cold welding of ultrathin gold nanowires

Cold welding of ultrathin gold nanowiresYang Lu1, Jian Yu Huang2, Chao Wang3, Shouheng Sun3 and Jun Lou1*

The welding of metals at the nanoscale is likely to have an important role in the bottom-up fabrication of electrical andmechanical nanodevices. Existing welding techniques use local heating, requiring precise control of the heating mechanismand introducing the possibility of damage. The welding of metals without heating (or cold welding) has beendemonstrated, but only at macroscopic length scales and under large applied pressures. Here, we demonstrate that single-crystalline gold nanowires with diameters between 3 and 10 nm can be cold-welded together within seconds by mechanicalcontact alone, and under relatively low applied pressures. High-resolution transmission electron microscopy and in situmeasurements reveal that the welds are nearly perfect, with the same crystal orientation, strength and electricalconductivity as the rest of the nanowire. The high quality of the welds is attributed to the nanoscale sample dimensions,oriented-attachment mechanisms and mechanically assisted fast surface-atom diffusion. Welds are also demonstratedbetween gold and silver, and silver and silver, indicating that the technique may be generally applicable.

Welding, with its historic development tracing back to theBronze Age, serves modern industry in many areaswhere metals are used1. When, in the 1940s, people

started to recognize cold welding as a general phenomenon, ithad already been practised for more than 700 years (refs 2,3).Unlike normal welding, in which liquids or molten phases needto be present, cold welding is a solid-state welding process inwhich joining takes place without fusion (the process of causing amaterial to melt with intense heat) at the interface. However, torealize cold welding in bulk metals, either a high applied normal/frictional load or an atomically clean at ductile surface in an ultra-high-vacuum environment are generally required. About twodecades ago, Whitesides and colleagues discovered that a metallicthin lm such as gold, supported on compliant elastomers, couldweld together at remarkably low loads under ambient laboratoryconditions3. Although its underlying mechanism was not fullyunderstood, this nding extended the cold-welding process intothe fabrication of a broad range of modern organic micro-electronic/optoelectronic devices, including organic light-emittingdevices (OLED) and photovoltaic cells4. Now, with extensiveresearch being conducted into nanoelectronic devices andnanoelectromechanical systems (NEMS), whether or not coldwelding can also be practised at the nanoscale has become aninteresting topic. In recent years, scientists have successfullyrealized the joining of individual low-dimensional nanostructuressuch as carbon nanotubes58 and metal/semiconductor-lled carbon nanotubes912, metal/semiconductor/ceramic nano-wires and nanoparticles1321, by either applying voltage/current5,7,10,14,17,20 or heating the sample stage9,15,21, or by focusinghigh-intensity electron or laser beams onto the joiningsection6,8,1113,16,18,19. Although these methods certainly have theiradvantages, such nanoscale welding techniques have alwaysinvolved local heating processes of some kind, which can bedifcult to control precisely at the relevant length scales, and maychange the underlying substructures and related properties ofthe original building blocks. Owing to these limitations, the ideaof cold welding, which joins nanostructures without heating, hasbecome an attractive solution for bottom-up assembly atthe nanoscale.

In this work, cold welding of individual gold nanowires wasperformed and monitored inside a high-resolution transmissionelectron microscope (HRTEM) equipped with NanofactoryTM

TEM-scanning tunnelling microscopy (STM) and TEM-atomicforce microscopy (AFM) sample holders (see Methods). Ultrathingold nanowires (diameters, 10 nm) were chosen, because theyare widely considered to be ideal candidates for achieving extremelydense logic and memory circuits in future molecular-scale intercon-nects22. The good resistance to oxidization of gold is another usefulproperty. Samples were selected from two sources. The rst typecomprises ultrathin gold nanowires with relatively small aspectratios, that is, which are 510 nm in diameter and 1050 nmin length (referred to as nanorods in this paper). They were originallyformed as ligaments that were cut off in situ from home-made porousgold nanostructures (Fig. 1a). The porous gold nanostructures wereobtained by means of a de-alloying process applied to a goldsilveralloy23. The second type of sample comprises the recently developedmicrometre-long ultrathin gold nanowires24, which are 39 nm indiameter (Fig. 1b). HRTEM imaging and selected area diffraction(SAD) showed that both samples had single-crystalline face-centredcubic (fcc) structures. The average interfringe distances for bothtypes of sample were measured to be 0.230.24 nm, correspondingto (111) lattice spacing (0.23 nm) of the fcc gold crystal.

Nanoscale cold weldingUsing the TEMSTM holder, both head-to-head and side-to-sidejoining procedures were performed for both sample types (Fig. 2aand b, respectively). Other welding geometries could also be rea-lized, such as head-to-side joining (see SupplementaryInformation). Each individual nanowire was manipulated using atungsten or gold STM probe (Fig. 2c) driven by the movablepiezoelectric head of the holder. The joining of two gold nanorodswas rst attempted in a head-to-head orientation (Fig. 3; see alsoSupplementary Movie S1). One nanorod was manipulatedtowards another, continuously adjusting the alignment until thetwo samples approached one another, head to head (Fig. 3a,b).When the front surfaces of the two nanorods came into contact,they welded together instantly (1.5 s, Fig. 3c,d). After welding,the image contrast of the welded nanorod became increasingly

1Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas 77005, USA, 2Center for Integrated Nanotechnologies(CINT), Sandia National Laboratories, Albuquerque, New Mexico 87185, USA, 3Department of Chemistry and Division of Engineering, Brown University,Providence, Rhode Island 02912, USA. *e-mail: [email protected]

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2010 Macmillan Publishers Limited. All rights reserved.

uniform (Fig. 3df), indicating continuous substructure evolutions(Fig. 3d,e) to smooth the surface. Once this process was complete,the STM probe was retracted (Fig. 3fh) along the direction indi-cated by the arrow, until it was fully separated from the sample. Itwas shown that the as-welded nanorod maintained its morphologyand structure in the free-standing state (Fig. 3i). Similar joining wasalso successfully achieved for micrometre-long ultrathin gold nano-wires; however, head-to-head welding for the gold nanorods waseasier to perform because there was no problem with buckling, aswas the case when manipulating longer nanowires.

For side-to-side welding, precise alignment is not required, andsuccessful joining of both types of sample was easily carried outmany times. By simply manipulating the nanowires so that they par-tially overlapped one another, side-by-side contact could be made,and cold welding would always occur quickly (Fig. 4a,b). In Fig. 4,one particular welding process is shown to nish within 34 s(Fig. 4b,c), and HRTEM imaging in Fig. 4d shows the structure of

the as-welded structure following the relaxation process. To checkits welding quality, in situ pulling of the as-welded nanowire(Fig. 4ej) was performed immediately after the rst joining.Surprisingly, the welded nanowire formed a neck and broke atanother location rather than at the joining section of the rstwelding. A comparison of the length of the nanowire remainingat the bottom during the pulling process (original, 21.5 nm inFig. 4a; after breaking, 25.7 nm in Fig. 4j) clearly demonstratesthat the as-welded nanowire broke at a location 4.2 nm abovethe original welding spot. This qualitative pulling result impliesthat the as-welded structure is as strong as the original nanowire.

For the same sample shown in Fig. 4aj, we also performed asecond welding in the head-to-head mode, and again pulled theas-welded nanowire until breaking occurred (SupplementaryMovie S2). During this pulling process, the sample formed a necknear the second welding zone (Fig. 4 k). Fast Fourier transform-ations (FFT) from images taken from both the welded segment

a b

Figure 1 | Two types of ultrathin gold nanowire samples used for cold-welding experiments. a,b, TEM images of an ultrathin gold nanorod (a, scale bar

5 nm) on a porous gold nanostructure, and micrometre-long ultrathin gold nanowires (b, scale bar 100 nm). Insets: corresponding HRTEM images showing

the crystalline structures. Scale bars, 5 nm. (Chemically fabricated micrometre-long nanowires are usually covered with a layer of surfactant (oleylamine).

Mechanical rubbing between two nanowires can effectively remove the residual surfactant on their surfaces before welding experiments.).

STM probeSTM probeSTM probe

a b

c

Figure 2 | Head-to-head and side-to-side cold-welding geometries. a,b, Schematics of two welding geometries for ultrathin gold nanowires: head-to-head (a)

and side-to-side (b) (d represents the virtual bending deection of the top nanowire when contacting the bottom nanowire). c, TEM image showing the

manipulation of a longer nanowire towards a short nanowire. Scale bar, 10 nm.

NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2010.4 ARTICLES

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and the remaining segment of the nanowire (which also containedthe rst welding zone) conrmed that the second welding zone andthe remaining part of the nanowire (Fig. 4l) were both single crystal-line, in the same ,111. orientation.

In situ mechanical and electrical measurementsTo quantitatively determine their strength, an in situ TEMAFMholder was used to perform tensile tests on as-welded nanowires,as illustrated in Fig. 5a. It should be noted that the twonanowire samples to be welded were obtained by breaking oneoriginal gold nanowire with a measured tensile strength of600+50 MPa (engineering stress). A side-to-side cold-weldingexperiment was then carried out. Following the completion ofthe joining and relaxation process (Fig. 5b,c), the as-welded nano-wire was pulled away from the AFM tip. Once again, the breakingpoint was not in the welding zone (Supplementary Movie S3).By measuring the deection of the AFM cantilever (DD,see Methods), a tensile strength of 580+40 MPa (engineeringstress) was obtained. This compares very well with the strengthof the original nanowire. In contrast, the tensile strength forbulk gold is normally 100 MPa (ref. 25). This drasticallyincreased mechanical strength of gold nanowires has previouslybeen demonstrated both experimentally and computationally26,27

(see discussions in Supplementary Information). The presentmeasurements clearly conrm that as-welded nanowires retainthe superior mechanical properties of the original single-crystalline nanowires.

Finally, in situ electrical measurements were conducted for theoriginal and as-welded nanowires using the TEMSTM holder,and their currentvoltage (I2V) responses were compared. InFig. 6a, a gold nanowire (length, 130 nm; diameter, 7 nm) wasbridged between gold probes. While applying a bias of 21 to1 mV, I2V measurements were carried out nine times, resulting

in an average electrical resistivity of 292.6+5.8 V nm. As-weldednanowires were then obtained by breaking the original nanowireand re-welding it, as shown in Fig. 6b,c. Eleven cycles of breakingand re-welding were performed in side-to-side or head-to-headmodes at different welding locations (because the as-welded nano-wires often broke at locations other than the previous weldingspot). Electrical measurements were carried out at least twice foreach as-welded nanowire. The corresponding averaged I2Vcurves are plotted in Fig. 6d, together with that of the original nano-wire. An average electrical resistivity of 298.1+14.5 V nm was cal-culated from all eleven successfully welded nanowires, clearlydemonstrating that the electrical resistivity changed very little witheach successful welding, and that the average resistivity of theas-welded samples was indeed very close to that of the originalnanowire. These results also compare very well with theresistivity results from ref. 24 (260 V nm) for the same micro-metre-long ultrathin gold nanowires. Even without making fullcorrection for the contact resistance, the electrical resistivities ofthese original and as-welded nanowires were already lower thanmany other types of gold nanowires with diameters rangingfrom 4 to 90 nm (for example, refs 2830), clearly suggestingthe great potential of using ultrathin gold nanowires as futureinterconnects and cold welding as an efcient nanoscaleassembly technique.

Comparison with fusion and macroscopic cold weldingIn contrast to other existing techniques for joining individualnanostructures, the demonstrated technique distinguished itself byrequiring no local heating process; that is, there was no need toapply bias or to use a dedicated heating stage. Additional heatingeffects from the electron beam can be ruled out31,32, because onlylow-intensity-spread electron beams were used for imaging(for further discussion, see Supplementary Information). The

0 s

22.5 s 37.5 s

21 s20 s

54 s

1 min 57 s1 min 34.5 s1 min 14 s

a cb

d fe

g ih

Figure 3 | Head-to-head welding of two gold nanorods. a,b, One nanorod (right) is caused to approach another (left) until their front surfaces come into

contact. ce, The welding process is completed within 1.5 s (c,d) followed by structure relaxation (d,e). fi, After withdrawal of the STM probe (fi), theas-welded nanowire is left in the free-standing state (i). Triangles indicate the front edges of the two nanorods. Arrows indicate the withdrawing direction

of the STM probe. Scale bars, 5 nm.

ARTICLES NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2010.4



cold-welding processes were faster than most other welding pro-cesses involving heating11,1315, and were completed close to roomtemperature, with no observable fusion occurring at the weldinginterface. As a result, the single-crystalline structures of the originaland as-welded nanowires were well maintained during the weldingprocess, with almost no defects or impurities introduced. The as-welded nanowires was at least as strong as the original nanowires,due to the fact that the welding zone had the same lattice structureand connected to the original wires with no observable grain bound-aries. It also appears that cold welding has very little effect onelectron conductionthis could again be attributed to the near-perfect welding zone formed during the process. More importantly,we have successfully extended this technique to other metal systemssuch as silversilver nanowires and goldsilver nanowires(Supplementary Figs S2,S3).

Unlike the traditional cold welding of bulk materials, whichnormally requires high load, the cold welding of the ultrathinnanowires described in this paper can occur easily in head-to-head welding experiments where there are matching crystallineorientations, and little external force is needed. In the side-to-side

welding mode, joining could occur by simply making mechanicalcontact between the nanowires to be welded, and no signicantdeformation due to contact was found in any of the experiments.Although the exact contact geometry and applied load were dif-cult to quantify at the joining interface, an estimate of the appliedstress in joining the nanowires in the side-to-side geometry(Fig. 2b) was attempted using simple beam theory. Assumingthat a small lateral deection of d 1 nm for a 100-nm-long nano-wire sample (Fig. 2c) resulted from the side-to-side contact, wherethe contact surface area was 10 nm2 and Youngs modulus forcommon gold nanowires (70 GPa) is used27, the calculatedapplied stress is only 4.7 MPa. This value is considered to bethe upper limit of the actual applied stress, because very littlelateral deection was observed in the actual experiments involvingside-to-side welding (Supplementary Movies S2,S3). This value,not only much smaller than the requirement for traditionalcold welding of bulk metals, is also smaller than the requiredpressure for cold welding of metallic thin lm (.100 MPa inref. 4). It should be noted that the reported pressure for coldwelding of gold thin lm (0.1 g cm22; that is, 9.8 Pa in ref. 3)

0 s 21 s 55 s 4 min36.5 s

5 min49.5 s

6 min8 s

6 min2.5 s

5 min57 s

5 min55 s

6 min10 s

a c eb d

f h jg i

k l

Figure 4 | Side-to-side welding of two gold nanowires. aj, Welding of two ultrathin gold nanowires was completed within 34 s by making side-to-sidecontacts (ac), followed by structure relaxation (cd) and in situ pulling of the as-welded nanowire (ej). The thin double-headed arrows in a and j indicate

the bottom nanowire length before and after the rst welding and pulling. The two broken nanowires in j were re-welded by making a second contact,

followed by a second pulling and breaking process. k,l, HRTEM images of the necking area during the second pulling (k) and the remaining nanowire at the

bottom after the second breaking step (l), respectively. Insets: diffraction patterns from the regions marked by squares in both images, calculated by fast

Fourier transformation (FFT). Again, the triangles indicate the two edges of the two nanowires before welding, and the thicker single-headed arrows indicate

the STM probe pulling direction. Scale bars, 5 nm.




represents the average stress over a large 1 cm 1 cm area, whichcould have many local contact points due to surface asperities.The actual pressure for individual asperities would probably bemuch higher. Therefore, this work may offer a nanoscopic viewof the initial stages of macroscopic cold welding for either bulkmetals or metallic thin lm.

Mechanisms of nanoscale cold weldingAtomic diffusion and surface relaxation, considered importantfactors in macroscopic cold welding, were obviously at play in theaforementioned nanoscale process2,3,33. It is well recognized thatthe diffusion barrier for a single metal atom on a metal surface isquite low (typically less than 1 eV)34. Thermal activation, even atroom temperature, is enough to overcome such low barriers, so iso-lated metal atoms can diffuse rapidly by means of surface diffusion.However, to create such isolated atoms demands a much higherenergy cost. It is the combination of the formation and diffusionenergy barriers that determines the cold welding observed in thiswork. The mechanical manipulation clearly provided the necessaryextra driving force to facilitate the cold welding and unication ofthe two nanowires.

We also believe that the oriented-attachment mechanism, asreported for PdSe nanocrystal15,35, was playing an important partin the welding occurring at close to room temperature. Evidencefor this is provided by the fact that cold welding always occurredeasily and instantaneously between two nanowires with the samegrowth orientation, in particular for those that were obtained bybreaking one original nanowire into two segments. Matching theorientation is therefore key to realizing successful welding. It willcertainly be very interesting to see if future work can verifywhether the aforementioned mechanically assisted surface-atom

diffusion alone could facilitate the cold welding of nanowires withdifferent crystal orientations. On the other hand, althoughPbSe nanocrystal particles still require low-temperature heating(100150 8C; ref. 15) for unication, the welding process reportedhere had no such requirement. This may be understood as follows.First, the mechanical manipulation of the nanowire, instead oflocal heating, helped to match the orientation of the samples inwhat was the beginning of the oriented-attachment process15.Second, as in traditional cold welding, the use of a clean surfaceunder conditions of high vacuum is an important factor2,3, andthe gold nanowire samples (particularly the freshly broken nano-wires) in the TEM chamber clearly satised this requirement.

ConclusionsWe have demonstrated that the cold welding technique hasthe capability to join ultrathin gold nanowires without introdu-cing defects. The welding occurs at close to room temperature,and its exceptional quality is attributed to the nanoscalesample dimensions, oriented-attachment mechanisms, as wellas mechanically assisted surface atom diffusions. This processrequires no heating or high load, and can be carried outrelatively quickly. More importantly, neither the mechanicalnor electrical properties of the nanowires were affected. Theseresults provide the rst atomic-scale visualization of the cold-welding process, revealing, for the rst time, the physical mech-anisms of the cold welding of nanowires. Combined with othernano- and microfabrication technologies3638, nanoscale coldwelding is anticipated to have potential applications in thefuture bottom-up assembly of metallic one-dimensional nano-structures and next-generation interconnects for extremelydense logic circuits.

Reference barNanowire sample

AFM cantilever

D

F

Loading direction

STM probea

b dc e

Figure 5 | In situ tensile strength measurements of the nanowelds. a, Schematic showing how the AFM cantilever acts as a force sensor (by measuring the

deection of the cantilever, DD) and the STM probe acts as an actuator while the attached nanowire sample is under tensile loading. b,c, Two gold nanowires

before and after cold welding on the TEMAFM holder. d, As-welded nanowire under the maximum load state. e, Broken nanowires at the steady state (note

that the breaking point is no longer the same as the initial contact point). The thin double-headed arrows in d and e indicate the relative displacements

of the AFM cantilever tip with respect to the reference bar. Triangles indicate the edges of the two nanowires before and during welding, and the thicker

single-headed arrows indicate the tensile loading direction. Scale bars, 10 nm.




MethodsIn situ TEM samples were prepared by adhering nanowires or nanorods ontotungsten or gold STM probes and then loading the probes into the TEMSTM orTEMAFM holders (NanoFactoryTM Instruments). Three-dimensional movementof the STM probes was driven by the piezo-electric heads of the holders. In additionto the normal manipulation and joining of the nanowires, the TEMSTM holdercould also apply a specic bias and measure the current response of the nanowiresample bridged across two gold STM probes.

The TEMAFM holder was used primarily for quantitative measurement ofnanowire strength, for which a silicon AFM cantilever beam with known springconstant (4.8 N m21) was deected by the clamped nanowire sample undertensile loading. Attachment of the sample to the AFM cantilever was carried outby pushing the nanowire against the AFM tip surface, which was coated with anadhesive layer, until strong bonding was formed. Because the deection of thecantilever was much smaller than its length, a linear relationship between DD(displacement of the AFM tip, equal to the cantilever deection) and F (forceapplied on the nanowire sample) was assumed. During the experiment, aselected area diffraction (SAD) centre-spot blocking bar, which was free ofmovement throughout the process of taking TEM images and videos, wasinserted as the reference for displacement measurements. The tensile strengthwas calculated as engineering stress. The stress calculation was reasonably

accurate (less than+10% error) by measuring cantilever deections in highmagnication TEM images.

All welding experiments were carried out using an FEITM Tecnai G2 F30 TEM,operating predominantly at 300 kV working voltage (lower working voltages werealso used to rule out the electron-beam heating effect; see discussions inSupplementary Information). During the welding experiments, no current waspassed through the sample, and a low-intensity electron beam was always used forimaging and video capturing.

Received 6 November 2009; accepted 13 January 2010;published online 14 February 2010

References1. Opening remarks. J. Am. Welding Soc. 1, 3 (1919).2. Freitas, R. A. & Gilbreath, W. P. (eds) Advanced Automation for Space Missions:

Proceedings of the 1980 NASA/ASEE Summer Study, Appendix 4C.1.(NASA, 1980).

3. Ferguson, G. S., Chaudhury, M. K., Sigal, G. B. & Whitesides, G. M. Contactadhesion of thin gold lms on elastomeric supports: cold welding under ambientconditions. Science 253, 776778 (1991).

4. Kim, C., Burrows, P. E. & Forrest, S. R. Micropatterning of organic electronicdevices by cold-welding. Science 288, 831833 (2000).

5. Jin, C., Suenaga, K. & Iijima, S. Plumbing carbon nanotubes. Nature Nanotech.3, 1721 (2008).

6. Wang, M., Wang, J., Chen, Q. & Peng, L. M. Fabrication and electrical andmechanical properties of carbon nanotube interconnections. Adv. Funct. Mater.15, 18251831 (2005).

7. Hirayama, H., Kawamoto, Y., Hayashi, H. & Takayanagi, K. Nanospot welding ofcarbon nanotubes. Appl. Phys. Lett. 79, 11691171 (2001).

8. Madsen, D. N. et al. Soldering of nanotubes onto microelectrodes. Nano Lett. 3,4749 (2003).

9. Wu, Y. & Yang, P. Melting and welding semiconductor nanowires in nanotubes.Adv. Mater. 13, 520523 (2001).

10. Dong, L., Tao, X., Zhang, L., Zhang, X. & Nelson, B. J. Nanorobotic spot welding:controlled metal deposition with attogram precision from copper-lled carbonnanotubes. Nano Lett. 7, 5863 (2007).

11. Misra, A. & Daraio, C. Sharp carbon-nanotube tips and carbon-nanotubesoldering irons. Adv. Mater. 20, 14 (2008).

12. Rodrguez-Manzo, J. A. et al. Heterojunctions between metals and carbonnanotubes as ultimate nanocontacts. Proc. Natl Acad. Sci. USA 106,45914595 (2009).

13. Xu, S. et al. Nanometer-scale modication and welding of silicon and metallicnanowires with a high-intensity electron beam. Small 1, 12211229 (2005).

14. Tohmyoh, H., Imaizumi, T., Hayashi, H. & Saka, M. Welding of Pt nanowires byJoule heating. Scripta Mater. 57, 953956 (2007).

15. van Huis, M. A. et al. Low-temperature nanocrystal unication through rotationsand relaxations probed by in situ transmission electron microscopy. Nano Lett.8, 39593963 (2008).

16. Kizuka, T., Yamada, K., Deguchi, S., Naruse, M. & Tanaka, N. Time-resolvedhigh-resolution electron microscopy of atomic scale solid-state direct bonding ofgold tips. J. Electron Microsc. 46, 151160 (1997).

17. Tohmyoh, H. A governing parameter for the melting phenomenon atnanocontacts by Joule heating and its application to joining together two thinmetallic wires. J. Appl. Phys. 105, 014907 (2009).

18. Kim, S. J. & Jang, D. J. Laser-induced nanowelding of gold nanoparticles. Appl.Phys. Lett. 86, 033112 (2005).

19. Moskalenko, A. V., Burbridge, D. J., Viau, G. & Gordeev, S. N. Electron-beam-induced welding of 3D nano-objects from beneath. Nanotechnology 18,025304 (2007).

20. Peng, Y., Cullis, T. & Inkson, B. Bottom-up nanoconstruction by the welding ofindividual metallic nanoobjects using nanoscale solder. Nano Lett. 9,9196 (2009).

21. Gu, Z., Ye, H., Smirnova, D., Small, D. & Gracias, D. H. Reow and electricalcharacteristics of nanoscale solder. Small 2, 225229 (2006).

22. Lu, W. & Lieber, C. M. Nanoelectronics from the bottom up. Nature Mater. 6,841850 (2007).

23. Ji, C. & Searson, P. C. Synthesis and characterization of nanoporous goldnanowires. J. Phys. Chem. B 107, 44944499 (2003).

24. Wang, C., Hu, Y., Lieber, C. M. & Sun, S. Ultrathin Au nanowires and theirtransport properties. J. Am. Chem. Soc. 130, 89028903 (2008).

25. Howatson, A. M., Lund, P. G. & Todd, J. D. Engineering Tables and Data 41, 2ndedn (Chapman and Hall, 1991).

26. Gall, K., Diao, J., Agrait, N. & Dunn, M. L. The strength of gold nanowires.Nano. Lett. 4, 24312436 (2004).

27. Wu, B., Heidelberg, A. & Boland, J. J. Mechanical properties of ultrahigh-strength gold nanowires. Nature Mater. 4, 525529 (2005).

28. Ramsperger, U., Uchihashi, T. & Nejoh, H. Fabrication and lateralelectronic transport measurements of gold nanowires. Appl. Phys. Lett. 78,8587 (2001).

1,000

1,000

1.0 0.5 0.0 0.5 1.0Bias (mV)

Original1st2nd3rd4th5th6th7th8th9th10th11th

Curr

ent (

nA)

500

500

0

a

b

d

c

Figure 6 | In situ electrical measurements of the nanowelds. ac, Gold

nanowire (length, 130 nm; diameter, 7 nm) bridged between gold probes

in initial (a), broken (b) and welding (c) states. Scale bars, 10 nm.

d, Comparison of the averaged IV curves of the original and as-welded gold

nanowires. Cold welding was successfully performed 11 times for the same

sample by repeated breaking and re-welding of the nanowire, as shown

in ac. The y-axis (current) error was 2% for the original nanowiremeasurement and 5% for each measurement of the as-welded nanowire.




29. Calleja, M., Tello, M., Anguita, J., Garcia, F. & Garcia, R. Fabrication of goldnanowires on insulating substrates by eld-induced mass transport. Appl. Phys.Lett. 79, 24712473 (2001).

30. Song, J. H., Wu, Y., Messer, B., Kind, H. & Yang, P. Metal nanowire formationusing Mo3Se3

2 as reducing and sacricing templates. J. Am. Chem. Soc. 123,1039710398 (2001).

31. Jose-Yacaman, M. et al. Surface diffusion and coalescence of mobile metalnanoparticles. J. Phys. Chem. B 109, 97039711 (2005).

32. Rez, P. & Glaisher, R. W. Measurement of energy deposition in transmissionelectron microscopy. Ultramicroscopy 35, 6569 (1991).

33. Kizuka, T. Atomic process of point contact in gold studied by time-resolvedhigh-resolution transmission electron microscopy. Phys. Rev. Lett. 81,44484451 (1998).

34. Sanders, D. E. & DePristo, A. E. Predicted diffusion rates on fcc (001) metalsurfaces for adsorbate/substrate combinations of Ni, Cu, Rh, Pd, Ag, Pt, Au.Surf. Sci. 260, 116128 (1992).

35. Cho, K. S., Talapin, D. V., Gaschler, W. & Murray, C. B. Designing PbSenanowires and nanorings through oriented attachment of nanoparticles. J. Am.Chem. Soc. 127, 71407147 (2005).

36. Zhong, Z., Wang, D., Cui, Y., Bockrath, M. W. & Lieber, C. M. Nanowirecrossbar arrays as address decoders for integrated nanosystems. Science 302,13771379 (2003).

37. Whang, D., Jin, S., Wu, Y. & Lieber, C. M. Large-scale hierarchical organizationof nanowire arrays for integrated nanosystems. Nano Lett. 3, 12551259 (2003).

38. Huo, F. et al. Polymer pen lithography. Science 321, 16581660 (2008).

AcknowledgementsY.L. and J.L. acknowledge the nancial support provided by the Air Force Ofce ofSponsored Research (AFOSR) YIP award FA9550-09-1-0084 and by National ScienceFoundation (NSF) grant ECCS-0702766. This work was performed, in part, at the Centerfor Integrated Nanotechnologies, a US Department of Energy, Ofce of Basic EnergySciences user facility. Sandia National Laboratories is a multiprogram laboratory operatedby Sandia Corporation, a Lockheed-Martin Company, for the US Department of Energyunder contract no. DE-AC04-94AL85000.

Author contributionsY.L., J.H. and J.L. conceived and designed the experiments. Y.L. performed the experiments.Y.L., J.H. and J.L. analysed the data. C.W. and S.S. supplied materials. Y.L. and J.L.composed the manuscript. All authors discussed the results and edited the manuscript.

Additional informationThe authors declare no competing nancial interests. Supplementary informationaccompanies this paper at www.nature.com/naturenanotechnology. Reprints andpermission information is available online at http://npg.nature.com/reprintsandpermissions/.Correspondence and requests for materials should be addressed to J.L.




Cold welding of ultrathin gold nanowiresNanoscale cold weldingIn situ mechanical and electrical measurementsComparison with fusion and macroscopic cold weldingMechanisms of nanoscale cold weldingConclusionsMethodsFigure 1 Two types of ultrathin gold nanowire samples used for cold-welding experiments.Figure 2 Head-to-head and side-to-side cold-welding geometries.Figure 3 Head-to-head welding of two gold nanorods.Figure 4 Side-to-side welding of two gold nanowires.Figure 5 In situ tensile strength measurements of the nanowelds.Figure 6 In situ electrical measurements of the nanowelds.ReferencesAcknowledgementsAuthor contributionsAdditional information

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cold welding of ultrathin gold nanowires

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