Nano Res

1

Synthesis and purification of long copper nanowires.

Application to high performance flexible transparent

electrodes with and without PEDOT:PSS Céline Mayousse, Caroline Celle, Alexandre Carella, and Jean-Pierre Simonato () Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-013-0397-4

http://www.thenanoresearch.com on December 12, 2013

© Tsinghua University Press 2013

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Nano Research DOI 10.1007/s12274‐013‐0397‐4

1

TABLE OF CONTENTS (TOC)

Synthesis and purification of long copper nanowires.

Application to high performance flexible transparent

electrodes with and without PEDOT:PSS.

Céline Mayousse, Caroline Celle, Alexandre Carella, and

Jean-Pierre Simonato*

CEA, LITEN / DTNM / LCRE, 17 rue des Martyrs, 38054

Grenoble, France

High performance flexible transparent electrodes based on random

networks of copper nanowires are reported. A crucial chemical

cleaning of copper nanowires with glacial acetic acid after the

hydrothermal synthesis is revealed, giving access to transparent

conductive films with excellent performances, typically 55 Ω/sq at

94% transmittance, without need of post-treatment. Encapsulation of

Cu nanowire networks with PEDOT:PSS also provides an efficient

route to very good transparent electrodes. Furthermore integration of

these electrodes into flexible touch sensors is reported.

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Synthesis and purification of long copper nanowires. Application to high performance flexible transparent electrodes with and without PEDOT:PSS.

Céline Mayousse, Caroline Celle, Alexandre Carella, and Jean-Pierre Simonato ()

CEA, LITEN / DTNM / LCRE, 17 rue des Martyrs, 38054 Grenoble, France

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT We demonstrate the hydrothermal synthesis of long copper nanowires based on a simple protocol. We show

that the purification of the nanowires is very important and can be achieved easily by wet treatment with

glacial acetic acid. Fabrication of random networks of purified copper nanowires leads to flexible transparent

electrodes with excellent optoelectronic performances (eg 55 Ω/sq at 94 % transparency). The process is

carried out at room temperature and no post‐treatment was necessary. Hybrid materials with the conductive

polymer PEDOT:PSS show similar properties (eg 46 Ω/sq at 93 % transparency), with improved mechanical

properties. Both electrodes were integrated in capacitive touch sensors.

KEYWORDS

Copper nanowires, Transparent electrodes, PEDOT:PSS, touch sensor, metallic nanowires

1. Introduction

Transparent conductive thin films are essential

components for touch screens, solar cells, organic

light‐emitting diodes and many other

optoelectronic devices. The fabrication of

transparent electrodes is currently realized with

transparent conductive oxides (TCOs), and notably

indium tin oxide (ITO) due to its high conductivity

(~ 10‐30 Ω/sq) at high optical transparency (90 %).

However, ITO is not able to meet the future

demands for transparent conductive films because

of the rising and volatile cost of indium, the

intrinsic brittleness of ITO and the necessary high

temperature processing for production. Therefore,

great efforts have been devoted to develop new

materials produced at low cost, low temperature

and processable over large areas. Several materials

have led to encouraging results so far, such as

carbon nanotubes (CNTs) [1–4], graphene [5],

conductive polymers and metallic nanowires [2,6,7].

Among those candidates, metallic nanowires are

especially promising. Indeed, transparent

electrodes based on silver nanowires (Ag NWs)

have been reported to compete well with TCOs

Nano Res DOI (automatically inserted by the publisher) Research Article

———————————— Address correspondence to: Caroline Celle, caroline.celle@cea.fr; Jean-Pierre Simonato, jean-pierre.simonato@cea.fr

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[8–14]. Up to now, mainly silver has been used.

However, copper appears as an interesting

alternative since it is almost as conductive as silver,

with a bulk resistivity of 1.59 nΩ.m (vs 1.67 nΩ.m

for silver). Moreover, copper is much more

abundant and far less expensive than silver (about

100 times cheaper). Based on these facts, there has

been recently a growing interest in the development

of one‐dimensional copper nanostructures,

especially copper nanowires (Cu NWs). Several

methods for preparing Cu NWs have been reported

such as chemical vapor deposition [15], template

assisted electrochemical synthesis [16,17] or

membrane processes [18]. All these routes have

usually complex reaction processes and often

require the use of toxic reducing agents such as

hydrazine [19–25]. Surprisingly, fabrication of

flexible transparent electrodes based on random

networks of Cu NWs has been scarcely studied up

to now. Wiley and co‐workers have pioneered Cu

NW based electrodes by preparing flexible films

exhibiting sheet resistance of 30 Ω/sq at 85%

transmittance [26,27]. Recently, Lu et al. [28] and

Peng et al. [29] have also reported very good

performances on glass or flexible substrates down

to 30 Ω/sq at 85 % and 51 Ω/sq at 93 %, respectively.

Transparent electrodes with remarkable

optoelectronic performances were also obtained

with the deposition of electrospun copper

nanofibers or nanotrough networks as reported by

Cui’s group [30,31]. However most of these

processes encompass significant drawbacks such as

high temperature, use of physical deposition

methods or use of toxic reducing agents, and

generally hard post‐processing treatments. Thus

there is still a need to develop processes compatible

with the fabrication of low‐cost large area devices

using Cu NWs and to integrate them into functional

devices. To our knowledge very few functional

devices using CuNWs have been reported to date

[29,32,33]. Hybrid materials with graphene

derivatives have also been reported recently,

showing interesting properties and ability to be

used in electrochromic devices or CdTe solar cells

[34,35].

Herein we report an environmentally friendly

synthesis to produce ultralong well‐dispersed

CuNWs. Electrode fabrication was realized at room

temperature without need of post‐treatment. We

show that after the synthesis of Cu NWs, a simple

cleaning with glacial acid acetic (GAA) in solution

improves dramatically the performances, typically a

sheet resistance of 55 Ω/sq at 94 % transparency

which compares well with ITO electrodes. Further

improvement of the electrodes was performed by

making composite thin films of Cu NWs and

poly(3,4‐ethylenedioxythiophene)poly(styrenesulfo

nate) also named PEDOT:PSS. To demonstrate the

practical use of Cu NW transparent conductive

films in functional devices, we also report herein

the fabrication of a capacitive touch sensor.

2. Experimental

Synthesis of Cu NWs

Cu NWs were synthetized according to a modified

published procedure. In a typical protocol, 0.53 g of

octadecylamine (ODA, Aldrich) and 0.13 g of

anhydrous copper chloride (CuCl2, Aldrich) were

added to 80 mL of deionized water. After 5 min of

stirring, the round bottom flask was sonicated for 45

min forming a blue emulsion. Subsequently, the

solution was transferred into a Teflon‐lined

autoclave and sealed for 140 h at 165°C. The reactor

was then cooled down to room temperature. The

supernatant was removed and the reddish settled

product was dispersed in deionized water. The

solution was then transferred into a separatory

funnel and washed successively with deionized

water, n‐hexane and ethanol. The final product was

kept under degased isopropanol to avoid oxidation

of the nanowires.

Before electrode fabrication, GAA treatment was

realized. The solution was centrifuged at 9000 rpm to

remove the solvent, and a large excess of glacial

acetic acid was added. After 10 min, GAA was

removed and the product was washed once with

isopropanol. Eventually, GAA treatment can also be

realized after deposition, by immersing the electrode

into a GAA bath for 10 min, and then drying on a

4

hotplate at 80°C.

Fabrication of electrodes

Electrodes were realized on

poly‐ethylenenaphthalate (PEN) substrates obtained

from Dupont Teijin (Teonex, 125 μm thickness).

Transparent conductive films were formed at room

temperature by filtration of a dispersion of Cu NWs

on a Teflon filter (Sartorius, 0.45 μm). The deposited

film was then transferred to the substrate by

applying uniform pressure. The Teflon filter was

peeled off to leave the Cu NW network on the

substrate. The fabrication process of the electrodes is

detailed in supplementary information (Fig. S‐1).

This method does not require heat or substrate

functionalization. In a parallel study, a solution of

PEDOT:PSS (Clevios, PH1000) in isopropanol was

spin‐coated at 1500 rpm for 50 s onto the nanowire

network. No GAA purification of the solution was

carried out in this case. The thin film was annealed

for 2 min at 100 °C in order to remove the solvent.

Fabrication of flexible capacitive sensors

Capacitive sensors were realized according to a

published procedure using Ag NWs [9]. Cu NWs

were transferred on a 3x3 cm² PEN substrate, and

eventually coated with PEDOT:PSS. Contacts with

external electronics were realized with silver paste

(Ag L200 from Ferro) and the conductive layer was

encapsulated through lamination of PSA film from

3MTM to prevent damaging of the Cu NW networks

and to avoid direct contact of the operators with

nanomaterials for safety precautions.

Characterization

Scanning electron microscopy (SEM) images were

obtained on a FEG‐LEO XL30 operating at 5kV. The

X‐ray diffraction (XRD) analysis was performed

using Siemens‐D5000 X‐ray diffractometer with a

grazing incidence angle of 1.0°. Silicon <100> 1‐10

Ω‐cm, 4 in. wafers were used as substrates for the

copper oxide removal experiments. Cu NWs were

dropped off on cleaned silicon surfaces. Experiments

were conducted on 1 cm² samples cleaved from the

substrate. X‐ray photoelectron spectroscopy (XPS)

measurements were conducted on a NOVA‐KRATOS

model equipped with Al Kα radiation source (1486.7

eV). Transmittance values were measured on a

Varian Cary 5000 spectrophotometer using the

substrate as reference. For haze determination, total

transmittance and diffuse transmittance were

determined using an integrating sphere. The sheet

resistance was measured using a four pin probe with

a Loresta EP resistivity meter. Adhesion test was

performed with scotch tape Crystal from 3M.

3. Results and discussion

The synthesis of Cu NWs was realized according to a

modified reported procedure [21,36]. This process is

extremely simple since it only requires copper

chloride, ODA and water. We found that long

experiments afford better results. After a reaction

time of 140 h, well‐dispersed long Cu NWs were

obtained. The morphology of the nanostructures was

studied by SEM. Figure 1 shows representative

images of the nanowires at different magnifications.

The diameters of the nanowires were in the range

50–400 nm. The nanowires were very long with

lengths up to hundreds of microns. Bar charts

showing distributions of Cu NW lengths and

diameters are presented in Fig. S‐2. Possibly, some

short NWs may originate from broken long NWs

during the purification process. Interestingly, the

nanowires were not straight but showed some

curvatures as observed in Fig. 1(a).

The XRD pattern of the Cu NWs is presented in

Fig. 1(c). The peaks measured at 2θ = 43.2, 50.3 and

74.1° can be indexed to face‐centered cubic (fcc), they

are consistent with those reported elsewhere

[20,29,36]. No peaks attributed to copper oxides such

as CuO and Cu2O could be detected.

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Figure 1 (a) and (b) SEM images of purified Cu NWs (c) XRD pattern of Cu NWs

We realized electrodes using as‐synthesised

NWs, but we found that the electrical measurements

varied over a wide range and were hardly

reproducible. This problem was not observed with

Ag NWs and was thus ascribed to the propension of

copper to oxidize readily with oxygen or to form

stable bonds with amine groups of ODA. We thus

decided to use a room temperature chemical

treatment which was claimed to remove copper

oxide from a copper surface while not affecting

underlying copper [37]. We indeed observed a large

improvement in terms of performances and

reproducibility by treating the nanowires with GAA

before or after the fabrication of random networks of

Cu NWs.

As observed on the SEM images in Fig. 2(a) after

many washes, remaining organics were present on

the substrate and the nanowires, leading to a highly

resistive transparent thin film. We assume that these

organics were ODA residues. After GAA treatment

for 3 min, the substrate was cleaned and the network

became conductive despite the presence of residual

traces on the nanowires (Fig. 2(b)). After 5 min of the

same treatment, a very smooth and cleaned surface

was observed (Fig. 2(c)). The electrical properties

were also considerably improved as reported in

Table 1. It can be noticed that even after 30 min of

immersion, Cu nanowires were not damaged and the

electrical properties almost unchanged.

Figure 2 SEM images of CuNWs (a) before GAA treatment, and after treatment for (b) 3 min (c) 5 min and (d) 30 min

Table 1 Influence of GAA time treatment on electrode sheet

resistance

The effect of GAA treatment is probably twofold.

First, the carboxylic acid species are prone to react

with basic amine groups of remaining ODA used

during the hydrothermal synthesis. Indeed ODA

plays both the role of reducing agent and capping

agent [36]. Thus a very thin layer could still partly

encapsulates the surface of nanowires, preventing

good conductivity notably through prevention of

ohmic contacts at nanowires’ junctions. By

protonating the amine, GAA certainly destabilizes

copper‐amine interaction, making easier the ODA

solubilization into the liquid phase. As mentioned

hereinbefore, GAA also removes traces of copper

oxides that could limit the overall conductivity.

Electrical resistance of Cu NW based electrodes

was observed to be stable after few days, however it

significantly increased after weeks. After 4 months,

the sheet resistance was around 20 kΩ/sq, which was

much higher than the initial value. Fortunately, by

applying GAA treatment to these electrodes, it was

possible to recover the initial conductivity,

highlighting the role of GAA through the

solubilization of superficially grown copper oxides.

GAA treatment

(min) none 3 5 30

Sheet Resistance

at T=94%

(Ω/sq)

> 106 128±90 15±4 17±7

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To further investigate the influence of GAA

treatment, XPS analysis was performed to

demonstrate the copper oxide layer removal.

Comparative experiments were realized on

air‐stored and freshly treated GAA samples. Figure

S‐4 shows XPS spectra in the region of Cu 2p3/2 for

both samples. Prior to GAA treatment, typical native

copper oxide is observed. The two peaks at 933.7 and

935.1 eV, correspond respectively to CuO and

Cu(OH)2 binding energies. The third main peak

contains contributions from both Cu(0) and Cu2O

(932.6 eV). From the O 1s peak, not shown here,

deconvolution allows to distinguish between

Cu(OH)2 (531.0 eV), Cu2O (530.3 eV), and CuO (529.7

eV). No significant amount of Cu(0) is detected on

the surface of the air‐stored sample, meaning that the

oxide layer is larger than the XPS analysis depth (few

nm). After GAA treatment, the peak centered at 932.6

eV corresponding to contribution of Cu(0) and Cu2O

dominates the Cu 2p3/2 spectrum. The pronounced

shoulder due to the presence of copper oxides ( CuO

and Cu(OH)2 ) is no more visible. According to the O

1s peak analysis, peaks assigned to Cu(OH)2 and

CuO are not detected, and a slight contribution of

Cu2O peak (~ 1.4 atom %) can probably be ascribed

to undesired oxidation during air transfer to the XPS

chamber. Our XPS results are in good agreement

with those reported with similar treatment on copper

film [37], they confirm that GAA treatment removes

a thin layer of insulating copper oxides, leading to

enhanced electrical conductivity of the electrodes.

The main characteristics of CuNW based

transparent electrodes are presented in Fig. 3. The

transmittance at 550 nm as a function of the sheet

resistance is plotted in Fig. 3(a). An excellent

trade‐off between transparency and conductivity

was obtained, particularly below 90 % transparency.

For instance, the electrodes exhibited sheet

resistances of 9, 17 and 55 Ω/sq at 88, 92 and 94 %

transparencies respectively. Above 97% transparency,

electrodes were not conductive indicating that

percolation threshold was exceeded. The nanowire

density is approximately 0.021 CuNW g.m‐2 at 94 %

transmittance (see Fig S‐3 for details). It is important

to note that from a processing point of view, our

protocol does not require the use of physical

processes like high temperature (≥170°C) thermal

annealing [29] usually combined with forming gas

post‐treatment [27,28,34]. This procedure gives direct

access to functional electrodes at low temperature, it

is thereby compatible with full solution process

fabrication of heat sensitive multilayered devices.

Figure 3(b) shows a comparison of the

transmittance of a Cu NW based electrode with other

transparent thin films of conductive materials. While

ITO or PEDOT:PSS electrodes absorb light in the

near‐infrared (NIR) region, electrodes made of Cu

NWs demonstrate almost constant transmittance

over a large wavelength range. This might be of

interest for devices such as solar cells with wide

absorption spectrum [38]. The relationship between

transmittance and haze factor is plotted in Fig. 3(c).

Haze factor is a standard metric for quantifying a

film’s light scattering efficiency. It is thus a critical

parameter for optoelectronic devices [13,39,40], but it

is important to mention that there is no ideal value

for haze factor since expectations for haze values are

entirely application dependent [7]. The haze is

defined according to Eq. (1), were Tdiff is the

percentage of light diffusively scattered and Ttot the

total light transmitted.

%

100 (1)

Haze values appear to be strongly dependent on

network density. Indeed a 11% haze value was

measured at 86% transparency, whereas only 4%

were obtained at 97% transparency This is not

surprising since high density of nanowires is

expected to cause more light scattering.

To conclude about the first part of this study, it

appears that the reported chemical treatment of Cu

NWs is particularly efficient, and provides an easy

route to achieve high performance Cu NW based

electrodes, in particular when compared to other

works as shown in Fig. 4.

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Figure 3 (a) Plot of transmittance (at 550 nm) vs sheet resistance for Cu NW electrodes. Inset shows a flexible electrode on PEN

substrate (b) Plot of transmittance vs UV-Vis-NIR wavelengths (pink, blue and grey areas respectively) for several well-known

transparent conductive materials. (c) Haze value as a function of Cu NW electrode transmittance (at 550 nm) (d) Picture of a 3x3 cm²

Cu NW based electrode on PEN substrate over CEA logo

Figure 4 Comparison of the best performances of transparent electrodes based on solution synthesised Cu NWs and obtained with different post-treatments. P=Pressure; P*=Plasma cleaner; H2, N2= hydrogen and nitrogen atmosphere respectively, T= Annealing; Vac= Vacuum. [33]: Pressure (4 GPa) and hydrogen annealing at 175°C for 30 min. [27]: Plasma cleaner and hydrogen annealing at 175°C for 30 min. [29]: Vacuum annealing at 200°C during for 1 h. [28]: Forming gas annealing at 300°C

In a second part of the study, we were interested

in the fabrication of hybrid films based on CuNWs

and PEDOT:PSS, which is a commonly used solution

processable polymeric conductor. As discussed

hereinbefore, electrodes made with as‐synthetized

Cu NWs were hardly conductive without acidic

treatment. We tried to associate this polymer with

CuNWs to improve the properties of the Cu NW

based electrodes since combination of PEDOT:PSS

and Ag NWs has already proven efficient [41–44].

Thus PEDOT:PSS was spin‐coated as a thin layer (50

nm) on top of the random networks of Cu NWs. To

investigate the electrical conductivity of the Cu NWs

/ PEDOT:PSS thin films in comparison of GAA

treated Cu NW networks, sheet resistances were

measured and plotted as a function of the

transmittance (Fig. 5(a)). The 50 nm spin‐coated layer

of pristine PEDOT:PSS exhibited a sheet resistance of

3x105 Ω/sq at T=98 %. Composite electrodes

presented sheet resistances of 46 and 24 Ω/sq at 93

and 91 % transparency respectively, similar to those

obtained with Cu NWs after GAA treatment. Their

optical properties were investigated by UV‐Vis‐NIR

spectroscopy. Figure 5(b) shows a noticeable

decrease of transmittance after PEDOT:PSS

deposition in the NIR range, in accordance with the

known absorption properties of the conjugated

polymer (also presented in Fig. 3(b) at higher

thickness). However in the visible range, some

fringes appear due to the PEDOT:PSS layer and the

transmittance is slightly smaller when compared to

Cu NWs only.

8

The significant modification of electrical conductivity

can be ascribed to several major phenomena. Firstly,

deposition of PEDOT:PSS could laminate the

nanowires, thus expanding the number of contacts

between NWs in the network. This was already

reported for Ag NWs [43,44]. The SEM images in

Figure 3(c) and 3(d) confirm that Cu NWs were

totally covered by PEDOT:PSS and the nanowires

were laid down on one another, ensuring improved

electrical connexions.

Figure 5 (a) Evolution of CuNWs electrode in function of wavelength before and after PEDOT:PSS coating. (b), Plot of

transmittance vs. sheet resistance for Cu NW electrode after acid treatment or PEDOT:PSS coating. (c), (d) SEM images of the

CuNWs covered by the PEDOT:PSS film at different magnification

However, to check this role of PEDOT:PSS, a

50 nm thick layer of insulating polymer

(polystyrene) was deposited on a thin film of

as‐synthetized Cu NWs. No improvement of

conductivity was observed. It can thus be deduced

that the lamination effect of spin‐coated

PEDOT:PSS layer is not sufficient on its own to

explain the large conductivity enhancement. This was also confirmed by coating GAA treated Cu

NW based electrodes with PEDOT:PSS: no

significant electrical conductivity improvement

was observed. The main reason can probably be

ascribed to the acidic character of PEDOT:PSS (pH

of the solution was measured at 2.5). It probably

acts like GAA through removal of some insulating

parts of native copper oxides and/or remaining

traces of ODA along the NWs, hence allowing

ohmic contacts at nanowire junctions. In order to

confirm the fact that PEDOT:PSS can remove native

copper oxide, we realized the following experiment.

We prepared a Cu NW based electrode and we

cleaned it with GAA (Rs=32 Ω/sq). After 60 days in

air, the sheet resistance increased up to 400 Ω/sq.

At this time the electrode was coated with a 50 nm

thick layer of PEDOT:PSS, and the initial

performances were recovered. This demonstrates

that PEDOT:PSS allows to regenerate electrodes

after a long oxidation time by removing insulating

copper oxides.

In addition to this point, the opening spaces of

the network were filled with an electrically

conductive polymer which contributes to the

overall conductivity by eventually building

conductive paths between non contacted

nanowires. Another significant advantage of the

hybrid material is the improvement of film

9

adhesion on the substrate. To assess the adhesion

strength modification, scotch tape tests were

performed on Cu NW based electrodes with and

without PEDOT:PSS coating. Pictures taken before,

during, and after scotch peeling are presented in

Fig. S‐5. Like for other metallic one‐dimensional

nanostructures, Cu NW network by itself shows no

significant adhesion onto plastic substrates (Fig.

S‐5(a)). However, addition of a 50 nm thick layer of

PEDOT:PSS leads to a mechanically resistant

conductive film with strong adhesion to the

substrate (Fig. S‐5(b)).

The electrodes based on random networks of

Cu NWs, either treated with GAA or covered with

PEDOT:PSS, were used to fabricate transparent

flexible touch sensors. In both cases, protective

films were applied for safety reasons onto the

conductive layers to avoid any potential physical

contact of users with the nanomaterials. This

encapsulation has also the advantage to limit water

and oxygen diffusion to the Cu NWs, consequently

the touch sensors were found to be functional for

months. The details of the realization and

operating conditions of the touch sensor have been

described in a previous study [9]. Briefly, when a

conductive object such as a finger is approaching or

touching the electrode through the protective

coating, the local electric field is modified and

induces a capacitance change. This variation is

measured using the relaxation oscillator method

(integrated with a MSP430 microcontroller from

Texas Instrument) as illustrated in Fig. 6. To our

knowledge this is the first flexible touch sensor

realized with Cu NW based transparent electrodes.

Figure 6 Use of Cu NW based electrodes on a capacitive touch sensor. The capacitance change is operated by approaching a finger

close to the surface, which modifies oscillation values (right: 362; left: 273)

4. Conclusion

In summary, we have developed a new protocol to

fabricate high performance transparent flexible

electrodes based on copper nanowires. The

nanowires are synthesized according to a

straightforward hydrothermal hydrazine‐free

method using only CuCl2, ODA and water. In order

to obtain excellent performances (eg 55 Ω/sq at 94 %

transmittance), an acidic chemical treatment is

operated to clean up the nanowires. The

performances are among the best values reported

for Cu NWs up to now. The electrode fabrication

process is carried out at room temperature from

nanowires in solution and there is no need for

post‐treatments such as thermal annealing or

forming gas. We also demonstrate the fabrication of

hybrid materials using PEDOT:PSS with similar

optoelectrical performances and improved

adhesion property. Finally, to demonstrate the

potential of these electrodes, capacitive touch

sensors were successfully realized.

Acknowledgements

This work was funded by DGA (French Ministry of

Defense) through a PhD grant to CM.

10

Electronic Supplementary Material:

Supplementary material (scheme of transfer

process, statistic distribution of diameters and

length of Cu NWs synthetized, evolution of

transmittance as density of Cu NWs, XPS spectra,

scotch tape test picures) is available in the online

version of this article at

http://dx.doi.org/10.1007/s12274‐***‐****‐*

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