ARTICLE
Received 20 Dec 2015 | Accepted 21 Mar 2016 | Published 19 Apr 2016

DOI: 10.1038/ncomms11402

OPEN

Nanoparticle chemisorption printing technique for
conductive silver patterning with submicron
resolution
Toshikazu Yamada1, Katsuo Fukuhara1, Ken Matsuoka1, Hiromi Minemawari1, Jun’ya Tsutsumi1, Nobuko Fukuda1,
Keisuke Aoshima1,2, Shunto Arai2, Yuichi Makita3, Hitoshi Kubo3, Takao Enomoto3, Takanari Togashi4,
Masato Kurihara4 & Tatsuo Hasegawa1,2

Silver nanocolloid, a dense suspension of ligand-encapsulated silver nanoparticles, is an
important material for printing-based device production technologies. However, printed
conductive patterns of sufﬁciently high quality and resolution for industrial products have notyet been achieved, as the use of conventional printing techniques is severely limiting.
Here we report a printing technique to manufacture ultraﬁne conductive patterns utilizing theexclusive chemisorption phenomenon of weakly encapsulated silver nanoparticles on a
photoactivated surface. The process includes masked irradiation of vacuum ultraviolet lighton an amorphous perﬂuorinated polymer layer to photoactivate the surface with pendant
carboxylate groups, and subsequent coating of alkylamine-encapsulated silver nanocolloids,
which causes amine–carboxylate conversion to trigger the spontaneous formation of aself-fused solid silver layer. The technique can produce silver patterns of submicron ﬁnenessadhered strongly to substrates, thus enabling manufacture of ﬂexible transparent conductive
sheets. This printing technique could replace conventional vacuum- and photolithographybased device processing.

1 National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, Tsukuba 305-8565, Japan. 2 Department of Applied
Physics, The University of Tokyo, Tokyo 113-8656, Japan. 3 Tanaka Kikinzoku Kogyo K. K., Tsukuba 300-4247, Japan. 4 Department of Material andBiological Chemistry, Yamagata University, Yamagata 990-8560, Japan. Correspondence and requests for materials should be addressed to T.Y.
(email: toshi.yamada@aist.go.jp) or to T.H. (email: t-hasegawa@ap.t.u-tokyo.ac.jp).

NATURE COMMUNICATIONS | 7:11402 | DOI: 10.1038/ncomms11402 | www.nature.com/naturecommunications

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M

etal nanoparticles can be densely suspended indispersion media if the metals are encapsulated byinsulating ligand layers1. These protect the active or
unstable bare metal surfaces and preserve the high surfacearea-to-volume ratio of the nanoparticles, preventing theself-aggregation2,3, coalescence4, welding5, decreased melting
temperature6,7 or liquid droplet-like deformation, especially forsilver nanoparticles (AgNPs) at room temperature8,9.
Concentrated silver nanocolloids, or nanometal inks,
composed of encapsulated AgNPs have recently attracted
considerable attention for applications in environmentallyfriendly printing-based device production (that is, printed
electronics) technologies10–15. Printed electronics technologies
are crucial in realizing a low-consumption society, as theysave resources, energy and time in the creation of ﬂexible,
large-area and ambient devices. The large-scale production ofmonodispersed AgNPs has progressed considerably16–19. Several
cases of direct chemical synthesis of encapsulated AgNPs werereported, utilizing thermolysis processing of precursor silver
complexes, the ligands of which later served as encapsulating
layers18,19. These techniques are promising for producing inks forprinted electronics, as they may allow predesigned encapsulating
layers with tunable coordination strengths while maintaining theequilibrium metastability of the ink.
Much effort has been dedicated to manufacturing ﬁne metal
wiring patterns by various printing methods, such as screen or
inkjet printing, with nanometal inks10–15. However, the obtained
printed patterns have not yet offered the material quality, pattern
resolution, substrate compatibility or substrate adhesion required
for most industrially available electronic devices20,21. Theselimitations result from the physisorption phenomena of theﬂuidic nanometal inks utilized. The inherent liquid nature of theinks results in unsatisfying resolution or quality of the printed
deposits, such as non-uniform layer thickness due to the coffeering phenomenon22–24. The incompatibility of the encapsulating
layer before and after the printing deposition is also critical.
While the layer is necessary for stabilizing the nanocolloid, it
must be removed after deposition to restore the metal
conductivity. To remove the encapsulated layer, annealing isused to fuse the AgNPs at high temperatures, although thisfrequently creates fatal voids and cracks in the conductive circuits
and distorts heat-sensitive ﬂexible plastic substrates. Considering
these difﬁculties and dilemma, an ideal method would restore theself-aggregating ability of bare AgNPs at predeﬁned positions byselectively removing the ligand encapsulation. Such a method has
not yet been reported.
In this study, we report an innovative printing principle tomanufacture ultraﬁne conductive patterns through an exclusive

chemisorption (that is, surface chemical reaction) of AgNPs thattriggers the self-aggregating ability of AgNPs on predeﬁned
photoactivated areas of a solid surface. For this purpose,
we utilize a stable, concentrated and low-viscosity silver
nanocolloid composed of AgNPs encapsulated in alkylamine
layers19. Alkylamine coordination is weaker than carboxylate
coordination, as demonstrated by the sintering temperature ofless than 150 °C. We use a photoactivated polymer surface withpendant carboxylate groups, which combine more strongly withbare silver surfaces. When the alkylamine-encapsulated ﬂuidic
silver nanometal ink contacts the photoactivated surface,self-fused solid silver layers form spontaneously and exclusively
on the photoactivated surface under ambient conditions. This
unique AgNP-based phenomenon enables the extremely simple
printing of large-area electronic circuits with submicron
resolution adhered strongly to ﬂexible plastic substrates.
Results
Printing process and printed products. A schematic of theprocess is presented in Fig. 1a,b. To produce a patterned
photoactivated surface, we used masked vacuum ultraviolet (VUV)
irradiation on the amorphous perﬂuorinated polymer poly[perﬂuoro(4-vinyloxy-1-butene)] (Cytop)25,26. We fabricated thin ﬁlms
of Cytop by spin-coating it on silica, polyethylene naphthalate(PEN) or polyethylene terephthalate (PET) substrates. The spincoated substrates were irradiated with VUV light at a wavelengthof 172 nm through a photomask. The polymer surface was thenexposed to the alkylamine-encapsulated silver nanometal ink (40–60 wt% dispersed in a 4:1 mixed solvent of n-octane and nbutanol) by blade coating under ambient conditions. A thin solid
silver layer eventually formed on the irradiated parts of thepolymer surface after the coating blade was swept, but theunirradiated parts remained bare (Supplementary Movie 1). As aresult, ﬁne silver patterns as shown in Fig. 2a–c were obtained. The
highest resolution obtained is a line width of 800 nm. The
resolution is 10-100 times higher than that obtained byconventional screen or inkjet printing3,4,10–15,20,21,23,24.
The as-printed silver layer shows a high conductivity of
B1.0  104 S cm  1 with no post-treatment by heat. The
conductivity gradually improves with time or by subsequent
low-temperature annealing (o80 °C) to values reaching
1.0  105 S cm  1. The thickness proﬁle of the silver layer is ﬂat
against the line cross section, as presented in Fig. 3a. The silver
layer is adhered to the substrate surface with an adhesive force
exceeding 5 MPa. These characteristics differ from those of thesilver layers obtained by conventional printing technologies.
For comparison, we created a deposit on the unirradiatedpolymer surface by a contact casting technique. The dried deposit

a
1

2

3

4

b
H
N

Alkylamine
hν

H

Cytop

CF2

F2C

FC

O

CF

O

O

Ag

CF2
C
F2

n

Figure 1 | Procedure and principle of the printing method. (a) Schematic of the process. (b) Underlying mechanism.
2

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a

b
5

3

0.8

c

d
UnirradiatedIrradiated
800 nmFigure 2 | Products of the printing method. (a) Optical micrographs of printed patterns of parallel lines with widths of 5, 3 and 0.8 mm, respectively.
(b) Optical micrograph of the printed silver pattern for source and drain electrodes of thin-ﬁlm transistor arrays. (c) SEM image of the printed parallel lines
with a width of 0.8 mm. (d) Expanded SEM image of the perimeter of the printed silver line pattern. Right: Schematic of fused and non-fused AgNPs.
Scale bar: (a) 50 mm; (b) 100 mm; (c) 5 mm; and (d) 100 nm.

acSurface height (nm)60

40
Polymer
layer

Ag
layer

20

0
–10 0 10–100
0Position (μm)

100

d

b

Surface height (nm)

1,000
Ag
layer
Polymer
layer

500

0
–100

0

100

–500
0
Position (μm)

500

Figure 3 | Comparison with conventional wet/dewet patterning. (a) Thickness proﬁles of the printed silver lines at different line widths by the present
printing technique with silver nanometal ink with alkylamine encapsulation on photoactivated polymer surface, and (b) by conventional wet/dewet
patterning with water-based silver nanometal ink with alkyl-carboxylate encapsulation. The proﬁle in (b) presents typical thickness distribution due to thecoffee-ring effect where the edge becomes thicker than the central region, while the proﬁle in (a) presents ﬂat distribution against the line cross section
with no area-size dependence. (c) Enlarged SEM images of cross-section of the printed silver layer with 60 wt% alkylamine-encapsulated silver nanometal
ink on photoactivated polymer surface, and (d) on unirradiated polymer surface. (c,d) Scale bar, 100 nm.

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was easily peeled off the polymer surface. The surface contact
angle of the dispersion solvent was B0° on the irradiated surface
and 34.3° on the unirradiated surface (Supplementary Fig. 1).
The difference in contact angle is similar to wet/dewet patterning
using surface-energy-patterned substrates27,28. The relatively highdewetting nature is indispensable in achieving ﬁne patterning,
because the nanometal ink is brieﬂy spread on but can becompletely swept away from the unirradiated part during theprocess. Nonetheless, the technique is essentially different fromtypical wet/dewet patterning regarding the area-size dependenceof thickness distribution (Fig. 3b) and the adhesive strength to thesubstrate surface.
Figure 3c presents an enlarged SEM image of the cross-section
proﬁle of the printed silver layer (with 60 wt% nanometal ink)
adhered to the irradiated polymer surface. The fused silver layer
with a smooth interface with the polymer layer is formed, and thespherical proﬁles of the AgNPs have completely disappeared.
The image indicates that the AgNPs are fully fused together,
despite the low-temperature (o80 °C) processing. In striking
contrast, the deposit produced by simple casting on theunirradiated polymer surface exhibits particulate features andvoids near the interface with the polymer layer (Fig. 3d). The
thickness depends not on the area size, but on the ink
concentration (Fig. 4a). In the surface proﬁles of the printed
silver layer (Fig. 4b–d), the presence of particulate features
depends on the layer thickness: The surface SEM image of thethin silver layer printed with 40 wt% nanometal ink has no
particulate features because complete self-fusion occurs, whereas
particulate features dominate the surface of the thicker silver layer
printed with 60 wt% nanometal ink. Note that the granularmorphology observed in Fig. 4b is also observed in a thin silver
layer fabricated by vacuum deposition on the same photoactivated Cytop ﬁlm surface (see Supplementary Fig. 2). Furthermore, a clear transition from fusion to non-fusion is observed at
the perimeter of the printed silver pattern (see Fig. 2b): The fusedsilver layer grown on the irradiated polymer surface isaccompanied by a peripheral non-fused layer of spherical AgNPs.
This indicates that a small amount of nanometal ink remains andadries on the neighbouring unirradiated area by capillarity, which
creates the residue of the peripheral non-fused layer.
Underlying mechanism of the printing process. To focus on the
microscopic aspects of the silver-layer formation, Raman spectra
as measured from the top surface of the silver layer and from thebottom interface with the irradiated polymer ﬁlm through thetransparent substrate are shown in Fig. 5a. Both sets of spectra
exhibit prominent and distinctly different vibrational features.
The measurement corresponds to surface-enhanced Raman
spectroscopy, in which the ligand molecules coordinated(or adsorbed) to the AgNP surface can be sensitively detected29.
The spectrum from the top surface exhibits four peaks at 1,135,
1,280, 1,395 and 1,590 cm  1 (Fig. 5a). These peaks likely relate to
the alkylamine group, associated with NH2 deformation, CH2
deformation and wagging, and CN stretching vibrations; theobserved features are much clearer than those of the former
report30,31. Well-ordered alkylamine coordination to the surfaceof silver nano-structures may be responsible for the chemicalenhancement of the surface-enhanced Raman spectroscopy
signal. In contrast, two peaks at 1,360 and 1,570 cm  1 are seenfrom the bottom interface, which clearly differ from those fromthe top surface (ii). Similar features were reported for a
carboxylate-coordinated silver surface32,33; the peaks wereassigned to symmetric and antisymmetric COO  stretching
and CH2 wagging vibrations. Similar features appear in othercases, as shown in Fig. 5a; one is from the bottom interface of asilver layer formed by the vacuum evaporation of silver on theirradiated polymer surface (iii), while the other is from the top
surface of the deposit obtained by casting of a different silver
nanometal ink with alkylcarboxylate encapsulation (iv). The
latter signal can be ascribed to the coordination of the carboxylategroup to the AgNP surface. The observed two broad peaks
are quite similar to those of (ii) and (iii). We conclude thatthe photoactivated area has pendant carboxylate groups, and thatthe bare surface of the evaporated silver ﬁlm is likely coordinatedby these carboxylate groups. Carboxylate groups on the150

Thickness (nm)After annealing
Before annealing
100

50

0

b

40

c

50
Ink concentration (%)

60dFigure 4 | Ink concentration dependence of thickness and surface proﬁles of the printed silver layers. (a) Ink concentration dependence of thickness ofthe printed silver layers. Error bars represent the range of data. (b) Top views of the printed silver layers with different layer thicknesses obtained byprinting nanometal ink with 40 wt%, (c) 50 wt% and (d) 60 wt% concentration, respectively. (b–d) Scale bars, 100 nm.
4

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a

b
i

10

ii
Adhesivity (MPa)iii

Raman intensity (a.u.)

iv

c

5

Contact angle (deg)

0

110

100
500

1,000

1,500

2,000

0

100

–1

200

300

400

–2

Raman shift (cm )VUV dose (mJ cm )Figure 5 | Evidence for chemical connection of the printed silver layer with photoactivated polymer surface. (a) Raman spectra (i) from the top surfaceand (ii) from the bottom interface of the printed silver layer on photoactivated polymer surface, (iii) from the bottom interface of the vacuum-evaporated
silver layer on the photoactivated polymer surface, (iv) from the top surface of the deposit obtained by simple casting of the alkyl-carboxylate-encapsulated
silver nanometal ink. (b) Adhesive strength of the printed silver layer to the substrate surface, and (c) water contact angle on the photoactivated polymer
surface, both plotted as a function of the VUV irradiation dose.

photoactivated area should connect easily to bare AgNP surfacesat room temperature, because the encapsulating alkylamine
molecules can be attached and detached at equilibrium
conditions. We conclude that the alkylamine coordination ischemically converted to carboxylate coordination on theirradiated polymer surface.
Regarding the origin of carboxylate group, VUV irradiation on
perﬂuorinated polymers has been reported to cause
photodegradation into several products, including carboxylic acidpendants on the polymer chain34–37. We obtained both X-ray
photoelectron spectroscopy (XPS) and electron spin resonance(ESR) spectra of the Cytop ﬁlms before and after VUVirradiation, as presented in Fig. 6. In the XPS spectra, theirradiation decreases the intensity of the peak at 290.5 eV andincreases that of the peak at 286 eV (Fig. 6a). This spectral
variation is ascribed to diﬂuoride carbons (  CF2  ) being
converted to different de-ﬂuorinated chemical species of carbon.
The ESR spectra also demonstrate the generation of peroxy
radicals (Fig. 6b). One possible photodegradation process isthe cleavage of the ether bond (C  O) within the ring unit of
the polymer to produce carbon radicals (  C) and acid ﬂuoride(  CF ¼ O) end groups by VUV irradiation. Through exposure
to air, the former would be converted into peroxy radicals,
while the latter would be followed by the hydrolytic formationof carboxylate groups. Thus, free carboxylate groups attached tothe polymer chain are generated by VUV photodegradation on
the surface of Cytop layers.
The irradiation-dose dependencies of the water contact angle
on the irradiated polymer surface and of the adhesive strength ofthe printed silver layer to the substrate are shown in Fig. 5b.
The decreased water contact angle by increasing the irradiationdose indicates the increased surface density of carboxylate groups.

With increased irradiation, the adhesive strength of the silver
layer to the substrate surface is increased considerably. Highconductivity is obtained by the high-enough irradiationdose (B100 mJ cm  2), while the conductivity becomes extremely
low at low irradiation dose (B20 mJ cm  2). We found that at
this dose level the coated nanometal ink forms a network-likepattern on the surface (see Supplementary Fig. 3). We conclude
that the printed silver layer is chemically bound to the polymer
layer through carboxylate coordination.
Based on all the observations above, we propose that when thealkylamine-encapsulated ﬂuidic silver nanometal ink contacts theirradiated polymer surface with pendant carboxylate groups, it
promotes the amine–carboxylate conversion for encapsulated
AgNPs that triggers the spontaneous formation of a self-fusedsolid silver layer. The following mechanism may be responsible
for the self-fusion process: as the alkylamine coordination isrelatively weak, the alkylamine would repeatedly attach to anddetach from the metal surface in the nanocolloid at room
temperature. When the nanometal ink makes contact with thecarboxylate-functionalized photoactivated surface, thermally
moving AgNPs are captured by the carboxylate coordination
when they collide with the photoactivated surface. When thetrapped AgNP density becomes sufﬁciently high at the surface,
the bare surfaces of the AgNPs make contact and fuse to form asolid silver layer. Actually, in the thermal analysis of driedalkylamine-encapsulated AgNP powder shown in Fig. 7, a notable
exothermic reaction, accompanied by a simultaneous weight loss,
was observed at a temperature lower than 100 °C and peakedaround 130 °C. It is most probable that the weight loss should be
due to the (endothermic) elimination reaction of alkylamine fromthe AgNPs, while it promotes the considerable exothermic
fusion reaction between AgNPs. The exothermic nature of the

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ab
Beforeirradiation

C1s

After
irradiation

295

290
285
Binding energy (eV)Integrated ESR signal (a.u.)

XPS signal (a.u.)

ESR signal (a.u.)

C1s

280

Immediately
after irradiation

After 25 days

310

×10
315
320
Magnetic field (mT)

325

Figure 6 | Spectral analyses of photoactivated polymer surface. (a) The carbon 1s XPS spectra are shown by black dotted curves for the spin-coatedCytop ﬁlms before (upper part of panel) and after (lower part of panel) the VUV irradiation. It is clearly observed that the irradiation affords a decrease in
the peak at 290.5 eV and an increase in the peak at 286.3 eV. The observed spectra were well ﬁtted by the sum (red solid curve) of four Gaussian lines(gray solid curves for each). Among them, the largest peak at 290.5 eV can be assigned as diﬂuoride carbons, while the peak at 286.3 eV can be assigned
as the carbons in structures resulting from deﬂuorination of the polymer. (b) First-derivative ESR spectra of a free-standing Cytop ﬁlm just after the VUVirradiation with a dose of 512 mJ cm  2 (solid curve) and 25 days after the VUV irradiation (dashed curve). The irradiation affords appearance of the ESR
signal due to peroxy radicals that are most probably formed by the reaction of carbon radicals with oxygen. ESR absorption spectra obtained by integrating
the ﬁrst-derivative signal are shown at the bottom. The upper panel is the spectrum of the ﬁlm just after the VUV irradiation, while the lower panel is the
spectrum of the ﬁlm 25 days after the VUV irradiation. These spectra are well ﬁtted by the sum (red solid curve) of three resonance components,
respectively; the former with g-values at 2.0252 (green curve), 2.0113 (blue curve) and 2.0062 (grey curve), and the latter with g-values at 2.0249 (green
curve), 2.0114 (blue curve) and 2.0071 (grey curve).

60

0

–2

40

–3
20

–4

Heat flow (μV)

Weight (%)

–1

–5
–6

100

200
300
Temperature (°C)

0400Figure 7 | Thermal analysis of AgNP powder. TG-DTA curves for the driedalkylamine-encapsulated AgNP powders. We used the AgNP powder forthe measurement immediately after the as-synthesized material was driedout under ambient conditions.

fusion reaction would promote the cascade formation of aself-fused solid silver layer at room temperature. In particular, theexothermic process should raise the local temperature at the top
Ag surface, which would make the alkylamine group detachedmore from the surface and thus promote additional adsorption of
6

AgNPs on the roughly bare Ag surface. We consider that the
scenario is quite consistent with the ink concentration
dependence of the layer thickness, as presented in Fig. 4a;presence of many more AgNPs in the vicinity of the Ag surface
should promote the adsorption of more AgNPs. Additionally, themobile nature of ligand molecules in the ﬂuidic ink would
effectively eliminate the ligand molecules from the self-fusedsilver layer, which differs from the usual sintering process for
aggregated AgNPs.
The ink stability can be kept good enough, in spite of therelatively weak alkylamine encapsulation and concomitantfacile chemisorption on the photoactivated surface. We consider
that the following facts should be a clue for understanding
the peculiar nature of the alkylamine-encapsulated silver
nanometal ink: Ink stability can be acquired at sufﬁciently
high concentration (for example, at 50 wt%). However, thestable dispersion is lost if it is diluted; AgNPs easily aggregate in
the diluted ink at 1.25 wt% (see Supplementary Fig. 4). It is
most probable that the high alkylamine concentration in
the high-concentration ink is indispensable for achieving
equilibrium colloidal states of the nanometal ink where thealkylamine can repeatedly attach to and detach from the Ag
surface in the ink.
Printing ﬂexible and transparent conducting electrodes.
The present technique enables the print production of

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a

c

100

Transmittance (%)(

90

i: This study
fit)
ii
iii
iv
v
vi
vii

80

70

100
101
102
103
104
Sheet resistance (Ω per square)

Resistance (Ω)

b
104
103
102

i
ii
iii
iv

101

r
TCE

102
103
Bending cycles

104

Figure 8 | Application of the printing technique to touch-screen sensor technology. (a) Plot of transmittance vs. sheet resistance for the printed silver
meshed line pattern with different line widths. Plots for other transparent conductor ﬁlms (ii: copper nanowires (CuNWs), iii: indium tin oxide (ITO), iv:
silver nanowires (AgNWs), v- PEDOT:PSS, vi: single-walled carbon nanotubes (SWNTs), vii: graphene) are shown for comparison39,40. (b) Results ofbending test for the printed silver meshed line pattern; resistivity is plotted as a function of the number of bending cycles at various concave (i) and (iii)and convex (ii) and (iv) bending radii (2.5 mm in i and ii, and 5.0 mm in iii and iv). (c) Photograph of an 18-cm-wide capacitive-type transparent ﬂexible
touch-screen sensor sheet, fabricated with a PET substrate.

submicron-resolution conductive patterns through the reaction of
the nanometal ink with the photoactivated polymer surface.
We call the process surface photoreactive nanometal printing.
The technique is applicable to the fabrication of many electronic
devices. Transparent conducting electrodes (TCEs) used for touch
screen technologies are one such class of devices, since TCEs canbe manufactured with only narrow conductive metal wires if theline width of the wire is comparable to the diffraction limit ofvisible light. To investigate the performance of printed patterns as
TCEs, two critical performance criteria of sheet resistance (Rs)and transmittance (T) are plotted in Fig. 8a, in comparison toother representative TCEs38–40. As shown, the present techniqueprovides the highest-performing TCEs, including the mostwidely used indium tin oxide TCEs. The obtained TCEs alsopresent a higher tolerance to bending than the PEN substratealone, as presented in Fig. 8b: The increased resistance in plots
with 2.5 mm bending radius at 4102 bending cycles is caused notby the deterioration of the printed silver pattern, but by thedegradation of the PEN substrate. It clearly demonstrates theapplicability of the technique for ﬂexible electronics. We
manufactured a capacitive-type touch-screen sensor of width
B18 cm (with a touch sensor line width of 2 mm at intervals of
300 mm, and frame line width of 50 mm) on a PET sheet, as
depicted in Fig. 8c. Sensing operations were successfully
performed with the sensor sheet (Supplementary Movie 2).
Discussion
The printing technique is simple, consisting only of masked VUVirradiation and blade coating of alkylamine-encapsulated
nanometal ink. The method utilizes a ligand conversion(chemisorption) for weakly encapsulated AgNPs that triggers
the self-fused formation of high-resolution patterned silver layers
adhered strongly to the substrate. The achieved resolution andquality has previously been impossible to produce by printing
techniques based on physisorption phenomenon of the ﬂuidic
inks. Considering the VUV wavelength, the resolution could be
improved further. The process does not require a vacuum
atmosphere, which is a main source of energy consumption in the
production of electronic devices. Additionally, the consumptionof nanometal ink can be minimized (to as small as B1–3 ml for a
10-cm2 substrate), as almost all silver contained in the ink is used
to transform into metal wires. The method is particularly
promising as a next-generation technology for producing
electronic products with ﬂexibility, large areas and arbitrary
shapes, as well as for the exploration of applications in which thinﬁlms or wiring patterns of precious metals are needed.
Methods
Preparation of silver nanometal ink. AgNPs were synthesized by thermal
decomposition of silver oxalate, Ag2(C2O4), as reported in the literature19. The
silver oxalate was activated by alkylamines (R-NH2) via the formation ofoxalate-bridged silver alkylamine complexes, [(R-NH2)Ag(m-C2O4)Ag(R-NH2)].
The oxalate-bridged silver alkylamine complexes underwent low-temperaturedecomposition at 110 °C with evolution of CO2, and alkylamine-encapsulated
AgNPs were generated in an almost quantitative yield. Various alkylamines wereadopted to activate the silver oxalate in order to prepare a stable, concentrated andlow-viscosity silver nanometal ink. In a typical case, N,N-dimethyldiaminopropane,
hexylamine and dodecylamine were mixed along with a trace amount of oleic acidto synthesize AgNPs that are dispersed independently in a mixed solvent of
n-octane and n-butanol (4:1 in volume). The AgNPs are in spherical shape, with amean dimension of 13.6 nm and a narrow size distribution (s ¼ 1.1 nm). In thisstudy, silver nanometal inks were prepared in the concentration range between 40
and 60 wt%.

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We also used silver nanometal ink with alkylcarboxylate encapsulation forreference measurements (Fig. 3b). The ink was obtained by thermal decompositionof silver dodecanoate (AgC12H24COO), according to the literature41. The
alkylcarboxylate-encapsulated AgNPs are spherical in shape, with a meandimension of B5 nm.
Fabrication of perﬂuorinated polymer layer. We used a silica plate, PEN or PET
ﬁlm as the base substrate. A thin layer of amorphous perﬂuorinated polymer,
poly[perﬂuoro(4-vinyloxy-1-butene)] (Cytop), was fabricated on the substrate byspin coating of diluted solution in ﬂuorinated solvent (CTL-809M, Asahi Glass Co.,
Ltd., Japan) at 2,000 r.p.m. for 60 s at room temperature. Then the ﬁlms on silica
were dried at 180 °C for 60 min, and those on PEN or PET ﬁlms were dried at 80 °C
in vacuum for 60 min.
Masked irradiation of VUV light. We used a UV dry processor (VUS-3150, ORC
Manufacturing Co., Ltd. Japan) for irradiating VUV light by an Xe2 excimer lamp.
The ﬁlms were set in a chamber ﬁlled with N2 gas with residual O2 lower than
300 p.p.m. Uniform irradiation is achieved by periodic motion of the sample stage
with a period of 20 s. The average dose rate was estimated at 6.4 mW cm  2, andthe irradiated power density was adjusted with a ﬁlter (synthesized silica plate
coated with thin Cr) and irradiation duration. A patterned photoactivated polymer
surface was produced by masked VUV irradiation through a photomask composedof a predeﬁned mask pattern of Cr on a synthesized silica plate.
Characterization of photoactivated polymer surface. Contact angle
measurements were conducted with a contact angle and surface tension analyser(FTA188; First Ten Angstroms, Inc., USA). Atomic force microscope images wereobtained with a scanning probe microscope (Dimension 3000 Nanoscope IIIa;
Bruker Co., Ltd., USA). XPS measurements were carried out using an XPS
apparatus (ULVAC-PHI 5000, ULVAC Inc., Japan) with monochromatic Al Ka
radiation (1,486.6 eV, 15 kV). Because the Cytop layer is highly insulating andeasily charged up by an incident beam, the ﬁlms were slightly coated by
vacuum-evaporated Ag to cover the partially polymer surface by island-shaped Ag.
All the measurements were conducted with a neutralizer, and the offset shift wascorrected by the reference signal of Ag 3d5/2 ( ¼ 368.3 eV). ESR measurements werecarried out using an X-band ESR apparatus (JES-FA200, JEOL Ltd., Japan). A thick
Cytop ﬁlm with a thickness of B25 mm was fabricated by repeated (eight times)
spin coating on a silicon wafer, and was peeled off from the wafer. The
free-standing ﬁlm was used in the ESR measurement. For this purpose, we used
another Cytop (CTX-809SP2, Asahi Glass Co., Ltd.), which enables the facile
peeling of Cytop ﬁlm from the wafer. We could not detect any ESR signal from theunirradiated polymer ﬁlm.
Coating of silver nanometal ink. We used a bar coater (AB3125, The Paul
N. Gardner Company, Inc., USA) for the blade coating of silver nanometal ink on
substrates with photoactivated polymer surfaces. We ﬁrst ﬁxed the substrate on atable, and deposited two or three drops of nanometal ink on the substrate surface
near its edge. Then the line edge of the blade composed of smoothly polishedglass-plate edge was put gently on the drops, and the blade was swept over thesubstrate surface at typical sweep rate of 2 mm s  1.
Characterization of the printed silver layer. We used a digital microscope(VHX-5000; Keyence Co. Ltd., Japan) for the optical microscope observations.
The SEM images were obtained with a scanning electron microscope with aﬁeld-emission gun (JSM-7000 F or -7500 F; JEOL Ltd.). The samples forcross-sectional SEM observation were prepared by an ion slicer (IB-09060CIS;
JEOL Ltd.), keeping the sample temperature at  150 °C. Electrical properties ofthe printed silver patterns were measured by four-terminal measurements using asemiconductor parametric analyser (E5270A; Agilent Technologies Co. Ltd., USA).
The thickness proﬁle was estimated by using a stylus proﬁler (Alpha-Step D-500;
KLA-Tencor Co. Ltd., USA) or by a scanning probe microscope (Dimension 3000
Nanoscope IIIa; Bruker Co., Ltd.). Raman measurements were carried out using a
confocal Raman microscope (inVia, Renishaw Co. Ltd., UK). The adhesion
measurements were carried out using a pull-off adhesion tester (PosiTest AT-A;
DeFelsko Co., USA), conformable to ISO 4624.
Characterization as a ﬂexible TCE. We fabricated various silver wire grid
patterns at uniform intervals of 250 mm with different line widths (15 kinds
between 0.8 and 100 mm) by the present surface photoreactive nanometal printing
technique on a silica substrate. Transmittance is estimated with a UV–vis
spectrometer (U-3500; Hitachi High-Tech Co. Ltd., Japan) by averaging thetransmittance in the wavelength range between 400 and 800 nm. Bending test,
conformable to JIS C5016, is carried out using a ﬂexing tester (FPC; Yasuda Seiki
Seisakusho Ltd., Japan) for the printed silver layer on a PEN substrate (Q65HA,
Teijin Dupont Films Ltd., Japan). The moving cycle was set at 0.72 s. The
measurements were terminated due to the generation of cracks in PEN substrateitself at the bending radii of 2.5 mm.
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Acknowledgements
We are grateful to Ayano Kato, Akiko I and Ichiro Hayakawa for their help in the printing
processes, to Dr Hiroyuki Matsui for his help in the ESR measurements, and to Dr Noritaka
Yamamoto and Dr Manabu Yoshida for their help in the adhesivity measurements. This
work was supported by Japan Science and Technology Agency (JST) through S-Innovation
program (S-Innova) and Next generation TEchnology transfer Program (NexTEP).

Author contributions
T.Y., K.F. and K.M. are responsible for printing production and characterization of all thesilver patterns and the materials. H.M. visualized the concept of the printing technique.
J.T. and N.F. helped the XPS measurement. K.A. and S.A. helped the adhesivity measurement. Y.M., H.K. and T.E. performed trial manufacture of the touch screen sensor.
Y.M., H.K., T.E., T.T. and M.K. synthesized the silver nanometal ink. T.Y. and T.H.
analysed and conceptualized the printing process and wrote the manuscript.

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How to cite this article: Yamada, T. et al. Nanoparticle chemisorption printing
technique for conductive silver patterning with submicron resolution. Nat. Commun.
7:11402 doi: 10.1038/ncomms11402 (2016).
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