nm, similar to the dimensions of the hydrogel matrix polymers, while most nanomaterials have dimensions significantly larger than those of the polymers.53-54 For example, a well-dispersed V2Se9 nanowire is chain shaped with a diameter of 1 nm and length of several micrometers.54 However, even though many types of single chain nanowires could be considered as new classes of reinforcement materials, only few related studies have been reported. In this paper, we report the fabrication of Mo3Se3--chain-nanowire-reinforced natural hydrogels by the polymer wrapping method (Figure 1a–d). An aqueous solution of Mo3Se3- nanowires was prepared by ion exchange of sonicated LiMo3Se3 nanorods in water. According to our previous report, the ion exchange chromatography can remove more than 99.98 % Li+ ion in LiMo3Se3 aqueous solution.55 The Mo3Se3- nanowire has a molecular-level diameter of ~0.6 nm and length of several micrometers, comparable to the diameter of the natural hydrogel chain.55-57 In addition, it has excellent mechanical strength such as elastic constant of 320 GPa (higher than steel) and flexibility.58 When the aqueous solution of Mo3Se3- nanowires was mixed directly with a gelatin solution, phase separation of Mo3Se3- nanowires from the gelatin matrix occurred in the composite structure, leading only to small improvements in their mechanical properties. In order to increase the chemical compatibility of the Mo3Se3- nanowire with gelatin, the surface of the Mo3Se3nanowire was coated with a small amount of gelatin polymer. Subsequently, the resulting gelatin-


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coated Mo3Se3- nanowire was mixed with an additional gelatin solution and thermally cured, yielding a phase-separation-free Mo3Se3--reinforced gelatin. This composite gelatin hydrogel exhibited significantly improved mechanical properties, including a tensile strength of 27.6 kPa, fracture toughness of 26.9 kJ/m3, and elastic modulus of 54.8 kPa, which are 3.7, 8.7, and 3.8 times higher than those of the pure gelatin hydrogel, respectively, with high reproducibility. In addition, the amount of added Mo3Se3- nanowires in the composite was as low as 0.01 wt%. The improvements in the mechanical properties are significantly larger than those of other composite hydrogels reinforced with 1D nanomaterials.18-38

Results and discussion A bulk 1D LiMo3Se3 crystal was synthesized by a solid-state reaction, followed by a substitution reaction through chemical vapor transport (See the supporting information for experimental detail, Table S1).55,59-60 The LiMo3Se3 crystal consists of Li+ cations and Mo3Se3- molecular chains with negative charges (Figure 1e). The Mo3Se3- chains are aligned along the Z axis, while the Li+ ions are located between the chains. The LiMo3Se3 crystals are obtained as black powders (Figure 1f) having rod-like shapes with diameters of 500–1,000 nm and lengths of 5–10 m (Figure 1g). Once the LiMo3Se3 crystal is dissolved in water, the Li+ ions are solvated by the water and the Mo3Se3atomic chains separated by sonication well disperse by the repulsive forces on their negatively charged surfaces (Figure 1h). The solution is dark red after exfoliation (see inset of Figure 1i). The morphologies and dimensions of the Mo3Se3- nanowires analyzed by TEM (Figure 1i) and AFM (Figure 1j) show that most LiMo3Se3 nanorods were exfoliated into Mo3Se3- chains having diameters smaller than 1 nm after ion exchange chromatography, as reported previously.55


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Gelatin type A, obtained by a partial hydrolysis of collagen derived from porcine skin, white connective tissues, and bones, is used as the natural hydrogel in this study. The gelatin produced from an acid-treated precursor is of type A and has a net positive charge at a low pH. We chose the Mo3Se3- nanowire with a negative charge on the chain surface owing to its high biocompatibility.61 The Mo3Se3--reinforced gelatin was prepared by adding the gelatin powder in the ion-exchanged Mo3Se3- nanowire solution and curing the mixed solution at 70 °C (Figure 2a; see Supporting Information for a detailed Experimental Section). The Mo3Se3--reinforced gelatin was brown owing to the dark-red color of the Mo3Se3- nanowire solution. The mechanical properties of the Mo3Se3--reinforced gelatin were measured using a universal testing machine (UTM; QC-508E). With the increase in the amount of Mo3Se3- nanowires in the composite gelatin in the range of 0.00 to 0.05 wt%, a gradual increase in the tensile strength was observed compared to that of the pure gelatin (Figure 2b). However, the fracture strain of the composite gelatin decreased with the increase in the amount of Mo3Se3- nanowires. The reduction in fracture strain could be related to the micro-network structure characteristics of the gelatin composite, which has been also reported for other composite hydrogels.62-63 In order to investigate the micro-network of the Mo3Se3--reinforced gelatin, the cross section of the freeze-dried composite gelatin was observed by SEM and compared with that of the pure gelatin hydrogel (Figure S1). As shown in Figure 2c and S1, both Mo3Se3--reinforced and pure gelatin samples have continuous fibrillar 3D network structures, typical for a hydrogel. The pure gelatin hydrogel has open pores, while the Mo3Se3--reinforced gelatin hydrogel has many micronetwork substances around the pores, which were identified by energy-dispersive X-ray spectroscopy as entanglement of Mo3Se3- nanowires (Figure S2). This indicates that the Mo3Se3nanowires and gelatin were not homogeneously distributed during the mixing and gelation


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processes, but were separated to their own phases (Figure 2d). It is well known that the surfaces of reinforcement materials should be chemically compatible with the matrix polymer to provide a homogeneous distribution of the reinforcement materials in the composite (Figure 2e).64 The Mo3Se3- nanowire has a negative charge on the chain surface, whereas gelatin has negative as well as positive residues yielding amphoteric characteristics and net positive charge. Therefore, the different surface characteristics of the Mo3Se3- nanowires from those of gelatin A caused phase separation of the nanowires from the matrix hydrogel. Several approaches have been suggested to modify the surfaces of nanomaterials through covalent and noncovalent modifications.65 As the covalent modification has been reported to affect the inherent mechanical, electrical, and optical properties of the nanomaterials,66 in this study, we focused on the noncovalent modification, which could successfully modify the Mo3Se3- nanowire surface without the formation of chemical bonds. Polymer wrapping of nanoparticles is one of the noncovalent modification methods effective to provide a nanoparticle surface chemically compatible with the polymer matrix.67-68 If nanoparticles are wrapped with short molecules or polymers having similar structures with those of the matrix molecules or polymers, the enthalpic interaction between the nanoparticle surface and organic or polymeric matrix is minimized, which yields homogeneously distributed nanoparticles in the matrix.69 In this study, type-A gelatin was selected as a wrapping polymer to increase the chemical compatibility and decrease the phase separation during the gelation between the Mo3Se3- nanowire surface and type-A gelatin matrix. The type-A gelatin polymer has positively charged side chains from arginine, lysine, hydroxylysine, and histidine residues, which are expected to strongly anchor to the negatively charged Mo3Se3- surface (Figure 2g, Figure S3, and Table S2).70 In addition, the type-A gelatin adsorbed on the surface of Mo3Se3- is expected to yield a good steric hindrance


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against the Mo3Se3- nanowire and good chemical compatibility with the type-A gelatin matrix (Figure 2f). To modify the surface of the Mo3Se3- nanowire with the type-A gelatin molecules, a gelatin polymer solution (with various concentrations) was poured into the Mo3Se3- nanowire solution. The resulting mixture solution was aged for 2 h under a constant stirring at 400 rpm (see Experimental Section in Supporting Information). Figure 2h shows the -potentials of the gelatinwrapped Mo3Se3- nanowires at various ratios of gelatin polymer/Mo3Se3- nanowire after the aging. The -potential of the pure Mo3Se3- nanowire was -48.1 mV, which became positive with the increase in the amount of polymer, and finally saturated at 7.2 mV at a gelatin/Mo3Se3- ratio of 25. These results indicate that the negative Mo3Se3- nanowires were wrapped with positive gelatin polymers during the wrapping process (Figure S4). The TEM image shows that at the gelatin/Mo3Se3- ratio of 25, the Mo3Se3- nanowire is fully wrapped with the gelatin polymer (Figure 2i and S5).

As the gelatin-wrapped Mo3Se3- nanowires were well dispersed and stable for a long time in the aqueous solution, they were used for the fabrication of the Mo3Se3- nanowire-reinforced gelatin hydrogel. Solutions of gelatin-wrapped Mo3Se3- nanowires prepared at different gelatin/Mo3Se3ratios were mixed with an additional gelatin polymer while maintaining the final Mo3Se3- nanowire concentration of 0.01 wt%. The resultant mixed solution was thermally cured at 70 °C to produce the Mo3Se3--reinforced gelatin hydrogel. The evaluation of the mechanical properties of the reinforced hydrogels showed that both tensile strength and fracture strain had maximum values at the gelatin/Mo3Se3- ratio of 25 (Figure S6). As this is an optimal ratio between the gelatin and Mo3Se3- nanowires with good mechanical properties, it was employed in further experiments.


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Based on the optimal gelatin wrapping ratio of the Mo3Se3- nanowires, the effects of the concentration of the Mo3Se3- nanowires on the mechanical properties of the reinforced hydrogel were investigated (Figure 3a). Figure 3b shows tensile stress–strain curves of the reinforced hydrogels with various concentrations of Mo3Se3- nanowires in the range of 0.00 to 0.05 wt%. With the increase in the amount of Mo3Se3- up to 0.01 wt%, both tensile strength and strain increased. It should be noted that not only the tensile strength but also the fracture strain increased for all samples prepared using the gelatin-wrapped Mo3Se3- nanowires, whereas the fracture strains of the samples prepared without the wrapping process decreased (Figure 2b). Above the Mo3Se3nanowire concentration of 0.01 wt%, both tensile strength and fracture strain started to decrease. The maximum tensile strength of the composite hydrogel at the optimal concentration of Mo3Se3was 27.6 kPa, which is approximately 367 % of that of the pure gelatin hydrogel (Figure 3c). The maximum toughness of the samples, obtained from the areal integration of the tensile stress–strain curves, was 26.9 kJ/m3, which is approximately 868 % of that of the pure hydrogel (Figure 3d). The elastic modulus (another important mechanical property) of the composite hydrogel also had a maximum value of 54.8 kPa at the Mo3Se3- nanowire concentration of 0.01 wt%, which is approximately 378 % of that of the bare gelatin hydrogel (Figure 3e). All of these mechanical properties of the composite hydrogel were highly reproducible (Figure S7 and S8). Although Mo3Se3- is reported to have issues with oxidation59, we did not find significant changes in the mechanical properties when we stored them in a refrigerated condition up to 5 days. When we stored them at room temperature, we started to observe some decrease of the mechanical properties after 3 days (see Figure S10-11). This suggests that our composite hydrogel does not have significant issue with stability.


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To elucidate the origin of the very large enhancements in the mechanical properties of the composite hydrogels, the microstructures of the freeze-dried composite samples were analyzed by SEM. Figure 3f and g shows cross-section microstructures of samples prepared with Mo3Se3concentrations of 0.003 and 0.01 wt%. The composite hydrogels had continuous fibrillar 3D network structures with no entangled Mo3Se3- separated from the matrix hydrogel, as in the pure hydrogel, indicating that most gelatin-wrapped Mo3Se3- nanowires were successfully incorporated into the gelatin hydrogel during the mixing and curing. In contrast, the sample with a Mo3Se3concentration of 0.05 wt% (Figure 3h), which exhibited reduced mechanical properties compared to the maximum values, had segregated Mo3Se3- nanowires around gelatin pores, as in the hydrogel composite prepared without gelatin-wrapped Mo3Se3- nanowires (Figure 2c). This shows that there are optimal concentration ranges of the gelatin-wrapped Mo3Se3- nanowires to produce a homogeneous structure of the gelatin hydrogel composite.

Finally, the improvements in the mechanical properties of the Mo3Se3--reinforced gelatin hydrogels were compared with those of reported composite natural hydrogels (Figure 4). The maximum increases in the tensile strength, elastic modulus, and toughness obtained for the Mo3Se3--reinforced gelatin hydrogel was 367 %, 378 %, and 868 %, respectively, which are considerably higher than those of most of the reported composite hydrogels. It is worth noting that the simultaneous increases in the tensile strength and tensile strain of the reinforced hydrogel, unlike in the other composites, provide a large enhancement in the toughness. This suggests that its inorganic characteristics, dimensions comparable to those of molecules, flexible mechanical properties, large surface area, and capability to interact with biomolecules make the Mo3Se3- chain nanowire an ideal nanomaterial to enhance the mechanical properties of natural hydrogels.


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Conclusion In conclusion, we developed a novel method to fabricate a phase-separation-free composite natural hydrogel using Mo3Se3- chain nanowires having dimensions similar to those of hydrogel matrix polymers. When the surface of the Mo3Se3- nanowire was wrapped with the gelatin polymer, the chemical compatibility of the Mo3Se3- nanowire with the gelatin matrix significantly improved, which enabled the fabrication of the phase-separation-free Mo3Se3--reinforced gelatin hydrogel. The phase-separation-free composite gelatin hydrogel exhibited very large improvements in the mechanical properties compared to those of the pure gelatin hydrogel. The improvements in the mechanical properties are significantly larger than those of other reported composite hydrogels reinforced with 1D materials.


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Figure 1. (a–d) Schematics of the phase-separation-free Mo3Se3--chain-nanowire-reinforced gelatin hydrogel: (a) Mo3Se3--chain-nanowire-reinforced gelatin hydrogel, (b) porous matrix of the phase-separation-free composite hydrogel, (c) intimate mixing and homogeneous distribution of Mo3Se3- nanowires and gelatin in the hydrogel matrix, and (d) crystal structure of the Mo3Se3nanowire. (e) Schematic of a single-crystalline LiMo3Se3 nanorod, (f) image of a LiMo3Se3 singlecrystal powder, (g) scanning electron microscopy (SEM) image of single-crystalline LiMo3Se3 nanorods, (h) schematic of dispersed Mo3Se3- chain nanowires, (i) transmission electron


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microscopy (TEM) image of dispersed Mo3Se3- chain nanowires and optical image of the Mo3Se3chain nanowire solution (inset), and (j) atomic force microscopy (AFM) image of the Mo3Se3chain nanowires and height profile along the dashed line (inset).


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Figure 2. (a–c) Comparison of the composite hydrogel, prepared simply by mixing Mo3Se3- chain nanowires and gelatin, with a pure gelatin hydrogel (15 wt%). (a) Images of the pure gelatin hydrogel, Mo3Se3- nanowire solution (0.01 wt%), and composite hydrogel. (b) Stress–strain curves of the pure gelatin and composite hydrogels with different amounts of Mo3Se3- nanowires. (c) Cross-section SEM images of the composite hydrogel. (d–i) Interaction between the Mo3Se3- chain nanowires and gelatin. (d) Schematic of the phase separation of the Mo3Se3- chain nanowire and gelatin in the composite hydrogel prepared by the simple mixing. (e) Schematic of the Mo3Se3chain nanowires embedded into the gelatin matrix in the composite hydrogel prepared by the wrapping method. (f) Electrostatic interaction between the negatively charged Mo3Se3- chain nanowires and positively charged residues of gelatin. (g) Structures of the positively charged


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arginine, lysine, hydroxylysine, and histidine residues in gelatin. (h) ζ-potential of the gelatinwrapped Mo3Se3- nanowires. (i) TEM image of the gelatin-wrapped Mo3Se3- nanowires prepared at a gelatin/Mo3Se3- ratio of 25.


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Figure 3. Mechanical properties of the composite hydrogels prepared by the wrapping method. (b) Stress–strain curves, (c) tensile stresses, (d) toughnesses, and (e) elastic moduli of the composite hydrogels prepared using different concentrations of Mo3Se3- nanowires. SEM images of the composite hydrogels having concentrations of Mo3Se3- nanowires of (f) 0.003, (g) 0.01, and (h) 0.05 wt%. The gelatin/Mo3Se3- ratio was fixed to 25. The final concentrations of gelatin are identical (15 wt%).


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Figure 4. Improvements in the mechanical properties of the composite natural hydrogels with various additive nanomaterials published in previous reports. (a) Tensile strength, (b) strain, (c) toughness, and (d) elastic modulus increases of each composite hydrogel, compared with those of the bare natural hydrogel.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Materials and detailed experimental methods; The SEM image of pure gelatin and Mo3Se3composite hydrogel (Figure S1); EDX analysis spectra of composite hydrogel (Figure S2); Structure of amino acid present in type A gelatin (Figure S3); Comparison of zeta potential values of pure gelatin and gelatin wrapped Mo3Se3- nanowires measured in the different pH conditions (Figure S4); TEM images of gelatin wrapped Mo3Se3- nanowires prepared with gelatin/SCAC ratio of 25 (Figure S5); The mechanical properties of composite hydrogels prepared by wrapping method in different gelatin/Mo3Se3- ratio (Figure S6); The mechanical properties analysis of different gelatin/Mo3Se3- ratio. All different ratio conditions were analyzed more than 5 (Figure S7); The mechanical properties analysis of composite hydrogel prepared using different concentrations of Mo3Se3- nanowire (Figure S8); SEM images of 0.01 wt% of Mo3Se3- nanowire composite hydrogels (Figure S9); The content of amino acids in gelatin type A (Table S2).

AUTHOR INFORMATION Corresponding Author *E-mail (J. Y. Choi): [email protected] *E-mail (J. H. Lee): [email protected] Author Contributions  The

first two authors (S. H. Kim and S. Oh) contributed equally to this work.


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Funding Sources This research was supported by the National Research Foundation (NRF) of Korea, funded by the Ministry of Science and ICT, for a Bio-inspired Innovation Technology Development Project (NRF-2018M3C1B7021997) and Nano-Material Technology Development Program (NRF2017M3A7B8065561). Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the National Research Foundation (NRF) of Korea, funded by the Ministry of Science and ICT, for a Bio-inspired Innovation Technology Development Project (NRF-2018M3C1B7021997) and Nano-Material Technology Development Program (NRF2017M3A7B8065561).

ABBREVIATIONS 3D, Three-Dimensional; 1D, One-Dimensional; TEM, Transmission Electron Microscopy; AFM, Atomic Force Microscope; UTM, Universal Testing Machine; SEM, Scanning Electron Microscopy


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Exceptional Mechanical Properties of Phase-Separation-Free Mo3Se3

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