Letter pubs.acs.org/NanoLett
Ultrahigh Tensile Strength Nanowires with a Ni/Ni−Au Multilayer Nanocrystalline Structure Boo Hyun An,† In Tak Jeon,† Jong-Hyun Seo,‡ Jae-Pyoung Ahn,‡ Oliver Kraft,§ In-Suk Choi,*,∥ and Young Keun Kim*,† †
Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 02792, Korea § Institute for Applied Materials, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen 76344, Germany ∥ High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 02792, Korea ‡
S Supporting Information *
ABSTRACT: Superior mechanical properties of nanolayered structures have attracted great interest recently. However, previously fabricated multilayer metallic nanostructures have high strength under compressive load but never reached such high strength under tensile loads. Here, we report that our microalloying-based electrodeposition method creates a strong and stable Ni/Ni−Au multilayer nanocrystalline structure by incorporating Au atoms that makes nickel nanowires (NWs) strongest ever under tensile loads even with diameters exceeding 200 nm. When the layer thickness is reduced to 10 nm, the tensile strength reaches the unprecedentedly high 7.4 GPa, approximately 10 times that of metal NWs with similar diameters, and exceeding that of most metal nanostructures previously reported at any scale. KEYWORDS: Ultrastrong nanowire, nanocrystalline Ni−Au, multilayer structure, microalloying
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introduce a multilayer structure to NWs to overcome, on the one hand, the extrinsic characteristic diameter length as constrained by typical 1D NW geometry, and on the other hand to produce a stable extremely fine-grained nanostructure. The concept of multilayer structures to enhance mechanical properties has been employed widely for thin films.14,15 More recently, researchers proposed a basic strengthening mechanism by adding layers to metal nanostructures.16 The high density of additional interfaces can block or confine dislocation propagation within the nanostructured metals leading to an increase in strength.16−21 In particular, incoherent interfaces effectively suppress dislocation propagation.15,18,22,23 While most results reported ultrahigh compressive strength, the tensile strengths of these multilayer structures did not exceed 3 GPa.8,24 This may be related to defects that are generated during the preparation of the nanostructures by focused ionbeam milling or brittle fracture along the interfaces within the structures. Here, we successfully synthesized ultrastrong Ni/Ni−Aumultilayer NWs (MNWs) with diameters over 200 nm. We intentionally chose Ni as our base material for the MNWs and
he mechanical properties of nanomaterials are largely determined by their size.1−3 The size effect of nanowires (NWs)the so-called “smaller is stronger” principlehas been extensively investigated.4−9 Because the strength of most metallic NWs increases by decreasing the diameter, researchers have investigated sub-100 nm diameter NWs with respect to their outstanding mechanical properties. For example, defectfree Pd and Au NWs with diameters from 40−180 nm possess tensile strengths ranging from 1 to 4 GPa,5,10 still having a reasonable ductility, and the strength of defect-free 40 nmdiameter Cu NWs reaches 6 GPa.6 However, reducing the NW diameter as a tool to increase their strength may restrict, on the other hand, the design of many small-scale structures,11,12 which require somewhat larger building blocks. Furthermore, most sub-100 nm NWs showing ultrahigh tensile strengths (over 2 GPa) are single-crystalline and defect-free, requiring complex, expensive vacuum-based fabrication processes.6 This study aims to develop a relatively simple process for producing extremely strong metallic NWs with diameters exceeding 100 nm. Since the strength of single-crystalline metallic NWs decreases, following a power-law relationship, with increasing diameter, the NW are designed to have nanocrystalline structure. However, it has been shown that such a configuration may lead to a “smaller is weaker” effect for single phase nanocrystalline NW.13 Therefore, in this work, we © XXXX American Chemical Society
Received: January 21, 2016 Revised: April 18, 2016
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DOI: 10.1021/acs.nanolett.6b00275 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Synthesis of Ni/Ni(Au) MNWs with varied thickness. (a) TEM images of Ni/Ni(Au) MNWs, categorized in three groups by controlling the thickness of each layer. Group 1: Thickness of both Ni and Ni(Au) layers are equivalent and less than 100 nm (Inset: compositional image of layers by electron diffraction scattering). Group 2: Either the Ni or Ni(Au) layer has a thickness exceeding 100 nm. Group 3: Pure Ni and Ni(Au) NWs without layer structures. (b) XRD patterns corresponding to each MNW in a. Peak positions of fcc-Au and fcc-Ni are indicated in orange and green lines, respectively, at bottom of figure. In Group 3, face-centered cubic (fcc) Ni peaks are observed in the Ni NW, while the broadening fcc Ni peaks with a high-intensity peak of fcc-Au (111) exhibit in the Ni(Au) NWs. For Group 1 and 2, the Bragg peaks appear to be a combination of the peaks of patterns from Group 3 Ni and Ni(Au) NWs.
Ni/Ni(Au) MNWs) are produced by controlling pulse intensity and deposition time, as shown in the representative transmission electron microscopy (TEM, Titan80-300 and Tecnai F20, FEI) images in Figure 1a. The diameters of the fabricated Ni-based NWs range from 190−240 nm, determined by the pore diameters of the AAO template. Three groups of Ni-based NWs were fabricated to investigate the effect of the multilayer nanostructure on the strength of the NWs. Group 1 contained Ni/Ni(Au) MNWs with distinct individual layers of Ni and Ni(Au) of the same thicknesses, varying from 100 to 10 nm. Group 2 consisted of Ni/Ni(Au) MNWs in which one layer is thicker (200 nm) than the other (70 or 100 nm). Group 3, as a reference, included separate Ni and Ni(Au) NWs without multilayer structures. We characterized the detailed microstructure of the Ni MNWs using X-ray diffraction (XRD, D/MAX-2500 V/PC, Rigaku) and TEM analyses. The XRD patterns of representative Ni-based NWs from Groups 1, 2, and 3 are shown in Figure 1b. From the bottom of Figure 1b, clear face-centered cubic (fcc) Ni peaks are observed in the profile of the Ni NW, while the XRD profile of the Ni(Au) nanowires is characterized by a shift and broadening of the fcc Ni peaks with a fairly sharp peak from fcc-Au (111). The peak broadening is indicative of a very fine grain structure. The grain sizes were estimated from XRD peak
Au as an alloying component because they enable the use of facile pulse electrodeposition in an anodized aluminum oxide (AAO, Anodisc 25, Whatman Ltd.) nanotemplate by electrodeposition in a bath consisting of nickel(II) sulfate hexahydrate (NiSO4•6H2O) and potassium dicyanoaurate (KAu(CN)2) in deionized water, with the solution pH adjusted to 3.0 by boric acid (H3BO3).25 Prior to the experiment, a 300 nm Ag layer was deposited by e-beam evaporation on one side of the AAO template to act as the working electrode; a thin Pt sheet was used as the counter electrode. The pulse electrodeposition was carried out under the current densities of 0.5 mA/cm2 and 10 mA/cm2, respectively. (The detailed information on the electroplating method is provided in Supporting Information.) Removal of the AAO nanotemplate was done in a 6 M NaOH solution for 30 min. The NWs were repeatedly rinsed of residual solvent with deionized water. Nanotemplate-assisted electrodeposition allows the fabrication of compositionally modulated microalloyed MNW arrays with high aspect ratios. A simple one-pot electrodeposition can incorporate small amount of Au atoms into the Ni matrix to form unique microalloyed Ni layers (hereafter denoted as Ni(Au) layers), which are strongly connected to but structurally different from pure Ni layers (hereafter denoted as Ni layers). The well-defined layer-bylayer structures of the Ni-based MNWs (hereafter denoted as B
DOI: 10.1021/acs.nanolett.6b00275 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 2. TEM analysis of Ni/Ni(Au) MNWs. TEM images, corresponding SAED patterns and HRTEM images of (a) Ni (100)/Ni(Au) (100) MNW, (b) Ni (25)/Ni(Au) (20) MNW, and (c) Ni (10)/Ni(Au) (10) MNW, respectively. The SAED patterns of Ni (100)/Ni(Au) (100) MNW show that only nanocrystalline Ni structure exists in Ni (100) layers; nanocrystalline Ni and Au structures mix in Ni(Au) layers in a. SAED patterns of Ni (25)/ Ni(Au) (20) MNW and Ni (10)/Ni(Au) (10) MNW show mixed nanocrystalline Ni and Au due to selective aperture in b and c. Inset in b: FFT patterns of each layer in Ni (25)/Ni(Au) (20) MNW corresponding to Ni and Au phases. The different microstructure in each layer may result in a distinct interface that effectively enhances the mechanical properties of the Ni MNWs.
broadening of the Ni (111) peaks using the Scherrer equation, providing values of approximately 15 and 3 nm, respectively, for the Ni and Ni(Au) NWs. For the latter, the value should be considered as a crude estimate since the peak shape is also influenced by the addition of the Au atoms. The occurrence of a separate Au peak indicates that the Au atoms are precipitating and forming their own grains. However, the shift of the Ni peaks relates to a change in lattice constant due to Au atoms in solid solution. This is somewhat surprising since the Ni−Au binary system is thermodynamically immiscible at room temperature, and a phase separation of Ni and Au is expected during the pulse deposition of the Ni(Au) layer.26 But apparently the growth rate does not allow for completing the separation. For detailed structural analysis, TEM analysis for the Ni and Ni(Au) NWs are provided in Supporting Information. The average grain size of nc Ni NWs is about 13 nm while the best estimate of the grain size for Ni (Au) NWs ranges between 2 and 7 nm, which is consistent with the XRD analysis results. The Bragg peaks of the X-ray diffraction pattern for Group 1 and 2 in the top five patterns appear to be a combination of the peaks of patterns from the Ni and Ni(Au) NWs of Group 3 with some subtle differences. For the NWs of Group 1 and 2 compared to Group 3, the Au (111) peak is stronger, and the Ni (111) peak is more asymmetric. These observations imply that the structure of the Ni and Ni(Au) layers in the Ni/Ni(Au) MNWs are overall consistent with those in the separate Ni and Ni(Au)
NWs. The stronger Au peaks indicate that the phase separation of Au and Ni is more pronounced in the MNWs. In order to investigate the microstructure in more detail, we performed TEM analysis on MNWs near the interfaces of the Ni/Ni(Au) in group 1, with layer thicknesses of 10, 20, and 100 nm. Figure 2 shows TEM images of Ni (100)/Ni(Au) (100) MNW, Ni (25)/Ni(Au) (20) MNW, and Ni (10)/Ni(Au) (10) MNW, respectively, in Figure 2a, b, and c. The selective-area diffraction patterns of all three Ni/Ni(Au) MNWs confirm the existence of crystalline structures in both the Ni and Ni(Au) layers, consistent with the XRD results. The selected area electron diffraction (SAED) patterns of Ni (100)/Ni(Au) (100) MNW in Figure 2a show that only nanocrystalline Ni structure exists in Ni (100) layers while nanocrystalline Ni and Au structures mix in Ni(Au) layers. The SAED patterns of Ni (25)/ Ni(Au) (20) MNW and Ni (10)/Ni(Au) (10) MNW in Figure 2b and c show mixed nanocrystalline Ni and Au because selective apertures cover both Ni and Ni(Au) layers. The high-resolution TEM image in Figure 2a and b with corresponding fast Fourier transform (FFT) patterns along the NW axes clearly depict different nanocrystalline structures across the Ni and Ni(Au) layers. While only nanocrystalline fcc Ni grains are observed in the Ni layer, nanocrystalline fcc Au and fcc Ni phases with smaller grain sizes coexist in the Ni(Au) layer. The different microstructures occurring in the two layers result necessarily in an incoherent interface consisting of Ni/Ni grain boundaries and Au/Ni phase boundaries. C
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Figure 3. Compositional analysis of Ni/Ni(Au) nanowires using 3D atomic probe. Elemental analyses on the MNWs are done by 3D atomic probe, preceded by sampling in sharp-tip-shape with FIB milling. Elemental distribution of Ni and Au within Ni (100)/Ni(Au) (100) MNW (a) and Ni (10)/Ni(Au) (10) MNW (d) clearly shows the distinct Ni and Ni(Au) layers in the Group 1 MNWs. Compositional line profiles along NW axial direction are plotted for Ni (100)/Ni(Au) (100) MNW (b) and Ni (10)/Ni(Au) (10) MNW (e) which quantitatively characterize incorporation of microalloyed Au in the Ni(Au) layers. Volumetric rendered images are given for Ni and Au in Ni (100)/Ni(Au) (100) MNW (c) and Ni (10)/Ni(Au) (10) MNW (f), which supports the segregation of the nanocrystalline Au phase in the Ni nanocrystalline matrix as speculated from TEM analysis. More 3D-rendered movies are located in Supporting Information, Movies S1−S4.
increased to decrease thicknesses of the layers to 10 nm, Au atoms could not be incorporated in the Ni as much as the case of Ni (100)/Ni(Au) (100) MNW. Thus, Au composition in the Ni(Au) layers is graded from 0 at. % to ∼3 at. % so as to decrease the apparent thickness of Ni(Au) layers as layer thicknesses are reduced. However, the structure of the thin Ni(Au) layer still includes the clusters of Au atoms well-dispersed in the nanocrystalline Ni matrix, as shown in Figure 3f, due to the immiscibility of Au. This implies that Ni MNWs with 10-nmthick layers still have incoherent multilayer structures with graded interfaces, similar to those in Ni MNWs with 100-nmthick layers. We characterized the mechanical properties of the microalloyed Ni MNWs, following the procedure depicted in Figure 4a from synthesis to mechanical testing. First, Ni/Ni(Au) MNWs were deposited into AAO nanotemplates, and the templates were removed with NaOH as an etchant. After harvesting a single MNW from the NW array using a nanomanipulator, the MNW was attached to a Force Measurement System (FMS, FMS-120, Kleindeck) for in situ tensile testing with imaging by a scanning electron microscope (SEM) in a dual-beam focused ion beam (FIB, Quanta 3D, FEI) system. Force and displacement were measured. Also, the diameter and length of each tested NW were determined and used to calculate engineering stress and strain. As shown in the representative stress−strain curves in Supporting Information, Figure S1, brittle fracture occurs without apparent plastic strain for all NWs. However, the fracture behavior of the MNWs can be categorized into two different types, depending on the angle of the fracture surface to the tensile axis, as shown in Figure 4b. Type 1 fracture occurs in Group 3, in which the NWs’ fracture
In many previous studies, multilayer metallic nanostructures with incoherent interfaces were fabricated by using the layer-bylayer deposition with distinct elements which results in the abrupt interface change both in element composition and structure.17,21,22 By contrast, our multilayer structure is fabricated by using microalloying-based electroplating method which incorporates secondary alloy element Au into Ni matrix making a graded composition profile across the interface. 3D atomic probe microscopy (LEAP 4000HR, CAMECA) analysis performed on the Ni/Ni(Au) MNWs provides further understanding of the formation of unique layered nanostructures by the incorporation of microalloyed Au. Figure 3a and d depict the atomic distributions of Ni and Au in Group 1 MNWs with layer thicknesses of 100 and 10 nm, clearly showing the distinct Ni and Ni(Au) layers. Figure 3b presents compositional profiles of Au and Ni across the layer interface for the Group 1 Ni/ Ni(Au) MNW with a layer thickness of 100 nm. The atomic concentration of Au, denoted by the orange line, is 0 at. % in the Ni layers and 15 at. % in the Ni(Au) layers. In between layers, a transition region approximately 5 nm thick occurs in which the composition of Au gradually increases from 0 to 15 at. %. Figure 3c presents a volumetric rendering image of isoconcentration, showing the clustering of Au atoms in the Ni(Au) layer, which supports the phase separation of the nanocrystalline Au in the Ni nanocrystalline matrix as speculated from TEM analysis (more 3D analysis is presented in Supporting Information, Movies S1−S4). As the layer thickness decreases from 100 to 10 nm, the maximum concentration of Au in a Ni(Au) layer decreases to approximately 3 at. %, as shown in the profile in Figure 3e. Because the frequency of the pulse D
DOI: 10.1021/acs.nanolett.6b00275 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. Mechanical analysis of Ni/Ni(Au) MNWs: (a) Procedure for preparing tensile test of single Ni/Ni(Au) MNWs is illustrated. (b) Two different types of brittle fracture behaviors of NWs are observed. Type 1: NWs’ fracture surface at 30−50° to tensile axis. Type 2: NWs’ fracture surface perpendicular to tensile axis. (c) Fracture stresses vs thicknesses of Ni layers and Ni(Au) layers of the MNWs show the size-dependent behavior. The tensile fracture strength in Group 1 NWs dramatically increases from 1.5 to 7.4 GPa by decreasing the layer thickness under 100 nm. (d) Tensile strength plot as a function of NW diameter is given to compare our results to reported tensile strength of metallic nanowires and nanopillars.5,6,28−31 The high tensile strengths of our MNWs have never been reported in any metallic nanostructures. (e) Tensile strengths of the Group 1 MNWs follow a power-law trend of fcc metals with respect to layer thickness.
NWs notably increases from 1.5 to 7.4 GPa even much higher than Group 2 and 3 NWs by reducing the layer thickness to below 100 nm. In particular, the tensile strengths for Ni MNWs with layer thicknesses under 30 nm are unprecedentedly high, compared to those for previously fabricated metallic nanowires,5,6,28−31 as indicated in Figure 4d despite their diameters far exceeding 100 nm. This high tensile strength has hardly been reported for any metallic materials and implies that multilayer nanostructures with incoherent and stable interfaces can achieve superior mechanical strength even under tensile loads for scaled-up metallic materials. While details of the strengthening and deformation mechanisms in metal nanostructures remain controversial, general agreement exists regarding the size effect of the strength of single crystals with respect to a characteristic length l if deformation is governed by dislocation-mediated processes.
surfaces are inclined at an angle of 30−50° to the tensile axis that can be induced by the shear-driven deformation. Type 1 fracture is also observed for some of Ni/Ni(Au) MNWs in Group 2. The MNWs of Group 2 show fracture surfaces with angles similar to those in Group 3 inside thick layers. By contrast, Type 2 fracture occurs in Group 1, with brittle fracture surfaces perpendicular to the tensile axes (also in Figure S5) which typically involves in Mode I brittle fracture. The ultimate tensile strengths are plotted in terms of the thicknesses of the Ni and Ni(Au) segments in Figure 4c. We found that the NWs displaying type 1 fracture have fracture strengths between 800 MPa and 1.52 GPa, similar to the strengths of pure Ni NWs with diameters of 360 and 200 nm in previous studies.13,27 While the fracture strengths of Group 2 and 3 NWs are higher than that of bulk Ni (140−195 MPa) by an order of magnitude, the tensile fracture strength in Group 1 E
DOI: 10.1021/acs.nanolett.6b00275 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters Then, the increase in strength, σf, follows typical power-law scaling behavior according to σf ∝ l−n, where n is often found to be close to 0.6 for fcc metals.2,31 However, the strength of nanocrystalline metals in small scale specimens and its size effect are strongly affected by interplaying among dislocation density, precipitate spacing, grain size, and specimen size.32−37 The complex structure of our MNWs, including nanosized alloyed grains, phase interfaces, and their multilayer structure, obscure the individual contribution of each microstructural feature to the strengthening of the MNW. Nevertheless, Figure 4d clearly indicates that for our nanocrystalline NW and MNW, the diameter is not the governing length scale. The grain size is not likely to be a major deterministic parameter for the size effect either because the grain size in Ni layer and Ni(Au) layer is similar to those of monolithic nc Ni NWs (∼13 nm) and nc Ni(Au) NWs (∼5 nm), respectively, regardless of the layer thickness. Instead, the layer thickness can be considered as the characteristic length constraining deformation and fracture processes of our MNWs because the strength of the Ni MNWs follows a power-law trend with respect to the layer thickness. Figure 4e plots tensile strength values for group 1 (values in the equidistance plane of Figure 4c) with respect to the layer thicknesses, in a logarithmic scale. A linear fit analysis provides n ≈ 0.7, which is consistent with n for fcc metals. Moreover, Supporting Information, Figure S2, collectively plots the failure stresses of Ni nanostructures (NW, MNW, nanopillar and etc.) from the literature together with our results with respect to their intrinsic characteristic length (diameter or layer thickness), in which all strength values lie well within the size effect trend lines provided by n ≈ 0.6 including the tensile strengths of our Ni MNWs as a function of their layer thickness. Therefore, we conclude that the incorporated Ni(Au) layers create a new intrinsic characteristic length in Ni NWs, significantly enhancing the tensile strength of Ni NWs. However, it should be noted that the observed trend in Figure S2 can only be considered as an indicator for the respective governing length scale and an empirical description of the observed size effect. It does not imply that the underlying physical mechanisms for deformation are the same for the largely different microstructures ranging from single crystals to nanocrystalline alloys as further discussed in Supporting Information.15,34,35,38,39 The tensile strength of our Group 1 NWs exceeded that of other nanocrystalline electrodeposited Ni and Ni alloys40 and even our maximum tensile strength is close to the lower end of the theoretical tensile strength. (Please see the detailed information on theoretical tensile strength in Supporting Information.) So, the question remains: why are the MNWs with layer thickness of less than 30 nm so much stronger? It is argued that this is related to the complicated deformation processes that occur in nanocrystalline metals simultaneously or sequentially, including grain boundary sliding, dislocation mediated plasticity, stress-driven grain boundary motion.41−43 In this fine nanocrystalline regime, the suppression of dislocation mediated plasticity in the small nanocrystalline Ni and Ni(Au) layers can be a major contribution to achieving a high strength. It can be argued that the extremely fine grain structure in the Ni(Au) layers provide a very high strength as dislocation processes are hindered. For such small grain sizes, very high stresses are required for the nucleation of dislocations in the context of the dislocation starvation model. When the nanocrystalline materials reached such a high strength, grain boundary mediated deformation typically starts operating.
Although stress-driven grain boundary motion can be the strength limiting mechanism for nanocrystalline metals, the two-phase microstructure in the alloy may suppress macroscopic grain boundary motion as a deformation mechanism in Group 1 contrary to Group 2 and 3 in which the tensile strength is not increased compared to Group 1 NWs. Here, our experiments indicate that the ultimate tensile strength increases drastically with the transition from shear-driven Type 1 to brittle Type 2 fracture. Type 1 fracture is obviously based on shear deformation which may be related to the formation of shear bands and grain boundary sliding.36,43,44 The size of these shear bands is naturally limited by the thickness of the layers in the MNWs because of the incoherent interfaces between the layers and the transition in grain size at the interface.23 Also, in the MNWs with the short spacing layers, the interfaces may restrict or localize stress-induced grain boundary motion including grain rotations and grain boundary sliding. Thus, it is argued that for small enough layers the shear deformation is not preferred to cause Type 1 fracture. Instead, brittle Type 2 fracture occurs at a much higher stress level which may lead to void formation and crack initiation at interface or boundaries in nanostructures.36,45 In conclusion, we fabricated multilayer metal NWs that are ultrastrong under tensile loading. The layer-by-layer structures were fabricated using a facile single-pot electrodeposition method. Systematic characterization showed that the layers with Au atoms incorporated into the Ni matrix formed different nanocrystalline phases. A series of in situ mechanical tests showed that tensile strength increases as layer thickness decreases. The deliberately tuned multilayer Ni NWs achieved the unprecedentedly high tensile strength of 7.4 GPa, even with total diameters far beyond 100 nm. Our results demonstrate that the incoherent but strongly bonded multilayer structures in NWs significantly improve the size-dependent mechanical behavior of nanocrystalline alloys by suppressing shear band formation or grain boundary mediated deformation as a possible failure mechanism of Type 1 fracture under tension while the stable graded interfaces can endure interface decohesion at such a high tensile strength. We believe this approach of fabricating microalloyed multilayer nanostructures can be generally applied in designing ultrastrong nanostructure materials. This may overcome the restrictions of the current scaling approach, bridging the gap between nano- and microscale structural materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00275. Representative stress−strain curves (Figure S1); size dependency plot of collective failure stresses of Ni nanostructures from literature and results of this work (Figure S2); TEM characterization of nanocrystalline (nc) Ni in nc Ni NWs (Figure S3); TEM characterization of Au clusters in Ni(Au) NWs before and after chemical etching of Ni elements (Figure S4); TEM images near the fractured surfaces of Ni (10)/Ni(Au) (10) MNW, and Ni (25)/Ni(Au) (20) MNW after tensile testing (Figure S5); synthesis methods of Ni/Ni(Au) MNWs (Figures S6 and S7) (PDF) F
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Description of theoretical tensile strength and threedimensional volumetric rendered movies (Movies S1− S4) are also supplied (ZIP)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: (I.S.C.)
[email protected]. *E-mail: (Y.K.K.)
[email protected]. Author Contributions
I.S.C. and Y.K.K. conceived and supervised the project. B.H.A., I.T.J., and Y.K.K. fabricated the NWs. B.H.A, I.T.J., and I.S.C. performed the mechanical tests. B.H.A., J.H.S, J.P.A. I.S.C., and Y.K.K characterized the structure of NWs. B.H.A, O.K, I.S.C., and Y.K.K. analyzed the data and wrote the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.K.K appreciates the funding by the National Research Foundation (NRF) of Korea (2015R1A2A1A15053002) funded by the Ministry of Science, ICT & Future Planning (MSIP). I.S.C. acknowledges the financial support through the internal research program of the Korea Institute of Science and Technology (2E26082) and also through a NRF of Korea (2015R1A2A2A04006933) funded by MSIP. O.K. is grateful for his support by the Robert Bosch Foundation.
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DOI: 10.1021/acs.nanolett.6b00275 Nano Lett. XXXX, XXX, XXX−XXX
