# Copper Nanowires for Transparent Electrodes: Properties, Challenges and Applications
## 4 Joining of Nanowires  
As already mentioned, junction resistance is a key factor that affects the conductivity of the whole network. Over the years, methods have then been developed to join, or weld, the NWs at the junctions, in order to eliminate, or cut down, the junction resistance. Most of these methods were firstly adopted for Ag NWs and involve the use of heat and/or light [54; 55].  
Such methods for Cu NWs also have the secondary effect of removing the oxide layer. [PERSON] et al. proposed a high intensity pulsed light to induce both deoxygenation and welding [56]. Figure 6a,b shows tilted SEM images before and after the pulsed light treatment, and it can be observed that NWs are joint together after the treatment. Moreover, increasing the number of pulses dramatically improves the optical and electrical performance of the network (Figure 6c) [56], although no data were presented above 85% transmittance. Using the same method, the same group fabricated a polyurethane (PU) composite [57]. Here, the pulsed light also softened the PU matrix allowing embedding the NWs.  
[PERSON] et al. proposed a hydrogen plasma treatment to weld the NWs together, and at the same time improve their stability towards oxidation [58]. Here, the plasma treatment allowed cleaning the NW surface from organic residues from the synthesis process and oxide. At the same time, plasmonic effects allowed light intensity enhancement at the NW  
Figure 5: (**a**) \(Rs\) vs. \(T\) comparison for pristine Cu NW and RGO films; (**b**) \(Rs\) vs. \(T\) comparison of pristine Cu NW films and NW-RGO composite films; (**c**) \(Rs\) evolution over time for pristine Cu NW and NW-RGO films at room temperature and at 60 \({}^{\circ}\)C. Adapted with permission from Ref. [49]. Copyright 2013 American Chemical Society.  
ends, causing thermal heating which induced melting [58]. Indeed, the Cu NWs had a plasmon resonance peak at 576 nm and a diameter of 60-100 nm, while the plasma had emission in the 570-610 nm range. [PERSON] et al. used flash light to both sinter Cu NWs, remove residues from the NW surface and induce adhesion to polyethyleneterephthalate (PET) by thermally melting the NW-PET interface (Figure 7) [59]. Similarly, plasmonic effects were exploited by [PERSON] et al. using a continuous 532 nm laser focused onto the NW network surface, for both preventing oxidation and welding the NWs [60]. Here, the Cu NWs had a diameter of 80-120 nm. Welding by electroplating was proposed by [PERSON] et al. who used the Cu NW film as electrode in a plating solution, then by applying voltage with a phosphor counter-electrode chemical welding was achieved [61].  
A welding method not involving the use of any form of light was also proposed by [PERSON] et al., who synthesized core-shell NWs by electrodeposition Ni, Ag or Au-Ag on the Cu NW surface [62; 63; 64]. Here the metal shell, other than providing protection from oxidation, also provided welding by joining the NWs at the junctions, as shown in Figure 8.  
Figure 6: Tilled SEM images of Cu NW before (**a**) and after (**b**) welding by high intensity pulsed light; (**c**) \(T\) vs. \(Rs\) plot of Cu NWs exposed to increasing nmber of light pulses. Adapted with permission from Ref. [56]. Copyright 2015 Royal Society of Chemistry.  
Figure 7: Schematics of the flash lamp treatment process, allowing junction welding (i), photochemical reduction (ii) and interlocking (iii). Adapted with permission from Ref. [59]. © 2021 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.  
In summary, methods for joining, or welding, Cu NWs mainly involve the use of light, which by thermal or plasmonic effects melt NWs at the junctions reducing the junction resistance. An electrodeposition method was also proposed, allowing welding as well as oxidation protection.