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Flexible Devices

Metallic Nanowire-Based Transparent Electrodes for Next Generation Flexible Devices: a ReviewThomas Sannicolo, Mélanie Lagrange, Anthony Cabos, Caroline Celle, Jean-Pierre Simonato,* and Daniel Bellet*

Transparent electrodes attract intense attention in many technological fields, including optoelectronic devices, transparent film heaters and electromagnetic applications. New generation transparent electrodes are expected to have three main physical properties: high electrical conductivity, high transparency and mechanical flexibility. The most efficient and widely used transparent conducting material is currently indium tin oxide (ITO). However the scarcity of indium associated with ITO’s lack of flexibility and the relatively high manufacturing costs have a prompted search into alternative materials. With their outstanding physical properties, metallic nanowire (MNW)based percolating networks appear to be one of the most promising alternatives to ITO. They also have several other advantages, such as solutionbased processing, and are compatible with large area deposition techniques. Estimations of cost of the technology are lower, in particular thanks to the small quantities of nanomaterials needed to reach industrial performance criteria. The present review investigates recent progress on the main applications reported for MNW networks of any sort (silver, copper, gold, coreshell nanowires) and points out some of the most impressive outcomes. Insights into processing MNW into highperformance transparent conducting thin films are also discussed according to each specific application. Finally, strategies for improving both their stability and integration into real devices are presented.

1. Introduction ........................................ 6053

2. Metallic Nanowire Networks: Synthesis, Fabrication and Physical Properties ..... 6053

3. Applications ........................................ 6059

4. Concluding Remarks and Outlook ........ 6070

From the Contents

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6053© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com

DOI: 10.1002/smll.201602581

T. Sannicolo, A. Cabos, Dr. C. Celle, Dr. J.-P. SimonatoUniv. Grenoble AlpesCEA, LITENF-38054 Grenoble, FranceE-mail: [email protected]

T. Sannicolo, Dr. M. Lagrange, Prof. D. BelletUniv. Grenoble AlpesCNRS, LMGPF-38000 Grenoble, FranceE-mail: [email protected]

1. Introduction

Transparent electrodes (TE) play a pivotal role in many modern devices, such as solar cells, displays, touch screens or transparent heaters.[1] The strong demand for transparent conductive materials (TCMs) has led industrialists and sci-entists to search for possible solutions involving indium tin oxide (ITO) replacement, which has been the most efficient and widely used TCM up to now. This is motivated by both economic and physical considerations: despite its very good optoelectronic properties, ITO is likely to become increas-ingly expensive due to the scarcity of indium.[2,3] Moreover, its brittleness is not compatible with the strong demands for flexible electronics.

In this context, nanostructured transparent conducting materials[4–8] such as carbon nanotubes, graphene and espe-cially metallic nanostructures, have been thoroughly explored and have attracted much attention in the past few years thanks to their promising properties and applications.[6,9,10] In particular, metallic nanowire (MNW) based percolating networks have been found to successfully combine high flexibility, high optical transparency and high electrical con-ductivity. Thanks to the very high aspect ratio of metallic nanowires, such networks can exhibit optoelectronic perfor-mances similar to ITO-based electrodes while consuming a substantially lower quantity of raw material: in order to reach optimized electrical resistance and optical transparency, the required quantity of indium is generally much higher in an ITO electrode than the quantity of silver required in a silver nanowire electrode. This latter quantity was measured as only several tens of mg m–2.[11,12] Moreover, MNW networks have been shown to be compatible with solution-based processing, low cost and large area deposition techniques.

Transparent conducting thin films are mainly used in optoelectronic devices, either to collect charges from nearby functional layers (solar cells) or to supply them with charge carriers (light emitting diode displays) while allowing light either to enter or escape the device. How-ever, MNW networks are also attractive for a wider range of applications, including transparent heaters.[11,13,14] In addition, MNW networks have also proven to be promising for electromagnetic applications such as stretchable radio frequency antennas[15,16] and transparent electromagnetic shielding.[17,18]

According to the wide diversity of possible applications, it appears unlikely that just one type of transparent elec-trode will be superior to all the others: each targeted market requires an appropriate trade-off between the material’s physical properties (electrical, optical, mechanical), stability (thermal, electrical, mechanical, chemical), sustainability, and, finally, certain technical and economic constraints regarding the processing methods available (synthesis, deposition, post-deposition treatments, etc.).

The present review aims at overviewing the main appli-cations for which the integration of MNW-based percolating networks is relevant. The important features and properties of such networks are reported in Section 2. The main results and future prospects are then discussed in detail in section 3 for each of the applications selected.

2. Metallic Nanowire Networks: Synthesis, Fabrication and Physical Properties

This part first reports the different methods for MNW syn-thesis and the fabrication of networks (Section 2.1). The properties of MNW networks depend strongly on the fol-lowing features: i/ individual NW properties (Section 2.2), ii/ the interconnections (junctions) between them, and finally iii/ network density (Section 2.3). The optimization of the physical properties of the networks by post-deposition treat-ments is also briefly described thereafter. The haziness of the MNW networks needs to be either minimized or maximized depending on the intended application and will be the subject of Section 2.4, while the mechanical flexibility of MNW net-works is discussed in Section 2.5. Finally, potential stability issues as well as hybrid nanocomposites based on MNW, as a relevant solution for overcoming them, are addressed in paragraph Section 2.6.

2.1. Synthesis of Metallic Nanowires (MNW) and Network Fabrication

The growth of metallic nanowires has already been described and reported by many groups.[19–27] In this review we will therefore only summarize the most important features. Sev-eral metallic nanowires (MNWs), such as copper, gold, or cupronickel, have been synthesized in solution and deposited in the form of networks with promising properties. However, silver nanowire (AgNW) networks have so far been the most commonly studied.[10,28–30] This stems from both the excellent physical properties of bulk silver, which is the most electri-cally conductive material at room temperature, and the rather straightforward synthesis scalability and reproducibility.

MNWs are mainly synthesized using the polyol process, but other methods, such as hydrothermal synthesis, may be preferred, notably for copper-based NWs.[31–33] In some cases, microwave activation may be used to advantage.[34] In the case of the polyol process, AgNWs are usually synthesized by reducing the Ag nitrate in the presence of polyvinylpyr-rolidone (PVP) in ethylene glycol.[21,35,36] PVP is the most widely used capping agent and is responsible for anisotropic surface passivation. It is adsorbed along the (100) surfaces of the growing AgNW while the (111) surfaces are free to grow. The lateral growth is then prevented thanks to this aniso-tropic adsorption. Figure 1a shows a typical AgNW observed

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by Transmission Electron Microscopy (TEM) revealing the presence of an amorphous thin layer (mostly seen as the PVP shell) surrounding the (100) AgNW surfaces. This type of synthesis generally yields MNWs with a typical diameter ranging from 20 to 150 nm and lengths from few to sev-eral tens of µm. Several companies have already proposed commercial MNWs in solutions, specifically AgNWs and to a lesser extent CuNWs. Some works are also devoted to ultrathin MNWs, mainly focused on AuNWs, with diameters as low as 2 nm.[37–39] Their high aspect ratio (>1000) appears promising. For instance, their small diameter helps prevent light scattering. Stability issues nevertheless still remain a major limitation for using them in devices. Moreover, the fact that the AuNW diameter is much smaller compared to the mean free path of electrons might be a limitation in terms of electrical performance.

MNWs can be dispersed in many solvents, usually water or alcohols. Inks can be formulated to be suitable for var-ious solution-based processing techniques in order to fabri-cate films with randomly oriented NWs. Of these methods, spin coating,[40,41] spray coating,[42] drop casting,[43–45] doctor blade casting,[46] dip-coating,[47] Mayer rod coating,[48–50] screen printing (serigraphy),[51,52] vacuum filtration[28,53,54] or brush coating,[55,56] are the most reported in the literature. Figure 1b shows a typical, dense AgNW network sprayed on to a polyethylene naphthalate substrate (PEN). The network is dense enough to provide many electrical percolating path-ways, while the free areas between the metallic NWs allow the light to go through, and the resulting film to exhibit high optical transparency.

As pointed out by Scardaci et al.[42] or Cao et al.,[30] either for metallic NWs or carbon nanotubes (CNTs), the choice of network fabrication method, as well as the experi-mental conditions, has a strong influence on the properties of the network. A trade-off often needs to be considered. For instance, for the spray method, the substrate should be heated high enough to allow the droplets to dry fast and prevent coalescence into larger droplets before drying. How-ever the heating temperature should be adapted to the type of substrate, remaining moderate for polymer substrates for example.[42,57] In addition, certain techniques, such as spin coating or vacuum filtration, can only be effective for fabri-cating films on small area substrates. The spray technique is compatible with up-scaling, as well as being cheap and pro-viding fast deposition of uniform MNW films on either rigid or flexible substrates (see Figure 1c and 1d). However, one possible limitation arises from the difficulty of spraying long MNWs as they tend to break in the ultrasound nozzle or become stuck and clog the spray.

2.2. Properties of Individual MNWs: Size Effects

Individual MNWs have very good electrical and thermal con-ductivity: silver is the most efficient electrical conductor, and gold, copper and even alloys such as Cu-Ni are also highly conductive. Even for small NW diameters, i.e., typically from 20 to 150 nm, the electrical conductivity is still very high. It decreases for diameters approaching the mean free path of

Jean-Pierre Simonato is Director of

Research at CEA, and Head of the

Laboratory of Synthesis and Integration of

Nanomaterials (LSIN). He is particularly

interested in the field of flexible transparent

conductive materials based on polymers or

nanomaterials.

Daniel Bellet is physicist and has worked in

different material fields such as single crys-

tal superalloys, porous silicon or by using

synchrotron micro-tomography. He became

Assistant-Professor at Grenoble University

in 1990 and is Professor at Grenoble INP

since 1998. He was junior member at IUF

from 1999 to 2004, and is now the director

of the Academic Research Community “En-

ergies” at the Région Rhône-Alpes since

2011. His research is now mainly focused on

transparent conductive materials.

Thomas Sannicolo is currently a PhD

candidate at Univ. Grenoble Alpes, France.

He is part of CEA-Liten and LMGP

laboratories, and his research involves the

study of the physical properties of metallic

nanowire percolating networks, as well as

their integration into functional devices. He

received his Master’s degree of Engineering

from the Grenoble Institute of Technol-

ogy in 2014, with specialization in physics,

optoelectronics, and nanosciences.

electrons in bulk metals (about 40 nm for Ag or Au). Bid et al., for instance, measured at both low and room tempera-ture the electrical resistivity of AgNW and CuNW.[59] These authors showed that when the NW diameter approaches the mean free path of electrons in the bulk material, the pro-portion of electrons undergoing surface scattering increases, leading to an increase in NW resistivity. For instance, at room temperature, the electrical resistivity of a 30 nm diameter AgNW is 25% higher than for a diameter of 100 nm.[59] A similar trend is observed for CuNW. However, this increase still appears reasonable for efficient integration, but could play a negative role when stability under high current density or rather high temperatures is required.[41]

From an optical point of view, a low diameter is benefi-cial for increasing the transmittance and decreasing the pro-portion of photons scattered by the NW.[60–62] Larger NW diameters are also associated with higher roughness, which often needs to be reduced for most optoelectronics devices,

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especially when very thin layers are concerned, such as in organic solar cells or OLEDs for instance.

Long MNWs help reach electrical percolation at a lower network density. Indeed, as described in Section 2.3, the critical density, associated with the percolation occurrence probability of 50% and defined as nc, is inversely propor-tional to the square of the NW length. Hence, the use of high aspect ratio nanowires appears of interest since it allows a decrease of the number of resistive junctions involved in the electrical percolating network. Meanwhile, it increases the optical transparency of the network. Several studies recently reported major advances in the synthesis of very high aspect

ratio (>1000) silver nanowires,[63–66] suit-able for fulfilling the industrial criteria of optoelectronic performances. However, it is rather challenging trying to maintain the integrity of very long NWs as they tend to break during the dispersion or deposition processes (especially when ultrasounds are used). Finally, a compromise needs to be found for the choice of the most suit-able NW dimensions in relation to target applications. The ability to control the dimensions of nanowires (both average values and size distribution[67]) during syn-thesis remains a crucial issue and a chal-lenge for future prospects for metallic nanowire networks.

2.3. Influence of Interconnections and Network Density on the Optical and Electrical Properties of MNW Networks

The interconnections between MNWs play a key role, strongly influencing the properties of the network.[68–70] Just after deposition, the electrical resistance of the MNW network is often high. This can be attributed to the non-efficient contacts

between NWs, and to the presence of organic residues such as the PVP shell at the contact points. This feature is sim-ilar to CNT based networks, but MNWs appear much more prone than CNTs to creating efficient interconnections: the network’s electrical resistance can be drastically reduced thanks to methods such as thermal annealing (see Figure 2a and 2b),[29,71] chemical treatments,[40] laser sintering,[72] light-induced plasmonic nanowelding (see Figure 2c and 2d),[73,74] or mechanical pressing (see Figure 2e and 2f).[45] Many papers report that such methods can lead to a dra-matic decrease in sheet resistance from 104 or 105 to about 10 Ω sq−1.[29,41,45] A decrease of this type can be attributed

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Figure 2. a,b) Scanning electron microscope (SEM) images of an as-deposited sample (a) and of a specimen annealed for 10 minutes at 300 °C (b). Scale bar is 1 µm. Reproduced with permission.[29] Copyright 2014, Royal Society of Chemistry. c,d) TEM images of silver nanowire junctions before (c) and after (d) optical welding. Scale bar is 50 nm. Reproduced with permission.[74] Copyright 2012, Nature Publishing Group. e,f) SEM images of AgNW electrodes before (e) and after (f) mechanical pressing at 25 MPa for 5 s. Reproduced with permission.[45] Copyright 2011, Springer Science + Business Media.

Figure 1. a) TEM observation of a single silver nanowire showing the amorphous layer surrounding the nanowires. It is mostly assumed that this shell is PVP. b) Plane-view SEM picture of an AgNW network. c) Typical flexible and transparent electrode fabricated with MNW networks. d) Typical flexible device (touch sensor) using MNW-based flexible TE. Reproduced with permission.[58] Copyright 2013, IOP Publishing.



to the activation of surface diffusion phenomena, leading to local sintering at the junctions between neighboring nanow-ires. Bellew et al.[69] and Nian et al.[70] have recently been able to measure the electrical resistance of nanowires and certain individual junctions after post-deposition treatments. Both investigations show that for optimized networks, the junctions do not seem to predominantly impact the network’s electrical resistivity any more, as they are associated with low electrical resistance.

Network density, defined as the number of nanowires per unit area, is also of primary importance. It may also be useful to consider the areal mass density (amd) simply expressing the metal mass per unit area in mg m−2.[28,41] This parameter has a strong influence on the optoelectronic properties of the network. For instance, optical transparency often appears to decrease linearly with network density,[60,75] while sheet resistance is proportional to (n − nc)

−γ where nc is the critical

density and γ is the percolation exponent.[76] This is well-demonstrated by the experimental measurements depicted in Figure 3a. Increasing the network’s amd leads to a linear decrease in optical transmittance and a power-law decrease in electrical resistance (linked to a non-linear increase in efficient electrical pathways available through the whole network). Moreover, such experimental measurements can be well fitted by relatively simple modeling approaches.[41] As shown first by De et al. and then by Lagrange et al., perco-lation theory dominates the electrical behavior of networks associated with low density.[28,41]

It is necessary to search for a trade-off in network den-sity. A network that is too sparse does not provide enough efficient pathways for electrons in the network to reach low sheet resistance values, while one that is too dense conducts efficiently but becomes too opaque. For many applications, network density should lead to high light

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Figure 3. a) Dependence of the experimental values of optical transmittance, electrical resistance, haze factor, and Haacke’s FoM of AgNW networks versus their areal mass density (amd). The continuous lines are associated with fits extracted from physical modeling by Lagrange et al.[41] b) Transmittance versus sheet resistance diagram for comparison between various TCMs: ITO,[3,28] graphene,[3,82–84] CNTs,[3,28,85] CuNWs,[25,48,73,86] Copper nanothrough,[87] AgNWs,[3,41,45,88–90] FTO.[91] The dashed lines correspond to different iso-values of Haacke FoM: 200, 100, 50, 10, 1 with units 10−3 Ω−1. c) Comparison of optical total transmittance in the UV–VIS–NIR region for a bare glass substrate (blue), AgNW network (black) and fluor-doped tin oxide (FTO, in red) which exhibit similar sheet resistance of about 11 Ω sq−1. MNW networks are still transparent in the NIR region. d) Calculated Haacke FoM (from experimental values) of AgNW networks versus their experimental haze factor. The dashed line is associated with a fit extracted from physical modeling by Lagrange et al.[41] Areas of interest are roughly represented for two applications which require either low or high haze factor values.



transmittance (≈90%), associated with low electrical resist-ance (≈10–100 Ω sq−1) through the whole electrode area. To efficiently assess the effects of network density, figures of merit (FoM) are often used. A classic FoM for consid-ering TE performances is Haacke’s FoM defined as Tr10/Rsh where Tr is optical transmittance (often considered at 550 nm) while Rsh is sheet resistance.[77] Another commonly used FoM is the ratio between DC conductivity, σDC, and optical conductivity αopt, the latter being proportional to the optical absorption coefficient.[28] These two FoMs are used to assess performance of a TE, specifically for comparing different TEs, with a higher value corresponding to a better trade-off between transmittance and sheet resistance. For the AgNWs presented in Figure 3a, the FoM is bell-shaped. The best amd appears to be close to 130 mg m−2.[41] This value depends on how the network is fabricated, and the dimensions of the MNWs. Other values have been reported in the literature, for instance 40 mg m−2 by Celle et al.[11] (in supplementary material section), 70 mg m−2 for De et al.,[28] and 331 mg m−2 for Göbelt et al., who used AgNWs of rather large diameters and small lengths.[78] Considering an amd range of values of 40–200 mg m−2 (compatible with the literature[41,70]) cor-responds to an equivalent homogeneous thin Ag layer that is 4–20 nm thick. To obtain similar optoelectronic perfor-mances, the usual thickness range associated with conven-tional ITO-based electrodes is 150–200 nm.[79] Given that In2O3 represents roughly 90 wt% in ITO, the corresponding indium amd range is roughly 750–1050 mg m−2, much higher than the required Ag amd in AgNW networks. Knowing that the price per unit mass of In and Ag are of the same order of magnitude, this shows that integrating MNW net-works as TEs in devices can be an interesting cost-efficient replacement for the conventional ITO or Ag grids used in solar cells.[78,80] Regarding the costs associated to raw mate-rials, AgNWs appear more competitive than ITO. Moreover, ITO is fabricated using an expensive high vacuum physical phase deposition process, whereas AgNW-based electrodes are compatible with low-cost solution-based deposition processes. It is actually difficult to compare fairly the final overall costs for various processes including all production costs (capital expenditure, operating expense, raw material prices, etc.). It is beyond the scope of this review, however some information dedicated to this specific point is available elsewhere.[81]

Figure 3b shows the characteristics (Tr vs Rsh) of several transparent conducting materials including ITO, FTO, CNTs, graphene, copper and silver nanowire networks. The discon-tinuous lines plotted in Figure 3b are associated with several iso-values of the Haacke FoM. Obviously, the target is in the top left corner of Figure 3b. By using adequate post-deposi-tion treatments, many research groups have reported metallic NW networks with optical transmittance of 90% and sheet resistance of less than 10 Ω sq−1,[41,70] showing that metallic nanowire networks can exhibit more competitiveness than other TCMs in terms of trade-off between transparency and electrical performances.

Finally, when compared to other conventional TCMs, MNW networks benefit from interconnections that enlarge the wavelength range for which the optical transparency is

very high. For instance, Figure 3c shows the optical transmit-tance of an AgNW network and one typical Transparent Con-ductive Oxide (TCO), i.e., Fluor-doped Tin Oxide (FTO). Both films have similar physical properties, a sheet resistance of 10 Ω sq−1 and an optical transparency of 90% at 550 nm once the glass substrate background has been removed. An interesting feature for the AgNW network is the high trans-mittance values in the near Infra-Red region (NIR) which is a relevant asset for certain applications, especially photovoltaic systems.

2.4. Light Scattering of MNW Networks

Another important parameter which needs to be considered for applications where TEs are involved is the haze factor (or haziness). This is an optical parameter that quantifies the amount of transmitted light scattered by the TE and is simply defined as the ratio between the diffuse part and total trans-mittance. Requirements regarding haziness depend greatly on the applications. For instance, touch panels and trans-parent heaters placed on windscreens or visors need a low haze factor (typically below 3%) to ensure comfort for the human eye, and to prevent blurred viewing, while the perfor-mance of solar cells is enhanced by a high haze factor asso-ciated with the TE used.[92,93] Scattering light enhances the optical path length of photons in solar cells, increasing their probability of being absorbed by the active area of the cell and then generating charge carriers.[94]

The haziness of MNW networks has thus been the sub-ject of several studies.[60,61] Experimental data show a linear increase in haze factor with network density, as is clearly shown in Figure 3a. In a similar way, the haze factor decreases in a linear manner as Topt increases.[41,60,75] Moreover, the haze factor depends significantly on the dimensions of the NW. For instance, Araki et al.[60] showed that using very long AgNWs (20–100 µm) helps decrease the haze factor, in agree-ment with Chang et al.[95] In the same vein, Preston et al.[61] observed experimentally that AgNWs with higher diameters lead to a higher haze factor in the visible range.

Unfortunately, the literature has not yet provided many detailed investigations capable of explaining these tendencies. Interestingly, Khanarian et al. reported that AgNW diameter appears to be the most important parameter morphologi-cally, determining both the transmission and haze of AgNW networks.[96] As discussed below, MNWs can be mixed with other materials to produce hybrids. A good example for var-ying haziness is the controlled incorporation of ZnO nano-pyramids into an MNW network film which makes possible highly tunable control of the scattering properties of this type of TE.[97]

Figure 3d shows the behavior of the AgNW networks’ FoM against the haze factor. Each experimental point is related to one specific value of areal mass density. The FoM is bell-shaped. Figure 3d can be seen as a guideline for selecting the most appropriate density value compatible with the target application, as well as the associated constraints regarding the haze factor.[41] Defining a new FoM taking into account the haziness of the MNW-based TCM studied would

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be very useful for selecting the appropriate physical param-eters for each specific application.

2.5. Flexibility of MNW Networks

Mechanical flexibility is one of the featured characteristics of metallic NW networks, paving the way for flexible elec-tronics. Unlike ITO and many other TCMs, their electrical conductivity is not affected much when subjected to bending tests (see Figure 4a) and they also show good responses in the case of stretching. For instance, Miller et al. reported that AgNW networks can withstand up to 76% tensile strain and 250 bending cycles of 15% strain with negligible increase in electrical resistance.[98] When AgNW networks are embedded in a transparent polyurethane optical adhesive and inserted into a flexible light-emitting electrochemical cell, the latter devices continue to emit light under bending, even with a radius of curvature of 1.5 mm (see Figure 4b).[98] Many other examples are provided in the applications sections, showing that MNW networks can be efficiently assimilated into a wide range of flexible devices. Finally, another very signifi-cant advantage of MNW networks is their ability to conform to non-planar surfaces. This may be useful in various appli-cations involving highly roughened surfaces. For instance, Figure 4c and 4d show that AgNWs are very ductile and are able to fit closely to the shape of a textured silicon surface, the latter being often used in the Si-based solar cell industry.

Figure 4e is another example, showing the conformability of AgNWs for smart con-ductive textile applications.[99]

2.6. Stability issues and hybrid nanocomposites based on MNWs

The stability of MNW networks has been mentioned as a possible limitation for their integration into devices. Stability in relation to different stresses is worth considering: thermal, chemical, electrical. When submitted to high temperatures and/or current conditions, MNW networks can have early failure rates[100] caused for instance by surface diffusion[29,101] and/or electromigration processes and/or modi-fications of surface chemistry. Figure 5a presents the morphology of an AgNW network after exposure to high tempera-tures. Plateau-Rayleigh instability occurs, leading to spheroidization, which is asso-ciated with a drastic increase in electrical resistance of several orders of magni-tude. Figure 5b shows an AgNW network after exposure to electric stress.[102] The combination of high relative humidity and high temperature appears critical as well,[103,104] while humidity-assisted annealing at low temperatures appears

to act favorably on lowering electrical resistance in AgNW networks.[105] Reports in the literature on MNW stability issues appear controversial and this can be explained by the different experimental conditions used, either for NW syn-thesis, solvent purity, network fabrication, post-deposition treatment or storage.[104,106] All these parameters can have a strong impact on network stability. For instance, May-ousse et al. showed that the sheet resistance of AgNW-based electrodes was insignificantly degraded after two years of storage in the dark and in air.[103] Given that investigations of MNW networks are relatively new, more background regarding environmental stability (relative humidity, natural light, storage) is still required. An important point that needs to be taken into account for MNW integration is that their stability will also have to be studied in the final product, after encapsulation. As discussed in more detail below, the use of nanocomposites can drastically improve the stability of NW networks.

The thermal stability of MNWs can be improved by increasing their diameters: the thinner they are, the lower the temperature associated with the nanowire’s thermal instability.[41] However, it seems that using hybrid mate-rials combining MNWs and thin layers of other materials is a much more promising way of drastically decreasing the effects of such instability. For instance, the use of reduced graphene oxide with either AgNWs[107] or CuNWs[108] has been shown to increase the robustness and stability of the resulting TE.

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Figure 4. a) Comparison of the relative electrical resistance of different types of TE under mechanical bending. The radius of curvature used was 5 mm. Reproduced with permission.[6] Copyright 2013, IOP Publishing. b) Flexible light-emitting electrochemical cell using a silver nanowire network embedded in a polyurethane optical adhesive as a transparent electrode. Reproduced with permission.[98] Copyright 2013, American Chemical Society. c,d) SEM images of AgNWs coated on textured Si surfaces. The nanowires can perfectly fit the shape of this uneven surface. e) SEM image of coated AgNWs on a polyester/rubber thread. Reproduced with permission.[99] Copyright 2015, Royal Society of Chemistry.



Embedding AgNW with either ZnO nanoparticles[109] or a TiO2 layer[110] was found to significantly improve the thermal stability. More specifically, using atomic layer depo-sition (ALD) makes possible the deposition of very con-formal and homogeneous thin protecting layers (just a few nm) without significantly impacting the transparency of the resulting TE, while improving the contact between the MNW network and the underlying substrate at the same time (see Figure 5c).[78]

From an applicative point of view, the use of hybrid materials combining MNW with either graphene, conduc-tive polymers, carbon nanotubes, TCOs or other materials can improve the performances of the final devices they are used in. Several examples given in the applications section are dedicated to hybrid materials which have several assets, as a result of taking advantage of improved stability as well as interface properties (adhesion, roughness, balance of the work functions, etc.). Many studies report enhanced perfor-mances for devices when hybrids are used instead of bare MNW networks.[78,109,112] For instance, highly stretchable and transparent conductors were obtained by combining the mechanical compliance of small CNTs and the high con-ductivity of AgNWs.[113] Another recent example is the use of atomic layer deposition of aluminum-doped zinc oxide on AgNWs leading to a very efficient nanocomposite-based transparent electrode used on top of Si-solar cells.[78] Other studies also considered hybrids between MNWs and other materials, such as graphene or conducting polymers, which pave the way for many possibilities in terms of architecting nanocomposites: hybrids should provide future innovations in terms of enhanced properties and integration into devices.

However, even though stability issues are most of the time seen as drawbacks for the applications, one can still take advantage of it. For instance, Bai et al. succeeded in fabricating nanostructured polyaniline (PANI) transparent chemical gas sensors by using AgNW networks as sacrificial templates (see Figure 5d).[111]

2.7. Integrating MNW Networks into Devices: Closing Remarks

The efficient integration of MNW networks does not rely only on their intrinsic optical transparency and electrical resist-ance. Many other features are crucial and often application-dependent. First, one problem of MNW networks is their poor adherence to the substrates. Studies have been conducted to overcome this drawback, for instance Kim et al. proposed dis-persing the nanowires with clay platelets prior to deposition which made it possible to create uniform networks with satisfac-tory adherence to the substrate.[114] Other solutions exist, such as surface oxidative pre-treatments or encapsulation of the net-works with graphene[115] or metallic oxide.[116] The roughness of the MNW network needs to be low to prevent possible short-circuits in very thin solar cells such as organic solar cells. This can be achieved for instance by pressing the networks mechani-cally[45] or coating them with a conducting polymer.[117,118] It is also well-known that some devices using MNW networks need proper alignment of energy bands between the adjacent layers in order to maximize in particular the efficiency of solar cells.[119] Homogeneity in the coverage of MNW networks is also primarily in order to improve the performances of the final device (an inhomogeneous layer in a transparent heater leads to hot spots for instance[114]). The thermal stability of the nanowires would also be critical if thermal annealing is required during manufacturing of the device (see Section 2.6), and sta-bility towards environment in any device needs to be evaluated. All these aspects should be thoroughly taken into considera-tion for efficient integration and are the object of many ongoing research activities around the world.

3. Applications

The properties and challenges linked to MNW networks were presented in the last section and we will now address

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Figure 5. Illustrations of MNW stability. a) SEM observation of AgNWs after thermal annealing up to 350 °C. Reproduced with permission.[29] Copyright 2014, Royal Society of Chemistry. b) Deterioration of AgNWs on a PEN substrate when submitted to electrical stress (constant current flow for two weeks). Induced modification of surface chemistry might also play a critical role in degradation. c) Scanning electron microscopy cross sectional image of an AgNW after encapsulation by a 100 nm thick layer of Aluminum-doped zinc oxide (AZO). Reproduced with permission.[78] Copyright 2015, Elsevier. d) Schematic illustration of a sacrificial AgNW network, etched by strong oxidants, and used as template for fabricating transparent conducting film of nanostructured polyaniline (PANI) network for chemical gas sensing application. Reproduced with permission.[111] Copyright 2015, Wiley-VCH.



how to integrate them into real devices. Every application has its own peculiarities, is subject to various problems and requires different properties of the MNW networks. We chose to address the applications of MNW networks in the six following categories: first the field of photovol-taic systems, second that of lighting, focusing on OLED devices, and third the field of transparent film heaters. The first three fields of application reviewed here are the main studies reported that deal with the integration of MNW networks into devices. We continue by reviewing the field of smart windows and displays, especially electrochromic and Polymer Dispersed Liquid Crystal (PDLC) devices, as well as touch screens, and then the field of electromag-netic devices, in particular RF antennas and electromag-netic shielding. The last section summarizes other original or recent concepts for applications integrating MNW-based electrodes. The whole section aims to ensure that this review is as exhaustive as possible on the different devices that have proved efficient when using MNW networks. How-ever, we are aware that certain applications may have been reported too recently in the literature, or may not have been brought to our attention. Beyond a simple list of the different applications that have been studied in the litera-ture, this review aims to emphasize the advantages of using MNWs in such devices and highlight future prospects for integrating MNW-based transparent electrodes into next-generation flexible devices.

3.1. Photovoltaic Applications

Photovoltaic systems, among other sustainable green energy technologies, have been developed to offer an alterna-tive to fossil fuel energy. Efforts are made, for instance, for reducing costs, improving efficiency and using abundant materials.[120] Light obviously needs to enter a solar cell while the latter also requires transparent contacts to collect the photo-generated charge carriers. Transparent electrodes are a key component in solar cells, directly influencing photo-conversion efficiency. Generally speaking, the mate-rials chosen must have several properties:[121] high electrical conductivity, high transparency, as well as band alignments, and the work function should be considered when the TEs are used as efficient Ohmic contacts.[119] For many decades, transparent conductive oxides (TCO) such as ITO, AZO or FTO have been used for solar cell applications.[5] For these materials, light absorption is associated with free carrier inter-band processes, resulting in a trade-off between con-ductivity and light transmission which both strongly depend on doping level: significant doping results in better electrical conductivity but in lower light transmittance. This type of trade-off should not be considered when dealing with MNW networks as their transparency versus wavelength depend-ency appears as a rather monotonous function (at least above 400 nm),[122] as shown in Figure 3c. However there is still, as for all applications implying transparency and conductivity, a trade-off between electrical resistance and optical transmission: for TCOs this is controlled by the film’s thickness, for metallic NW networks, the key parameter is

network density (or areal mass density), as developed in Section 2.3.

The drawbacks of ITO mentioned in Section 1 have led to research into emerging transparent electrodes which may have major advantages. Metallic nanowire networks have been shown to be serious candidates for this purpose,[123] as described below, and also appear to be a TE associated with a lower energy pack back time when compared with ITO for organic solar cells.[124]

One of the main topics that has attracted interest recently is the development of flexible solar cells with the use of simple and low cost methods of cell printing.[125,126] Unlike ITO, AgNW networks are flexible and retain their electrical properties intact when bending, as seen earlier. Andrés et al. have recently synthesized high aspect ratio AgNWs, seen to be a low-cost alternative to ITO in organic solar cells (OSC) whereas they induce only a small decrease in the power con-version efficiency.[125] Guo et al. obtained conversion effi-ciency up to 5.81% by using AgNWs as the top electrode in tandem OSCs.[126] Using AgNWs now appears compatible with the development of solar cells that can be fully printed. For instance, Angmo et al. fabricated single and tandem cells almost entirely in ambient conditions compatible with the roll-to-roll method, which is an easily scaled up deposi-tion method for flexible substrates.[127] Other reported works have also proved that AgNW networks can be used effec-tively as the transparent top electrode in OSCs[51,89,128] with excellent bending capacities within this application.[129] These results show that AgNW networks can compete with ITO in terms of photovoltaic performances, but have much greater flexibility and solution-processed fabrication.

In addition to high transparency, low electrical resistance and excellent flexibility, another asset of AgNW networks is their haziness. Scattering light increases the efficiency of the solar cells by increasing the length of the photon path-ways within the cell, making possible a higher probability of photon absorption. This feature was already studied in 2008 by Lee et al. for example, and AgNW networks were inte-grated for this purpose into organic solar cells with good per-formances compared to when TCOs were used.[130]

Another primary aspect concerning the integration of MNW networks as TE are the plasmonic effects which can be exploited in order to improve the performance parameters of ultrathin photovoltaic active layers, as recently reviewed by Petoukhoff et al.[131] Plasmonics is the study of the interac-tion between electromagnetic fields (such as sunlight) and free electrons in a metallic material (such as in MNW net-works for example). This leads to light management through light trapping or localization in the thin-film active layer. In other words, these plasmonic effects can favor enhanced collective scattering, which can result in an increased short-circuit current density in thin solar cells. Generally speaking, thanks to partial oxidation and/or the presence of interfa-cial layers, both the structure and surface work function of the plasmonic electrodes can be controlled, making it pos-sible to adjust the optical and electronic properties of the plasmonic electrode.[131] Solution-processed ultrafine gold nanowires (AuNW), for example, were used as plasmonic antennas in organic P3HT:PCBM photovoltaic cells by

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Oo et al.,[12] resulting in a clear increase in the short-circuit current density.

As depicted in Figure 6a–c, AgNW-based electrodes were also used to fabricate polymer solar cells that are semi-transparent in the visible region[132–134] (for example, 66% transparency at 550 nm[135]). While semi-transparent in the visible range, the cell is nevertheless highly absorbant in the IR range. As seen previously, AgNW networks, unlike TCOs, are highly transparent in this region (see Figure 3c), allowing the cell to be very efficient by absorbing in the IR and being semi-transparent for the human eye. While this approach is of interest, integration of such electrodes into windows can be delicate because of the haziness which needs to remain low.

Another interesting example of the integration of AgNW-based TEs was recently reported by Göbelt et al.[78] who used AgNW-AZO composite transparent electrodes in wafer-based Si solar cells. When compared with reference solar cells using thermally evaporated silver grid electrodes, a clear increase in the short circuit current density was reported (28 instead of 16.3 mA cm−2), as well as a drastic reduction in the amount of silver used: at least 95% of the amount of silver can indeed be saved when using an AgNW-AZO com-posite instead of the usual Ag grids.

A recent investigation also reported the use of AgNW networks as front electrodes for a fully roll-to-roll processed flexible ITO-free organic photovoltaic cell.[136] The stability of the devices was tested and it was shown that replacing the PEDOT:PSS as the front electrode with an AgNW net-work increased operational stability by up to 1000%.[136] Finally, the use of sort of MNWs other than Ag has also been reported by Wiley’s group. They successfully integrated entirely solution-processed CuNi NW based-films as anode in OPV devices exhibiting efficiencies of 4.9%.[40] Generally, the integration of other MNWs than Ag ones as transparent elec-trodes with good properties and high stability still remains a challenge in front of photovoltaic applications,

3.2. Lighting (Organic LEDs)

The vast majority of lighting devices using MNW network transparent electrodes concerns organic light-emitting

diodes. For this reason, this application will be investigated in depth in this section. However, Miller et al. recently used AgNW/optical adhesive coatings as transparent anodes in a light-emitting electrochemical cell (LEEC).[98] The lighting performances of the end device were found to be similar to those of the comparable ITO-based structure, as well as high levels of flexibility and stretchability (see Figure 4b). This is the only example of LEECs with AgNWs reported so far to our knowledge.

In organic or polymer-based light-emitting diodes (LEDs), the global architecture has similarities with that of solar cells. The active layer is sandwiched between two electrodes, at least one of which is transparent. Contrary to solar cells, charge carriers are not extracted but driven to the active layer, which is responsible for the recombination of electron-hole pairs to produce photon emission. The use of organic component (OLEDs) or polymers (PLEDs) as the active layer paves the way for the production of flexible light-emitting devices compatible with an entirely solution-based and low cost process. In order to offer appropriate solutions for the target applications, e.g., portable electronic rollable displays or conformable lighting panels, not only the active layer but also the interfaces and especially the trans-parent conducting layer have to be highly flexible. Large angle bending cycles with no alteration in conductivity, trans-parency or light emission properties are essential. All these needs can be fulfilled by using MNW-based electrodes.

In the very first studies examining silver nanowire-based OLEDs, good electroluminescent properties were reported.[71,137–141] Zeng et al. replaced the commonly used Glass/ITO layer with an AgNW/PVA (polyvinyl alcohol) mixture in the architecture of the OLED device.[137] In order to prevent electrical leakage and to adjust the anode work function to enhance hole injection, a buffer layer of PEDOT:PSS a few nanometers thick was deposited between the AgNW network and the active layer. When combined with other post-deposition treatments such as thermal annealing or mechanical pressing, it helps to decrease the surface roughness of the AgNW-based electrodes. In the first study, the AgNW/PVA-based OLED devices were found to be slightly less luminous than the reference ITO-based OLED device but showed higher power efficiency when

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Figure 6. a) J–V curves of a 64 cm² 16-cell module with illumination from both the bottom ITO and top AgNW electrodes. Reproduced with permission.[134] Copyright 2015, Wiley-VCH. b) Photograph of a semi-transparent module with an active area of 64 cm2. Reproduced with permission.[134] Copyright 2015, Wiley-VCH. c) Semi-transparent module with an AgNW top electrode and an active area of 11 cm².



operated at low current density (<140 mA cm−2). Yu et al. observed a similar trend when using AgNW/Polyacrylate transparent electrodes.[138] The same team managed to pro-duce blue, green, red[139] and even white[140] phosphorescent PLEDs with electroluminescent performances (current den-sity, luminance, driving voltage, current efficiency) all higher than those associated with the ITO-based reference device. Gaynor et al. were able to produce very efficient ITO-free white OLEDs using an AgNW/PMMA (poly(methyl meth-acrylate)) composite electrode.[141] The whole device showed one of the highest levels of luminous efficacy reported so far for an ITO-free white OLED, i.e., over 30 lm W−1 at a bright-ness of 1000 cd m−2, and exhibited two other major optical characteristics: color-independent emission while increasing the viewing angle (see Figure 7a) and an almost perfect Lambertian emission. Patterning of AgNW meshes also proved efficient for the fabrication of thermal-evaporated OLEDs.[142] As in previous studies, increased efficiency of the device was observed, and can be attributed to an enhanced scattering effect of the AgNWs.

Another key challenge is the ability of the AgNW-based OLED devices to keep their electroluminescent perfor-mances unaffected when exposed to deforming strains, which has been dealt with remarkably by Pei’s group.[143] Both stretchability (see Figure 7b,d,e) and bendability (see Figure 7c) of elastomeric polymer light-emitting diodes (EPLEDs) with a rubbery AgNW/PUA (poly(urethane acrylate)) composite as the transparent electrode were studied in depth. The resulting stretched device was found to withstand strain up to 120% while displaying fairly uniform

light emissions across the entire luminous area. It also showed significantly improved efficiency in the stretched state. A module of 5×5 EPLED-based pixels was manufac-tured for the first time, showing the ability of this kind of architecture to face the technical challenges required for stretchable OLED displays.

As mentioned above, lowering the work function, avoiding potential current leakage with the active layer, and enhancing the flexibility are some of the key challenges that need to be faced when integrating an MNW-based trans-parent electrode into an OLED device. In order to eliminate the energy level mismatches that occur with the adjacent layer, an n-type hole injection layer (HIL) can be inserted between the AgNW-based transparent electrode and the hole transport layer (HTL). In the case of MNW-based elec-trodes, this process makes it possible to smooth the surface of the electrode at the same time, which is necessary to avoid shorts.

Using this process, Lee et al. manufactured a phospho-rescent OLED device with promising performances such as a very low turn-on voltage (3.6 V) and very high cur-rent and power efficiencies (44.5 cd A−1 and 35.8 lm W−1 respectively).[90]

Similarly, AgNWs were embedded into a colorless poly-imide (cPI). The resulting composite film showed very advan-tageous mechanical properties and its thickness could be reduced (<10 µm) without affecting the high flexibility.[144] The efficiency of the final phosphorescent OLED (ph-OLED) was not significantly impacted when repeated folding stresses with a down-to-30-µm bending radius were applied.

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Figure 7. a) Photographic image of four functioning white tandem OLEDs at 1000 cd m−2. Reproduced with permission.[141] Copyright 2013, Wiley-VCH. b–d) Current density and luminance characteristics of an elastomeric polymer light-emitting diode (EPLED) device working at 12 V with increasing strain (b). Image of an EPLED (original emission area of 3.0 × 7.0 mm2, biased at 12 V) wrapped around the edge of a piece of 400 µm thick cardboard (c). Photographs of an EPLED (original emission area, 5.0 × 4.5 mm2) biased at 14 V at 0% (d) and 120% strains (e). Reproduced with permission.[143] Copyright 2013, Nature Publishing Group. f) Demonstration of a flexible OLED on a CuNW-GFRHybrimer film: plotting luminance vs voltage (L-V). A reference OLED device on ITO/glass was tested for comparison. Reproduced with permission.[147] Copyright 2014, American Chemical Society.



Song et al. also showed that projecting intense pulsed light irradiation on to silver nanowires deposited on cPI can help reduce the surface roughness of such TEs, leading to a signifi-cant decrease in the leakage current when integrated into the ph-OLED device.[145] Finally, performing reverse bias condi-tioning to Ag nanowire based-OLED devices was reported by Lee et al. as another interesting option for decreasing and stabilizing the leakage current.[146]

Very few examples of CuNW integration into OLED applications have been reported so far. For instance, Im et al. embedded CuNWs into a Glass Fabric Reinforced plastic film (GFRHybrimer film) and the resulting plat-form was successfully integrated for the very first time into a flexible OLED device with performances almost identical to those of the ITO based-reference device (see Figure 7f).[147] Proper encapsulation is a crucial point when trying to integrate CuNWs in general, as copper is readily prone to oxidation, leading to degradation of the electrode’s performances. This was achieved by manufacturing stable core-shell nanowires, e.g., Cu@Cu4Ni nanowires, used in a transparent elastomeric form for segments of the external circuits of OLEDS.[148]

Finally, AgNW percolating networks have also been used as TEs in inorganic light-emitting diodes (AlGaNP- and GaN-based LEDs [149–151]) showing that even without the need for flexibility, AgNWs are still regarded as a promising alternative to ITO. The recent fabrication by Cheng et al. of composite fibers made of DCY (Double-Covered yarn) cov-ered with AgNWs and encapsulated in PDMS is particularly interesting.[152] This conductive hybrid material was found to be ultra-stretchable and used as a wiring system to success-fully integrate several LEDs in series. This type of realization paves the way for the fabrication of entirely transparent and stretchable optoelectronic complex circuits with MNW net-works as the very first building block.

3.3. Transparent Film Heaters

Transparent film heaters (TFHs) are transparent conducting films that can heat by voltage application.[153] They make possible a variety of applications, for instance in defrosting or defogging vehicle windows,[154] outdoor panel displays[156] or more generally devices exposed to temperature variations. Historically, TFHs were one of the first applications envis-aged for transparent conducting materials (TCMs). They were used to de-ice aircraft windscreens during World War II, enabling the aircraft to fly at much higher altitudes.[157] After extensive studies based on TCOs, TFHs made from emerging nanomaterials have started to arise in the past few years. Carbon-based nanostructures such as graphene[158] and carbon nanotube[159] (CNT) networks, metallic nanowires (MNWs) as well as hybrid materials[160,161] have recently been reported. The reasons for this development are the same as for other transparent electrode (TE) applications: to reduce fabrication costs, extend to large surfaces, increase flexibility and, in this special case, give the possibility for heating at low voltages.

MNW networks are among the most promising mate-rials for TFH applications. In the literature, AgNW networks have been the main sort of MNW networks proposed for TH applications in the last years.[11,114,162] They are able to increase their temperature by several tens of degrees using an operating voltage of less than 12 V, which is convenient for most devices.[11,13,163] as well as producing large-area uni-form heating[153] and a fast thermal response.[11,14] They do not show any significant resistance change when subjected to bending.[164] Furthermore, they can be used for surfaces with complex geometries and shapes (helmet visors, wind-screens…), as the processes can lead to conformable deposi-tions. In addition to the previous criteria (heating properties, transparency and flexibility), the development of future elec-tronic devices will possibly require high stretchability for wearable electronics.[163]

Very few studies have so far focused on MNWs other than silver. Chen et al. fabricated TFHs based on Cu–Ni core–shell nanowires, Ni being used to enhance resistance to oxidation.[165] Li et al. investigated metallic microwires and reported a transparent heater made of a Cu wire/Al2O3/poly-imide composite film with high stability against electromigra-tion, oxidation, bending and stretching.[166]

Studies have focused especially on producing high tem-perature elevations with low voltages.[114,162] Kim et al. for instance obtained interesting results with samples of Rs = 10 Ω sq−1, Tr ≈ 90% and could increase the film’s tem-perature to 70 °C with a voltage as low as 5 V.[114] They optimized the sample properties by using clay platelets to improve the uniformity of the NW distribution on the sub-strate and by using thin (20–40 nm) and long (20–40 µm) nanowires. The samples were able to bend with a radius of curvature of 10 mm without inducing any degradation to the conductivity. The clay platelets also led to better adherence of the NW to the substrate: the samples were able to pass the “tape test”, without being removed from the substrate, which is generally not the case for deposited AgNWs.

The first report of AgNW networks used as transparent heaters from Celle et al.[11] studied the temperature eleva-tion of the films while applying different voltages, as reported in Figure 8a. A thermochromic display was also designed by coupling AgNWs used as transparent heaters with a thermo-chromic ink (see Figure 8b,c). The power dissipated in the material (V²/R) is directly related to the steady-state tem-perature by means of a balance between the Joule effect and heat loss: conduction to the substrate, convection to the sur-rounding air and radiation from the hot surface.

Sorel et al.[13] described the theory of Joule heating in a TFH composed of AgNWs. Considering that the temperature is uniform over the whole sample, the energy balance can be written as follows (conduction losses to external parts of the system being neglected):

21 2 0

1 24

04

I R mCdT t

dt A h h T t T

A T t Tσ ε ε ( )( )( ) ( ) ( )

( ) ( )

= + + −

+ + − (1)

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The term on the left is the power input to the TFH. The first term on the right is the heat that is responsible for the temperature elevation of the sample of mass m and specific heat C, the second term represents the heat losses by convec-tion (A and h are respectively the specimen area and the con-vective heat-transfer coefficients) and the third one the heat losses by radiation (1 and 2 design the MNW network and the substrate, respectively). At low temperature ranges, as in the experiment presented in Figure 8a, the radiation term can be linearized and then this energy balance has the following analytical solution, demonstrated in Sorel’s paper:[13]

T t 1 1 exp /02

T I RA mC Atα

α( )( ) ≈ + − −

(2)

with α the heat transfer constant, which is related to the time constant σ equal to mC/αA. This solution shows that there is indeed a transitional state at small t, and then a steady-state temperature is achieved, which depends on the voltage and resistance of the sample. This approach is vali-dated by the experimental results displayed in Figure 8a. The steady-state temperature is thus directly proportional to the applied power density,

2I RA , as observed experimentally.[14]

The key parameters in TH are therefore the steady-state temperature Tstab, which needs to be high while applying an operating voltage as low as possible, to limit power con-sumption, and response time, which needs to be as short as possible. Response time is considered as the time needed to achieve 90% of Tstab. The phenomena limiting these key parameters are heat loss (convection, conduction and radia-tion), which also tend to increase with temperature.[13,158,164]

One of the major problems of MNWs is their low adher-ence to substrates and their thermal and electrical sta-bility during TFH operations.[29,100] To address this issue, Li et al.[164] for instance created a polymer composite resisting temperatures as high as 230 °C, while allowing adhesion that could pass the tape test. The samples were also mechanically stable as bending cycles did not have an impact on the heating temperature. Ji et al.[14] also coated

AgNW networks with PEDOT:PSS for the same reasons of thermal stability and improved adhesion, and Zhang et al. with graphene microsheets.[161] The latter shows that tem-peratures as high as 230 °C can be reached in a reproduc-ible manner.

Another drawback that has been identified for MNW networks in TFH applications is their high degree of hazi-ness, especially when dense networks are considered, since haziness increases with network density.[41,60] This can be a problem for applications for window defrosting, as the user needs a clear view of the outside. One of the solutions for reducing haziness is hybridization of the heaters with CNTs that have a lower diameter or by using long NWs.[160]

Spatial uniformity is also an essential requirement for preventing hot spots in TFHs, especially in applications such as car and aircraft windscreens. Solutions have been pro-posed for this problem: the work of Kim et al., as shown in Figure 9a,b, has made possible good spatial uniformity over the entire metallic network area thanks to the use of clay platelets.[114]

To finish with, the fabrication of stretchable TFHs is a clear challenge for the integration of AgNW network-based electrodes in future wearable electronic devices. Hong et al.[163] have recently demonstrated that AgNW network/PDMS electrodes can have good heating properties as well as stretchability: excellent reliability was observed at a tempera-ture of 60 °C and strain of 60%.

In conclusion, while several interesting studies have been reported since the first fabrication of TFH composed of AgNW networks in 2012,[11] there is still some need to enhance electrical/thermal stability and to ensure the homo-geneity of the heating surface. TFHs that are simultaneously transparent, non-hazy, flexible, stretchable, stable, and with a short reaction time, are the object of ongoing research. An important target is the possibility of covering non planar sub-strates, even with a complex relief, as this would be a clear breakthrough. Associated with flexibility, this will offer pos-sibilities for combining new design properties. This is particu-larly interesting in textile and medical applications.

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Figure 8. a) Temperature achieved by an AgNW-based transparent heater (TH) with Rs = 33 Ω sq−1 at various voltages (on PEN substrate). Insert: heating and cooling rate at 5 V. Reproduced with permission.[11] Copyright 2012, Springer Science + Business Media. b,c) Simple thermochromic device composed of an AgNW network: (b) at room temperature, (c) when a voltage of 6 volts is applied between opposite electrodes.



3.4. Smart Windows and Displays

The aim of this part is to review the use of MNW net-works as transparent electrodes in display applications and for controlled-transmittance windows, often called “smart windows”. As already mentioned in Section 3.3, coupling MNW networks as transparent heaters with thermochromic ink[167] is an interesting way of designing flexible and low cost thermochromic displays and smart windows, capable of competing with materials studied earlier, such as doped vana-dium dioxide (VO2).[155] In this section we mention recent results first on electrochromic and Polymer-Dispersed Liquid Crystal (PDLC) devices, and then on touch screens.

3.4.1. Electrochromic and Polymer-Dispersed Liquid Crystal (PDLC) Devices

“Electrochromic” stands for materials with the ability to modify their optical properties in a long-term and reversible way by changing their oxidation states when subjected to electrical fields. Either metal oxides (WO3, NiO) or polymers can be convenient.[168–170] In PDLC devices, electrical fields allow the liquid crystals droplets – dispersed in a solidified polymer matrix – to align so that the device is in its trans-parent configuration. Otherwise, the random dispersion of the droplets results in a scattering of the incident light as a result of the mismatch of the refractive indices at the bound-aries of the droplets:[171] the device then turns opaque.

Both electrochromic and PDLC devices can be used in a wide variety of optical switchable technologies,[172] such as information displays, electronic paper-like displays, anti-reflectance mirrors and smart windows. The latter help improve energy efficiency, safety and personal comfort in buildings, vehicles or aircraft by regulating incident energy and harsh lighting. This is technology getting closer and closer to households and which has already been developed by several companies.[173,174]

Both devices require transparent electrodes, either to change the oxidation level of the electrochromic material or to apply voltage and align the liquid crystals in the case of PDLCs. Here again, ITO is the most commonly used trans-parent electrode. But the need for highly flexible displays and smart windows capable of conforming to complex non-planar surfaces has encouraged research into cheap and flexible alternatives to ITO. Moreover, it seems that ITO degrades when subjected to voltammetry cycles in electrochromic cells which is, of course, a serious issue regarding the aging of devices.[175]

Here again, MNWs, and especially AgNW percolating networks, seem to be promising candidates. For instance, Yan et al. successfully integrated stretchable conductors made of AgNW/PDMS elastomer matrix into a WO3 electrochromic display device.[176] The final device could sustain consider-able stretching stress (with associated strain of 50%) as well as twisting and folding stress without being damaged. Figure 10a shows the final device being stretched up to 50%

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Figure 10. a) Electrochromic display devices – in both colored and bleached state – using an AgNW/PDMS matrix as the elastic electrodes. The device has high resistance to stretching stress. Reproduced with permission.[176] Copyright 2014, American Chemical Society. b,c) Example of a PDLC smart window using silver nanowire-based electrodes in the “on state (85 V)” (b) and “off state (0 V)” (c). Reproduced with permission.[178] Copyright 2015, Elsevier.

Figure 9. a,b) Infrared images of AgNW-based film heaters (50 × 75 mm2) exhibiting hot spots due to the self-aggregated nanowires within the network (a), while uniform heat distribution can be obtained thanks to the use of exfoliated clays, leading to improved spatial network uniformity (b). Reproduced with permission.[114] Copyright 2013, Wiley-VCH.



strain in both colored and bleached states. Another example of successful integration of AgNWs in a transparent flex-ible electrochromic device was recently provided by Huang et al.[177] The use of AgNW-based electrodes in PDLCs was also recently investigated. Hosseinzadeh Khaligh et al. sand-wiched PDLC layers between either AgNWs embedded in poly(ethylene terephthalate) PET or ITO electrodes with similar sheet resistance (50 Ω sq−1).[178] Pictures of the resulting smart windows using AgNW networks are provided in Figure 10b (“on state”) and 10c (“off state”). Despite the fact that the conduction level of both the ITO and AgNW-based electrodes was similar, the final AgNW-based smart window could be modulated over a larger transparency range and at lower voltage supply when compared to the similar ITO-based architecture.[178] Another example of a thermo-sensitive PDLC smart window using AgNW networks was recently reported by Wee et al.[179] In this case, the AgNW network was used as a transparent film heater to transfer heat to the thermo-sensitive active layer.

3.4.2. Touch Screens

Let us now focus on integrating MNWs into transparent touch screens, while their integration into touch sensors used as strain sensors (mostly non-transparent devices) will be detailed in Section 3.6.

Smart phones, tablets and notebooks have become ubiquitous in our day lives at home or at work. Worldwide market forecasts estimate that the surface area of manu-factured touch screens will double between 2014 and 2025, representing more than 80 km2 in 2025.[180] Moreover, the touch screen market is increasing ten times faster than other

displays. This field of research is strongly correlated to market trends for flexible displays. A lot of advertisements on this subject from Apple,[181] Samsung,[182] LG, Toshiba, Lenovo, etc.[183] are strong indicators for this trend. To address the flexibility issue, technological interest in finding a replace-ment for ITO is focused on research into flexible transparent conductive materials. Touch Displays Research Inc. forecasts that the non-ITO transparent conductor market will reach up to $13 billion by 2023.[183]

Korean research institutes (KAIST and KETI) are heavily involved in the integration of MNWs into flex-ible touch screens (see Figure 11a and 11b). Of the MNWs, copper ones cannot easily be used in this field because of their reddish tint.[184] Most of the papers reported focus on capacitive sensing related to the change in capacitance when fingers interact with the electrode. The only resistive sensing that fulfills the criteria of transparency is reported by the work of S.H. Ko’s team.[185,186] Beyond resistive sensing which makes possible low costs and high resolution, capaci-tive detection can also ensure both multi-touch features and durability.[182] Depending on the technology involved, capacitive touch sensors can be made up of one or two trans-parent and conductive layers for respectively single[184,187] or double-sided sensors.[188,189] The former requires resolution at the millimeter scale whereas the latter is down to hun-dreds of micrometers. For AgNW networks, caution should be taken to prevent the divergence of the electrical proper-ties in narrower lines (<250 µm in width) because orienta-tion and alignment will also in this case govern the properties of percolative networks (not only random NW distribu-tion).[188,190] AgNW patterning (line/column, see Figure 11c) is of prime importance for high performance touch screens.

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Figure 11. a,b) Highly flexible AgNW-based multi-touch panel developed at the KETI Institute. Reproduced with permission.[187] Copyright 2015, Royal Society of Chemistry. c) AgNW-based capacitive sensing films overlaid on a LED display for Smiley face drawn by finger touch on real device. Reproduced with permission.[189] Copyright 2014, American Chemical Society. d,e) Optical image of an AgNW network patterned using laser ablation (d) and associated capacitive touch screen (e,f). Reproduced with permission.[188] Copyright 2016, Elsevier.



Several patterning strategies such as direct laser ablation (see Figure 11d),[188,190] shadow mask[184] or chemical etching using the photolithography process[182] have been reported. Capacitance change in AgNW-based devices has increased from a few %[187] to more than 40%[182,191] in the last couple of years. Multi-touch sen-sors similar to smartphone technology with device sizes up to 5 inches as shown in Figure 11e and 11f have recently been published by Dalton et al.[188]

Capacitive touch modules using AgNW transparent conductive films, man-ufactured in a real production environ-ment and with both high manufacturing yields (over 90%) and high reliability in various environments (temperature, rela-tive humidity and power), were reported in a very relevant paper by Fried et al.[192] Pei et al. published interesting results on healable touchscreens in which AgNWs are embedded into a thermally stable thin layer on a healable polymer substrate.[189] To increase reliability, further investi-gations of aging in several environmental tests on functional devices and improvement of the optoelectrical properties of hybrid AgNW-based electrodes[193] should be carried out to confirm these first promising studies. For reading applications, the integration of very low haze electrodes is mandatory. The use of high length or small diameter MNW-based electrodes should be considered to reduce the haze factor.[62,194,195]

However, despite the promising results reported for AgNW-based touch panels, emulation of new, fully flexible or rollable touch screens is time-to-market dependent on our ability to make the external parts of the entire device, such as the electrical circuits and the battery, flexible as well.

3.5. Electromagnetic Devices

3.5.1. Electromagnetic Shielding

Metallic nanowire (MNW) networks also attract intense attention for electromagnetic applications. In this section, we focus on the ability of MNW electrodes to efficiently atten-uate electromagnetic (EM) waves. The shielding effect can originate from reflection of EM waves at the surface of the shielding materials (reflection losses), from absorption of EM waves (absorption losses), and from multiple reflection losses taking place inside the shielding material itself. A major chal-lenge is to reduce the EM interference (EMI) that is likely to cause a noise signal in electrical circuits or even the malfunc-tion of the latter. EM shielding also finds markets in reducing the risks of hacking at home, as well as for companies or military purposes. The limiting factors for some of these new markets with regard to integrating them into transparent surfaces such as windows are the material costs and the low optical transparency. In addition, the market wants to orient toward light materials. Interest in 1D nanoparticle-based

materials has thus grown for EM shielding applications. The use of MNW percolating networks seems to be a promising route for fulfilling these requirements and offering appro-priate shielding devices likely to be integrated into trans-parent windows, touchscreens or even windscreens.

Gelves et al. first demonstrated the ability of CuNWs to efficiently shield electromagnetic signals.[196] They prepared cell-like CuNW networks embedded into a polystyrene matrix and found the EMI shielding effectiveness (SE) of the resulting nanocomposite to be higher than 20 dB in the range of 8–12 GHz and at a relatively low concentration of copper nanowires (1.3 vol%, i.e., roughly twice the percola-tion threshold). By putting a smartphone inside a paper box reshaped with a thin layer of AgNWs, Yang et al. also evi-denced the ability of the box to shield the RF signal from the smartphone when dialing a number (see Figure 12a).[197]

Yu et al. studied the impact of silver content on the ability of AgNWs/PVA and AgNWs/epoxy films to efficiently shield radio-frequency (RF) signals and made the comparison with films made of silver nanoparticles (AgNPs).[198] AgNWs were found to be more effective at low material content, compared to nanoparticles, considering that lower resistances could be achieved with a smaller amount of Ag as predicted by the percolation theory. By using AgNPs instead of AgNWs, the amount of Ag needed to achieve shielding effectiveness of more than 20 dB (>99%) for all the frequency ranges (from 3 to 17 GHz) was multiplied by four.[198]

Hu et al. also performed EM shielding tests on poly(ethersulfones) PES/AgNW/PET sandwich-structured films with good flexibility and transparency properties.[199] By playing with the density of the AgNW network, they obtained several films with different optoelectrical per-formances leading to specific EMI shielding performances (see Figure 12b). While increasing the density of the network, the transparency gradually decreased and the sheet resist-ance dropped (down to 1.8 Ω sq−1) while providing satisfying EMI shielding properties – more than 25 dB at a frequency

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Figure 12. a) Photographic comparison of the radio frequency (RF) field strength when dialing an Apple iPhone 4 smartphone. Top: normal (no shielding); bottom: the cell phone is shielded with a piece of single-sided, AgNW-covered Fuji Xerox paper and grounded by the zinc plate substrate. Reproduced with permission.[197] Copyright 2011, Wiley-VCH. b) Electromagnetic interference shielding effectiveness of PES/AgNW/PET sandwich-structured films with different associated AgNW network sheet resistance and transparency. Reproduced with permission.[199] Copyright 2012, American Chemical Society.



of 8 GHz. The optical transparency of the associated films was not affected too much and remained reasonable (at least 70% at the wavelength of 550 nm).

Ma et al. recently manufactured lightweight AgNW/polyimide composite foams.[200] The 3D structure consists of a microcellular structure with AgNW fillers. In order to reinforce attenuation of the electromagnetic wave, a 2D per-colating network of AgNWs was spray-deposited on the top of the structure. By adjusting the AgNW content and their aspect ratio, the resulting foams were capable of attenuating EM signals even with very low content of silver nanowire fillers (the maximum EMI SE was found to be 45−16 dB in the range of 30 MHz–1.5 GHz).[200]

Another sort of architecture was recently proposed by Kim et al.[201] They deposited a percolating network of copper nanowires by electroless plating on the top of a AgNW/polyimide composite in order to improve the electrical con-ductivity.[201] Good shielding properties (up to 55 dB in the range of 0–1.5 GHz) and resistance to cycling bending tests (10 000 cycles at a radius of 3 mm) were achieved. The optical transparency of the whole device decreased with increased plating time, but the transparency associated with the shielding performances mentioned above still remained acceptable (58%).

Using MNWs to efficiently shield EM waves has already been demonstrated by several research teams. The main advantages of MNWs over materials such as metallic nano-particles,[198,202] CNTs[203–205] and graphene[206] rely on the smaller amount of material required to efficiently shield the signals, the high level of transparency and finally the flex-ibility. Due to the very specific and original architecture of MNWs, the exact nature of the interaction between the inci-dent EM waves and MNWs has still not yet been fully under-stood. Fundamental approaches might be required to better control the shielding performances of the final devices and to manufacture more complex MNW architecture dedicated to specific applications such as transparent Frequency Selec-tive Surfaces (FSS),[207] or optimization of RF signal trans-mission through double glazed windows with reinforced thermal insulation. Finally, the shielding behavior of MNW percolating networks under mechanical stress (stretching, bending, twisting) still need to be studied further.

3.5.2. Radio Frequency Antennas

AgNW networks have also been studied over recent years as a means of developing flexible radio frequency (RF) antennas, which can be integrated into many devices such as smartphones, automotive navigation systems, wireless network systems, wearable systems for detecting motion and health monitoring, or radio frequency identification (RFID) systems. One of the most well-known flat surface RF antennas, patch antenna, is generally made of thin metallic films. Replacing these films with a percolating MNW network would: i/ drastically decrease the amount of metal required for the electrode (thus lowering the manufacturing costs), ii/ induce an increase in the flexibility of the device (useful for improving the antenna’s portability) and iii/ increase trans-parency in the visible range. However, although flexible and even stretchable RF antennas based on MNW networks have already been fabricated, most research reported so far has not dealt with the optical transparency of the resulting antenna. The MNW networks used for this application are also subject to the trade-off between transparency and con-ductivity imposed by the density of the nanowires, as the electrical level is directly related to the electromagnetic (EM) performances of the resulting devices. Reaching high transparency levels without affecting the EM performances of the end device seems to be a serious challenge.

Several teams have already examined the possibility of using AgNW-based materials as the radiating elements in antennas. For instance, Yang et al. integrated a pair of AgNW-based RFID tags on to a battery-powered LED device and showed that the electrical resistance of the whole AgNW net-work is a critical parameter that directly influences the read range distance.[197] Moreover, it has been demonstrated by Komoda et al. that the morphology of the antenna and espe-cially the smoothness of its surface is a crucial parameter that has a significant influence on the signal losses of the antenna at high-frequency radio.[208,209] They succeeded in printing an AgNW paste-based antenna measuring 3 cm in length with low signal loss, on to a flexible PET film. This antenna was integrated into a radio-controlled car (see Figure 13a) and could be controlled efficiently within a distance of 10 m. When their resistivity, surface roughness and thickness are

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Figure 13. a) A commercialized radio-controlled car with a printed AgNW paste monopole antenna on a PET substrate. Reproduced with permission.[208] Copyright 2012, Royal Society of Chemistry. b) Measured frequency response of reflection coefficient for the AgNW/PDMS microstrip patch antenna for various tensile strains. Reproduced with permission.[211] Copyright 2014, American Chemical Society. c) Photograph of a stretchable transparent RF antenna based on wavy AgNWs. Reproduced with permission.[212] Copyright 2016, American Chemical Society.



optimized, AgNW-based mixtures can be used efficiently as the building blocks for RF antennas with low return losses. However, the literature has not yet provided much informa-tion regarding the radiation performances (gain and radia-tion pattern) of transparent electrodes composed of solely MNW percolating networks, which is of great importance for obtaining feedback on the electromagnetic limitations of MNWs and therefore learning how to use them properly in RF applications.

The flexible and stretchable properties of AgNW-based materials make it possible to control the frequency response of antennas by changing the dimensions of the radiating ele-ment’s structure, paving the way for wireless strain sensing applications. Rai et al. were the first to elaborate a stretch-able RF antenna with PDMS as the dielectric substrate and an AgNW network as the radiating element.[210] More recently, Song et al. studied the stretchable and reversible properties of such AgNW/PDMS antennas in depth: for this purpose, a 3-GHz-microstrip patch antenna and 6 GHz-2 ele-ment patch arrays were fabricated and tensile strains from 0% to 15% were applied to the antennas leading to a linear shift in the resonant frequency as the strain was increased (see Figure 13b).[211] The reversibility of the deformation of these antennas could be demonstrated and their spectral properties were found to be almost the same both before and after deformation tests such as bending, twisting and rolling. Finally, Kim et al. recently managed to enhance the stretch-ability and resistance to cycling deformation tests of such antennas by using wavy AgNW networks.[212] This technique made it possible to reduce the quantity of AgNWs used, resulting for the first time in the fabrication of an optically transparent RF antenna (see Figure 13c) with reasonable radiation performances.[212]

To conclude, using flexible MNW-based materials in RF applications seems promising. In order to use them appro-priately as radiating elements in RF devices, better under-standing of the EM behavior of MNWs in this range of frequencies is required, for instance evaluating both dielec-tric and conduction losses. More in-depth studies relying on transmission lines made of MNWs might help obtain this pre-cious information. In addition, detailed study of the MNW network density necessary for achieving reasonable radi-ating performances is still essential. This would be helpful for assessing the potential MNWs have (and their limitations) for being efficiently integrated into flexible and transparent RF antennas, while transparent antennas made of MNWs still remain a serious challenge.

3.6. Other Applications

Several other applications can benefit from the properties of silver nanowire (AgNW) networks, especially in the fields of energy (storage or harvesting), flexible sensors or health-care. In addition to their promising electrical performances, AgNW-based materials are also seen as promising in these fields thanks to their transparency, conformability, how easily they can be integrated, and finally their flexibility and stretchability.

Even though photovoltaic systems are the most com-monly reported application in the field of energy, AgNWs have also made it possible to improve supercapacitors. Combined with other materials, such as graphene, metallic oxide[213] or hydroxide,[214] PEDOT:PSS,[215] 1D AgNWs have really high performances as advanced electrode materials in devices with high energy and power densities.[213] These electrochemical energy storage devices (supercapacitors) can be considered to be serious alternatives to flexible batteries (e.g., Li ion), paving the way for self-powered sensors.

In the field of sensing devices, electrochemical sensing[216] with transparent AgNW networks was used for fast and low limit detection of hydrogene peroxide. However, most studies dealing with MNW networks and sensing devices have so far focused on strain sensors,[217,218] even though the transpar-ency requirements were not always satisfied in certain cases. Combining the latest generation of elastomeric materials with AgNW networks promotes the development of low-cost, highly sensitive and stretchable strain sensors used in a wide range of applications, from the structural health monitoring of bridges, aircraft, to the human health monitoring systems or artificial e-skin. Ho et al.[219] have described an original tun-able strain gauge whose sensitivity is controlled by the areal density and waviness of a percolating AgNW network. An all-in-one self-powered system made up of a patchable trans-parent strain sensor to monitor human activities was recently published by Hwang et al.[220] As shown in Figure 14a–d, the same AgNW polymer nanocomposites were involved in the elaboration of the strain sensor, the supercapacitor and the triboelectric nanogenerator.

Stretchability is one of higher added-values of this new generation of MNW-based electrodes. For trans-parent systems, stretchability of between 50–90% has been reported.[221,222] The highest stretchability, up to 300%, was measured by Jeong et al.[223] for a non-transparent AgNW-based piezoelectric generator (rubber piezoelectric composite in between very long AgNW electrodes). This outstanding feature of conductive materials broadens the field of applications available to AgNWs, for instance in stretchable heaters (AgNW-elastomer composite) for point of care articular thermotherapy,[224] wearable and invisible human-machine interfaces[225] or for long-term health moni-toring.[226] Stretchability is also required for electrically con-ductive textile applications either on threads[99] or fabrics.[227]

AgNW-based electrodes have overcome some of the macroelectronic hindrances associated with the emer-gence of transparent and flexible thin film transistor (TFT) arrays. This has been successfully demonstrated in both organic[228,229] or metallic oxide[110,230,231] TFTs: in the latter case, the AgNWs were integrated as a transparent source and drain electrodes,[110,230] or in a more original way as a bus line in the active channel,[231] and TFTs with electron mobility of more than 100 cm² V−1 s−1 (40 times higher than without AgNWs) were reported. In the former case, a fully transparent, high performance n and p-type organic field effect transistor (FET) stacked with AgNW mats as the elec-trodes was demonstrated by Lee et al.[228] (see Figure 14e). In addition, a 3D hollow framework of AgNWs as the source and drain electrodes induced a 4-fold increase in the

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hole mobility against the use of an Ag thin film continuous electrode.[229] This can be explained by the enhancement of the contact area between the AgNWs and the semicon-ductor leading to an increase in charge injection efficiency (see Figure 14f and 14g). Photonic devices such as UV pho-todetectors[232] have also benefited from the transparency and conformability of AgNW-based electrodes. By simply elaborating an AgNW contact with other 1D nanostructures such as ZnO[233] or Ge nanowires,[234] Unalan’s team has demonstrated high performance, fully transparent all-NW UV photodetectors.

Finally, we can mention coupling AgNW plasmonic effects with surface-enhanced raman spectroscopy (SERS) measurements: this has made it possible to develop ultra-sensitive spectroscopic sensors with near-perfect absorption

across the whole visible spectrum, a detection limit at the ppb level[235] and real time measurements.[236]

All the promising features and proofs of concept reviewed in this section are likely to encourage the emer-gence of a new generation of advanced flexible devices. There is no doubt that further topics will soon be the focus of atten-tion thanks to the use of MNW networks.

4. Concluding Remarks and Outlook

The aim of this review is to demonstrate the considerable potential of MNW networks and their promising integration into devices in various fields of application such as energy, lighting, thin film heaters, etc. Beyond their excellent optical

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Figure 14. a–d) Schematic descriptions of AgNW-based patchable integrated devices (a) of TriboElectric NanoGenerator TENG (b), charging the SuperCapacitors SC (c), and strain sensors (d). Reproduced with permission.[220] Copyright 2015, American Chemical Society. e) Output characteristic of a transparent organic transistor made of AgNW-based source-drain electrodes. Reproduced with permission.[228] Copyright 2014, Royal Society of Chemistry. f,g) Schematic diagram showing charge injection of OFETs with (f) Ag thin film and (g) AgNW electrodes. Reproduced with permission.[229] Copyright 2015, American Chemical Society.



and electrical properties, these networks may present high flexibility, the ability to be conformably deposited and even stretchability, all of which are breakthrough aspects for the development of many innovative devices.

The nanowires (NWs) can be produced at large scale in solution, and processed at room temperature and ambient pressure using large area printing techniques. This gives access to low-cost fabrication processes. Different methods for optimizing the properties and improving stability have been proposed, for instance post-deposition processes to reduce the electrical resistance or using hybrid materials to limit potential thermal and chemical instabilities. MNW networks have been particularly well-studied for solar cells, LEDs, touch screens and film heaters. In each case, they proved efficient with results at least similar to ITO-based references, associated with the possibility of addressing any flexibility issues.

Further studies are still ongoing to maximize electrode efficiency and reduce their costs. First, fine-tuning synthesis to control the dimensions of the NWs, i.e., both diameter and length with very low standard deviation, is desirable because the expected properties of the networks, such as haziness or roughness, are application-dependent, and rely on their con-stitutive individual building blocks. For instance, low haze materials are preferred for displays, which has led to current research focusing on the synthesis of NWs with small diam-eters, typically less than 30 nm.[62,237] Second, developing NWs with properties similar to those of metals cheaper than Ag could be interesting. Recent developments in Cu-based nanowires are particularly promising and should decrease the material cost. Hybrid materials (for instance core-shell NWs or MNWs coated with graphene) also appear to be of interest, notably with regard to stability issues. Whatever the mate-rial, stability remains a key challenge for these electrodes. Although evaluating the intrinsic stability of these materials with regard to environmental pollutants or electrical stresses is interesting for understanding their potential limitations, the main point is to consider the electrode in its final envi-ronment in the devices, usually sandwiched in a multi-layer stack. Up to now, very successful integrations of MNWs have been demonstrated, but stability studies in real environments and real device uses are still lacking, and will undoubtedly be the next challenge for proving that these materials can be confidently integrated into industrial products.

Another aspect that must be developed is the potential risk that could arise from industrial use of MNW. Although it can be anticipated that there are many societal and individual benefits from MNW-enabled technology, safety aspects need to be tackled. There are already existing studies on the tox-icity of the nanomaterials themselves[238–241] and others are ongoing. Furthermore, potential exposure to MNWs as a result of them being released from commercial products is also currently under investigation.

The technology of innovative flexible transparent con-ducting materials based on MNWs is gaining increasingly in maturity. More efforts in fundamental research are clearly essential for understanding the scope and limitations of the materials, and continuing to develop advanced alternatives to existing technologies. At the same time, integrating MNWs

into functional devices has already proved efficient in many demonstrations, and paves the way for various industrial applications in the foreseeable future.

Acknowledgements

This work was supported by the FICHTRE ANR-13-RMNP-0015-01 project and European Joint Doctorate FunMat. The authors would also like to acknowledge the European Community through the funding support of the H2020-ICT 29–2014 LEO project. PhD grants supporting this work were provided by the DGA (French Min-istry of Defense). The authors would like to warmly thank the fol-lowing for fruitful discussions: Y. Bréchet, C. Jiménez, D.P. Langley, D. Muñoz-Rojas and N.D. Nguyen. They also thank S. Berson and T. Lescouet for collaboration on organic PV cells.

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Received: August 3, 2016Revised: September 16, 2016Published online: October 18, 2016

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