Laser-induced single point nanowelding of silver nanowires
Shuowei Dai, Qiang Li, Guoping Liu, Hangbo Yang, Yuanqing Yang, Ding Zhao, Wei Wang, and Min Qiu
Citation: Applied Physics Letters 108, 121103 (2016); doi: 10.1063/1.4944699
View online: http://dx.doi.org/10.1063/1.4944699
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APPLIED PHYSICS LETTERS 108, 121103 (2016)
Laser-induced single point nanowelding of silver nanowires
Shuowei Dai,a) Qiang Li,a),b) Guoping Liu, Hangbo Yang, Yuanqing Yang, Ding Zhao,
Wei Wang, and Min Qiuc)
State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering,
Zhejiang University, Hangzhou 310027, China
(Received 25 August 2015; accepted 10 March 2016; published online 22 March 2016)
Nanowelding of nanomaterials opens up an emerging set of applications in transparent conductors,
thin-film solar cells, nanocatalysis, cancer therapy, and nanoscale patterning. Single point nanowelding (SPNW) is highly demanded for building complex nanostructures. In this letter, the precise control of SPNW of silver nanowires is explored in depth, where the nanowelding is laser-induced
through the plasmonic resonance enhanced photothermal effect. It is shown that the illumination
position is a critical factor for the nanowelding process. As an example of performance enhancement, output at wire end can be increased by 65% after welding for a plasmonic nanocoupler. Thus,
single point nanowelding technique shows great potentials for high-performance electronic and photonic devices based on nanowires, such as nanoelectronic circuits and plasmonic nanodevices.
C 2016 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4944699]
V
The welding of nanomaterials has been investigated
intensively because of its potential applications in transparent conductors,1–7 thin-film solar cells,8 touch-panels,9–11
wearable electronics,12 and cancer therapy.13 So far, different approaches have been pursued to weld the junctions,
such as thermal annealing,14,15 light-induced (including nonchromatic lamp16 or laser17–25) interaction, mechanical
pressing,26 and cold welding.27–30 These methods are mainly
intended for large-scale nanowire meshes. The metal nanowires are also one of the essential building blocks for nanophotonics and nanoelectronics.31,32 They can be constructed
as integrated plasmonic circuits in nanophotonics.31 They
also function as electrical connections in nanoelectronics.32
The nanowire–nanowire joining to realize single point nanowelding (SPNW) is thereby important and needs to be examined. Whether the methods used for large-scale nanowire
meshes can be directly extended to SPNW needs to be studied. This is because extremely large beam size is usually
adopted in these methods. While for SPNW, the illumination
position can be a crucial factor in welding. Lu et al. proposed
a cold welding method to connect ultrathin gold nanowires
together.33 The electrical Joule heat could also be conducted
to weld nanowires by applying voltage/current.34,35 The
Joule heating is often detrimental to the underlying substructures. Moreover, the as-mentioned methods require extra
electrodes to contact nanowires directly. Welding through
laser-induced photothermal interaction appears to be a competitive alternative in terms of non-contact processing and
minimum thermal damage to the substrate. To be specific, it
relies only on the localized heat generated by the plasmonic
resonance at the surface of metallic nanowires.
In this work, the light-induced SPNW based on plasmonic
enhanced photothermal effect is explored in depth. To gain a
better understanding of photothermal effect on the nanowirebased structures, we first investigate laser illuminating on an
individual nanowire. A heat transfer model is adopted to
quantitatively characterize the thermal maps in nanowires.
The SPNW is then applied to form functional nanostructures such as nanocouplers and nanojunctions. Finally, the
optical output of nanocoupler is tested to demonstrate that
the plasmon coupling efficiency could be improved after
nanowelding.
In order to make the described nanowires, we used the
method reported in Ref. 36. The nanowires are synthesized
in a chemical mixture of polyvinylpyrrolidone (PVP) and
ethylene glycol (EG), and have a diameter around 300 nm.
These thick silver nanowires show low propagation loss
because only a small portion of field is confined inside the
silver nanowires, and are thus widely used as plasmonic
waveguides.31,37 This widely used method for Ag nanowire
synthesis results in a nanoscale coating of insulating PVP.
The Ag nanowires are first dropcast on the glass substrate.
For a typical experiment, the diameters of Ag nanowires
range from 270 nm to 370 nm, with lengths between 15 lm
and 35 lm.
Next, the nanowelding of silver nanowires is studied. A
schematic of the experimental setup is illustrated in Fig. 1. A
continuous-wave laser operating at 532-nm wavelength is
adopted as a light source. The laser beam passes through a
a)
S. W. Dai and Q. Li contributed equally to this work.
[email protected].
c)
[email protected].
b)
0003-6951/2016/108(12)/121103/5/$30.00
FIG. 1. Schematic of the experimental setup.
108, 121103-1
C 2016 AIP Publishing LLC
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Dai et al.
Appl. Phys. Lett. 108, 121103 (2016)
FIG. 2. Photothermal morphology
change of an individual nanowire. (a)
Schematics of the illumination geometry on a glass substrate. (b) and (d) are
optical images after middle illumination and side illumination, respectively. Scale bar is 10 lm. (c) and (e)
are SEM images of the middle region
after middle illumination and the
whole nanowire after side illumination,
respectively. Scale bar is 200 nm. The
green and red arrows denote the beam
center position and the shrinkage
direction, respectively.
beam attenuator to control the incident power (around
200 mW) and then a mechanical beam shutter to control the
processing time. A quarter wave-plate transforms the linear
polarized light into circular-polarized one. After that the
light propagates directly into the optical microscope and is
then focused with a waist radius of about 430 nm using a
100 objective. A CCD camera is mounted on the microscope to monitor the sample. The shutter time of the laser
processing is set to be 1 s considering the easy-controllability
of our mechanical shutter, while the actual welding process
happens on microsecond scale. The exact, desired welding
location is achieved by an electrically controlled stage where
the Ag nanowires are placed. Throughout this study,
circular-polarized light is used. During the nanowelding, the
nanoscale PVP coating is easily removed because of the
large temperature increase.
Figure 2 shows the photothermal morphology changes
for different incident positions, which could be divided into
two categories: middle illumination and side illumination.
The latter refers to the case where the beam center is close to
the endface of the nanowire, as Fig. 2(a) depicts. By setting
the incident power as 150 mW, the Ag nanowire seems to be
unchangeable after middle illumination (Fig. 2(b)). Scanning
electron microscope (SEM) image shows that the focused
light nearly cuts the nanowire at the beam center (Fig. 2(c)).
In contrast, for the side illumination case, the left end of the
Ag nanowire apparently shrinks along the nanowire (Fig.
2(d)), transforming into a ball-like end (Fig. 2(e)).
To further explore the temperature distribution in the
nanowelding process, the Finite Element Method (FEM)
based commercial software Comsol Multiphysics is used
here. This method was also adopted in Refs. 38–40. All
physical and thermal data are taken from COMSOL libraries.
The thermal analysis of the process is described by a transient heat transfer equation written as Csq T/ t ¼ k䉮2T þ Qs.
T is the temperature of the nanostructure, which is time and
space dependent. Cs, q, and k are specific heat capacity, density, and thermal conductivity, respectively. Qs is the heat
source per unit volume. Csq T/ t is the time derivative of
thermal energy per unit volume. k䉮2T is heat flux going
outside the unit volume due to temperature gradient. Our
previous work on nanoparticle system has shown that this
equation is still valid from the temporal perspective.39 The
heat source per unit volume Qs can be calculated as
Qs ¼ 12 e0 xImeðxÞjEj2 , where e0 and e(x) are vacuum permittivity and gold permittivity, respectively, x is the angular
frequency of the light, and E is the electric field. By obtaining the electric field also from the commercial software
Comsol Multiphysics, the heat source is obtained, and correspondingly, the temperature distribution can be obtained by
solving the heat transfer equation. Figure 3(a) exhibits the
corresponding temperature distribution for middle and side
illumination shown in Figs. 2(b) and 2(d). The melting point
of bulk silver is 1235 K. However, the melting point of Ag
e e
FIG. 3. (a) Temperature distribution as a function of distance along the
nanowire. The dashed green lines represent the melting point of Ag nanowire. The inset shows the illumination positions. (b) SEM image of the
enlarged nanorod-shaped terminal. The incident power is 150 mW. Scale bar
is 1 lm. The green arrow denotes the position of the beam center.
e e
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Appl. Phys. Lett. 108, 121103 (2016)
FIG. 4. Selective nanowelding of a
nanocoupler. Optical images of nanowire unit A and unit B (a) before and
(b) after micromanipulation. Scale bar
is 2 lm. SEM images after laser illumination when (c) aiming at contact at
140 mW power and (d) deviating from
contact at 120 mW power. Scale bar is
200 nm. The insets sketch the positions
of light spot. The radius of the green
circle is size of one beam waist. (e)
and (f) are corresponding temperature
profiles. The insets illustrate the end of
the nanowires, where the green arrows
stand for the heat flux.
nanowire has been found to be lower than that of the corresponding bulk materials. Previous study by Alvaro Mayoral
has demonstrated that the melting point of the Ag nanowire
is around 873 K,41 which is denoted as a green dashed line in
Fig. 3(a). We then review the peak values of both the curves
in Fig. 3(a). For the middle illumination case, the temperature profile is highly symmetric with a peak value of 862 K
(red curve in Fig. 3(a)), a little bit lower than the melting
point. For the side illumination case, the beam center is
located 2 lm away from the nanowire end with the peak
value rising to 1200 K (blue curve in Fig. 3(a)). Thermal conductance determines the heat transfer efficiency to the surrounding. Since the thermal conductance of Ag nanowire is
much higher than that in the air as well as in the glass substrate, a large amount of heat accumulates in the region
between the beam center and the terminal, thus elevating the
temperature significantly.
It is worth noting that the shrinkage length, as Fig. 3(b)
depicts, could be distinguished through the residual traces,
which have also been observed by Polyakov et al.42 From
the simulated temperature curve, we can see that the length
of the silver nanowire where the temperature is above the
melting point is around 4.4 lm, which is in good accordance
with the measurement in SEM image. The conceptual interpretation can be analyzed as follows. First, the position of
the beam center decides the location of peak temperature.
Once nanowire’s temperature is higher than the melting
point, it transforms into liquid phase entirely. Meanwhile,
for some parts of the nanowire, the temperature is slightly
below the melting point. The large temperature gradient
(108 K/m) determines the shrinkage direction of the nanowire. Those melted amounts of silver in liquid phase tend to
shrink into a ball-like structure in order to obtain a minimum
surface energy state,43 while the silver in solid phase remains
unchanged, acting as a crystal nucleus for the melted silver
to resolidify due to the strong surface tension. The actual
welding process happens on the microsecond time scale.
The discussion of photothermal effect on individual
nanowires serves as an excellent guideline for nanowelding.
Figure 4(a) illustrates two nanowire units before micromanipulation. The diameters are both 280 nm. The lengths for
unit A and unit B are 11.3 lm and 10.2 lm, respectively. A
fiber probe using a flame-heated drawing technique44 is
applied to push unit A towards unit B so as to form a Ybranch nanocoupler (Fig. 4(b)). In order to weld two nanowire units together, one may naturally set the beam center at
the contact point. However, our experimental result in Fig.
4(c) implies that this kind of illumination may not lead to the
welding due to the formation of a ball-like end as discussed
before. The inset of Fig. 4(e) clearly denotes that the heat
flux along the nanowire outweighs the heat flux directing
upon another nanowire. To avoid the “negative” heat effect
at the end of nanowire, we adjust the beam center to deviate
from the contact point instead. In Fig. 4(d), the beam center
is located approximately one waist size of a Gaussian beam
away from the contact point. Owing to the light intensity nature of the Gaussian beam, the plasmonic heat is excited
locally and most of the power absorption occurs within the
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Appl. Phys. Lett. 108, 121103 (2016)
FIG. 5. Dark-field optical images of a
silver nanocoupler excited by the
976 nm laser (a) before nanowelding
and (b) after nanowelding. Scale bar is
2 lm. (c) Light-field optical image of a
silver nanocoupler. The green arrow
denotes the position of the beam center
for nanowelding. Scale bar is 2 lm. (d)
SEM image of the welded nanocoupler. Scale bar is 200 nm.
radius of one beam waist. For unit B, this localized heat not
only helps to form a relative-high heat flux between two
nanowires (see inset in Fig. 4(f)) but also keeps the unit B
thermally stable. The importance of controlling the beam
center position is to ensure the temperature at the joint is
higher enough to initiate surface melting but lower than the
melting temperature to avoid liquidation.
Apart from the welded nanocoupler built by basic nanowire units, the SPNW could also be realized in the case of
cross junctions (see supplementary material Fig. S1.45). To
show the potential to improve the performance of photonic
devices after nanowelding, we characterize the optical output
of a typical nanocoupler, as Fig. 5 illustrates. A tapered optical
fiber is employed to conduct the 976 nm laser light with the
quasi-circular-polarization guided modes.31 Through directional coupling, the light is coupled into the nanowire to excite
the plasmonic transmission in the nanocoupler. Figure 5(a)
reveals that the excitation light is directed into nanowire with
a large scattered light spot. Compared to the optical output at
wire end 2, the output at wire end 1 is weaker due to the weak
coupling and propagation loss. For the output at wire end 1,
we calculate its intensity change before and after nanowelding. The intensity is increased by 65% as Fig. 5(b) exhibits,
indicating an enhanced coupling efficiency between the two
nanowires. The wire end 3 in Fig. 5(c) forms a contact point
with another wire, and the corresponding welded region is
shown in Fig. 5(d). Note that the roughness of Ag nanowire
surface is due to the sputtered Au ions aiming to increase sample conductivity under SEM. Meanwhile, the optical output at
wire end 3 is dimmer after nanowelding, which indicates
more photons is coupled into another nanowire instead of
being scattered into the free space, thus improving the coupling efficiency. Further investigations of optical properties of
welded nanowires will be included in our future works.
In conclusion, we have proposed a non-contact laserinduced nanowelding method to join the nanowire-based
structures. The power intensity, illumination position, and
the topology of the nanowire could be determined deliberately. This high degree of control over light–matter interactions is derived from the fact that the plasmonic heat
generated by Ag nanowires is spatial localized, thus laying
importance on the illumination position. To realize nanowelding effectively, the temperature distribution for the basic
unit is instructive as proved by thermodynamic simulations.
As such, a large variety of complex on-chip nanodevices
could be fabricated by means of laser illumination, which
may be extended to the welding of other basic components,
nanoplate, and nanorod. Our work could be the basis of the
design in next-generation interconnection for extremely
dense logic circuits, waveguides, and addressable routers.
This work is supported by the National Natural Science
Foundation of China (Grant Nos. 61425023, 6157517,
61235007, and 61275030).
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