In situ Investigation of Defect-free Copper Nanowire Growth - Nano

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In situ Investigation of Defect-free Copper Nanowire Growth Ting Yi Lin, Yong-Long Chen, Chia-Fu Chang, Guan-Min Huang, Chun-Wei Huang, Cheng-Yu Hsieh, Yu-Chieh Lo, Kuo Chang Lu, Wen-Wei Wu, and Lih-Juann Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03992 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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In situ Investigation of Defect-free Copper Nanowire Growth Ting-Yi Lin a, Yong-Long Chen a, Chia-Fu Chang a, Guan-Min Huang a, Chun-Wei Huang a, Cheng-Yu Hsieh b, Yu-Chieh Lo a, Kuo-Chang Lu c, Wen-Wei Wu a, and Lih-Juann Chen d a

Department of Materials Science and Engineering, National Chiao Tung University 1001 University Road, Hsinchu 300, Taiwan

b

Material and Chemical Research Laboratories, Nanotechnology Research Center, Industrial Technology Research Institute, Hsinchu 310, Taiwan

c

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan d

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan

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Correspondence and requests for materials should be addressed to W.W.W (email: [email protected])

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ABSTRACT The fabrication and placement of high purity nanometals, such as one-dimensional copper (Cu) nanowires for interconnection in integrated devices have been among the most important technological developments in recent years. Structural stability and oxidation prevention have been the key issues, and the defect control in Cu nanowire growth has been found to be important. Here, we report the synthesis of defect-free single-crystalline Cu nanowires by controlling the surface-assisted heterogeneous nucleation of Cu atomic layering on the graphite-like loop of an amorphous carbon (a-C) lacey film surface. Without a metal-catalyst or induced defects, the high quality Cu nanowires formed with high aspect ratio and high growth rate of 578 nm⁄s . The dynamic study of the growth of heterogeneous nanowires was conducted in situ with a high-resolution transmission electron microscope (HRTEM). The study illuminates the new mechanism by heterogeneous nucleation control and laying the groundwork for better understanding of hetero surface-assisted nucleation of defect-free Cu nanowire on a-C lacey film.

KEYWORDS: defect-free, Cu nanowires, a-C lacey film, hetero surface-assisted nucleation, in situ TEM

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Introduction Over the past several decades, copper (Cu) has been the most common interconnection material used in various applications.1-5 Due to the rapid shrinkage of size in electronic devices, poor stability and microstructural defects of Cu nanowire, the increase in the electrical resistance and current density exacerbate the detrimental joule heating effect. In addition, more severe Cu atomic electromigration shortens the lifetime of the electronic devices.6 Under these circumstances, the synthesis of defect-free nanostructure can be beneficial to improving stability in future Cu nanowire devices.7,

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The demand for an effective reduction of the nanodevice’s energy consumption also leads to the demand for single-crystal Cu nanowire synthesis. In electronic industries, most fabrication methods of Cu nanowire devices were reported with the use of a chemical precursor as the catalyst. 1, 9 Since the diameter

control of the catalyst particles and the substrate conditions could be affected together, the synthetic accuracy of nanodevices could be hampered by the catalyst. In addition to the limitation of the catalyst-induced vapor–liquid–solid (VLS) mass transport process,10 the uneven surface-to-volume ratio of conical cylindrical nanowires is the main destabilizing factor of current transmission for many applications. Therefore, catalyst-free methods have been recently utilized to synthesize nanowires. Several synthesis processes have been designed with defect engineering for the study of catalyst replacement on nanowire nucleation dynamics. In the case of self-catalytic metal oxide nanowire nucleation by using an environmental transmission electron microscope (E-TEM), the evaporated precursor activated by thermal energy becomes the nuclease with ionic and covalent bonds. In the bonding of metal oxides, a lower annealing temperature and stable redox reaction rate contribute to the layer-by-layer atom formation with the induction of defects.11, 12 The stability of the nucleation with a specific diameter control has been clearly reported in the metal oxide nanowire synthesis process. However, the growth control of pure metal nanowires is relatively difficult without the means to lower the nucleation barrier. Mechanisms

for

nanowire

growth13,

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defect-catalyzed growth , as well as vapor–solid (VS) growth , have been studied. The nucleation dynamic models indicate that catalytic interfaces, typical dislocations (twin structures11, edge12 and screw dislocations16) and other defects are necessary to decrease the nucleation barrier for the anisotropic nucleation of nanowire growth.17-20 In other words, typical dislocations and defect conditions play a critical role in the catalyst-free method of nanowire growth. As reported previously, the five-fold twinned Cu nanowire in a Cu crystallization system is the most common nanostructure with high aspect ratios, but internal stresses appear in unstable 3


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structures with an angle deviating from the ideal 70.5° between twin boundaries.21-26 To solve those problems, Liu et al. has demonstrated the successful synthesis of Cu nanorods and nanowires through the vacuum vapor deposition method.27 In this work, a new approach to control the heterogeneous synthesis mechanism is presented. More specifically, this study was undertaken to investigate the heterogeneous nucleation behaviors of different Cu nanostructures on the a-C substrate and to control the synthesis of the defect-free crystallization system of the Cu nanowire growth. In the classical theory of crystal nucleation, a nucleus seed leads to the crystallization in different nucleation behaviors.22, 28, 29 Controlling the nucleation condition at the initial stage of seed crystallization is the critical step for the defect-free Cu nanowire growth. Our objective in this study is to address and investigate the seed crystallization model of defect-free Cu nanowire growth. According to the limited critical crystallization radius and the decrease in activation energy of Cu nucleation, defect-free Cu nanowires could be formed on the a-C lacey film.30, 31 The growth system described here could serve as the basis for the study of a specific heterogeneous reaction between the Cu and a-C interface. With the in situ investigation, the method is demonstrated to be practical through the study of defect-free Cu nanowire fabrication, providing knowledge for understanding hetero surface-assisted nucleation on a-C lacey films.

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Experimental Methods The defect-free Cu nanowires were synthesized in the ultrahigh vacuum (UHV) environment of a JEOL 2000V UHV-TEM, which lowered the effective melting point of Cu and induced Cu adatoms migration and crystallization at approximately 1000 K. The catalyst-free method of nanowire growth was conducted on the a-C substrate with a high-ratio mixed graphite-like carbon (C) structure as the heterogeneous nucleation system. Based on the thermal annealing control in the UHV environment, the Cu nanowires were formed in three steps, Cu adatoms migration, Cu seed crystallization and Cu nanowire growth. Our results were confirmed by the in situ observation in the JEOL 2000V UHV-TEM. Electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy (EDS) chemical analysis were performed with a JEOL JEM-ARM200F. Simultaneously, molecular dynamics (MD) simulation of Cu migration and nucleation, in contrast to the theoretical calculation of critical crystallization radius, is conducted to confirm the nucleation advantage by hetero surface catalysis in the supporting information. The simulated bonding construction analysis and Raman scattering of the a-C substrate also help further the understanding of the heterogeneous nucleation system of defect-free Cu nanowires formed on the a-C substrate.

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Results and Discussion In the present study, the synthesis method involves heating the sample in the UHV system to lower the melting point and provides the thermodynamic nucleation energy for Cu nanowire growth.32-34 With oxygen desorbed in the vacuum system, thermal annealing under UHV drove the pure Cu source to evaporate from the Cu oxide reduction on a Cu grid and crystallize to be the seed crystal through contact-induced Cu droplets on the a-C lacey film of the Cu gird. The schematic illustration of the vacuum vapor deposition (VVD) process is shown in Fig. 1a. Fig. 1b shows the unit square area of the copper grid with the assigned locations I, II, III moving from the edge to the center. Different Cu nanostructures in Fig. 1c-e (large particles, nanoparticles and nanowires) reflect the crystallization behaviors in different nucleation conditions at the assigned locations from the edge to the center of a unit square of the copper grid (I, II, III in Fig. 1b). The different nucleation behaviors of Cu crystallization on the a-C substrate represent the rare heterogeneous nucleations through hetero interface control, especially for the anisotropic nucleation of Cu nanowire growth. The elemental mapping by EDS in Fig. 1e indicates the remarkable difference between the a-C lacey film and the Cu nanostructures in Fig. 1f and 1g. The findings mentioned above indicate that the interconnection between Cu nanostructures and the a-C lacey film is the supporting foundation of the crystal growth. To clarify the heterogeneous nucleation reactions of those Cu nanostructures under the premise that the only interaction between C and Cu is the van der Waals force, a deeper understanding of the a-C substrate is critical. Although many experiments on graphene and graphite structures on the annealing a-C substrate have been performed35, few studies demonstrated the a-C substrate as an intriguing support in the heterogeneous nucleation system. Here, we utilized the MD simulation of the a-C substrate to investigate the bonding construction (C sp2 / sp3 hybridized bonds) transformed in the UHV annealing process. Additionally, we confirmed the Raman spectrum analysis with an increasing G peak and decreasing D peak to demonstrate increased hybridization of the C sp2 bond and decreased structural defects to create new crystal bonding on the a-C film.36 These findings correspond to a high ratio of graphite-like C structure substrates, shown in Fig. S1-b. With the additional information shown in Fig. S1, the a-C substrate provided for the synthesis in our work is mostly composed of the meshed graphite-like C structures. The diverse behaviors of the different nucleation systems A, B and C (A: Cu flowing along the edge of the a-C lacey film, B: the Cu nanowire growth and C: the Cu nanoparticle coarsening) are shown by the time-series TEM images in Fig. 2a (the 6


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real-time video is shown in the supporting information Movie S1). The uneven distribution of the Cu nanostructures on the a-C substrate indicates the high mobility of Cu migration in the thermal annealing process. Once the Cu evaporation was substantially developed, the volume of the Cu droplet increased by contact-aggregation, and the high nucleation barrier could be reduced to form the larger Cu crystal at the hetero surface. The crystallization volume of Cu nanostructures increases with an increased supply of atoms from the Cu grids. Both droplet aggregation and nanoparticle coarsening behaviors are driven by the reduction in surface energy. On the other hand, the anisotropic growth behavior of the Cu nanowire growth stands out. Obviously, Cu nanowire nucleation could be recognized as the main nucleation reaction and the growth technique is very attractive, especially with a high efficiency growth ratio of 860% from 13 s to 23.2 s and a growth rate of 578 nm⁄s for the nanowire from 21.7 s to 22 s, including the incubation time, as shown in Fig. 2b. However, the growth kinetics are limited by the thermodynamic consideration of the total free energy. Following the investigation of Cu nanowire growth starting at 57 s in Fig. 3a (the complete video is in supporting information Movie S2), the growth of the nanowire’s length of becomes sluggish, with a corresponding increase in width of approximately 30% in the intermediate annealing process from 180 s to 300 s. The dynamic system would be saturated without the supply of more thermal energy; and the nucleation behavior would be transformed to compete with the total surface energy of system in the thermodynamic equilibrium state. Raising the evaporation rate from 349 s dramatically affected the supply of Cu atoms in the heterogeneous system to continue the dynamic growth of Cu nanowire. Statistical analysis of the nanowire growth rate in Fig. 3b clearly shows two kinds of growth mechanisms competing with each other. The controlling anisotropic growth behavior of the Cu nanowire can be extracted from systems exhibiting heterogeneous nucleation based on the dependence of temperature and precursor flux (shown in Fig. 3c). Simultaneously, there is another significant finding on the growth kinetics of the Cu nanowire. In the investigations of the growth kinetics, the connected condition of two Cu nanowires provides the explanation for the change in nanowire growth, as the in situ TEM video shows in supporting information Movie S3. Once the connected condition is cut off, the dynamic system immediately transformed, as the in situ observation result shows in supporting information Fig. S2 and Movie S4. From the observations, the interface of the Cu nanowires and the a-C substrate surface are connected. We propose that the connecting condition is critical for driving the anisotropic dynamics of the heterogeneous nucleation; once the connecting condition between nanowire and a-C substrate transformed, the initial axial growth of 7


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the nanowire ended. For a further detailed investigation of the growth of the Cu nanowire, a high-angle annular dark field scanning transmission electron microscopy (STEM-HAADF) measurement was carried out. The inset image in Fig. 4a shows the contrast difference on the base side of the Cu nanowire. To further discuss the details of the interconnection between the Cu nanowire and a-C lacey film in Fig. 4b, we provide the HRTEM image of the clear interface between the Cu nanowire and surface decoration in Fig. 4c. The surface decoration, composed of high-ratio graphite-like C structures of mixed layers, and the loop spacing between the layers with the average interlayer spacing of 0.344 nm, is close to the distance between graphite layers; we believe the surface decoration is the graphite-like loop (as shown in Fig. 4c) and related to the heterogeneous nucleation dynamics of the Cu nanowire growth. The heterogeneous interconnection of the graphite-like loop and Cu nanowire could be inferred to provide the critical nucleus seed for the Cu nanowire with an almost perfect match between the C hexagon spacing and the nearest distance in the closely packed plane of Cu, {111}. Under the conditions of heterogeneous interaction between Cu and a graphite-like loop, the Cu atomic layering construction and the arc determining the graphite-like loop created the foundation for Cu nanowire growth with a lower free energy barrier of nucleation, as shown in Fig. 4c. Simultaneously, the layer limitation of the graphite-like loop feature also resisted the Cu nucleation on the {111} plane and effectively enhanced the anisotropic ability of Cu crystallization. The graphite-like loop increased the solidification point of the Cu seed crystal and was attached to the a-C substrate. The corresponding EELS mapping indicates the location of the seed crystal by the oxide-signal contrast and confirmed the composition of the seed crystal, shown in Fig. 4d. Fig. 4c and 4d show the oxide layers in both nanowire samples. The surface oxide was formed by self-oxidation during exposure to the atmosphere, but the remarkable oxide signal decay of the EELS analysis indicates the influence of the Cu seed position at the nanowire surface; more detailed information is shown in Fig. S3. The cross-sectional geometry of the nanowires is also affected by the nucleation mechanism. Here, we provide complete information regarding the two-dimensional nucleation planes of the perfect single-crystal Cu nanowires by SEM and TEM analysis from different orientations. The cross-sectional geometry of the Cu nanowire shows the hexagonal column morphology with symmetry {111} and {002} plane in Fig. 4e; Fig. 4g and h are compared with the top view SEM image in Fig. 4f. The size dependence of the stability on the nanowire cross-sections has been widely studied; the construction of the nanowire cross-sectional geometry is enclosed by a low-index13. We confirmed the growth has a rectangular section bounded by the {002} 8


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plane as the lateral facets with the [110]-type of anisotropy axis, and the surface diffusion on {111} surface drives the growth of the defect-free Cu nanowire. These findings involving the graphite-like loop reflect the low binding energy and fast diffusion for Cu atoms or small Cu clusters on terraces of the basal plane. According to the melting point reduction and the Cu fusion heat change, the critical crystallization radius ratio of the Cu free droplet between 1000 K and 300 K is 3.029, which supports the increased critical radius in the UHV heterogeneous nucleation system. (The theoretical calculation is shown in the supporting information S4.) As the condition confirmed, the MD simulation of the Cu heterogeneous nucleation C shows the gradation crystallization rate of the Cu crystal is linear, with 29.97 ns and the dynamic simulation of the Cu migration and nucleation (shown in supporting information Fig. S5) also illustrates the limited probability of nucleation on the Cu nanowire surface from the vapor phase due to the high diffusion rate. For the Cu nanowire growth at 1000 K, we believe the Cu adatoms became clusters to reduce the mean free path and nucleated on the top plane of nanowire by surface diffusion. Unlike the typical dynamic mechanism of nanowire growth, the specific heterogeneous interaction between Cu and the a-C substrate induced the nucleation of defect-free Cu nanowires, as shown in the time-lapse TEM images of Fig. 5a to c. The synthesis mechanism is divided into three stages in Fig. 5d to f, which can be described as the solidification of Cu droplet formed on the a-C substrate, the Cu atomic layering to form the based layer as the foundation of seed crystal, and the nucleation dynamics of Cu nanowire driven by the surface diffusion along the close-packed {111} plane on the nanowire surface. Due to the limitation of the connecting formation between the graphite-like loop and the foundation of nanowire, the lateral nucleus on the {111} surface transformed to the two-dimensional nucleus on the {110} plane by surface diffusion and induced hexagonal column Cu nanowire growth. This high efficiency synthesis of defect-free Cu nanowire simplifies the fabrication by utilizing the a-C lacey film, which is beneficial for Cu nanowire applications. The new approach of controlling heterogeneous synthesis and providing effective modification of Cu nanowire nucleation probability without the inclusion of defects or catalysts is an important finding for one dimensional nanomaterial growth.

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Conclusions In conclusion, the highly efficient synthesis of defect-free Cu nanowire growth has been investigated in situ via UHV-TEM. The growth technique without inclusion of defects or catalysts contributed to the high aspect ratio of nanowire growth in a heterogeneous nucleation system. During the UHV thermal annealing process, the stacking relation between Cu and the graphite-like loop was the critical basis of the defect free Cu nanowire growth. The kinetic process of the Cu nanowire two-dimensional nucleation on {110} top plane was driven by the Cu surface diffusion on the {111} nanowire surface. The growth kinetic model has been corroborated with in situ HRTEM, EELS and EDS. These results indicate that the anisotropic properties of the lowest surface energy and the seed crystallization reactions on the a-C lacey film surface drive the nucleation dynamics of high quality Cu nanowires.

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ASSOCIATED CONTENT Supporting Information Materials composition analysis by EELS and Raman spectrum, MD simulation report and the thermodynamic calculation of critical nucleation radius (pdf), plus four videos (avi) of the in situ TEM measurements.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Author Contributions T.-Y.L. fabricated the sample and conducted the in situ reaction experiments. T.-Y.L. and Y.L.C. performed the MD simulation and the theoretical calculation for the experiments. T.-Y.L., C.-W.H. and W.-W.W. conceived the study and designed the research. T.-Y.L. and G.-M.H. analyzed the diffraction data and atomic structure. C.-F.C performed the fabrication of FIB sample. C.-Y.H performed the EELS analysis. T.-Y.L., K.-C.L., W.-W.W. and L.-J.C. wrote the manuscript. T.-Y.L. performed the experiments with the support from Y.-C.L, K.-C.L., W.-W.W. and L.-J.C.

ACKNOWLEDGMENT The authors acknowledge the support by the Ministry of Science and Technology through grants 103-2221-E-009-222-MY3, MOST 106-2628-E-009-002-MY3

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FIGURES

Figure 1. Schematic illustration of various Cu nanostructures investigated under UHV-TEM. (a) The illustration of evaporation, deposition and surface migration pathways of forming the Cu droplet in the VVD system. (b) Unit square area of copper grid showing the observation area from the edge to the middle. (c-e) TEM images showing the effect of different Cu nanostructures on the resultant products under the same environmental conditions. (f-g) EDS mapping analysis of the copper nanostructures in TEM image (e) distinguishing the a-C lacey film (f), and the copper nanostructures (g).

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Figure 2. Characteristics of Cu nucleation via the in situ TEM observation. (a) In situ TEM investigation of the growth of Cu nanostructures via real-time observation from 13 s to 23 s. System A, B and C shows the Cu droplet flowing along the edge of a-C film lacey structures, the growth of Cu nanowire and the Cu nanoparticle coarsening, respectively. (b) Statistical analysis of nucleation kinetics of different Cu nanostructures in three different synthesis systems.

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Figure 3. Kinetics of Cu nanowire growth. (a) TEM image sequence from 57 s to 409 s, with heating at 700 and at 750 . The growth behaviors could be divided into two stages. During the observation, the growth in length is initially the dominant growth mode. Dotted lines denote the length statistics benchmark of the Cu nanowire. (b) The statistical analysis shows the sluggish growth in width and the rapid growth in length promoted by raising the supply of thermal energy. (c) The statistical analysis shows the aspect ratio change in different growth kinetics by thermal condition control. (The temperature was raised from 700 to 750 at the intermediate process at approximately 325 s to 328 s.)

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Figure 4. SEM, HRTEM and EELS analysis of defect-free Cu nanowires from different observation orientations. (a) TEM image of the Cu nanowire with an insert of the STEM-HAADF image showing the contrast image featuring the structure on the base side of the Cu nanowire. (b) SEM image showing a side view of the bottom-up growth of the Cu nanowire connecting to the a-C lacey film. (c) HRTEM image of the interface between the nanowire seed crystal and graphite-like loop on the a-C surface. (d) Elemental mapping by EELS on the base side of the Cu nanowire. (e) Cross-section sample of the Cu nanowire produced by E-gun and FIB fabrication. (f) SEM image showing top view of the hexagonal column Cu nanowire located on the edge of the a-C lacey film. The cross-section HRTEM images are shown in (g) and (h); the crystal geometry of the Cu nanowire consists of four {111} plane and two {200} plane along [1 10] zone axis.

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Figure 5. Growth mechanisms of the Cu nanowire through the adatoms surface migration, droplet crystallization and nanowire outgrowth on the a-C substrate. (a) In situ TEM observation area of the seed crystal nucleus (inside the red square) at the step edge position. (b) Critical crystallization of seed crystal formed. (c) Cu nanowire bottom-up outgrowth along the stable growth orientation. Schematic illustration of Cu adatoms migration on the a-C film surface (d) by increasing the concentration of the Cu source, Cu droplet crystallization (e) leads to nanowire seed crystallization at the hetero surface. (f) The Cu nanowire growth by the Cu adatoms surface diffusion along the nanowire {111} surface and the concentration gradient increased across the nucleation barrier of the system. REFERENCES

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of a Microcrucible Mechanism of Nanowire Growth. Science. 2014, 344, 623-626. 18. Chou, Y. C.; Hillerich, K.; Tersoff, J.; Reuter, M. C.; Dick, K. A.; Ross, F. M. Atomic-Scale Variability and Control of III-V Nanowire Growth Kinetics. Science. 2014, 343, 281-284. 19. Wei, Z.; Xiaochuan, D.; Charles, M. L. Advances in nanowire bioelectronics.

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31. Dong, L.; Tao, X.; Hamdi, M.; Zhang, L.; Zhang, X.; Ferreira, A.; Nelson, B. J. Nanotube Fluidic Junctions: Internanotube Attogram Mass Transport through Walls.

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For TOC only 131x87mm (299 x 299 DPI)


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Figure 1. Schematic illustration of various Cu nanostructures investigated under UHV-TEM. (a) The illustration of evaporation, deposition and surface migration pathways of forming the Cu droplet in the VVD system. (b) Unit square area of copper grid showing the observation area from the edge to the middle. (c-e) TEM images showing the effect of different Cu nanostructures on the resultant products under the same environmental conditions. (f-g) EDS mapping analysis of the copper nanostructures in TEM image (e) distinguishing the a-C lacey film (f), and the copper nanostructures (g). 133x92mm (300 x 300 DPI)


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Figure 2. Characteristics of Cu nucleation via the in situ TEM observation. (a) In situ TEM investigation of the growth of Cu nanostructures via real-time observation from 13 s to 23 s. System A, B and C shows the Cu droplet flowing along the edge of a-C film lacey structures, the growth of Cu nanowire and the Cu nanoparticle coarsening, respectively. (b) Statistical analysis of nucleation kinetics of different Cu nanostructures in three different synthesis systems. 136x129mm (299 x 299 DPI)


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Figure 3. Kinetics of Cu nanowire growth. (a) TEM image sequence from 57 s to 409 s, with heating at 700 ℃ and at 750 ℃. The growth behaviors could be divided into two stages. During the observation, the growth in length is initially the dominant growth mode. Dotted lines denote the length statistics benchmark of the Cu nanowire. (b) The statistical analysis shows the sluggish growth in width and the rapid growth in length promoted by raising the supply of thermal energy. (c) The statistical analysis shows the aspect ratio change in different growth kinetics by thermal condition control. (The temperature was raised from 700℃ to 750℃ at the intermediate process at approximately 325 s to 328 s.) 134x95mm (299 x 299 DPI)


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Figure 4. SEM, HRTEM and EELS analysis of defect-free Cu nanowires from different observation orientations. (a) TEM image of the Cu nanowire with an insert of the STEM-HAADF image showing the contrast image featuring the structure on the base side of the Cu nanowire. (b) SEM image showing a side view of the bottom-up growth of the Cu nanowire connecting to the a-C lacey film. (c) HRTEM image of the interface between the nanowire seed crystal and graphite-like loop on the a-C surface. (d) Elemental mapping by EELS on the base side of the Cu nanowire. (e) Cross-section sample of the Cu nanowire produced by E-gun and FIB fabrication. (f) SEM image showing top view of the hexagonal column Cu nanowire located on the edge of the a-C lacey film. The cross-section HRTEM images are shown in (g) and (h); the crystal geometry of the Cu nanowire consists of four {111} plane and two {200} plane along [1 ̅10] zone axis. 98x191mm (299 x 299 DPI)


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Figure 5. Growth mechanisms of the Cu nanowire through the adatoms surface migration, droplet crystallization and nanowire outgrowth on the a-C substrate. (a) In situ TEM observation area of the seed crystal nucleus (inside the red square) at the step edge position. (b) Critical crystallization of seed crystal formed. (c) Cu nanowire bottom-up outgrowth along the stable growth orientation. Schematic illustration of Cu adatoms migration on the a-C film surface (d) by increasing the concentration of the Cu source, Cu droplet crystallization (e) leads to nanowire seed crystallization at the hetero surface. (f) The Cu nanowire growth by the Cu adatoms surface diffusion along the nanowire {111} surface and the concentration gradient increased across the nucleation barrier of the system. 131x87mm (299 x 299 DPI)


In situ Investigation of Defect-free Copper Nanowire Growth - Nano

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