# Copper Nanowires for Transparent Electrodes: Properties, Challenges and Applications
## 2 Electrical and Optical Properties of Cu NWs  
When a thin film of nanomaterials is deposited onto a surface, such nanostructures form an interconnected network. When such nanostructures are electrically conductive, like in the case of metal NWs, electrical conduction can be measured only if there are conduction paths between electrodes, i.e., if the network is above the percolation threshold. It can be shown that, for such types of networks, the optimal conductivity is achieved for the structures with the longest length [25; 26]. This is because electrons do not just travel ballistically through a single NW, but have to hop from one to another through junctions between electrodes. Junction resistance is thus another fundamental parameter that determines the overall network conductivity, more than the intrinsic material conductivity [27]. At the same time, for maximum transparency, the structures must have the smallest possible diameter [25; 26]. Thus, the highest possible aspect ratio, intended as a length/diameter ratio, has to be sought after for the maximum conductivity associated with maximum transparency. Indeed, the scope of many synthetic methods has been to produce ultralong NWs. Moreover, a range of methods have been developed to reduce the junction resistance by melting the structures at the junction itself. The first report in which Cu NWs were used as transparent electrodes appeared in 2010 from [PERSON] et al. [18]. Here, Cu NWs were prepared by a chemical method, reducing Cu(NO\({}_{3}\))\({}_{2}\) by hydrazine in aqueous medium containing also NaOH and ethylenediamine at 80 \({}^{\circ}\)C. The authors obtained NWs with 90 nm average diameter and 9 \(\upmu\)m average length, as shown by scanning electron microscopy (SEM) images in Figure 1. While _Rs_ was as low as 15 \(\Omega\)/sq at \(T=65\%\), no data were reported at higher \(T\), due to the large diameter of the NWs [18]. Such results were later improved by changing the synthesis method, using a heated reaction environment (80 \({}^{\circ}\)C) only for a short time, adding then polyvinylpyrrolidone (PVP) and cooling down the mixture in an ice bath [19]. In this way, NWs grew longer and thinner, with 20 \(\upmu\)m length and 52 nm diameter, yielding networks with \(T\) up to 90% corresponding to _Rs_ = 186 \(\Omega\)/sq. Other synthetic methods to improve NW morphology were also reported. [PERSON] et al. reported a synthesis in a liquid crystalline medium using hexadecylamine and cetyl trimethylammonium bromide (CTAB) and Cu acetylacetonate as precursor [28]. Such synthesis allowed the formation of NWs with 78 nm average diameter and length over 100 \(\upmu\)m, with _Rs_ = 90 \(\Omega\)/sq at 90% transmittance. Compared to Ref. [19], for the same \(T\) the halving of _Rs_ is most likely due to the much longer length, despite the wider diameter. [PERSON] et al. obtained very thin NW (16 nm) with 40 \(\upmu\)m length carrying out a reaction in oleylamine using a Ni-based catalyst [29]. A high transmittance of 93% was reported,associated with a low \(Rs\) of 51 \(\Omega\)/sq. Compared to previous reports, the improvement here is mainly due to the very small NW diameter. Another synthetic method to produce very long NWs was reported by [PERSON] et al., who used octadecylamine and CuCl\({}_{2}\) reacting at 165 \({}^{\circ}\)C for 140 h, to obtain films with \(Rs\) = 55 \(\Omega\)/sq at \(T\) = 94% [30]. Unlike many other reports, the authors here also study haze, intended as the amount of light that is scattered rather than transmitted. While haze is detrimental for applications like displays or LED as they decrease transmittance, this can be seen as beneficial for solar cell applications as it allows the scattered light to be reabsorbed and thus utilized for energy production. Haze evaluation is thus related to the specific application. It was found that haze is somehow related to the transmittance, and is minimum for maximum transmittance [30]. Interestingly, the authors report diameters of a few hundred nm associated with lengths lower than 50 nm.