The Synthesis and Coating of Long, Thin Copper Nanowires to Make Flexible, Transparent Conducting Films on Plastic Substrates

This Communication describes the synthesis of long ( > 20 μ m), thin ( 500 mA cm − 2 ), were stable in air for over one month, and could be bent 1000 times without any degradation in their properties. Indium tin oxide (ITO) is the transparent conductor of choice for most applications because of its high transmittance and conductivity, but it is scarce, brittle, and expensive. [ 1a , 2 ] The cost of ITO fi lms is due not only to the fact that indium is a rare and costly material, but also because ITO must be deposited in an ineffi cient, low-throughput, vapor-phase coating process that, at 0.01 m s − 1 , is 1000 times slower than wet-coating processes such as newspaper printing. The limitations of ITO and other transparent conducting oxides have motivated a worldwide search for fl exible, low-cost alternatives that can be deposited from liquids at coating rates orders of magnitude greater than vapor-phase coating processes. Solution-coated fi lms of carbon nanotubes (CNTs) are one fl exible alternative to ITO, but to date they have a relatively low transmittance and sheet resistance due to their absorbance of light and the poor electrical contact between nanotubes. [ 3 ] The performances of solution-coated fiof graphene are generally inferior to that of CNT fi lms. [ 4 ] Solution-coated fi lms of silver nanowires (AgNWs) have a transmittance and sheet resistance close to ITO, but silver is also scarce and expensive. [ 5 ] Copper (resistivity ρ = 1.59 n Ω m) is nearly as conductive as silver (1.67 n Ω m), but it is 100 times less expensive and 1000 times more abundant. [ 6 ] Motivated by these fundamental advantages of copper, we have recently reported a scalable synthesis of copper nanowires (CuNWs) and fi ltered them from solution to make transparent conducting fi lms. [ 7 ] However, the properties of the fi lms were not as good as those made with AgNWs; at a sheet resistance of 15 Ω sq − 1 , the transmittance of a AgNW fi lm was about 85%, while that of a CuNW fi lm was only 65%. Three reasons why the CuNW fi lms were not as conductive as the AgNW fi lms were the relatively short lengths (10 ± 3 μ m), large diameters (90 ± 10 nm), and aggregation of the nanowires. Here we report a new synthesis that produces longer, thinner, well-dispersed CuNWs. These CuNWs were incorporated into an ink that could be coated onto a clear, plastic substrate to give a fl exible, transparent conducting fi lm with properties equivalent to fi lms of AgNWs. In contrast to ITO, these fi lms can withstand severe mechanical deformation and remain highly conductive. To grow nanowires in our previous study, we kept the reaction mixture at a constant temperature (80 ° C) throughout the nucleation and growth of the CuNWs. [ 7 ] This resulted in the formation of nanowires that were relatively short ( 90 nm in width). Furthermore, the capping agent used in the synthesis, ethylenediamine (EDA), was found to be a poor dispersant for the CuNWs. As a result, aggregation of the CuNWs made it diffi cult to fabricate uniform fi lms with high optical transmittance and low electrical resistance. In an effort to improve the properties of transparent conducting fi lms made from CuNWs, we developed a new synthesis to produce longer, thinner CuNWs that are also well dispersed. In order to grow longer, thinner wires, we heated the reaction mixture only a short time (about 3 min at 80 ° C) in order to induce reduction of copper ions (as indicated from a change in the reaction color from blue to clear). Polyvinylpyrrolidone (PVP) was then added to this mixture to prevent the CuNWs from aggregating and this mixture was quickly cooled in an ice bath. This process allowed the CuNWs to grow at a lower temperature, resulting in a longer, thinner morphology. Interestingly, if PVP was added to the reaction before heating, only copper nanoparticles formed. In contrast, the addition of PVP to the reaction after the 3 min heating stage did not prevent nanowires from forming, but instead prevented the CuNWs from forming aggregates that could not be dispersed. Once well-dispersed CuNWs were obtained, we wanted to demonstrate a scalable coating method for fabrication of transparent electrodes with CuNWs. In previous work, we fi CuNWs from solution onto a membrane and transferred them onto a glass slide coated with clear glue. Although this method enables the fabrication of fi lms from precise amounts of relatively clean nanowires, it is not a practical method for fabricating transparent electrodes over large areas. Spray coating is a potentially scalable method for fabrication of electrodes from