-
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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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.
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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.
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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.
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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.
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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
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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.
NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2010.4 ARTICLES
NATURE NANOTECHNOLOGY | VOL 5 | MARCH 2010 |
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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.
ARTICLES NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2010.4
NATURE NANOTECHNOLOGY | VOL 5 | MARCH 2010 |
www.nature.com/naturenanotechnology224
2010 Macmillan Publishers Limited. All rights reserved.
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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