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Nano-soldering of magnetically aligned three-dimensional nanowire networks

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 21 (2010) 115604 (7pp)

doi:10.1088/0957-4484/21/11/115604

Nano-soldering of magnetically aligned three-dimensional nanowire networks Fan Gao1 and Zhiyong Gu1,2,3 1

Department of Chemical Engineering, University of Massachusetts Lowell, One University Avenue, Lowell, MA 01854, USA 2 Nanomanufacturing Center, University of Massachusetts Lowell, One University Avenue, Lowell, MA 01854, USA E-mail: Zhiyong [email protected]

Received 23 December 2009, in final form 1 February 2010 Published 24 February 2010 Online at stacks.iop.org/Nano/21/115604 Abstract It is extremely challenging to fabricate 3D integrated nanostructures and hybrid nanoelectronic devices. In this paper, we report a simple and efficient method to simultaneously assemble and solder nanowires into ordered 3D and electrically conductive nanowire networks. Nano-solders such as tin were fabricated onto both ends of multi-segmented nanowires by a template-assisted electrodeposition method. These nanowires were then self-assembled and soldered into large-scale 3D network structures by magnetic field assisted assembly in a liquid medium with a high boiling point. The formation of junctions/interconnects between the nanowires and the scale of the assembly were dependent on the solder reflow temperature and the strength of the magnetic field. The size of the assembled nanowire networks ranged from tens of microns to millimeters. The electrical characteristics of the 3D nanowire networks were measured by regular current–voltage ( I –V ) measurements using a probe station with micropositioners. Nano-solders, when combined with assembling techniques, can be used to efficiently connect and join nanowires with low contact resistance, which are very well suited for sensor integration as well as nanoelectronic device fabrication. (Some figures in this article are in colour only in the electronic version)

of methods are normally slow, expensive, and difficult to scale up into manufacturing processes. Even though self-assembly and atomic/molecular level manipulation have achieved significant progress, many challenges still remain. For example, in many cases the selfassembled structures are fragile, not permanent, and difficult to transfer to another surface, and thus it is difficult to form functional circuit or integrated nanoelectronics. One hurdle to overcome in the processing and manufacturing of nanoelectronic devices is that, the assembled nanostructures have to be connected/interconnected to each other and communicate in a functional way so that complex electronic device can be made. To date, some efforts have been made in the interconnect formation of nano-components, for example, focused electron beam (FEB) or focused ion beam (FIB), which is enabled by strongly emitted electron beams with a focus spot less than 1 nm in diameter and localized thin metal layer deposition [11–13]. However, this technique suffers from contamination of the uncompleted

1. Introduction Self-assembly is a process that enables a disordered system to organized structures on all scales ranging from molecules to galaxies [1–4]. In the field of nanoscience and nanotechnology, self-assembly has emerged as a new enabling bottom–up approach in generating well-defined 1D, 2D, and 3D structures using such nano-building blocks as nanotubes, nanowires, nanoparticles, polymers, and molecules [5–8]. Based on the self-assembly principles, previous efforts including molecular forces, fluidic flow, electrical and magnetic assisted assembly, and many new methods mimicking biological phenomena, have shown promise in integrating nano-units into ordered and complex structures [8]. On the other hand, molecular or atomic level manipulation has been enabled by AFM or optical tweezers [9, 10], which can be used to perform such tasks as pushing, pulling, cutting or aligning nanoscale objects. Compared with self-assembling methods, these types 3 Author to whom any correspondence should be addressed.

0957-4484/10/115604+07$30.00

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© 2010 IOP Publishing Ltd Printed in the UK

Nanotechnology 21 (2010) 115604

F Gao and Z Gu

2.3. Nanowire assembly and soldering

precursor decomposition and high operating cost. Annealing or sintering, which has been used considerably in forming connections between electronic components, may damage or destroy many useful structures or components, especially for hybrid structures with both organic and inorganic materials. Significant improvements are needed in order to achieve effective nanoscale interconnections with electrical and thermal functionality [14–16], and efficient and effective methods to integrate nanoelectronic devices are of immediate necessity. It is well known that solders and soldering technique are widely used in electronic assembly and packaging. In recent years, lead-free nano-solders have received significant attention and have shown promise in the assembly and integration of micro- and nanoelectronic devices [14, 15]. In this paper, we demonstrate a simple technique to solder multi-segmented nanowires into large-scale conductive 3D networks, in combination with a magnetic field assisted assembly technique. The tin solders were successfully fabricated onto two ends of multi-segmented nanowires by a template-assisted electrodeposition method. These nanowires with solder ends were then assembled and soldered together in a fluidic system with an external magnetic field control. After assembly and soldering, the electrical conductance of the nanowire networks was measured by standard current–voltage techniques and excellent conductivity was achieved.

Solder reflow is a process used in the electronics industry to solder electronic components or chips onto printed circuit boards (PCBs). The solder reflow process is normally done using a temperature-controlled and automatic reflow oven. There are normally four stages of a solder reflow process: preheat, thermal soak, reflow and cooling. Solder manufacturers normally supply the reflow profiles to industrial customers. In this report, we developed a new process for simultaneous nanowire assembly and soldering in a liquid medium with a high boiling point. To compare this new reflow method with the traditional soldering process, a control experiment was conducted for comparison, in which the solder nanowires were reflowed on a silicon (Si) substrate in an inert atmosphere. This control experiment was close to the standard solder reflow process in traditional electronics assembly and manufacturing. Figure 1(a) presents the schematic of the experimental setup for the solder nanowire reflow process on a Si surface, while figure 1(b) shows the setup for the liquid-phase assembly and solder reflow, where a hot plate with temperature and magnet control was used. 2.3.1. Solder reflow of nanowires on Si substrates in an inert atmosphere. In the case of solder reflow on Si substrates in an inert atmosphere, a droplet of nanowire suspension in ethanol was put onto a piece of Si wafer of size 1 cm × 1 cm. Upon solvent evaporation, the prefabricated nanowires were kept on the Si surface. The sample was then sent into a temperature programmable tube reflow oven, as shown in figure 1(a). Nitrogen (N2 ) gas was first purged for 10 min to keep the tube environment oxygen-free and avoid nanowire oxidation. After purging, the pre-set temperature profile was run in a similar way to the industrial reflow profile. Briefly, the tube temperature was increased from room temperature (∼25 ◦ C) to about 230 ◦ C, slightly below the melting point of tin, in a time duration of about 7–8 min. Since there was no flux used in this case, there was no clear distinction between the pre-heat step and the thermal soak step. The temperature was then increased from 230 ◦ C to the maximum of 250 ◦ C, being kept there for a short while, and then lowered to 230 ◦ C again. The total duration in this reflow step was ∼5 min. After the reflow step the oven was cooled down to ambient temperature in about 15– 20 min. The actual temperature inside the tube was monitored with a thermometer. The dwell time (the period above the solder melting point) was minimized, normally within 5 min, to reduce inter-diffusion of metals and ensure optimum solder reflow properties. After the heating system cooled down to room temperature, the Si wafer with nanowires was taken out for characterization.

2. Experiments and methods 2.1. Chemicals and materials Commercial plating solutions of gold (Au) (Techni Gold25E), nickel (Ni) (Techni Nickel Sulfamate Bath RTU), and tin (Sn) (Techni Tin) were purchased from Technic, Inc. The tin plating solution was prepared by using the Techni tin solution together with a corresponding make-up solution and antioxidant (also obtained from Techni). Dichloromethane, triethylene glycol (TEG) and ethanol were purchased from Fisher Scientific. All the chemicals were used as received. Nitrogen gas (Industrial, 99.8%) was purchased from Airgas East. Polycarbonate membranes were purchased from Whatman (now part of GE Healthcare). 2.2. Nanowire fabrication Multi-segmented nanowires with solder materials were fabricated by a template-assisted electrodeposition method. In principle, the templates can be polycarbonate (PC) or anodic aluminum oxide (AAO). In this research PC templates were used and the nominal pore size was 50 nm. Different segments/layers of the nanowires, including gold (Au) and nickel (Ni), were prepared by sequential plating through changing the electrolytic solution after each segment. Tin (Sn) was used as a model solder material in the nanowires. After nanowire fabrication, the PC templates were dissolved by dichloromethane and then cleaned by dichloromethane and ethanol, each for at least three times. The detailed fabrication process has been described in previous papers by our group [17].

2.3.2. Nanowire assembly and soldering in a liquidphase without a magnetic field. Liquid-phase based reflow is a promising method for large-scale 3D self-assembly, which not only provides the fluid medium for assembly in three dimensions but also supplies sufficient temperature for soldering nanowires. Gu et al has reported three-dimensional electrically interconnected nanowire networks formed by 2

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F Gao and Z Gu

Figure 1. Schematic of nanowire reflow and assembly with nano-solders. (a) Experiment setup for nano-solder reflow on a Si substrate in an inert atmosphere; (b) experiment setup for liquid solder reflow (container not real size).

and the speed was carefully controlled. During the heating process, the higher speed of the magnetic field will dominate the direction of fluid flow.

diffusion bonding in a liquid medium [18]. Here, we used a similar experimental setting, but introduced solder material onto the nanowires and used a liquid medium for the solder reflow process (we will call this the ‘liquid reflow’ method thereafter). The temperature profile of the liquid reflow was similar to the reflow on a Si wafer, which was heating the solder material to above the melting point and forming nanowires solder junctions. The solvent used in liquid reflow was triethylene glycol (TEG) with a boiling point of 285 ◦ C, higher than that of the tin material (232 ◦ C). Figure 1(b) shows the schematic of the experimental setup for a liquid reflow process. A small glass vial with 0.5 ml TEG was pre-heated to 120 ◦ C on a hot plate for 5 min to warm up the solvent. Ten drops of the nanowire suspension were then put into the glass vial. The vortex of the TEG medium helped mix and disperse the nanowires in the medium. The solvent temperature dropped to about 70–80 ◦ C due to the addition of the ‘cold’ nanowire suspension, and it took about 4–5 min to reach 120 ◦ C again. The temperature was then heated to 200 ◦ C, which was similar to the thermal soak zone. After that, the temperature was increased from 200 to 250 ◦ C, which took about 4–5 min. The peak temperature of 250 ◦ C was kept for 1– 2 min. After the liquid reflow process was finished, the sample was cooled to room temperature and ethanol was used to wash the samples. The actual temperature inside the liquid solvent was monitored by a thermometer.

2.4. Instruments for nanowire fabrication and property characterization Nanowire fabrication was performed on a Princeton Applied Research (PAR) Model 362 Potentiostat. Vapor-phase nanosolder reflow was carried out in a Thermo Scientific Lindberg Blue M tube furnace with programmable temperature control. De-ionized water was obtained from a Barnstead E-pure system (Model # D4541) at 18.2 M cm. Optical images of the nanowires fabricated were obtained with an Olympus CX41 microscope equipped with a DP-71 CCD camera. A JEOL JSM-7401F field-emission scanning electron microscope (FESEM) was used to characterize the size, surface morphology and composition of the nano-solders and nanowires. Current– voltage measurements were conducted on a Rucker & Kolls 667 prober station connected to a Keithley 2400 source meter.

3. Results and discussion By using the template-assisted electrodeposition method, solder material such as tin has been fabricated onto multisegmented metal nanowires. In order to form solder joints and interconnects between nanowires, one desired structure is to synthesize solders on both ends of the nanowires. To optimize solder reflow, two types of nanowires were fabricated. Figure 2 shows the SEM images of the two types of multisegmented nanowires, with a solder layer on both ends, that have been fabricated by electrodeposition method. Those nanowires were fabricated in PC templates with a nominal diameter of 50 nm, and the length was controlled at around 5–6 μm. Figure 2(a) shows a symmetric structure with two solder ends in the configuration of ‘solder–barrier layer– base layer–barrier layer–solder’ with tin (Sn), nickel (Ni) and gold (Au) serving as solder material, barrier material and base material, respectively. The different segments can be clearly distinguished by the contrast from the image. This type of structure is suitable for solder reflow on a Si wafer in an inert atmosphere, as shown in figure 1(a). In

2.3.3. Nanowire assembly and soldering in a liquid phase with a magnetic field. Magnetic field assisted assembly has become a powerful tool in aligning nanowires that contain magnetic components. Magnetic nanoparticles and nanowires such as Co, Ni, Fe and alloys can be manipulated in an external magnetic field. In this report, a rotating magnetic field was generated by a combined hot-plate magnetic-stirrer device, where solder nanowires were assembled and soldered in a pre-heated TEG solvent, and the stirring rate was controlled by the knob. The movement of nanowires should be along the direction of the rotating fluid flow driven by the external magnetic field, since each nanowire contains a long nickel segment which mimics a tiny magnetic stirrer. When the solder nanowire suspension was slowly dropped into the preheated organic solvent, the rotating magnetic field was initiated 3

Nanotechnology 21 (2010) 115604

F Gao and Z Gu

Figure 2. Multi-segmented nanowires with nano-solder on both ends. (a) Sn–Ni–Au–Ni–Sn nanowires for solder reflow in an inert atmosphere; (b) Sn–Au–Ni–Au–Sn nanowires for liquid-phase reflow. (Scale bar: 1 μm.)

Figure 3. (a) SEM image of nano-solder reflow on Sn–Au–Ni–Au–Sn nanowires on a Si substrate in N2 . The inset is a magnified image showing many solder balls and joints formed during solder reflow. (b) SEM image of nano-solder reflow on Sn–Au–Sn nanowires on a Si substrate in N2 . The inset is the SEM image of one single Sn–Au–Sn nanowire before reflow.

Sn solder segments melted and formed almost spherical solder balls onto the nanowires. During this process, the Ni layer was critical for the prevention of significant diffusion between Sn and Au. In order to demonstrate the effectiveness of the Ni diffusion barrier layer, we fabricated nanowires without a Ni segment (figure 3(b) inset) as a control experiment. Figure 3(b) shows that, without any Ni barrier layer, the ‘solder–base layer–solder’ (‘Sn–Au–Sn’) nanowires had dramatic diffusion between Sn and Au during solder reflow, through which the nanowire structure was severely damaged. For the solder reflow of the Sn–Ni–Au–Ni–Sn nanowires, solder joints were formed on some of the nanowires when two nanowires were close to each other, which indicates the technical feasibility of interconnect formation between nanowires using lead-free nano-solders. However, because the nanowires were attached on the Si surface during preparation and could not move freely later on, there was no assembly during the process and the joints were formed by randomly aligned nanowires. Even though these nanowires were soldered or partially soldered, it is difficult to use them directly or peel them off from the substrate, since the van der Waals attraction is strong enough to keep them on the Si surface. Thus, it is difficult to integrate them into nanoelectronic devices without effective manipulation or pattern transfer.

principle, other substrate can also be used, such as copper, gold, or other metallic or non-metallic substrates. The nickel segment on the nanowires can effectively avoid the significant diffusion/dissolution between tin and gold, as demonstrated in previous research [17]. This structure has been fabricated and used in the solder reflow in nitrogen. Figure 2(b) presents another symmetric structure with two solder ends, ‘solder–wetting layer–functional layer–wetting layer–solder’ with Sn, Au and Ni as solder, wetting and functional material, respectively. This type of structures serves a different purpose to the one shown in figure 2(a), and is a good model system for 3D assembly and reflow. The magnetic nickel segment in these nanowires is the functional layer and can be easily controlled in a magnetic field. A gold layer is good for solder wetting and bonding formation on the functional nickel layer. If necessary, other functional segments, such as sensor or electronic components, can be fabricated on the nanowires to enable device integration. We had reported that the reflow environment is critical for wafer level nano-solder reflow, and nitrogen can effectively prevent the solder nanowires from severe oxidation [17]. For this reason, all the wafer level 2D reflow on the Si substrate was processed in nitrogen. Figure 3(a) shows the result of reflowed Sn–Ni–Au–Ni–Sn five-segment nanowires in nitrogen. The 4

Nanotechnology 21 (2010) 115604

F Gao and Z Gu

Figure 4. 3D random nanowire networks formed in a liquid reflow without any magnetic field. (a) Overview of the random nanowire networks composed of Sn–Au–Ni–Au–Sn nanowires; (b) SEM image of some solder joints formed; (c) one Sn solder reflowed on the nanowire.

scale up the magnetic assembly process to obtain large-scale structures, because most of the research so far involved the manipulation of ferromagnetic nanowires on surfaces, which possess many drawbacks, such as substrate confinement, solvent evaporation, droplet edge accumulation, etc, which mostly limit nanowire movements to only one or two dimensions. Also, after magnetic alignment, the assembled structures were difficult to transfer and post-process by other techniques. The liquid reflow technique developed in this report eliminates the edge and substrate effect so that nano-elements can freely move and form large-scale 3D structures. During this assembly process, if solder materials are present at both ends of the nanowires, the assembled nanowires will be joined together by the solder materials and permanent interconnects will form upon cooling. Figure 5(a) shows the liquid reflow results with magnetic field control, demonstrating 3D well aligned and almost parallel nanowire network structures on a very large-scale. Figures 5(b) and (c) are higher magnification SEM images of a local zoomed in area showing two different types of joints between nanowires. One is a head-to-head joint (figure 5(b)) and the other is head-totail joint (figure 5(c)). Due to the unique network structures, both bondings are present and contribute to the formation of the conductive structures. It is seen that some degree of diffusion occurred on the Sn–Ni–Au–Ni–Sn nanowires during assembly and soldering, and both the gold and nickel segments were difficult to discern after joining. This may be due to the intermetallic diffusion between gold and tin during the temperature ramp-up step of the liquid reflow process and/or when the temperature went over the melting point of tin. The melted tin may move along the nanowires in the liquid solvent and diffuse over the surface of the nickel segment. However, overall the nanowire structures were very well maintained and good solder joints were formed between the nanowires. In 2D reflow on Si wafers, the nickel segment in the nanowires can be viewed as a barrier layer to avoid diffusion between tin and gold, while in the 3D liquid reflow process, the nickel segment served as a functional ferromagnetic stirrer to control the liquid flow along a certain direction. During the liquid reflow, several factors may affect the nanowire network

Fluidic assembly based on solder materials has shown promise in integrating macro- and micro-scale materials and in forming 3D functional electronics or devices [19, 20]. In principle, it is possible for this technique to be scaled down to the nanometer level. In order to form 3D nanowire structures, a liquid medium is desirable. High boiling point solvents can be used, which provide a similar reflow environment to a nitrogen purge. Also, there is much less oxygen in liquid than in the air, thus avoiding solder material oxidation. More importantly, the fluidic flow will provide the necessary force to enable 3D assembly of nano-components in a largescale. In this work, many nano-solder joints were formed successfully between multi-segmented nanowires in a liquid reflow process. Five-segment nano-solder nanowires ‘Sn–Au– Ni–Au–Sn’ were investigated by a liquid reflow at 250 ◦ C, which is nearly 20 ◦ C higher than the melting point of tin (232 ◦ C) but still below the boiling point of the solvent TEG (285 ◦ C). This optimized structure, with a nickel segment in the middle of the nanowire, is a good system to realize real interconnections between nanowires. The results of liquid reflow assembly in the form of 3D nanowire structures are presented in figure 4, and the morphology of the sample before liquid reflow can be found in figure 2(b). Figure 4(a) shows the result of randomly assembled nanowire networks in the liquid reflow without magnetic field control. Figure 4(b) is a higher resolution SEM image showing that the solders reflowed on nanowires and some joints were obtained. Since the nanowires were not aligned, those joints between nanowires were formed by chance and it is not easy to get large-scale aligned structures. The small-scale alignment of nanowires mostly came from the nanowire fabrication process (due to non-uniformity of the PC template). Figure 4(c) presents a perfect single spherical solder formed on one end of a nanowire, which indicates that the TEG solvent can effectively protect solder material from oxidation and help the wetting of solder onto the nickel segment. Magnetic alignment and assembly of ferromagnetic nanowires have been studied and developed by several research groups to achieve hierarchical structures on substrates or between electrodes [21–26]. However, it was difficult to 5

Nanotechnology 21 (2010) 115604

F Gao and Z Gu

Figure 5. SEM images of large-scale 3D nanowire networks formed in a liquid reflow with the aid of an external magnetic field. (a) Overview of well aligned nanowire networks composed of Sn–Au–Ni–Au–Sn nanowires; (b) ‘head-to-head’ bonding by tin solder; (c) ‘head-to-tail’ bonding by tin solder.

Figure 6. I – V characteristics of soldered nanowire networks at room temperature. (a) Optical image of the nanowire network; (b) I – V results.

from the liquid medium to gold electrodes which were lithographically formed on a Si/SiO2 substrate. Figure 6 shows the current–voltage ( I – V ) characteristics of a magnetically aligned and soldered Sn–Au–Ni–Au–Sn nanowire network. Figure 6(a) is the optical image of the nanowire network on patterned gold electrodes, and the positions where the micropositioner located are visible. For the sample shown, the size is in the millimeter scale. In figure 6(b), the black line indicates the result for blank electrodes without any nanowires on them, so the current was almost zero, indicating an open circuit. When the soldered nanowire network was transferred onto the gold electrodes, the measured resistance was around 1–1.2 k and the network showed a very conductive nature. This is contributed by the many solder joints (as shown in figure 5) in the large-scale 3D nanowire network and the low contact resistance of the solder joints. Since the gold electrodes were inert and the contact area between nanowire network and electrodes was large, the contact resistant between the nanowire network and electrodes is relatively small and thus negligible. Therefore, the total resistance from the I –V measurements was dominated by the nanowire network, which must be influenced by the number of nanowires and the quality of interconnected solder joints.

formation and the scale of the network formed. First, the concentration of nanowires is important in determining the final size of network; the more nanowires added in the liquid medium, the larger the size of network obtained; Second, the rate of the rotating magnetic field is also important for liquid reflow assembly. The multi-segmented nanowires contain interfaces between different layers where limited stress force can be tolerated, and too high a speed of magnetic rotation may affect or even destroy the multi-segmented structure of the nanowires. For example, since the solder segments will become liquid above the melting point, too high a speed may lead to the separation of the solder droplets from the magnetic segment and thus the assembled structures become damaged. Third, the length of each nanowire segment may affect assembly and soldering since the higher rate of rotating magnetic field needs to be applied by shorter ferromagnetic segments fabricated on nanowires to ensure sufficient driving force in the direction of nanowire movement. After the liquid-phase assembly and solder reflow, the electrical properties of the 3D ordered nanowire networks was measured at room temperature. Before measurement, the nanowire networks were washed several times using ethanol. The visible nanowire network clusters were easily transferred 6

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F Gao and Z Gu

In principle, this liquid-phase solder reflow technique can be extended to multi-segmented nanowires with other functional segments such as diode or sensing elements. Other assembling techniques can also be combined with this liquid solder reflow method to form other types of large-scale 3D structures. More accurate alignment or complex structures may also be facilitated by coating nanowires with organic molecules or polymeric materials which will lead to potential biosensor or biotechnology applications.

[8] Gu Z and Gracias D H 2008 Nanowire assembly and integration Nanofabrication Fundamentals and Applications ed A A Tseng (Singapore: World Scientific) pp 187–211 [9] Avouris P 1995 Manipulation of matter at the atomic and molecular levels Acc. Chem. Res. 28 95–102 [10] Agarwal R, Ladavac K, Roichman Y, Yu G, Lieber C and Grier D 2005 Manipulation and assembly of nanowires with holographic optical traps Opt. Express 13 8906–12 [11] Banhart F 2001 The formation of a connection between carbon nanotubes in an electron beam Nano Lett. 1 329–32 [12] Molhave K, Madsen D N, Dohn S and Boggild P 2004 Constructing, connecting and soldering nanostructure by environmental electron beam deposition Nanotechnology 15 1047–53 [13] Valizadeh S, Abid M, Hern´andez-Ram´ırez F, Rodr´ıguez R A, Hjort K and Schweitz J Aa 2006 Template synthesis and forming electrical contacts to single Au nanowires by focused ion beam techniques Nanotechnology 17 1134–9 [14] Cui Q, Gao F, Mukherjee S and Gu Z 2009 Joining and interconnect formation of nanowires and carbon nanotubes for nanoelectronics and nanosystems Small 5 1246–57 [15] Zheng W and Jacobs H O 2006 Self-assembly process to integrate and connect semiconductor dies on surfaces with single-angular orientation and contact-pad registration Adv. Mater. 18 1387–92 [16] Lee J-Y, Connor S T, Cui Y and Peumans P 2008 Solution-processed metal nanowire mesh transparent electrodes Nano Lett. 8 689–92 [17] Gao F, Mukherjee S, Cui Q and Gu Z 2009 Synthesis, characterization, and thermal properties of nanoscale lead-free solders on multisegmented metal nanowires J. Phys. Chem. C 113 9546–52 [18] Gu Z, Ye H, Bernfeld A, Livi K J T and Gracias D H 2007 Three-dimensional electrically interconnected nanowire networks formed by diffusion bonding Langmuir 23 979–82 [19] Zheng W, Buhlmann P and Jacobs H O 2004 Sequential shape-and-solder-directed self-assembly of functional microsystems Porc. Natl Acad. Sci. 101 12814 [20] Gracias D H, Tien J, Breen T L, Hsu C and Whitesides G M 2000 Forming electrical networks in three dimensions by self-assembly Science 289 1170–2 [21] Whitney T M, Searson P C, Jiang J S and Chien C L 1993 Fabrication and magnetic properties of arrays of metallic nanowires Science 261 1316–9 [22] Tanase M, Bauer L A, Hultgren A, Silevitch D M, Sun L, Reich D H, Searson P C and Meyer G J 2001 Magnetic alignment of fluorescent nanowires Nano Lett. 1 155–8 [23] Yoo B, Rheem Y, Beyermann W P and Myung N V 2006 Magnetically assembled 30 nm diameter nickel nanowire with ferromagnetic electrodes Nanotechnology 17 2512 [24] Hangarter C M and Myung N V 2005 Magnetic alignment of nanowires Chem. Mater. 17 1320–4 [25] Hangarter C M, Rheem Y, Yoo B, Yang E and Myung N V 2007 Hierarchical magnetic assembly of nanowires Nanotechnology 18 205305 [26] Ye H, Gu Z, Yu T and Gracias D H 2006 Integrating nanowires with substrates using directed assembly and nanoscale soldering IEEE Trans. Nanotechnol. 5 62–6

4. Conclusion Multi-segmented nanowires with tin solder at both ends have been fabricated by a template-assisted electrodeposition method. These nanowires were successfully assembled and joined together in a liquid-phase solder reflow process. In particular, by using the magnetic field assisted method, the nanowires were self-assembled into large-scale 3D network structures, which were conductive due to the low contact resistance of the solder joints formed. The method developed here and the 3D nanowire network structures have great potential in enabling sensor and hybrid device applications.

Acknowledgments Financial support from the NSF Center for High-rate Nanomanufacturing (CHN) is greatly acknowledged. We thank Mr Xiaopeng Li for the design and fabrication of the microelectrodes.

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