Site-selective core/shell deposition of tin on multi-segment nanowires for magnetic assembly and soldered interconnection

The field of nanotechnology continues to grow with the ongoing discovery and characterization of novel nanomaterials with unconventional size-dependent properties; however, the ability to apply modern manufacturing strategies for practical device design of these nanoscale structures is significantly limited by their small size. Although interconnection has been previously demonstrated between nanoscale components, such approaches often require the use of expensive oxidation-resistant noble metal materials and time-consuming or untargeted strategies for welded interconnection such as laser ablation or plasmonic resonance across randomly oriented component networks. In this work, a three-segment gold–nickel–gold nanowire structure is synthesized using templated electrodeposition and modified via monolayer-directed aqueous chemical reduction of tin solder selectively on the gold segments. This core/shell nanowire structure is capable of directed magnetic assembly tip-to-tip and along substrate pads in network orientation. Upon infrared heating in a flux vapor atmosphere, the solder payload melts and establishes robust and highly conductive wire–wire joints. The targeted solder deposition strategy has been applied to various other multi-segment gold/nickel nanowire configurations and other metallic systems to demonstrate the capability of the approach. This core/shell technique of pre-loading magnetically active nanowires with solder material simplifies the associated challenges of size-dependent component orientation in the manufacture of nanoscale electronic devices.

Traditional interconnect strategies for electronic device manufacture typically involve the mounting of components via wetted solder contact [28,29] or thermal/ultrasonic welding of connective wiring [30,31]: micron-scale techniques which even now are challenged by reliability issues related to ongoing efforts at miniaturization [32,33] and environmental conscientiousness [34].Metallic pillaring between components [35] and sintered welding [36] have been conducted at the nanoscale as three-dimensional (3D) additive manufacturing techniques [37], mitigating some issues of miniaturization by increasing component density with innovative packaging.However, to fully realize the potential of nanoscale building blocks and their novel capabilities [5][6][7], new joining strategies must be considered which enable deliberate geometric configurations [38] while accommodating the unique size-based challenges inherent to their assembly [39,40] and interconnection.One-dimensional metallic nanowires fundamentally resemble conventional wiring components in device packaging; however, their nanoscale diameter and large aspect ratio exclude traditional lever arm-directed wiring approaches [41], instead favoring self-assembly methods such as magnetic alignment [42].In the literature, hierarchical arrangement of nanowires [43] has been found to improve electrical properties for engineered devices [44,45].Techniques such as magnetic trapping [46] and network alignment have enabled controlled orientation [47] of nanowire building blocks in the construction of advanced nanoelectronic sensing [48], optoelectronic [49], diode [50,51], and thermoelectronic devices [52].Although nanoscale components possess many useful properties due to their small size and high surface area, their use in device design inherently involves numerous nanoscale interfaces with significant contact resistance [53][54][55].
Nanowire interconnection strategies make these arrangements more permanent and mitigate the issue of contact resistance.Nanojoining has been demonstrated in the literature by localized surface melting via light-based resonance techniques such as radiative laser-ablation [56][57][58][59][60] or plasmonic confinement [61,62] across a broad population of overlapping nanowires.Given their small diameter, thin film metallic deposition can also enable functional interconnection between nanowire components, as demonstrated between individual wires under an electron microscope [63] or across a randomly oriented array [64].Joint structures have been established between metallic nanowires via the surface tension of an evaporating suspension medium [65], through additive manufacturing in sol-gel followed by annealing [66], or simply through cold-welded bonding directed by electron microscopy [67].The alignment and joining may even be facilitated by a scaffolding material such as graphene oxide [68].These novel interconnect strategies capitalize upon the high surface activity of nanomaterials through the application of unconventional driving forces.However, most nanojoining applications involve random orientation of components for scaled network interconnection in a welded orientation, which limits their versatility and scalability as a more universal manufacturing strategy.
This work reports a technique for the magnetic assembly and soldered interconnection of self-assembled onedimensional nanowire structures into permanent connective arrays tip-to-tip and between interdigitated metal electrodes.This functionality was enabled by site-selective deposition of tin solder material onto the gold segments of gold/nickel/gold multi-segment nanowires, targeted at the tips of each wire to enhance interconnection reliability.The nickel segment acted as a magnetic carrier for assembly, while the bordering gold segments provided a compatible surface for solder deposition.Shell deposition specificity was achieved by application of a compositionally selective protective monolayer along the nickel segment, establishing a uniform 'Q-tip' like structure upon solder deposition as in figure 1.The connective capability of these 'building block' structures was established via demonstration of magnetic self-assembly and melted interconnection, characterized by resistance change of the nanowires across an interdigitated electrode pad before and after soldering.This work presents a technique for the deliberate magnetic orientation and soldered interconnection of nanowire structures in a manner analogous to bulk components, enabled by material functionality.

Materials
The anodic aluminum oxide (AAO) membrane was purchased from Whatman.Silver-and nickel-plating solutions were purchased from Technic Inc., and gold plating solution (3.7 g l −1 ) from Thermo Fisher Scientific.The silver plating solution had the primary ingredients potassium silver cyanide and potassium cyanide, the nickel was composed primarily of nickel sulfamate, nickel bromide and boric acid, and the gold contained aurate(1−),bis(cyano-c)-,potassium and ammonium hydroxide.Sacrificial silver etching solution involved a mixture of methanol (99+%) and hydrogen peroxide (35 wt.% solution in water) from Acros Organics, and ammonium hydroxide (A.C.S. reagent, 28.0%-30.0%)from Avantor.Sodium hydroxide (extra pure, pellets), tin sulfate (97%), sodium borohydride (99%), and azelaic acid (98%) were all purchased from Acros Organics.Ethanol (absolute, 200 proof) and sodium dodecyl sulfate (99%) were purchased from Fisher Chemical.Harris Stay-Clean inorganic acid type liquid flux was used for soldering.All chemicals were used without further purification.

Synthesis of multi-segment nanowires
Synthesis of the core gold-nickel-gold multi-segment nanowire structure was completed via sequential electrodeposition templated by silver-capped AAO.A silver deposition of 400 nm was established on one side of the AAO membrane via CHA 6 Pocket Electron Beam Evaporator to act as a capping layer for templating.The AAO membrane was sealed beneath a glass vial with an O-ring to house the plating solutions, with a silver wire used for silver deposition and platinum wire for all other plating.
A sacrificial silver layer was first electrodeposited into the template above the silver capping layer, to fill the branching bottom portion of the template and improve nanowire dispersion and uniformity, plated at sequential currents of 2 mA followed by 0.5 mA for 900 s each.The gold segments proceeded at 1 mA for 600 s, targeting a length of 0.5 µm, and the nickel at 8 mA for 600 s, targeting a length of 3 µm, to establish a goldnickel-gold pattern as in figure 1.Other nanowire sequences were synthesized by modifying the sequential ordering or plating time of the nickel and gold segments.
Following synthesis, the silver deposition layer and sacrificial silver portion of the nanowire were etched by immersion in a mixture of methanol, ammonia, and hydrogen peroxide as described in our previous publication [69].The AAO template was then dissolved by sonication in an aqueous 1 M sodium hydroxide solution, followed by several water washes.The released nanowires were stored in ethanol for further processing.

Site-selective solder deposition
The nickel segment was protected from solder deposition by the selective coating of azelaic acid (5 mM) chemisorbed to its oxidized surface in ethanol solution [70].Following monolayer formation, the nanowires were cleaned by three ethanol washes to remove any residual azelaic acid and subsequently added to a glass beaker at a total volume of 10 ml DI water.Sodium borohydride (0.06 g) was dissolved into this reaction vessel to act as reducing agent, stirred by an IKA RW20 digital mechanical mixer set to 300 rpm.In a separate vessel, a solder precursor solution was prepared by dissolution of tin sulfate (0.05 g) and sodium dodecyl sulfate (0.06 g) surfactant in 10 ml DI water.This precursor solution was added to the reaction vessel dropwise, allowing for precise seeded growth of tin selectively on the gold segments.After an initial dropwise deposition period, the tin solution was added more rapidly with a KD Scientific Legato 110 syringe pump to grow the shell.Additional details are provided in figure S1 (Supplementary data).After growth had completed, the solution was centrifuged and cleaned with three washes each of water and ethanol.Finally, the nanowires were separated from any unattached tin material by placing the centrifuge tube over a magnet and pipetting away the supernatant (and suspended tin particles).

Magnetic assembly and soldering
Nanowire assembly was driven by magnetic alignment via the nickel segment onto an interdigitated electrode surface of gold over chromium with 10 µm gap and 20 µm finger on Sitall substrate (NanoSPR).An aliquot of 1 µl of the nanowire suspension was deposited onto the substrate under a magnetic field of ∼125 mT (measured by Tunkia TD 8620 Handheld Digital Gauss Meter), enabling magnetic assembly between the interdigitated electrodes.This droplet was established on the surface for a period of 30 s to allow for settling and van der Waals adhesion of the nanowires to the substrate, after which the excess liquid was removed via pipette to prevent misalignment through surface tension adhesion during drying, following the 'coffee-ring' effect as detailed in the literature [71].
An infrared soldering method was applied under a flux atmosphere to enable solder melting of the nanowire payloads, a nanowire joining method established by our prior work [72].After drying, the assembled nanowires on the electrode surface were positioned onto a preheating station, alongside a separate silicon wafer with 10 µl liquid flux, which raised the temperature of the sample to ∼100 • C prior to soldering.Within a closed environment, an infrared soldering gun was set to a temperature of 250 • C and powered on for a period of 60 s.During this time, the flux evaporated from the surface of the silicon wafer, establishing a lightly corrosive environment to clean any surface oxidation on the nanowires and enable effective solder joining.

Equipment and characterization
The templated electrodeposition of nanowires was achieved using a Princeton Applied Research VersaSTAT 4 electrochemical station.Solder melting was performed using an Aoyue Int 710 focused IR welding station.Imaging and elemental mapping of the core/shell structure before and after melting was performed with a JEOL JSM 7401F field emission scanning electron microscope (FE-SEM) with x-ray micro-analysis via EDAX Genesis XM2 imaging system.A Philips CM12 transmission electron miscroscope (TEM) was used for additional imaging of the nanowire structures.The ImageJ software package was used alongside SEM imaging to quantitatively monitor the diameter change of the coated gold tips.Electrical resistance was measured with a Keithley 2400 source meter.X-ray diffraction (XRD) was performed with an AXRD Benchtop Powder x-ray Diffractometer (Proto Manufacturing Inc.) with a copper anode to analyze the crystallographic structure of the gold-nickel-gold and core/shell nanowires.The 2θ range was from 30 • to 90 • (0.02 • step) with a dwell time of 2 s.The identification of peaks and patterns was based on the Crystallography Open Database (COD) reference cards cited in the text.

Nanowire synthesis and monolayer formation
Characterization via SEM confirmed the presence of nanowires with three distinct segments following sequential templated electrodeposition of the gold-nickel-gold (Au-Ni-Au) nanowires and their subsequent release, in figure 2. In this work, the nickel segment of the nanowire was chosen to act as the magnetic carrier to manipulate the core/shell structure, in fitting with previous work in this area [73].In the literature [74], other magnetic segments have been synthesized in multi-segment nanowires including iron [75,76] and cobalt [77,78], depending on the application.The gold segments were selected to act as a suitable deposition site for tin solder, which would resist corrosion and not form a chemisorbed monolayer with the azelaic acid.To enhance the visualization, false coloring of the SEM image was performed on the single wire inset with green to represent nickel and yellow to represent gold.The length of each segment was determined using ImageJ analysis of the SEM results in figure 2(a).A deposition time of 10 min at a current of 1 mA resulted in an average gold length of ∼0.44 µm.At a deposition time of 10 min under 8 mA of induced current, the average nickel segment length was ∼2.8 µm.Considering all three segments, the multi-segment nanowires had an average length of ∼3-4 µm.Between each plating step, the reaction vessel was washed thoroughly five times with DI water.As the site-selectivity of the core/shell deposition depended upon composition of the individual segments in monolayer formation, having pristine boundaries between nanowire segments was significant.
Under SEM, conductive elements with a higher atomic number resolve more brightly; thus, three distinct segments can be observed within each nanowire, in fitting with the gold-nickel-gold plating sequence.Composition of each segment was confirmed with EDS line-mapping, as shown in figure 2(b).These results demonstrate the presence of sequential gold-nickel-gold segments with clear compositional distinction, matching the expected orientation based on the electrodeposition procedure, with segment lengths corresponding to the size analysis performed above.
Three, four, and five-segment nanowires consisting of sequential gold and nickel segments of various lengths were synthesized following the same procedure to test the solder deposition strategy on different segment orientations, to confirm compositional selectivity distinct from geometric effects on deposition.All synthesized nanowires were immersed in an azelaic acid solution in ethanol at ∼5 mM.Carboxylic acids react with oxidized nickel [79] to form a stable intermediate, resulting in a chemisorbed monolayer selectively over the nickel nanowire portion.Theoretically a similar monolayer could be established over any compatible metal oxide surface.A thin nickel oxide layer was expected to be present around the nickel segment due to natural corrosion from atmospheric exposure and contact with dissolved oxygen in water during cleaning.In order to qualitatively evaluate the formation of this monolayer, a nickel layer was electrodeposited directly onto a copper plate, which was then exposed to the same azelaic acid treatment.A simple contact angle visualization on the nickel surface with a water droplet showed an estimated initial angle of 64 • which changed to 77.2 • after coating.The surface becoming slightly more hydrophobic is likely due to adhesion of both carboxylic acid end groups onto the nickel, exposing the hydrocarbon chain of the azelaic acid which has a hydrophobic nature.This monolayer was established in order to protect the nickel from tin deposition by steric hindrance of the initial tin nucleation seeds during synthesis.Following monolayer adhesion, the nanowires were cleaned by three ethanol washes and three water washes with short, pulsed sonication (∼1 s per wash) to remove any physisorbed azelaic acid from the gold segment.

Site-selective solder deposition
Site-selective solder deposition along the gold nanowire tips proceeded according to a 'flipped' chemical reduction mechanism, established in consideration of the limitations of the nickel-protective monolayer, to achieve a highly compositionally selective core/shell in a 'Q-tip'-like orientation as observed under SEM in figure 3(a).Tin was selected as the shell material due to its prevalence in the soldering industry as the primary component of many lead-free solder alloy compositions [28,80,81] such as tin-silver [82] or tin-silvercopper [83].Deposition of the tin solder layer was targeted selectively on the gold tips of the nanowires, leaving the middle nickel segment uncoated.False coloring of the SEM image after core/shell deposition designated the tin shell as blue, with green continuing to represent nickel.
Dropwise addition of tin precursor solution (with stabilizing surfactant sodium dodecyl sulfate) into an excess of sodium borohydride reducing agent (with the suspended nanowires) caused an instantaneous reduction of tin coupled with diffusive spreading of nucleation sites into the reaction vessel, allowing for controlled growth of very small seeds.Each aliquot addition was observed to cause a change in the solution color of the reaction vessel, which was used to iteratively optimize the addition amount until the reaction solution became translucent yellow after a single droplet.This translucent yellow suspension indicated the plasmonic characteristic of very small metal seeds (<5 nm) which geometrically confine the resonant behavior of incidentally excited plasmons [84].The growth of very small seeds was targeted to allow for steric hindrance of their adhesion to the nickel surface by the chemisorbed monolayer of azelaic acid.With continued addition of metal precursor solution, these precipitated tin seeds would undergo Ostwald ripening and crystal growth into discrete nanoparticles, alongside a corresponding change in solution color to opaque grey.This behavior has been observed and documented in the literature and experimentally within our research group in the synthesis of tinalloy nanoparticles [85,86].Without further addition of precursor solution, the solution color was observed to change from translucent yellow back to clear, indicating that the seeds were no longer present in suspension.It follows that if the seeds did not undergo ripening and additional growth, they had likely deposited onto the nanowire surface.Given the presence of a protective monolayer on the nickel segment and having not observed any extraneous tin growth within the reaction vessel, it was inferred that the tin seeds were nucleating onto the gold segments of the nanowire structure.Each subsequent aliquot was only added after the solution had completely returned to clear, which generally took approximately two minutes.In this manner, the total amount of tin in the reaction vessel was controlled to direct solder growth onto the nanowire surface.A total of 15 aliquots were added, which was an empirically determined value optimizing the thickness of the final core/shell against the uniformity of its growth.The addition of fewer initial seeding aliquots resulted in a thinner coating, while adding too many caused extraneous growth between nanowires into bundles.A summary schematic of the mechanism employed in this work is shown in figure S1 (Supplementary data).
After establishing a layer of tin seeds on the gold surface, involving 15 sequential aliquots (75 µl each) over a 30 min period, the tin precursor solution was added more rapidly to promote uniform shell growth over the initial seeded layer.This more rapid addition of precursor was only possible because of the presence of a seeded tin layer on the gold segments; otherwise, this would result in tin growth on the nickel as well.The optimal addition was identified empirically to be 4 ml injected at 23 ml h −1 , resulting in the uniform core/shell growth shown in figure 3(a).This optimized core/shell structure was additionally characterized with transmission electron microscopy to show a detailed outline of the extent of tin growth on the gold portion, as in figure S3 (Supplementary data).There was some observable roughness on the nickel portion under TEM, which may indicate that some small tin seeds were capable of adhering to the nickel segment.This likely indicates that the monolayer did not uniformly cover the entire nickel surface.However, the degree of monolayer coverage was sufficient to prevent growth of the tin seeds on the nickel and achieve site-selective growth on the gold segments.A subsequent growth period of 30 min following addition was found to improve the thickness and uniformity of the final shell.The quantitative analysis of shell size to determine these parameters is shown with additional detail in figure S4 (Supplementary data).
After deposition and allowing for 30 min of growth period, the nanowires were centrifuged and cleaned with three water and three ethanol washes.This experimental procedure was repeated without modification on all length and segment order variations of gold-nickel nanowires synthesized.The presence and confinement of tin solder onto the three-segment gold-nickel-gold nanowire was confirmed with EDS line-mapping, shown in figure 3(b).Because the electron beam penetrates the sample, the gold was still resolved even though it was encapsulated by tin.The core/shell nanowire retained the three-segment gold-nickel-gold orientation, with the presence of a new tin signal confined to the gold portions of the scan.In combination with the SEM imaging, these results demonstrate the presence of a gold-selective tin shell following chemical reduction.
The nanowire crystalline structure was characterized via xray diffraction before and after deposition of the core/shell, with results shown in figure 4. The presence of gold (111), ( 200), ( 220), (311), and (222) peaks and their relative intensities correspond to the COD [87] card number 9013043 for gold in the control diffractogram.Similarly, the presence of nickel (111), (200), and (220) peaks and their relative intensities correspond to COD card number 1512526.These peaks were observed in the core wire and were retained following the deposition of the core/shell.The apparent increase in intensity of Au (200) and Ni (111) can be explained by the new presence of Sn (220) and Sn (211) in the same region.The tin (200), ( 101), ( 220), ( 211), (301), ( 112), (400), ( 420), (411), and (312) peaks observed in the core/shell sample and their relative intensities correspond to COD card number 7222460 for the structure of beta tin, with the noted presence of AuSn intermetallic (102), ( 110), ( 200), ( 201), ( 103), ( 202), ( 211), (212), and (114) peaks (COD card number 9008883) indicating solid-state diffusion between the inner gold core and the outer deposited tin layer.Peaks were compared to reference powder diffractograms generated from CIF files cited in the COD using the Vesta software package [88].
To examine the effects of segment length and orientation on the mechanism of core/shell formation, several variations on the initial three-segment gold-nickel-gold nanowires were synthesized with the templated AAO electrodeposition procedure by modification of the deposition time and precursor solution ordering.Single segment gold nanowires were synthesized by the deposition of gold at 1 mA for 45 min.In order to study the impact of segment orientation/geometry, 'inverse' nickel-gold-nickel nanowires were synthesized by the sequential deposition of nickel at 8 mA for 5 min, gold at 1 mA for 20 min, and again nickel at 8 mA for 5 min.These parameters were selected in order to maintain the same overall deposition time of nickel and gold per nanowire, but with the inverse orientation of nickel-gold-nickel.A two-segment nanowire was synthesized, maintaining the overall length of each segment by plating gold at 1 mA for 20 min followed by nickel at 8 mA for 10 min.The impact of segment length on core/shell deposition was studied by synthesizing a threesegment nanowire with gold deposition lengths of 20 min at 1 mA on either end of the wire structure.Through the sequential plating of gold and nickel two times each at 1 mA for 10 min and 8 mA for 10 min respectively, a four-segment nanowire was synthesized.Similarly, a five-segment nanowire batch was synthesized, through the sequential plating of three gold segments (1 mA for 10 min) and two nickel segments (8 mA for 10 min).SEM imaging of these structures is shown in figure 5.
The compositionally selective core/shell synthesis technique was replicated successfully on each of the multisegment nanowire variations, verified via SEM in figure 6. Exposed gold segments were uniformly coated in tin regardless of length, orientation, or nanowire sequence complexity.However, without the presence of a nickel segment (as in figure 6(a)), there was a tendency for the wires to aggregate together and grow into large tin clusters.Thus, the presence of the nickel segment additionally served to stabilize the nanowire suspension and improve uniformity of the final core/shell structure.The inverse core/shell deposition resulted in solder deposition along the internal gold segment (figure 6(c)), indicating that the targeted gold deposition was compositionally directed by the monolayer rather than driven by preferential deposition to the geometric tips of the nanowire structure.Similarly, targeted solder deposition was observed in the two-, four-, and five-segment nanowire batches (figures 6(b), (e) and (f)), showing the reproducibility and reliability of the selective coating on gold segments.The protective effect was preserved even with more complex structures with additional nickel segments.Finally, with a longer gold segment (figure 6(d)), the tin shell deposited to cover the entire length of the gold tips.
In figure S5 (Supplementary data), a four-segment goldnickel-silver-nickel nanowire was synthesized and treated with the same deposition procedure to establish a shell on both the gold and silver segments for comparison.EDS line mapping analysis showed the expected orientation of goldnickel-silver-nickel, with tin selectively deposited on the gold and silver portions.The protective effect was preserved on the nickel segments, although visually the coating of tin was thicker and more uniform on the gold portions.This may be due to the higher corrosion resistance of gold, which prevents  the establishment of a monolayer from the carboxylic acid.This work demonstrated the possibility of depositing a solder shell on other multi-segment nanowire compositions, provided an appropriately selective monolayer treatment can be identified.For the purposes of solder bonding, the gold segments were identified as the ideal material based on the thickness of the coating.

Magnetic assembly and nanowire interconnection
The purpose of the site-selective core/shell synthesis technique was the targeted deposition of solder material onto the tips of the magnetically manipulable multi-segment nanowires.Upon exposure to an external magnetic field, the nanowires could be directed to assemble tip-to-tip, positioning the solder loaded areas for optimized melted interconnection.In previous work, we have demonstrated broad scale 3D network orientation of drop cast magnetic Sn-Au-Ni-Au-Sn nanowires and commented on the limitations of the technique, including solvent evaporation [89] and coffee-ring misalignment.Although this issue has been widely studied in the literature, its suppression remains an ongoing area of research [90].While functionally simple and intuitive at optimized conditions, the magnetic assembly of nanoscale wire-like structures is mechanistically complex and must be precisely controlled by magnetic field strength and loading percentage of nanowires in suspension.Previous work with gold-nickel-gold nanowires has demonstrated tip-to-tip alignment of multiple nanowires in sequence, assisted by regularly spaced substrate pads [91].In this work, tip-to-tip alignment was achieved experimentally with the solder-loaded three-segment nanowires by careful modification of the magnetic field strength and suspension concentration.
After cleaning, the core/shell nanowire sample was diluted with a 50/50 mixture of water and ethanol by volume until the suspension had no apparent color.A 1 µl droplet of the suspension was positioned on top of the interdigitated electrode surface under a magnetic field of ∼125 milli-Tesla (mT), as in figure 7. Within a short time period of 30 s, the nanowires were presumed to have settled on the surface of the electrode and the excess liquid from the droplet was removed by micropipette.Any residual liquid dried rapidly, within another 30 s, leaving behind the assembled nanowires with minimal disturbance.This experimental mitigation strategy was designed to avoid the damaging droplet-edge accumulation of nanomaterial via the coffee-ring effect.Optical microscopy as in figure S6 (Supplementary data) confirmed alignment of multiple nanowire assemblies connecting the gold electrode pads.
Given their high surface activity, surface oxide growth on nanoscale solder components could prevent solder joint formation during melting [85].We have previously demonstrated solder melting and wire bonding between multisegment nanowires with incorporated tin solder segments following an infrared heating mechanism [92].In this work we additionally applied a flux atmosphere to polish the solder shell, rather than directly fluxing the nanowires.The direct fluxing typically employed in micron-scale solder applications can be significantly corrosive to nanoscale components.Instead, a flux vapor atmosphere was established by depositing liquid flux on a silicon wafer separate from the sample, which provided polishing during reflow by the evaporation of flux [93].
In addition to an external oxide shell, the XRD results of figure 4 indicated the presence of an intermetallic AuSn layer between the gold and tin.In previous work we have studied solder reactions in one-dimensional copper-tin diffusion couples of various sequential orientations [94].Solidstate metallic diffusion occurs more rapidly at higher temperatures, as has been demonstrated in the literature between noble metals and tin [95].As gold/tin solder alloy melts at a higher temperature than pure tin, expansion of this intermetallic layer could also suppress droplet formation and inhibit solder joining.Infrared radiation of the core/shell sample localized heating along the nanowire surface, minimizing substrate damage during droplet formation and also theoretically minimizing solid-state diffusion between the gold and tin [94].After magnetic assembly and solder reflow, the ∼4-5 µm nanowires connected the electrode gap, allowing for identification of the morphology evolution from tip-to-tip wire bonding and wire-pad substrate bonding through melting of the deposited solder shell, as in figure 7. Despite the flux atmosphere approach, some flux residue is visible coating certain wires.However, the morphology change following reflow can be identified in the nanowires bridging the gaps by the droplet shape of the tips and the wetting behavior between individual components.Nanowire assembly and solder bonding was achieved using both three-segment (figure 7(c)) and five-segment (figures 7(d)-(f)) core/shell nanowires.The fivesegment nanowires were capable of consistently bridging longer gaps through magnetic assembly while maintaining solder contact with nearby wires due to the presence of an additional solder loaded segment.Additional unique morphologies associated with melting are shown in figure S7 (Supplementary data), including more visually apparent solder reflow behavior with excess loading and higher magnification of melted reflow connecting the nanowire to the substrate pad.The five-segment nanowires were capable of some unique geometric soldered orientations, including X-and T-shaped joints and three dimensional assemblies.
Melting on the interdigitated electrode enabled a quantitative assessment of electrical resistance change as a function of solder joining.The substrate 'fingers' connected to larger conductive pads, which served as contact sites for multimeter probes.Resistance of the substrate pad (which was initially an open circuit) was monitored after nanowire deposition before and after infrared heating, to provide a numerical indication of nanowire interconnection.In one representative melting experiment shown in figure 8, the as-assembled core/shell nanowires had a resistance of ∼120 MΩ before reflow, due primarily to contact resistance between the assembled nanowires and the contact pads.After infrared heating, the resistance dropped to ∼100 Ω, indicating melted solder joining between wires and to the electrode pads.Assuming that the 10 µm electrode gap was bridged by a single-file assembly of nanowires, composed of 40% gold and 60% nickel, with a diameter of 300 nm, we would expect an ideal resistance value of approximately 7 Ω based on a simple wire calculation (and assuming only one assembly bridges the gap).The final value observed in this work was significantly higher, which indicates the influence of the various soldered interfaces present in the final bonded structure.However, this final resistance value was several orders of magnitude lower than the initial resistance, which reflected the impact of contact resistance both among the nanowires and between the nanowires/electrode pad and the ability of soldering to mitigate this issue.Note the break in the x-axis indicates the time during which the sample was melting, which took approximately 200 s.This significant drop in resistance across the electrode was replicated in other similar samples with starting resistances ranging from ∼3 to 150 MΩ.After soldering, the resistance values dropped to below 5 kΩ for all electrodes tested (with the lowest recorded value ∼50 Ω) with some variability in the final resistance value depending on the uniformity of the assembly (e.g., number of assembled wires and patterns) and the amount of solder material present in the sample.As discussed above, the presence of intermetallic AuSn and potential surface corrosion can both prevent uniform solder melting and bonding to the electrode pad, which introduces some variability in behavior.However, this significant drop in resistance resembles our prior work in the bonding of multi-segment nanowires to solder loaded gold contact pads, indicating melted reflow of the solder.In combination with the morphological evolution observed under SEM, these results demonstrate metallic contact after solder bonding between the nanowire components [91].Contact resistance is one of the major issues associated with nanoscale device design [54], and the ability to solder nanoscale components to mitigate this issue could be valuable to a variety of industries considering advanced device designs on the nanoscale.

Conclusion
Compositionally selective solder deposition onto the gold segments of multi-segment nanowire structures was achieved using a 'flipped' reductive synthesis with experimentally optimized staging of precursor addition.This approach was enabled by the establishment of a nickel-selective azelaic acid monolayer to provide protection against adhesion and nucleation of small tin seeds during reduction.SEM imaging enabled visualization of the Q-tip like structure of the final core/shell, while EDS mapping and XRD verified the tin solder composition of the deposited material.This shell deposition was replicated on other metal surfaces and segment orientations, including pure gold nanowires, four-fivesegment nanowires, and silver nanowire segments to demonstrate the reproducibility of the site-selectivity.After synthesis, the core/shell nanowires were assembled across an interdigitated electrode using a magnetic field of approximately 125 mT and careful control of nanowire deposition to prevent surface tension driven aggregation.Infrared radiation of the material under a vapor flux environment melted the solder payload and joined the nanowire structures across the electrode pads while minimizing the two confounding factors: potential corrosion of the solder payload and growth of the gold/tin intermetallic layer.The consistent drop in resistance after melting, in addition to the change in morphology of the material visualized via SEM, demonstrated the presence of a viable solder payload and the feasibility of this technique for solder bonding between multifunctional nanowires.

Figure 1 .
Figure 1.Schematic illustration of (a) silver-capped anodic aluminum oxide (AAO) template.Sequential electrodeposition of gold, nickel, and gold within the template results in the multi-segment wire structure shown.The nanowires are released by silver etching and dissolution of the template.Once released (b), the wires are selectively coated with tin solder by monolayer-directed site-selective deposition.The solder coated wire structures are capable of magnetic alignment and soldered interconnection upon heating.

Figure 2 .
Figure 2. (a) Scanning electron microscopy of three-segment gold-nickel-gold nanowires with inset single wire.Elemental analysis of a single gold-nickel-gold nanowire via (b) EDS line-scan analysis reveals the presence of the gold-nickel-gold metal segment sequencing in fitting with the electrodeposition procedure.The line scan sample is shown in figure S2(a) (Supplementary data).

Figure 3 .
Figure 3. (a) Scanning electron microscopy of core/shell three-segment tin-coated gold-nickel-gold nanowires with inset single wire.Elemental analysis of a single core/shell nanowire via (b) EDS line-scan analysis reveals the presence of the gold-nickel-gold metal segment sequencing with the tin signal overlapping the gold segments, corresponding to the core/shell structure.The line scan sample is shown in figure S2(c) (Supplementary data).

Figure 4 .
Figure 4. X-ray diffraction analysis of core/shell nanowires (top) and control gold-nickel-gold nanowires (middle).The core/shell diffractogram shows the additional presence of tin and gold/tin intermetallic crystal structures.Calculated reference powder diffractograms (bottom) were generated from the CIF files cited in the Crystallography Open Database using Vesta.

Figure 6 .
Figure 6.SEM imaging of core/shell structures of (a) pure gold nanowires, (b) two-segment gold-nickel nanowires, (c) inverse nickel-gold-nickel nanowires, (d) long gold gold-nickel-gold nanowires, (e) four-segment gold-nickel-gold-nickel nanowires, and (f) five-segment gold-nickel-gold-nickel-gold nanowires with single wire insets.False coloring of the segments continued to use green for nickel and blue for tin.

Figure 7 .
Figure 7. Substrate (a) and magnetic alignment (b) schematic representation for deposition of core/shell nanowires on interdigitated electrode.(c) Three-segment core/shell nanowires were aligned across a 10 µm gap and melted with infrared heating.(d), (e) Five-segment nanowires were aligned across a 10 µm gap and melted with infrared heating.(f) Five-segment nanowires were capable of longer chain assemblies connecting two 10 µm electrode gaps after soldering.

Figure 8 .
Figure 8.The recorded resistance value across the interdigitated electrode dropped from ∼120 MΩ to ∼100 Ω (as shown in inset) after solder melting of the assembled core/shell nanowires.